Supplementary Material
Enhanced field emission properties of doped
graphene nanosheets with Layered SnS2
Chandra Sekhar Rout,1*Padmashree D. Joshi,2 Ranjit V. Kashid,2 Dilip S. Joag,2 Mahendra A.
More,2 Adam J. Simbeck,3 Morris Washington,3 Saroj K. Nayak,1, 3*Dattatray J. Late4*
1
2
3
School of Basic Sciences, Indian Institute of Technology, Bhubaneswar751013, India
Center for Advanced Studies in Material Science and Condensed Matter Physics, Department of
Physics, University of Pune, Pune 411007, India.
Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy,
New York 12180, USA
4
Physical & Materials Chemistry Division, National Chemical Laboratory, Pashan Road, Pune
411008, India
*Corresponding
*
authors:
csrout@iitbbs.ac.in
(C.S.R),
nayaks@rpi.edu
(S.K.N),
and
dj.late@ncl.res.in
/
datta099@gmail.com (D.J.L.)
Keywords: Layered materials, Graphene, SnS2, Doping, Field emission, Density functional
theory.
1
EXPERIMENTAL METHODS
(a) Preparation of SnS2 sheets:
SnS2 sheets were synthesized by a one-step hydrothermal reaction. In a typical reaction
process, 3 mMNa2SnO3.3H2O (Sigma-aldrich, 99%) and 15 mM
thioacetamide (C2H5NS,
Sigma-Aldrich, ≥ 99%)were dissolved in 40 mL DI water and stirred for 1 hour at room
temperature by using a magnetic stirrer. The solution was transferred to a 50 mL stainless steel
autoclave, heated up to 200⁰C, and kept for 24 hours. After cooling naturally, the product was
filtered, washed with DI water, and dried in vacuum at 60⁰C for 6 hours.
(b) Preparation of SnS2/RGO composites:
The SnS2/RGO composite was synthesized by the same hydrothermal reaction condition
as that forSnS2nanosheets. 8 mL of 5 mg/mL GO solution was added to the mixture of
Na2SnO3.3H2O and thioacetamide, and the total volume of the solution was maintained at 40
mL. The same processes mentioned for the SnS2 sheets were followed. The GO solution
synthesis was performed by a modified Hummer’s method as reported earlier.1,2-4
(c) Characterizations:
The samples were characterized with X-ray diffraction [XRD (Rigaku, Minislex model)]
equipped with the following: Ni filtered Cu K radiation (40 kV, 100 mA, λ= 0.15418 nm),
scanning electron microscopy [SEM (FEI ESEM QUANTA 200 3D)], and high resolution
transmission electron microscopy [HRTEM (FEI TECNAI TF-30)]. The samples were also
characterized by a Micro Raman spectrometer (Lab RAM HR Raman microscope) with a laser
excitation wavelength of 514.5 nm and laser power of 1 mW.
2
(d) Field emission:
The field emission studies of few-layered SnS2nanosheetsand the SnS2/RGO
nanocomposite were investigated independently in an ultra-high vacuum (UHV) chamber at the
base pressure of ~1x10-8 mbar as described earlier.1
A modified F-N equation31,32 is used for the analysis of field emission current density versus
applied electric field data from the F-N plot for SnS2 and SnS2/RGO composite. The modified
F-N equation is as follows,
𝐽 = 𝜆𝑀 𝑎𝜑
𝑏𝜑 3⁄2
𝐸 𝛽 𝑒𝑥𝑝 (−
𝑣 )
𝛽𝐸 𝐹
−1 2 2
… (1)
Where, λM macroscopic pre-exponential correction factor, J is the current density, E is the
applied average electric field (surface field), a and b are constants (a = 1.54×10-6 AeV/V2, b =
6.83 eV-3/2Vnm-1), 𝜑 is the work funtion of the emitter,  is the field enhancement factor, and υF
(correction factor) is a particular value of the principal Schottky-Nordheim barrier function υ.
The field enhancement factor is calculated from the slope (m) of F-N plot and work function
(𝜑) of emitter (calculated from DFT which is discussed below), using following equation,
𝛽=
(−6.8 × 103 )𝜑3⁄2
𝑚
(2)
COMPUTATIONAL METHODS
To model the SnS2/RGO structure, free-standing SnS2, free-standing graphene, and a
SnS2/graphene composite system were considered (Figure 5) within first-principles density
DFT.3,4 The DFT based Vienna abintio simulation package (VASP)7-10 was employed with the
projector-augmented wave (PAW)11,12 method to describe the electron-ion interaction. The
3
exchange-correlation energy is described using the local density approximation (LDA).13 For
SnS2 (graphene) a plane-wave basis energy cutoff of 350 eV (600 eV), a 12×12×1 (18×18×1) Γcentered k-point grid, and at least 10 Å of vacuum was necessary to see convergence in the total
energy on the order of 1 meV per unit cell. Furthermore, the partial occupancies of the orbitals
were described using the tetrahedral smearing method. For the SnS2/graphene composite system
a 12×12×1 k-point sampling was employed and at least 20 Å of vacuum is included. The
geometry and cell parameters were fully optimized for the SnS2 unit cell using the conjugate
gradient method where the total energy convergence threshold was10-4 eV and the maximum
force allowed on any atom was 0.01 eV/Å. The unit cells of each two-dimensional (2D) structure
were then multiplied and combined to form the composite supercell: 2×2 for SnS2 and 3×3 for
graphene, resulting in a total of 30 atoms (4 Sn, 8 S, and 18 C). In addition, the graphene cell
was compressively strained by 1.6% in order to match-up the lattice constants of the two layers.
The electronic character of graphene was preserved under this small amount of strain, consistent
with the literature,14 i.e. the Dirac cone is preserved and no gap is opened up, hence graphene
remains semi-metallic. No further lattice mismatch or rotational considerations were taken into
account for the supercell. Lastly, in order to account for spurious dipole interactions between
periodic images, dipole corrections wereconsidered,15 but such corrections were found to be
small (less than 1 meV per atom). The previous set of parameters and basic methodology has
proven to be reliable in past theoretical studies of metal dichalcogenide-graphene composites.1
4
References
1
C. S. Rout, P. D. Joshi, R. V. Kashid, D. S. Joag, M. A. More, A. J. Simbeck, M.
Washington, S. K. Nayak and D. J. Late, Sci. Rep. 3, 3282 (2013).
2
S. Ratha, C.S. Rout, ACS Appl. Mater. Interf. 5, 11427 (2013).
3
M. Thripuranthaka D. J. Late, ACS Appl. Mater. Interf.6, 1158 (2014).
4
M. Thripuranthaka, R. V. Kashid, C. S. Rout D. J. Late, Appl. Phys. Lett.104, 081911 (2014).
5
W. Kohn and L.J. Sham, Phys. Rev.140, 1133 (1965).
6
P. Hohenberg and W. Kohn, Phys. Rev. B 136, B864 (1964).
7
G. Kresse and J. Furthmuller,Comput. Mater. Sci.6, 15 (1996).
8
G. KresseandJ. Furthmuller, Phys. Rev. B54, 11169 (1996).
9
G. Kresse and J. Hafner, Phys. Rev. B47, 558 (1993).
10
G. Kresse and J. Hafner, Phys. Rev. B49, 14251 (1994).
11
P.E. Blöchl, Phys. Rev. B50, 17953 (1994).
12
G. KresseandD. Joubert, Phys. Rev. B59, 1758 (1999).
13
J.P. Perdew and A. Zunger, Phys. Rev. B23, 5048 (1981).
14
S. M. Choi, S.H. Jhi, and Y.W. Son, Phys. Rev. B81, 081407 (2010).
15
J. Neugebauer and M. Scheffler, Phys. Rev. B46, 16067 (1992).
5
400
200
0
498
495
492
489
486.55
486
Sn 3d
483
480
Binding energy (eV)
161.6
S 2p 3/2
Binding energy (eV)
168
Sn 3d5/2
494.98
Sn 3d3/2
Intensity (a.u)
C 1s
S 2s
S 2p
600
Sn 4d
Sn 3p
800
S 2p
162.7
S 2p 1/2
Intensity (a.u)
1000
Sn 3s
Intensity (a.u)
Sn 3d
Figure S1: XPS spectra of SnS2
166
164
162
160
158
Binding energy (eV)
6
400
200
162
486.5
Sn 3d5/2
492
489
486
483
480
C 1s
S 2p
C 1s
Intensity (a.u)
161.5
S 2p 3/2
164
495
Binding energy (eV)
162.6
S 2p 1/2
Intensity (a.u)
166
494.9
498
0
Binding energy (eV)
168
Sn 3d3/2
Intensity (a.u)
C 1s
S 2s
S 2p
600
Sn 4d
Sn 3p
800
Sn 3d
284.5
1000
Sn 3s
Intensity (a.u)
Sn 3d
Figure S2: XPS spectra of SnS2/RGO
160
Binding energy (eV)
158
294 292 290 288 286 284 282 280
Binding energy (eV)
7
Brief background of Research work on field emission investigations of Layered SnS2:
Graphene and its analogous layered materials have received tremendous attention of researchers
for their potential applications in energy storage, energy conversion, catalytic activity and
sensing devices.1-4 Attributed to the versatile coordination characteristics of tin (Sn) and sulfur
(S), SnS2 nanomaterials can exist in one-dimension (1D) chains to two-dimension (2D) sheets
and even three-dimension (3D) open frameworks and discrete molecular species.5-8 Oxidation
states of tin may vary from +4 to +2 and -2, -1, and 0 for sulfur. Various possible local
coordination geometries around the tin center vary from trigonal pyramidal to tetrahedral,
trigonal-bipyramidal and octahedral. In addition the geometry around sulfur can vary from
terminal, v-shaped to trigonal pyramidal.5-8 Depending upon the oxidation state, SnS2 exist in
two types of structures namely SnS and SnS2, which feature either close-packing or an open
structure decorated by regular arrays.5 The SnS2 commonly referred to as mosaic gold, is a
layered material with a peculiar Cadmium iodide (CdI2)-type (trigonal-1T) structure with a
pseudo-hexagonal unit cell (a = 0.36486 and c = 0.58992 nm) consisting of a S-Sn-S triple layer,
in which the individual triple layers are held together by the van der Waals interactions.7 The
SnS2 layer can be viewed as composed of all-edge sharing octahedral SnS6 building units with
the sulfur atoms exhibiting three-fold coordination and local trigonal-pyramidal symmetry.5
The SnS2 layered materials has attracted considerable attention in recent years because of
their visible band-gap of 2.2-2.35 eV, as well as being nontoxic, inexpensive and chemically
stable compound.5-7 Due to their superior electrochemical properties and high surface area, SnS2
nanosheets9,10 and their composites with carbon nanotubes11 and graphene12,13 have been widely
investigated for Li-ion battery application. The high surface area and high percentage of
8
disordered surface atoms in chemically synthesized SnS2 nanosheets has enabled them to act as
visible-light driven photocatalysts,6,14 high response photodetector, gas sensors and other
optoelectronic nanodevices.14-16 Also, SnS2 nanosheet membranes have recently been employed
for fabricating field-effect transistors and other logic devices.17,18 SnS2 with its wide range of
applications to technology has even been fascinated as a possible cold cathode material.
Field electron emission is the ejection of electrons by quantum mechanical tunneling
from a conducting/semiconducting emitter upon application of a very high field (~10 6V/cm). In
view of their applicability as cold cathodes, metallic emitters are known for their inherent
tendency to deliver high current densities whereas semiconductors are known for stable
operation in the saturation regime. Reports on various semiconducting field emitters exhibit
comparatively lower current densities though (due to limited conductivity); hence, researchers
are trying to explore field emission characteristics from other superior semiconducting field
emitters. Recently efforts have been put forward by various groups for improving the electrical
conductivity by preparing metal-semiconductor composites.19 Researchers are now interested in
exploring the field emission properties of 2D nanostructures due to their planar structure, which
is ideal for flat/planar device technology such as electron sources in flat panel displays. Layered
materials like graphene,19-23 its composites24 and RGO25 are well established 2D field emission
cathodes and have potential application in field emission based cathodes. Promising field
emission properties of 3D MoS2 nanoflowers like morphology and few layer MoS2 nanosheets
has also been reported.26 We have recently reported field emission studies on WS2 and a
WS2/RGO composite.27 The study revealed that WS2/RGO nanocomposites show superior field
emission performance in comparison with their semiconducting counterparts due to overlapping
9
of structure.27 This study has inspired us to further investigate field emission performance in
different chalcogenides and their composite with RGO.
10