applied
sciences
Article
Recent Progress on Irradiation-Induced Defect
Engineering of Two-Dimensional 2H-MoS2
Few Layers
Guang-Yi Zhao 1, * , Hua Deng 2 , Nathaniel Tyree 3 , Michael Guy 3 , Abdellah Lisfi 3 ,
Qing Peng 4 , Jia-An Yan 5 , Chundong Wang 6 and Yucheng Lan 3, *
1
2
3
4
5
6
*
Nuclear Science and Technology Center, School of Physics, Jilin University, Changchun 130012, China
Department of Chemistry, Morgan State University, Baltimore, MD 21251, USA; Hua.Deng@morgan.edu
Department of Physics and Engineering Physics, Morgan State University, Baltimore, MD 21251, USA;
natyr1@morgan.edu (N.T.); miguy2@morgan.edu (M.G.); Abdellah.Lisfi@morgan.edu (A.L.)
Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, USA;
q@qpeng.org
Department of Physics, Astronomy, and Geosciences, Towson University, Towson, MD 21252, USA;
jyan@towson.edu
School of Optical and Electronic Information, Huazhong University of Science and Technology,
Wuhan 430074, China; apcdwang@hust.edu.cn
Correspondence: zhaogy@jlu.edu.cn (G.Z.); yucheng.lan@morgan.edu (Y.L.); Tel.: +1-443-885-3752 (Y.L.)
Received: 14 January 2019; Accepted: 8 February 2019; Published: 16 February 2019
Abstract: Atom-thick two-dimensional materials usually possess unique properties compared
to their bulk counterparts. Their properties are significantly affected by defects, which could be
uncontrollably introduced by irradiation. The effects of electromagnetic irradiation and particle
irradiation on 2H MoS2 two-dimensional nanolayers are reviewed in this paper, covering heavy ions,
protons, electrons, gamma rays, X-rays, ultraviolet light, terahertz, and infrared irradiation. Various
defects in MoS2 layers were created by the defect engineering. Here we focus on their influence
on the structural, electronic, catalytic, and magnetic performance of the 2D materials. Additionally,
irradiation-induced doping is discussed and involved.
Keywords: MoS2 ; 2D material; irradiation; review
1. Introduction
With the discoveries of zero-dimensional buckyballs in the 1980s [1] and one-dimensional
carbon nanotubes in the 1990s [2,3], various nanomaterials have been synthesized and characterized.
Their physical and chemical properties are unique compared with counterpart bulks (graphite,
diamond, and amorphous carbon) because of size-induced quantum effects, enabling their great
potential in various fields. Two-dimensional (2D) graphene was reported in the 2000s [4] and excellent
electrical behaviors were claimed. Since then investigations of 2D layers were initialized and rapidly
developed in recent decades. At present, the 2D layer family spans conductors (such as graphene),
insulators (such as hexagonal boron nitride), and semiconductors. Transition-metal dichalcogenide
(TMDC) materials, including MoS2 , WS2 , MoSe2 , and WSe2 , are layered semiconductors and have been
fabricated into 2D semiconducting layers. MoS2 2D layers have novel potential applications in novel
nano-optoelectronics, optical sensors, catalysts, energy storages, and environments. Additionally,
MoS2 2D layers show thickness-dependent semiconducting behaviors, being the most interesting and
important among TMDC 2D layers. Bulk MoS2 is a semiconductor with an indirect energy band gap of
1.2 eV while mono-layer MoS2 is a direct band-gap semiconductor with an energy band gap of 1.9 eV
Appl. Sci. 2019, 9, 678; doi:10.3390/app9040678
www.mdpi.com/journal/applsci
Appl. Sci. 2019, 9, 678
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due to 2D quantum confinement. The MoS2 layers have also important applications in valleytronics—a
new type of electronics where information is encoded by wave quantum number of electrons.
MoS2 low-dimensional materials have been investigated over the past 50 years. MoS2 few-layers
(4–5 layers) were first prepared by a peeling process from bulk crystals in the 1960s [5].
Tribological properties of MoS2 films have been studied later in a vacuum or dry air as solid lubricants
since the 1990s [6,7]. Compared with bulks, the friction coefficient of MoS2 films was reduced, and the
wear life of the films was enhanced. Catalytic properties of MoS2 nanoparticles [8] and films [9,10] have
been investigated from the 1980s. It is believed that edge sites of MoS2 nanomaterials play important
roles in electrochemical and catalytic behaviors. MoS2 atomically thin materials, especially mono-layer
MoS2 , have been widely investigated in recent years because of their sizable direct band gap, high
charge-carrier mobility, and excellent mechanical flexibility [11,12]. The materials have potential
applications in the next generation flexible electronics [11,13,14], elastic energy storage[12], field-effect
transistors [15,16], electronic switches [13], electronic devices such as chemical biosensors [16,17],
and optoelectronic devices [18] such as photodetectors and solar cells [19]. It was demonstrated
that chemically active defects, such as sulfur vacancies, significantly control or tune their catalytic
activities [20], electronic transports [21–23], and optical properties [24].
Various forms of irradiation and particle fluxes modulate the density of sulfur vacancies of
chemical-vapor deposition (CVD) grown and mechanically exfoliated MoS2 sheets, affecting their
chemically active defects. Therefore, irradiation effects have been carried out in recent years.
Additionally, these MoS2 devices and sensors have been used in harsh environments, and it is necessary
to investigate their tolerance under irradiation. Radiation effects on two-dimensional graphene were
recently reviewed [25,26]. Defect engineering and defect-induced properties of other atom-thick
mono-layers were summarized as well [26]. The impacts of irradiation on 2H MoS2 two-dimensional
materials are reviewed here.
1.1. MoS2 2D Materials
MoS2 has two main types of natural phases: hexagonal structure (2H type) with the space
group of P63 /mmc (D46h , No. 194) and the trigonal structure (3R type) with the space group of R3m
(C53v , No. 160). The 2H phase contains two layers per unit cell, a = 3.15 Å and c = 12.30 Å. Similar to
graphite, the 2H type crystals can be easily cleaved to form (0001) layers because of weaker van
der Waals forces between (0001) layers. The 3R phase contains three layers per unit cell, stacking in
rhombohedral symmetry with trigonal prismatic coordination, a = 3.16 Å and c = 18.33 Å. A third
metastable phase, 1T-MoS2 with a tetragonal symmetry, was also artificially produced by intercalating
2H-MoS2 with alkali metals [27–29]. The artificial 1T phase and natural 3R phase are metastable at
room temperature and transform to the 2H phase on heating [27] or IR irradiation [30] or microwave
irradiation [31]. This work focuses on the 2H MoS2 phase. 1T and 3R polytypes will not be discussed.
Molybdenum disulfide (2H-MoS2 ) is a transition-metal dichalcogenide with a melting point of
2375 ◦ C. The bulk material is chemically stable in dilute acids and oxygen and insoluble in water.
Figure 1a shows the crystallographic structure of 2H-MoS2 bulks. There are two S-Mo-S layers per unit
cell stacked in hexagonal symmetry. Each S-Mo-S layer consists of two hexagonal planes of sulfur atoms
and an intermediate hexagonal plane of molybdenum atoms. The sandwiched molybdenum atom
plane is coordinated through ionic-covalent interactions with the sulfur atoms, forming a stable MoS2
mono-layer as shown in Figure 1b. The MoS2 mono-layers are held together by weak van der Waals
interactions. The forces between the mono-layers are not very strong [32], and the interatomic forces
within a mono-layer are sufficient for thermodynamical stability [33]. Therefore, 2H-MoS2 has been
widely used as a dry lubricant under high temperature due to the microscaled chemical properties.
Appl. Sci. 2019, 9, 678
(a)
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(b)
Figure 1. (a) Crystallographic structure of 2H-MoS2 bulks and (b) Three-dimensional structure of
2H-MoS2 mono-layers.
Figure 2 shows the calculated band gap of MoS2 bulks and that of MoS2 few-layers. The bulk
2H-MoS2 crystal is an indirect band-gap semiconductor with a band gap of 1.23–1.29 eV [32,34–36].
MoS2 bulks can be employed as cathode materials in batteries [29]. MoS2 bulks show very poor
catalytic activities for hydrogen evolution reactions. The band gap of MoS2 is a function of layer
thickness as shown in Figure 2b–d, and unperturbed by substrate interactions. The mono-layer MoS2
has a direct band gap of 1.8 eV [37]. The band structure changes from the indirect-gap in bulks to the
direct-gap in MoS2 mono-layers, being confirmed from experiments [38,39]. The thickness-dependent
electronic structure change is closely associated with many corresponding changes in chemical and
physical properties. For example, the transition for indirect-to-direct band gap is correlated with
changes in mechanical strength, spin density, bond energy, electrical conductivity, and the properties
of transistor and sensor devices. MoS2 nanoparticles can adsorb various compounds, including
tetrahydrothiophene, thiophene, benzothiophene, dibenzothiophene, and their derivatives [40].
The chemical [29,41] and physical properties [42–44] of MoS2 few-layers were recently reviewed.
These novel properties of MoS2 2D materials make them suitable 2D candidates in environmental
applications [45,46], catalysts (such as electrocatalysts [47–51] and photocatalysts [48–53]), energy
storage (such as lithium ion batteries [48,49,54], supercapacitors [49,54], elastic energy [12], and
solar cells [49]), as well as sensing [55]. MoS2 mono-layers were also fabricated into ultrasensitive
photodetectors [56,57], and integrated circuits to perform the NOR logic operation [58], besides being
employed in drug delivery [59], thermoelectrics [60,61], piezotronics [62,63], osmotic nanopower
generators [64], and valleytronics [65].
Because of the novel properties and wide applications of 2H-MoS2 2D layers, MoS2 2D layers
have been prepared by various physical or chemical methods [41,44–46,66–69]. Individual MoS2 layers
were usually obtained by the micromechanical cleavage technique [70] because of weak interactions
between sulfur layers and strong intralayer interactions of MoS2 bulks. The mechanically cleaved MoS2
layers are usually highly crystalline with low defect density. Massive MoS2 layers have successfully
been produced through the liquid-phase exfoliation [71,72]. MoS2 few-layers can also be fabricated
from CVD. Interested readers can refer to the references herein.
The MoS2 -based devices will be used in harsh environments from basic science to industry, such
as near nuclear plants/reactors and nuclear medicine imaging. Radiations may damage MoS2 layers
by breaking atomic bonds and ionizing atoms. To use the MoS2 devices reliably under irradiation, it is
desirable to explore the irradiation effects on MoS2 layers.
Appl. Sci. 2019, 9, 678
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conductor renders krelax ) 0, le
of luminescence that is only lim
rate kdefect. In MoS2 of all thickn
transition at the K point rema
energy.
The unusual electronic struct
the resulting unique optical beha
ters of d-electron orbitals that c
and valence bands. Our theoret
electronic states of different w
orbitals with different spatial
conduction band states at the K
posed of strongly localized d orbi
have minimal interlayer coupling
in the middle of the S-Mo-S un
FIGURE 4. Calculated band structures of (a) bulk MoS2, (b) quadFigure 2. Calculated
band
structures
of (a) bulk MoS2 , (b) quadri-layer MoS2 , (c) bi-layer MoS2 , and
rilayer
MoS
2, (c) bilayer MoS2, and (d) monolayer MoS2. The solid
states near the Γ point and the
(d) mono-layer MoS
The solid
arrows
indicate
the lowest
Bulk MoS2 is
arrows
indicate
the lowest
energy
transitions.
Bulkenergy
MoS2 istransitions.
charac2 [39].
originate from a linear combin
by anband
indirect
bandgap.
The direct
excitonic
transitions
characterized by terized
an indirect
gap.
The direct
excitonic
transitions
occuroccur
at high energies at K
at high energies at K point. With reduced layer thickness, the
atoms and antibonding pz orbita
point. With reduced layer thickness, the indirect band gap becomes larger, while the direct excitonic
indirect bandgap becomes larger, while the direct excitonic transistrong interlayer coupling and th
transition barely changes.
For
mono-layer
MoS2 in (d),
it 2becomes
a direct band-gap
in d, it becomes
a direct semiconductor.
tion barely
changes.
For monolayer
MoS
on layer thickness.
bandgap
semiconductor.
This dramatic
change
of electronic
The horizontal solid
lines in
each panel indicate
the valence
band
maximumstruc(VBM) and tively
the dotted
ture in monolayer MoS2 can explain the observed jump in monolayer
conclusion, our study revea
lines indicate the conduction band minimum (CBM). The solid blue arrows indicate theIn
lowest
photoluminescence efficiency.
of
photoluminescence
in MoS2
c
energy transitions. Reprinted with permission from Reference [39]. Copyright
2010 American
consistent
with
the
theoretical
pre
Chemical Society.
likely to change appreciably with respect to bulk value,
bandgap transition in going from
1.2. Irradiation Sourcesbecause the direct excitonic transitions remain at the same
MoS2. Such behavior, arising fro
energy. Therefore, the enhanced photoluminescence in
in MoSof
2, may also arise i
Figure 3 shows the spectrum of electromagnetic irradiation. Cosmic rays aretions
composed
monolayer has to be attributed to a dramatically slower
metal
dichalcogenides.
It point
particularly high-energy particles and photons. Primary cosmic rays are composed primarily of 99%
electronic relaxation krelax. The decrease of interband relaxcontrolling
electronic
structure
nuclei (about 90% protons, 9% alpha particles, and 1% nuclei of heavier elements) and about 1%
ation rate strongly suggests a substantial change in MoS2
exploiting
rich d-electron physic
solitary electrons. The energy of the cosmic rays is high up to 1020 eV for the primary
cosmic rays.
electronic structure when going from the bulk to monolayer.
to engineering novel optical be
When entering the Earth atmosphere, the primary cosmic rays collide with atoms and molecules
Electronic structure of bulk and monolayer MoS2 has
bonded materials and holds prom
to produce secondary cosmic particles with lower energy and electromagnetic
waves, including
beenpreviouslyinvestigatedthroughabinitocalculations.20,21
applications.
gamma-ray, X-ray, neutrons,
protons, electrons,
and improved
alpha particles.
The energy
of the generated
Recent theoretical
results using
algorithm
and
gamma rays can be 50computation
MeV. These primary
and secondary
cosmic rays
shouldbulk
damage microelectronic
power predict
that the indirect
bandgap
Acknowledgment.
We thank
devices because of sufficient
energy,
causing
soft
errors
in
electronic
integrated
especially
MoS2 becomes a direct bandgap semiconductor when circuits,
helpful
discussion
and
Professor
21
in small-scale devices.
Here,toseveral
ion irradiation
from GeV to
thinned
a monolayer,
whichand
canelectromagnetic
readily accountirradiation
for
measurements. The work was
meV are reviewed onour
2Dexperimental
MoS2 few-layers.
Physical properties
of MoS
films are
also
2 bulks and thick
observations.
For a detailed
comparison
Science
Foundation
CAREER aw
covered for comparison
with
those
of
2D
MoS
few-layers.
2
between experiment and theory, we calculated the band
(F.W.) and by DOE-BES Grant D
structures of bulk MoS2 and ultrathin MoS2 layers with
and T.L.). Y.Z. is supported by a
acknowledges support from Scu
different thicknesses. We employed density functional
periori di Padova and the EAP.
theory with generalized gradient approximation using the
22
PWscf package. The calculation
results are displayed in
THz
REFERENCES AND NOTES
Figure 4. It is shown that the direct excitonic transition
(1)
Novoselov, K. S.; Jiang, D.; Sche
energy at Audio
the Brillouin zone K point barely changes
with
Cosmic
V. V.; Morozov, S. V.; Geim, A. K.
rays
layer thickness, but the indirect bandgap increases mono102, 10451.
(2)
Novoselov, K. S.; Geim, A. K.; M
tonically as the number of layers decreases. Remarkably,
son,
M. I.; Grigorieva, I. V.; Dub
the indirect transition energy becomes so high in mono2005, 438, 197.
layer MoS2 that the material changes into a two-dimen(3)
Zhang, Y.; Tan, Y.-W.; Stormer,
201.
sional direct bandgap semiconductor. The variation of the
(4)
Yoffe, A. D. Adv. Phys. 2002, 51
electronic structure in few-layer MoS2 is in accord with
(5)
Ayari, A.; Cobas, E.; Ogundadegb
our experimental data. With the increase of the indirect
2007, 101, No. 014507.
Figure 3. Spectrum of electromagnetic irradiation.
(6)
Gourmelon, E.; Lignier, O.; Hado
bandgap
in thinner MoS2, the intraband relaxation rate
J. C.; Tedd, J.; Pouzed, J.; Salard
from the excitonic states decreases and the photolumiCells 1997, 46, 115.
nescence becomes stronger. In the case of monolayer
(7)
Ho, W. K.; Yu, J. C.; Lin, J.; Yu, J.
5865.
MoS2, a qualitative change into a direct bandgap semi© 2010 American Chemical Society
1274
DOI: 10.1021/nl903868
Appl. Sci. 2019, 9, 678
5 of 53
1.3. Irradiated MoS2 Materials
Defects are created in MoS2 few-layers when the layers are fabricated by exfoliation technique and
vapor deposition growth when layers are transferred from substrate to substrate, and placed under
electromagnetic radiation and energetic particle fluxes. The defects typically deteriorate the device
quality of MoS2 layers. On the other hand, defects in MoS2 also have a beneficial impact on material
properties. For instance, defects have been shown to be the dominant dopant in MoS2 , with natural
defect variation allowing n-type and p-type regions to coexist across distinct regions of the same
sample [73]. The sulfur vacancies can lower Schottky barrier heights of MoS2 [73]. Therefore, it is
crucial to fully understand defects in MoS2 to develop a reliable MoS2 -based electronics. In this paper,
irradiation-related defects of MoS2 few-layers are focused on.
Up to now, MoS2 2D layers have been irradiated under different irradiation sources. Table 1
lists some typical irradiation effects of MoS2 materials. The details are summarized in the following
sections. The particle irradiation and electromagnetic irradiation will be reviewed first on structural
properties and defects, then theoretical explanations of the irradiation mechanism are discussed. The
irradiation-induced band structures, electric, catalytic, and magnetic properties will be summarized at
the end. To understand the irradiation-induced defects well, the synthesis/preparation details of the
as-prepared MoS2 layers will be described in each section in addition to irradiation.
Appl. Sci. 2019, 9, 678
6 of 53
Table 1. Radiation Induced Defects in 2H MoS2 .
MoS2
Irradiation
Particle Energy
Dose/Fluence
Irradiation Source
Irradiation Conditions
Results
Ref.
200 µm thickness
electron
0.7 MeV
150–600 kGy
electron accelerator
RT
created S vacancies
[74]
created S vacancies and Mo vacancies, diamagnetic to
ferromagnetic phase transition
[74]
formed crystalline islands
[75]
200 µm thickness
electron
2.0 MeV
20 layer
electron
5 keV
100–250 kGy
electron accelerator
RT
50 nA for 15 min
SEM beam
in vacuum
106 –109
electron
60 keV
induced 2H/1T phase transition
[76]
∼10 layer
electron
3–15 keV
n/a
EPMA
in vacuum
broke the inversion symmetry
[77]
mono-layer
electron
80 keV
n/a
TEM beam
in vacuum
removed top and bottom S atoms
[78]
electrons/nm2 /s
STEM beam
400–700
◦C
mono-layer
3000
electron
/nm2
in vacuum
mono-layer
electron
200 keV
TEM beam
in vacuum
created S vacancies, increased electric resistance
[79]
mono-layer
electron
15 keV
280 µC/cm2
EBL
in vacuum
produced local strain and changed band structure
[80]
mono-layer
electron
80 keV
40 A/cm2
TEM beam
in vacuum
produced holes and Mo5 S3 nanoribbons
[81]
amorphous 5–7 layer
electron
1 keV
1–10 min
EBI
in vacuum
crystallized
[82]
mono-layer
U238
1.14 GeV
4000 ions/cm2
heavy ion accelerator
in vacuum
total damaged
[83]
micron thickness
Ar+
500 eV
2.26 × 1015 ions/cm2
plasma
UHV
produced S vacancies
[84]
bi-layer
Ar+
500 eV
1014 –1015 ions/cm2
plasma
UHV
produced S vacancies and MoS6 vacancy clusters
[84]
mono-layer
Ar+
500 eV
2.26 × 1015 ions/cm2
plasma
UHV
damaged
[84]
200 µm thickness
proton
3.5 MeV
5 × 1018 ions/cm2
Singletron facility
RT
preserved lattice structure, produced defects, changed
magnetic moments
[85]
few-layer
proton
10 MeV
1012 –1014 ions/cm2
MC-50 cyclotron
n/a
decreased electrical conductance
[86]
particles/cm2
mono-layer
proton
100 keV
1012 –1015
LEAF
n/a
created defects
[87]
bi-layer
proton
100 keV
6 × 1014 particles/cm2
LEAF
n/a
created defects
[87]
bulk
He2+
1.66 MeV
900 MGy
ion accelerator
n/a
changed Raman scattering slightly
[88]
nanosheet
He2+
1.66 MeV
900 MGy
ion accelerator
n/a
invariant
[88]
few-layer
He2+
30 keV
1018 ions/cm2
FIB beam
in vacuum
mono-layer
He2+
3.04 MeV
8 × 1013 particles/cm2
PTA
n/a
mono-layer
He+
30 keV
1012 –1016 ions/cm2
HIM
in vacuum
produced S vacancies
[91]
mono-layer
He+
30 keV
1013 –1017 ions/cm2
NFM
in vacuum
generated disorder
[92]
bulk
Ar+
500 eV
3 × 10−3 ions/cm2
STM
UHV
removed S atoms
[93]
micron thickness
Ar+
500 eV
1014 –1015
n/a
UHV
produced S vacancies at low doses and Mo/MoS6
vacancies at high doses
[84]
powder
Ar+
3 keV
n/a
n/a
removed S atoms
[94]
ions/cm2
0.1 A/cm2
milled or damaged
[89]
produced defects
[90]
Appl. Sci. 2019, 9, 678
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Table 1. Cont.
MoS2
Irradiation
Particle Energy
Dose/Fluence
Irradiation Source
Irradiation Conditions
200 nm
Ar+
180 keV
1015 –1017 ions/cm2
IBAD
200 ◦ C in vacuum
Results
Ref.
induced amorphization
[95]
[84]
bi-layer
Ar+
500 eV
1014 –1015 ions/cm2
n/a
UHV
produced S vacancies at low doses and Mo/MoS6
vacancies at high doses
mono-layer
Ar+
500 eV
1014 –1015 ions/cm2
n/a
UHV
produced S vacancies at low doses and Mo/MoS6
vacancies at high doses
[84]
mono-layer
Ar+
500 eV
3–11 µA/cm2
sputter beam
vacuum
selectively removed S atoms without significantly
depleted Mo atoms
[96]
mono-layer
Ar+
13.56 MHz
n/a
RF plasma
RT
produced 2H/1T phase transition
[97]
mono-layer
Ga ion
n/a
n/a
PAMBE
450 ◦ C in vacuum
doped Ga, reduced binding energy
[98]
mono-layer
Ga ion
30 keV
1013 ion/cm2
FIB beam
in vacuum
produced sub-nm pores and vacancies
[99]
bulk
Xe
91 MeV
10 ions/µm2
GANIL
n/a
formed nano-hillocks
[100]
few-layer
Xe
91 MeV
15 ions/µm2
GANIL
n/a
formed nano-incisions
[100]
ions/µm2
GANIL
n/a
formed nano-incisions
[100]
EBIT
n/a
induced pits and hillocks
[101]
HIRFL
vacuum, RT
formed hillocks
[102]
TOF-SIMS
UHV
formed defects
[103]
mono-layer
Xe ion
91 MeV
mono-layer
Xe ion
25–30 keV
1–4 layer
Bi ion
0.45–1.23 GeV
mono-layer
Mn ion
25 keV
15
1010 ion/cm2
1010 –1012
ions/cm2
1012 –1014 ions/cm2
109 –1011
ions/cm2
bulk
C60
20–40 MeV
micron thickness
γ-ray
n/a
5 × 109 R
TA
RT
modified structures
[104]
RGIF
in air
no damaging effects
[105]
50–132 layer
γ-ray
662 keV
5000 photons
137 Cs
source
5–8 layer
γ-ray
∼1.2 MeV
120 Mrad
60 Co
source
RT in air
unaffected
[106]
RT in air
converted to MoOx
[107]
1–3 layer
X-ray
10 keV
6 Mrad
XRD
RT in air
film
UV
n/a
n/a
mercury lamp
in O2 (ozone)
mono-layer and multilayer
UV
n/a
1010 –1013 photos/cm2
deuterium lamp
multilayer
laser
λ = 514 nm
1–20 mW for 1–100 s
laser
few-layer
laser
λ = 532 nm
300 mW
diode laser
in air or vacuum
mono-layer
laser
λ = 800 nm
20–50 mJ/cm2
Ti:sapphire laser
RT
no noticeable degradation
[108]
formed oxygen-sulfur bonds
[109]
vacuum
no structural damage, no oxidation
[110]
RT in air
stable or damage depending on laser power
[111]
patterned and thinned
[112]
damaged or unaffected depending on irradiation intensity
[113]
EBI: electron-beam irradiation; EBIT: electron-beam ion trap; EBL: electron-beam lithography system; EPMA: electron probe micro-analyzer; FIB: focused ion beam; GANIL: The
Grand Accélérateur National d’Ions Lourds (Large Heavy Ion National Accelerator); HIM: helium-ion microscope; HIRFL: heavy ion Research facility in Lanzhou; IBAD: ion-beam
assisted deposition; LEAF: low-energy accelerator facility; NFM: NanoFab microscope; PAMBE: plasma-assisted molecular beam epitaxy; PTA: Pelletron tandem accelerator;
RF: radio frequency; RGIF: Reactor Gamma Irradiation Facility; RT: room temperature; S: sulfur; SEM: scanning electron microscope; STEM: scanning transmission electron
microscope; STM: scanning tunneling microscope; TA: Tandem accelerator; TEM: transmission electron microscope; TOF-SIMS: time-of-flight secondary ion mass spectrometer; UHV:
ultra-high vacuum; XRD: X-ray diffractometer.
Appl. Sci. 2019, 9, 678
012101-4
8 of 53
Mishra et al.
structural degradation of MoS2 layers. However, softening of
2.1
1
1
E2g and enhancement of I(A1g)/I(E2g ) indicates towards
dia
2.1. Swift-Heavy Ion Irradiation
p-type doping of ML-MoS2 after N2 - and Ga-irradiation.
is 0
shifted
toward lower
Fig. 4(a)
shows to
Movery
3d5/2
ble
Swift-heavy ions are usually accelerated in particle
accelerators
highpeaks
energies,
typically
binding
energies
for
Ga-irradiated
samples
relative
to
that
of
pol
in the MeV or GeV range. They have sufficient kinetic energy and mass to penetrate solid materials
pristine
ML-MoS
4(b) atoms,
shows induce
Ga 2p heating,
for pristine MLwe
2. Fig.
along straight lines. Therefore, heavy ions release
sufficient
energy
displace
and
Ga
irradiated
sample.
The
existence
of
Ga
2p
MoS
hav
2
1/2
permanently modify crystal structure, and leave tracks
of heavily damaged structure.
and Ga 2p3/2 core levels at 1144 and 1117 eV, respectively,
2.6
2.1.1. Uranium-238 Ion Irradiation
reveals the incorporation of Ga into MoS2 post Galite
irradiation.
It isuranium
important
out that
thefound
short duration
Exfoliated MoS2 mono-layers were irradiated
by 1.14 GeV
(238to
U)point
ions [83].
It was
by
(30
s)
and
the
use
of
low
Ga
flux
beam
equivalent
that its electrical properties were significantly changed and the MoS2 -based transistor was destroyed pressure
8
FIG. 2. (a) Mo113d, (b) Mo 3p
S22p HRXPS
spectra
of
pristine-MoS
and
plasma
treated
(for
1
min
and
3
min)
layere
2
3/2/N 1s, and (c)(BEP)
2
of 6 10 Torr during Ga-irradiation ensured no
ret
at a fluence of 4 × 10 ions/cm (4000 ions/µm
).
band spectrum for pristine-MoS2 and N2 -irradiated ML-MoS2 samples for (e) 1 min and (f) 3 min.
deposition of Ga adlayers on layered-MoS2. Moreover, for
gat
2.1.2. Gallium Ion Irradiation
Ga-irradiated MoS2 the reduced value of binding energy of
Mo
Ga-irradiated
(2-step
treated)
MoS2 sam
physical damage to the layered-MoS
remain
aisprocess
2 will(0.2
VBM
eV)
obtained
0.2
as
shown
in
Fig.
4(c),
which
bet
Michra
et al. [98] challenge,
deposited which
p-type may
mono-layered
MoS
c-sapphire3(a).
substrates
using uplift
CVD in the I(A )/I(E
optimization
be addressed
by2 on
adopting
addition,
◦ C has
samples and 1g int
similar
trend
obtained
for(equivalent
NIn2 -irradiated
techniquethe
and
irradiated
the
layers
at
450
for
30
s
at
a
low
Ga
flux
beam
pressure:
and
2.31,
is
obtained
for Ga-irradiate
low
brightness
mode
of
nitrogen
plasma
irradiation
for
a
hence confirming
theproduced
realization
ofap-type
ML-MoS2.17,34
ele
6 × 108 torr)
in
ultra-high
vacuum
(UHV)
conditions.
Ga
ions
were
from
plasma-assisted
treated sample, respectively, as compare
longer time.
Such
reduction
in
binding
energy
of
VBM
also
confirms
ins
MBE system. Figure
4 shows
the Raman spectra
andof
X-ray
photoelectron
spectra
of
pristine (XPS)
ML-MoS
Further,
we investigated
the effect
Ga-irradiation
onspectroscopy
2. Also, increase in F
theespecially
absence Aof1g inelastic
scattering
of electrons
at the surface
Mo
Ga-irradiated
MoS2 .mono-layers.
Raman peaks,
mode, shifted
after3(b)
the
Ga-irradiation.
Fig.
after Ga-irradiation shows the
ML-MoS
2 Fig. 3(a) shows the Raman spectroscopy of
5/2
of
ML-MoS
.
In
Ga-irradiated
samples
sulfur
vacancies
and
Mo-3d pristine-,
peaks shifted
towards lower
Ga-irradiated
samples
pristinedegradation of M
2
ing to
asthat
wellofasthepossible
Ga-irradiated,
andenergies
2-step,fori.e.,
Ga-irradiation
for relative
1/2 and Ga-2p3/2
were
occupied
by Ga
atomsatand
hence
enabling
the p-type
afo
MoS2 mono-layer.
New
core
levels
were
observed
1117
eV
and
1144
Ga-2p
Fig. 3(c) shows eV,
the room temperatu
30 s (step 1)/N2 -irradiation for 1 min (step 2) treated MLbehavior.
Fig.
4(d)
shows
the
energy
band
diagrams
of
prisrespectively,
the incorporation
reduced
the
of Ga into the MoS2 layers. The Ga-irradiation
MoSrevealing
Ga-irradiated
samples. PL
tine, N2 -, and
2 samples. Similar to N2 -irradiated MoS2, softening in
1
1
the
and
p-type
ML-MoS
realized
N2 -been
and observ
tine
ML-MoS
value of binding
energy
of
0.2
eV.
Room
temperature
photoluminescence
(PL)
spectroscopy
indicated
1
2
2
E2g phonon mode by 0.93 cm (2.83 cm ) is obtained for
with both methodsbyhas
that the optical properties of the MoS2 layers were
changed. in MBE. The reported electronic bandgap of
Ga-irradiation
N2
2. Charged Particle Irradiation
(b)
Figure 4. (a) Raman spectra and (b) Mo-3d XPS of pristine- and Ga-irradiated mono-layered MoS2
samples [98]. Reprinted with permission from Reference [98], with the permission of AIP Publishing.
Thiruraman et al. [99] synthesized single-layer MoS2 triangular-flakes via a halide-assisted
powder vaporization method and transferred over holes. The suspending MoS2 mono-layers
were irradiated by Ga+ ions in a focused ion beam (FIB). The Ga+ ion dose varied from
6.25 × 1012 ions/cm2 to 2.50 × 1013 ions/cm2 . Figure 5 shows high angle annular dark-field (HAADF)
images of the MoS2 mono-layers before and after Ga+ ion irradiation for different doses: 0
(pristine), 6.25 × 1012 ions/cm2 , 8.16 × 1012 ions/cm2 , 1.11 × 1013 ions/cm2 , 1.60 × 1013 ions/cm2 ,
and 2.50 × 1013 ions/cm2 . Statistical analysis showed that Ga+ ion irradiation produced pores
with average and maximum diameters of 0.5 nm and 1.0 nm. Within the irradiation dose range,
the nanopore density increased with increasing doses. With increasing ion doses, nanopores started to
merge, resulting in larger and irregularly shaped pores. For the lowest dose (6.25 × 1012 ions/cm2 ),
FIG. 3. (a)
pristine-, Ga
Ga-irradiatio
(step 2) treat
Intensity rati
modes, i.e.,
of E12g for Ga
(c) Photolum
Ga-irradiated
temperature.
such as type, density and edge termination of defects, is required but canno
only the techniques above. In this context, Surwade et al. mentioned th
Appl. Sci. 2019, 9, 678
9 of 53
optical signatures were observed in differently prepared defective graph
most of the atomic pores were single-molybdenum-based vacancies, while the amount of missing
sulfur varied. With an increasing Ga+ ion dose, the number of double-molybdenum-based
vacancies
9
increased, and some triple-molybdenum-based vacancies were also found. When the Ga+ ion
dose reached to 2.50 × 1013 ions/cm2 , the density of larger pores (with diameter size > 0.8 nm)
increased. Metal atomic vacancies were formed with sulfur vacancies while few topological defects
and amorphous regions were observed.
transport properties of the membranes varied.
Figure 5. HAADF images of the pristine and the 30 keV Ga+ ion-irradiated MoS2 mono-layers with
different ion doses [99]. Reprinted with permission from Reference [99]. Copyright c 2018 American
Chemical Society.
Figure
2. Aberration corrected scanning transmission electron micr
2.1.3. Xenon Ion Irradiation
Madauβ et al. [100] fabricated ultra-thin mechanically exfoliated MoS2 sheets on SiO2 substrates
and irradiated them using 91 MeV Xe ions. Figure 6a shows2 Atomic force microscopy (AFM) images
of the track morphology in MoS2 with various thicknesses. In the case of bulk-like MoS2 , chains
of nanosized hillocks were generated and protruded from the surface. For few-layer MoS2 sheets
(thickness under 10 nm),
2 individual hillocks were created and mixed with incisions. In even thinner
MoS2 sheets, i.e., tri-layered, bi-layered, and mono-layered MoS2 sheets, the major part of the surface
track consisted of a continuous incision. The length of the incision varied among tri-layered (3L),
bi-layered (2L), and single-layered (1L) sheets. Compared with the surface track length of mono-layers,
the length of the incisions was reduced by ∼25% for a bi-layer and by ∼50% for a tri-layer. Additionally,
the surface tracks in tri-layered and bi-layered MoS2 were accompanied by protrusions before and
after the central incision. Mono-layered MoS2 surface tracks generally consisted only of incisions and
no protrusions. Figure 6b shows lattice information around an incision (length 100 nm) created by a
projectile. The extended incisions were oriented along the direction of the incoming ion beam and
extremely narrow (less than 10 nm). The edges of the incision were relatively straight at an atomic
scale. The surrounding lattice remained undisturbed while the violent atomic displacements took
place inside the ion track core. Several mechanisms were proposed to explain the ion irradiation
damages, such as electrostatic repulsion between the atoms in the ionized region, exciton self-trapping
28,29 changes of the interatomic
causing a local lattice distortion, non-thermal melting caused by significant
potentials, and phase transitions such as melting due to a thermal spike. It was assumed that the
hillock chains in bulk-like MoS2 sheets consisted of nonstoichiometric MoS2 which re-solidified after a
phase transition caused by the thermal spike. The nanoscaled incisions should have originated from
characterization of single-layer MoS . (a) AC-STEM image of the pris
irradiated MoS with different ion doses. (b) High magnification AC-ST
vacancies with different atomic configuration. These images were used to
analysis of defects shown in Figure S7 and described in the text.
In 2D systems, the type of vacancy-defects introduced by ion irradiation c
the ion characteristics and kinetic energy.
For electron irradiation of M
ACS Paragon Plus Environment
Appl. Sci. 2019, 9, 678
10 of 53
the material 2D
evaporation.
It was reported that the threshold for grazing incidence
of Xe
Mater. 4 (2017) 015034
L Madauß
et alions was
2.0 keV/nm.
2D Mater. 4 (2017) 015034
L Madauß et al
(a)
Figure 4. Transition of SHI modifications obtained in MoS2 (91 MeV Xe, Se = 19 keV nm–1, Q  1) with varying thickness from
bulk-like down to single layer. The direction of the ion beam is depicted by the white arrow. (a) Bulk-like MoS2 shows periodic chains
(b)
Figure
STEM–MAADF
images
irradiatedMoS
(91 MeVwith
Xe, nominal
angle thicknesses
of incidence Q from
freestanding single
layer
0.2) bulk-like
Figure
6. 5.
(a)
AFM images
oftaken
Xe from
irradiated
varying
down
to
2
of MoS2 suspended on a TEM grid with holes of one micron diameter as shown in the inset in (a). The direction of the ion beam is
◦
single
layer
(grazing
angle
θ
≤
1
)
deposited
on
SiO
substrates
and
(b)
STEM
images
of
irradiated
depicted by the white arrow. The brighter features are most likely remnants
of the chemicals used in the production of suspended
2
black stretch seen
in (a)(grazing
and (b) is a part
of anθincision
the MoS2 middle
sheet by the(left
projectile.
The part
MoS2, see methods.
◦ ),into
freestanding
MoS2 The
mono-layers
[100]
angle
≤ 0.2cut
showing
panel)
and end
visible in (a) and (b) is roughly 20nm long. The round white spots are polycrystalline and/or disordered, and have an apparent lattice
2
(rightspacing
panel)
of an
by one
Energy
Xe
ions:between
91 MeV.
ions/µm
between
2.8incision
and 3.2Å, created
consistent with
them ion.
being Mo
clustersof
(the
spacing
latticeFluence:
planes in bcc10–15
Mo varies
between .
2.23
and
3.85
Å,
depending
on
the
orientation).
(b)–(d)
Atomically
resolved
images
of
the
incision
show
that
the
MoS
lattice
outside
2
The directions of the ion beam in (a) are marked by white arrows. The black stretches in (b) are
of the ion track remains intact.
parts of an incision cut into the MoS2 mono-layer by the projectile. The round white spots in (b) are
molybdenum clusters. Reprinted with permission from Reference [100]. 1L: mono-layer; 2L: bi-layer;
c under
3L:however
tri-layer.
Copyright
terms
of the
Creative
Commons
Attribution
3.0the
license.
not show
any protrusions.
While
complete surface
individual
hillocks
were the
absent
in these
stu-
dies [28]. The striking periodicity of the hillocks found
track length in these three different MoS2 samples is
Hopster
al.observed
[101] deposited
MoS2 mono-layers
on KBr(100)
substrates
and irradiated
them
here haset
been
in several crystalline
materials comparable,
the length
of the incisions
is reduced by
35
+
40
+
with highly
chargedexplanation
Xe ions (Xe
kineticelecenergy
of 25.4
and
with
and a possible
in termswith
of a varying
≈25%
for a keV
bilayer
andXe
by ≈50%
for kinetic
a trilayer.energy
More- of
tronic
in-between
crystal
layers has beenThe
dis-fluences
over, theranged
morphology
of 5the
as well. 2 .
38.5 keV)
in density
a vacuum
at room
temperature.
from
×incisions
109 –2 ×changes
1010 ions/cm
cussed
in observed
detail in [29].on
This
almost While the incisions in a single layer usually appear as a
Hillocks
were
thefeature
MoSdisappears
2 mono-layer surfaces. The MoS2 mono-layers were exfoliated
completely in thin, few layer MoS2 sheets (thickness clean cut with straight edges (see figure 1(b)), bilayer
from a single crystal under ambient condition while no details were provided, so it is hard to compare
under 10 nm)—although individual hillocks are still and trilayer incisions appear more irregular. In parttheir results
with
created.
The other
surfacegroups.
track is now a mixture of irregu- icular for the trilayer in figure 4(c), it can be seen that
larly formed hillocks/protrusions and incisions, the
the incisions appear more like a series of roundish
holes with a diameter of about 10nm each.
bilayer MoS2, see figures 1(b) and 3(a).
The data presented above immediately leads to an
Guo et al. [102] deposited 1–4-layer-ed MoS2 on silicon substrates capped with SiO2 . The
question from a physics as well as from an
In even thinner MoS2 crystallites, three layers and interesting
mechanically
exfoliated
were irradiated
by 209 Bi
ionsofwith
of the
0.45–1.23
2 nanosheets
point
view:energies
How does
substrateGeV
below, the
major partMoS
of the
surface track consists
of a application
10
12
2
and fluences
of 1 ×
10 –3.6
10 of
ions/cm
invaries
a vacuum
at room
temperature
under in
normal
incidence.
influence
the fabrication
of incisions
SL-MoS
continuous
incision.
The×
length
the incision
2? The
between trilayer
bilayer
(2L)observed
and single on
layer
(1L)
substrate
does indeed
play
a decisive role,
as attributed
we will
The hillock-like
latent(3L),
tracks
were
the
surface
of irradiated
MoS
and
2 few-layers
as shown in figure 4(c). The surface tracks in tri- and show further below, but it is by no means a prebilayer MoS2 are accompanied by protrusions before requisite. In figure 5 we show scanning transmission
and after the central incision. Single layer MoS2 sur- electron microscopy (STEM) medium-angle-angularface tracks generally consist of incisions only and do dark-field (MAADF) images from freestanding layers
2.1.4. Bismuth
Ion Irradiation
latter of which
resemble the ones found in single- and
6
Appl. Sci. 2019, 9, 678
11 of 53
to the ionization and excitation of energy transfer from 209 Bi ions to the electron system. The induced
damages shifted the Raman A1g peak to a higher frequency and increased the intensity ratio between
the A1g and E12g modes.
2.1.5. Manganese Ion Irradiation
Mignuzzi et al. [103] mechanically exfoliated natural bulk MoS2 and deposited it on Si substrates
covered with SiO2 . The exfoliated MoS2 mono-layers were bombarded with Mn+ (kinetic energy of
25 keV) with different ion doses (1012 –1014 ions/cm2 ) in a UHV. The resulting average inter-defect
distance ranged from 1 nm to 10 nm. The generated defects activated new Raman modes around
227 cm−1 , broadened Raman peaks, and shifted Raman modes.
2.1.6. Gold Ion Irradiation
Zhai et al. [114] bombarded MoS2 (0001) bulk surfaces using Au ions with 13.4 MeV. Under ion
doses of 1.0 × 1013 ions/cm2 , 1.0–3.5 nm craters were generated on the surface.
2.1.7. Silver Ion Irradiation
Bhattacharya et al. [115] irradiated magnetron-sputtered amorphous MoS2 films of thicknesses
50–750 nm with a 5 × 1015 cm−2 dose of 2 MeV Ag ions. The sliding life of Ag ion-irradiated films
increased ten-fold to thousand-fold compared to as-sputtered films. The improvement in wear life
was correlated with a significant improvement in adhesion of the films with the substrates and a small
increase in the density of the ion-irradiated films.
2.1.8. C60 Ion Irradiation
Henry et al. [104] irradiated MoS2 bulks under 20–40 MeV C60 ions at 300 K. Irradiation was
performed at normal incidence and at high fluences (3 × 1011 ions/cm2 ), as well at a grazing incidence
of 20◦ and a low fluence of 109 ions/cm2 . The structural modifications occurred in the vicinity of the
projectile paths.
2.2. Argon Ion Irradiation
Argon ion beams (500 eV) were employed as early as 1987 [9] to irradiate MoS2 crystalline films
(cleaved from a single crystal) with a thickness of 0.7 µm. The Ar bombardments created sulfur
vacancies and reduced molybdenum of MoS2 edge surfaces.
Inoue et al. [93] studied defects generated by Ar+ ion irradiation of MoS2 bulk surface by
scanning tunneling microscopy (STM). A clean MoS2 surface was prepared by cleaving the surface
layers of a MoS2 bulk crystal with Scotch tape. The cleaved clean MoS2 was then degassed in a UHV
chamber by electrical heating at 538 K for half an hour to provide a clean and contamination-free
surface. The sample was irradiated with Ar+ ions at an energy of 500 eV and an irradiation density of
75 × 1011 ions/cm2 . The incident angle of Ar+ ion was ±2–3◦ from the normal to the MoS2 surface.
The irradiated MoS2 sample was heated at 583 K for an hour by direct current heating through the
substrate to remove residual ions on the sample surface after irradiation. Low-density individual
defects were observed. Some surface defects were formed by removal of sulfur atoms from the top
MoS2 surface. Some defects may originate from the hybridized dangling bond composed of the Mo-4d
orbital and removal of MoS2 layer fragments.
Bae et al. [84] MoS2 prepared slabs with a thickness of several microns from MoS2 crystals using
the mechanical exfoliation method and then irradiated them with Ar+ ions with an energy of 500 eV in
an UHV, with fluences of 5.65 × 1014 ions/cm2 and 2.26 × 1015 ions/cm2 . The incident angle of Ar+ was
almost normal to the surface of the samples. Residual argon on sample surfaces was removed by heating
at 583 K for one hour in an UHV after the irradiation process. Figure 7 shows the Raman spectra of Ar+
Appl. Sci. 2019, 9, 678
12 of 53
irradiated samples. After Ar+ irradiation, additional broad satellite peaks appeared in the lower-frequency
1 and A peaks, located at 377–381 cm−1 and 404–406 cm−1 respectively.
side of both the E2g
1g
The Raman satellite peaks shifted to lower wave-numbers while their intensity increased with
increasing Ar+ doses. First-principles calculation showed that new satellite modes were related to
molybdenum vacancies, sulfur vacancies, or MoS6 vacancy clusters. The lower shift of the satellites
1 mode should have come from the large cluster vacancy. The A and E1 mode of MoS
of the E2g
2
1g
2g
materials remained unchanged before and after the irradiation. It is believed that the new peaks were
induced by lattice defects introduced by Ar+ irradiation. Additionally, the intensity of the satellite
1 and A modes of
peaks increased with the increasing irradiation dose and dominated over the E2g
1g
pristine MoS2 , indicating the formation of molybdenum vacancies or MoS6 vacancies.
Baker et al. [94] irradiated MoS2 powders under a 3 keV Ar+ ion beam with a current density
of 0.1 µA/cm2 . The Ar+ ion-beam bombardment caused the preferential sputtering of sulfur and
produced an amorphous MoSx phase of depth of 3.8 nm. The Mo/S ratio increased with increasing
bombardment time, as shown in Figure
8. REV. APPLIED 7, 024001 (2017)
PHYS.
M.A. Baker et al.r Applied Surface Science 150 (1999) 255–262
256
duction of coatings which exhibit low friction and
Arq bombardment and then present a simple method
Intensity [arb. units]
+
404
Ar -ion
fluence
fect-induced satellites appear
good wear resistance and
are thus
capable of dry
of accurate stoichiometric determination, using only
2
412
1
[ions/cm
]
machining.
Of
the
solid
lubricants
tried
to
date,
the XPS Mo 3d 5r2 and S 2p 3r2 peak positions. With
and E2g modes of MoS2 . The
MoS 2 shows the most promise in providing the
regard to the preferential sputtering of S, changes in
378
fect-induced peaks increases
lubricating properties necessary to be
used for dry
the binding energy and lineshape of the XPS Mo 3d
387
x3 monitored as a function of
applications.
and S 2p peaks have been
sed. We report the resultsmachining
of
Accurate determination of the 15MoS x stoichiomMoS x composition during the formation of the sul406
2.26 × 10
on the defective monolayer
etry is of crucial importance in relating the composiphur depleted altered layer. Investigations of this
tion both to coating deposition conditions and to 412 nature have been performed on oxides Že.g., Refs.
strate Raman activities under
386
w15–17x. and transition metal sulphides w18x, but to
mechanical properties. Studies of sputter-deposited
x3
381
cess for the defect-induced
MoS x films in which the stoichiometry has been
the authors’ knowledge, this is the first of such study
14
quantified have used5.65
Rutherford
on MoS 2 . Resulting from the data analysis, a simple
ht, based on the comparison
× 10 Backscattering
Spectroscopy ŽRBS. w10–12x or Electron Probe Mimethod of accurate stoichiometric determination usof the Raman spectra with cro
theAnalysis ŽEPMA. w2,10x. Both of 386
these tech- 412ing only the Mo 3d 5r2 and S 2p 3r2 peak positions
niques however, are limited in certain
was developed and will be presented. This method is
bring chemical and physical
E 12g thin Afilm
1g
14 analysis depth.
applications by their relatively large
useful to the analyst as it can be employed to deternalysis of defective structures
0.00
×
10
X-ray Photoelectron spectroscopy ŽXPS. and Auger
mine the stoichiometry after atmospheric exposure of
s.
Electron spectroscopy ŽAES. are two of the most
the MoS x film and precludes the need to fit the Mo
common techniques used for determining the compopeak.
350 but have not400
450
500
sition of thin film materials,
been
-1
widely used for determining MoS x stoichiometry.
Raman shift [cm ]
SERVATION OF
The main reason for this is that S is preferentially
2. Experimental
MoS2 SURFACES
from MoS 2 w8,13,14x.
+ -irradiated MoS micron crystals
Figuresputtered
7. Raman
spectra
of
(bottom)
MoS
crystalline
Ar
2of pristine bulk,
2
FIG.
Raman
spectrastudy
bulk
MoS
and
2 (bottom)
This article
will 1.
undertake
a quantitative
of
Two
standard
bulk
MoS 2 samples
were prepared
14
2
15 ions/cm
2 [84]. 2Reprinted with
þ
14
at fluences
of (middle)
5.65
× 10
and
(top)
2.26
×
10
first
of MoS2 samples. The
preferential
sputtering
of
S from
MoSions/cm
under
3
keV
for
analysis.
The
bulk
samples
were
a
MoS 2 powder
Ar
-irradiated
bulk
MoS
at
fluences
of
5.65
×
10
ions=cm
2
2
permission
Reference
[84].
Copyright
15 c 2017 by2 the American Physical Society.
chanical exfoliation from
the from(middle)
and 2.26 × 10 ions=cm (top).
using adhesive tape in the
are referred to as bulk since
rons. These cleaved MoS2 are
satellite peaks appear in the lower-frequency side of both
e (17.3 × 7.9 mm2 ) with sliver
the E12g and A1g peaks. The Raman intensity of these
be heated by passing direct
satellite peaks increases when the Arþ dose increases.
bstrate in an ultrahigh vacuum
This indicates that the new peaks are induced by lattice
ples are first degassed at 538 K
defects introduced by the Arþ irradiation. The maxima
re a clean and contaminationof the defect-induced peaks in the lower-frequency side of
pe of samples are made from
E12g and A1g peaks are at 377–381 and 404–406 cm−1 ,
diameter is 3–5 μm) chemirespectively.
calation in a hexane solution
Similar satellite peaks have been previously observed in
. [28,29]. Mono- to few-layer
defective MoS2 samples [15,16,18], and were rationalized
the colloidal solution of the
by resonance and finite-crystal size effects. In the resonanceubstrate. Then these samples
Fig. 1. The transient in surface composition showing the preferential etching of S from a MoS powder sample2 when exposed to a 3 keV
+ ion beam [94].
effect model, defect-induced peaks are assigned
E1u Ar
and
FigureAr8.ionSurface
to ato
3 keV
at the energy of 500
eV in beam. composition of MoS2 powders when exposed
1
The atomic percent
was calculated
energy-dispersive
spectroscopy.
B1u modes,
which from
are Davydov
pairs of X-ray
E2u and
A1g modesReprinted from
carried out with fluences
of
c
Reference
[94].
Copyright
1999,
with
permission
from
Elsevier.
observed by Sekine et al. [24] in resonance Raman specions=cm2 , estimated from the
troscopy. However, these resonance peaks can be observed
mes. The incident angle of Arþ
only when the incident photon energy is tuned to the exciton
mal to the surface of the bulk
energies of MoS2 , which is not the case in ordinary Raman
d is applied perpendicular to
scattering spectroscopies (such as ours) using the same light
diation process, residual argon
source at the off-resonant photon energy. On the other hand,
2
q
Fig. 3b shows XRD spectra of as-deposited and ion
Fig. 3b shows XRD spectra of as-deposited and ion
irradiated coatings. The as-deposited coating had two
irradiated coatings. The as-deposited coating had two
peaks: a broad peak at 13.528 and a low 2 u peak at 11.028.
peaks: a broad peak at 13.528 and a low 2 u peak at 11.028.
Both peaks have been observed previously in IBAD MoS 2
Both peaks have been observed previously in IBAD MoS 2
w22x. The LD and HD ion irradiated coatings exhibited
w22x. The LD and HD ion irradiated coatings exhibited
only a narrower Ž002. peak at 14.258, with reduced intenonly a narrower Ž002. peak at 14.258, with reduced intenAppl.tests.
Sci.Arrow
2019, 9, sity
678
dosage diagram
was increased.
The sliding
UHD‘‘stripe’’
coatingtests.
spectra
sity as dosage was increased. The UHD coating spectra 13 of 53
diagram of reciprocating sliding ‘‘stripe’’
Fig. as
2. Schematic
of reciprocating
Arrow
ndicates test starting location, turnaround points are
marked ‘‘start’’
indicates test
starting
location, turnaround points are
showed
no crystalline
MoS
showed no crystalline MoS 2 peaks at all. The peak at
2 peaks at all. The peak at
circles, and sliding cycles for the track segments are
indicated
circles, and sliding
for the track
.
13.528
inbytheopen
as-deposited
coatingcycles
is identified
as segments
the Ž002are
13.528 in the as-deposited coating is identified as the Ž002.
numbered.
peak of MoS 2 , and yields d-spacings similar to those
peak of MoS 2 , and yields d-spacings similar to those
reported
other sputtered
MoS 2 2films.
Recently,
Dunn et
reported for other sputtered MoS films. Recently, Dunn et
[95]fordeposited
layers
nts Žinstead of the normal two in a Wahl
recipro- et al.
turnaround
points ŽinsteadMoS
of the normal
two (roughly
in a recipro- 200 nm) on steel and Si 2substrates via ion-beam
al. w34x have attributed the low 2 u peak in the as-deposited.
al. w34x have attributed the low 2 u peak in the as-deposited
.,
th the extra turnaround pointassisted
in the middle
cating track,
withproduced
the extra turnaround
point
in the middle
, fully dense with basal-oriented microstructures.
deposition.
The
MoS
layers
were
2 resulting from
coating
to widely spaced MoS basal planes
coating to widely spaced MoS 2 basal planes resulting from
le wear tracks run to the same number of
and Ž2. multiple wear tracks2 run to the same number of
defects
in layers
the coatingwere
crystal structure.
These defectswith
were
defects in the2+
coating
structure.ofThese
MoS
irradiated
ionscrystal
at doses
1 ×defects
1015were
ions/cm2 ,
are obtained with minimal The
numberdeposited
of tests
sliding 2cycles
are obtained then
with minimal
number of tests 180 keV Ar
removed in the ion irradiated coatings, sharpening and
removed in the ion irradiated coatings, sharpening and
a needed.
and 2coating area needed.16
16
2
Ž0025. ×
shifting, the
peak
that of crystalline
MoS 2 . temperature
shifting the Ž002
towards thatFigure
of crystalline
1 × 10 ions/cm
and
10towards
ions/cm
at room
in .apeak
vacuum.
9a MoS
shows
X-ray
2.
ar tracks and transfer films on balls were
Coating wear tracks and transfer films on balls were
Fig. 4 is a montage of multi-beam images from crossFig. 4 is a montage of multi-beam images from crossally with a Nomarski microscope.
Coating (XRD)
examined
optically of
withthe
a Nomarski
microscope. and
Coatingion-irradiated coatings. The intensity of the (002)
diffraction
patterns
as-deposited
sectional TEM of as-deposited, LD, and UHD coatings.
sectional TEM of as-deposited, LD, and UHD coatings.
wear track depths were measured to within
thickness
and wear track depths were measured to within
4a◦ and
4b are taken
with increasing
the objective
aperture
Fig. 4a
and 4b The
are taken
with plane
the objective
peak at Fig.
13.5
decreased
with
irradiation
doses.
basal
peakaperture
(2θ = 11◦ )
..
ŽMI..
Michelson interferometry ŽMImain
10 nm using
Michelson interferometry
encompassing the transmitted beam and Ž002. reflections,
encompassing the transmitted beam and Ž002. reflections,
disappeared after
the4c irradiation.
9b shows
spectra
ofwithout
the samples.
The main
while Fig.
is taken without an Figure
objective aperture.
SAED Raman
while Fig.
4c is taken
an objective aperture.
SAED peaks
patterns
inset in each image were taken from the same area
patterns inset in each image were taken from the same area
3.
Results
of the Raman asspectra
were kept after the irradiation, while
their intensity decreased with increasing
each image.
as each image.
Ž
. shows basal-oriented,
The
as-deposited
coating
Fig.
4a. showsthe
basal-oriented,
The of
as-deposited
coatinglayers
analyses of coatings
3.1.
Structural
analyses
of Žcoatings
doses. To explore
the
irradiation
effects,
microstructure
the MoS
characterized by
2 Fig. 4a was
nanocrystalline domains of MoS 2 , with numerous forked
nanocrystalline domains of MoS 2 , with numerous forked
high-resolution
transmission
electron
microscopy
(HRTEM)
as
shown
in
Figure
9c.
The
w
x
w
x
sited, LD and HD coating thicknesses, meaas-deposited,
LD and23,34
HD coating
thicknesses,
meaand The
terminated
plane defects
. The horizontal
fringes
and terminated plane defects 23,34 . The horizontalas-deposited
fringes
were all roughly 200 nm, while the UHD
sured byinMI,
all correspond
roughly 200
nm, while S–Mo–S
the UHD
observed
the were
images
to individual
observed in the images correspond to individual S–Mo–S
MoS
consisted
of
basal-oriented
and
nanocrystalline
MoS
with
horizontal
(002)
fringes.
After an
2
2
. reflec0 nm thick. We attribute the 20 nm reduccoatingand
wasare180
We attribute
the Ž20
reduclayers
duenmtothick.
interference
between
002nm
layers and are due to interference between Ž002. reflec2Fig.
ess to sputtering losses Žroughly
that pre- dose
tionofin
toions/cm
sputtering
that
pre-became
tions.
The
SAED
inset inlosses
4aŽroughly
consists
of two
tions. The
SAED pattern and
inset inthe
Fig.surviving
4a consists of crystalline
two
irradiation
1thickness
×
1015pattern
, some
regions
amorphous,
Ž002.
ile Code calculations. rather than coating
dicted
by Profile
Code diffuse
calculations
rather thanto coating
sets
of arcs.
The larger,
arcs .correspond
sets of arcs. The larger, diffuse arcs correspond to Ž002.
regions
(still with
theandbecause
basal
orientation)
were
embedded
in the
amorphous
matrix.
After a high
ecause the as-deposited IBAD
MoS 2 coatcompaction,
the
as-deposited
reflections,
the smaller,
inner arcs areIBAD
due
toMoS
expanded
reflections,
and the
smaller, inner arcs
are due to expanded
2 coatw35
x.
ense w35x.
ings are
fullyspacing
dense16
2
basal
plane
arising
from fork
and termination
basal plane spacing arising from fork and termination
irradiation dose
ofw345x.spectra
× 10of three
ions/cm
, the MoS regions
were nearly amorphous, although few
tra of three coatings Žas-deposited, LD and
Raman
coatings Žas-deposited, LD 2and
defects
defects w34x.
ŽFig. 4b
. shows
wn in Fig. 3a. All spectra exhibited
Raman regions
UHD
are
shown
Fig.the3a.surviving
Allcoating
spectra Žexhibited
Raman regions
The. TEM
imagein of
LD
Fig.
4b. shows
The TEM
image of randomly
the LD coatingafter
crystalline
existed.
The
crystalline
oriented
the
high dose
w36x,embedded
o those for molybdenite w36x, but intensity
bandsdomains
similar to
molybdenite
but intensity
small
of those
basal for
oriented
crystallites
in
small domains of basal oriented crystallites embedded in
The
irradiation-reduced
microstructural
degraded
tribological
behaviors
ecreased in the ion irradiatedirradiation.
coatings.
of amorphous
the bands decreased
in the ioncrystalline
irradiated
coatings.
an
matrix. Remaining
regions show change
an amorphous
matrix. Remaining
crystalline
regions showof MoS2
coatings, such as accelerated wear and eliminated lubrication.
4
(a)
4 Wear 237 (2000) 1–11
K.J. Wahl et al.r
K.J. Wahl et al.r
4 Wear 237 (2000) 1–11
(b)
K.J. Wahl et al.r Wear 237 (2000) 1–11
2q
Ža. XRD and
. Raman
3. Ža. XRD and Žb. Raman spectra from IBAD MoS 2 coatings,
as-deposited
and
Fig.ion
3. irradiated
withŽb180
keV Ar
spectra
ions.
from
with/ cm
1802keV Ar 2q ions.
0 ions
/ cm2
5 xirradiated
1016 ions
1015
ions IBAD
/ cm2 MoS 2 coatings, as-deposited and ion
(c)
Figure 9. (a) XRD patterns, (b) Raman spectra, and (c) cross-sectional HRTEM images of MoS2 coatings
on Si substrates as-deposited and irradiated with 180 keV Ar2+ -ions [95]. Insets: selected-area electron
diffraction (SAED) patterns. Reprinted from Reference [95]. Copyright c 2000, with permission
from Elsevier.
Murray et al. [116] irradiated mechanically exfoliated MoS2 few-layers (5–15 layers) under
low-energy Ar (200 eV and 1 ×1013 –1015 ions /cm2 ). The electric resistance increased 100 times
after a high dose of irradiation because of induced defects.
Bae et al. [84] also prepared MoS2 bi-layers by chemical exfoliation from MoS2 micron powders in
a hexane solution of butyllithium. The prepared bi-layers were then irradiated by Ar+ ions with an
energy of 500 eV in an UHV, with fluences of 5.65 × 1014 ions/cm2 and 2.26 × 1015 ions/cm2 . Figure 10
1 modes
shows Raman spectra of the MoS2 bi-layers before and after Ar+ irradiation. The A1g and E2g
were broadened. It was claimed that a new broad peak emerged at approximately 373 cm−1 , which
1 peak. There was a broad feature at around 435 cm−1 . The mode was
was 10 cm−1 away from the E2g
fewer fork and termination defects. SAED of the LD film
forkAtand
defects. SAED
the LDFig.
film4c., the At the highest implantation dose ŽUHD, Fig. 4c., the
At
the
highest implantation
doseofŽUHD,
4c., the
ŽUHD,
fewer fork
andirradiation.
termination
defects. SAED
the
LDFig.
filmfewer
thetermination
highest
implantation
doseof
relatively
after
the
The
new
vibration
mode
was
identified
as astillunique
signature
. reflections. In comparison
Ž002. reflections.
shows only
well defined Ž002enhanced
coating
contained
crystalline
regions.
Basalshows
planes
only wellstill
defined
In comparison
coating
contained crystalline
regions. Basal planes
Ž002. reflections.
shows only
wellstill
defined
In comparison
coating
contained
crystalline regions.
Basal planes
Ž
.
Ž
.
with Ž002. reflections in Fig. 4a, these reflections are
were
oriented
in
many
directions,
and
100
fringes
were
with
002
reflections
in
Fig.
4a,
these
reflections
are were
were oriented in many directions, and Ž100. fringes were
with Ž002. reflections in Fig. 4a, these reflections are
were oriented in many directions, and Ž100. fringes
ofshifted
theoutward
sulfur
vacancies.
sharper and
in reciprocal
space. This imalso observed at several orientations. Circular sharper
and shifted outward in reciprocal space. This imalso observed at several orientations. Circular areas of
sharper and
shifted outward in reciprocal space. This im-areas of
also observed at several orientations. Circular areas of
2
Ža. as-deposited, and after irradiation with 180 keV Ar 2q ions to Žb. 1 = 10 15 ionsrcm2
Fig. 4. Cross-section TEM images from IBAD MoS 2 coatings Ža. as-deposited, and after irradiation with 180 keV Ar 2q ions to Žb. 1Ž =
10 15Fig.
ionsrcm
4. Cross-section
TEM images from IBAD MoS
2 coatings
Fig. 4. Cross-section TEM images from IBAD MoS 2 coatings a. as-deposited, and after irradiation
with 180 keV Ar 2q ions
to Žb. 1 = 10 15 ionsrcm2
ŽLD. and Žc. 5 = 10 16 ionsrcm2 ŽUHD.. Fringes running left to right correspond to MoS 216basal planes
ŽS–Mo–S..
ŽLD. and Žc. 5 = 10 16 ionsrcm2 ŽUHD.. Fringes running left to right correspond to MoS 2 basal planes ŽS–Mo–S..
ŽLD. and Žc. 5 = 10 ionsrcm2 ŽUHD
.. Fringes running left to right correspond
to MoS 2 basal planes ŽS–Mo–S..
plies that the Ž002. spacing of the crystalline regions has
amorphous material can also be seen by their light
pliesconthat the Ž002. spacing of the crystalline regions has
amorphous material can also be seen by their light conplies that the Ž002. spacing of the crystalline regions has
amorphous material can also be seen by their light concontracted and that these regions have far fewer defects
trast. Moreover, basal planes appeared at the circumfercontracted and that these regions have far fewer defects
trast. Moreover, basal planes appeared at the circumfercontracted and that these regions have far fewer defects
trast. Moreover, basal planes appeared at the circumferthan in the as-deposited coating. In addition, the inner
ence of several of the circles. The SAED pattern than
shows
in athe as-deposited coating. In addition, the inner
ence of several of the circles. The SAED pattern shows a
than in the as-deposited coating. In addition, the inner
ence of several of the circles. The SAED pattern shows a
reflection seen in the SAED pattern in Fig. 4a has vanstrong, diffuse background with weak, diffuse spots
near seen in the SAED pattern in Fig. 4a has vanreflection
strong, diffuse background with weak, diffuse spots near
reflection seen in the SAED pattern in Fig. 4a has vanstrong, diffuse background with weak, diffuse spots near
ished, consistent with XRD spectra in Fig. 3 and previous
the transmitted beam. These spots are due to remaining
ished, consistent with XRD spectra in Fig. 3 and previous
the transmitted beam. These spots are due to remaining
ished, consistent with XRD spectra in Fig. 3 and previous
the transmitted beam. These spots are due to remaining
results w34x.
crystalline regions and can be indexed as Ž002. reflections.
results w34x.
crystalline regions and can be indexed as Ž002. reflections.
results w34x.
crystalline regions and can be indexed as Ž002. reflections.
Appl. Sci. 2019, 9, 678
DEFECT-INDUCED VIBRATION MODES OF Arþ - …
14 of 53
PHYS. RE
+
monolayer models contai
construct a 5 × 5 monol
~375 386
consider three typical vaca
~451
~399
the S vacancy, and the Mo
Fig.
3, panels (c)–(e). Va
15
2.3 × 10
×2
images of the monolayer
405
spurious interaction. Elec
386
436
properties are calculated
ulation package (VASP) co
14
tional theory with the l
0.0 × 10
1
A1g
E 2g
exchange and correlation
wave method. The plane
350
400
450
500
and the first Brillouin zo
-1
Raman shift [cm ]
~
Monkhorst-Pack k-points
Figure 10. Raman spectra of MoS2 bi-layers before (bottom) and after (top) Ar+ irradiation at the
FIG. 2. Raman spectra of chemically exfoliated bilayer MoS2
structural relaxation is 1
fluence of 2.26 × 1015 ions/cm2 [84]. Reprinted withþpermission from Reference [84]. Copyright c
flakes before (bottom) and after Ar irradiation at the fluence of
relaxed structures, the har
2017 by the American Physical
Society.
2.3 × 1015 ions=cm2 (top). Notice that the bilayer intensity is
lated within frozen phon
+ of the
multiplied
by aMoS
factor
of 2 (×2) due to the weakening
Bae et al. [84]
claimed that
500 eV.modes are obta
2 mono-layers were destroyed by Ar irradiation with
vibration
+
Raman
signal.
Chen et al. [117] sputtered MoS2 mono-layers with Ar ions (500 eV and emission current
of dynamical
20 mA),
the
matrix. Ram
defect peaks of Raman scattering were observed and its hydrogen evolution performance was enhanced.
Raman off-resonant activit
1
However, no details
reported
literature.
434were
cm−1
for theinAthe
1g and E2g modes, respectively. Notice
end [33], and the polarizab
Zhu et al. [97] treated CVD-grown MoS2 mono-layers under radio-frequency (RF) argon plasma
that the peaks are not as sharp as for the bulk cases as the
polarizability routine of th
with 13.56 MHz at room temperature for 40 s. The very weak Ar plasma had enough kinetic energy
signal
is
very
weak,
and
that
there
is
also
a
broad
peak
at
Raman
are then
to trench the S-Mo bond −1
while no enough energy to etch molybdenum and sulfur atoms.
Thus, intensities
the
2
436
cm
.
Upon
irradiation,
also
the
bilayer
system
shows
α~ yy corresponding
to our o
plasma treatments induced the lateral sliding of the top S layer and the 2H phase transited
to the
the emergence of new defect-induced vibration modes.
1T phase.
We first calculate pho
, inRaman
A broad
new2 mono-layers
peak emerges
at approximately
cm−1
Ma et al. [96]
grew MoS
on SiO
treated
them
a vacuum
2 substrates and 373
intensities for the
2.
+ ions is
under 500 eV Arwhich
at shifted
room temperature.
The beam
density
µA/cm
by approximately
10 current
cm−1 from
thewas
E12g3.1–11.2
in Fig. 4. There
are two R
0
The molybdenum
content
remained essentially
while theand
amount
of
2 layers
peak.
Thisofisthe
theMoS
same
defect-induced
peak as constant
is observed
the A1 mode, which a
sulfur decreased significantly during irradiation. The sulfur content of the layers
be reduced to
−1 could
for the bulk system. The broad feature
at
435
cm
is
spectroscopy
of MoS2 .
50%. The average sputter yield was 0.03 sulfur atom per Ar+ ion. During the irradiation procedure,
relatively enhanced. In the monolayer domain of the
vibration mode and the
MoS2 mono-layers were selectively de-sulfurized while the basic physical structure of the MoS2
chemically exfoliated samples, no clear signals correspondvibration mode, as show
remained largely intact. The photoluminescence
(PL) 0yield decreased as sulfur atoms were removed
0
or
the
A
modes
are
found
after
ing
to
either
the
E
frequencies, 391.7 and 40
1
by the Ar+ irradiation.
þ
irradiation,
indicating
that
the
Ar
irradiation
destroyed
our
+
with previous
theories an
Ar plasma was also employed to thin MoS2 sheets at room temperature [118]. It was reported
that
+
flakes.
monolayer
MoS
that
the monolayer vibrati
2
the top MoS2 layers were entirely removed
by the Ar plasma while the bottom MoS2 layers
remained
largely unaffected. Thus, MoS2 multi-layers with controllable thickness, even MoS2 single-layers,
to bulk or few-layer mod
could be prepared
reliably
(with almost
100% success
rate). OF DEFECTIVE
III.
PHONON
VIBRATION
MODES
different due to differe
Chen et al. [117]
MoS2 mono-layers
using aRAMAN
liquid exfoliation
method, and annealed
AND THEIR
SPECTRA
MoSprepared
2 SURFACES
perpendicular to the surfa
them at 300–450 ◦ C in a vacuum. The treated layers were then deposited on Au substrates and exposed
induced vibration modes
the irradiation-induced
defects
in theinfluencing
MoS2 the
to 0.5 keV Ar+ ions To
for 1identify
min. Defects
were introduced into the
MoS2 layers,
electronic
within
one monolayer even
surfaces,
we
perform
a
first-principles
calculation
of
MoS
2
structures of MoS2 . Figure 11 shows XPS spectra of irradiated MoS2 mono-layers. Peaks shifted
406
436
Bilayer
Intensity [arb. units]
Ar -ion fluence
2
[ions/cm ]
because of induced defects.
(a) Slab model
(b) Pristine
(c) Mo vacancy
(d) S vacancy
(e) MoS6 vacancy
20 Å
FIG. 3.
monola
extende
The top
vacancy
vacancy
at the c
Appl. Sci. 2019, 9, 678
15 of 53
ACS Nano
Figure 11. X-ray photoelectron spectra of the Mo-3d and S-2s in the MoS2 mono-layers on Au
substrates before and after Ar+ sputtering. XPS of Au-4f electrons were shown as a standard reference.
Reprinted with permission from Reference [117]. Copyright c 2018 American Chemical Society.
2.3. Neon Ion Irradiation
Maguire et al. [92] also irradiated CVD-grown MoS2 mono-layers with Ne+ ions with an energy
of 30 keV. With increasing Ne+ ion doses, the A1g and E2g modes were quenched and broadened,
indicating the growing disorder induced by the Ne+ ions.
2.4. Alpha-Particle/Helium-Ion Irradiation
Alpha particles consist of two protons and two neutrons which are tightly bound together bound
together, which is identical to a helium ion (He2+ ). They are produced either from particle accelerators
in the form of helium-ion beams or from the process of alpha decay of alpha-particle-emitting
radionuclides. Alpha particles have been widely studied in radiopharmaceutical therapy that
is a promising treatment approach under active pre-clinical and clinical investigation [119,120].
Figure 3. XPS and Raman characterization of samples before and after introducing defects by Ar+ sputtering for
Decayed alpha particles
havespectra
a kinetic
energy
MeV,
which induce
defects
and
raygenerally
photoelectron
of the
Mo 3d,ofS about
2s, and5Au
4f electrons
in the MoS
2/Au sample after annealing at
even ‘cut’ the MoS2 nanosheets.
sputtering, and sputtering with subsequent annealing at 450 °C. ΔE for the upper two and the lower two plots
annealing at 300(helium
°C and 450
°C in UHV
and after(1.66
sputtering.
of the
MoS2/Au
Isherwood et al. [88]
studied
thesample
effectsafter
of alpha-particle
nuclei)
irradiation
MeV) The MoS2 Raman pe
spectra are normalized by the corresponding A1′ peaks. The measured data are shown as dots. Fitted individua
on both bulk and liquid-phase exfoliated MoS2 nanosheets using Raman spectroscopy and
shown as gray and colored curves, respectively. (c) Raman shift and fwhm of E′ and A1′ peaks for the MoS2/Au
energy-dispersive X-ray°C
spectroscopy.
Liquid-phase
exfoliated
MoS2oflayers
more
and 450 °C and
after sputtering.
(d) Illustration
energywere
band often
alignment
fordefective
the MoS2/Au sample. The left
sputtering,
and
the
right
one
shows
the
bands
after
sputtering.
than mechanically exfoliated
MoS
and
CVD-grown
MoS
layers.
Besides
water,
the
solvent
used
2
2
during liquid exfoliation could be retained between the MoS2 layers during ultrasonication and
of photoemission binding energy
(Figure 3a). The intensity ratio of the Mo and S photoemission
annealing/cleaning post-procedures. Raman spectroscopy showed a small blueshift of the E12g and
properties for MoS2 on Au substra
peaks did not change after annealing at 450 °C (Figure S3b).
A1g modes in the MoS2These
bulk under
a high
absorbed dose (∼900 MGy), accompanied
by a small
mean that
MoStotal
the defects
introduced by ion irrad
2 remained stable and without detectable
broadening of both peaks.
The of
exfoliated
MoSannealing
tolerant the
thanband
the bulk
2 layers were
alignment between Mo
formation
defects upon
alone.less
Thisradiation
is consistent
material. Both spectroscopies
proved
presence
of in
amorphous
carbon3b,c).
in the exfoliated
MoS2 decreased by 0.8
with Raman
resultsthe
presented
later
the paper (Figure
binding energy
However,
thermal
annealing changed
the coupling
between
unchanged after sputte
membranes, which could
be formed
by radiolytic
amorphization
of residual
solventremained
and retained
MoS
beam
for
1
min. In contrast to this
and
the
substrate
by
improving
the
contact
between
2
within the exfoliated nanosheets.
38
sputtering,
no
MoS
and
the
substrate
in
MoS
/Au,
MoS
/SLG/Au,
and
2 freestanding mechanically2 exfoliated2 MoS few-layers in a helium-ionchange in ΔE was d
Fox et al. [89] irradiated
at 300 °C and 450 °C. Re-anneali
MoS2/BN/Au or by introducing defects into the 2reducible
microscope. Figure 12 shows HRTEM images of a MoS2 few-layer. The hexagonal structure of
450 °C does not change the band
substrate CeO2 in MoS2/CeO2/Au. Both effects significantly
the pristine MoS2 waschange
destroyed
to analignment
amorphous
state.
Energy-dispersive
X-ray spectroscopy
To provide more quantitative in
the band
between
MoS
2 and the substrate
(EDX) indicated that preferential
sputtering
of sulfur
in theperformance,
mechanically exfoliated
MoSsputtering,
in MoS2 after
we perfor
and can have
a significant
impactoccurred
on the device
2
since many
devices
dependofonsulfur measurements
few-layers. Its stoichiometry
waselectronic
modifiedand
by optoelectronic
the preferential
sputtering
at nanometer on the MoS2/Au
38
Figure 3b shows the R
band more
alignment
of MoS
2.
scales. It was believedthe
thatenergy
He2+levels
ions and
transfer
energy
to sulfur
atoms than to sputtering.
molybdenum
after
300
°C
and 450 °C annealing
As
stated
in
the
Introduction,
ion
irradiation
is
another
way
atoms because of the lighter mass of sulfur atoms. Electric properties of the MoS2 layers were altered
A1′ modes of MoS2 are easily ob
to create defects and defect-induced electronic states in 2D
to be semiconducting, metallic-like, or insulating depending on doses. When the dose was above
normalized by A1′ modes. As can
not show considerable difference
annealing, but after sputtering, E
shifted, and both are widened (Fig
(at around 362 cm−1) on the lef
materials. Heavy ion irradiation is shown to introduce extended
defects such as incisions and folds in 2D materials including
MoS2.50 Here we focus on lattice defects that can be created by
low-energy ion irradiation. We introduced lattice defects into
SL MoS2 by low-energy Ar+ sputtering and studied the change
2573
Appl. Sci. 2019, 9, 678
16 of 53
Let
Nano Letters
(2.56 ± 0.05) × 1018
ion/cm2 ,
that the crystal(milling)
structure of thewas
MoS2 observed.
was controllably Figure
the balls,12c
the more
efficient the exchange of energy is.
complete removalresults
of material
shows
amorphized by the He+ irradiation process.
helium ion is much lighter than both sulfur and molybden
Stoichiometry
Tuning.
To
gain
a
better
understanding
of
but
is
still
much
closer
in mass to sulfur than molybden
He2+ -ion fabricated freestanding nanoribbons in MoS
few-layers.
The
edges
of
the
milled
regions
may
2
the He+ irradiation process, chemical composition analysis was
After the collision sulfur atoms will have a lot more energy t
2
+
energy-dispersive
spectroscopy
atoms.
This preferential sputtering of sulfur
be amorphous or crystalline, depending on He performed
beamusing
sizes.
MoS2 X-ray
layers
were(EDX).
milled molybdenum
when the
dose
Doses from (1.00 ± 0.02) × 1016 to (1.60 ± 0.03) × 1020 He+/
been observed for both electron and argon ion beams in
2
17However,
2,
useddose
to irradiate
mechanically
exfoliated,
free- ) ×
cm2 were
past.
was higher than (1.30 ± 0.03) × 1018 ion/cm
the
wasa below
(1.00
± 0.02
1014,37
ion/cm
our technique
enables localized stoichi
Letter
Letter . When
standing, few-layer MoS2 sample. The irradiated region of MoS2
etry modification and areas with a range of stoichiometries w
MoS2 layers were extensively damaged but not iscompletely
etched
away.
displayed in the high-angle annular dark-field (HAADF)
produced within a region of just a few tens of nanometers
doses above (2.56 ± 0.05) × 1018 He+/cm2 (region
complete removal of material was observed for this sampl
Nanostructure Fabrication. For few-layer samples
MoS2, the interaction volume can be approximated to
interaction area (see Supporting Information for an in
tigation of sample thickness effect). In this case, the He+ pr
size plays a crucial role on the lateral damage extension
crystal structure of milled nanosystems. An experiment
designed in order to better understand the relationship betw
Nano Letters
Lettersize and lateral damage extension in the mate
the probe
This enables control of the width of the amorphous e
2 nm
2 nm
region. As an example, defocus was used to adjust the pr
size to 12 ± 1, 5.9 ± 0.7, and 1.7 ± 0.2 nm (details on
probe
size measurement can be found in the Suppor
+
Figure 2. Stoichiometry alteration of an MoS2 flake by He irradiation.
Information). In Figure 3, three different edges were fabric
(a) STEM image of a freestanding MoS2 flake irradiated with 15
+
on a mechanically exfoliated, freestanding MoS2 flake. E
different He doses. Each region was irradiated with twice the dose of
edge was fabricated with a different probe size. Each pr
the previous region, starting with a dose of (1.00 ± 0.02) × 1016 He+/
delivered the same dose of (1.90 ± 0.04) × 1018 He+/cm2.
cm2 at region 1. Regions 9 ((2.56 ± 0.05) × 1018 He+/cm2) and above
were milled. (b) Concentration of Mo and S for different He+ doses as
structure of each edge was imaged in high-resolution T
extracted from EDX analysis. The error bars were obtained by the
Figure 3a−c corresponds to the ∼12, ∼5.9, and ∼1.7 nm pr
fitting of EDX data.
sizes, respectively. The lattice structure was not affected
distance (white line) from the edge which is approxima
equal to the probe size (red circle).
(labeled from 1 to 15) was irradiated with twice the dose of the
16
We simulated the distribution of He+ ions for the t
previous region with a starting dose of (1.00 ± 0.02) × 10
2 nm
+
22 nm
18
+
different
probe sizes used in the experiment. During ima
He /cm at region 1. At doses of (2.56 ± 0.05) × 10 He /
2
the2 He+ beam is moved from pixel to pixel
(region 29)+and above milling was observed. EDX spectra 18 and milling,
+ /cm
Figure 12. HRTEM image of (a) pristine andcm
(b)
He
-irradiated
(
(
1.00
±
0.02
)
×
10
He
) of Gaussian distributions was simulate
raster scan. An array
from each of the regions labeled 1−8 were recorded. The
+ doses
to account
for beam overlap during scanning (Suppor
evolution of
of the
stoichiometry
of the under
material with
respect to He2order
1
+
1 MoS2 few-layers [89]. (c) Stoichiometry
freestanding
an
MoS
flake
different
as
2 an
the Raman
E 2g andspectrum
A1g modes
of pristine
MoSof2 supported
an MoS
grum
Heof. (a)
of the
E 2g and∼6A1glayer
modes
pristine ∼6onlayer
supported
on
Information Figure S4). The milled MoS2 edge was assume
is shown in Figure
2b. It was observed that as the
the He+2 dose
2
+ EDX analysis. (d) HRTEM
1
+
be found
where the dose drops below (1.30 ± 0.03) ×
dose
increased
to 1017(left
He+/cm
(whereand
the sample
is
He+dose.
from
images
of crystalline
panel)
amorphous
(middle
he
E12g (b)
and FWHM
A1g modes
as acalculated
irradiation
dose. of
The
bars
ctively.
of the
Efunction
modes
as a function
Heerror
irradiation
The
error
bars
2g and Aof
1g He
He+/cm2; a dose below this value was found experimentall
predominantly amorphous), preferential sputtering of sulfur
2
+
2
+
. image
(d)right
High
resolution-TEM
MoS
EM
of pristine MoS
(d)image
Highofresolution-TEM
image
of MoS
. (c)image
High-resolution
TEM2and
of panels)
pristine
extensive damage but not complete milling. The p
the He
sample
occurred.
Intuitively
clear why sulfur
MoS2. nanoribbons
fabricated
from
milling.
He it isenergy:
30 keV.produce
Reprinted
2 after from
2 after
2
at which the crystal structure was assumed to remain intact
should be subjected to preferential erosion compared to
. Scale bars are 2 nm.
c 2015 The
with permission from Reference [89]. Copyrightmolybdenum.
American
where the ion dose drops below (1.00 ± 0.02) × 1017 He+/
mass of sulfurChemical
atoms is three Society.
times less than
as determined by Raman spectroscopy. Our simulation pred
that of molybdenum. Upon collision, He+ transfers more
an amorphous region that extends 7, 3.5, and 1.0 nm for th
energy to sulfur than to molybdenum. As in the collision of
± 1, 5.9 ±substrate.
0.7, and 1.7 ± 0.2 nm probes, respectively.
spherical objects
in classical
mechanics,supported
the closer the masses
Fox et al. [89] also irradiated MBE-grown pristine
MoS
onof an MgO
2 6-layers
scanning TEM (STEM) image in Figure 2a. Each region
(a)
(b)
(c)
(d)
resolution. A
milling approach
was milling
also developed.
resolution.
A nanoscale
approach was also developed.
as spintronics
andnanoscale
Freestanding
ribbons
of
MoS
with
pristine
crystal
structure
Freestanding
ribbons
of
MoS
What
is currently
with
pristine
crystal
Figure
4. He+ 2fabricated
freestanding
nanoribbons
in
2D materials.
(a) TEM
image of astructure
9 nm wide crystalline MoS2 nanoribbon.
(b) A 5 nm wide
2
+
With increasing
He doses, the full2width
at half
maximum
(FWHM) of the E1a 7 (the
in-plane Mo-S
nm wide crystalline
amorphous MoS2 nanoribbon. (c) An amorphous nanoribbon with a minimum width of less than 1 nm. (d) STEM image of2g
andof widths
to 2 nanoribbon.
∼7
nm(e) Awere
fabricated.
A
new
density
structuraldown MoS
and
widths
down
to
∼7
nm
were
fabricated.
A
new
9
nm
wide
crystalline
Mn
O
nanoribbon
milled
with
HIM.
(f)
TiO
edge
milled
with
HIM.
(g)
An
array
of
10
nm A
wide peaks
2
3
2
vibration)
increased,
indicating
more in-plane defects during irradiation. The FWHM of
1g
nanoribbons milled
withbeen
HIM. achieved
Mn
controllability
ashasevidenced
by electrical
2O3achieved
nd pattern
nanoscalehas been
controllability
as evidenced by electrical
2
+
decreased
with
increasing
doses,
inferring
material
removal
with
high
He
doses.
metallic-like,
or insulating
n 10 characterization.
nm wide, with Semiconducting,
characterization.
Semiconducting,
metallic-like, or insulating
Tongay
al. [90]
irradiated
exfoliated
with a high-energy He2+
2 mono-layers
MoS2 can be obtained
by2etirradiation
with different
doses of
ed orientation.
MoS
can
be obtained
bymechanically
irradiation
with
differentMoS
doses
of
13 ions/cm2 . Upon the He2+ particle irradiation at different
+ MeV)
beamthat
(3.04
with
doses
to 8is ×a 10
He+beyond
. We illustrate
is aillustrate
generalized
nanomodification
ve this
the
Hethis
. We
thatup
this
generalized
nanomodification
also
patterning
samples
Mn2O3 and
TiOeV
.
doses,
aapproach
new PL
peak
appeared
atsamples
1.78
and
the
many approach
chemicalbyand
by
alsoof
patterning
of
Mn
O
and TiO2. intensity of this new peak increased
2
2 3integrated
Structural
Modification.
The
change
in
crystal
structure
to produce
MoSwith
Structural
Modification.
The
change
in
crystal
structure
the
irradiation
dose.
The
new
peak
became
stronger
and broader after thermal annealing at
2
was approaches
analyzed 500
by ◦Raman
spectroscopy
and
TEM.
The
two
Figure
3.
HRTEM
images
of the
edges of three
regions in the same
MoS flake
with three different probe sizes. (a−c) M
was
analyzed
by
Raman
spectroscopy
and
TEM.
The
two
. These
C, which thermally introduced sulfur vacancies.
The integrated
intensity
offreestanding
the main
PLmilled
peak
with 12 ± 1, 5.9 ± 0.7, and 1.7 ± 0.2 nm He probes, respectively. The probe size is indicated by the red dashed circles overlaid on each image.
characteristic
Raman
peaks
for
pristine
MoS
are
shown
in
characteristic
Raman
peaks
for
pristine
MoS
olycrystalline
or too
are
shown
in
white
dashed
lines
approximately
show
the
edge
of
the
amorphous
region
and
agree
well
with
the
simulated
2
2
at 1.90 eV increased three-fold while
the PL peak position
shifted to a higher energy by 20 meV after values for the damage extension
1a.
This
sample
was
grown
on
an
MgO
substrate
by
5309
Figure
1a.
This
sample
was
grown
on
an
MgO
substrate
by
f MoSFigure
nanoribbons
2
the irradiation. Calculations estimated that approximately one defect per 100 unit cells was generated
molecular
beam epitaxy
(MBE).
The
peak
positions
were
molecular
beam
epitaxy
(MBE).
The
peak
positions
were
ransmission
electron
2 irradiation
−1
under
1013cm
dose.
−1
be
382.3the
and8 ×
406.6
for theand
E12g406.6
(the cm
in-plane
measured
toions/cm
be 382.3
and measured
phase of to
these
for
the E12g (the in-plane
2+ beams
14
+ 30 keV He
Klein
et
al.
[91]
employed
toelectrical
irradiate
MoSHe2 + dose
mono-layers
on SiO2 /Si
Figure
Electrical
characterization
He modified
devices. (a) Double log
plot of
resistivity versus
for a substrate supported,
A1g5.(the
out-of-plane
vibration)
Mo−S
vibration)
and ofAMo−S
(the
out-of-plane
Mo−S
vibration)
sion. Mo−S
Also, vibration)
TEM is andmechanically
1g The
letters S, I,12and M 16
correspond to regions
with semiconducting, insulating, and metallic-like behavior,
exfoliated bilayer MoS2 flake.
2
under
various
doses
10
–10
ions/cm
invalues
asample
scanning
helium-ion
microscope.
modes, respectively.
These
agree
well
with
the accepted
values
its requirement
ofsubstrates
a
modes,
respectively.
These
well
with
the
accepted
respectively.
Inset:
Resistivity
measurement
asagree
a of
function
of
gate
bias. The
resistivity
of the pristine
(dashed red line)
was widely tunable.
The
2
35sample irradiated with 1017 He+/cm
35high
showed
littlethe
gate response
(solid vibrational,
green line). (b) A 20luminescent,
nm wide, electrically contacted,
MoS2 nanoribbon. properties
The
from
the literature,
demonstrating
the
quality
of
our
Helium-ion
bombardments
affected
intrinsic
and
valleytronic
from
the
literature,
demonstrating
the
high
quality
of
our
g, lack
of scalability,
ribbon is shown with red false coloring indicating the measured region, and the gold electrodes are also colored. The blue dashed line shows the
+
sample.adopted
A plotofofthe
theatomically
full width
atwithhalf-maximum
(FWHM)
bar
is 500
(c)of
Current−voltage (FWHM)
plots for the indicated
isolating
cut made
the
s a widely
sample.
A thin
plot
ofHethe2beam.
fullScalewidth
at nm.
half-maximum
of doses as well as for the 20 nm wide MoS2
MoS
sheets.
nanoribbon.
All of the results were +measured at 0 V back gate.
1 function
+
E12gchemical
and A1g peaks
as
a
of
He
dose
is
shown
in
ture23theand
the
E
and
A
peaks
as
a
function
of
He
dose
is
shown
Maguire 2g
et al. [92]1ggrew MoS2 mono-layers on SiO2 substratesinby a CVD technique and irradiated
Figure
1b.(FIB)
Thethem
widthwith
of
these
peaks
was
observed
change
forangle
simulation
data
corresponds
wellofwith
thetoprobe
radiiwas
(half
the
remains entirely intact. In Figure 4b, a 5
structure
of the MoS
2
+
Figure
width
these
peaks
to change
for
used ion
beam
He 1b.
atThe
an
energy
30 keV
and
an+observed
of incidence
of2 0◦ . These irradiation doses ranged
14
+ of the2of
Further
analysis
extension
of the
14 amorphous
2 nm wide ribbon with an amorphous structure can be seen. The
doses nanoscale
above (1.00 ± FWHM).
0.02)
×
10
He
/cm
(The
dose
error
doses
above
(1.00
±
0.02)
×
10
introduce
He
/cm
(The
dose
error
region
from
the milled 2edges
local FFTs2 .from
width of the narrowest
structure
that the
can two
be reproduced
1×
1013
ions/cm
to 1was×done
1017using
ions/cm
With increasing
He2+ ion
doses,
characteristic
25
estimatedfrom
variation
of by
beam
current,
seeof
the
wasTEM
estimated
the
variation
beam current,
the
images. This
data
can be
found in the
Supporting
consistentlysee
is lessthe
than 1 nm, while the structure is no longer
probe.was
However,
a by the
Raman
peaks
quenched
and
broadened,
reflecting
the
growing
disorder
with
increasing
irradiation.
Information
(Figure
S5).
The
strong
agreement
between
the
crystalline
MoS
(see
Figure
1c).
The
narrowest
crystalline
2
in the
Supporting
Information).
discussion
of beam
stability
in the
Supporting
Information).
meter discussion
probe size ofarebeam stability
experimental
results at
and
simulation
that irradiation
for this
nanoribbon
was 7 nm wide
as shown with
in Figure
4d. MoS2 doses.
+
1−1indicates
+
1
A
new
peak
appeared
227
cm
after
the
and
its
intensity
increased
increasing
With greater He doses,
the(which
FWHMs
ofdoses,
theirradiated
E the
Aa 1g
peaks
With
greater
Heand
FWHMs
ofdose
the E 2gribbons
and are
A1gtheoretically
peaks stable down to widths of ∼1 nm.19 We
2g and
sample
is thin
was
with
sufficient
Helium-ion
beams
were
also
employed
to mill MoS
Some
of MoS2during
few-layers
speculate
that [121].
a heating
effect regions
may be occurring
for
milling) control
of the
edge damage
range can be achieved
increasetoand
decrease,
respectively.
2 layers
een shown
be an
increase
and
decrease,
respectively.
1 the size of the He+ probe. 1
milling,38 which might destabilize ribbons narrower than 7
by adjusting
2+ sub-nanometer
increase
the
E
peak
FWHM
is
due
to
the
wereofthinned
to
mono-layers
under
30
keV
He
ion
beams.
+
The
increase
of
the
E
peak
FWHM
is
due
to
the
density The
of structural
2g
2gHe dose ((3.10 ±
With the probe size (1.0 ± 0.5 nm) and
nm. Our future work will aim to address this question.
+
of more0.06)
in-plane
defects
during
The
/cm
optimized,
airradiation.
few-layerdefects
MoS2 flake
was
× 1018 He
The edges of The
these crystalline ribbons exhibit well-defined
introduction
of2) more
in-plane
during
irradiation.
ose.27 introduction
This is because
selected and
nanoribbons
were
fabricated.
InisFigure
4a,
a 9 nm
lattice configuration, indicating a subnanometer precision of
peakthan
FWHM
because
the
A
peak
not
only
A
peak
FWHM
decreases
because
the
A
peak
is
not
only
muchA1g
larger
an decreases
1g
1g ribbon is shown. The well-defined crystal lattice fringes 1g structuring. More importantly, this fabrication technique is not
wide
affectedinbymore
the presence
ofacross
defects
but
is
alsodemonstrating
very
sensitive
tois also very
affected
by
presence
of defects
butcrystal
m, resulting
span
the the
whole
ribbon,
that
graphene. A 9 nm wide Mn2O3 nanoribbon
limitedsensitive
to MoS2 or to
36
thickness.
is changing
samples
by material
thickness.in36our
This
is changing
in our samples
by
material
use oflayer
its ∼0.35
nm Thislayer
5310
DOI: 10.1021/acs.nanolett.5b01673
+
Nano Lett. 2015, 15, 5307−5313
with of
high Heremoval
doses. with
This high
demonstrates
theThis
ability
of
He+ doses.
demonstrates
the ability of
the removal
fabrication
+
+
He irradiation
alter irradiation
the crystal tostructure
defect structure by defect
ne nanoribbons
with to He
alter theby crystal
introduction
and material
removal.andFurther
Raman
data Further
is
introduction
material
removal.
Raman data is
fabricated
by this
presented
S1. The crystal
structure
was crystal
directlystructure was directly
in Figure
S1. The
e useful
in orderin toFigure presented
high-resolution
imaging of a freestanding
observedTEM
by high-resolution
TEM imaging of a freestanding
ement.observed
However,bythe
mechanically
exfoliated
sample.
Figure
1c
shows
the
hexagonal
mechanically
exfoliated
sample.
Figure 1c shows the hexagonal
dge orientations and
+
+ of pristine MoS . In Figure 1d, at a He+ dose of (1.00
structure
+
2
DOI: 10.1021/acs.nanolett.5b
Nano Lett. 2015, 15, 5307
ions/cm2. The Mo peak is given in (a)
and (b) and S peak in (c) and (d). The fitted spectra along with the constituent
peaks and experimental points are also
shown.
Appl. Sci. 2019, 9, 678
17 of 53
2.5. Proton Irradiation
Mathew et al. [85] prepared MoS2 flakes with a thickness of 200 µm and irradiated them at room
temperature using a 3.5 MeV proton ion beam. The Raman spectra of the pristine and irradiated
samples at a fluence of 5 × 1018 ions/cm2 are shown in Figure 13. A new peak at 483 cm−1 was
clearly visible in the irradiated samples. The appearances of the mode at 483 cm−1 along with the
broadening of the mode at 452 cm−1 indicated the presence of lattice defects due to proton irradiation
of the samples. The FWHM of the E12g and A1g modes did not increase in the irradiated MoS2 ,
indicating
preserved
the to
near-surface
modesstructure
becomewas
Raman
activeindue
resonance region
effect,ofasthe irradiated samples.
chi-square iteration
program that
usingthe lattice
29 after the proton irradiation, showing the enhanced
The
intensity
ratio
of
A
/E
enhanced
16%
2g
observed
by Sekine et al. These extra phonons are Davyan–Gaussian functions with a Shir1g
1
interaction
of electrons
X-ray
g the Mo 3d doublet,
the peak sepaand
A1gphotoelectron
modes.29 Thespectroscopy
broad peak indicated there were
dov with
pairsAof
the E2g
1g phonons.
1
canand
be the
order ofvalence
LA(M)of irradiated samples
ea ratio for 5/2 and
3/2 spin-orbit
at 452 cm
zone-edge
phononsobserved
in the irradiated
samples
the second
molybdenum
30
1
phonon.
ned to be 3.17 eV was
and higher
1.5, respecIn
the
irradiated
sample,
a
peak
at
483
cm
is states affected their
than +4. The irradiation-induced changes of structures and chemical
1
nding constraints for
the S 2p
3/2
clearly visible. Frey et al. reported a peak at 495 cm in
magnetic
moments.
5 eV and 2, respectively.25,26 The
.7 eV observed in the pristine spec(a)
d as Mo 3d5/2 and 3d3/2, while the
V in Fig. 3(a) is the sulphur 2s
pectrum in Fig. 3(b), apart from the
additional peaks at 229.6 eV and
peak observed at 229.6 eV in Fig.
the total Mo signal, which could be
r than þ4. The binding energy posi(V) have been reported to be 2 eV
V).26 We found a peak at 229.6 eV
hich is only 1.0 eV above that of the
(b)
e spectrum of S consists of S 2p3/2
eV and 162.5 eV and another two
eV. The peak at 163 eV in the irrag. 3(d) had increased by 6% in inthe pristine sample.
nature of the induced defects and
gained using Raman spectroscopy,
for characterizing ion irradiation
ne and graphite in a recent study.27
pristine and irradiated samples at a
1
cm2 are shown in Fig. 4. The E2g
1
A1g mode at 411 cm Figure
are clearly
13. RamanFIG.
spectra
of (a)spectra
irradiated
(b)MoS
pristine
MoS2 nanosheets
under
4. Raman
of (a) and
pristine
MoS2 at
a flu- a proton fluence of
1
2 and (b) irradiated
w frequency sides of E2g
and A1g
2 [85].
The deconvoluted
modes
arepermission
labeled in the
ence
of Reprinted
5 1018 ions/cm
5 × 1018 ions/cm
from2. Reference
[85], with
the
ofspecAIP Publishing.
observed in the deconvoluted spectrum; the fitted curve with constituent peaks and experimental points are
2
given.
and B1u phonons;
ive E1u
Kim etthese
al. [86] also
micromechanically
exfoliated MoS2 few-layers from a bulk MoS2 crystal and
fabricated them into MoS2 field-effect transistor (FET) devices on highly doped Si substrates coated
with SiO2 . Source and drain electrodes were made by depositing Au/Ti electrodes. The FET
Sep 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
devices were irradiated with a 10 MeV proton beam under fluences of 1012 , 1013 , and 1014 ions/cm2 .
Sufficiently high irradiation fluences decreased the electrical current and conductance of the devices
while low dose did not change them significantly. The threshold voltage was shifted towards the
positive gate voltage direction under proton irradiation. However, these changes were recovered
over a time scale of days. It was believed that proton irradiation changed the SiO2 /MoS2 interface
states, and the interface trap states at the SiO2 /MoS2 interfaces affected the electrical behaviors of the
FET devices.
Wang et al. [87] exfoliated MoS2 bi-layers, transferred them to silicon nitride membranes, and
irradiated them at 100 keV protons (H+ ) with a fluence of 6 × 1014 particles/cm2 . Figure 14a-b shows
the PL spectra of the bi-layers. Both the suspended and substrate-supported bi-layers showed almost
spectra of MoS2 ex
the direct bandgap emission after irradiation and the emer3.04 MeV a-particle
gence of a defect-induced sideband peak at 1.71 eV. As menpower (i.e., deposit a
tioned above, defects induced by the radiation trap the
pared to the 100 keV
excitons and cause them to be redshifted from the main
here.
direct band emission at 1.85 eV. For the suspended region,
In summary, we
Appl. Sci. 2019, 9, 678
18 of 53 cence and direct band
we understand this decrease in intensity due to defects,
which cause non-radiative recombination and shorten the
flakes. For all substra
lifetime of the photoexcited carriers. Under all conditions,
suspended samples, th
the PL intensity
in the
of the
and the indirect emis
complete suppression of the indirect emission (1.55–1.60
eV) after
thesubstrate-supported
irradiation and region
the emergence
MoS2 is much weaker than that in the suspended region, as
of a defect-induced sideband peak at around 1.70 eV.
The direct band emission at 1.83 eV increased due to decoupling of
shown in Fig. S8 of the supplementary material, consistent
tion). In all samples
1.6× after irradiation while the substrate-supported
direct
band
emission
by we
a factor
of 2.7.defect-induced sideba
see a 2
with the
original
reports
of Makincreased
et al.19 Here,
in the direct bandgap
emission
at disappeared
1.85 eV for the for annealed out by heat
After being annealed at 300 ◦ C for 1 h in argon,increase
the defect-induced
sideband
peak
substrate-supported
regionafter
after irradiation.
Asindicating
with the sus-that pended monolayer Mo
both kinds of bi-layers. The indirect band emission
was suppressed
annealing,
pended region, we observe an increase in the defect-induced
by a factor of 2.8, l
the bi-layers underwent an irreversible indirect-to-direct
band-gap
et al.et[87]
peak at around
1.7 eV, astransition.
was reportedWang
by Tongay
al.11also recombination centers
examined
MoS
and four-layers. TheseAfter
multi-layers
showed
as
bi-layers.
annealing the
samplessimilar
at 300 behaviors
C
forPhys.
1 h in
argon,
the (2017)
2 tri-layers
that, after annealing, t
131101-3
Wang
et al.
Appl.
Lett.
111, 131101
suppressed, indicating
material from an indir
bandgap semiconduct
(c)
See supplementa
data including Rama
results.
This research w
1402906 (B.W.) and
No. DE-FG02-07ER4
to acknowledge sup
Institute of Optical N
ION2) (S.Y.). The por
Corporation was fun
development program
(d)
1
G. F. Knoll, Radiation
Hoboken, NJ, 2010).
2
R. Dhall, Z. Li, E. Kosm
195702 (2016).
3
J. S. Ross, S. F. Wu, H. Y
Q. Yan, D. G. Mandrus, D
1474 (2013).
4
R. Dhall, M. R. Neupane
Moore, R. K. Lake, and S
5
T. Y. Kim, K. Cho, W. P
T. Lee, ACS Nano 8(3), 2
6
C. X. Zhang, A. K. M. N
Fleetwood, M. L. Alle
Pantelides, IEEE Trans. N
7
FIG.
2. PL spectra
of bilayer
MoS2 of
taken
before andMoS
after proton
irradiation,
J. Lee, M. J. Krupcale,
FIG. 5. PL spectra
monolayer
MoS
after
irradia-irradiation,
FIG.
3. of
PL
spectra
of trilayer
MoS2before
taken and
before
andproton
after proton
Figure
14. PL
spectra
bi-layer
over
holes
and
(b)2 taken
deposited
on
substrates,
2 (a) suspended
and after annealing.
tion, and after
023106 (2016).
andannealing.
after annealing.
and mono-layer MoS2 (c) suspended over holes and (d) deposited on substrates taken before and
calculations
by Lake
andirradiation
coworkers with
established
that of
an6 × 1014 particles/cm2 , and after annealing [87].
after 100 keV
proton
a fluence
increase
in thefrom
interlayer
separation
of just 1 Å of
is sufficient
Reprinted
[87], with
the permission
AIP Publishing.
to induce an indirect-to-direct bandgap transition in this
4
which
experimentally
AFM
material,
Wang et
al. was
[87]confirmed
also exfoliated
MoSwith
2 mono-layers and transferred them to silicon nitride
measurements.4 While these DFT calculations provide a
membranes
with holes.
freestanding
MoS
2 mono-layers were then irradiated at 100 keV
qualitative explanation
of theThe
mechanism
underlying
this
protons
(H+ ) withbandgap
fluences
rangingwe
from
2 × currently
1012 particles/cm2 to 6 × 1014 particles/cm2 . Figure 14c
indirect-to-direct
transition,
do not
have PL
an atomistic
of the ion interaction
the latshows
spectrapicture
of suspending
MoS2 with
mono-layers
after proton irradiation with a fluence of
tice 14
of this material. The
suppression of the indirect bandgap
2
6 ×emission
10 particles/cm
with and without annealing, compared with those taken before the irradiation.
is potentially useful for fabricating optoelectronic
Thedevices
intensity
of
the
direct
band-gap
emission
suspending
mono-layers decreased two-fold after the
such as LEDs and solar
cells. Spectra
taken of
after
an
13
2
protons/cm
and
a
intermediate
proton
fluence
of
6
10
irradiation due to the irradiation-induced defects. The defects caused non-radiative recombination
fluence of 6 1015 protons/cm2 are also shown in
andheavier
shortened
the lifetime of the photo-excited carriers. On the contrary, the intensity of the direct
the supplementary material. For the heaviest fluence, the PL
band-gap
emission
at 1.85 eV
increased
the irradiation for the substrate-supported regions, as
intensity decreases, indicating
that
the effectsafter
of radiationinduced
defects eventually
outweigh
the indirect-to-direct
shown
in Figure
14d. After
being annealed
at 300 ◦ C for 1 h in argon, the defect-induced sideband
bandgap
transition.
peak disappeared in the bi-layers and mono-layers, and the direct band emission reverted to its original
Figures 3 and 4 show MoS2 samples with 3 or more
pre-irradiated
intensity
both kinds
of MoS
2 layers.
layers in thickness
(after for
irradiation
and before
annealing),
which, again, show slightly enhanced direct bandgap emis2.6.sion,
Electron
Irradiation
complete
suppression of the indirect emission at around
1.4 eV, and a broadened, red-shifted defect induced sideband
irradiated
MoS
single multicrystals with a thickness of about 50 µm by using
at Han
aroundet1.7al.
eV.[122]
In addition,
the spectra
of 2suspended
high-energy
electrons
in
ambient
conditions
at
room temperature. The electron dose was 300 kGy
layer MoS2 look very similar to those of substrate-supported
14
2
(See
Fig.
S12
in
the
supplementary
multi-layer
MoS
2
(6.70 × 10 electrons/cm ) and the acceleration energy of electrons was 0.7 MeV. There was a
material), reflecting the decreased sensitivity to substrate
interactions as the material approaches the bulk limit. After
annealing, the indirect emission remains suppressed, indicating that an indirect-to-direct bandgap transition is stable
FIG. 4. PL spectra of MoS2 with four layers taken before and after proton
irradiation, and after annealing.
against annealing at 300 C. Also, the defect-induced sideband
Appl. Sci. 2019, 9, 678
19 of 53
negligible reduction of sulfur XPS intensity compared to the molybdenum XPS intensity of the samples
after the irradiation. However, 1T-like defects were generated in the MoS2 surface, inducing a weak
ferromagnetic state at room temperature and improving transport properties.
from ref. (3)) show fine distinction between them, substantially undulating domains are
Han et al. [74] prepared single-crystalline MoS2 lamellae with a thickness of 100 µm. The lamellae
observed atwere
the MFM
image of the electron-irradiated
sample atconditions
the 150 kGy with
the
electron-irradiated
in ambient
at room
temperature. Figure 15 shows HRTEM images
accelerationof
energy
0.7 MeV (b). And
then these domains
are morehoneycomb
evident at the increased
theofirradiated
samples,
showing
lattices of the MoS2 . The vacancies depended on the
acceleration
energy
and
dose.
Without
irradiation,
electron dose of 300 kGy (c). But, they are weakened at the further increased electron doses of the lattice of crystalline MoS2 was a honeycomb
few(e),defects,
shown
in Figure
After
400 (d) andwith
600 kGy
respectively.
Consistently,
the MFM15a.
images
of thebeing
increasedelectron-irradiated with 150 kilogray(kGy) dose at
0.7 MeV acceleration energy, additional defects were produced. Double-sulfur vacancies (VS2 ) were
acceleration energy of 2.0 MeV also exhibits similar change depending on the electron dose
more frequently observed than the monosulfur vacancies (VS ) after electron irradiation. Slightly
between 100 (f) and 250 kGy (g). This is in good agreement with the change of the
displaced molybdenum atoms were occasionally found, as marked by the red arrow in Figure 15b.
magnetizations of Fig. 2, even though the MFM results mainly concern on the magnetic
Under electron irradiation with a 300 kGy dose at 0.7 MeV acceleration energy, the honeycomb lattice
property along
theheavily
out-of-planedistorted
direction. On(Figure
the other hand,
is notable
that, while
the
was
15c)itand
perfect
honeycomb
lattices were rarely observed. The numerous
VSbetween
and V
vacancies
distorted
honeycomb
and the irradiation considerably decreased the
magnetizations
theS2two
different acceleration
energiesthe
have an
order difference inlattice
Fig.
lattice
parameters.
V
defects
were
more
frequently
observed
under the electron irradiation with a
S2no significant influences on the MFM images
2, the high acceleration energy of 2.0 MeV has
100 kGy dose at 2.0 MeV. With an increasing dose of 2.0 MeV electrons, the VS and VS2 concentration
(Figs. 5(f) and 5(g))and
compared
to those
5(b) spacings
and (c)) of
the low
acceleration
energy of (b) Zoomed image
(110) planes
with(Figs.
the lattice
of 2.71
Å and
1.58 Å, respectively.
increased significantly. Molybdenum vacancies were produced too. The lattice parameters were
0.7 MeV. This means
thethe
SQUID
MFM results
bulkMoS
and2 single
surface
sensitive,
in (a)that
shows
typicaland
honeycomb
latticeare
of the
crystal.
The estimated lattice or
also significantly increased. Under electron irradiation at higher energy of 10 MeV, different types
respectively.
distance between the two neighboring Mo atoms is of a = 3.12 Å (c) During the TEM
of defects
were produced. Under all irradiation, the vacancy densities increased with the electron
defects were generated.
measurements,
monosulfur
(VS’s) and line
irradiation
dose
and electron
energy.
VS2
(a)
(b)
(c)
(d)
(e)
Figure 15. Zoomed HRTEM images of MoS2 lamellae
without
electron
(a)electron
and with
electron
Figure S10.
(a) HRTEM
image ofirradiation
the H(ii) sample:
irradiation
with 250 kGy dose at
Figureimage
S7. (a)
image of 2the
L(i)inset
sample:
electron
irradiation
with 150 kGy dose at 0.7
Figure S6. (a) HRTEM
ofHRTEM
the pristine
FFT
image
displays
hexagonally
irradiation
with (b)MoS
150. kGy
dose
at 0.7
MeV
energy, (c) 300 kGy dose at 0.7 MeV energy, (d) 100 kGy
2.0 MeV accelerating energy. The fringes of HRTEM image indicate that the number of the
accelerating
energy.the
More
are obtained.
Especially,
VS2 (100)
vacancies
are frequently
arranged spots withMeV
a 1×1
structure,
where
innerdefects
andand
outer
spots250
correspond
to the
dose
at 2.0 MeV
energy,
(e)
kGy dose
at 2.0 MeV
energy [74]. The distance between two
MoS2 layers is about mono- or bi-layers. FFT inset image shows weak additional spots in the
neighboring
molybdenum
atoms
is
3.12
Å
(a
top
panel),
3.24
Å (b top panel), 2.94 Å (c top panel),
observed than the VS vacancies after electron irradiation. The fringes of HRTEM image
hexagonal pattern,
relating to The
the very
increased Vin
VMo
defects. (c) The lattices
S, Vthe
S2, and
3.23
Å
(d
top
panel),
and
3.22
Å
(e
top
panel).
Defects
are
circled.
VSmuch
vacancies
(a)
bottom
indicate that the number of the MoS2 layers is about mono- or bi-layers. FFT inset image
Figure
S8.
(a)
HRTEM
image
of
the
L(ii)
sample:
electron
irradiation
with
300
kGy
dose
at
panel come from the irradiation
of the
electron
beam
observations.
Reprinted
from
[74],
also significantly
increased
compared
to
that ofwith
the pristine
2. at with
Figure
S9. (a)
HRTEMare
image
ofduring
the H(i) sample:
electron
irradiation
100
kGyMoS
dose
displays an additional spot in the hexagonally arranged spots due to the increased defects. (b)
0.7 AIP
MeV accelerating
energy. The fringes of HRTEM image indicate that the number of the
the permission of
Publishing.
2.0 MeV accelerating energy. The fringes of HRTEM image indicate that the number of the
The honeycomb lattices are increased compared to that of the pristine MoS2. (c) The red arrow
MoS2 layers is about bi- or tri-layers. There rarely exist honeycomb lattices. (b-d) The distorted
MoS2 layers
is about References
fiveor six-layers.
VS2-like defects are more frequently observed
Rotunno
et al. [75]
mechanically
exfoliated
MoS
2 20-layers and transferred them to carbon-coated
indicates
the displacement
of Mo
atom, but these defects
are occasionally
found.
patterns are frequently obtained due to the presence of the many VS and VS2 vacancies. In
1.
S.
Yan,
W.
Qiao,
X. around
He, X. the
Guo,
L. Xi, and
W.
Zhong,of
Appl.
Phys.
106, 012408
to the case of L(x).
(b-d) The lattices
vacancies
areimage
increased
except
the Lett.
copper grids. Figure 16a showscompared
a transmission
electron
microscopy
(TEM)
a typic
MoS
2
contrast to the case of L(i), the lattices are considerably decreased compared to that of the
(2015). of
few-layer. The multi-layers werecase
then
under
a the
scanning
microscopy
beam
of (c).irradiated
The different intensities
main peaks electron
in the line profiles
of (c) and (d)(SEM)
are
pristine
MoS2.
with an accelerating
voltage
ofattributed
5 keVto and
an electron-beam
currentB.of
50
nA J.for
15
min.
Many
S. Tongay, S. S. Varnoosfaderani,
R. Appleton,
Wu,
the electron2.irradiation-induced
gliding S or Mo atoms.
This suggests
thatand
theA.
VS2F.- Hebard, Appl. Phys.
crystalline islands were formed throughout theLett.multi-layers
after the irradiation, as shown in
like defects are quite similar to101,
the 123105
electron(2012).
beam-induced 1T phase. The details will be
Figure 16b. The selected-area electron diffraction (SAED) patterns indicated that the islands were
3. S. W. Han, Y. H. Hwang, S.-H. Kim,W. S. Yun, J. D. Lee, M. G. Park, S. Ryu, J. S. Park,
elsewhere.
metallic molybdenum. Under reported
high-energy
(200 keV) electron irradiation, the MoS2 layers were
D.-H. Yoo, S.-P. Yoon, S. C. Hong, K. S. Kim, and Y. S. Park, Phys. Rev. Lett. 110,
drastically modified, and their sulfur stoichiometry was changed, inducing the formation of the
(2013).
molybdenum nanoislands, as shown in Figure 16c.247201
In both
cases, massive surface sulfur depletion was
induced together with the consequent formation of molybdenum nanoislands.
2D Mater. 3 (2016) 025024
2D Mater. 3 (2016) 025024
2D Mater. 3 (2016) 025024
E Rotunno et al
E Rotunno et al
Appl. Sci. 2019, 9, 678
20 of 53
(a)
(b)
(c)
Figure 16. TEM images of (a) un-irradiated, (b) 5 keV electron-irradiated, and (c) 200 keV
electron-irradiated MoS2 20-layers [75]. The strong reflection spots of the inset SAED come from
the MoS0.6 layers while the weak ones from metallic Mo. Reproduced with permission under the
Creative Commons license. Copyright c 2016 IOP Publishing.
Kim et al. [82] deposited five to seven atomic layers of MoS2 on a SiO2 /Si wafer at room
temperature using the RF magnetron sputtering method. The thickness of the deposed amorphous
MoS2 layers was about 4 nm. The films were then exposed under a collimated electron beam with an
energy of 1 keV for 1–10 min at room temperature without any additional heating processes. HRTEM
Figure 4. (a) HR-TEM image of a MoS2 flake after irradiation. Mo nanocrystals circled in red. The FFT is reporte
images indicated that the as-deposited
MoS2 was
amorphous
(left
panel
17a).profile
Under
the pattern in (b).
diffraction pattern
of the
irradiatedin
area.Figure
(c) Radial intensity
of the diffraction
Figure 5. (a) TEM image of a MoS
flake
after
SEM
irradiation. (b) Electron diffraction pattern of the irradiated area. (c) Radial
2
Figure
1. (a) HAADF-STEM
image of the typical
MoS
before
andpattern
a close view
edge (b). (c) High-from amorphous to crystalline
2 flake
intensity
profile
ofirradiation
the diffraction
in atoms
(b).of the flake
electron
beam
irradiation
(EBI)
of
1
min,
the
random
re-arranged
resolution TEM image revealing the perfect MoS2 honeycomb structure (inset). (d) Electron diffraction pattern.
structure, forming MoS2 crystal domains with a size about 5 nm (middle panel in Figure 17a). However,
reported in the case of electron irradiated GaN microThis is in agreement with the
longer irradiation time would
crystalline
domains
panel
in Figure 17a) because of the
spacingdamage
is. Therefore,
the formation
of
a large(right
hex-and
structures
[23]
devices [24] due to the breaking of tion provided in figure 2.
of μm
and
thickness
of around
20 monolayers.
In by Raman scattering,
of MoS
compensation
of the
intrinsic
n-type
2, (i.e.bond
agonal
Moiré
pattern
means
that
these the
crystallites
pre- was
Mo-S
breaking.
The
amorphous-crystalline
transformation
confirmed
Therefore, we can conclude th
Mg–H complexes.
doping).
ordercloser
to assess
theMoS
thickness
of the
we carefully
sent a lattice parameter
to the
one modifications
than
in flakes,the
2 The
1 and
S depletion,
flakes
undergo
at the atomic
as
shown
in
Figure
17b.
The
two
prominent
Raman
peaks
of
MoS
,
the
in-plane
mode
E
the the remaining metall
2
2g
Different works [19, 20] have recently
reported
the previous
case. inspected their edge. The flake termination, in fact, is
level have been studied by means of high resolution
in small nanoislands on the surfac
that capping
or encapsulating
layers
with
the irradiated
is because
out-of-plane
modeMoS
A1g2 ,single
didThe
notdiffraction
appear
in
theof(figure
as-deposited
film
its amorphous nature.
notpattern
abrupt
1(b))
butarea
present
clear steps of
with
in agreement with [16]. The form
TEM and
electron
diffraction.
1 experireported
in figure
5(b)
and
its
rotational
averaged
graphene
during
electron
beam
irradiation
thickness
that produce
different
image
intenlands demonstrates the impossib
The peak intensities of
the
E2g
and
A1different
modes
increased
dramatically
after
the
electron
irradiation
In
figure
4(a)
a
high
magnification
TEM
image
of
g
intensity
profile insity.
figure
5(c).
Comparing
the profile
ments has beneficial and protective effects,
decreasing
The
study
of the
steps
intensity
a rough
the irradiated
area isgives
reported.
Nanometric
crystals doping
at
room
temperature.
Their
XPS
measurements
provided
a
stronger
proof
that
the
amorphous
filmsinversion from intrinsic n
in figure
5(c) withestimation
the one reported
in figure
4(c) the
the generation of defects and possible lattice
breaking
of the flake
thickness
accuracy
of between the holes increasing the concentration of s
(circled
in red)within
can bean
clearly
seen in
electron
irradiation.
Figure
shows the
XPS
spectra of MoS2 samples
appearance
of a new
set ofmonolayers.
reflections
can
be 17c
seen.resolution
sulfur vacancy causes a deep acc
and/orcrystallized
amorphization.under the 1 keV
a few
Atomic
TEM
imaging
(bright
areas) produced
in the
continuous MoS2 film
Inray
particular,
theenergy
two strong
peaks
at
2.61The
andMo-3d peak was de-convoluted
band-gap
Incharacterized
this work we test with
the structural
and
composian Al-K
with
an
of
1486
eV.
into [17]). Modifying drastic
assessed that the flakes
didregion).
not undergo
structural
α
(gray
Moirè any
fringes
also appear where these
1.50under
Å werehigh
not present in the TEM irradiated sample.
chiometry induces the spontaneo
tional three
stabilitychemical
of MoS2 multi-layer
flakes
flakes,
testicrystals
overlap
with
the
underlying
MoS
transformation
during
the
exfoliation
process
and
bonding states
of Mo-Mo,
Mo-S,
and Mo-O
in from
the as-deposited sample.
After 1 aforementioned
min of
2
The small
lattice distance
difference
measured
Mo nanoislands.
dose electron irradiation. We experimentally
deterfying
that 2there
is a closereported
relationship,
both in terms of
retain
theconsisted
perfect
2H-MoS
structure
in from
electron
irradiation,
the
Mo-3d
spectrum
only
of
two
peaks
originating
the
states
of the
the
diffraction
pattern
is
consistent
with
the
presence
mine the modifications induced by electron-beam figure 1(c) and its inset.
orientation
and
lattice
parameters,
between the
only
found
evidences
of
of the dramatically
large Moiré
shown in thepeak
image.We
SEM irradiation
Mo-S
Mo-O
bonds,
with
increased
intensity
of the
irradiation
to and
the crystal
lattice
and stoichiometry
of pattern
flake. Similarly, the
molybdenum
nanograins
andMo-S
the MoSbonds.
presence
of several
ripplocations
Besides these the
strong
contributions,
the weak [21] induced by 2
In this section we report on the effe
multi-layer
MoS
flakes
exfoliated
from
geological
The nanocrystals
are responsible
for the weak
dif- only the
S-2p peak2 from the as-deposited
sampleÅwere
de-convoluted
into
S-S and
S-Mo bonding
while
exfoliation
process, which
reflection at 2.23 the
assigned
to metallic
Mo, isappear
still as straight line in
dose on the structural properties o
molybdenite via the conventional tape method. The figure 1(a).
spots,The
indicated
by the also
white arrows,
observed
S-Mo bond remained in the
1 min electron-irradiatedfraction
sample.
authors
checked
the
samples
present.
electron
irradiation in a
Figure 6. Illustration
of the protective
effect
of
C coating beam
on
experiments are carried out both in a transmission
both
the fast Fourier
(FFT)
of figure
4(a),
flake after
the SEM
irradiation.
thetransform
MoS2pattern
The
of
the
electron
diffraction
The two
strong peaks
atsymmetry
2.61
and 1.50
Å in
cannot
be with
microscope.
under
TEM
at
400
keV
and
then
measured
the
Raman
scattering
a
532
nm
laser.
It
was
not
clear
electron microscope (TEM) and in a scanning electron of figure 1(d) is also
as anwith
inset,the
and2Hin phase,
the electron diffraction
In this case also we observed
Sy consistent
compound.
attributed to any known Mo or Moxreported
if the inamorphous
films
were
changed
under
the
TEM
electron-beam
irradiation
microscope
order to study
different
irradiation
pattern
of
the
same
region
reported
inand
figurelaser
4(b). irradiation.
tion of many crystalline islands all
Further studies arebeing
ongoing
order
a =inb =
2.74toÅidentify
and γ =them.
120°.
In figure 4(c)effect
the radial
intensity
profile
ofthe
thethree
dif- through
regimes. LastKarmakar
but not least we
presence
of irradiation
etfind
al. that
[77]theinvestigated
the
few-layers
shownare:
in figure 5(a).
to 2the
labels
regions
During the
experiment
we
Theelectron
experimentirradiation
hasincidentally
been carried out According
in of
situ MoS
inside
fraction
pattern
is reported. It has been obtained by
a nanometric amorphous carbon layer,
unwillingly
The
noticed
that
a
very
thin
carbon
coating
is
able
to
prothe TEM;
the beam-reducing
apertures by
havemechanical
been
micro-Raman scattering. The few-layered
MoS2allsamples
were prepared
exfoliation andMoirè fringes, resulting fr
integrating
the pattern
intensity
along
azimuthal of the
deposited during the TEM experiments,
nanocrystals
of the
flake is with the unde
(1)
Free
standing
MoSthe
tect minimizes
the MoS2 flakes
and completely
prevent
the
for2 flake; this portion
−
7
removed
and
the
electron
beam
has
been
focused
as
irradiated
a vacuum
of 10of×the
10 Mo torr.
The
thickness
of the few-layers
was
about
7 nm,
which
direction.
the damage
induced under
by the electron
beam
irradiation.
exhibit
incarbon
this case a pattern with
located
over
one
of
the
holes
of
the
TEM
mation
grains.
This
effect
is
clearly
much as possible in order
to maximize
the electron
The two
strongest
peaks
at
2.74 and
1.58before
Å are theand
was equivalent to about ten
layers
of MoS
shows
the
Raman
scattering
after
than
one observed
in the TEM
coated
copper
grid,
therefore
here
nothecarbon
is
described
in figure
6 showing
a MoS18
2 . Figure
2 flake after the
dose.
As a result, an(100)
areaand
of few
ofofsquare
respec(110)hundreds
reflections
the MoS
1
2 flake,
It
can
be
useful
to
recall
that th
present
either
below
or
over
the
sample.
SEM
irradiation.
irradiation.
In addition to the in-plane E
A1girradiated.
modes, extra Raman modes appeared
nanometers
has been uniformly
2g and out-of-plane
Results
and discussion
The
weak reflection at 2.23 Å, marked by the iodicity depends on the difference
Its surface is almost entirely coveredtively.
by Mo
grains.
The flake
isthe
standing
overunder
the carbon
grid.
after being irradiated under 5 keV, 7 keV,The
and
10 keV,
showing
the(2)contrast
maximum
change
keV of the two crystals form
damaged
area
shows
a grainy
black
arrow,
is consistent
withinthe
(110)
reflection
of 10
parameters
Three regions, having different grain densities
can be
STEM image reported
in figure
2(a), and itin
canthe
be breaking of inversion
TEM irradiation
similar
the two
parameters, the lar
metallic
Mo.
irradiation. Obviously, the
electron-irradiation-induced
defects,
resulting
noticed. They have been labeled and black dashed lines (3) The circle is a contamination spot. This region
was
easily
distinguished
from
the
otherwise
continuous
Figure 1(a) shows the typical appearance
of
the
MoS
2
have been drawn
imagetotoany
highlight
them. in the spectra
previously
observedthe
in the
TEM, before
symmetry. The 15 keV irradiation
did on
notthelead
changes
because
electron
beamthe SEM
4
flakes studied in this work. We limited our attention to contrast exhibited by the pristine region surroundpassed
the
few-layers
without
creating
any
damage.
large multilayer flakes having lateral dimension of tens ing it.
5
Matsunaga et al. [80] irradiated triangular-shaped MoS2 mono-layers grown on SiO2 /Si substrates
2 acceleration voltage of 15 kV and an areal dose of 280 µC/cm2 charge.
under an electron beam with an
Figure 19a shows an optical image of a mono-layer MoS2 crystal. The right half region was covered
by polymethyl methacrylate (PMMA) and was not electron irradiated. Raman scattering indicated
1 peaks of MoS . However,
that both regions (with and without irradiation) exhibited the A1g and E2g
2
the two peaks were blue-shifted in the exposed region, as shown in Figure 19b. It was suggested that
the electron-beam exposure introduced a local compressive strain in the MoS2 crystal. The induced
Appl. Sci. 2019, 9, 678
21 of 53
strain would further change the band structure. The PL peak of the exposed left region, ∼1.85 eV, was
blue-shifted 40–50 meV compared with that of unexposed right region (1.81 eV). Figure 19c shows the
PL mapping of PL intensity. It was stated that the averaged PL spectrum of the unexposed region of the
MoS2 mono-layer could be characterized by a broad peak at 1.81 eV while the exposed region of the
same MoS2 sample showed a narrower feature at 1.85 eV, corresponding to a blue shift of 40–50 meV.
www.nature.com/scientificreports/
www.nature.com/scientificreports/
(a)
3. HR-TEM
images
of EBI-treated
films.
(a) Cross-sectional
HR-TEM
image
of 1EBI-treated
min
FigureFigure
3. HR-TEM
of EBI-treated
MoS2 MoS
films.2 MoS
(a) Cross-sectional
HR-TEM
image
of 1 min
Figure
3.images
HR-TEM
images
of EBI-treated
(a) Cross-sectional
HR-TEM
image
of 1EBI-treated
min EBI-treated
2 films.
MoS2Plan-view
film.2Plan-view
HR-TEM
images
of
(b)the
as-deposited
sample,
(c) 1EBI-treated
min
sample,
and (d)
MoS2 film.
images
of the
(b)the
as-deposited
sample,
(c) 1sample,
min
sample,
andsample,
(d)
MoS
film.HR-TEM
Plan-view
HR-TEM
images
of
(b) as-deposited
(c) 1EBI-treated
min EBI-treated
and (d)
min10EBI-treated
sample.
Insets
showpatterns
FFT patterns
of
the of
areas
as dashed
squares.
10 min10EBI-treated
sample.
Insets
showInsets
FFT
of the
areas
marked
as dashed
squares.
min EBI-treated
sample.
show
FFT
patterns
the marked
areas
marked
as dashed
squares.
1the E1 peak
−1 cm−1 with
−1an average value of
min1EBI-treated
sample.
The Raman
at 381–382
min EBI-treated
The
of the
E12g dispersed
peakatdispersed
at 381–382
cmaverage
with value
an average
1 min 1EBI-treated
sample.
Thesample.
Raman
shiftRaman
ofshift
the of
Eshift
381–382
cm
with an
of value of
2g dispersed
2g peak
−1
−1
−1
−1
−1
−1
−1 cm−1.
,cm
and
the
Raman
the
A
peak
dispersed
at
406–408
cm
with
an
average
value
of
406.77
381.44381.44
cm 381.44
, and
the Raman
of
the
A
peak
dispersed
at
406–408
cm
with
an
average
value
of
406.77
cm−1.
, cm
and the
Raman
shift
ofshift
the of
Ashift
peak
dispersed
at
406–408
cm
with
an
average
value
of
406.77
cm
.
1g
1g
1g
−1
−1
−1
The average
peak difference
(∆k)25.33
was
25.33
cm
,cm
which
corresponding
theto
value
of
six of
atomic
layers
or bulk
The
average
peak(∆k)
difference
(∆k)cm
was,25.33
, which
corresponding
six atomic
layers
or bulk
The average
peak
difference
was
which
corresponding
to theto
value
ofthe
six value
atomic
layers
or bulk
Even
thetime
EBI
time
was
to (c)
510and
min,
did
not
change
signifi
antly
as
summarised
Even
when
the
EBI
timeincreased
wastoincreased
to 510and
10∆k
min,
∆k
did
not change
antly
as summarised
in
MoS2.MoS
Even2.MoS
when
the
EBI
was
increased
5 and
min,
∆k
did
not
change
signifi
antlysignifi
as
summarised
in in
(b)2. when
Figure
5. XPS
spectra
ofcrystallinity
the
(a)the
surveyof
spectrum,
(b)
Mothe
3d spectrum,
and (c)samples
S 2p spectrum
from
as- improved
Table
1. Based
onRaman
this
we estimated
that
the
crystallinity
the
EBI-treated
was
Table
1.
Based
oncomparison,
this
comparison,
we
estimated
that
the
of
EBI-treated
samples
was
improved
Table 1.
Based
onFigure
this
comparison,
we estimated
that
crystallinity
of
EBI-treated
samples
was
improved
1.
spectra
of as-deposited
and EBI-treated
MoS
filmsthe
according
to
irradiation
times
of
1,
5,
and
deposited and 1 min EBI-treated samples.
10 min.
Figure
17.
(a)through
Plan-view
HRTEM
images
ofthe
the
as-deposited
sample,
1 min
sample,
antly
through
the atomic
rearrangement
Mo
atoms
from amorphous
to a crystalline-layered
signifi
antly
the atomic
rearrangement
of
the
MoS and
S atoms
from
amorphous
to a crystalline-layered
signifisignifi
antly
through
the
atomic
rearrangement
of the of
Mo
and
S and
atoms
from
amorphous
to a EBI-treated
crystalline-layered
and
10
min
EBI-treated
sample
[82].
Insets
show
fast
Fourier
transformation
(FFT)
patterns
of the
structure.
structure. structure.
FWHM (cm )
Average (cm )
High-resolution
TEM
analysis
was
performed
in
order
to
confirm
the
crystalline
microstructure
areasHigh-resolution
marked
as
dashed
squares.
(b)
Raman
spectra
of
as-deposited
and
EBI-treated
MoS
samples
TEM
analysis
was
performed
in
order
to
confirm
the
crystalline
microstructure
High-resolution
TEM
analysis
was
performed
in
order
to
confirm
the
crystalline
microstructure
of
theof theof the
2
Sample
A
E
A
∆k
E
1after
min
19.3
11.5
381.44
406.77 in
25.33
MoSfor
films
as
discussed
thein
results
of
the
Raman
analysis.
Figure
3(a)
shows
a
cross-sectional
TEM TEM
MoS
films
after
EBI,
asin
discussed
results
of
the
Raman
analysis.
Figure
3(a)
shows
a
cross-sectional
MoS2 films
asEBI,
discussed
the
results
ofthe
the
Raman
analysis.
Figure
3(a)
shows
a
cross-sectional
TEM
2after
an2EBI,
irradiation
time
of
1,
5,
and
10
min
[82].
(c)
Mo-3d
XPS
spectrum
(left
panel)
and
S-2p
Table 2. Area ratio of bonding states after the deconvolution of Mo 3d and S 2p spectrums and the calculatedXPS
5 min
17.6
11.6
381.25
406.69
25.44
valueobtained
of the S/Mo ratio from
the room-temperature
as-deposited and 1 min EBI-treated samples.
of a 10synthesised
MoS
film
on
a
SiO
/Si
wafer
by
sputtering
with
EBI
a as a
image
of(right
a synthesised
MoS
film
on
a
SiO
/Si
wafer
obtained
by
room-temperature
sputtering
with
imageimage
of aspectrum
synthesised
MoS
film
on
a
SiO
/Si
wafer
obtained
by
room-temperature
sputtering
with
EBI
as
a
min
19.1
12.1
381.422
406.64
25.22
2
2
2
2
2
panel) from as-deposited and 1 min electron-irradiated samples [82]. Reproduced with asEBI
post-process
for
1Five
min.
toFive
seven
atomic
layers
of
MoS
formed
parallel
to
theto
substrate
awith
thickness
of
post-process
for
min.
to seven
layers
of2, AMoS
formed
parallel
the
substrate
aofthickness
of
post-process
for Table
1 min.
toFive
seven
atomic
layers
of average
MoS
formed
parallel
to the
substrate
with
awith
thickness
1. FWHM
of E 1
and
A peaks
in
Raman
spectra
(Fig.atomic
1) and
values
of E
peak
positions,
2
2
the composition4.0
of MoSInternational
. If both S and Mo have thelicense.
same sticking coeffici t on the SiO /Si substrate, the film
permission
under
the(Fig.Creative
Commons
Attribution
and ∆k
in Raman
map
data
2)of
for 1,
5, andMoS
10 min EBI-treated
samples.
nm.
Even
though
the
the
crystal
domain
was
limited
tosticking
about
5this
nm,
iswas
clear
evidence
woulddomain
belimited
MoS
. However,
S hasabout
a limited
lower
coeffici
t than
Mo,
the film
slightly
t in
~4though
nm.
Even
though
the
size
the2 MoS
was
to about
5this
nm,
this
isdeficie
clear
evidence
~4 nm.~4Even
the size
ofsize
the
MoS
crystal
domain
was
to
5 nm,
isand
clear
evidence
ofsulphur
theof the of the
2 crystal
2of
under the normal sputtering conditions. For S to have a sticking coeffici t of 1, two S atoms would have to react
with a Mo atom
as they
arrive
onby
the
substrate.
Otherwise,
theyPlan-view
would
desorb,
and a sulphur-defi
ient MoS images
rearrangement
from from
amorphous
to a crystalline
structure
theprocess.
EBI
Plan-view
TEM
atomic
rearrangement
amorphous
to a structure
crystalline
by
theprocess.
EBI
process.
Plan-view
TEM images
atomicatomic
rearrangement
from amorphous
to a crystalline
bystructure
the
EBI
TEM
images
film would be formed. To maintain the stoichiometry of MoS , the S/Mo ratio arriving at the substrate should be
etsulphide
al.
[81]
prepared
aand
MoS
mono-layer
micromechanical
cleavage,
as
shown
inFigure
Figure
20a.3(b)
>2.by
Toalso
maximise
the heating
sticking
coeffici
t of3(b)–(d),
S Fig.
in our in
process,
werespectively.
kept3(b)–(d),
the sputter
power
asrespectively.
low
as possible,
at
about Figure
theLiu
and
and
10
min
samples
are
shown
in
3(b)–(d),
respectively.
3(b)
(H S)1gas
and 1
vaporised
ambient
. However,
these processes
require
toFig.
a high
ofas-deposited
the
as-deposited
and
1sulphur
102EBI-treated
min
EBI-treated
samples
are
shown
Fig.
of the of
as-deposited
and
10
min
EBI-treated
samples
are
shown
in
Figure
3(b)
the working
W, and maintained
temperature of up to 700–750 °C. Several researchers have reported a direct 20
sputtering
process with
a MoSdistance
tar- as high as possible, at up to 10–15 cm.
As
shown
in Figs 1 andas-deposited
3, the a
Raman
spectra and HR-TEM
results confi
med
that keV
the as-deposited
MoS film
MoS
mono-layer
was
then
exposed
in
vacuum
under
a
80
electron
beam
shows
that
the
amorphous
nature
was
observed
over
the
entire
sample.
However,
crystallites
with
shows
that
the
amorphous
nature
was
observed
over
the
entire
as-deposited
sample.
However,
crystallites
with
showsThe
thatwell-crystalline
the
amorphous
nature
was
observed
over
the
entire
as-deposited
sample.
However,
crystallites
with
2
get at a relatively low substrate temperature
of 300–350 °C, but as-depositedremained
MoS films
show poor
crystallinity
amorphous.
However,
we obtained a 2D layered structure for the MoS film after only electron irracompared to other methods
. Recent2studies on the post-processing of as-deposited
MoS
films
thermal
diation
with
an energy
of with
1 kV for
1 min,
as illustrated
in Fig. EBI-treated
6(a). Th s is very surprising
that such low-energy
a size
~5
nm
were
distributed
in
an
area
of
20
nm
×
20
nm
for
the
1
and
10
min
samples,
as
shown
aofsize
of
~5
nm
were
distributed
in
an
area
of
20
nm
×
20
nm
for
the
1
and
10
min
EBI-treated
samples,
as
shown
a sizewith
of
~5an
nm
were
distributed
in
an
area
of
20
nm
×
20
nm
for
the
1
and
10
min
EBI-treated
samples,
as
shown
intensity
of
40
A/cm
.
Initial
defects
and
holes
were
induced
in
the
MoS
sheet
once
exposed
annealing and laser treatment showed improved crystallinity, but the problems
of with
high1temperature
and
highrearrangement for MoS crystallisation.2No additional thermal heating
electrons
kV encouraged
atomic
cost
remain.
process was applied to the substrate, and the temperature resulting from energetic electron bombardment of the
in3(c)
Fig.in
3(c)
and
(d).
Thsuggest
s indicates
a startling
transformation
of
theunder
MoS
film 2amorphous
from
to a crystalline
Fig.
3(c)
and
Th
indicates
a startling
transformation
of
the
MoS
filmofamorphous
from
amorphous
to
crystalline
in Fig.
and
(d).
Th
s indicates
as startling
transformation
the
MoS
film
to
a ofcrystalline
2from
the experimental
as a small
substrateof
didexposure
not
exceed
100 °C2irradicondition
EBI withirradiation,
an
energy
1 kV for 1 min,
In
this
paper,
we(d).
a simple
method ofAfter
sputtering
and81
post-processing
with
electron
beam
to
the
electron
beam
(Figure
20b).
a
second
under
the
electron
the
shown
Fig.amorphous
6(b). For thestructure
experimental
substrate temperature was measured by direct contact with
ation
(EBI)
to obtain
crystallinewithin
MoS films at
amin.
low temperature
below
100
°C. in
The
of results, thehowever,
structure
the
EBI
1In
the
10EBI-treated
min
EBI-treated
sample,
theofarea
the of
amorphous
by
theprocess
EBI
process
within
min.
In
the
10
min
sample,
theof
area
the amorphous
structure
bystructure
theby
EBI
process
within
1 min.
the1In
10
min
sample,
however,
thehowever,
area
the
amorphous
a thermocouple.
The EBI-treated
substrate
temperature
as-deposited MoS films can be transformed into a crystalline nature after only
electron beam
irradiation
(EBI) exceeded 300 °C only after the EBI process for 10 min. These results
holes
spread
rapidly
(Figure
20c)
and
extended
into
big
holes
withsample.
diameters
of
3–6
nm
(Figure
20d)
an
energy
ofincreased
1 kV for
1increased
min. There
have been
several
reports
onto
exploring
the
evolution
of EBI-treated
materegion
waswith
slightly
compared
to
of
the
1EBI-treated
min
Th sbe
may
from
the
Mo–S
was
slightly
compared
ofstructural
the
1EBI-treated
Th
sbe
may
from
the
Mo–S
min
region
was region
slightly
increased
compared
to
that
ofthat
the
1that
sample.
Th sample.
s may
from
thebe
Mo–S
min
rials from amorphous to crystalline as they are being
transformed
at the
local| DOI:10.1038/s41598-017-04222-6
region through the application of a
6
Scientific
Reports
| 7: 3874
abonds
103
sbreaking
irradiation.
The
MoS
converted
to
Motime.
after
bonds
breaking
owing
to
the
excessive
energy
transfer
with
the
longer
irradiation
time.insets
The
insets
of3(b)–(d)
Fig.long-term
3(b)–(d)
high-energy
electron
beam
of several
hundred
kilovolts
tomono-layer
several
megavolts
in the
transmission
electron
microscopy
2transfer
5 S4 nanoribbons
owing
to
the
excessive
energy
transfer
with
the
longer
irradiation
time.
The
insets
of
Fig.
3(b)–(d)
bondsafter
breaking
owing
to
the
excessive
energy
with
longer
irradiation
The
of
Fig.
(TEM)
. Interestingly, our EBI process makes it possible to stimulate the atomic rearrangement of amorphous
show
theFourier
fast
Fourier
transform
(FFT)
patterns
from
the
areas
marked
as dashed
squares.
They reveal
that the
MoS
underFourier
the conditions
of a relatively
low electron
energy
of 1the
kV
andareas
short
timemarked
of 1the
min. We
think
show
thefilm
fast
transform
(FFT)
patterns
from
areas
marked
as dashed
squares.
They
reveal
that the
showelectron
the
fast
transform
(FFT)
patterns
from
as that
dashed
squares.
They
reveal
that the
irradiation.
this EBI technique can be easily applied to scale up the synthetic process of MoS films considering its compaticrystal
domain
of
the
MoS
film
transformed
from
an
amorphous
state
into
a
hexagonal
lattice
structure.
The
crystal
domain
the
MoS2such
film
transformed
from an amorphous
state
into alattice
hexagonal
latticeThe
structure.
The
crystal domain
ofbility
the
MoS
film
from
amorphous
state MoS
into a hexagonal
structure.
FFT FFT
2processes
with
conventional
PVDtransformed
as sputtering
and an
evaporation.
2 of
Komsa
et
al.
[78]
prepared
freestanding
single-layer
samples
by
mechanical
exfoliation
of FFT
2
patterns
of
theof
as-deposited
showed
typical
amorphous
characteristics
widearing.
halo
ring.
In
contrast,
patterns
theDiscussion
as-deposited
sample
showed
typical
amorphous
characteristics
wideIn
halo
ring.
In contrast,
patterns
of the
as-deposited
samplesample
showed
typical
amorphous
characteristics
with awith
wideawith
halo
contrast,
Results
and
naturaldiffraction
MoS2 1bulk
crystals,
observed
a MoS
sheet
under
an 80 keV
electron
beam
in aofvacuum
on
2 and
shows
the
Raman
spectra
ofand
MoS
synthesised
using
sputtering
the EBI process
withsample
differspots
in
thepatterns
FFT
patterns
of
the
1EBI-treated
indicated
that
the
c-axis
the of
crystal
min
sharpFigure
diffraction
spots
in
thefilms
FFT
patterns
of
the
1EBI-treated
min
EBI-treated
sample
indicated
that
the crystal
sharp sharp
diffraction
spots
in
the
FFT
of
the
1bymin
sample
indicated
that
the
c-axis
ofthe
thec-axis
crystal
ent irradiation times. In the as-deposited film grown at room temperature with a working pressure of 5 mTorr,
an
aberration-corrected
HRTEM.
It
was
experimentally
observed
that
the
top
and
bottom
sulfur
atoms
two
prominent
Raman
peaks
of
MoS
,
i.e.
the
in-plane
mode
(E
)
and
out-of-plane
mode
(A
),
did
not
appear
structure
(space
group
P6
/mmc)
was
perpendicular
to
the
substrate.
The
FFT
pattern
in
Fig.
3(d)
shows
that
the
structure
(space
group
P63perpendicular
/mmc)
was perpendicular
to theThe
substrate.
The FFT
pattern
in Fig. that
3(d)the
shows that the
structure (space
group
P63/mmc)
to the substrate.
FFT pattern
in Fig.
3(d) shows
3 was
because of its amorphous
nature. However,
after the EBI process with as-deposited sample, the peak intensities of
crystalline
ordering
hexagonal
symmetry
of
10EBI-treated
min
sample
was somewhat
inferior
toofthattoofthat of
the
E for
(~381
cm for
) and
A for
(~407
cm
) modes
increased
dramatically
without
an additional
thermal
heating
crystalline
ordering
hexagonal
of
the
10EBI-treated
min
EBI-treated
sample
was
somewhat
inferior
crystalline
ordering
hexagonal
symmetry
ofsymmetry
the
10the
sample
was
somewhat
inferior
to
that
min
were
removed
under
the
80
keV
electron
beam
and
vacancies
were
generated
in
the
MoS
mono-layers
2
process. As the EBI process time was increased from 1 min to 10 min, the peak intensities of the E and A bands
the 1EBI-treated
sample.
min
the
min EBI-treated
sample.
the 1 under
min
sample.
decreased
without peak
shifts. The
highest intensities of the E and A peaks for the 1 min EBI-treated sample
the1EBI-treated
electron
irradiation.
The sulfur vacancies agglomerated into line defects due to migration of
indicated that the amount of Mo–S bonding may decrease with a longer irradiation time. The difference between
4(a)
shows
the
atomic
force
microscopy
(AFM)
profiles
at
the at
edges
ofas-deposited
theof
as-deposited
4(a)
shows
the
atomic
force
microscopy
profiles
the as-deposited
FigureFigure
4(a) Figure
shows
the
atomic
force
microscopy
(AFM)
height
at
edges
ofthe
theedges
and and and
the Raman
shifts of
the
E and
A peaks
(∆k)
was ~25
cm , which
is typically
shown(AFM)
forheight
sixprofiles
atomicheight
layers
or the
the
defects
[123].
MoS . MoS films. The average thickness of the MoS film slightly decreased after the EBI process
min1EBI-treated
minbulkEBI-treated
films.
average
offilm
the2(FWHM)
MoS2 film
slightlyafter
decreased
after
the EBI process
1 min 1EBI-treated
MoS
The
the
MoS
slightly
decreased
the EBI
process
2 MoS
For
the 1,25, films.
and 10 min
EBI-treated
samples, The
asthickness
shown
in Table 1, of
thethickness
full
width
at half 2
maximum
2average
values
of4.0
E were
19.3,Figure
17.6, and 19.1
cm , respectively,
and
those forAFM
A were 11.5,
and 12.1
cm
from
toThe
nm.
4(b)
and
(c)
show
images
of(FWHM
the, respecas-deposited
min
samples.
from
4.5
nm
to
4.0
nm.
Figure
4(b)
and
(c) images
show
AFM
images
of
the as-deposited
and 1EBI-treated
min EBI-treated
samples.
from 4.5
nm4.5
to nm
4.0
nm.
Figure
and
show
AFM
of11.6the
and 1 and
min 1EBI-treated
samples.
tively.
FWHM
of the4(b)
EBI sample
was(c)
higher
than that
of the exfoliated
monolayer
MoS as-deposited
of E :
2.8 cmroughness
, the
FWHM
of A : 4.7
cm )at.measured
However,
thethe
crystallinity
of our
1of
minthe
EBI-treated
sample
atthe
room
temper- changed
However,
the
measured
at
centre
region
of
the
MoS
films
signifi
antly
with
the
EBI
time.
However,
roughness
at
the
centre
region
of
MoS
films
changed
signifi
antly
with
the
EBI
time.
However,
the
roughness
measured
the
centre
region
MoS
films
changed
signifi
antly
with
the
EBI
time.
2
2 : ~14 cm ) with a 2
ature was superior to that of sputtered MoS films (FWHM of E : ~50 cm , FWHM of A
The roughness
AFM
roughness
(Ra) values
of
the
SiO
and as-deposited
were almost
the at
same
0.524
The AFM
roughness
(Rthe
of
the
SiOand
and as-deposited
were
almost
the at
same
0.524 and
The AFM
(R
of
SiO
as-deposited
samplesample
weresample
almost
the same
0.524
andat and
2 substrate
a) values
2 substrate
a) values
2 substrate
2 samples
Scientific
Reports
| 7: 3874 |respectively.
DOI:10.1038/s41598-017-04222-6
0.523respectively.
nm,
RThe
of
1, the
5,10and
min10EBI-treated
were 0.696,
0.572
and
0.523
nm, respectively.
of
1, 5,10and
min EBI-treated
samples
were
0.5720.541
and nm,
0.541 nm,
0.523 nm,
The RaThe
values
of Rthe
1, the
5, and
min
EBI-treated
samples
were
0.696,
0.5720.696,
and 0.541
nm,
a values
a values
respectively.
Because
of
theof
atomic
rearrangement
from amorphous
to crystalline
MoS
,
the
roughness
inevitably
respectively.
Because
the atomic
rearrangement
from amorphous
toMoS
crystalline
MoS
,
the
roughness
inevitably
respectively.
Because
of the
atomic
rearrangement
from amorphous
to crystalline
,
the
roughness
inevitably
2
2
2
2
Mo 3d
−1
1
2g
−1
1
1g
1g
2g
1
2g
S 2p
Sample
Mo–O
Mo–S
Mo–Mo S–S
S–Mo
S/Mo ratio
as-deposited
11.54
44.53
43.93
57.23
42.68
2.24
1 min-EBI
8.63
91.37
—
—
100
2.00
1
2g
1g
1g
2
2
2
2−x
18
2
2
19
2
23
2
2
20–22
2
2
24
2
2
2
25–28
2
2
2
1
2g
2
1
−1
2g
1g
1
1
29
2
2g
1
2g
5
−1
2g
1g
1g
1g
2g
−1
1g
1
−1
1g
−1
−1
1g
1
2
−1 30
2
c
Reports
| 7:| 3874
DOI:10.1038/s41598-017-04222-6
ntific
| 7:|3874
| DOI:10.1038/s41598-017-04222-6
orts
| Reports
7: 3874
DOI:10.1038/s41598-017-04222-6
1
2g
−1
1g
2g
−1
4
4
4
Article
images denote regions where the channel current is enhanced by
ce
hift
the electric field generated by the tip. Most importantly, this
he
response is not observed at the edge of the (Cr/Au) electrodes
ed
but is ratherAppl.
located
in the
channel interior. The response
Sci. 2019,
9, 678
22 of 53
ACS Nano
Article
Article
ful
becomes increasingly
prominent when Vtip‑dc is increased,
J. Appl. Phys.
117, 135701 (2015)
miwith profile.
the electrostatic
n-type
nature
of theimages
MoSdenote
(notregions
shown
4−10
2 the
whereforce
the channelimages
currentdenote
is enhanced
regionsbywhere the channel current is enhanced by
EFM, in consistent
contrast, detects
the
EFM,
force
in contrast,
detects
electrostatic
reIn sample
Figurebetween
1c, theasthe
right
electrode
the
FET
serves
asathe
e AFM tiphere).
and the
surface
a phase
AFM
shift
tip and of
thethe
sample
surface
as
phase by
shift
electric field generated
the tip. the
Most
electric
importantly,
field generated
this
by the tip. Most importantly, this
ns
grounded
source
theonVboth
applied response
to
the information
left
ilever oscillations.
Spatial
information
of and
the cantilever
oscillations.
the
Spatial
on both
is notelectrode.
observed
at thethe
edge of response
the (Cr/Au)
is not
electrodes
observed at the edge of the (Cr/Au) electrodes
d is
nd chargeIndistribution
the
potential
surfaceand
is captured
charge
distribution
isincaptured
but but
isatrather
locatedthat
the channel but
interior.
is rather
The located
responsein the channel interior. The response
Figure 1d,at this
configuration
is reversed,
ittheis surface
clear
mplementary
inproviding
suchremains
complementary
a powerful
providing
a prominent
powerful when
becomes
increasingly
becomes
Vtip‑dc increasingly
is increased, prominent when Vtip‑dc is increased,
the measurements,
SGM response
pinned measurements,
near
the right
contact.
r understanding the electronic
properties
for understanding
of semithe
electronicwith
properties
of semiconsistent
the n-type
nature of consistent
the MoS2 with
n-type nature of the MoS2 (not shown
(not the
shown
(See section 1 ofmethod
the
Supporting
Information
for awe
discussion
ofmeasurenanostructures. In this work, we
conductor
apply these
nanostructures.
measure- In this
work,
apply
here).
In Figure
1c,these
the right
electrode of
here).
the FET
In Figure
serves1c,
as the
the right electrode of the FET serves as the
the
dependence
of
the
SGM
response
on
drain
bias.)
This
pinning
iques
potential
techniques
distributions
to probe the current
andsource
potential
in to probe the current andment
grounded
anddistributions
the Vd is appliedgrounded
source
and the Vd is applied to the left electrode.
to the left
electrode.
effect suggests the
formation
domain
structure
ETs.
in MoS
2 FETs. of some fixed
ed
In Figure
1d, this
configuration is reversed,
In Figure
but it1d,
is clear
this configuration
that
is reversed, but it is clear that
within the channel, in spite of the fact that
the the
SGMtransistor
response was
remains pinned near
the SGM
the right
response
contact.
remains pinned near the right contact.
ces
11
AND DISCUSSION
AND DISCUSSION
fabricated from aRESULTS
single-domain
MoS2 crystal.
(See section 1 of the Supporting Information
(See section
for a discussion
1 of the Supporting
of
Information for a discussion of
onthe dependence
of the
drain
dependence
bias.) Thisof
pinning
the SGM response on drain bias.) This pinning
In parts
andThe
b of
Figure
2, of
we
EFM
images obtained
menon
here isa demonstrated
phenomenon
in Figure
1,interest
inshow here
is demonstrated
inSGM
Figureresponse
1, in on the
ile of interest
suggests
theour
formation
of some effect
fixed suggests
domain structure
the formation of some fixed domain structure
plot the SGM
response of with
which
one the
ofwe
our
plot
back-gated
the
SGM response
of one
of
back-gated
simultaneously
SGM
mapping
foreffect
opposite
polarities
of
hin
within
thedetails
channel,
in spite
of the factwithin
that the
thetransistor
channel, was
in spite of the fact that the transistor was
(see
for bias.
furtherThese
transistors
detailsimages
on these
(seeallow
the
devices
Methods
for
further
on these
devices
the drain
us to construct
the
evolution
11
as, the Methods
fabricated
from
a single-domain
MoS2 crystal.
fabricated from a single-domain MoS211 crystal.
fabrication).
This
response
and
was
their
obtained
fabrication).
by
conThis
response
was
obtained
by
conof the potential variation along the channel length,
asa we
indicate
ed
In parts
and
b of Figure
2, we show EFM
In parts
images
a and
obtained
b of Figure 2, we show EFM images obtained
measuring
the transistor drain
tinuously
currentmeasuring
(Id) while
the transistor
drain
current
(Id) while
the plots
shown
Theofplots
reveal
eseAFM tipin over
simultaneously
with
the SGMthin
mapping for
simultaneously
opposite polarities
with the
of SGM mapping for opposite polarities of
he
the surface
rastering
of underneath
the atomically
the AFMthe
thin
tipEFM
over scans.
the
surface
the
atomically
two
consistent
features,
the
first
ofwere
which
a workfunctionthe is
drain
bias. These
images
construct
drain bias.
the evolution
These images allow us to construct the evolution
his
this way, we
were
able to construct
channel.
(for
Infixed
this way,
tip bias,
we
able
to
construct
(for fixed
tip allow
bias, us to the
of
potential
variation
along the channel
of the
length,
potential
as we variation
indicate along the channel length, as we indicate
induced
potential
difference
∼0.4 eVofbetween
thechange
MoS
of the
current
change
) the(Ispatial
) caused
variation
thethe
current
Vtip‑dc
espatial
1 variation
SGMof
SGM) caused
2(Iand
inelectrical
the
plotssetup
shown
underneath
the EFMinscans.
the plots
The shown
plots reveal
underneath the EFM scans. The plots reveal
ence
tip. gold
The electrical
by
setup
the used
presence
during
ofelectrode.
these
the tip. The
used
during
at the surface
of each
The
second
feature
is a these
or of thethe
two
consistent
features,
two consistent
is a workfunctionfeatures, the first of which is a workfunctionnts is shown
in Figure
1b,inand
measurements
further details
is on
shown
this
in Figure
1b,
and occurring
further
details
on first
this of which
sudden
drop
potential
within
the
channel
interior,
at the
ese
inducedParts
potential
of ∼0.4
betweenpotential
the MoSdifference
of ∼0.4 eV between the MoS2 and
be found in the Methods. Parts
setup
c and
can dbeoffound
Figurein1the Methods.
c anddifference
d of Figure
1 eVinduced
2 and
the gold
thechannel
surface of
each electrode.
the
Thegold
second
at thefeature
surface
is of
a each electrode. The second feature is a
to SGM scans taken over correspond
the channeltoregion
SGMfor
scans taken
overatthe
region
for
sudden
drop
in potential
the channel
sudden
interior,
dropoccurring
in potential
at within the channel interior, occurring at
). Brightdirections
areas in these
areaswithin
in these
rections of the drain bias (Vddifferent
of the drain
bias (V
d). Bright
Figure 18. Micro-Raman
spectra of 10-layered
MoS2 sheets
(black
curves)
and
after (red curves)
FIG. 7. Micro-Raman
spectra measured
within before
the 25 lm
channel
(see
text).
irradiation [77].
Raman-shift versus frequency is plotted, both before (black line) and after
(red line) irradiation.
(a)
(b)
(c)
energy irradiation. Therefore, we infer that optimal energetic
electron irradiation can be utilized for increasing the carrier
density of MoS2 devices.
We have also investigated the stability of enhanced conductivity with time, which was found to reduce gradually
and stabilize above the pre-irradiation value after a few days.
It is well-known that the lifetime of MoS2 devices is limited.
ance. Above 10 KeV, the
If the devices are kept at ambient atmosphere, they get deacHowever, as discussed eartivated
veryof
soon.
It electron-beam
is observed
that
such
deactivated
devi- was performed
the effect
irradiation
Figure
19.
Optical
image
a2mono-layer
MoS
crystal.
The
electron-beam
exposure
performed
) Optical
image of a of
monolayer
Figure
MoS
3. (a) is
Optical
taken(a)
just
image
after
ofEBL
a monolayer
and development.
MoS
crystal,
Thetaken
just
after2EBL
exposure
and development.
was
performed
The
onelectron-beam
exposurewas
on
2 crystal,
side
of
the
crystal
so
that
the
PMMA
only
the
resist
left
side
remains
of
the
on
crystal
the
right
so
that
side.
the
(b)
PMMA
EFM
image
resist
(V
remains
=
4
on
V,
the
lift
right
height
side.
=
60
(b)
nm)
EFM
of
image
the
MoS
(V
=
4
V,
lift
height
= 60 nm) of the MoS2
tip‑dc
2
tip‑dc
ces
can
also
be
activated
after
irradiation.
rom
30
KeV
plot.
In
princion
only
the
left
side
of
the
crystal
while
the
right
side
is
covered
to
avoid
electron
exposure.
(b)
Raman
n in panel (a), obtained after removal
crystal shown
of all remaining
in panel (a),
resist.
obtained
Inset:after
magnified
removal
view
of of
allthe
remaining
area identified
resist. Inset:
by themagnified
green square
viewinofthe
the area identified by the green square in the
1 Raman
st(G)
after
EBL
andline
development.
The
exposure
wasand
performed
The
blue
dashed
corresponds
main
toimage.
theelectron-beam
boundary
The of
bluethe
between
dashed
line
the2 exposed
corresponds
unexposed
to the boundary
regions.
between
(c) Raman
the exposed
spectra
and
of the
unexposed
exposed
the exposed
for
low-energy
electron
mapping
MoS
crystal
shown
inon(a),
indicating
the Raman
shift regions.
of the (c)
E2g
peakspectra
as a of
function
of
1
crystal,
indicating
the
Raman
shift
of
the
E
crystal,
peak
as
indicating
a
the
Raman
shift of the E12g peak as a
(blue)(b)
areas
of the
(red)
crystal.
and
the
Raman
unexposed
scattering
(blue)
map
areas
of
the
of
the
MoS
crystal.
(d)
Raman
scattering
map
of
the
MoS
ne unexposed
the right side.
EFM
image
(V(d)
=
4
V,
lift
height
=
60
nm)
of
the
MoS
2
2
2g
tip‑dc
2
position.
(c)
PL
mapping
of
the
MoS
crystal
at
a
wavelength
of
670
nm.
Reprinted
with
permission
rease
density
and/
(e) carrier
PLmagnified
spectra obtained
function
at the
different
of area
position.
positions
(e) PL
within
spectra
thegreen
obtained
unexposed
at (labeled
different
positions
as (i))within
and exposed
the unexposed
(ii−iv) (labeled
regions here as (i)) and exposed (ii−iv) regions
gposition.
resist. in
Inset:
view
of
identified
by
the
square
in2thehere
4. PL
Effects
on
vibrational
properties
after
irradiation
crystal
at
a wavelength
ofmap
670
at a American
wavelength ofChemical
670 nm.
panel (f).
PL map and
of theunexposed
MoS
identified
in
panel
(f).
ofnm.
the MoS
2from
2 crystal
cexposed
Reference
[80].
Copyright
2016
Society.
etween
the(f)exposed
regions.
(c)(f)Raman
spectra
of the
the
present
situation,
since
1
shift of the E 2g peak as a
ttering map of the MoS2 crystal, indicating the Raman
9732To
9732of irradiation-induced
DOI: 10.1021/acsnano.6b05952
DOI: 10.1021/acsnano.6b05952
investigate the impact
vacancy
we may
donsnot
across
SiO2, (labeled
ACS Nano 2016, 10, 9730−9737
ACS Nano 2016, 10, 9730−9737
within
the unexposed
here as (i)) and exposed (ii−iv) regions
2
creation on vibrational properties, we have also carried out
ength
670 nm.
bulkofMoS
2, viz., 200 cm /V
micro-Raman
(MR) measurements before and after irradiaer 9732
density (n) at a particular
DOI: 10.1021/acsnano.6b05952
ACS Nano 2016, 10, 9730−9737
tion. Due
to restricted electron beam diameter, the damage on
ng the relation n ¼ Gl/leA,
flake is localized and changes in the Raman spectra can,
nd A is the vertical crosstherefore, be seen only around the irradiated area. The
passage. Table IV presents
Raman-shifts before and after irradiation are plotted in Fig. 7.
ults on flakes, where the
In addition to the in-plane E12g and out-of-plane A1g modes,
, differential conductance
extra modes have appeared for 5, 7, and 10 KeV. From 15
ensity is evident for low-
ented across the 25 lm channels on
V) are plotted for flakes of similar
ed) irradiation, with electron beam
he figure. Current value above 2 V
ensitivity range of the instrument.
n irradiation on transport properties of few-layered flakes of MoS2. Suffixes 1 and 2 stand for before and after irradiaenergy of electron irradiation, conductance, and differential conductance both before and after irradiation and carrier
S | DOI:
NS
| DOI:10.1038/ncomms2803
10.1038/ncomms2803
ARTICLE
Appl. Sci. 2019, 9, 678
a
b
bb
(a)
Monolayer
Monolayer
Monolayer
g
h
k
Bilayer
ARTICLE
ARTICLE
23 of 53
(b)
Bilayer
Bilayer
Bilayer
2 nm
(c)
2 nm
2 nm
(d)
g
0s
h
23
2 nm
Figure
HRTEM images of a MoS2 mono-layer
under electron irradiation [81]. (a)35
MoS
23
35sss2 mono-layer
00s20.
23
hh
23
ssirradiation.
35sss iji (FFT) image. (b) The initial59
i
k with
without
Inset: fast Fourier transformation
MoS2 sheet
small irradiation-induced vacancies as highlighted by the arrows. (c) Large holes extended from the
small vacancies upon electron irradiation of 108 s. (d) Larger holes extended from the small vacancies
upon electron irradiation of 261 s. Reprinted with permission under a Creative Commons license.
89
Parkin et al. [79] prepared MoS2 single-crystalline mono-layers by CVD method and irradiated
them with 200 keV electrons in an aberration-corrected TEM. Most generated defects were single
sulfur vacancies. The irradiation-induced defects were quantitatively related to the electron dose with
the actual defect concentration. Figure 21 shows the defect-induced stoichiometry as a function of
electron dose. The elemental composition within the illuminated region was measured by electron
X-ray dispersive spectroscopy (EDS). The EDS measurements indicated that the S/Mo atomic ratio
was very
close
the stoichiometric 2:1 ratio
before
irradiation. The M/Mo
59
89
s lelectron
113ss ratio decreased
59
sss to
89
113
kk
113sremoval
s l of
1 |ofInthe
situ
fabrication
suspended
sub-nanometre
ribbons.
l doseFigure
with the 89
irradiation
because
sulfur
under
electronmolybdenum-sulfide
irradiation. It should be
noted
locates at the
region
labelled
by arrow.
Scale
100 mm.supporting
(b) TEM images of the thin
that the EDS intensity ofregion
the molybdenum
signal
remains
constant
during
thebar,
irradiation,
the fact that the main effect
of irradiation
to create
vacancies
while
molybdenum
atoms
by fringe
countingwas
at the
edge. sulfur
The straight
edge
verifies
that the membrane
is single laye
were much more difficult
be sputtered.
bytocontrast.
Scale bar, 100 nm. (c) High-resolution TEM data obtained on the unfolded regio
Electron-irradiation-induced
transformation
of MoS
was also reported at high
2 layers
is shown inphase
the inset.
(d) The initial
MoS
2 membrane with small irradiation-induced vacan
◦ C in a vacuum [76]. Electron irradiation initiated the phase transition
temperatures, such as 400–700
from the small vacancies upon 80 kV electron irradiation of another 108 and 261 s. (g–l) T
of 2H-MoS2 at high temperatures,
and the
transformation
increased
with increasing
electron
sub-nanometre
ribbon
under 80 kVratio
electron
irradiation.
In (c–l) scale
bar, 2 nm.
doses above 40 × 106 –100 × 106 electrons /nm2 , depending on the temperature. It was also found that
MoS2 layers were damaged when the total electron dose exceeded 5.0 × 108 –1.1 × 109 electrons/nm2
with the accelerating voltage of 60 kV.
breaking (Supplementary Movie 1), indicating an excellent periodic flu
mechanical ribbons.
strength.
Our
first-principles
simulations
alsoThe
predict
nofofsuspended
suspendedmolybdenum-sulfide
molybdenum-sulfidesub-nanometre
sub-nanometre
ribbons.(a)
(a)
few-layer
MoS22flake
flake
onTEM
TEMgrid.
grid.
The
thinnest vertical dev
AAfew-layer
MoS
on
thinnest
fide
sub-nanometre
ribbons. (a) A few-layer
MoS
flake onhas
TEM
grid.
The thinnest
2
S rectangle
that
the
ribbon
a
respectable
tensile
strength
up
to
30
GPa
labelledbybyarrow.
arrow.Scale
Scalebar,
bar,100
100mm.
mm.(b)
(b)TEM
TEMimages
imagesofofthe
thethinnest
thinnestregion.
region.The
Thethickness
thicknessofofthe
themembrane
membranecan
canbe
bedetermined
determined
m.belled
(b) TEM images
of the thinnest
region.
The
thickness
of
the
membrane
can
be
determined
Peierls dist
and canisisbe
stretched
for
9% and
before
breaking
without
plastic
dge.
Thestraight
straight edgeverifies
verifies thatthe
themembrane
membrane
single
layer.The
The
folded
and
unfolded
regioncan
can be
be further
further
determined
e.theThe
single
layer.
folded
unfolded
region
determined
membrane edge
is single layer.that
The folded
and unfolded
region
can
be
further
determined
deformation
(Fig.
4a).
Its
Young’s
modulus
is
calculated
by
fitting
m.(c)
(c)High-resolution
High-resolutionTEM
TEMdata
dataobtained
obtainedon
onthe
theunfolded
unfoldedregions.
regions.The
Thecorresponding
correspondingfast
fastFourier
Fouriertransformation
transformation(FFT)
(FFT)image
image
m.
btained
on the unfolded regions.
The corresponding
fast Fourier
transformation
(FFT) curve
image
the
linear
elastic
region
of
stress-strain
to
be
about
300
GPa,
membranewith
withsmall
smallirradiation-induced
irradiation-inducedvacancies
vacanciesas
ashighlighted
highlightedby
bythe
thearrows.
arrows.(e,f)
(e,f)Larger
Larger holes
holes extended
extended
initialMoS
MoS
eheinitial
2 2membrane
16.extended
mall
irradiation-induced
vacancies
as highlighted
bythat
the arrows.
(e,f)
Larger
holes
Discussion
close
to
of
pure
MoS
sheets
Further
calculations
on
the
2
pon80
80 kVelectron
electronirradiation
irradiationofofanother
another108
108and
and261
261s.s.(g–l)
(g–l)Time
Timeseries
seriesofofthe
theformation
formationand
andgrowth
growth for
for aa suspended
suspended
on
notherkV
108 and 261
s. (g–l) Time series Mo
of the
formation
and growth
for
a suspended
As the tra
S
ribbon
with
hybrid
functional
predict
an
indirect
band
gap
5 42 nm.
r 80kV
kVelectron
electronirradiation.
irradiation.InIn(c–l)
(c–l)scale
scalebar,
bar,
2 nm.
l)80scale
bar, 2 nm.
with a desirable value of 0.77 eV (Fig. 4b). This gap value not only similar stru
promises the optical adsorption into the infrared region but also may be fab
Article
Appl. Sci. 2019, 9, 678
24 of 53
Figure 21. EDS measured stoichiometry, showing an initial value of S/Mo = 2.01 ± 0.07, as expected
for CVD-grown MoS2 where a small amount of sulfur vacancy (∼0.5%) can be present after growth [79].
Reprinted with permission from Reference [79]. Copyright c 2016 American Chemical Society.
2.7. Plasma Irradiation
Plasma has been employed to remove impurities and contaminants from surfaces by the collision
energy of gas molecules and the chemical action on impurities and contaminants. Various gases (such
as argon, oxygen, hydrogen, and nitrogen, as well as their mixtures) have been employed to carry
out the cleaning procedures. The plasma is generated by using high-frequency voltages (typically
kHz-MHz) to ionize the working gases. The activated species in plasma, including atoms, molecules,
ions, electrons, free radicals, metastables, react with contaminants on surfaces. Short-wave ultraviolet
(vacuum UV), whose energy is very effective to break most organic bonds of surface contaminants, is
produced in plasma too. The technique has been utilized to modify surfaces of MoS2 few-layers in
recent years.
2.7.1. Active Nitrogen N2∗
Michra et al. [98] investigated oxygen-plasma-irradiated MoS2 mono-layers deposited on
◦ C. Figure
S spectrum ofsapphires
monolayer
a holey
carbon
grid. (b)
Measured
at 450MoS
shows Raman
scattering
of irradiated
MoS2 EDS
mono-layers. Raman
2 on 22
−
1
1
−1 towards a lower
A1g mode
shifted
1.79 the
cm Mo
towards
a higherremains
wavenumber
and E2g (c)
1.11 cm
e. The S intensity
drops
while
intensity
constant.
EDSmin of N2 plasma
irradiation.
The peak intensity ratio I A /I and FWHM of
01 ± 0.07, aswavenumber
expected after
for 3CVD-grown
MoS
2 where a small amount of ES
1 peak increased with plasma irradiation time. XPS investigations indicated that N-Mo bonds
the E2g
pattern of pristine,
monolayer MoS2 on a 100 nm thick Si3N4 substrate. (e)
formed during
the irradiation
energies
(calculated
from
Mo-3d5/2 , Mo-3p3/2 , and
gives an S sputtering
cross-section
ofand
75 the
± binding
10 barn.
(e) Sum
of the
intensity
∗
3/2
S-2p
peaks) shifted towards lower binding energy after N2 irradiation. The valence band maximum
cross-section(VBM)
of 120
± 20 barn. Note that we fit the SAED data only at low
reduced to 0.9 eV after 1 min of irradiation and 0.5 eV after 3 min of irradiation from 1.0 eV of
ds
07
np
or
vs
n
2,
n
1g
1
2g
pristine MoS2 mono-layer.
Azcatl et al. [124] doped MoS2 with nitrogen through a remote N2 plasma surface treatment.
Nitrogen covalently bonded to MoS2 upon nitrogen plasma exposures and substituted chalcogen
sulfur of MoS2 . The nitrogen doping converted MoS2 to p-type and changed electrical properties. The
nitrogen concentration in the doped MoS2 was controlled through adjusting N2 plasma exposure time.
XPS measurements indicated that binging energies of Mo-3d5/2 and S-2p3/2 decreased with increasing
exposure time.
2
Mono-layer MoS2 heterojunctions,
such as intrinsic GaN/p-type MoS2 heterojunction, were also
irradiated by N2 plasma under UHV conditions [98]. The values of VBM were reduced to 0.5 eV for
−1 MoS2 layers.
−1
N2∗ irradiated
We have carried out first-principles DFT calculations to
establish the microscopic origin of the experimentally observed
Raman peak shifts as a function of the electron irradiation dose.
Monolayer, pristine MoS belongs to space group P6m2 (No.
187), and its two characteristic Raman active modes are E′
(∼384 cm ) and A′1 (∼403 cm ), as discussed previously. 17−19 Creating monosulfur vacancies reduces the
symmetry of the system, and the symmetry assignments of
Raman modes change as well. For simplicity and consistency,
modes in the opposite direction has been predicted to be
caused by p-type doping in ML-MoS2.26 As pristine MLMoS2 prone to have S vacancies, hence the incorporation of
N atoms is favorable during N2 -irradiation.26,27 Therefore,
enhancement of compressive strain with increasing25 ofN53
Appl. Sci. 2019, 9, 678
vacancies a
lenging esp
Using our
reduction i
carriers. Fu
Figure 22. Raman spectraof pristine and N2∗ -irradiated MoS2 mono-layers [98]. Reprinted from
Reference [98], with the permission of AIP Publishing.
2.7.2. Active Oxygen O2∗
ARTICLE
Oxygen plasma species include ionized oxygen atoms O+ , excited oxygen atoms O∗ , ionized
oxygen molecules O2+ , metastable excited oxygen molecules O2∗ , ozone O3 , ionized ozone O3+ , excited
ozone O3∗ , and free electrons. Therefore, oxygen plasma can effectively clean and etch MoS2 2D
materials, introducing defects and doping oxygen to MoS2 2D layers.
Nan et al. [125] treated MoS2 layers under oxygen plasma irradiation with 13.56 MHz and 5 W
under 5 Pa. As shown in Figure 23 and the inset, the PL intensity increased with increasing plasma
irradiation time. The PL enhancement could be increased as high as 100 times over. Considered the
unchanged Raman scattering and XPS information, it was concluded that oxygen plasma introduced
defects and oxygen bonding in MoS2 , enhancing PL intensity.
PL Intensity
10K
15K
10K
5K
0
ation and charge density differysisorbed on perfect monolayer
Figure 23. PL spectra of mono-layer MoS2 after oxygen plasma irradiation with different
Figure 5. PL spectra of monolayer MoS2 after oxygen
d on defective monolayer
MoS2[125]. The
durations
change
of PL intensities
with the plasma
irradiation
time is of
shown in the inset.
plasma
irradiation
with different
durations.
The change
vacancy (b,d). The positive
and
c 2014isAmerican
Reprinted with permission
from Reference
[125].irradiation
Copyright times
Society.
PL intensities
with plasma
shown inChemical
the
wn in red and blue, respectively.
4
2
3
inset.
(c) et
and
0 and 1 10 e/Å for
Chen
al. [126] treated MoS multi-layers under oxygen plasma in a reactive ion etcher. Initial
2
n-type MoS2 layers were selected-area doped to p-type, forming p-n junctions in MoS2 . The fabricated
monosulfur vacancies are the
ructures,21 and therefore our
a monosulfur vacancy on the
ures of the O2 molecule adsupercell of monolayer MoS2
be a promising method for manipulating the optical
properties of MoS2. Here we adopt mild oxygen plasma
(13.56 MHz, 5 W, 5 Pa) irradiation to controllably
introduce defects in MoS2. It has been reported that
plasma irradiation can easily introduce S vacancies
in MoS2.23,24 At the same time, the oxygen ions are
Nano Letters
Appl. Sci. 2019, 9, 678
Nano Letters
26 of 53
highly rectifying diodes exhibited high forward/reverse current ratios and a superior long-term
stability at ambient conditions.
Kang et al. [127] treated MoS2 mono-layers with oxygen plasma. The photoluminescence evolved
Nano Letters
Nano Letters
from a higher intense to completely quenched with increasing plasma exposure time because of a
direct-to-indirect band-gap transition. The MoS2 lattice was distorted after oxygen bombardment and
MoO3 disordered regions were generated in the MoS2 flakes.
Islam et al. [128] deposited MoS2 mono-layers (mechanically exfoliated from crystals) on
Si/SiO2 wafers and exposed them under oxygen plasma for several seconds. The plasma
treatments were carried out at a power of 100 W operating at 50 kHz, using a gas mixture of
oxygen (20%) and argon (80%) with a pressure of 250–350 mTorr. Raman spectroscopy and XPS
spectroscopy indicated that MoS2 mono-layers were oxidized and MoO3 was produced in the
reaction, 2MoS2 + 7O2 → 2MoO3 + 4SO2 . The resulting MoO3 -rich domains significantly decreased
the mobility and conductivity of the fabricated MoS2 mono-layer devices.
Ye et al. [129] exposed CVD-grown MoS2 mono-layers under oxygen plasma at a pressure of
10 Torr with oxygen gas. Figure 24 shows the morphologies of MoS2 exposed to oxygen plasma for
10 s, 20 s, and 30 s. After 10 s of oxygen plasma treatments, short and isolated cracks were observed on
the continuous basal plane. Angles of those connected cracks were around 120◦ . After 20 s of oxygen
plasma treatments, cracks became longer and were connected to each other, forming a continuous
Figure
1. treated
Morphology
monolayer
Figureof1.
MoS
Morphology
synthesized
monolayer
by CVD
method
MoSmost
on angles
Si/SiObetween
by CVD method
and transferre
on Si
2cracks
2 synthesized
2 substrate
network.
When
for 30 of
s, the
widths
the
wereoffurther
enlarged,
◦
monolayers.
(B)
Optical
microscopy
monolayers.
image
(B)
of
Optical
monolayer
microscopy
triangles.
i
image
of
CVD
grown
triangular
image
MoS
of
CVD
grown
triangular
MoS
2 2 was decomposed into even2 smaller fragments with
the interconnected cracks were 120 , and the MoS
a thickness
ofintensity
∼0.7
nm
with
thicknessasofme
∼
cannumber
be observed.
(C) edges.
AFM image
canstructural
beofobserved.
CVDchange
grown
(C)significantly
MoS
AFM2 with
image
of CVDthe
grown
MoS
2 (inset
a greater
of exposed
The
decreased
of
A1g a profile),
1
shows
its
perfect
hexagonal
shows
lattice
its
with
perfect
2H
structure.
hexagonal
(E)
lattice
Pho
w
HAADF
image
of
monolayer
HAADF
MoS
image
of
monolayer
MoS
2
2 shorter wave-numbers
and E2g Raman modes of the MoS2 mono-layers,
and shifted the A1g mode to
were test.
transferred
to thetheremovable
were transferred
glassy
carbon
to the
electrode
removable
for fur
g
electrochemical
test.wave-numbers.
MoS2 monolayers
electrochemical
MoS
2 monolayers
and the
E12g mode to longer
The exposure
also
decreased
PL intensity.
The changes
thePL
glassy
carbon
electrode,
showing
the glassy
thecarbon
high
electrode,
showing
well
maintained
the high coverage
morphology
and well
aftermtr
MoSthat
2 on
2 onoxygen
of the MoS
Raman
and
spectra
indicated
plasma
can coverage
lower
theand
MoS
2 crystal symmetry
−1
−1
and
(H)
PL
intensity
map
at
and
680
(H)
nm
PL
of
intensity
the
MoS
map
triangle
at
680
in
inset
nm
of
of
the
(G)
MoS
show
triangle
high
quality
in
inset
of ot
cm
cm
and increase the lattice distortion, which can be attributed to the 2defects that may benefit MoS22 as
electrode.catalyst.
electrode.
electrochemical
Figure 1. Morphology of monolayer
Figure 1.
MoS
Morphology
of monolayer
by CVD method
MoS2 synthesized
on Si/SiO2 substrate
by CVD method
and transferre
on Si
2 synthesized
monolayers.
(B) Optical
microscopy
monolayers.
image
(B)
of
Optical
monolayer
microscopy
triangles.
i
image of CVD grown triangular
image
MoS
of2 CVD
grown triangular
MoS
2
of ∼0.7
nm2 (inset
with a profile),
thicknessasofme
∼
can be observed. (C) AFM image
can beofobserved.
CVD grown
(C)MoS
AFM2 with
imagea thickness
of CVD grown
MoS
shows of
its monolayer
perfect hexagonal
lattice itswith
perfect
2H structure.
hexagonal (E)
lattice
Phow
HAADF image of monolayer
HAADF
MoS2 image
MoS2 shows
were test.
transferred
to the removable
were transferred
glassy carbon
to the
electrode
removable
for fur
g
electrochemical test. MoS2 monolayers
electrochemical
MoS2 monolayers
the glassythecarbon
high coverage
electrode,and
showing
well maintained
the high coverage
morphology
and well
aftermtr
MoS2 on the glassy carbon electrode,
MoS2 on showing
680(H)
nmPL
of intensity
the MoS2map
triangle
at 680
in inset
nm ofofthe
(G)MoS
show
triangle
high
quality
in
inset
of ot
cm−1 and (H) PL intensity map
cm−1atand
2
electrode.
electrode.
Figure 24. SEM images of CVD-grown MoS2 mono-layers with oxygen plasma treatments for 0 s,
10 s, 20 s, and 30 s oxygen plasma exposure [129]. Both the density and width of the cracks increased
Figure 2. Morphology and structure
Figure 2.
characterization
Morphology and
of CVD
structure
grown
characterization
MoS2 with
oxygen
of CVD
plasma
grown
treatment.
MoS2 witS
with exposure time. Reprinted with permission from Reference [129]. Copyright c 2016
American
with
0,
10,
20,
and
30
s
oxygen
with
plasma
0,
10,
exposure,
20,
and
30
where
s
oxygen
the
cracks
plasma
appear
exposure,
in
the
where
basalthp
monolayer
MoS
monolayer
MoS
2
Chemical Society. 2
cracks increase with longer exposure
cracks increase
time. STEM
with longer
imagesexposure
(E,F) show
time.
theSTEM
fresh edges
imagesinside
(E,F)the
show
monolayer
the fres
these exposed
edgesedges
are found
in MoS
at2these
, with exposed
120° angle.
edges
(G)
in Raman
MoS2, with
and (H)
120
and S2 terminated edges are found
and S2atterminated
and shifted peaks after oxygen
andplasma
shiftedexposure.
peaks after oxygen plasma exposure.
monolayer.
shows higher improvement
shows
thanhigher
that treated
improvement
by O2 than
plasma
that treated
by O2 Aberration-co
plasma
tronedges.
microscopy (STEM)
because of the higher density
because
of the
of exposed
the higher
edges.
density of the exposed
characteristic exciton peaks of MoS2 monolayer whose energy separaFor the confocal PL and Raman spectroscopy measurements, a labtion is due to spin-orbit splitting at the top of the valence band at the
made laser confocal microscope with a spectrometer was used. With a
K point of the 1st Brillouin zone [19]. The integrated PL intensity is uni0.9 NA objective, the focused spot diameter of the laser light was
form across the entire area of the pristine MoS2 monolayer. Fig. 1(e) and
approximately 300 nm. Scattered light was collected using the same
(f) show the integrated absorption intensity mapping image and averobjective and guided to a 50 cm long monochromator equipped with
aged absorption spectrum of the pristine MoS2 monolayer, respectively.
a cooled CCD through an optical fiber with a 200 μm core diameter,
The two peaks in the absorption spectrum, at 619 and 669 nm, correwhich acted as a confocal detection pinhole. The excitation lasers
Appl.
Sci.
2019,
9,
678
27 of 53
spond to B and A excitons, respectively, and are similar to the peak
were the 514 nm laser line of an Ar gas laser, with a typical laser
positions observed in the PL spectrum (Fig. 1(d)). The large peak at
power applied to the sample of 100 μW and an acquisition time of
~460 nm may be understood as a broadening of the high-energy exciton
500 ms per pixel. Confocal absorption spectral imaging was conducted
peaks due to phonon–electron coupling [25].
using the same system with a tungsten-halogen lamp as the light
Dhall
et
al.
[130]
treated
MoS
few-layers
(15
layers
with approximately 12 nm thickness) in
2
Fig. 2(a) shows optical microscope images of the pristine MoS2
source. The experimental details of the confocal absorption spectral immonolayer
of MoS2 monolayers
with oxygen
plasma for
aging are explained
in the
information
previous
oxygen
plasma
forsupplementary
3 min. The
plasmaof awas
generated
by and
flowing
air past treated
an electrode
supplied
times ranging from 10 to 120 s. All of the MoS2 monolayers showed simpaper [22].
with 20 W of RF power at 200 mTorr. The PL efficiency
of the few-layers was increased after the
ilar morphologies. Fig. 2(b) shows integrated PL intensity mapping images obtained from each sample. The integrated PL intensity was found
3. Results and
discussion
plasma
treatments.
to be decreased with oxygen plasma treatment. Fig. 2(c) shows the peak
Kim et al. [131] grew triangular MoS mono-layers
on SiO /Si substrates by CVD technique. The
position of the 2PL corresponding to A excitons, which indicate that the
Fig. 1(a) shows an optical microscope image of a pristine2triangular
peak position
of the PL corresponding
A excitons was
gradually
on a SiO2/Siwere
substrate.
Highly
crystalline MoS
monoMoS2 monolayer
MoS
then
subjected
to 2plasma-oxygen
treatment
for timesto ranging
from
10 sredto
2 mono-layers
layers typically grow with a triangular shape due to the hexagonal latshifted with increasing oxygen plasma treatment duration.
120
s.
Ultra-high
pure
oxygen
gas
(99.9999%)
was
activated
by
a
RF
plasma
cell,
and
the
working
Fig. 3(a) shows the PL spectra of the MoS2 monolayers subjected to
tice structure of such crystals [23]. Confocal Raman spectroscopy was
3 Pa
[11,12,19,24]
(Fig. 1(b)). pure
oxygen
plasmagas
treatment
durations.
The PL
spectra were
used to identify
the1.3
number
of−
MoS
2 layers
pressure
was
× 10
under
an 1ultra-high
oxygen
flowforofdifferent
2 sccm.
Optical
properties
averaged from the inside region of each sample. The intensities of the
Consistent peak positions and a shift in the in-plane (E 2g) and out-ofof
the
such
as in
PL
and
scattering,
changed to
with
time.decreased
Figure
25a
) Raman modes of the
vibrations
each
MoS2Raman
were observed
plane
(A1gmono-layers,
peaks corresponding
A andtreatment
B excitons gradually
with
increasingand
oxygen
plasmaMoS
treatment
duration, possiblytreated
due to increased
A1g peaks
in
[12,24]. The
wavenumber
difference
the E12g and
shows
optical
images
of between
the pristine
MoS
mono-layer
these
mono-layers
with
2
2
the Raman spectrum has been widely used to identify the number of
lattice distortion and defect density as a result of oxygen plasma bomoxygen
plasma
10 s,the30value
s, 60
s and
120 s.
Figure
shows
integrated
mapping
images
bardment
[17].
PL emissionsPL
are intensity
caused by radiative
recombination
layersfor
because
of this
difference
varies
as a 25b
MoS2 crystalline
radiative
recombination
function of the
number
of layers.
For the
film in Fig. 1(a), the
peak
posiupon
photoexcitation.
In MoS2 monolayer,
obtained
from
each
of
the
mono-layers.
The
integrated
PL
intensity
decreased
with
the
oxygen
plasma
dominates as it only requires one recombination step for the electron
tion of A1g is 404.8 cm−1 and the difference from the E12g peak is aptreatment
time.
Figure
25c
showswith
thea MoS
peak
position oftothe
mainback
PLtopeak.
The main
position
of the
, which is
consistent
proximately 18.4
cm−1
transition
its equilibrium
state. peak
Therefore,
the observed
PL
2 monolayer.
quenching
upon
oxygen
plasma
treatment
suggests
that
the
recombinaFig.
1(c)
and
(d)
show
the
integrated
PL
intensity
mapping
image
and
PL spectra gradually red-shifted from 674 nm to 692 nm with an increasing oxygen plasma treatment
averaged PL spectrum of the pristine MoS2 monolayer, respectively.
tion process in the pristine MoS2 was changed by the bombardment of
duration.
It was
suggested
shallow
defect states
were generated
by distortion
the oxygen
plasma
treatments.
oxygen,
generating lattice
and defects.
Previous
studies of
The PL spectrum
exhibited
peaks at 624that
and 674
nm, corresponding
to
Fig. 2. (a) Optical microscope images of a pristine MoS2 monolayer and monolayers subjected to oxygen plasma treatment for different durations. (b) Confocal PL spectral mapping images,
indicating integrated PL intensity, of pristine and treated MoS2 monolayers. (c) Confocal PL spectral mapping images, which show the position of the peak corresponding to A excitons, of
pristine and treated MoS2 monolayers. The numbers represent the wavelength.
2
Figure 25. (a) Optical images, (b) confocal PL spectral intensity mapping images, and (c) spectral peak
mapping images of pristine and treated MoS mono-layers [131]. The color represents PL intensity
in (b) and wavelength of PL peak in c. Reprinted from Reference [131]. Copyright c 2015, with
Please cite this article as: M.S. Kim, et al., Photoluminescence wavelength variation of monolayer MoS2 by oxygen plasma treatment, Thin Solid
from Elsevier.
Filmspermission
(2015), http://dx.doi.org/10.1016/j.tsf.2015.06.024
2.7.3. Active Hydrogen H2∗
Ye et al. [129] exposed CVD-grown MoS2 mono-layers in hydrogen plasma at different
temperatures (400–700 ◦ C), as shown in Figure 26. The MoS2 mono-layer were treated to expose more
active sites in the basal plane of MoS2 and to improve catalytic activity. No significant changes were
observed on the MoS2 when the annealing temperature was 400 ◦ C. The MoS2 was then H2 -treated
at 500 ◦ C and small triangular holes with sizes around 1–4 µm appeared. High-density holes with
sizes around 10–20 nm were omnipresent in the basal plane of MoS2 . High-density triangular-shaped
holes became the prominent part in the original mono-layer MoS2 at 600 ◦ C. 700 ◦ C H2 -treatments
led to severe MoS2 decomposition. The hydrogen plasma treatments decreased the peak intensities of
Raman scattering and PL spectra, indicating that hydrogen treatments could increase the defects and
edges in mono-layer MoS2 .
Appl. Sci. 2019, 9, 678
Nano Letters
28 of 53
Nano Letters
Nano Letters
Nano Letters
Letter
Letter
Figure 26. SEM images of mono-layer MoS2 with 400 ◦ C, 500 ◦ C, 600 ◦ C, and 700 ◦ C H2 annealing,
respectively, showing the appearance of small triangle holes [129]. Reprinted with permission from
Reference [129]. Copyright c 2016 American Chemical Society.
2.7.4. Other Molecules
Fluoride
Chen et al. [126] treated MoS2 multi-layers under fluoride plasma (SF6 , CHF3 , and CF4 ) for
Figure 4. p-doping
Structure characterization
Figure
ofp-n
CVD
4. Structure
grown2MoS
characterization
Figure
of
CVD
4.treatment
Structure
grown atMoS
characterization
different
hydrogen
Figure
of CVD
4.treatment
Structure
(A−D)
grown atMoS
SEM
characterization
different
with temperatures.
hydrogen
of monolayer
of CVD
treatment
(A−D)
grown atMoS
SEM
different
with temperatures.
hydrogen
of monolay
trea
2 with hydrogen
2 with temperatures.
2 images
2 images
selected-area
to form
MoS
junctions
and
diodes.
°C2 H
with
400, 500,respectively,
600, and 700
showing
°C2 H
with
annealing,
400,
appearance
500,respectively,
600,ofand
small
700
showing
triangle
°C2 H
with
the
annealing,
400,
appearance
STEM
500,respectively,
600,
images
ofand
small
(E,F)
700
showing
triangle
°C
show
H2holes.
highthe
annealing,
appearance
STEMrespectively,
images
of small
(E,F)
showing
triangle
show hol
hig
the
MoS2 with 400, 500, 600, and 700
MoS
MoS
MoS
2 annealing,
2 the
2holes.
with
abundant
exposed
edges
and
with
step-edges.
abundant
(G)
exposed
Raman
edges
and
(H)
and
PL
with
step-edges.
spectra
abundant
show
(G)exposed
Raman
the
edges
and
(H)
andPL
with
step-edges.
spectra
abundant
show
(G)exp
R
t
density nanometer-scale holes formed
density
inside
nanometer-scale
the MoS2 layers
holes
formed
density
inside
nanometer-scale
the MoS
holes
formed
density
inside
nanometer-scale
the MoS
holes
formed
inside
the MoS
2 layers
2 layers
2 layers
decreased intensity after hydrogendecreased
annealingintensity
caused after
by thehydrogen
defectsdecreased
as
annealing
revealed
intensity
caused
by SEM
after
by and
thehydrogen
STEM.
defectsdecreased
as
annealing
revealed
intensity
caused
by SEM
after
by and
thehydrogen
STEM.
defects as
annealing
revealedcaused
by SEM
by and
the STEM.
defects as revealed
Chlorine
Murray et al. [116] irradiated exfoliated MoS2 few-layers (5–15 layers) under low-energy Cl
(200 eV and 1 × 1013 –1015 ions/cm2 ). The natural n-type MoS2 layers were doped to p-type and the
electric conductivity was reduced after irradiation.
Phosphorus
Nipane et al. [132] reported a compatible, controllable, and area selective phosphorus plasma
immersion ion implantation process for p-type doping of mechanically exfoliated MoS2 layers using
PH3 –He plasma. Homogeneous p-n MoS2 junction diodes were fabricated.
Figure 5. HER property characterization
Figure 5.ofHER
CVDproperty
grown MoS
characterization
Figure
hydrogen
5.ofHER
CVD
annealing
property
grownatMoS
different
characterization
Figure
temperatures.
hydrogen
5.ofHER
CVD
annealing
(A)
property
grown
LSVatMoS
figure
different
characterization
shows
temperatures.
hydrogen
that of
MoS
CVD
annealing
grown
LSVatMoS
figure
different
shows
temperature
hydrogen
that Mo
2 with
2 with
2 with
2 (A)
2 with
has better
500 °C
catalytic
hydrogen
activity
annealing
than under
400
has and
better
500600
°C
catalytic
°C.
hydrogen
(B)activity
Tafel
annealing
plots
than under
show
400
has and
better
hydrogen-annealed
500600
°C
catalytic
°C.
hydrogen
(B)activity
Tafel
annealing
MoS
plots
than
show
400
has
lower
and
better
hydrogen-annealed
Tafel
600catalytic
°C. (B)activity
Tafel
MoS
plots
than
show
400
lower
and
hydro
Ta
60
2 has
2 has
Galliumunder 500 °C hydrogen annealingunder
revealing
improvedanyelectrochemical
treatment,
revealing
activity
improved
of MoSany
astreatment,
a slope
HER catalyst
revealing
activity
after
improved
of
hydrogen
MoSany
asannealing.
treatment,
a HER catalyst
revealing
activity
afterimproved
of
hydrogen
MoS2 electrochemical
asannealing.
a HER catalya
slope than MoS2 without any treatment,
slope than
MoS2 without
slope than
MoS
than
MoS
2 without
2 electrochemical
2 without
2 electrochemical
Mono-layer
MoS
such
as
intrinsic
GaN/p-type
were
also
2 heterojunctions,
24 heterojunction,
density
density
became
the prominent
part
density
inbecame
triangular-shaped
the prominent
partd
about 4 orders
of magnitude
about
less than
4 orders
the amount
of magnitude
used
about
in
less than
4 orders
the amount
oftriangular-shaped
magnitude
usedMoS
about
in
less holes
than
orders
the amount
oftriangular-shaped
magnitude
used in
less holes
than
the amount
used in holes
24−26
24−26
24−26
24−26
the
original
monolayer
MoS
the
original
monolayer
MoS
original
monolayer
literatures.
literatures.
literatures.
literatures.
film,
while
700
°C
led
to
more
film,
while 700
°C led MoS
to mo
fi
irradiated by Ga plasma under UHV conditions [98]. The values of VBM were
reduced to 0.2 eV
for
2
2 the
2 th
40
In addition to oxygen plasma In
exposure,
additionhydrogen
to oxygenannealing
plasma In
exposure,
addition
hydrogen
to sample
oxygenannealing
plasma In
exposure,
addition
hydrogen
tomass
oxygen
annealing
plasma
exposure,
hydrogen
annealing
severe
decomposition
severe
and
sample
loss
decomposition
(Figure
4D).severe
and
mass
sample
loss
decomposition
(Figure 4D).s
Ga-irradiated
2 layers.
is also anMoS
effective
method isforalsodefect
an effective
engineering
method
of is
2D
foralsodefect
an
effective
engineering
method
of is
2D
for
defect
an
effective
engineering
method
2D
for
defect
2D
This
can be explained
byalso
the
decomposition
This
can be explained
of of
MoS
when
the
decomposition
the
Thisengineering
can be explained
of of
MoS
when
the de
th
T
2by
2by
43
materials. Previously, hydrogen
materials.
annealing
Previously,
has beenhydrogen
used materials.
to annealing
Previously,
has been
used materials.
tothan
annealing
Previously,
been
usedatmosphere.
tothan
annealing
been
usedatmosphere.
tothan 500
temperature
ishydrogen
higher
500
temperature
°Chas
at hydrogen
ishydrogen
higher
500
temperature
°Chas
at hydrogen
is higher
te
create
simple
hexagonal
holes
create
or
complex
simple
hexagonal
fractal
geometric
holes
create
or
complex
simple
hexagonal
fractal
geometric
holes
create
or
complex
simple
hexagonal
fractal
geometric
holes
or
complex
fractalto
geometric
Compared
to
defects
induced
Compared
by
oxygen
to
plasma,
defects
the
induced
triangular
Compared
by
oxygen
plasma,
defects
theinduced
triangulCb
Combination
patterns in graphene, which can
patterns
even in
form
graphene,
graphene
which
nanoribcan
patterns
even in
form
graphene,
graphene
can
patterns
evenanneal
in
form
graphene,
which
can even
form
holes
createdwhich
bynanoribhydrogen
holes
isgraphene
created
not that
bynanoribuniform.
hydrogen
Besides
anneal
holesisgraphene
created
not that
bynanoribuniform.
hydrogenBesid
ann
h
40−42
40−42
40−42
bons for electronic usage.40−42
bons
for the
electronic
bons
for the
electronic
usage.
bons
for the
electronic
usage.
Here,
similar usage.
strategy
was
Here,
similar
strategy
was
Here,
similar
strategy
was
Here,
the
strategy
was
the
microscale
images
observed
the
by
microscale
SEM,
STEM
images
was
observed
further
the similar
by
microscale
SEM,
STEM
images
was
observe
furthth
Jadwiszczak
et al. [133] exposed
mechanically
exfoliated
MoS
few-layers
toimage
ansites
O
–Ar
plasma
used to etch MoS2 in order toused
to etch
used
to etch
used
to etch
MoS
expose
moreMoS
active
orderintothe
expose
moreMoS
order
tothe
expose
more
order
tothe
exposelevel.
more
sites
the
used
toactive
image
the2inmorphology
used
change
toactive
the22in
morphology
used
change
toactive
image
of MoS
the2 in
morphology
u
at
atomic
at
atomic leve
2 in sites
2 in sites
2ofinMoS
43
43
43
43
(O2 :Ar basal
= 1:3)
2–28
The plane
frequency
oftothe
was
13.52
oxide
phase
was
planefor
of MoS
basal
of MoS2 activity.
basal
plane
of MoS
basal
plane
ofAn
MoS
its catalytic
and
improve
A plasma
its
catalytic
and
toSTEM-HAADF
improve
AMHz.
its
catalytic
and
toSTEM-HAADF
improve
A planeits
A planeF
Figure
4E,F2 activity.
shows
Figure
images
4E,F
shows
of the
basal
Figure
ofcatalytic
images
4E,Factivity.
shows
of the STEM-HAA
basal
2 ands.to improve
2 activity.
series of SEM images of H2series
of CVD
SEM grown
images MoS
of H2 2series
SEM2 grown
images MoS
of H2 2series
of CVD
SEM
images MoS
of H
are
etchedof CVD
are
etched
grown
etched
CVD
grown
MoS
MoS
sample
annealed
at 500
°C. 2High-density
sample
annealed
holes
atwith
500size
°C. 2High-density
sample MoS
annealed
holes
with
500siM
°
2 2are
2 areat
generated
under the plasmaetched
exposure,
changing
thein MoS
electrical
conductivity
and
carrier
mobility
shown in Figure 4A−D. Whenshown
the annealing
in Figuretemperature
4A−D. When
was
shown
the annealing
Figuretemperature
4A−D.
shown
theomnipresent
annealing
in around
Figuretemperature
4A−D.
When
theomnipresent
annealing
around
10−20 When
nm was
are
10−20
in the basal
nm was
are
plane.
This
around temperature
10−20
in the basal
nm was
are
plane.
omnip
Th
a
of the 2D
400materials.
°C, no significant changes
400were
°C, observed
no significant
on the
changes
MoS
400
°C, observed
no
significant
on the
changes
MoS
400
°C, observed
no
significant
oneffective
the
changes
MoS
observed
oneffective
the
MoS
indicates
that
hydrogen
annealing
indicates
is anthat
hydrogen
way
annealing
to form
indicates
is anthat
hydrogen
way
to forin
2 were
2 were
2 were
2 anneal
triangles (Figure 4A). As the temperature
triangles (Figure
increased
4A). As
to 500
the
temperature
°C,
triangles (Figure
increased
4A). As
to 500
the temperature
°C,
triangles
(Figure
increased
4A). As
toits500
the
temperature
°C,
increased
toits500
°C,
edge
enriched
MoS
edge
enriched
MoS
edge
enriched
MoS
e
benefit
electrochemical
benefit
electrochemic
∗ plasma
2 that would
2 that would
2 that wo
Mishra
et
al.
[98]
irradiated
MoS
under
N
for
1–3
min
and
then
under
Ga
flux.
The
VBM
2 with
performance.
Similar
totriangles
the
performance.
oxygen
Similar
treatment,
to the
the
performance.
oxygen
plasma
Similar
treatment,
p
the triangles start to be etchedtheand
triangles
small 2triangular
start to beholes
etched
theand
triangles
small
triangular
start to beholes
etched
with
theand
small
triangular
start toplasma
beholes
etched
withand
small
triangular
holes
withto the th
decrease
ofarethe
peakThese
intensity
decrease
in 1−4
Raman
ofare
and
the
PL
peak
spectra
intensity
isphenomena
also
decrease
in Ramanofare
and
the
PL
peak
spectra
intensity
is aldi
size around 1−4 μm appear. These
size around
phenomena
1−4 μm
areappear.
analogous
These
size around
phenomena
1−4 μm
appear.
analogous
size
around
phenomena
μm
appear.
analogous
These
analogous
was changed.
observed
here,
which
observed
support
here,
thewhich
conclusion
further
that
observed
support
here,thewhich
conclusion
th
o
to the morphology change ofto graphene
the morphology
when graphene
change of
was
to graphene
the morphology
when graphene
change
of
was
tocan
graphene
thefurther
morphology
when
graphene
change
of
wascan
graphene
when
graphene
wascan furth
Bhimanapati
et al. [134] treated
MoSAr/H
layers
in
UV–ozone.
The
ozone
treatment
increased
2 vertical
annealing
could
the
annealing
defects
could
edges
increase
hydrogen
in
the
annealing
defects
could
edges
inch
treated by hydrogen annealingtreated
with different
by hydrogen
annealing
treated
with different
byhydrogen
hydrogen
Ar/H
annealing
treated
with increase
different
byhydrogen
hydrogen
Ar/H
annealing
with
different
Ar/H
rate
rate
2 flow rate
2 flow rate
2 flow and
2 flow and
monolayer
MoS
monolayer
MoS
monolayer
MoS
m
ration. When temperature
further
ration.increased
When
temperature
to 600 °C,
further
highration.increased
When
temperature
to 600
°C,
highration.
increased
When
temperature
to by
600
°C,
high- increased
to 600
°C,
super wettability
and enhanced
hydrogen
evolution
reaction
of2. further
the
material
changing
edge
2. further
2. high-
chemistry and surface defects.
1100
1100
DOI: 10.1021/acs.nanolett.5b04331
1100
Nano Lett. 2016, 16, 1097−1103
Nguyen et al. [19] treated MoS2 layers in UV-ozone and then fabricated the layers into organic
photovoltaic cells. The open-circuit voltage, fill factor, and power conversion efficiency increased
significantly compared with un-irradiated MoS2 -based solar cells.
3. Electromagnetic Irradiation
Electromagnetic irradiation refers to the waves of the electromagnetic field, including gamma
rays, X-rays, ultraviolet, (visible) light, infrared, microwaves, and radio waves. The frequency and
wavelength of the electromagnetic irradiation is shown in Figure 3.
DOI: 10.1021/acs.nanolett.5b043
1100
Nano Lett. 2016, 16, 1097−11
Appl. Sci. 2019, 9, 678
29 of 53
3.1. Gamma-ray Irradiation
It is generally accepted that MoS2 macroscopic materials are stiff under gamma-ray irradiation.
There is little literature on γ-irradiated bulks. It was reported that MoS2 powders were resistant to a
γ-ray irradiation with a dose of 5 × 109 R (1.29 × 106 C/kg) [105].
Lee et al. [106] mechanically exfoliated MoS2 2D layers (50–132 layers) and exposed them to
5000 γ-ray photons with 662 keV. It was found that the resonance frequency of the layers upshifted
immediately after γ-ray exposure and returned to their initial frequency after 60 h. The procedure was
repeatable. It was assumed that γ-ray photons generated charges on MoS2 and caused electrostatic
forces between MoS2 and substrates, resulting in electrostatic tension and deflection of MoS2 layers.
The MoS2 multi-layers were stiff under the γ-ray and showed no irradiation damage.
Ozden et al. [107] prepared MoS2 multi-layers (5–8 layers) through the vapor phase sulfurization
and exposed the layers to a γ-ray irradiation of 60
27 Co source (γ-ray energy: 1.1732 MeV and
1.3324 MeV) with a dose of 120 Mrad. The irradiation was carried out at room temperature in an
ambient atmosphere. The X-ray photoelectron spectra indicated that MoS2 layers were converted to
molybdenum oxide (MoOx ) after the irradiation. It is plausible that the γ-ray displaced or knocked
out S atoms while leaving molybdenum atoms unaffected. Maybe the as-produced sulfur vacancies
were filled with oxygen atoms to form MoOx .
3.2. X-ray Irradiation
There are few reports on X-ray irradiation of MoS2 . Only one paper [108] reported that MoS2
mono-layers were irradiated with 10 keV X-rays with varying total ionizing doses. It was reported
that the MoS2 mono-layers were robust to X-ray radiation, withstanding doses of up to 6 Mrad doses
without any noticeable degradation of optical properties. It is generally accepted that X-rays do not
affect MoS2 few-layers.
Zhang et al. [135] fabricated single-layer MoS2 FETs and irradiated them under 10 keV X-ray
exposure. At room temperature in air, the drain current of the devices decreased significantly under
X-ray irradiation up to 10 Mrad. Effective threshold voltage and mobility were degraded with X-ray
irradiation dose. It stated that the degradation was consistent with the generation of negatively
charged surface states during the X-ray exposure.
3.3. Ultraviolet Light Irradiation
Azcatl et al. [109] treated MoS2 crystals under ultraviolet–ozone exposures at room temperature.
Oxygen-sulfur bonds were formed at the top sulfur layer of the MoS2 surface without breaking
sulfur-molybdenum bonds. Li et al. [136] investigated optical behaviors of azobenzene-functionalized
MoS2 mono-layers and exfoliated multi-layers on Au substrates. It was reported that UV light could
tune doping and the Fermi level of the hybrid structures. Lu et al. [137] irradiated liquid-exfoliated
MoS2 nanosheets in aqueous solutions and found that the MoS2 layers were oxidatively etched because
of photon-induced powerful OH∗ radicals.
Singh et al. [138] illustrated multilayer MoS2 FETs under ultraviolet light in N2 atmospheres. The
multi-layers were micromechanically exfoliated from natural MoS2 crystals and deposited on SiO2
coated silicon. The wavelength of the UV light was 220 nm and the average intensity was 10 mW/cm2 .
Nitrogen gas alone did not affect the electrical properties of MoS2 nanosheets, but nitrogen gas in the
presence of UV light remarkably affected the electrical properties of MoS2 nanosheets. Charge-carrier
mobility, carrier density, and drain current were enhanced after exposure to nitrogen gas under UV
light irradiation because of a possible doping effect. Detailed investigations [139] showed that the
charge-carrier density of single-layer, bi-layer, and few-layer MoS2 nanosheets were reversibly tuned
with nitrogen and oxygen gas in the presence of ultraviolet light. The device performance was adjusted
by exposure to gases in the presence of UV light.
Appl. Sci. 2019, 9, 678
30 of 53
McMorrow et al. [110] fabricated single-layer and multilayer MoS2 (mechanically exfoliated) on
Si substrates into FETs. It was found that the electron mobility increased with UV exposure up to
3.4 ×1010 –2.2 ×1013 photos/cm2 in a vacuum. Raman spectroscopy showed no significant crystalline
radiation damage or oxidation degradation under UV exposure. Irradiation-hard MoS2 FET devices
are expected.
3.4. Visible Light Irradiation
Liu et al. [140] tested photocatalytic performance of N-doped MoS2 nanoflowers under visible
light. It was reported that the N-doped MoS2 nanoflowers showed excellent photocatalytic activities
and durability on the elimination of the organic pollutants under visible light irradiation.
Laser has been widely employed in Raman scattering of MoS2 few-layers. Laser has also been used
to thin multilayered MoS2 down to a single-layer two-dimensional crystal [141,142] (power density up
to 80–140 mW/µm2 ) and generate ripples of MoS2 [142,143]. It is reported that the upper atoms of
MoS2 layers can be removed by high-power lasers because of laser ablation [112,141].
Early investigation indicated that natural MoS2 crystals do not undergo any kind of
significant laser-assisted oxidation when exposed to high laser power (up to 32,000 W/cm2 )
(wavelength: 632.8 nm) [144] while their Raman mode intensity changes slightly with laser power.
For microcrystalline MoS2 powders, Raman scattering indicated that MoS2 oxidized to MoO2 under
a high-power laser. Raman mode intensity and position were also significantly affected by the
laser power.
Paradisanos et al. [113] mechanically exfoliated MoS2 few-layers from a bulk natural crystal and
subsequently deposited them on Si/SiO2 wafers, irradiated the few-layers under a pulsed laser with
800 nm wavelength and 1 kHz repetition rate. No modification of MoS2 mono-layers was observed up
to a certain single-pulse fluence of 50 mJ/cm2 (2.5 mW) while damage occurred beyond the fluence via
the material ablation. Further work indicated that MoS2 mono-layers were practically unaffected by a
low-power (600 µW) pulsed laser irradiation when exposed to 103 pulses at a fluence of 20 mJ/cm2
(lower than the damage threshold). However, the MoS2 mono-layers were damaged upon 105 pulses at
20 mJ/cm2 . The A1g and E2g intensities were almost constant with pulse times, then rapidly decreased
at a critical exposure time. The abrupt decrease possibly came from ablation and eventual sublimation
of the MoS2 atoms.
Gu et al. [111] in situ studied Raman scattering of MoS2 layers during the laser thinning
of MoS2 . Due to the high surface-area-to-volume ratio, thinner MoS2 layers are less stable and
easier to decompose under high-power laser irradiation because of laser-induced thermal effects
and sublimation.
Tran Khac et al. [145] irradiated MoS2 layers under laser power of 1 mW, 5 mW, and 10 mW for
an exposure time of 60 s. No significant changes were observed in topographic images of the irradiated
regions under the 1 mW laser. However, significant amounts of particles or adsorbates were formed
on the MoS2 surface after irradiated under 5 mW and 10 mW lasers.
To minimize thermal effects on atomically thin MoS2 during Raman spectral measurements, low
laser powers (0.14 mW to 2 mW) are usually employed [145] to avoid potential laser-induced local
surface temperatures.
Lu et al. [112] employed a focused laser beam to directly pattern MoS2 mono-layers and few-layers.
Focused laser beam irradiation modified and thinned MoS2 layers to create well-defined structures
and controllable thickness.
Alrasheed et al. [146] used a 532 nm laser to irradiate mechanically exfoliated MoS2 few-layers
(less than 5 layers) on SiO2 /Si substrates. The power ranged between 0.93 and 8.3 mW and the
treatment time was 0.01–180 s. In ambient conditions, MoS2 nanosheets were etched and amorphous
MoS2 redeposited on the nanosheets at low laser powers while the few-layers were oxidized and MoS2
nanoparticles formed at high laser powers. The nanoparticle formation and oxidation were dependent
Appl. Sci. 2019, 9, 678
31 of 53
on the number of layers and laser exposure time. The Raman intensity and Raman peak width changed
with laser treatments, as shown in Figure 27.
ACS Applied Materials & Interfaces
Research Article
Figure 27. Raman intensity and linewidth (FWHM) of (a) mono-layer-ed (1L), (b) tri-layer-ed (3L), and
(c) 5-layer-ed (5L) MoS2 nanosheets laser-treated in ambient conditions. Reprinted with permission
from Reference [146]. Copyright c 2018 American Chemical Society.
3.5. Infrared Light Irradiation
Fan et al. [30] irradiated lithium intercalated 1T MoS2 layers with thickness of a few microns under
Figure 5.a Raman
intensity,
linewidth
shift of (a)
1L, (b) 3L, and
(c)MoS
5L MoS
in converted
ambient conditions.
2 nanosheets
near-IR
laser (780
nm)(FWHM),
in air atand
ambient
temperature.
The
were laser-treated
completely
to
2 layers
2H counterpart under the irradiation.
2H MoS2 multi-layers were grown through pulsed mid-infrared laser deposition [147]
(wavelength of 7.0–8.2 µm). No any IR damage was reported.
3.6. Terahertz Wave Irradiation
MoS2 multi-layers were fabricated into terahertz modulators [148] applied in high-speed
communications. Terahertz conductivities of MoS2 few-layers were experimentally measured [149,150]
from THz spectroscopy.
However, there was no report on terahertz damage on MoS2 .
Theoretical simulations indicated that the THz absorption of mono-layer MoS2 was very low and the
maximum THz absorption of mono-layer MoS2 was approximately 5% [151]. The sum of reflection
and absorption losses of mono-layer MoS2 was lower than that of graphene by one to three orders
of magnitude.
Figure 6. AFM images and Raman measurements on 1L MoS2 nanosheets laser-treated in vacuum. (a) Optical image and (b) AFM image of laser3.7. Microwave
Irradiation
treated MoS
2 nanosheet. Higher-magnification AFM images of each treatment locations for (c) 1 min and (d) 30 s laser treatments. (e) Raman
spectra before and after the 30 s laser treatment.
Few-layer MoS2 were fabricated into two-dimensional nanoelectromechanical systems (NEMS)
as ultralow-power, high-frequency tunable oscillators and ultrasensitive resonant transducers [152].
These devices can operate in the very high frequency band (up to ∼120 MHz). Ultra-thin MoS2 was
also fabricated to MoS2 transistors operating at gigahertz frequencies [153,154]. All these devices
worked at microwave range. Therefore, it is necessary to characterize the microwave damages to MoS2
few-layers.
It was reported that microwave can accelerate catalytic reactions [155] because of the formation
of hot-spots within catalysts. However, no significant differences were detected between microwave
irradiated MoS2 catalysts (size in microns) and untreated MoS2 during the sulfating reaction [155].
Zhao et al. [156] irradiated MoS2 powders under a microwave (1 kW power, 2450 MHz) at 200 ◦ C.
It was reported that the (001) basal planes of the MoS2 crystal structure were cracked into (100) edge
planes under microwave irradiation, creating additional active edge sites. The Raman intensity of
microwave irradiated materials increased significantly after microwave treatments. The irradiated
0 efficiently because of the microwave-induced cracks.
couldand
capture
Figure 7.MoS
AFM2 images
Raman Hg
measurements
on 4L MoS nanosheets laser-treated in vacuum. (a) Optical image and (b) AFM image of laser2
treated MoS2 nanosheet. Higher-magnification AFM images of each treatment locations for (c) 30 s and (d) 2 min laser treatments. (e) Raman
spectra before and after the 2 min laser treatment.
instant thinning for laser powers of 4.7 and 8.3 mW regardless
of the treatment time. However, for 0.93 mW, we see the
formation of anomalous particles around the treatment sites. In
fact, we observe nanoparticles forming around all the treatment
18108
DOI: 10.1021/acsami.8b04717
ACS Appl. Mater. Interfaces 2018, 10, 18104−18112
Appl. Sci. 2019, 9, 678
32 of 53
Xu et al. [31] treated chemically exfoliated MoS2 nanosheets in an inert nonpolar solvent of
1,2-dichlorobenzene under 2.45 GHz microwave irradiation at 130 ◦ C in an inert atmosphere, with an
output power of 500 W. 2H MoS2 nanosheets converted to the 1T phase in minutes.
4. Other Irradiation
4.1. Ultrasonic Wave Irradiation
MoS2 nanosheets have been prepared by ultrasonication through liquid exfoliation
technique [19,41,46,71]. The physical properties were affected by ultrasonication conditions.
For example, Gao et al. [157] sonicated MoS2 powders in N,N-dimethylformamide for 2–10 h to
chemically exfoliate MoS2 nanosheets. Crystalline MoS2 few-layers were produced after 2 h sonication
and the size of the nanosheets decreased gradually with increasing sonication time. The exfoliated
MoS2 nanosheets showed ferromagnetic properties at room temperature, in contrast to pristine
MoS2 bulks which showed diamagnetism only. The saturation magnetizations of the ultrasonicated
nanosheets increased with the ultrasonication time and the decreasing crystalline size.
MoS2 nanomaterials were also prepared by ultrasonicating slurrys of molybdenum hexacarbonyl
and sulfur in 1,2,3,5-tetramethylbenzene (isodurene) with a high-intensity ultrasound (20 kHz) under
Ar atmosphere [158]. The nanostructured MoS2 had a higher surface area and showed higher catalytic
activities for thiophene hydrodesulfurization.
4.2. Thermal Irradiation
MoS2 bulks sublimate at 450 ◦ C and melt at 2375 ◦ C. MoS2 crystals are not stable in the presence
of oxygen, resulting in MoO3 . It was reported that the material oxidized at 315–375 ◦ C in air and
converts to MoO3 at 400 ◦ C [144].
MoS2 mono-layers are not very stable at ambient conditions either. Peto et al. [159] examined
atomic-resolution STM images of the basal plane of mechanically exfoliated MoS2 mono-layers
after one month and one year of ambient exposure (in air, at room temperature and ambient light).
STM measurements clearly revealed modifications in the atomic structure of the MoS2 basal plane
during the ambient exposure. New point-defects were formed after one month of the ambient exposure
and the defect concentration increased 20–30 times. The point-defects increased 100–500 times after
one year of exposure. The oxidation of MoS2 basal planes yielded 2D MoS2− x Ox layers.
Chen et al. [117] annealed the liquid-exfoliated MoS2 mono-layers on various substrates (SiO2 , Au,
graphene, BN, and CeO2 ) in a vacuum. Thermal annealing did not introduce any noticeable defects
into MoS2 layers up to 450 ◦ C and MoS2 remained stable, as shown in Figure 11. Other group reported
that thermal annealing in a vacuum caused S vacancies in MoS2 at 500 ◦ C [90].
Donarelli et al. [160] annealed MoS2 crystals in UHV. Single sulfur vacancies were generated
in bulks while the S/Mo ratio in the bulk did not change with the annealing temperature up to
400 ◦ C. They also examined the liquid-exfoliated MoS2 few-layers. The S/Mo ratio of MoS2 few-layers
gradually decreased once the annealing temperature increased up to 400 ◦ C. Two kinds of sulfur
vacancies (single and double) were introduced in the molybdenite layers. The threshold for the
formation of double vacancies typically occurred upon thermal annealing at 200 ◦ C.
Nan et al. [125] mechanically exfoliated MoS2 mono-layers from bulk crystals and transferred
them to Si wafers with SiO2 capping layer. The mono-layers were then annealed at temperature of
350 ◦ C or 500 ◦ C for 1 h in a vacuum (0.1 Pa). Figure 28a shows PL intensity image of an as-prepared
MoS2 mono-layer, which was uniform across the whole sample. After being annealed for 1 h at
350 ◦ C in a vacuum, the PL intensity was enhanced by 6-fold (Figure 28b). At the same time, the
PL peak blue-shifted after the annealing. Figures 28c-d show another MoS2 mono-layer before and
after annealing at 500 ◦ C for 1 h in a vacuum. The PL image becomes highly inhomogeneous after
the annealing. More detailed investigation indicated that the PL enhancement and inhomogeneity
Appl. Sci. 2019, 9, 678
exposed
to ambient
air.physical
At the same
time, the
and explained
by the
adsorption
of PL
O2peak
and
blue
to ∼1.81
eV, as
shown
by
the
green
curve
H2O shifts
molecules
on MoS
(p-type
doping,
with
O
2
2 as
18
inthe
Figure
1g. These
phenomena
been reported
dominant
contributor).
Thehave
as-prepared
MoS2 is
and explained by the physical adsorption of O2 and
H2O molecules on MoS2 (p-type doping, with O2 as
the dominant contributor).18 The as-prepared MoS2 is
to
30 and 89after
times,
respectively)
therespectively
pumping
original
values.
Furthermore,
the
and they
are very
stable after
those
locations
drop
slightly
∼30 min
with a only
power
of ∼0.5
m
respectively) after the pumping
and they 33are
very stable after
of 53
∼30 min with a power of ∼0.5 m
should come from the reaction of oxygen (due to not very high vacuum condition) with MoS2 at high
temperature and followed chemical bonding of oxygen molecules to MoS2 .
(a)
(b)
(c)
(d)
Figure 28. PL intensity images of as-prepared (a,c) and annealed MoS2 mono-layers in a vacuum for
◦ CPL
intensity
images
ofpermission
monolayerfrom
MoS2Reference
: (a) as-prepared,
(b) annealed
c in vacuum for 1 h at 3
1 h at (b) 350 ◦ C and Figure
(d) 5001.
[125].
Reprinted
with
[125]. Copyright
down
to
0.1
Pa.
The
images
have
the
same
color
bar;
PL
intensity
images
of
another monolayer Mo
2014 American Chemical Society.
annealed in vacuum for 1 h at 500 C, (f) after pumped down to 0.1 Pa. The images have the same color b
from locations AD in the images; (h) change of normalized PL intensities (as compared to the origi
Yamamoto et al. [161] mechanically exfoliated MoS2 single-layers and few-layers from MoS2 bulk
BD throughout
the images
annealing
and pumping
process.
Figure
intensity
of monolayer
MoS
2: (a) as-prepared, (b) annealed in vacuum for 1 h at 3
crystals, deposited them onto 1.
300PLnm
thick SiO
2 /Si substrates. The MoS2 layers were then exposed to
down to 0.1 Pa. The images have
the
same
color
bar; PL intensity images of another monolayer Mo
◦
◦
an Ar/O2 mixture at temperatures
ranging
The flow
rates
of Ar
O2 were
annealed
in vacuum
forfrom
1 h at27
500CC,to(f)400
afterC.
pumped
down
to 0.1
Pa. and
The images
the
’
NAN ET AL.
VOL. 8have
NO.same
6 ’ color
5738b
1.0 L/min and 0.7 L/min,
Raman
measurements
indicated
that the oxygen
fromrespectively.
locations AD
in thespectroscopy
images; (h) change
of normalized
PL intensities
(as compared to the origi
BD throughout
pumping
process.
treatments led to triangular
etch pits onthe
theannealing
surfaces and
of the
atomically
thin MoS2 . MoS2 was etched
preferentially along the crystallographic directions of the zigzag edges with a preferential termination,
resulting in uniform orientations
with
NAN ET AL. of the pits, as shown in Figure 29a–e. The pit size increased
VOL.
8 ’ NO. 6 ’ 5738
increasing oxidation exposure (Figure 29f), and the growth rate was larger at higher temperature.
However, the numbers of etch pits per unit area was uncorrelated with oxidation time, oxidation
temperature, and MoS2 thickness but varied significantly from sample to sample. It was assumed that
the oxidative etching in MoS2 layers on SiO2 was initiated at intrinsic defect sites. Additionally, oxygen
exposure above 200 ◦ C significantly diminished the electron density in MoS2 single-layers and oxygen
treatments at 400 ◦ C resulted in conversion of MoS2 layers to MoO3 platelets. No molybdenum oxide
(MoO3 ) was detected after oxygen treatments below 340 ◦ C.
Tongay et al. [162] annealed MoS2 single-layers in a vacuum. Figure 30 shows room temperature
PL spectra of a MoS2 mono-layer annealed at 450 ◦ C in a vacuum for different annealing time. A 40 min
annealing at 450 ◦ C enhanced the PL intensity by over 50 times. The FWHM of the PL peak decreased
and the PL peak position shifted slightly with annealing time. The thermal annealing did not degrade
the crystalline quality of the material.
Appl. Sci. 2019, 9, 678
34 of 53
The Journal of Physical Chemistry C
Article
f
2. (a−d)
AFM images of
and
MoS
at 320 °C
Figure
1. (a) An
of single-layer
trilayer (3L), and(1L),
four-layer
(4L)Figure
MoS(2L),
at 1L
320
°C 2L
for
3 h. 2 oxidized(4L)
2 on SiO
2 after oxidation
Figure
29.AFM
(a)image
AFM
images(1L),
of bilayer
MoS(2L),
bi-layer
tri-layer
(3L),
and
four-layer
2 single-layer
for surrounded
(a) 1, (b) 3,by(c)
4, andlines
(d)on
6 h.
bars are 2 μm. (e) The
The inset shows an optical image of this flake before oxidation.
(b−e) Close-up images of the areas
dashed
(b)The
1L,scale
(c) 2L,
◦
average
distance
r
from
the
center
to
the
apex
of
triangular
pits as a
with triangular
(d) on
3L, and
(e)2 4L
in the oxidation
panel (a). The scale
bars areC
500for
nm. 3
(f)hSchematic
of hexagonal
lattice an
of theoptical
MoS2 structure
SiO
after
at 320
[161].drawing
The inset
shows
image
of thispits.
flake before
function
of oxidation
time. The layers
red line
is fit.
Two dashed lines indicate the (1̅010) S and (101̅0) Mo edges. The little blue circles correspond to
2 S atoms
in outer constituent
above
andThe inset is an AFM
oxidation.
(b–e)interior
Close-up
of the
areas
surrounded
bythat
dashed
lines
onispit(b)
1L,
(c)
2L, (d)MoS
3L,2 after
image
of the
a typical
triangular
formed
on
single-layer
islands suggests
Mo (101̅
0) edge
more
stable,
and
below
the Mo (red circles)
constituentimages
layer. Evidence
for MoS
2 nanocrystal
for 4 h. as
The
scale bar
300(g)nm.
that it is decorated by S atoms in one of two possible configurations that cannot be distinguished oxidation
in our experiment
described
in is
text.
Profiles
and
(e)
4L
in
the
panel
(a)
[161].
The
scale
bars
are
500
nm.
(f)
The
average
distance
r
from
the
center
of pits along the dashed lines in (b−e).
to the apex of triangular pits as a function of oxidation time [161]. The inset is an AFM image of a
no obvious simple corr
The observed oxidati
SiO2 are in sharp contra
on the same SiO2 surf
SiO2 results in circular
unlike atomically thin
supported graphene i
single-layer being the
pits in single-layer grap
the surface, and the n
time and temperature. T
graphene on SiO2 is du
charged impurities in
impurities is significan
thickness. Thus, for thic
is predominantly activa
the etch pits have nearl
layer depth.36 The oxid
similar in character to t
rather than graphene
oxidative etching of atom
defect sites on the surfa
of the density of pits
after oxidation at variou
from 106 to 109 cm−2,
reported density of intr
atoms such as tungste
crystal,37,38 indicating th
initiating etching.
Previous scanning p
emission measurement
oxidation leads to the
basal plane surface of b
microcrystalline MoS2
results in a peak at 820
terminal oxygen atom
normalized intensity o
oxidation temperature
820 cm−1 in pristine sin
(black line). However, t
the Si peak at ∼520 cm−
even at 340 °C for 2 h
conclude that the peak
stretching mode in M
mode of MoS2.42 This i
MoO3-related peaks su
spectrum of oxidized M
of MoS2 does not ch
suggesting that no M
spectroscopy clearly
remains after oxidation
of MoO3 formation, th
and 06),h.18,19,23,26
After oxidation
for exact
an hour,
etch pits with an average
points to Mo-edge4,(101̅
(3M Water-Soluble Wave Solder Tape 5414). The thicknesses
though
the
structure
3
2
nm
on the
size edge
of 6.3(and
× for
10locations
triangular
formed
onatomic
single-layer
MoS
after oxidation
4 h.areThe
scale
barsurfaces
is 300(Figure
nm. 2a).
identified bypit
optical
contrast,
force
of typical
MoS2 were
of the 2reconstructed
offormed
additional
sulfur
Nano
Additional
oxygen likely
treatment
leads on
to lateral
growth of the
microscopy (AFM), and
RamanLetters
spectroscopy.30,31 To remove
atoms
terminating
the
Mo-edge)
depends
the
c 2013 American
Reprinted with permission from Reference [161]. Copyrighttriangular
Society.
as shown
inChemical
Figure
2b−d.
The distance r from
19,23,26
adhesive residue, all samples were annealed in an H2/Ar
Further
work
chemical environment andpits,
substrate.
the center
to the apex
of themicroscopy
triangular or
pits increases almost
mixture for 2 h at 350 °C. The flow rates of Ar and H2 are 1.7
using high-resolution
transmission
electron
linearly with a growth rate of approximately 70 nm/h, as shown
and 1.8 L/min, respectively. This hydrogen treatment leads to
scanning tunneling microscopy could resolve the issue and
in Figure 2e, but the density of pits is nearly constant during
no chemical modification of the MoS2 basal plane (see
also elucidate the electronic and magnetic properties of these
the oxygen treatment, indicating that the oxidative etching is
Supporting Information for an AFM image and Raman spectra
edges.
not initiated homogeneously but at specific sites on the surface
of MoS2). After preannealing MoS2 samples in H2, they were
Figure 1g shows the profiles of the pits along the dashed lines
of atomically thin MoS2.
exposed to an Ar/O2 mixture at temperatures ranging from 27
in Figure 1b−e. The pits are mostly single-layer-deep
(∼0.7
Figure 3a−d shows AFM images of MoS samples of various
to 400 °C. The flow rates of Ar and O2 are 1.0 and 0.7 L/min,
nm) on single- and few-layer MoS2, indicating a very high2
thicknesses after oxidation at 320 °C for 1 h. In Figure 3a, the
respectively. The nanoscale structure of oxidized MoS2 was
degree of anisotropy in etching along the basal plane versus the
density of etch pits formed on the single-layer MoS2 film is 7.5
characterized by AFM in tapping mode using silicon cantilevers
double-layer-deep
c-axis, though we×do
cm−2, while observe
the pit density
on single-layer MoS2 in Figure
106occasionally
with a nominal tip radius of >10 nm (NCH, Nanoworld), and
Figure 3e,f).
pits
on
few-layer
MoS
3b
is22samples
orders of(see
magnitude
larger(The
than larger
that in Figure 3a. Figure
the composition and oxidation state were determined using
in
Figure
1g
is
artifactthickness with etch
depth
of
the
pits
on
single-layer
MoS
2
Raman spectroscopy (Horiba Jobin Yvon Raman microscope)
of single- toan4-layer
3c shows a MoS2 flake
caused
by
the
limitation
of
the
tapping
mode
AFM
with 2400 gratings per mm and a solid state laser with a fixed
pits on the surfaces. The density of pits onto4-layer MoS2 is 3.5
determine the thickness
of −2an, which
atomically
thin than
membrane
on
excitation wavelength of 532 nm.
is larger
the densities
on surfaces of
× 108 cm
7
rough SiO2.32) The
AFM images
any
cleartrilayer
sign of (2.7 × 108 cm−2)
cm−2
) and
single-layer
(9.0do×not
10show
the reaction products (presumably MoO3); we discuss this in
RESULTS AND DISCUSSION
25645
dx.doi.org/10
more detail below.
Figure 1a shows a typical AFM topographic image of atomically
Our MoS2 crystals are expected to have a 2H structure,1,2
thin MoS2 supported on SiO2 after oxygen annealing at 320 °C
where the triangular lattices of adjacent layers are 180°-inverted
for 3 h (see the inset for an optical image of this flake). The
relative to each other. Therefore, the triangular pits formed on
oxygen treatment results in etch pits on the surfaces of singlethe surfaces are also expected to have 180°-inverted
and few-layer MoS2. We observe the formation of etch pits even
orientations among even and odd numbers of layers. Such
after air annealing (see Supporting Information for an AFM
trends can be seen in Figure 1a. However, we also observe the
image),
suggesting
that
the etching
process
is independent
Figure
1. of
Drastic
enhancement
photoluminescence
of
monolayer TMDs by thermal annealin
Figure
30. PL
spectra
mono-layer
MoSof2 in
forroom-temperature
different
annealing
time [162].
Theintensity
anneal
temperature
triangular pits
with same
orientations
on even
and
odd layerthe partial pressure of oxygen gas. As shown in Figure 1b−e,
as
a
function
of
annealing
time.
The
anneal
is
at
450
°C
in
vacuum.
laser excitation intensity is fixed at
MoS
number-thickness
regions
(see
Supporting
Information
for anThe
◦ C in amonolayer
2
c
is
450
vacuum.
Reprinted
with
permission
from
Reference
[162].
Copyright
2013
American
the shape of the pits is triangular and their orientations are
AFMspectrum
image), suggesting
that itand
is the
top surface
that is monolayer MoS2. (Inset) Raman spe
PL measurements.
(b)observations
Normalized PL
for pristine
optimally
annealed
flat terrace. These
identical over each atomically
Chemical Society.
continuous
across the layer-number-thickness
indicate that the triangular
shapesmonolayer
of the pits reflect
latticePL intensity
enhancement
of monolayer MoSeboundary.
thermal anneal at 250 °C for 15 min. T
annealed
MoSthe
2. (c)
2 upon
Because of this ambiguity, we cannot be certain
of the
of the MoS2 basal plane
and that
thePL
edges
of the pitsenhancement
thermal
anneal
at 300 °C for 10 min. The peak shift
by surface
∼17 meV.
(d)
intensity
of
monolayer
WSeorder
2 upon
correlation
between
the
stacking
of
MoS
layers
and
the
2
along theIrradiation:
zigzag directions
with only a single
chemical
4.3.areHybrid
UV–Ozone
Treatments
meV.
All
the
PL
measurements
are
taken
at
room
temperature
in
ambient
condition.
orientations
of
the
triangular
pits;
however,
the
observations
of
termination, that is, terminated on either the Mo-edge (101̅0)
only a single etch-pit orientation within a single terrace, and the
or S-edge (1̅010) (see Figure 1f). The observation of only three
Ultraviolet–ozone
technique
dry-cleaning
method
to clean
surfaces by the
of opposite
orientations
for material
different layer
preferred
edge orientations rulescleaning
out armchair-oriented
edges is a observation
thicknesses
within a single
strongly
that theof oxidation by
for which there of
are contaminants
six possible identical
edges. Our
decomposition
through
ultraviolet
irradiation
andcrystal,
the suggests
chemical
action
termination is globally determined to be along only one of the
experiments are unable to resolve whether the preferred edge
ozone.
During
cleaning,
organic
compounds
decompose
to volatile substances by ultraviolet
Mo or S terminated
zigzag edges.
is the Mo-edge
or S-edge;
however,surface
evidence from
other studies
■
lights and by strong oxidation of ozone. The
has been employed
to25643−25649
irradiate 2D MoS2
25644 technique
dx.doi.org/10.1021/jp410893e
| J. Phys. Chem. C 2013, 117,
mono-layers [163,164] and few-layers [109,165–167].
Wang et al. [164] rapidly treated MoS2 1–3 layers that were mechanically exfoliated from crystals.
These was no any change before and after the UV–ozone treatments. It was assumed that the irradiation
effect was below detection because of the rapid treatment (30 s) and nitrogen protection that reduced
high oxidation and mobility change. Le et al. [165] prepared MoS2 nanosheets using a sonication
exfoliation method and UV–ozone treated the MoS2 multi-layers for 15–30 min. The MoS2 layers
were converted to MoOx . Burman et al. [167] treated liquid-exfoliated MoS2 few-layers (15–16 layers)
in UV–ozone atmosphere for 1.5 h. It was found that the Mo/S atomic ratio decreased, and atomic
percentage of oxygen increased with the expose time. Azcatl et al. [109] exposed the mechanically
exfoliated MoS2 layers under ultraviolet-ozone at room temperature. They found that oxygen-sulfur
bonds were formed at the top sulfur layer while the sulfur-molybdenum bonds were kept besides
removal of adsorbed carbon contamination from the MoS2 surface.
Figure 2. Effects of exposure to different gas species on the PL intensity of annealed monolayer TMDs. (a) Change in
annealed but measured in vacuum value) upon exposure to H2O alone, O2 alone, and ambient air. The pressure of these ga
Torr, respectively. Trion X− and exciton X0 peak positions are indicated. (b) Modulation of the PL intensity of monolayer Mo
purging (50 and 100 Torr, respectively) and pumping. (c) Modulation of the PL intensity of monolayer WSe2 as a function
cross section (barn)
cross section (barn)
evolution of a MoS2 shee
for practically all TMDs is within the energies commonly
First, freestanding singl
used in TEM studies.
The displacement thresholds for chalcogen atoms in the
pared by mechanical exfo
(top) layer facing the beam proved to be considerably higher
tals, followed by characte
Appl. Sci. 2019, 9, 678
35 of 53
than for the bottom chalcogen layer, as the displaced atom is
a Si þ 90 nm SiO2 subs
‘‘stopped’’ by the other layers. However, after a vacancy is
TEM support film (Quan
The ultroviolet–ozone
technique
was
also
used
to
mill
MoS
few-layers
to
mono-layers
2
created in the bottom layer, the threshold energy for the top
[32]. [166].
The TEM grid wa
Compared to mechanically
exfoliated
MoS
single-layers,
the
PL
intensity
of
the
UV–ozone
milled
2
S atom in MoS2 to be displaced
and fill the vacancy is about
evaporating
isopropanol
MoS2 mono-layers was enhanced by 20–30 times.
8.1 eV. This is similar in magnitude to the threshold for
was etched with KOH. Ab
displacingand
S atom
from Simulations
the bottom layer (6.9 eV), and thus
imaging was carried out
5. Irradiation Mechanism
Theoretical
formation of vacancy columns should be possible even at
TITAN microscope at a
Besides the experimental work reviewed in the previous sections, there are lots of theoretical
80
kV
when
lattice
vibrations
are
accounted
for.
T
for
The contrast difference b
approaches to investigate the irradiation mechanism of MoS2 2D nanosheets. d
transition
metals
even higher,
sinceenergy
they are
to initial
clearly
detectable in the
Komsa et al.
[78] calculated
the is
displacement
threshold
and bonded
the minimum
kinetic
six neighbors
similarly
by the Susing
layer.theFor
single-layer nature of th
energy of the recoil
atom to kickand
an atom
from stopped
MoS2 mono-layers,
density-functional
about 20 eVexchange-correlation
is required to displace
contrast would be ident
theory with theinstance,
Perdew-Burke-Ernzerhof
function.Mo
Theatom
sulfur displacement
threshold energy
of MoS
to incident-electron
energies
from
its2 was
site 6.9
in eV,
thecorresponding
MoS2 lattice,
which corresponds
toof about
This90iskeV.
also confirmed in
About 20 eV was
required
to displace
molybdenum
atom from
site in MoS2 mono-layer
electron
energy
of 560one
keV.
Naturally, under
suchits
conditions
cessive diffraction spots f
lattices, corresponding
to
an
electron
energy
of
560
keV.
Therefore,
the
sulfur
sub-layers
were
easier
the S sublattice is quickly destroyed. Formation of transition
ent intensity,
whereas for
destroyed whilemetal
it wasvacancies
unlikely to
generate
molybdenum
vacancies
in
molybdenum
sub-layers
is thus considered highly unlikely.
[18]. The analyzed inten
under TEM observations (where 200 keV electron beams were employed). Figure 31 shows the
With regard to possible vacancy agglomeration under
spots was found to be 1:0
calculated cross-section of sulfur atoms. Lattice vibrations were taken into account, assuming
continuous irradiation, we found that creation of a vacancy
During continuous ima
a Maxwell-Boltzmann velocity distribution. The cross denotes the experimentally determined
alter
theshows
formation
energy
in athe
neighboring
sites energies.
ber of The
vacancy sites (excl
cross-section fordoes
MoS2not
. The
inset
the same
data for
larger
range of electron
in
the
semiconducting
TMDs.
Thus,
we
do
not
expect
by crack forma
atom displacement cross-section was estimated from the electron threshold energy bypanied
using the
accelerated
formation
of
large
vacancy
clusters.
In
the
shrinkage
of the membra
McKinley-Feshbach formalism.
sputtered atoms as in Ref
20
ing was found to be 1.8 b,
150
with the calculated cross s
that the theoretical estim
100
15
cies in the parameters of t
50
distribution) at energies b
0
In Figs. 4(d) and 4(e) w
100
150
200
50
10
electron energy (keV)
[33] for the single and
based on atomic structu
from the DFT calculation
5
the experimental TEM im
can clearly be distinguis
0
contrast relative to the
60
65
70
75
80
85
90
95
100
pristine area. We find t
electron energy (keV)
ratios are 0.9 (0.9) for a
Figure 31. Cross-section for sputtering a sulfur atom from MoS2 mono-layers as calculated through
FIG. 3 (color online). Cross section for sputtering a sulfur
single
the McKinley-Feshbach formalism and the dynamical values of the displacement thresholds
[78]. and 0.2 (0.2) for t
atom from MoS2 , WS2 , and TiS2 sheets
as
calculated
through
Having shown that va
Reprinted with permission from Reference [78]. Copyright c 2012 by the American Physical Society.
the McKinley-Feshbach formalism and the dynamical values of
under electron irradiatio
the displacement
displacementthreshold
thresholds.
Dotted
are thewith
data
for the observations.
The calculated
energy
was inlines
agreements
experimental
they could be consecutiv
static lattice,
and solids
linesobserved
are the results
of calculations
wheremicroscopes with
Single sulfur vacancies
(VS’s) were
frequently
under transmission
electron
cies deliberately introdu
lattice voltage
vibrations
taken
into 200
account,
a the
Maxwellan 200 kV acceleration
(the are
electron
energy,
keV isassuming
higher than
displacement energy of
calculate the formation e
velocity
The cross
denotes
the experisulfur atoms (90Boltzmann
keV) while lower
thandistribution.
that of molybdenum
atoms,
560 keV).
and consider dono
mentally
determined
cross of
section
for MoS
inset shows for
the MoS2MoS
2 . The
Table 2 compares
formation
thresholds
five kinds
of defect
configurations
bulks2 and
As, and Sb; double accep
same
data forfor
a larger
of electron
energies.
mono-layers. The
thresholds
single range
S vacancies
were very
low, both in bulk and mono-layers.
Single-line S-vacancy and double-line S-vacancy configurations are generated with increasing single
S-vacancy concentration by prolonged electron irradiation. In addition, the formation thresholds for
035503-3
Mo-vacancy and DIV are not very high (18–29 eV).
Appl. Sci. 2019, 9, 678
36 of 53
Table 2. Comparison of formation thresholds in eV for various vacancy configurations of MoS2 bulks
and mono-layers [77].
Comfiguration
Bulk (eV)
Mono-layer (eV)
single S-vacancy
single Mo-vacancy
single-line S-vacancy
double-line S-vacancy
DIV
14.4
24.8
39.2
66.2
28.8
5.7
18.8
30.5
62.3
23.3
DIV: di-vacancy comprised of nearest neighbor single sulfur vacancies and single molybdenum vacancies.
Kretschmer et al. [168] combined analytical potential molecular dynamics with Monte Carlo
simulations to simulate helium and neon ion irradiation of MoS2 nanosheets deposited on SiO2
substrates. Their simulation indicated that substrates governed the defect production of irradiated
MoS2 layers. The irradiation-induced defect production was dominated by backscattered ions and
sputtered substrate atoms, rather than by the direct helium and neon ion impacts.
Molecular dynamics simulations were also employed to study the production of defects in MoS2
mono-layers under noble gas ion irradiation, such as He, Ne, Ar, Kr and Xe [169]. Sulfur atoms were
sputtered away predominantly from the top or bottom layers by the ion irradiation, depending on the
incident angle, ion type, and ion energy.
6. Irradiation-Induced Properties
Pure 2H-MoS2 crystalline bulks are indirect band-gap semiconductors and show diamagnetic
from 10 K to room temperature. Their physical properties are sensitive to structures and defects.
Irradiation produces defects in MoS2 few-layers and affects their properties significantly. Below, the
optical properties, electronic properties, catalytic properties, and magnetic properties are summarized.
6.1. Band Structures
Bulk MoS2 is a semiconductor with an indirect band gap of 1.23 eV [36,170]. The band gap of the
material depends on thickness [171], as shown in Figure 2. Figure 32a shows the band structure of
MoS2 mono-layers without vacancies. MoS2 mono-layers have a direct band gap of 1.8 eV, resulting in
strong absorption, PL bands [38,39], and electroluminescence [172] near 680 nm.
Defects affect the band gap of MoS2 layers significantly. Figure 32b–c shows calculated band
structures of MoS2 mono-layers with defects. Negative (S vacancies) and positive (Mo vacancies)
charges transferred to the orbitals of nearest neighbor Mo or S atoms around the vacancies, resulting in
additional states at the bonding level and at mid-gap of the band structures. The K-point direct band
gap remained intact for MoS2 mono-layers with S vacancies, while the gap value was reduced. The
direct band gap at K-point was reduced too for MoS2 mono-layers with Mo vacancies.
6.2. Electric Properties
The density-functional method and the Green’s function approach indicated that structural
defects and grain boundaries were principal contributors to electric transport properties of MoS2
mono-layers [173]. Atomic vacancies could significantly reduce the conductance of the 2D layers.
In experiments, MoS2 2D materials were deposited on various substrates and connected to metal
electrodes to fabricate FETs to detect electric properties under irradiation.
Durand et al. [174] studied electrical transport properties of CVD-grown MoS2 mono-layers on
SiO2 /Si substrates. The carrier density was significantly increased, and the mobility was reduced
under a low-energy electron irradiation (5.0 keV) in an UHV environment. It was believed that
the electron-irradiation generated defects in MoS2 layers and caused Coulomb potentials at the
MoS2 /SiO2 interfaces.
135701-3
Karmakar
et 135701-3
al.
Karmakar
135701-3
Karmakar
et al. et al.
J. Appl. Phy
Appl. Sci. 2019, 9, 678
37 of 53
FIG. 1. Total b
ent vacancy c
monolayers, viz
gle SV, SL S-v
single Mo-vaca
cancy compris
vacancies (DIV
C high-symmet
onal Brillouin
are plotted w
Appearance of
states for differ
tions is evident
(a)
(b)
(c)
intact
for a
assumptions
may
approximately
only
for S-vacancies,
intact for remains
SV, remains
albeit
with
assumptions
may band
approximately
hold
only
for S-vacancies,
intact
forreduce
SV,
may
approximately
hold hold
only
forremains
S-vacancies,
structure for
different
vacancy configurations
in MoS
Figure 32. Total assumptions
2 mono-layers (a) without
SL DL
and
DL gh
thesulfur
other
vacancy
configurations
need
accounting
ofboth
both
SL along
and
have both
a C-point
direct
the other
vacancy
configurations
needconfigurations
a proper
of a proper
SL
and
have
the
other
vacancy
need
a proper
accounting
of DL
vacancy,
(b) with
single
vacancies,
and
(c)accounting
single
molybdenum
vacancies
Γ-M-K-Γ
DIV
cases,
direct
b
isolated
atom
reference
energies.
The
isolated
reference
enerDIV
cases,
direct
band-gap
at
K-point
is re
isolated
atom
reference
energies.
The
isolated
reference
enerhigh-symmetry directions
of hexagonal
Brillouin
zone [77].
DIV cases, direct band-g
isolated atom
reference
energies.
The isolated reference enerIn some
studie
gies
for
individual
Mo Sand
S atoms
are 1.89
and
1.08
eV, like
some
Refs.
14 and
35
gies for individualgies
Mofor
andindividual
S atoms
are 1.89
1.08
eV,1.89
In some
studies,
likw
Mo
and
atoms
are
and In
1.08
eV,studies,
◦ C through
Lu
et
al.
[175]
grew
MoS
single-crystalline
few-layers
on
SiO
/Si
substrates
at
700
2
2
gated
MoS2single
(0001
respectively,
arenegligible.
not negligible.
gated MoS2 (0001)
surface
respectively, whichrespectively,
are not
negligible.
gated
MoSwith
surS
whichwhich
are not
2 (0001)
CVD method, and then transferred them to pre-patterned SiO2 /Si substrates with Au electrodes
For
DL
vacancies,
it
was
indicated
that
the
created
vacancy
induced
gap
For DL vacancies,For
it was
indicated
that
the
created
vacancy
induced
gap
states
are
situated
de
DL vacancies, it was indicated that the created vacancy induced gap stat
by the poly(methyl methacrylate)-assisted method. The fabricated MoS2 devices were then
cancy
get arranged
a staggered
configuration
bytheseindicates
This
indicates
that
cancy may get arranged
in amay
staggered
configuration
by configuration
This
indicates
levels
may
as
may
get
arranged
in a in
staggered
by that
thatact
these
electron-irradiated cancy
in a scanning
electron
microscope
with a 30 keV
electron beam.
The This
electron
removing
S
atoms
alternatively
from
two
nearest
neighbor
conduction.
With
s
removing
S atomsµC/cm
alternatively
from that
two the
nearest
neighbor
conduction.
With single
SV, we
too single
observ
atoms
from
two nearest
neighbor
With
2 . It S
dose was 100–1800 removing
was
foundalternatively
threshold
voltage
shifted to
the negative conduction.
side and
lines.
We
have
observed
thatformation
thestagformation
energies
of stagsuch
in
lines.
We have
observed
that
the
formation
energies
of
such
deep
states
insuch
addition
to
thestates
additio
lines.
We
observed
that
the
energies
of stagdeep deep
states
in
addi
the mobility
increased
upon
thehave
increasing
electron
doses.
gered
and columnar
DLsame
defects
areonto
ofHowever,
thex membranes,
same
order.
However,
incr
geredParkin
and etcolumnar
DL
are of
order.
with
increasing
S-vacancy
conce
gered
anddefects
columnar
DLthe
defects
are
of
the
same
order.
However,
with with
increasin
al. [79]
deposited
single-crystalline
MoS
SiN
contacted
2 mono-layers
Therefore,
in
the
present
study,
the
DL
configuration
consists
DL
cases
and
also
Therefore,
the present
configuration
consists
DL
cases
and also
inwith
presence
Moin(M
Therefore,
in the DL
present
study,
the
DL configuration
consists
DL
cases
and ofalso
p
them to Auin
electrodes.
Thestudy,
fabricated
device
was
then
irradiated
under
a TEM
electron
beam
of
columnar
S
vacancies.
In
addition,
in
Ref.
17,
calculated
(DIV)
vacancies,
an
of
S found
vacancies.
In
addition,
in
Ref.
17,
calculated
(DIV)
vacancies,
antibonding
gap
states
are
200columnar
keV. It was
that
the
electrical
resistance
increased
with
the
electron
irradiation
dose.
of columnar S vacancies. In addition, in Ref. 17, calculated
(DIV) vacancies, antibon
prediction
ofactual
the
actual
electron
required
to getcontribute
contribute
to doping
prediction
electron
beam
energy
required
tobeam
get energy
contribute
to
In
addition,
these
addit
Kim etofal.the
[86]actual
prepared
few-layer
MoS
FETs,
andbeam
investigated
the
effect
of
under
prediction
of the
energy
required
to irradiation
getdoping.
to doping.
In a
2 electron
MeV proton
exposures.
The
electrical
properties
of
the
devices
were
nearly
unchanged
in
response
a
sustained
production
of
these
defects
is
very
high
(more
spin-polarized
and
S
a10sustained
production
of
these
defects
is
very
high
(more
spin-polarized
SO-coupled and
con
a sustained production of these defects is very
high (more and spin-polarized
andthus
SO-co
12
2
to low
protons/cm
). The
current
conductance
the
devices
thanKeV).
80 (10
KeV).
the successive
experimental
sections,
wenetic
netic
order
forvacanc
syste
than
80fluence
KeV).proton
In than
theirradiation
successive
experimental
sections,
we andnetic
order
forofsystems
with
different
80
In
theInsuccessive
experimental
sections,
we
order
for systems
w
13 –1014 protons/cm2 ). The electrical changes
significantly
decreased
after
high
fluence
irradiation
(10
Onestudy
recent
have
observed
that
in practical
situation,
electron
beam
of
One
recent
onfirs
se
have observed that
in practical
situation,
electron
beam
of electron
One recent
first-prin
have
observed
that in
practical
situation,
beam
of first-principles
contributed
to proton-irradiation-induced
traps
in the SiO
substrate
layers
and atin
the
interfaces
2introducing
face
defects
in
Mo
far
lower
energy
is
capable
of
modification
of
also
includes
the
eff
face
defects
MoS
far
lower energy
is
capable
of
introducing
modification
of
face
far lower energy is capable of introducing modification of
2 defects in MoS2 al
between the MoS2 layers electronic
SiOproperties
2 layers. structural
which
indicates
th
in
different
structural
forms.
of
MoS
which
indicates
that
adsorption
ofthat
S ato
in
different
forms.
electronic
properties
of and
MoSthe
2
which
indicates
ad
in
different
structural
forms.
electronic
properties
of
MoS
2
2
Islam et al. [128] measured electrical properties
of MoS2 mono-layers exposed under oxygen
Figure
1
exhibits
a
comparison
of
the
impact
of
defects
localized
shallow
Figure 1 exhibits aFigure
comparison
of the
impact of defects
localized
shallow localized
state at the
valence
ba
1 exhibits
a comparison
of the impact
of defects
shallow
state
plasma. The plasma was generated from a gas mixture of oxygen (20%) and argon (80%) under a
,
revealing
on
the
total
band-structure
of
monolayer
MoS
understandably
in
,
revealing
on the total band-structure
of
monolayer
MoS
understandably
in
contrast
with
the
presen
2
revealing
theattotal
band-structure
of2 monolayer
MoS2of, the
understandably in contrac
pressure of 250–350on
mTorr
50 kHz.
Figure 33a shows
the ID -VDS curves
device at a back-gate
appearance
ofatadditional
states
both
at bonding
level
atleading
leading
toresults
deep
ga
appearance
of additional
statesofboth
bonding
level
and
atcurves
leading
to deep
pr
appearance
additional
both
atDS
bonding
levellinear
and
atand gap
deep
gap sta
voltage VG = 40 V for
various plasma
exposure states
time. The
ID -V
were
around
thestates.
zero toThe
mid-gap.
These
states
signify
the
contribution
from
addirent
investigation
mid-gap.
These
states
signify
the states
contribution
addirent
investigation
also
support the modific
mid-gap.
These
signify from
the the
contribution
from
addi- ofrent
investigation
also sa
bias, showing
Ohmic
behaviors.
Figure
33b demonstrates
exposure-time
dependence
electric
negative
(S-vacancies)
or positive
(Mo-vacancies)
levels
described
tional
negative
(S-vacancies)
orexponentially
positive
(Mo-vacancies)
levels
as described
indrain
Ref.
36,aswhere
tionaltional
negative
(S-vacancies)
positive
(Mo-vacancies)
levels
as described
inforR
resistance.
The resistance
increased
withor
increasing
plasma
exposure
time. The
charges
transferred
toorbitals
the
orbitals
of nearest
neighbor
(nn)than
layer,
in addition
charges
transferred
to the
orbitals
of to
nearest
neighbor
(nn)
layer,
in (nn)
addition
to four
the
state
charges
transferred
the
of
nearest
neighbor
layer,
inshallow
addition
to thn
current and
mobility
decreased
exponentially
with
plasma
exposure
time,
dropping
more
Mo
or
S-atoms
around
the
vacancies.
Charge
transport
in
edge,
defect
states
Mo
or of
S-atoms
around
vacancies.
transport
inIt wasedge,
defect
states
are also
observed
to a
orders
magnitude
only
a totalaround
of 6 Charge
s plasma
exposure. Charge
claimed
thatinthe
significant
Moafter
or the
S-atoms
the vacancies.
transport
edge,
defect
states are
for
low
carrier-density
regime
can
be
explained
by
MoS
conduction
band
degradation
of
electronic
properties
was
caused
by
the
creation
of
insulating
MoO
-rich
disordered
for
low
carrier-density
regime
can
be
explained
by
MoS
conduction
band
edge.
2
MoS2 for low carrier-density regime can be explained 3by
conduction band edge.ed
2
hopping
through
such
vacancy-induced
localized
gapFor each
vaca
domains inthrough
the MoShopping
sheet upon
oxygen plasma
exposure.
hopping
vacancy-induced
localized gap- localized
For each
configuration,
inc
2 such
through
such vacancy-induced
gap- vacancyFor
each
vacancy
16
16
16
Figure
1
implies
that
the
K-point
direct
band-gap
states.
coupling
reduces
implies Figure
that the1 K-point
band-gap
states. Figure 1 states.
coupling
reduces coupling
the total reduces
energy of
implies direct
that the
K-point direct
band-gap
thethe
to
TABLE
II.
SO-coupled
non-collinear
magnetic
moment
(m) zcomponents
along
x, y,z and
z directions
TABLE II. SO-coupledTABLE
non-collinear
magnetic
moment
(m)magnetic
components
along
x, y,components
and
directions
moments
for res
nea
II. SO-coupled
non-collinear
moment
(m)
along and
x, y,resultant
and
directions
and
atoms
and
for
the
entire
system.
atoms and for the entireatoms
system.
and for the entire system.
Configuration
Configuration
mx, my, mz (nn Mo)
Configuration
(nn Mo)
x, m
y, mz
Total
m (nn Mo)
mx, mym
,m
z (nn Mo)
Total
(nn Mo)
x, my, mz (entire system)
Total m
(nnmm
Mo)
mx, m
ource–drain bias plasma exposure time leads to the gradual increase of the
tunnel barrier raised by the effective medium semiconductor
2 device. The ID
h the increase of
4 Sci. 2019, 9, 678
Appl.
ound to be 10
.
2
lated to be 6 cm
D/dVG), where L is
d CG ¼ 303rA/d is
h 3r 3.930 is the
¼250 nm) is the
38 of 53
f the same device
nied view of the
(pristine MoS2)
stingly, the drain
Figure 33. (a) ID -VDS curves of the single-layer MoS2 device after different plasma exposure time [128].
Fig. 3 (a) ID vs. VDS characteristics curve for the single-layer MoS2
ncreasing oxygen (b) Resistance
of the device as a function of plasma exposure time [128]. Republished with permission
device after different plasma exposure times. (b) Resistance of the
of
RSC
Publisher,
Reference [128]. c 2014. Permission conveyed through Copyright Clearance
seen in Fig. 2(b) device as afrom
function of plasma exposure time. The green line is the
Center, Inc.
is displayed in a linear fit of the logarithmic resistance as a function of exposure
Properties
duration.
5 nA for the 6.3.
as-Catalytic
Molybdenum is relatively abundant and cheaper than noble metals. Thus, molybdenum
compounds have been used as electrocatalysts and photoelectrocatalysts to supersede noble metal
catalysts. It was found that MoS2 edges were catalytically active for hydrogen evolution reaction
Nanoscale, 2014, 6, 10033–10039 | 10035
(HER) while the basal surfaces were catalytically inert. So MoS2 nanomaterials have been employed
as catalysts to generate hydrogen [69,176–179] because of more catalytic edges. MoS2 nanoparticles
were first used as photocatalysts [180] and electrocatalysts [181] to generate hydrogen in the 2000s.
The catalytic properties were controlled by edge sites of the MoS2 materials. MoS2 2D films were more
active and employed as photocatalysts [182] late in 2016 for water purification,
Irradiation can introduce defects in MoS2 layers, which create more HER active sites. Sulfur
vacancies can activate and optimize hydrogen evolution [20]. Additionally, electrical conductivity can
be tuned by the introduced defects. Therefore, catalytic activities of MoS2 HER can be significantly
enhanced by irradiation defect engineering. Tao et al. [183] treated the CVD-grown MoS2 thin films
by Ar plasma. The active site density of MoS2 thin films increased five times to 7.74 × 1016 cm2 after
the Ar plasma treatments. HER performance was enhanced significantly too. However, long-time Ar
treatments etched MoS2 , not benefited the HER behaviors. The MoS2 sheets were also treated by O2
plasma [183]. The O2 plasma treatments led to the formation of both Mo-O and S-O bonds, generating
more active sites with the increasing plasma time, enhancing the HER activity significantly.
6.4. Magnetic Properties
2H-MoS2 bulks are temperature-dependent diamagnetic [184]. It was reported that irradiation
improved magnetic ordering in single crystals of MoS2 [85]. Karmakar et al. [77] investigated
electron-irradiation-induced magnetic behaviors of MoS2 crystals. The irradiation was carried out
under 30 keV electrons with exposure time of 30 min. The effective magnetic moment of single crystals
increased from 0.42 µB per Mo-ion to 1.11 µB per Mo-ion on account of irradiation.
Han et al. [74] investigated magnetism of electron-irradiated MoS2 single crystals.
The diamagnetic MoS2 single crystals transformed into ferromagnetic state after irradiation up to room
temperature, as shown in Figure 34. The irradiation-induced magnetic phase transition was largely
attributed to the strain around the irradiation-induced vacancies.
Appl. Sci. 2019, 9, 678
39FIG.
of 532. Comparison of magne
curves after subtracting the diam
or paramagnetic background fr
data of Figs. 1 and S1, supplem
material. The electron dos
changed at low (a), (c) and high
acceleration energies (see Table
Figure 34. Magnetization curves of crystalline MoS2 lamellae with a thickness of 100 microns after
various electron irradiation [74]. Left panel: under low-energy electron irradiation. L(i): 150 kGy and
0.7 MeV; L(ii): 300 kGy and 0.7 MeV; L(iii): 600 kGy and 0.7 MeV. Right panel: under high-energy
electron irradiation. H(i): 100 kGy and 2.0 MeV; H(ii): 250 kGy and 2.0 MeV. Magnetic fields were
parallel to ab-plane or c-plane of the lamellae. Reprinted from Reference [74], with the permission of
AIP Publishing.
Mathew et al. [85] irradiated MoS2 sheets with a thickness of 200 µm at room temperature using a
3.5 MeV proton ion beam. The pristine sample was diamagnetic in nature. After irradiated at a fluence
of 1 × 1018 ions/cm2 , ferromagnetic ordering was induced in the MoS2 , as shown in Figure 35a. More
detailed work indicated that the induced magnetization depended on the irradiation dose. Figure 35b
shows the magnetization of irradiated MoS2 crystalline flakes that were measured at 300 K and 5 kOe
as a function of proton ion fluence from 1 × 1017 ions/cm2 to 5 × 1018 ions/cm2 . The magnetization
of the pristine flakes was negative as well as in irradiated flakes at a fluence of 1 × 1017 ions/cm2 . The
magnetization became positive after the sample was irradiated at a fluence of 2 × 1018 ions/cm2 and
increased with the irradiation doses. However, the magnetization decreased when the flakes were
irradiated at a fluence of 5 × 1018 ions/cm2 .
Mathew et al. [85] also exposed MoS2 crystals (200 µm thickness) under 0.5 MeV proton
irradiation at an ion fluence of 1 × 1018 ions/cm2 (same fluence at which magnetic ordering was
observed using 2 MeV protons) while ferromagnetism was not observed in the low-energy proton
irradiation. After annealing at 350 ◦ C for 1 h in Ar flow, a weak magnetic hysteresis loop was
observed. The weak magnetic signal was also observed in the irradiated samples at a lower fluence of
2 × 1017 ions/cm2 and a lower energy of 0.5 MeV proton irradiation. Based on these phenomena, the
appearance of observed magnetism in proton-irradiated MoS2 flakes were due to a combination of
defect moments arising from vacancies, interstitials, deformation and partial destruction of the lattice
structure, such as the formation of edge states and reconstructions of the lattice.
Zhang et al. [185] reported weak ferromagnetism phenomenon in MoS2 nanosheets and attributed
FIG. 3. HRTEM images of (a) the pristine MoS2 and (b)–(e) electron beam-irradiated samples (see Figs. S4–S8, supplementary material). (f)–(j) Hist
the magnetic
properties
tosize
thedistribution
presenceofof
atoms.irradiation
Later, first-principles
computations
summarize
that the
theunsaturated
countable atomsedge
after electron
complies with a Gaussian
profile (red curves) with the peak p
centers at around 1.4
Å for thecalculations
pristine MoS2. were carried out to predict ferromagnetism in zigzag
and density-functional
theory
nanoribbons of MoS2 [186–188], magnetism in MoS2 clusters [189], and magnetic edge state of
hydrodesulfurizated MoS2 particles [190]. It was generally accepted that [191] zigzag MoS2 edges were
ferromagnetic and metallic whereas armchair MoS2 edges were nonmagnetic and semiconducting.
The predication was experimentally approved in MoS2 few-layers [192]. It stated that grain boundaries
or defects in the nanosheets were responsible for the ferromagnetism of MoS2 nanosheets.
Appl. Sci. 2019, 9, 678
102103-2
Mathew et al.
Appl. Phys. Lett. 101, 102103 (2012)
(b)
(blue), and 10 K (red)
irradiated MoS2 at a fl
ions/cm2. An enlarged
isotherms near the orig
inset. The variation of
40 of 53
fluence is shown in (
and field cooled (ZFC
ization vs temperature
an applied field of 500
ated MoS2 sample
5 1018 ions/cm2. Th
estimated magnetic
temperature plot near
shown in the inset. 2
magnetization at 300 K
as a function of io
1 1017 ions/cm2 to 5
of coercivity is found to increase with ion fluence at all temperatures used in this study, although at 10 K, it is almost
constant. A large variation of coercivity with temperature
indicates the presence of long-range magnetic ordering in
the irradiated samples. The zero field cooled (ZFC) and field
cooled (FC) magnetizations at an applied field of 500 Oe are
given in Fig. 2(c). An insight into the nature of magnetic
ordering (ferro- or ferri-magnetic) can be gained by analyzing the variation of susceptibility with temperature. The
inverse of susceptibility plot is shown as an inset of Fig.
2(c). The value of Curie temperature (Tc) is estimated to be
895 K. The nature of the curve near Tc, a concave curvature with respect to the temperature axis, is characteristic of
ferrimagnetic ordering whereas for a ferromagnetic material,
this curvature near Tc would be convex.23 The variation of
magnetization at 300 K with an applied field of 5 kOe is plotted in Fig. 2(d) for different fluences. The magnetization of
the pristine sample and sample irradiated at a fluence of
1 1017 ions/cm2 are negative, at a fluence of 1 1018 ions/
cm2, the sample magnetization becomes positive and at
2 1018 ions/cm2, the magnetism in the sample increases
further, and at a fluence of 5 1018 ions/cm2, it has
FIG. 1. M vs H curve (a) at 300 K and (b) at 10 K for a pristine and irradiated
decreased. The dependence of magnetization on the irradiaFigure
35. (a) M vs H curve for a pristine without
irradiation and an irradiated MoS micron-thick
MoS
2 after subtracting the substrate Si contribution.
tion fluence observed in Fig. 2(d) (the bell shaped curve) 2
18
2
indicates
thatmagnetization
the role of the implanted
and thus of
the proton fluence
flake atordering
a fluence
ofto5the×presence
10 protons/cm
and (b)
as protons
a function
magnetic
can be due
of defects such
effect of end-of-range
for the observed magnetism is
as atomic vacancies,
displacements, and
saturation of 18
a va17 protons/cm
2 to
2 [85]. defects
from
1
×
10
5
×
10
protons/cm
Reprinted
from
Reference
minimal, as it had been shown in the case of proton- [85], with the
cancy by the implanted
same sample was subseDownloaded
08 Sepprotons.
2012 The
to 139.184.30.132.
Redistribution
subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
irradiated HOPG.19,20,24 It was demonstrated that 80% of the
quently irradiatedofatAIP
cumulative
fluences of 2 1018 and
permission
Publishing.
measured magnetic signal in the 2 MeV Hþ irradiated
5 1018 ions/cm2 to probe the evolution of induced magneHOPG originates from the top 10 nm of the surface.24 To
tism with ion fluence.
radiation-induced
modification in our samples near the
Magnetizations
asstated
a functionthat
of field
isothermsedges
at a flu- of probe
Recent
work
zigzag
MoS
2 few-layers can induce ferromagnetism. Therefore,
surface region, we used XPS and Raman spectroscopy.
ence of 5 1018 ions/cm2 are shown in Fig. 2(a). An
defects,
such
sulfur
vacancies,
canis given
convert
diamagnetic
2H-MoS
nanosheets
ferromagnetism [193].
The modifications
in atomic 2bonding
and core-level
enlarged
viewas
of the
M-H curves
near the origin
in
electronic structure can be probed using XPS. The XPS specthe inset and the decrease of coercivity with increasing temIrradiation
could
create
numerous
defects
in
MoS
2D
materials
as
discussed
in
the previous sections.
tra of a 2
pristine sample and the sample irradiated at a fluence
perature is clear from the plot. A plot of coercivity vs ion flu18
2
ions/cmfew-layers
are shown in Figs.
3(a)–3(d). FittingMore
of
of 5 of
10MoS
ence at variousshould
temperatures
is givenmagnetic
in Fig. 2(b). The
value
So irradiation
affect
properties
significantly.
fruitful outputs
2
are expected in irradiated MoS2 few-layers.
7. Conclusions and Outlook
FIG. 2. (a) M vs H curve at various temfrom 400 K (black), 95 K
Semiconducting MoS2 2D layers have unique transportperatures
properties
have
been applied in
(blue),
and 10 K (red) forand
a pristine
and
irradiated MoS at a fluence of 5 10
various devices. Their band-gap, doping, photonic, electric,
electronic,
chemical, and
ions/cm
. An enlarged viewmagnetic,
of the M-H
isotherms near the origin is shown in the
bio-properties can be tuned effectively and simply by defect engineering.
exposures,
inset. The variation Various
of coercivity vsirradiation
ion
fluence is shown in (b). (c) Zero field
and
field
cooled
(ZFC
and
FC)
magnetincluding swift-heavy ions, argon ions, alpha particles, protons, electrons, electromagnetic waves, can
ization vs temperature measurements in
applied field ofproperties
500 Oe for an irradi-essentially. Various
significantly induce defects in MoS2 few-layers to manipulate an
various
ated MoS sample at a fluence of
10 ions/cm . The inverse of the
defects, mainly sulfur vacancies and molybdenum vacancies, 5estimated
have
been
generated/created in MoS2
magnetic susceptibility vs
temperature plot near T (900–950 K) is
layers. MoS2 layers can be significantly activated, functionalized,shown
and
modified. The irradiation-induced
in the inset. 2(d) The value of
at 300 K with H ¼ 5 kOe
defects are beneficial for several applications including: solar magnetization
cells
[19,194–196],
batteries [54,197,198],
as a function of ion fluence from
1 10 ions/cm to 5 10 ions/cm .
supercapacitors [199], thin film transistors [15,16,80,86,110,138,200–205], sensors [17,55,56,106,206–210],
hydrogen generators [8,47,50,69,134,176,177,183,211,212], and applications in thermoelectrics [213–216],
piezotronics [63], valleytronics [65], and environments [45,49]. MoS2 2D layers were damaged
simultaneously under the irradiation and lost their unique properties. Therefore, it is critical to
control the irradiation exposure to tune MoS2 properties by the irradiation-induced defect engineering
while avoid potential damages.
Besides MoS2 2D layers, the irradiation techniques can be employed to tune physical properties
of other disulfide 2D materials, such as NiS2 urea-electrocatalysts [217], WS2 hybrid catalysts [218],
as well as selenide water-electrocatalysts [219], perovskite electrocatalytic nanoparticles [220], C3 N4
catalytic materials [221], hydroxide nanosheet battery-electrodes [222], oxide photocatalysts [223],
low-dimensional thermoelectric materials [224,225], graphene and carbon nanotubes [226–228], and
even porous electrodes [229,230].
18
2
2
18
2
2
c
17
2
18
2
Downloaded 08 Sep 2012 to 139.184.30.132. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions
Author Contributions: G.Z. and Y.L. conceived the idea, designed and went through with the manuscript.
H.D. wrote the section of alpha-particle irradiation. N.T. and Y.L. wrote the section of UV irradiation. M.G. and
Y.L. wrote the section of proton irradiation. A.L. was in charge of the section of magnetic properties. Q.P. took
charge of the theoretical analysis on irradiation. C.W. summarized background of MoS2 materials. J.-A.Y. was in
charge of Raman scattering. G..Z. and Y.L. wrote other sections. All authors provided constructive comments on
the manuscript.
Appl. Sci. 2019, 9, 678
41 of 53
Funding: Y.L. is partially supported by the Army Research Laboratory under Cooperative Agreement Number
W911NF-12-2-0022. A.L. is thankful to the financial support from the National Science Foundation through
research grant NSF DMR 1206380. C.W. thanks the financial support from the National Natural Science
Foundation of China (grants 51502099), the National Key Research and Development Program of China (Grant
No. 2017YFE0120500), and the Hubei “Chu-Tian Young Scholar” program.
Acknowledgments: The views and conclusions contained in this document are those of the authors and should not
be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory
or the U. S. Government. The U. S. Government is authorized to reproduce and distribute reprints for Government
purposes notwithstanding any copyright notation herein.
Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
The following abbreviations are used in this document:
2D
AFM
CBM
CVD
EBI
EBIT
EBL
EDS
EDX
EPMA
FET
FFT
FIB
FWHM
GANIL
HAADF
HER
HIM
HIRFL
HRTEM
IBAD
LEAF
MBE
NEMS
PAMBE
PL
RF
RT
SAED
SEM
STEM
STM
TMDC
TEM
TOF-SIMS
UV
UHV
VBM
VUV
XPS
XRD
two-dimensional
atomic force microscopy
conduction band minimum
chemical vapor deposition
electron-beam irradiation
electron-beam ion trap
electron-beam lithography
electron X-ray dispersive spectroscopy
energy-dispersive X-ray spectroscopy
electron probe micro-analyzer
field effect transistor
fast Fourier transformation
focused ion beam
full width at half maximum
The Grand Accélérateur National d’Ions Lourds (Large Heavy Ion National Accelerator)
high angle annular dark-field
hydrogen evolution reaction
helium-ion microscopy
heavy ion Research facility in Lanzhou
high-resolution transmission electron microscopy
ion-beam assisted deposition
low-energy accelerator facility
molecular beam epitaxy
nanoelectromechanical systems
plasma-assisted molecular beam epitaxy
photoluminescence
radio-frequency
room temperature
selected-area electron diffraction
scanning electron microscopy
scanning transmission electron microscopy
scanning tunneling microscope
Transition-metal dichalcogenides
transmission electron microscopy
time-of-flight secondary ion mass spectrometry
ultraviolet
ultra-high vacuum
valence band maximum
vacuum ultraviolet
X-ray photoelectron spectroscopy
X-ray diffraction
Appl. Sci. 2019, 9, 678
42 of 53
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