Tunable Emission from Two-dimensional
Semiconductor with Platelet Optical Antennas
by
Yingyi Yang
Submitted to the Department of Mechanical Engineering
in partial fulfillment of the requirements for the degree of
Master of Science in Mechanical Engineering
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2
Tunable Emission from Two-dimensional Semiconductor with
Platelet Optical Antennas
by
Yingyi Yang
Submitted to the Department of Mechanical Engineering
on Aug 30, 2015, in partial fulfillment of the
requirements for the degree of
Master of Science in Mechanical Engineering
Abstract
The remarkable properties of two-dimensional semiconductors allow for abundant research and applications in electronic and optoelectronic area. Monolayer transition
metal dichalcogenides (TMDC), such as MoS 2 , are direct bandgap semiconductors
which hold promises in valley-based optoelectronic applications or light-emitting devices. However, the weak light-matter interaction in this atomically thin slab leads
to low absorbance (- 3%) and quantum yield (~ 10-3). Although the quantum yield
of monolayer MoS 2 is higher compared with its few-layer counterparts, it is still significantly lower than that would be expected for a direct-gap semiconductor. Here,
we explore the possibility of tuning the spontaneous emission of monolayer MoS 2 by
coupling it with plasmonic platelet antennas, which can both convert freely propagating light into nanoscale and transmit radiation power into free space efficiently. The
antennas used are single crystalline and the plasmonic modes are obtained both by
near-field imaging and numerical simulation. The tunability of photoluminescence by
nearby antenna is analyzed in details. This thesis proposes an economic and promising way to tune the emission of low-quantum-yield emitters, such as MoS 2 , while
preserving both A and B exciton peaks. This ultrathin structure can facilitate the
development of on-chip emitters and valley-based devices.
Thesis Supervisor: Nicholas X. Fang
Title: Associate Professor
3
4
Acknowledgments
I would like to sincerely express my gratitude to Professor Nicholas X. Fang for his
inspiration and support in my research and academic study during the past two years
at MIT. His patient guidance was important for the development of my project. I
would also like to thank Matt Klug, who initiated the synthesis of platelet antennas
and introduced chemical bench work to me, Anshuman Kumar, who helped me with
physical concepts and offered suggestions at critical moments, Yoon Kyung (Eunnie)
Lee, who helped me with simulations and shared her positivity. I thank Steven Kooi
and William DiNatale at ISN for their help in the experimental setup, and Dr. Dafei
Jin, who tested my samples in our collaborator's lab. I would also like to thank Chu
Ma and Zheng Jie Tan, who shared the moments of happiness and hardships, and Dr.
Jun Xu, Dr. Qing Hu, Dr. Sam Hoon Nam, for their help in the lab. Apart from the
names mentioned above, I am thankful to visiting professor Xiang Xiong, who shared
his knowledge in optics and his optimism towards life.
This work is made possible through the financial support by the NSF (grant
CMMI-1120724) and AFOSR MURI (Award No. FA9550-12-1-0488).
5
6
Contents
1
Introduction
11
1.1
11
M otivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.1
Emerging optoelectronic applications of two-dimensional semiconductors........
1.2
2
3
.............................
11
1.1.2
Weak light-matter interaction in two-dimensional semiconductors 14
1.1.3
Wide applications of optical antennas . . . . . . . . . . . . . .
16
Thesis outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
1.2.1
Preparation and optical properties of MoS2 flakes . . . . . . .
19
1.2.2
Silver single crystalline platelet antennas . . . . . . . . . . . .
19
1.2.3
Tuning the emission of monolayer MoS2 by platelet antennas
19
Preparation and optical properties of MoS 2
21
2.1
Exfoliation of few-layer MoS 2 flakes . . . . . . . . . . . . . . . . . . .
21
2.2
Optical properties of MoS 2 . . . . . . . . . . . . . . . . . . . . . . . .
24
2.3
Transfer MoS 2 onto different substrates . . . . . . . . . . . . . . . . .
28
Silver single crystalline platelet antennas
3.1
31
Synthesis of colloidal silver nanoplates
. . . . . . . . . . . . . . . . .
31
3.1.1
Why colloidal silver nanoplates
. . . . . . . . . . . . . . . . .
31
3.1.2
Direct reduction and etching . . . . . . . . . . . . . . . . . . .
34
3.1.3
Polyol process . . . . . . . . . . . . . . . . . . . . . . . . . . .
36
3.1.4
Seed-mediated growth
40
. . . . . . . . . . . . . . . . . . . . . .
7
3.2
3.3
High-order plasmon modes in silver triangular platelet by Electron
Energy-Loss Spectroscopy (EELS) . . . . . . . . . . . . . . . . . . . .
43
3.2.1
STEM-EELS acquisition and EELS maps . . . . . . . . . . . .
43
3.2.2
EELS mapping of plasmon modes in individual triangular antenna 45
Finite-difference time-domain (FDTD) simulation of high-order plasmon modes in triangular antennas . . . . . . . . . . . . . . . . . . . .
4 Tuning the emission of MoS 2 by platelet antennas
4.1
4.2
47
55
Antenna-emitter coupled system for tunable photoluminescence
. . .
55
4.1.1
Horizontal dipole coupled with spherical antenna
. . . . . . .
56
4.1.2
Model MoS 2 coupled with disc antenna . . . . . . . . . . . . .
59
4.1.3
Experimental verification of photoluminescence enhancement .
66
Time-resolved spectroscopy and preliminary experimental results . . .
72
4.2.1
Time-resolved spectroscopy
72
4.2.2
Time-resolved photoluminescence of monolayer MoS 2
. . . . . . . . . . . . . . . . . . .
. . . . ..
5 Summary and outlook
74
77
5.1
Sum m ary
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
77
5.2
O utlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
79
8
List of Figures
1-1
Atomic representation of TMDC structure. (a) Schematic representation of typical TMDC structure (b) Atomic structure of hexagonal
symmetry from Ref [2]
. . . . . . . . . . . . . . . . . . . . . . . . . .
12
1-2
Simplified electronic band structure for MoS 2 from Ref [22] . . . . . .
12
1-3
An ultrasensitive monolayer MoS 2 photodetector. (a) Schematic view
of monolayer MoS 2 photodetector (b) Time-resolved photoresponse of
the device from Ref [11]
1-4
. . . . . . . . . . . . . . . . . . . . . . . . .
13
Monolayer MoS 2 LED and light emission characteristics of the device
from [261 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
1-5
Atomic and electronic structure of monolayer and bilayer MoS 2 [281
15
1-6
Absorption and photoluminescence of MoS 2. (a) Absorption spectra
.
for MoS 2 from Ref [27] (b) PL spectra and quantum yield of MoS 2 from
Ref [22]
1-7
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15
Wide range applications of optical nanoantennas. (a) Germanium photodetector enhanced by dipole antenna [501 (b) Single-molecule fluorescence enhancements by a bowtie nanoantenna
[39]
(c) Near-field Ra-
man imaging of single-walled carbon nanotubes [55] (d) Unidirectional
emission of quantum dot coupled to nanoantenna
1-8
. . . . . . . . .
17
Antenna enhanced transmission efficiency from the transmitter to the
receiver (adapted from [441)
2-1
[47]
. . . . . . . . . . . . . . . . . . . . . . .
18
. . . . . . . . . . . . . . . .
21
Trilayer stack for better optical contrast
9
2-2
Optical contrast pf the trilayer stack. (a) Contrast versus illumination
wavelength and SiO2 thickness (b) Contrast at 550nm illumination
.
22
2-3
Optical micrograph . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
2-4
AFM characterization. (a) AFM image (b) Height profile . . . . . . .
23
2-5
Band structure of MoS 2 from [21].
(a) Bulk MoS 2 (b) Quadrilayer
MoS 2 (c) Bilayer MoS 2 (d) Monolayer MoS 2 . . . . . . . . . . . . . .
2-6
24
Schematic diagram of confocal Raman spectroscopy (Courtesy from
http://www.chm.bris.ac.uk/pt/diamond/stuthesis/chapter2.htm)
. .
25
2-7
Crossover from indirect to direct band gap . . . . . . . . . . . . . . .
26
2-8
PL curves for MoS 2 with different number of layers
. . . . . . . . . .
26
2-9
Raman spectra of MoS 2
. . . . . . . . . . . . . . . . . . . . . . . . .
27
2-10 Frequency difference identification of number of layers . . . . . . . . .
28
2-11 Transfer graphene onto hBN flakes from Ref [641 . . . . . . . . . . . .
29
2-12 The method I use to transfer MoS 2 onto different substrates
. . . . .
29
. . . . . . . . . . . . . . . . .
30
2-13 Transfer MoS 2 from Si0 2 /Si onto gold
3-1
Localized surface Dlasmon resonance (LSPR). (a) Schematic illustration of LSPR (Courtesy from https://gregemmerich.wordpress.com/2012/11/16/surfaceplasmon-resonance-technology-overview-and-practical-applications/)
((b)
Enhancement of local field (Simulated by FDTD) . . . . . . . . . . .
3-2
Permittivities and quality factors of different metals.
32
(a) Real and
imaginary parts of permittivity (b) Quality factor of LSPR for a metal/air
interface from Ref [491
3-3
. . . . . . . . . . . . . . . . . . . . . . . . . .
32
Summary of the shapes, LSPR absorption peaks, applications and
methods for synthesis of Ag nano structures 149] . . . . . . . . . . . .
33
3-4
HRTEM images of silver nanoplates made by direct reduction and etching 35
3-5
Resonance of the silver nanoplates made by direct reduction and etching. (a) Absorbance of the solution prepared by adding different amount
3-6
of H 2 0 2 (b) Different colors of the as-prepared solution . . . . . . . .
35
HRTEM images of different reaction time (a) 9h (b) 21h (c) 30h . . .
37
10
3-7
Characterization of Ag nanoplates solution after 9h reaction time. (a)
UV-Vis spectrum (b) FDTD simulation of the extinction cross section
of the particle in the air and water (edge=100nm, thickness=12nm)
3-8
.
38
Characterization of Ag nanoplates solution after 21h reaction time. (a)
UV-Vis spectrum (b) FDTD simulation of the extinction cross section
of the particle in the air and water (edge=200nm, thickness=24nm)
3-9
.
38
Remaining particles after centrifugation . . . . . . . . . . . . . . . . .
39
3-10 Improved yield with white light illumination.
(a) Particles formed
when light is totally blocked (b) Particles formed under high-power
white light illumination . . . . . . . . . . . . . . . . . . . . . . . . . .
39
3-11 Formation of anisotropic silver nanoplates. (a) Formation of hexagonal
plate (b) Formation of triangular plate (inset: SAED pattern)
3-12 TEM imaging.
. . . .
40
(a) Schematic illustration of TEM imaging and (b)
Electron diffraction (Courtesy from Internet) . . . . . . . . . . . . . .
41
3-13 Characterization of silver nanoplates synthesized by seed-mediated growth.
(a) High-concentrated silver nanoplates (b) Silver nanoplates dried
down on the TEM grid . . . . . . . . . . . . . . . . . . . . . . . . . .
42
3-14 UV-Vis spectrum of the absorbance of Ag nanoplates (inset: final solution) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
43
3-15 Illustration of STEM-EELS imaging. (a) Schematic representation of
STEM-EELS from Ref [90] (b) Typical EELS spectrum from Ref [67
3-16 Data acquisition in STEM-EELS from Ref [90] . . . . . . . . . . . . .
44
45
3-17 EELS maps of plasmon modes in single triangular antenna (credit: Dr.
Ritesh Sachan)
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-18 FDTD simulation setup
. . . . . . . . . . . . . . . . . . . . . . . . .
46
48
3-19 Optical resonance of triangular antenna with edge length 200nm and
height 40nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
49
3-20 Plasmon eigenmodes of 200nm triangular antenna: dipole mode (697nm)
and quadrupole mode (410nm) (upper: total field; lower: z-component
of the field) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
50
3-21 Optical resonance of triangular antenna with edge length 400nm and
height 40nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-22 Plasmon eigenmodes of 400nm triangular antenna:
51
1100nm, 550nm
and 458nm (upper: total field; lower: z-component of the field) . . . .
51
3-23 Optical resonance of triangular antenna with edge length 600nm and
height 40nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
52
3-24 Plasmon eigenmodes of 600nm triangular antenna: 1505nm, 730nm,
596nm and 490nm (upper: total field; lower: z-component of the field)
4-1
Quantum yield enhancement for emitters with different intrinsic quantum yields . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-2
59
Scattering of a single disc in the air (diameter=80nm, thickness=6nm)
and the photoluminescence of MoS 2 . . . . . . . . . . . . . . . . . . .
4-4
58
Emission enhancement. (a) Emitters with different intrinsic quantum
yields (b) Emitter with q0 = 1 . . . . . . . . . . . . . . . . . . . . . .
4-3
53
Simulated field enhancement.
60
(a) Representative field enhancement
when gap=6nm (b) FDTD simulation setup . . . . . . . . . . . . . .
61
4-5
Classical picture of dipole oscillation
62
4-6
Representative FDTD simulation for quantum yield enhancement
4-7
Simulated photoluminescence enhancement (no spacer layer). (a) Av-
. . . . . . . . . . . . . . . . . .
. .
64
erage field enhancement under the antenna (b) Average radiative and
nonradiative decay rate enhancement (c) Average quantum yield enhancement (d) Theoretical PL enhancement
4-8
. . . . . . . . . . . . . .
Simulated radiative and nonradiative power with and without spacer
layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-9
66
67
Simulated photoluminescence enhancement (with A12 0 3 as spacer layer).
(a) Average field enhancement under the antenna (b) Average radiative
and nonradiative decay rate enhancement (c) Average quantum yield
enhancement (d) Theoretical PL enhancement . . . . . . . . . . . . .
12
68
4-10 Measured photoluminescence enhancement for different A1 2 0
3
thick-
ness. (a) 6nm (b) 8nm (c) 10nm (d) 16nm (e) 18nm (f) Ratio of PL
intensity (black dots: PL intensity enhancement; red dots: average PL
intensity enhancement) . . . . . . . . . . . . . . . . . . . . . . . . . .
4-11 A and B exciton peaks fitting using Lorentz model y = yo+2A 4_
69
2+2
(dash line) before (solid black line) and after (solid red line) PL enhancem ent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
71
4-12 PL curves when the platelet antennas are in direct contact with monolayer M oS 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
.
71
4-13 Working principle of pump-probe setup. (a) Degenerate pump-probe
setup (b) An illustration of differential transmission and reflection at
a certain time delay using lock-in detection [1021 . . . . . . . . . . . .
73
4-14 Operating principles of a streak camera [1031 . . . . . . . . . . . . . .
73
4-15 Time-resolved PL measurement. (a) Experimental setup (b) Measured
point spread function (PSF) of the system (c) Measured data (black
dots) and fitted curve (red solid line) (The red curve was obtained by
t-tp
setting the time-resolved signal as y = Ae- TO , and then convolved
with PSF curve. The fitting parameters are A, to and To.)
(d) Real
time-resolved PL signal . . . . . . . . . . . . . . . . . . . . . . . . . .
13
75
14
Chapter 1
Introduction
1.1
1.1.1
Motivation
Emerging optoelectronic applications of two-dimensional
semiconductors
The remarkable properties of two-dimensional materials-flexibility, transparency, strengthallow for abundant research and applications in electronic and optoelectronic area
[1, 2, 3] . Transition metal dichalcogenides (TMDCs) are layered materials with strong
intralayer bonding and weak interlayer bonding, which enables them to be exfoliated
into two-dimensional layer with thickness on the order of a single atom
[4].
Compared
with graphene, TMDCs such as MoS 2 , MoSe 2 , WS 2 and WSe 2 have band gaps that
change from indirect to direct in monolayer [5], providing significant advantages in
optoelectronic applications including photodetectors, sensors, photovoltaic and light-
emitting devices [6, 7, 8, 9, 10, 11, 12, 13, 14]. Flexible optoelectronic devices holds
promise in wearable electronics and transparent displays [7, 15, 16, 17]. The flexibility
also results in the tunability of bandgap [18, 19, 20]. Their ability to absorb and emit
light is directly affected by their electronic band structure. For direct bandgap semiconductor, they can readily absorb photons with energy greater than the bandgap
energy, and emit photons with energy corresponding the the bandgap. On the other
hand, the absorption and emission should be assisted by phonon in indirect bandgap
15
semiconductor, because extra momentum is needed for optical transition in different
valleys. In terms of MoS 2 , the crossover from indirect to direct bandgap is observed
in changes in absorption and photoluminescence [21, 22, 23]. This shift leads to 104
times increase in the quantum yield [22].
(a)
(b)
r
*AS
'S
Jo
214
*Ma
Figure 1-1: Atomic representation of TMDC structure. (a) Schematic representation
of typical TMDC structure (b) Atomic structure of hexagonal symmetry from Ref [2]
H
K
I
H
A
C1
E3
V2
Figure 1-2: Simplified electronic band structure for MOS 2 from Ref f22]
Their direct bandgap in the visible range makes TMDCs attractive as the lightabsorber in photovoltaic devices and photodetector [24, 11]. The conduction and
valence-band edges of several TMDCs are compatible with typical electrodes, so the
16
electrons can transport across the Schottky barrier in the heterojuction under biased
voltage [2].
(b)(b A60
(a)
____________
IVM--70V
V'so
VSV
0
M0
200
TV". (s)
200
40
Figure 1-3: An ultrasensitive monolayer MoS 2 photodetector. (a) Schematic view of
monolayer MoS 2 photodetector (b) Time-resolved photoresponse of the device from
Ref [11]
An ultrasensitive monolayer MoS 2 photodetector with high photoresponsibility is
shown in Figure 1-3 [11]. When the device is illuminated by photons with energy larger
than the bandgap, the light is absorbed and electron-hole pairs are thus generated.
They can be extracted by applying a drain-source bias, resulting in the formation
of photocurrent.
The device shows high photoresponsibility because of the direct
bandgap, and low dark current due to the presence of the bandgap. In addition,
photodetection of different wavelengths, i.e. detection of green light with single- or
double-layer, and detection of red light with trilayer, can be achieved by using MoS 2
of different layers [8].
Electroluminescence (EL) and photoluminescence (PL) are two important applications of MoS 2 [25, 21]. Photons are emitted in response to electrical field in EL. This
mode works in light-emitting diode (LED). On the other hand, photons, typically of
higher energy, are absorbed and re-emitted in PL. Similarly, the direct bandgap of
monolayer MoS 2 makes it an ideal candidate as active light-emitting layer. A monolayer MoS 2 LED with a vertical p-n junction has been demonstrated in Figure 1-4
[261. The heterostructure composes of n-type MoS 2 and p-type silicon, with the former serving as electron injection layer and the latter serving as hole injection layer.
17
The electron-hole pairs can radiatively recombine in the junction under forward bias.
The presence of A- peak originates from the tightly bond trions in MoS 2 [27]. This
heterojunction diode can also operate as photovoltaic device, in which the process is
reversed in that light is absorbed and photocurrent is generated.
2000
eunited light
A
-
n-type
monolayer
10000
eo
wu
65o
;;7o
seo
W&"Iesth (sm)
Figure 1-4: Monolayer MoS 2 LED and light emission characteristics of the device
from [261
An interesting property of TMDCs such as MoS 2 is their spin, orbit and valley
interactions [28, 29, 30, 31]. Monolayer MoS 2 belongs to D3h point group which lacks
inversion symmetry as can be seen from its atomic structure in Figure 1-5, while
bilayer MoS 2 is centrosymmetric [32]. This leads to a strong spin-orbital splitting
of valence band maximum at K valley, and the resulting two PL peaks are called A
and B excitons. Moreover, the spin degeneracy is lifted at the band edge. Therefore,
A exciton can be selectively excited under circularly polarized light, which provides
the additional momentum needed to satisfy the optical selection rule [28, 29j. This
effect is stable thanks to the large spin-orbital interaction, which is quite distinctive
in TMDCs and may lead to many promising applications, such as transistor [33, 34].
1.1.2
Weak light-matter interaction in two-dimensional semiconductors
In the previous section, TMDCs are shown to be promising candidates in optoelectronic applications. However, for optoelectronics and photonics applications, lightmatter interaction is a critical issue because the typical interaction length is smaller
18
a
wa
d
Mo
b
V
M -V2
K
M
v32
.. -V2
r
-/2
V
-- -- -a
K'
K
Decomied waty aMd sowi
Spin rivilain
opt
I
Coupled valeyv- Win excionic absorUon
C
W12
W2
-
IV2
W2
B
A
A
4
K
KK
Figure 1-5: Atomic and electronic structure of monolayer and bilayer MoS 2 [28]
than the wavelength of light. Atomically thin nature of monolayer thickness poses
a challenge due to the weak light-matter interaction [351. From the reported experimental results in Figure 1-6, we can that both absorption [27] and quantum yield [22]
.
are poor in monolayer MoS 2 slab
(a)
A
0.08
40
(b)
-
S400
300
0.06-
200
a
-1001
4
6
4
-wla
0
-
0.02
-"m1y
-80 -40 0 40 80
Gate voltage (V)
0.00'
1.6
1.7
2.0
1.9
1.8
Photon energy (eV)
2.1
2.2
1.4
1.8
1.6
Phoon Energ (WV)
2.0
22
Figure 1-6: Absorption and photoluminescence of MoS 2 . (a) Absorption spectra for
[22]
MoS 2 from Ref [27] (b) PL spectra and quantum yield of MoS 2 from Ref
The absorbance normalized by the number of thickness shows that monolayer
part of refractive
MoS 2 has quite low absorption. This is intuitive due to the imaginary
19
index, which leads to the exponential decay of field in MoS 2. On the other hand,
although the quantum yield of monolayer MoS 2 is higher compared with its few-layer
counterparts, it is still significantly lower, i.e. up to 10-3, than that would be expected
for a direct-gap semiconductor. It should be mentioned that the quantum yield can
be improved if the sample is suspended or transferred onto hBN flakes [28].
Given this inherent drawback in this atomically thin slab, I use optical antennas
to boost the light-matter interaction. They are widely used to enhance the photoemission of the nearby emitters [36, 37, 38, 39, 40, 41, 42, 431. These antennas can
convert freely propagating electromagnetic waves into localized energy, typically on
nanometer scale [44, 45, 46], which is compatible with the thickness of MoS 2 flakes.
Furthermore, they can also serve as transmitters to direct the radiative wave [47, 48].
In general, these antennas achieve the manipulation of optical field in a nanometer
scale. Nobel-metal nanostructures can serve as antennas in the visible range due to
to the collective oscillation of electrons in the conduction band in resonance with optical field [49]. Among all types of metallic nanostructures, I use silver nanoplates as
antennas because of the following reasons: First, the resonance frequency is readily
tunable by changing the lateral size; second, the imaginary part of refractive index
is small for silver, resulting in negligible ohmic loss; third, the platelet geometry is
compatible with two-dimensional semiconductors.
I. .
Wide applications of optical antennas
Light can be focused by dielectric materials, which cannot focus light to area smaller
than half of the wavelength.
However, optical antennas with feature size on the
order of nanometers have many important applications because of the presence of
"hotspots". This property holds promise for improving the efficiency of photodetec-
tion [50], light emission [51, 52], biological and chemical sensing
[53],
energy harvest-
ing [541 and spectroscopy [551. The goal of optical antenna design is to optimize the
energy transfer between the localized source and free-space radiation.
In a general antenna problem consisting of a transmitter and a receiver, the antenna can be used to enhance the transmission efficiency, and the antenna efficiency
20
(b)' 1.500
(a)
0
(C)
0
40
W0
s0
S*
(d)
(b)
4
a
___0
____
2
Figure 1-7: Wide range applications of optical nanoantennas. (a) Germanium photodetector enhanced by dipole antenna [501 (b) Single-molecule fluorescence enhancements by a bowtie nanoantenna [39] (c) Near-field Raman imaging of single-walled
carbon nanotubes [55] (d) Unidirectional emission of quantum dot coupled to nanoantenna [471
can be represented as [441:
Prad _
P
Prad
Prad + Poss
where P is the total power dissipated by the antenna, Prad is the radiated power
and Pos, is the power dissipated through other channels, such as by absorption of the
antenna. Moreover, the transmission can also be improved by directing the power of
radiation [44], but this is not the focus of this thesis. Another important antenna
parameter is the antenna aperture, which is denoted by the absorption cross-section
np1 2
0 =
where uo is the cross-section of the dipole-like receiver, nr
(1.2)
is the unit vector in the
direction of the dipole absorption axis, EO is the incident field at the receiver and
E is the enhanced field when the receiver couples with the antenna
[44].
Take the
application of optical antennas in photodetector as an example, the optical antenna
21
Radiation
Antenna
Transmitter
Figure 1-8: Antenna enhanced transmission efficiency from the transmitter to the
receiver (adapted from [44])
can increase the absorption cross-section and hence the light flux on a detector, contributing to a high signal-to-noise ratio. Furthermore, the absorption depth limits
the thickness of the thin-film devices, while optical antennas can be used to reduce
the critical dimensions of the devices. This is particularly promising for photovoltaic
devices, which can have lower carrier collection length and impurity recombination
while keeping the same optical absorption [541.
Generally, optical antennas have become a critical tool in terms of light manipulation at the nanometer scale. In this thesis, I will mainly discuss how to incorporate
single crystalline platelet antenna with the emerging 2D semiconductor-MoS 2 , in order
to improve its photoluminescence and give full play to its strength in the application
of on-chip compact devices 1561.
1.2
Thesis outline
In this thesis, I will discuss about both the experimental results and the theoretical
calculation regarding controlling the emission of monolayer MoS 2 by optical platelet
antennas. The thesis is divided into three chapters. In the first and second chapter, I
will discuss the preparation of MoS 2 flakes and silver platelet antennas respectively,
as well as their optical properties. In the third part, I will build a model to study the
coupling between MoS 2 and antenna. The theoretical model can also be applied to
22
other dipole-emitter system to investigate the quenching or enhancement of PL.
1.2.1
Preparation and optical properties of MoS2 flakes
This section demonstrates the micro-mechanical exfoliation of MoS 2 flakes. Then the
thickness is measured by atomic force microscope (AFM), and the optical properties
are measured by Raman spectroscopy and micro-PL. The strong correlation between
the layered structure and the evolution of optical properties is discussed in details.
Furthermore, I have also tried to transfer MoS 2 onto other substrates which may
extend the application of MoS 2 in optoelectronic field.
1.2.2
Silver single crystalline platelet antennas
In this section, I will document several methods I used to synthesize single crystalline
platelet antennas. The advantages and disadvantages of these methods are shown
by both extinction spectra and transmission electron microscope (TEM) characterization. The plasmon modes in single crystalline triangular plates are mapped by
Electron Energy-Loss Spectroscopy (EELS). In the last subsection, the high-order
plasmon modes are generated by Finite-difference time-domain (FDTD) simulation.
1.2.3
Tuning the emission of monolayer MoS2 by platelet antennas
This section aims at obtaining the theoretical PL enhancement in MoS 2-antenna
system. The emission of MoS 2 is controlled by manipulating two processes: excitation
and emission. Separating excitation and emission factors has remained a challenge,
so their relative contributions are often studied by simulations [43]. I initially have
analytically investigated a simple model containing a horizontal dipole and a spherical
antenna. This basic scheme gives two pieces of information: first, the enhancement
and quenching of PL is governed by the dipole-antenna distance; second, the PL
of emitters with lower intrinsic quantum yield have larger potential to be enhanced
by the antenna nearby. The excitation rate enhancement is obtained by calculating
23
the field enhancement at the dipole location, while the quantum yield enhancement is
obtained by calculating the rate enhancement of different decay channels. In addition,
the tunability of PL is verified by experiments.
resolved PL results are also shown.
24
Finally, some preliminary Time-
Chapter 2
Preparation and optical properties of
MoS 2
2.1
Exfoliation of few-layer MoS 2 flakes
Similar to graphene, MoS 2 flakes can be obtained by mechanical exfoliation due to
the weak interlayer van der Waals force [4]. The bulk MoS 2 is bought from SPI, and
the scotch tape is from 3M. The exfoliated MoS 2 flakes are transferred onto a silicon
oxide substrate, which is cleaned with Piranha solution and rinsed with IPA and then
blow dried with N2. Here 300nm silicon thermal oxide wafer is used because it could
provide relatively good optical contrast. This can be confirmed by considering the
interference in a trilayer stack and normal incidence in Figure 2-1.
MoS2
SiO
Si
Figure 2-1: Trilayer stack for better optical contrast
The reflectance is [57, 58]
25
riei(01+02)
R
ei(1+02)
+
+
rir
where q 1 =imnidi and 02
r
2
e -(01
02)
2 e-i(01-02)
=
+
r 3 e i(o1 +2)
+
rir
+
rir
3 e-i(01+02) +
2 r 3 ei(1 -02)
r 2 r 3 ei(*1-02)12
(2.1)
2rin2 d2 are optical path difference in MoS 2 and SiO 2
layers. Since we compare the regions with and without MoS 2 flakes, the contrast can
be defined as
(2.2)
C - R(ni = 1) - R(ni)
R(n, = 1)
The contrast versus illumination wavelength and SiO 2 thickness is given in Figure
2-2.
.1)02
01
41.
910
1W
4W
Miasan'
W~2 ftk~noss(rrn)
Figure 2-2: Optical contrast pf the trilayer stack. (a) Contrast versus illumination
wavelength and SiO2 thickness (b) Contrast at 550nm illumination
Under white light illumination, it is reasonable to choose 300nm silicon oxide
substrate because 550nm wavelength is most sensitive to human eyes.
Figure 2-3
shows one representative optical microscopy image.
Based on this figure, the number of MoS 2 layers can be estimated. The light blue
region most likely corresponds to monolayer, while the whiter regions correspond to
few-layer flakes
[591.
The thickness of MoS 2 is further confirmed by atomic force microscopy (AFM)
working under tapping mode. Under tapping mode, there is attractive force between
the tip and the sample. Here the AFM is Dimension 3000 in ISN at MIT. Since the
26
Figure 2-3: Optical micrograph
thickness of monolayer MoS 2 is typically under 1nm, laser alignment is important.
The motion of the tip is adjusted delicately by a feedback loop. The height information is recorded by the color scale in Figure 2-4. The background noise has been
removed by low pass filtering.
log-
(b)
02
04
2
1,08
14
8
-1c0
Figure 2-4: AFM characterization. (a) AFM image (b) Height profile
The height profile shows that monolayer MoS 2 is around 0.8nm, which agrees well
with literature
159, 211.
Large-area MoS 2 can be grown by Chemical vapor deposition (CVD) [60, 61, 621,
27
m
which greatly improves the yield of high-quality monolayer samples.
2.2
Optical properties of MoS 2
The optical properties of MoS 2 attracts much attention not only because it has direct
band gap is in the visible range, but also arise from its relatively large spin-orbital spitting in the valence band maximum
[32]. Figure 2-5 is the calculated layer-dependent
band structure of MoS 2 [21]. The smallest energy transition is indicated by the arrow.
Bulk MoS 2 has an indirect bandgap. The direct transition at K valley has higher energy in few-layer MoS 2. With the reduced layer thickness, the direct transition almost
shows no change while the indirect bandgap becomes larger.
b
d
C
'
F
I
r
MK
r r
MK
r
r
mK
r
r
MK
r
Figure 2-5: Band structure of MoS 2 from [21]. (a) Bulk MoS 2 (b) Quadrilayer MoS 2
(c) Bilayer MoS 2 (d) Monolayer MoS 2
Confocal Raman spectroscopy is used to characterize the optical properties of
MoS 2 flakes. The setup shown in Figure 2-6 is Horiba LabRAM with 532nm excitation
laser. The pinhole aperture in front of the light source greatly reduces the laser spot
size on the sample. The size of the pinhole can be adjust accordingly. Another
pinhole aperture between the objective lens and the image plane rejects the rays
from out-of-focus points on the sample and works as a spatial filter. Both Raman
and photoluminescence (PL) signals can be detected by this setup. They correspond
28
to different frequency regions. Raman effect is a form of inelastic light scattering.
When the sample is illuminated by the laser, some vibration states will be excited
and the sample will be in a virtual energy state. Then the resulting scattered photon
can either have lower or higher energy than the incident photon. The former case
is designated by Stokes shift, while the latter case is designated as anti-Stokes shift.
Such signal is close to the original incoming laser signal because the phonon vibration
modes have significantly lower energy compared with photon energy.
On the other
hand, the PL signal is away from the incoming laser signal.
Beam splitter and notch filter
Pinhole
Gratig
opentre
Ught
Fabry- Perot etalon
opedre
CCD
detector
Sample
Pnh ole
Laser in
Objective
Somple
Figure 2-6: Schematic diagram of confocal Raman spectroscopy (Courtesy from
http://www.chm.bris.ac.uk/pt/diamond/stuthesis/chapter2.htm)
2
.
Figure 2-7 and 2-8 show the PL curves corresponding to different layers of MoS
It is clear that few-layer MoS 2 has three peaks. They are A exciton, B exciton and
indirect transition [22]. A and B excitons are direction transitions from conduction
band to the valence band at K valley. The indirect transition shows a blue shift when
the number of MoS 2 layers decreases [21]. Hence, for monolayer MoS 2 , the indirect
transition gap is larger than direct transition gap, and MoS 2 becomes direct band
gap. This makes its emission more efficient, as can be seen from its relatively higher
quantum yield [22].
29
-
-
-
Va.
4L
4*
46
I?
p.
I
miS
4.2
.4
00
liz
'I
II
4
m
mao 611
1;0
M
710
O
I
I
so5
Mlo
411
2L
I
Sm
12
goL
02
44
:4
0
64
?"
Wal
m in
"05
U 11 e M
am
A"
"a
11MI
Figure 2-7: Crossover from indirect to direct band gap
Bulk
3500
3L
2L
3000
CD
.
4000
!
SH
---
-
I
'4
11
2500
2000
1500
E
.a-
1000
500
a'
550
600
650
700
750
800
850
900
950
1000
Wavelength (nm)
Figure 2-8: PL curves for MoS 2 with different number of layers
30
The A exciton is at around 670nm, while the B exciton is at around 623nm. So the
spin-orbital splitting is about 140meV. For monolayer MoS 2 , the indirect transition
peak is absent, providing us a way to determine the number of layers [22].
Another reliable way to identify the thickness of MoS 2 is Raman spectroscopy [631,
which can characterize the lattice vibration modes in materials. Typically, when the
sample is optically excited by laser beam, there will be some certain phonon modes
which leads to inelastic scattering. This vibration energy is much lower than electron
transition energy, and the corresponding frequency can be determined by measuring
the energy difference between incident light and the collected light. This inelastic
scattering process will generate Raman shifts, which are like the fingerprint of the
material. This can be seen from the Raman spectra of MoS 2 in Figure 2-9.
Ag
bulk
2.0
4L
1.
2L
0-5
1L
0-0
360
370
360
390
400
410
420
430
Ran= shifcm')
Figure 2-9: Raman spectra of MoS 2
In the phonon spectra, E29 mode corresponds to in-plane lattice vibration, while
A 19 mode corresponds to out-of-plane vibration. When the number of layers is reduced, the out-of-plane vibration will be weaker due to the decreasing restoring force,
which arises from the interlayer van der Waals force. Monolayer sample has in-plane
1
vibration mode at around 385cm- 1, and out-of-plane mode at around 404cm- . The
31
frequency difference of these two modes in Figure 2-10 can also be used to determine
the number of layers. For monolayer, the difference is around 18cm-
1
to 19cm-1,
according to the literature [63].
26
26
24.
is
19
IS~
1
3
2
4
buk
Ttwcknes(L)
Figure 2-10: Frequency difference identification of number of layers
2.3
Transfer MoS 2 onto different substrates
Originally MoS 2 flakes are exfoliated onto silicon oxide substrate because it provides
better adhesion and contrast. But sometimes we need other substrates, such as hBN,
photonics crystal and gold. hBN is used to increase the quantum yield of MoS 2 [28],
.
and silver or gold can be used to investigate the coupling between plasmon and MoS 2
In this section, I will demonstrate the technique of transferring MoS 2 onto gold.
There are many ways to do the transfer. The key point is that we should add
a sacrificial layer, which enables MoS 2 to detach from the original substrate.
One
example to combine graphene and hBN flakes is demonstrated in Figure 2-11 [64].
In this paper, graphene is exfoliated onto PMMA layer, which is spin-coated onto a
water-soluble layer. After the stake is immersed in the DI water, graphene is detached
32
from the substrate, and then a new glass slide is used to carry the graphene/PMMA
layer.
Glass slide
.,PMMA
Water-soluble
2
layer
Graphene
S,/St02
Dl water
Floating
PMMA ;
,
BN
Graphene
(iv)
(0
Figure 2-11: Transfer graphene onto hBN flakes from Ref 1641
Here I use similar method to transfer MoS 2 flakes. But since exfoliation MoS 2
onto a polymer layer will degrade the contrast and thus add difficulty to the thickness
identification, I use the method shown in Figure 2-12.
PMMA
S1O2
spin coating
exfoliation
0A
NaOH
dissolve PMMA by
bath
new substrate
acetone
Figure 2-12: The method I use to transfer MoS 2 onto different substrates
In a transferring process, 200nm PMMA is spin-coated onto the MoS 2 /SiO 2 /Si
stack. Then the sample is immersed into a NaOH bath, and the solution heated at
33
80 degrees. After around 30min, the SiO 2 layer is etched away and the PMMA/MoS 2
will float on the solution. Then a new substrate is used to carry PMMA/MoS 2 , and
later PMMA is dissolved by acetone. The optical microscope images in Figure 2-13
show that a certain MoS 2 flake is successfully transferred onto gold substrate. We
can also see that the color of MoS 2 is different when it is on gold substrate, because
the change of refractive index affect the optical interference in this multilayer stack.
Figure 2-13: Transfer MoS 2 from SiO2 /Si onto gold
There are some points we need to concern about in the transfer. Firstly, there
should be strong adhesion between MoS 2 flakes and PMMA layer, so a post-bake is
needed after spin-coating. Secondly, there will be some PMMA residue after rinsing
the sample with acetone. One way to remove these residue is to use oxygen plasma
etching. In general, this technique works well. I have also tried HF etching, which
is not only inefficient but also dangerous. Since MoS 2 itself is mechanically flexible,
it can be transferred onto other flexible substrates, such as PET and PDMS, which
holds promise in flexible display and optoelectronic applications.
34
Chapter 3
Silver single crystalline platelet
antennas
3.1
3.1.1
Synthesis of colloidal silver nanoplates
Why colloidal silver nanoplates
Metallic nanostructures can serve as antennas to convert freely propagating light
into localized electric fields with nanometer precision. This arises from the collective
oscillation of conduction electrons in the metal nanostructure in resonance with the
incident light. With delicate control of the size and shape of the nanostructures, light
can be effectively manipulated at nanoscale based on different needs. This is known
as localized surface plasmon resonance (LSPR) [49, 65, 66].
Among other metals, i.e. Au, Cu, Li and Al, which can support surface plasmons in the visible and near-infrared regions, silver might be the most important one
because it offers many advantages.
A simple way to illustrate LSPR is Mie theory, which deals with the extinction
(absorption plus scattering) cross section of the nanostructure:
Cext = 24[
2
R 3 Em 3
A
1
(E, + 2Em)2 +
35
E,2
]
(3.1)
(b)
(a)Electroc
40
Figure 3-1: Localized surface plasmon resonance (LSPR). (a) Schematic illustration
of LSPR (Courtesy from https://gregemmerich.wordpress.com/2012/11/16/surfaceplasmon-resonance-technology-overview-and-practical-applications/)
((b) Enhancement of local field (Simulated by FDTD)
-
(a)A
30
~30-
-
25-
0-20
430
Ol35
-Si
2010
__
__
___
401
36o
I
-2-10.
o e0i~o7oo
s&0
4w0
WaveimnGo, (nm)
a00
_
0-3 D 460csoc7oS
am70
aim
waelnh (nm)
(b)
10
-YV-
CUa
-0Pt
0.1
200
400
am0
g0(
W"000ngt (nm)
1000
1200
Figure 3-2: Permittivities and quality factors of different metals. (a) Real and imaginary parts of permittivity (b) Quality factor of LSPR for a metal/air interface from
Ref [49]
36
where R is the radius and Em is the dielectric constant of the medium surrounding
the nanostructure. The strong resonance condition is satisfied when Er is close to
-2Em, and Ej is close to zero. This is only possible for metal. The dispersion curve of
permittivity in Figure 3-2 shows that silver is a good candidate for LSPR thanks to
its negative real part of permittivity and small imaginary part of permittivity, which
is less lossy compared with gold. Moreover, silver has higher quality factor over a
wide spectrum. All these denote that silver has less damping and stronger plasmon
resonance. Admittedly, in practical application the material properties should also
be taken into consideration. For example, silver can be easily oxidized in the air, so
sometimes gold is used for plasmonic applications.
Shape
Illustration
320 -450
Sphere and
quasi-sphere
LSPR 8
0
4o- 48o
Cube and
truncated cube
-o
aher on
Octahedon and
truncated octahedron
Method of Synthesis
Ref#
SERS; LSPR sensing Polyol process (single-crystal); 58, 85, 95, 330,
358
Citrate reduction (quasi-sphere)
assembly
214.37
Seedmediated growth
SERS; LSPR sensing
assembly
Pote
ro h
5 3 70
Polyol
process,
113
TO*
tetr
Appikcationsb
V
+lght-medliated
350- 450
SERS
Ught-mediaed Growth
400 - 500
Assembly
Polyo process
see-mediated
growth,
growth
105, 334,346,
364
Bar
350-900
SEAS
Poyol process
93
Spheroid
350 - 900
SEAS
Polyol proom
93,529
500-700
-
Polyol process
81,112
Polyol process
73
Seed-madiated growth.
light-mediated growth;
citrate reduction
101
Right
bipyramid
-
Beam
Decahedron
Electron transport
*
350.- 450
-
Wave guiding;
electronics. SEAS
assembly
Seed-mediated growth
17,47,63,83,98.
99,100,22. 2.44
256.329, 357
350 - 1000 SERS. LSPR sensing
Light-mediated growth;
polyoprocess
18,92,111.123,
126,134,183
Branched structures
400-1100
SEAS
Seed-mediated growth
251
Hollow structures
3W - 00
SERS; LSPR sensing
Template-directed growth
131,137.46,
Wis and rod
Polygonal plates
and disc
380 - 40
Figure 3-3: Summary of the shapes, LSPR absorption peaks, applications and methods for synthesis of Ag nano structures 149]
37
Among the rich variety of shapes, i.e. sphere, cube, octahedron, rod, polygonal plates, the platelet shape possesses an extreme degree of anisotropy due to its
unique two-dimensional geometry, which provides them with high tunability of localized surface plasmon resonance covering the visible spectrum. Many techniques
[67] have been used to investigate the plasmonic excitation in plasmonic nanostructure, such as Electron Energy Loss Spectroscopy (EELS) [68, 69, 70, 71, 72, 73, 74],
Cathodeluminescence (CL) [75, 76, 77, 781, Near-field Scanning Optical Microscopy
(NSOM) [79] and two-photon photoluminescence (TPPL) [801. When the information of dark modes and bright modes are obtained by these techniques, their coupling
behavior with quantum emitters can thus be retrieved [68]. Compared with samples
fabricated by lithographic methods, these colloidal single crystalline nanoparticles
have smoother surface and fewer defects, which may otherwise degrade their plasmon
resonance quality [44]. In addition, the presence of grain boundaries in amorphous
samples is also a drawback when electron beam is used to probe the plasmonic modes.
Typical synthesis approaches to produce nanoplates with well-controlled morphologies
include polyol process [81, 82], seed-mediated growth [83, 84] and light-induced synthesis [85, 86, 87]. In this section, I will document some chemical synthesis methods
I have used to obtain silver nanoplates. The methods are abundant and summarized
in Figure 3-3 [49].
3.1.2
Direct reduction and etching
In all the methods I have tried, I use silver nitrate (AgNO 3) as precursor. For typical
face-centered cubic (fcc) nobel metal, truncated nanocubes and multiple twinned particles are thermodynamically favored structures. So the kinetic control of reduction
process is a key factor in obtaining nanoplates [88].
In a standard synthetic approach, sodium borohydride (NaBH 4 ) serves as the
reducing agent, and sodium citrate serves as the capping agent which preferentially
binds to the (111) face. Hydrogen peroxide (H 2 0 2 ) is added as the etchant to dissolve
the unstable nanoparticles of other structures. Hence, the remaining silver particles
will favor planar growth and form plate structures, as shown in the High-Resolution
38
reTransmission Electron Microscopy (HRTEM) images in Figure 3-4. During the
a
action process, poly(vinylpyrrolidone) (PVP, M, = 29, 000 g/mol) is added as
into
stabilizer to reduce the plate size distribution. When NaBH 4 is rapidly added
that
the precursor solution, the solution initially turns light yellow, an indication
changes
spherical particles are formed. With the next several seconds, the solution
the
from yellow to red, green and blue. When the ratio of NaBH 4 to H 2 0 2 is changed,
dynamic
resultant color of the solution will also be different. This demonstrates the
equilibrium between the reduction and etching.
and etching
Figure 3-4: HRTEM images of silver nanoplates made by direct reduction
of the
Since the reaction process is very fast, it is difficult to control the size
the wide
particles within tolerance. The wide resonance peak in Figure 3-5 indicates
size distribution of the particles.
3.1.3
Polyol process
In a typical synthesis, a polyol serves as both solvent and reducing agent.
At the
initial stage of the reduction process, Ag atoms form small clusters of fluctuating
structure.
After these clusters grow larger, they become more stable and evolves
twinned and
into one of the three predominant structures: single crystalline, single
AgNO 3 by
multiply twinned. Here I modify the protocol in the paper which reduces
slow, the
the hydroxyl end group of PVP [81]. Since the reduction is substantially
products
nucleation and growth has been turned into kinetic control, and the final
can deviate from the thermodynamically stable ones. After the reaction is stopped,
39
(a)
'
-Out
(b)
H202
SutA H202
4--
10uL H202
20u H2O2
12
-
10-
-B6uL H2021
40uL H202
O~L
h0LOk
0%R
080,4
0.2
00
300
400
500
000
700
300
1000
500
wavelngi(nm)
Figure 3-5: Resonance of the silver nanoplates made by direct reduction and etching.
(a) Absorbance of the solution prepared by adding different amount of H 20 2 (b)
Different colors of the as-prepared solution
the particles are separated from the solution by centrifugation, and then dispersed
with ultrasonic dispersion. This process is repeated three times to guarantee that
most of the polymer is removed. Then the particle solution is diluted and dropped
cast onto the TEM grid. As can be seen from the HRTEM images in Figure 3-6, the
size of the triangular plates increases with time.
The yield of the triangular plates is not high, as is shown in the HRTEM images.
This situation does not change when I vary the concentration of the reactants. The
UV-Vis spectra in Figure 3-7 and 3-8 also show the presence of the spherical particles,
which have resonance at around 400nm, regardless of the size of the particle. To obtain
a sense of the size and the corresponding resonance of the plate, Lumerical FDTD is
used to calculate the extinction cross section of a single triangular particle both in
the air and water. The TFSF plane wave source is used. The edge length of the plate
is 100nm and the thickness is 12nm. The small peak at around 380 nm corresponds
to out-of-plane quadrupole resonance, and the peak at around 420 nm corresponds
to in-plane quadrupole resonance. This can be seen by placing a 2D profile monitor
perpendicular to the plate. All the resonance peaks will redshift in the water, which
agrees with the prediction of Mie theory. In general, this method realizes the size
control of the triangular plate.
40
Figure 3-6: HRTEM images of different reaction time (a) 9h (b) 21h (c) 30h
41
(a)
'(b)
3 6.
4- 30.
E
4-i
j20
Is,
no
4
o
6
;
0
30
io
4S
of
7M
50
o
100 100Io
.ngt=onm)
Wre-1.nomm>
Figure 3-7: Characterization of Ag nanoplates solution after 9h reaction time. (a)
UV-Vis spectrum (b) FDTD simulation of the extinction cross section of the particle
in the air and water (edge=100nm, thickness=12nm)
(a)(b)'
300
2j2
wan!M)
WWWWW11M)
Figure 3-8: Characterization of Ag nanoplates solution after 21h reaction time. (a)
UV-Vis spectrum (b) FDTD simulation of the extinction cross section of the particle
in the air and water (edge-2O0nm, thicknesszz24nm)
In order to remove the spherical particles, I have tried two methods. One is by
membrane filter, and the other is by centrifugation. But they do not work well. For
membrane filter, the particle will stay in the holes and hindering the filtering process.
In terms of centrifugation, since the particles to be separated have different weight
and shape, it is theoretically possible to separate them by centrifuge.
Each time,
the precipitation is separated from the solution and then dispersed by ultrasonic
dispersion.
After many times, the remaining is mostly spherical particles.
This
method is not so efficient, so I try to improve the yield by modifying the reaction
42
Figure 3-9: Remaining particles after centrifugation
process.
Many researchers reported that light can induce the transform of spherical particles to triangular plates by plasmonic excitation [85, 86, 87]. In this process, highpower laser or LED sources with single wavelength is good choice because of its size
selectivity. However, since we do not have these light sources, I use broad band and
high-power white light source instead.
Although it does not have size selectivity,
Figure 3-10 indicates that the yield of the triangular plates is improved a little bit.
Figure 3-10: Improved yield with white light illumination. (a) Particles formed when
light is totally blocked (b) Particles formed under high-power white light illumination
Hexagonal plates also form during the growth process.
This proves the planar
growth of the plate. In addition, selected-area electron diffraction (SAED) is implemented by inserting the aperture in the back focal plane. Figure 3-12 can help us
43
Figure 3-11: Formation of anisotropic silver nanoplates. (a) Formation of hexagonal
plate (b) Formation of triangular plate (inset: SAED pattern)
better understand this imaging process. Here the smallest aperture can select
lum
area, so a single triangular plate is selected in this area for electron diffraction. When
the Bragg's Law 2dsinO = nA is satisfied, the electron beam diffracted by a certain
set of crystal planes will form constructive interference. Hence, there are corresponding rotationally-symmetric diffraction spots centered around the central point on the
back focal plane. All the diffraction pattern belongs to the zeroth order Laue zone.
Since the electron beam is normally incident on the sample, the six-fold symmetric
diffraction pattern proves that the plate is single crystalline and the upper and lower
faces are (111) plane.
The Ag triangular plates synthesized by this method have been used in EELS
mapping because of their well-defined shape and large edge length.
3.1.4
Seed-mediated growth
In this subsection, I will document the method I use to grow Ag nanoplates which
can meet my needs, i.e. narrow size distribution and with resonance frequency close
.
to the absorption /emission of MoS 2
Unlike the methods discussed in the previous section, the nucleation and growth
steps are separated in the seed-mediated growth, allowing for manipulation of the
44
(a) Electron source
High voltage
(b)
Aperture
-
Anode
I Double
Condenser-w
lens
EDS
Specimen.
Objective
Detection of
secondary
and reflected
radiation
Aperture
Images
Electron lens
Transmitted
signal image
Final image
D2etection
of
Energy-lost electrons
Bragg's Law
dsin 0 = nA
EELS
Figure 3-12: TEM imaging. (a) Schematic illustration of TEM imaging and (b)
Electron diffraction (Courtesy from Internet)
size and shape of the resultant nanostructures.
This method is highly versatile in
that both the small variation in the chemical concentration and the mixing speed will
affect the shape of final products. Therefore, the concentration of the reactants is
kept very low in order to slow down the reaction and bring it under control.
In a typical synthesis process, two steps are involved. In the first step, a suspension of spherical seeds are prepared as follows: silver nitrate (AgNO 3 , 0.11mM,
11mL) and trisodium citrate (Na3 CA, 2.05mM, 11mL) are prepared under magnetic
stirring. Then the solution of sodium borohydride (NaBH 4 ) is added rapidly into
the solution. The stirring is kept for about 10min and then the solution is aged at
room temperature overnight. This is the seed solution. In the second step, 100mL DI
water is mixed with AgNO 3 (5mM, 2.5mL) and PVP (Mw=29, 000, 0.7mM, 7.5mL),
Na 3 CA (30mM, 7.5mL) and 0.2mL of the previous seed solution. Then the solution
45
is rapidly combined with L-ascorbic acid (AA, 1mM, 62.5mL) under rigorous stirring. The color slowly changes from yellow to cyan. Then the solution is cleaned by
centrifugation to remove the excessive polymer and concentrated for future use. In
this process, Na 3 CA serves as the capping agent which binds to the (111) plane of
the seeds, and AA is a gentle reducing agent which can deposit the Ag ions in the
solution onto the seeds, thus leading to the planar growth of the particles. The shape
and size of the plates are characterized by HRTEM and shown in Figure 3-13.
In
order to obtain the thickness of the plates, the droplet is dried down onto the grid so
that the plates can stand vertically.
Figure 3-13: Characterization of silver nanoplates synthesized by seed-mediated
growth. (a) High-concentrated silver nanoplates (b) Silver nanoplates dried down
on the TEM grid
In Figure 3-14, the absorbance of the as-prepared solution is measured by UV-Vis
spectroscopy.
Since the resonance is around 750nm in the water, I do not need to
grow the plates any further.
If larger plates are needed, the solution prepared in
,
the second step can serve as seeds in the next round growth. The precursor AgNO 3
capping agent Na 3 CA and reducing agent AA should be added in the same way in
each round.
Although there is still a small amount of spherical particles as is indicated by the
small 400nm peak in the UV-Vis spectrum [85], the size distribution of the nanoplates
is improved compared with the previous methods.
Therefore, the Ag nanoplates
synthesized by seed-mediated growth is used as antennas to transmit and extract
.
light from MoS 2
46
,
,
400
500
,
,
600
700
,
1.00.9
0,80.7-
0
0.5-
0.4
0.3
0.2
0.1
300
800
900
1000
Wavelength(nm)
Figure 3-14: UV-Vis spectrum of the absorbance of Ag nanoplates (inset: final solution)
3.2
High-order plasmon modes in silver triangular
platelet by Electron Energy-Loss Spectroscopy
(EELS)
3.2.1
STEM-EELS acquisition and EELS maps
As is illustrated in Figure 3-15, EELS is performed in a Scanning Transmission Electron Microscope (STEM), in which the beam is focused and scanned over the sample
area to collect loss spectra. The main advantage of the electron microscope is the
nanometer spatial resolution, allowing for the near field mapping of plasmonic nanostructures. STEM-EELS is a useful technique to obtain the high-resolution mapping
of plasmonic modes, as well as probe the photonic local density of states
[891. In this
section, I will discuss about the working principle of EELS, and its application in
probing the plasmonic modes. Moreover, I will present the experimental results from
our collaborator Dr. Ritesh Sachan in Oak Ridge Laboratory.
The elastically scattered electrons only change their momentum, while the inelas-
47
(a)
convuqme
an&0- 10 mmd
1.-.0-
('b10
sin
9.2An.0
-0.1
0 .
0
200
300
400
500
600
Energy loss hw(eV)
Figure 3-15: Illustration of STEM-EELS imaging. (a) Schematic representation of
STEM-EELS from Ref [90] (b) Typical EELS spectrum from Ref [67]
tically scattered electrons will transfer energy to the sample. The former are collected
by dark field detectors, i.e. High-Angle Annular Dark-Field (HAADF) detectors, from
which the Z-contrast images can be acquired. The latter will either excite the core
or valence electrons, as well as collective modes (LSP and SPPs), depending on the
energy of interest. The core-loss region has energy loss higher than 50eV, and the lossloss region has energy loss lower than 50eV. The zero loss peak (ZLP) corresponds to
an energy loss of OeV. Since typical plasmon energy is of the order of several electron
volts, the FWHM of ZLP is an important factor in determining the resolution of the
energy.
In terms of data acquisition, the focused electron beam scans over the sample
and stops for a defined time at each position. In normal imaging mode, elastically
scattered electrons are collected by HAADF detector. Electrons that do not change
their trajectory are dispersed in the energy-filter and the dispersive plane is projected
onto the CCD camera. The camera records the spectrum for each position sampled
by the electron beam. The resultant data is a three-dimensional data cube, including
two spatial coordinates and one energy loss spectrum.
illustration of the data acquisition process.
48
Figure 3-16 gives a clear
EEL epectrun
EELS maps
Figure 3-16: Data acquisition in STEM-EELS from Ref [90]
This data cube allows for the extraction of energy loss spectrum at a certain
position, as well as the EELS map of a certain energy loss.
3.2.2
EELS mapping of plasmon modes in individual triangular antenna
As is mentioned before, plasmon resonance enables the concentration of light into subdiffraction limited volumes. EELS has the potential to generate a full mode spectrum
of plasmonic nanostructures, including bright modes and dark modes. Dark modes
are present in coupled-antenna structure [681. Their decay channel is via absorption
instead of radiative damping, resulting in the quenching of nearby emitters. It is thus
important to study the spectral location of such plasmon modes. In this section, I
will show the experimental results from our collaborator Dr. Ritesh Sachan in Figure
3-17. The triangular plates are synthesized by the polyol process mentioned above.
49
Figure 3-17: EELS maps of plasmon modes in single triangular antenna (credit: Dr.
Ritesh Sachan)
50
The intensities are proportional to the z-projected local photonic density of states.
Multiple-order plasmon modes are discernible for all particles.The rightmost column
of the images shows the bulk plasmon mode, which is 3.65eV. This mode slightly
redshifts compared with the typical Ag bulk plasmon resonance, which is 3.8eV. This
may arise from the presence of the substrate, i.e. TEM grid, which increases the
effective permittivity of the surrounding environment. Higher order mode has larger
resonance energy. Also, it can be seen that for the same order of plasmon mode, the
resonance energy is smaller in antenna with larger edge length. I will discuss this in
details in the next section in accompany with the FDTD simulation results.
3.3
Finite-difference time-domain (FDTD) simulation of high-order plasmon modes in triangular
antennas
As is discussed in the previous sections, nobel metal particles can support LSPR, and
the optical excitation of higher-order harmonics, i.e. multipolar plasmon resonances,
becomes possible. Such high-order plasmon modes have resonance frequency in the
visible and near-infrared regions.
In this section I will demonstrate the spatial distribution of the high-order resonance calculated by FDTD. In order to calculate the plasmon eigenmodes, a circularly polarized excitation is used and the field profile monitor is placed 5nm above
the structure. The light source contains two orthogonal plane waves with a 90 phase
difference. Total-field scattered-field (TFSF) source is used in order to calculate the
absorption, scattering and extinction cross section based on the recorded total and
scattered power.
Here the edge length of the equilateral triangles are 200nm, 400nm and 600nm
respectively, and their thickness is 40nm. They have round tips with radius 10nm
to mimic the practical case. The calculated resonance curves, and the eigenmodes
corresponding to the peaks are shown in the Figure 3-19, 3-20, 3-21, 3-22, 3-23 and
51
Figure 3-18: FDTD simulation setup
3-24.
For 200nm triangular antenna, the dipole mode resonance is at 697nm and the
quadrupole mode resonance is at around 410nm, which can be visualized by the zcomponent of the electrical field.
For 400nm triangular antenna, the dipole mode resonance is at 1100nm, the
quadrupole mode resonance is at 550nm, and the octupole mode resonance is at
around 458nm.
For 600nm triangular antenna, there are more eigenmodes due to larger edge
length.
Four of them are shown in the figure, and their resonances are 1505nm,
730nm, 596nm and 490nm.
The resonant behavior of the triangular platelet antenna arises from the interference of excited, reflected and transmitted wedge plasmons.
In analogy to light
propagation, the collective oscillation of the electrons is like standing wave, and the
wave vector varies for different eigenrmodes. Once we get the wave vectors and the
corresponding energy, we can plot the dispersion curve of this kind of plasmon modes
[70]. It is obvious that the resonance depends on the size, shape and material of the
plate. For larger plate, there can be multiple field maxima along the edge, as long as
the resonance frequency is smaller than that of the bulk plasmon mode.
Here the optical excitation only shows qualitative agreement with the EELS data.
If the particular triangular plate is excited by the circularly polarized light, only
52
2.0x101 '
1
.
1
---- ---- absorption
-------scattering
extinction
1.5x107-
0
1.0x10
."--.'..-"'"
-
5.0x10"
-
---- Res .
0.0
400
600
800
1000
1200
1400
Wavelength (nm)
Figure 3-19: Optical resonance of triangular antenna with edge length 200nm and
height 40nm
certain modes can be excited [91]. Quantitative analysis can be realized by using an
array of dipoles to mimic the case of electron excitation [76], which can excite all the
modes in the plasmonic structure. This is not my focus and will not be discussed in
details in this thesis.
53
II
Figure 3-20: Plasmon eigenmodes of 200nm triangular antenna: dipole mode (697nm)
and quadrupole mode (410nm) (upper: total field; lower: z-component of the field)
54
5.0x1043
-
I
I
i
I
I
absorption
---- scattering
-----
-- eXtinction
4.Ox107
C)
3.0x1 0-13
a)
-
-
I-
10
14=
-
~
2.0x1~-13
1.0x10-1
- -------
0.0
400
600
800
1000
1200
1400
Wavelength (nm)
Figure 3-21: Optical resonance of triangular antenna with edge length 400nm and
height 40nm
r
als
Figure 3-22: Plasmon eigenmodes of 400nm triangular antenna: 1100nm, 550nm and
458nm (upper: total field; lower: z-component of the field)
55
1.4x10-12
.
.
1.0x10
I
I
------ absorption
-
1.2x10 12 -
I
------ scattering
extinction
12
C
o
C
8.Ox10-1-
(
6.0x10 13
4.0x10--
2.0x10
300
600
900
1200
1500
1800
2100
2400
Wavelength (nm)
Figure 3-23: Optical resonance of triangular antenna with edge length 600nm and
height 40nm
56
-
Figure 3-24: Plasmon eigenmodes of 600nm triangular antenna: 1505nm, 730nm,
596nm and 490nm (upper: total field; lower: z-component of the field)
57
58
Chapter 4
Tuning the emission of MoS 2 by
platelet antennas
4.1
Antenna-emitter coupled system for tunable photoluminescence
In photonics, spontaneous emission arises from dipole oscillations in the exited state
of atoms, molecules or quantum dots. When an optical antenna nearby acts as a local
probe, the emission from a single molecule can either couple to the far field via the
antenna or quench through ohmic loss. Compared with high-quantum-yield emitters,
the brightness enhancement for low-quantum-yield emitters is much higher, because
their intrinsic quantum yield has a larger potential to be enhanced by the antenna
[39].
In this section, I will first prove this rule by calculating the fluorescence enhancement based on a simple system, i.e. a single horizontal dipole placed right below a
spherical silver antenna. Then I will use FDTD to numerically calculate the photoluminescence enhancement of MoS 2 induced by a disc antenna.
59
-I
4.1.1
Horizontal dipole coupled with spherical antenna
This section mainly deals with the analytical results adapted from reference [92, 931, to
address the PL tunability of the nearby emitter with different intrinsic quantum yield.
The fluorescence enhancement factor can be expressed as the product of excitation
rate enhancement and quantum yield enhancement [94]. Therefore, the tunability of
emission can be achieved by controlling both excitation and emission processes.
Aeyexc qIq
fF=
(4.1)
The total decay rate of the excited dipole in the absence of antenna is the sum
of radiative decay and nonradiative decay. They are both intrinsic properties of the
molecule. The intrinsic quantum yield is defined as
0
(4.2)
Yr
q0 q
Yr+YNr
This definition is based on the so-called differential quantum yield
[95].
When
the antenna is present, it will induce additional decay channel Ybs, due to ohmic loss.
This can result in the total spontaneous decay rate enhancement
'Yp/ -yp,
which is
called Purcell factor [40, 96, 97, 98]. The quantum yield is this case becomes
Yr
q =
-
-
_Y +'-r+ las
(4.3)
1
Y10
_Yr
qO
The decay rate enhancement can be represented by the energy dissipation rate
enhancement, i.e. g
and
= g
0
fro
=
This relationship will be discussed using
P
classical harmonic oscillator in the next subsection.
The curvature of the antenna can be neglected when the distance between dipole
and antenna is sufficiently small. Then the nonradiative decay rate enhancement can
be calculated using electrostatic image theory
(4.4)
Ybs
3--
yr,
16
p+ 2 + 2p
1 p.(2)-1
=
E(w 2 ) + 1 k2z 3
(.
where z is the dipole-antenna distance, p is the dipole moment and w2 is the dipole
60
frequency.
Because the scattered power P, from a particle much smaller than wavelength
mainly arises from the particle's induced dipole, the induced dipole can be expressed
as
Pinduced =
k
(w2)
EO
(rp, ro;w2)p
(4.5)
denotes the dyadic Green's function in free space
+-+1
p(ikr -ro|)(46
_ex
(rO) = k22VVG(r, ro), G(r, ro) =
-
rol(4.6)
47rlr - rol
The radiative decay rate enhancement is
r
P + Pinduced12 = Inp+ kj' 2 a(W2) (rp, ro; W2)np2
(4.7)
The quasi-static polarizability of a spherical particle is
S(w2 )
=
- 1I
(rWa2)
E(w 2 ) + 2
4
and the resultant radiative decay rate enhancement is
Yr = 1 + 2
_r
a3
E(w2) - 112
(a + Z)3 E(W2) + 2
(4.9)
For a horizontal dipole, pz = 0, the nonradiative rate enhancement becomes
7abs
-=
'yO
3
-Im
16
E(w 2 ) - 1
1
E(w 2 ) + 1 k3z 3
(.0
On the other hand, the excitation rate yexc is determined by both the incident
laser field E0 and the secondary field induced by the antenna E,. In terms of weak
excitation, Yexc 0c lp - [Eo + E,] 2. The dipole's local excitation field can be expressed
in terms of free-space Green's function
E
=
E0 + k
+O
(ro,rp;wi)"i(w)Eo
61
(4.11)
The excitation rate enhancement is
7'exc
- [ I + kg
E
-Inp
exc
(ro, rp; wj)a(wi)nEO] 2'(.2
inp nEo 2
Substitute the polarizability of spherical particle into the above equation, the
excitation rate enhancement becomes
7exc = 11+ 2
a3
E(wI) - 1
(4.13)
2
(a +Z)3 E(Wi)+2
Yexc
Here w, and w 2 are excitation and emission frequencies, respectively. In order to
compare this case with the case of MoS 2 , I assume the radius of the silver spherical
antenna is a = 40nm, and the emission wavelength of the dipole is 670nm. The
+ 1.3247i, according to Palik data and ignoring
permittivity of silver is E = -16.561
the dispersion 199].
The quantum yield enhancement curve in Figure 4-1 shows that the enhancement
factor is much larger for low-quantum-yield emitters. The quantum yield increases
monotonously for q0
1 emitter.
q=1
C
q1=O.1
_q=0.1
*
-
q1=0.001
04
E
3
010
30
20
40
50
80
gaponm)
Figure 4-1: Quantum yield enhancement for emitters with different intrinsic quantum
yields
Therefore, the total emission enhancement is also much larger for low-quantum62
0
yield emitters. In particular, for q
=
1 emitter in Figure 4-3, the enhancement is
largest when the dipole-antenna distance is around 10nm. Furthermore, when the
distance is smaller than 4nm, the enhancement factor is less than 1, which indicates
the emission quenching due to the dominant of ohmic losses.
(a)
*
(b)
q~t
i0
q~OOO1
40
0
a
0
to
0
30
40
to
We
C
A0
40
(a) Emitters with different intrinsic quantum
Figure 4-2: Emission enhancement.
yields (b) Emitter with q0 - 1
This simple analytical model containing horizontal dipole and spherical antenna
provides qualitative illustration on how the emission enhancement will change with
respect to the dipole-antenna distance, and when the emission quenching or enhancement will happen.
4.1.2
Model MoS 2 coupled with disc antenna
The analysis in the previous section proves that the enhancement or quenching of
PL intensity can be controlled by varying the dipole-antenna distance. Besides, the
emitters with lower intrinsic quantum yield has larger potential to be enhanced by
the nearby antenna.
Here I use the disc antennas which are synthesized by seed-mediated growth,
because they have relatively narrow size distribution as is shown in Chapter 3. In
addition, there is an overlap between the scattering spectrum of a single disc and the
.
photoluminescence spectrum of MoS 2
63
W
05
'WI
WaveeMngth(nm)
Figure 4-3: Scattering of a single disc in the air (diameter=80nm, thickness=6nm)
and the photoluminescence of MoS 2
In the model, I assume there is no coupling between antennas. Besides, experiments have demonstrated that the emission orientation of monolayer MoS 2 is in-plane
[1001. So MoS 2 flake can be represented as in-plane dipole. But since the disc antenna
has finite size while the dipole is just a point source, both the excitation rate and
quantum yield enhancement should be averaged over different locations under the
antenna.
The excitation rate enhancement is represented as
Yexc
-Eexc
2(4.14)
2
EO .npI
7Or |Ec
-n2
7e~xc
Because MoS 2 is excited by 532nm laser in the experiment, 532nm plane wave is
used as light source in the simulation. From the expression of the field enhancement,
we know that z-component of the field will not excite in-plane dipole, so we only need
to consider x-component and y-component of the field.
Here assume the dipole is
oriented in x direction for simplicity. The field enhancement is sampled at different
locations under the antenna. In FDTD simulation, the antenna is placed at a certain
distance above the Si0 2 /Si substrate. The field profile is sampled by a 8 by 8 grid,
64
and the dimension of each square is 10nm by 10nm, just cover the region under the
antenna. The field intensity (E2) at the center of each square is recorded and averaged
by Matlab code. Since 532nm incidence does not match the resonance frequency of
the antenna, we can anticipate that the enhancement is not so high.
(a)
IE/EIF
(b)
|E, IEl.
EI6
IE~ 'E,
|E 14E
I
Figure 4-4: Simulated field enhancement. (a) Representative field enhancement when
gap=6nm (b) FDTD simulation setup
The quantum yield enhancement is calculated by connecting it with the energy
dissipation rate. This has been proved in details in Novotny's book
[94].
Here I will
use a classical model to capture the main idea and illustrate their relationship.
In terms of dipole radiation, the radiated power into far field can be determined by
integrating the Poynting vector over a closed spherical surface. The average radiated
power is
47rEoE 3c3
(4.15)
The rate of energy dissipation is given by
P= dW = wIm[p* - E(ro)]
2
dt
(4.16)
where E is the sum of the dipole field EO and the scattered field E,: E(ro) =
Eo(ro) + E, (ro). Since PO represents the radiated power in homogeneous environment
65
W
(4.17)
Po = -Im[p* - Eo(ro)]
2
The normalized rate of energy dissipation can be obtained
=
Po
+ 6rE 31Im[p* -E,(ro)]
|p1 2 k
(4.18)
which depends on the secondary field at the dipole after it has been scattered in
the environment.
The classical picture of spontaneous decay can be described by considering an
harmonically oscillating dipole whose intrinsic frequency is at its emission frequency.
As the dipole oscillates, it will radiate energy and the dipole moment will decrease.
The decay time corresponds to the time when the dipole's energy decreases to 1 of
its original value.
E
Figure 4-5: Classical picture of dipole oscillation
In the homogeneous environment, i.e. in the absence of antenna, the equation of
motion for the oscillating dipole is
d2p(t) + YO p(t) + w2p(t)
0
(4.19)
-N1/O)te-To0/2]
(4.20)
=
where -yo is the damping constant.
The solution for dipole moment is
p(t) = Re[poe -
The damping rate not only attenuates the dipole moment but also shifts the
resonance frequency. Assume the damping rate is much smaller than the oscillation
66
frequency, i.e. -yo < wo, the average energy is the sum of the kinetic and potential
energy
m [wjOp(t) 2 +pt2
W (t)
=
Then the lifetime is ro
=
2
2
=
m
W2
12 e
Yt
(4.21)
1/70.
When connecting the energy dissipation with the power of dipole oscillation, we
0
should introduce the intrinsic quantum yield q , which denotes the fraction of energy
transformed to radiation, so we have
W(t = 0) - W(t) = q 0
Po(t')dt'
(4.22)
The decay rate in the homogeneous environment can thus be expressed as
'Yo = q 0
1
(4.23)
2q2
3
47rFO 3mc
In an inhomogeneous environment, i.e. in the presence of antenna, the dipole will
experience a driving field that arrives back to the dipole after it has been scattered
in the environment. In this case, the equation of motion is
d2
dp(t) +
d
_2
dp(t) + W2p(t) = -E(t)
(4.24)
Here the secondary field is similar to the one discussed in the derivation of radiated
power. Considering the form of dipole moment in the homogeneous environment case,
we can use the following trial solutions for this differential equation
p(t) = Re[poe-i"te-yt/ 2]
(4.25)
E,(t) = Re[Esei"te--yt/ 2
(4.26)
Comparing the imaginary part of the left hand and right hand side, plugging in
0
the expression for -yo, and set q = 1, we have
67
-1+
yo
|p!2 k
(4.27)
Im[p* - Es(ro)]
Thus, we can get the connection between normalized decay rate and normalized
radiated power
-
(4.28)
PO
Yo
This is the essential relationship used in calculating quantum yield enhancement
by FDTD simulation.
The quantum yield enhancement is
f
0Yr__-
-
fQ
7
fr
______
q
(;r +
^a )q0 + (1q
0
)
(fr+
where the radiative decay rate enhancement is fr
rate enhancement is
fnr =
7.
-
fnr)qO+ (1- q()
(4.29)
, and the nonradiative decay
They are independent of the intrinsic nonradiative
decay channel inside the emitter, so they can be obtained by using a dipole source
with q0 = 1 in FDTD. A representative simulation setup with the dipole source placed
at a certain position is shown in Figure 4-6.
Figure 4-6: Representative FDTD simulation for quantum yield enhancement
In a typical simulation, one small box around the dipole source is used to record
the total power, while another box enclosing the dipole and antenna is used to record
68
the radiation power into free space. The nonradiative power is thus the difference
between these two recorded power. In addition, the radiative power in the absence
of antenna is also recorded, by the same box. All the power recorded correspond to
dipole emission at 670nm. Since the parameter we care about is the optimal distance
between the dipole and the antenna, the small box should be small enough and not
touch the antenna. The mesh size is 2nm, so the gap is multiple times mesh size. Since
the dimension of small box is 4nm, when the gap is smaller than 4nm the calculation
does not make sense, and the quantum effect should be taken into account. Therefore,
I change the gap from 6nm to 20nm in the simulation. Furthermore, the enhancement
factor is calculated by scanning the dipole in the different locations under the antenna
and taking the average value, similar as before.
After obtaining the average radiative and nonradiative decay rate, the quantum
yield can be calculated based on these two values. I plot the quantum yield enhancement for emitters with different quantum yield to emphasize the effect of antenna on
low-quantum-yield emitter, such as MoS 2 . Here I assume that different emitters have
the same emission frequency.
It can been seen that the antenna can result in both radiative and nonradiative
rate enhancement. When the gap is smaller than 8nm, the nonradiative decay channel
is dominant. Then both radiative and nonradiative rate enhancement decrease as the
gap increases. The maximum quantum yield enhancement happens at gap 8nm. This
trend shows qualitative agreement with the analytical result in the previous section.
Taking the product of the field enhancement and quantum yield enhancement, we
can get the total average emission enhancement fI
=
fEfQ. The enhancement can
be up to 18 times, and then drop rapidly when the gap increases.
The limitation of this model is that we cannot get the enhancement factor when
the gap is sufficiently small. So we do not know where the emission quenching will
happen. Here the simulation result serves as a guidance to the experiment in which
.
a spacer layer is required to tune the PL intensity of MoS 2
69
(b)
(a)
30
1.55
-
1.5
2-
1.45
20
1.35
0-
radiatve decay
nonradiave decay
415
2
10
1.25
12
.
.2
-.-.
-.
4
6
8
10
12
gap
14
16
18
20
22
4
6
(nm)
8
10
14
12
gap (nm)
=0
20
to
16
22
Average__uantumyiedenhancem(d)
(C)
20
is
14
4
16
.. ,..qR.2
12
h=.
'0
114
12
C
0
4E
1
4
6
1
CL
12
1%6
102
26 810
14han0.m04
Figure 4-xprimuaetlvrfiin
photoluminescence enhancemn(ospcrly).a)A-
ment
In order to verify the effect of gap thickness on the tunability of PL, aluminum oxide
was deposited on MoS 2 by atomic layer deposition (ALD). Precursors were trimethyl
aluminum (TMA) and H2 0 and the ambient temperature was 17500. The thickness
of A1 0 was controlled by the cycles of deposition. Each cycle will deposit 1.9 Ato
2 3
2.0 A A12 03 . After ALD, the thickness of the spacer layer ranges from 6nm to 2Onm,
which follows the simulation above.
Since the refractive index of Al 2 03 is about 1.768 in the visible range, it will
70
affect both the excitation and emission of MoS 2 . This effect can be seen by placing
the dipole at a certain position under the antenna and compare the radiative and
nonradiative power.
I
60
- -- -
50
I
radiative decay (with
spacer)
nonradiative decay (wii spacer)
radiative decay (witout spacer)
50- nonradiative decay (without spacer)
30
40
20
600
700
650
750
Wavelength (nm)
Figure 4-8: Simulated radiative and nonradiative power with and without spacer layer
From the simulation results, we can see that when we add the spacer layer, the
radiative decay will be very small compared with nonradiative decay for a dipole
emitting at 670nm. The maximum decay power also shows a shift. The simulation
setup was the same as the previous section, except that a A12 0 3 was added in the
the
gap. The simulated PL enhancement is about 5.3 times, which is smaller than
case when no spacer layer is added. This agrees with the fact that radiative power
becomes very small.
The micro-PL measurements were conducted on the as-deposited samples. 10
the
pL droplet of platelet antenna solution was drop-cast onto the samples. Then
the
samples were washed with DI water and blow-dried with nitrogen gas to avoid
formation of clusters.
For each sample, three curves were collected with the laser spot probing different
locations of the sample, and then baseline was subtracted from the signal. The
photoluminescence enhancement can be clearly seen in Figure 4-10. The difference
71
(b) ;
(a) 1.5
1.45
25
-
1.4
-
-
decay
radiative
w-onradiathva dacay
1.35
4..
1.3
0
2
1t5
1.25
-4OC 10
1.15
1.1
5
1.05
4
6
8
10
14
12
gap (nm)
1
18
20
22
'4
6
8
10
12
6
8
10
14
12
gap (nm)
14
gap (nm)
16
18
16
18
20
22
(d) a
(C)
4.5
4
I2.5
2
2
0.5
4
6
8
10
12
14
gap (nm)
16
15
20
22
4
20
Figure 4-9: Simulated photoluminescence enhancement (with A1 2 0 3 as spacer layer).
(a) Average field enhancement under the antenna (b) Average radiative and nonradiative decay rate enhancement (c) Average quantum yield enhancement (d) Theoretical
PL enhancement
72
22
(a)
(b) 300
Mo%+S n A603
MoS+&W AgAO+ AGNPLJ
MoS+Snm AO,
MoS+Omn A2 3 +AgNPL,.-.
MoS 2+8nm AO+AgNPL-2
M*S+Bm A,0,+A-NPL_3
250
-
250
MoS2+n A0,+
AGNPL2
MoS%+m AO,+ AGNPL
-
200
150
150
I-
100
100
50
0
Al
(C)
650
000
750 600
Mutag
700
50
900 950
1000
600
550
650
(nm)
(d) 300
40
30
MoS2+1
-
AO,
MoS,10tnm A1,0)ADPL_1
SMOS61VM A1,#AOWANPL_2
3S
--
MaS,*10em AI,O#AOtPL_3
.
,
MoS+1GInm A203,+AgNPL-3
0 2+1ni AIIAgNPL
200
I
,
mAO
MS,+6nm AI.,0+AgNPL-t
250
0
.
.
",- MoSm.1
100
950
700 750 600 50
Vwvogeh (nm)
.
550
1-:
25
20 0
150
115
100-
0
0
,
.
,
,
000 950
600 650
700
600
750
650
g00
950
100
Waveb.ngU (nm)
If)
.
.
0
560
1000
.
.
2.4
.
300
0050
wawIngh (nim)
750
700
)
k
650
00
.
550
MoG.1Inm A,60,
2.2
Mo6 3.10nm A1,0,+AgNPLI
250
Moa,0Inm AkO4ANPL_2
MOS,+&MAI,0,+ANPL_3
0
2
a
b
0
U
100
-'
1.6
*
200
1.2
10
12141is
560
600 050 700
750
Boo
850
OOD
950
1000
wvelngh (nm)
6
p
Is
Gap (nm)
Figure 4-10: Measured photoluminescence enhancement for different A1 2 0 3 thickness.
(a) 6nm (b) 8nm (c) 10nm (d) 16nm (e) 18nm (f) Ratio of PL intensity (black dots:
PL intensity enhancement; red dots: average PL intensity enhancement)
73
in the maximum PL intensity from the same sample arises from the variation of
plate distribution on MoS 2 . The irregular sharp peaks in Figure 4-10(c) is due to the
occasional presence of particle clusters. The average ratio of PL intensity was denoted
by red dots. In general, the ratio of PL intensity shows qualitative agreement in both
trend and magnitude with the simulated results, except that the sample with 12nm
spacer layer deviates from the trend. The maximum enhancement can be more than
2 times.
The discrepancy between experiments and simulation may arise from many factors. First, the coverage of the antennas on MoS 2 must be smaller than the scenario
in the simulation, in which I only consider the local field right below the antenna.
Second, the collection efficiency rn, which denotes the probability of the photons reaching the detector, should be included in the PL enhancement factor. This is related
to the radiation pattern of the dipole-antenna system, NA of the microscope and
the sensitivity of the CCD camera etc. Furthermore, in the simulation model I have
treated MoS 2 as a horizontal dipole and ignored its optical properties, but actually it
has high refractive index around n = 6+ 2i [101] which will shift the resonance of the
antenna. Despite of the low enhancement, these single crystalline platelet antennas
still hold promise in improving the photoluminescence of monolayer MoS 2 , which is
a low-quantum-yield emitter.
Another advantage is that the feature of B exciton is still discernible. This is
illustrated in Figure 4-12 where A and B excitons are fitted with Lorentz model.
This feature is important because the spin-orbital splitting leads to many promising
applications, as is summarized in Chapter 1.
Furthermore, according to the analytical results in the previous section, PL quenching is expected to happen when the gap between the antenna and the emitter is sufficiently small. However, PL enhancement is still present in Figure 4-13 when the
.
antennas are in direct contact with monolayer MoS 2
74
300
250
200
150
U)
C
a)
C
100
50
0I
550
750
700
650
600
800
850
900
1000
950
Wavelength (nm)
Figure 4-11: A and B exciton peaks fitting using Lorentz model y = yo +2_ 42W2
(dash line) before (solid black line) and after (solid red line) PL enhancement
i
1000
MnI
900
2
MS2 +AgNPL
800
700
600
C"
500
.ijj
C
a)
400
300
200
100
0
5 50
I
I
600
650
700
750
800
850
900
950
1000
Wavelength (nm)
Figure 4-12: PL curves when the platelet antennas are in direct contact with monolayer MoS 2
75
4.2
Time-resolved spectroscopy and preliminary experimental results
4.2.1
Time-resolved spectroscopy
The dynamic properties of materials on a femtosecond timescale can help us determine
their decay rate. Pump-probe spectroscopic techniques are important ultrafast optical
tools for us to understand the dynamic electronic or optical properties.
In such
technique, a high-intensity pump pulse perturbs the sample from equilibrium, then
a weak probe pulse measures the change in transmission and reflection at a certain
time delay [1021. The time delay depends on the pump and probe path length, which
can be controlled by the delay stage. Sampling technique is used to reconstruct the
time-resolved photoluminescence.
One of the critical parameters is the minimum
time resolution, which is limited by the pump and probe durations and the interval
between the time delays.
Figure 4-14 shows a representative degenerate pump-probe setup. The pump and
probe wavelength are the same, and they are separated by polarization optics. The
material properties, i.e. differential transmission or reflection, are sampled by the
weak probe pulse which should not significantly change the material properties. If we
use lock-in detection, the change can be reflected in the voltage. The pump beam is
chopped at frequency
fe,
while the probe beam is not chopped. If we use a 50% duty
cycle, half of the probe beam will be incident on the sample without the perturbing
of the pump beam.
The differential transmission or reflection is related to the change in the absorption, which depends on the joint density of states of the material [102]. So we can
imagine that the pump-probe spectroscopy can be used to measure the change in
electron occupation states. In order to obtain the full time-resolved dynamics, multiple measurements at different time delays are required. This technique is obviously
inefficient. An alternate method of rapidly acquiring signals employs a streak camera
demonstrated in Figure 4-15. The optical field coming from the sample is incident
76
Deteilelwk-in
(a)
(b)
sunplifler
w" M
Painpe'
Vddte
S
mpllttr
Sample
hobe beam
0
2
-4
4
2
Modulaor
3
I
S
<50fS
__
_
I
I
V
_
us
In1Pulse
~Cccmm-nor
0
4
0
A""kweI~e
-2
2
Figure 4-13: Working principle of pump-probe setup. (a) Degenerate pump-probe
setup (b) An illustration of differential transmission and reflection at a certain time
delay using lock-in detection [1021
Spe
nTORWge
Weph
signaW
0
.2
Entrane Sat/
Phokcahoe
4!
ACe
6I
Defiechon ic
S
0
00
4W0
SW
mwns*y (Couft)
400
~300
20010
4kW'
520 540
~vfeolgV (Nun)
Figure 4-14: Operating principles of a streak camera [103]
77
on the photocathode that can emit electrons through photoelectric effect. The electrons then pass through a time-dependent field, and then a time-to-space mapping is
obtained on a phosphor screen. Therefore, axis in the perpendicular direction corresponds to temporal axis, while the axis in the horizontal direction corresponds to the
wavelength of incident light.
4.2.2
Time-resolved photoluminescence of monolayer MoS 2
The reported decay time of MoS 2 is around 100ps [104, 105], and it is temperatureand substrate- dependent [104, 28]. The Time-resolved PL curve of bare MoS 2 was
measured by Dr. Dafei Jin and our collaborator in UC-Berkeley. The sample was
pumped by the femtosecond laser (Inspire), and the signal was collected by the streak
camera (Hamamatsu C10910-02). The measured signal was the convolution of the
signal from the sample and the point spread function (PSF) of the system. After
deconvolving the PSF from the detected signal, we can obtain the real time-resolved
PL signal y = 1.216e-.17 . The decay time 0.917ps is substantially smaller than the
reported data. Since the nominal time resolution of the streak camera is ips, this
result would not be valid. Figure 4-16 shows the experimental setup and the obtained
data. The unexpected short decay time may arise from the substrate we use. If h-BN
substrate is used or the sample is suspended, we may expect higher quantum yield
factor, which denotes the enhancement of the total spontaneous emission rate -y,,p
[40].
78
,
and thus longer decay time [28, 22]. Thus we will not seek to measure the Purcell
(a)
(b)
100
s0
40
20
0
0
2
4
6
Tim. (ps)
8
10
I
(d)1 4
(C)
.2
1o0
0
60
0.4
0.4-
20
a .2
0
0
2
4
6
a
10
12
T"me (PS)
TWO (PS)
Figure 4-15: Time-resolved PL measurement. (a) Experimental setup (b) Measured
point spread function (PSF) of the system (c) Measured data (black dots) and fitted
curve (red solid line) (The red curve was obtained by setting the time-resolved signal
as y = Ae- ro , and then convolved with PSF curve. The fitting parameters are A,
to and To.) (d) Real time-resolved PL signal
79
80
Chapter 5
Summary and outlook
5.1
Summary
In this master thesis, I have investigated the interaction between optical antenna
and emitter, by studying the enhanced spontaneous emission from monolayer MoS 2
with single crystalline platelet antennas. This basic scheme can also be applied to
many other antenna-emitter systems, such as antenna-coupled quantum well and
quantum dot, to develop more efficient emitters. For monolayer MoS 2 , which is both
a promising two-dimensional semiconductor and a low-quantum-yield emitter, some
researchers have reported the enhanced emission from MoS 2 with lithographically
defined plasmonic nanoantennas [106, 107] and plasmonic nanocavity [108] during
the time of my work. Here I demonstrate a more economic and promising way in
tuning the emission of MoS 2 by colloidal platelet antennas.
My work can be summarized as follows:
First, I seek to colloidally synthesize single crystalline silver platelet antennas for
high-quality near-field imaging. The antennas of this particular shape have tunable
resonance ranging from 350nm to 1000nm, covering the whole visible spectrum. The
main methods that can be used to obtain polygonal plates are polyol process, lightmediated growth and seed-mediated growth [49]. With polyol process, I can obtain
platelet antennas with well-defined polygonal shapes, and the yield has been improved
with high-power white light illumination. These plates have already satisfied the re81
quirements of near-field imaging by CL and EELS. Although light-mediated growth
seems to be a promising way to obtain platelet antennas with narrow size distribution,
all the documented methods use high-power LED and laser. Albeit with wide size
distribution, these plates grown by polyol process can serve as broadband absorbers.
Then I turn to seed-mediated growth to get platelet antennas with narrow size distribution and resonance close to the emission line of MoS 2 . The plates synthesized in
the first-round growth have resonance around 750nm in the water. The as-prepared
platelet antennas have thickness below 10nm. Furthermore, I have qualitatively compared the high-order plasmon modes in triangular plates with the EELS data from
our collaborator. Since the incident source is circularly polarized light and the plate
has 3-fold symmetry, only certain resonant modes can be excited [91J.
Second, I have investigated the thickness-dependent properties of MoS 2 . The samples are prepared by micro-exfoliation onto a Si0 2 /Si substrate to enhance the optical
contrast. Besides the contrast, both micro-PL and Raman spectra can offer abundant
information on the layer-dependent optical properties of MoS 2 . The crossover from
indirect bandgap in bulk MoS 2 to direct bandgap in monolayer MoS 2 leads to the
absence of indirect transition in the PL curve. In addition, the decreasing restoring
force results in the softening of the out-of-plane phonon mode in the Raman spectra.
In order to widen the application of MoS 2, I have also tried to transfer the flakes onto
gold substrate by spin coating PMMA as a sacrificial layer.
Third, I have been looking for a feasible way to study the photoluminescence
enhancement in this antenna-emitter coupled system. It is well known that both the
decay rate and PL intensity of the emitters can be enhanced by the nearby lossy
medium, such as metallic structure and photonic crystal, which can increase the local
photonic density of states. Both the total spontaneous emission rate enhancement,
i.e. Purcell factor, and the PL intensity enhancement arise from the enhancement
of radiative and nonradiative decay. The nonradiative decay can come from various
decay channels, which may be induced by plasmonic nanocavity, nanowire, grating,
antenna, etc. In my case, the only nonradiative decay channel is due to the ohmic
loss in the silver platelet antenna.
Initially, I seek to measure the time-resolved
82
PL of MoS 2 and the corresponding Purcell factor, but the short decay time, which
agrees with its low quantum yield, hinders the further investigation of decay rate
analysis. Then I turn to study the enhanced photoluminescence in a simple system
containing a horizontal dipole and a spherical antenna. The analytical results provide
two important pieces of information: first, the PL of low-quantum-yield emitter has
larger potential to be enhanced by the antenna; second, photoluminescence quenching
and enhancement is governed by the dipole-antenna distance. Next I have studied
the tunable emission in my system by both experiments and FDTD simulation. The
numerical simulation is conducted by simulating both the excitation and emission
processes.
Here I treat MoS 2 as an in-plane dipole in line with the experimental
observation reported by the literature, and scan the dipole at different locations under
the antenna to take account of the geometry effect.
The presence of the spacer
layer shifts the plasmon resonance and thus changes the PL enhancement. The PL
enhancement in the experiments is a bit smaller than the simulated results. This may
arise from the simulation model and the so-called collection efficiency of the PL setup.
In general, the PL enhancement versus spacer layer thickness qualitatively agrees with
the trend and magnitude in the simulation. One limitation of the numerical model
is that it cannot tell when PL quenching will happen because there should be a
finite distance between the antenna and the box recording the total power. In the
experiments, I have not observed the quenching effect when the antennas of same
-
concentration are in direct contact with MoS 2
5.2
Outlook
In general, I have demonstrated an economic and promising strategy to tune the
spontaneous emission of the emerging two-dimensional semiconductor MoS 2 . The
maximum PL enhancement can be more than 2 times. Besides, both A and B exciton peaks are discernible after enhancement. Since the spin-orbital splitting and the
resulting A and B excitons carry abundant information, it can facilitate the advancement of valley-based optoelectronic applications of MoS 2 [28, 33]. With the platelet
83
antennas, there is no need to transfer MoS 2 onto other substrates, such as photonic
crystal [109] and hBN [28], to improve the quantum yield. Moreover, the ultrathin
platelet antenna together with monolayer MoS 2 hold promise for the development of
on-chip emitters.
84
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