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Research Article
Enhancement of Room-Temperature Photoluminescence and Valley
Polarization of Monolayer and Bilayer WS2 via Chiral Plasmonic
Coupling
Gaohong Liu, Xuanli Zheng, Haiyang Liu, Jun Yin, Congming Ke, Weihuang Yang, Yaping Wu,*
Zhiming Wu,* Xu Li, Chunmiao Zhang, and Junyong Kang
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*
ABSTRACT: Transition-metal dichalcogenides with intrinsic
spin-valley degree of freedom have enabled great potentials for
valleytronic and optoelectronic applications. However, the degree
of valley polarization is usually low under nonresonant excitation at
room temperature due to the phonon-assisted intervalley
scattering. Here, achiral and chiral Au arrays are designed to
enhance the optical response and valley polarization in monolayer
and bilayer WS2. A considerable band edge emission with 7 times
increment is realized under the resonant coupling with Au dimerprism arrays. Valley polarization enhancement is quantitatively
predicted by the inherent mechanisms from elevated electromagnetic field intensity and radiation efficiency and further realized
in polarized photoluminescence. A tunable valley polarization up to
30.0% is achieved in bilayer WS2 under a nonresonant excitation at room temperature. All of these results provide a promising route
toward the development of room-temperature valley-dependent optoelectronic devices.
KEYWORDS: WS2, chiral plasmonic coupling, photoluminescence, Raman, valley polarization
■
symmetry and interlayer coupling.11,13,14,25 The indirect optical
transition process greatly depletes the phonon mode at the Γ
point and brings in exceptionally robust valley polarization in
bilayer WS2.14 However, since the indirect band gap structure
and the nonradiative recombination channels produced by the
interlayer coupling, the light emission of bilayer WS2 is
seriously suppressed.
Previous studies have provided various strategies to improve
the valley polarization of WS2. Resonant excitation can
effectively reduce the intervalley scattering and promote the
valley polarization.13 Nevertheless, appropriate pumping
condition with certain energy as well as low-temperature
environment is generally needed. Stacking monolayer TMDCs
with graphene is beneficial for valley-polarized excitons owing
to the quick radiative recombination, while serious photoluminescence (PL) quenching and short exciton life are
inevitable.26 Electrical, magnetic, and strain have also been
INTRODUCTION
Valleys refer to the extreme points of energy in the momentum
space, which are perceived as an additional degree of freedom
besides of charge and spin.1−3 Valleytronic materials, such as
transition-metal dichalcogenides (TMDCs), hold a great
potential for manipulating the polarization optovalleytronic
applications.4−9 WS2 is a layered TMDC material with
intralayer S−W−S covalent bonding and interlayer van der
Waals (vdW) stacking.10 Monolayer WS2 is a direct band gap
semiconductor, possessing two inequivalent valleys at K and K′
points in the hexagonal Brillouin zone due to the broken
inversion symmetry and strong spin−orbital coupling, which
offer a promising platform for detecting and tuning the valley
degree of freedom.11−14 Valley-polarized pumping is a feasible
approach to distinguish any single valley through the unique
optical selection rules, i.e., the left- and right-handed circularly
polarized (LCP and RCP) photons coupling to the interband
transitions in the K′ and K valley, respectively.15,16 However,
due to the phonon-assisted intervalley scattering and the
intervalley electron−hole interaction, it is difficult to obtain a
clear valley contrast at room temperature.17−24 Moreover, the
excitation photon energy needs to be carefully controlled to
suppress the loss of the valley polarization.13,20 Bilayer WS2 is
more superior in valleytronic devices because of its structural
© 2021 American Chemical Society
Received: April 11, 2021
Accepted: July 2, 2021
Published: July 14, 2021
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Research Article
Figure 1. (a, b) SEM images of the fabricated Au(I) and Au(II) arrays on SiO2/Si substrates. Insets are the AFM images with the scale bar of 200
nm. The height profiles are measured along the red lines. (c) Normalized extinction spectra of the Au(I) and Au(II) arrays measured at room
temperature. (d, e) SEM images of WS2 transferred on Au(I) and Au(II) arrays, respectively. (f, g) Raman spectra of the monolayer WS2 (MLWS2) and bilayer WS2 (BL-WS2) taken in the areas without and with the Au(I) and Au(II) arrays beneath, respectively. (h, i) PL spectra of the
ML-WS2 and BL-WS2 taken in the areas without and with the Au(I) and Au(II) arrays beneath, respectively.
employed to manipulate the valleys.27−30 However, both the
efficiency and controllability are still far from practical
applications. A universal approach for realizing considerable
valley polarization at room temperature is of fundamental
importance. Resonant plasmonic coupling can effectively
enhance the interaction between light and materials, which
has been considered as a promising strategy to enhance the
optical response and manipulate the polarization of twodimensional (2D) layer materials.31,32 Sergio Catalan-Gomez
et al. fabricated monolayer MoS2-gallium hemispherical
nanoparticle (Ga NP) hybrids, which increased the photoluminescence (PL) intensity of monolayer MoS2 by more than
9 times.33 By embedding monolayer MoS2 into a compact
plasmonic nanocavity, Jiawei Sun demonstrated a 48.7% valley
polarization of in MoS2 at room temperature.34 By utilizing a
chiral stacked Au plasmonic nanorod, Te Wen et al. obtained a
high valley polarization up to 47% in monolayer MoS2.35
However, there are very few reports on the study of plasmonic
enhancement of the valley polarization characteristics in WS2,
especially the chemical vapor deposition (CVD)-grown films,
which are beneficial for the scalable device applications.
Moreover, the thickness dependence of the enhancement effect
is still lacking. Concerning the superior spin and valley
properties of bilayer WS2, it is significantly important to study
its resonant plasmonic enhancement. It is also exciting to tailor
the valley polarization with plasmonic coupling and the chiral
light−matter interactions.
In this work, enhanced optical response and valley-polarized
PL are investigated in heterostructures with Au plasmonic
arrays and CVD-grown monolayer/bilayer WS2 at room
temperature. Two kinds of Au arrays composed of achiral
single units and chiral dimer units are designed. Surfaceenhanced Raman scattering (SERS) and PL intensity are
demonstrated with both structures, and even more pronounced for the latter. Strongly increased valley polarization is
predicted by analysis by the electromagnetic field intensity
distribution from the finite-difference time-domain (FDTD)
method and further realized under nonresonant optical
excitation (532 nm), especially with the chiral plasmonic
coupling for bilayer WS2.
■
RESULTS AND DISCUSSION
Large-area single-prism (Au(I)) and dimer-prism (Au(II))
arrays are fabricated through the nanosphere lithography
approach on SiO2/Si substrates (Figure S1). Figure 1a shows
the scanning electron microscopy (SEM) and atomic force
microscopy (AFM) morphologies of the Au(I) arrays, which
exhibit a P6mm symmetry. Each prism unit possesses an inplane triangular shape with side length of about 130 nm and
height around 60 nm, as measured from the height profile
along the red line in the inset. Different from Au(I), Au(II) is
composed of one large and one small prism with an in-plane
triangular shape (Figure 1b). The height is around 60 nm for
both, and the side lengths are 120 and 70 nm, respectively.
Normalized extinction spectra for the two structures are
measured at room temperature (Figure 1c) that show a wide
extinction range above 600 nm for both. The surface plasmon
resonance peak is approximately centered at 630 nm for Au(I)
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arrays and red-shifts to 640 nm for Au(II) arrays, almost
consistent with the absorption edge of monolayer and bilayer
WS2, respectively. Compared with that of Au(I) arrays, the
resonance peak intensity of Au(II) arrays increases to about 1.5
times, which means that the Au(II) arrays can be more
effective for enhancing the optical response of WS2.
Monolayer and bilayer WS2 with high quality is then
transferred onto the top of Au arrays through a typical wettransfer approach. The SiO2/Si, monolayer, and bilayer regions
can be well distinguished from the different contrasts in the
SEM images in Figure 1d,e. The rotation angle between the
two stacking layers is about 60°. Raman and PL spectra are
measured and compared to study the SERS effect and
enhanced PL at room temperature. The signals are taken for
WS2 in the areas covering and uncovering the Au arrays,
respectively. The Raman spectrum of monolayer (bilayer) WS2
on SiO2/Si substrate contains two characteristic peaks located
at 358.8 (357.1) cm−1 and 418.8 (419.4) cm−1, as shown in
Figure 1f,g, which correspond to the in-plane (E2g1) and outof-plane (A1g) vibration modes, respectively.36 In the WS2/Au
area, ∼1.7 cm−1 redshift for the peak position of the E2g1 mode
and ∼0.7 cm−1 for the A1g mode are observed. This can be
attributed to the electron−phonon coupling caused by the
strong near-field interaction, the increased electron concentration, and the interfacial stress as well.37,38 It is observed that
the Raman signal is significantly enhanced in both WS2/Au
systems, demonstrating a typical SERS effect. The intensities of
bilayer are all higher than those of the monolayer, and the
signal enhancement is more pronounced for Au(II) arrays than
that of Au(I) arrays.
Compared with the SERS effect, the enhancement PL is
even more pronounced when introducing the Au prism arrays
(Figure 1h,i). Pristine monolayer WS2 without the plasmonic
coupling has a single PL peak centered at 617 nm, consistent
with its neutral exciton emission.39 When coupling with Au(I)
and Au(II) plasmonic arrays, the PL intensity is more than 4
and 7 times for WS2, respectively. Simultaneously, the peak
positions are red-shifted slightly, which can be mainly
attributed to change of the dielectric environment40 and the
plasmon−exciton coupling interaction.41,42 The PL uniformity
is confirmed by the mapping images in Figure S2, where the
enhancement is globally larger for monolayer WS2 on Au(II)
arrays than on Au(I) arrays. The PL enhancement may be
accompanied by the fluorescence quenching effect because the
WS2 is directly contacted with Au. To clarify the competitive
relation between these two mechanisms, PL enhancements
with and without a 1.0 nm Al2O3 insulating layer between WS2
and Au are compared, as shown in Figure S3. The
enhancement effect is only slightly improved when adding
the insulating layer, which suggests that plasmonic coupling
effect plays a dominant role in the PL enhancement. Different
from the monolayer results, the PL intensity of original bilayer
WS2 on SiO2/Si is much smaller, and simultaneously the peak
positions have a redshift for about 12 nm owing to the
interlayer coupling. The PL intensity is enhanced by more than
3 and 3.5 times with Au(I) and Au(II) arrays, respectively. In
addition to the surface plasmonic effect, the interaction
between Au and the underlying top layer of WS2 will weaken
the interlayer coupling within the bilayer WS2. Thereby, the
nonradiative recombination is suppressed and the PL intensity
is distinctly increased. The global uniformity of PL enhancement is also demonstrated by the mapping images in Figure S2.
Although the enhancement is not so high as that of monolayer
WS2 due to the indirect band gap and interlayer coupling, it
still provides a promising strategy for the light emission of
bilayer WS2.
The notably SERS effect and enhanced PL emission
demonstrate that the Au prism arrays have a distinct plasmonic
coupling effect with the 2D WS2. To reveal the mechanism of
different Au structures, electromagnetic field intensity distributions of monolayer and bilayer WS2/Au heterostructures
under 532 nm are simulation through the FDTD method, as
shown in Figure 2a−d and 2e−h, respectively. In both
Research Article
Figure 2. Near-field intensity distribution of (a−d) monolayer and
(e−f) bilayer WS2/Au heterostructures at 532 nm (log scale). (a, e)
and (c, g) Cross sections along the x−y plane on the WS2/Au
interface for WS2/Au(I) and WS2/Au(II), respectively. (b, f) and (d,
h) Cross sections along the x−z plane on the black dashed lines in (a)
and (c), respectively.
heterostructures, the near-field intensity distributions are
notably concentrated on WS2/Au and Au/SiO2 interfaces,
especially on the edge and tip of the Au prisms. For both
monolayer and bilayer WS2, WS2/Au(II) heterostructures
show a higher electromagnetic field intensity than that of the
corresponding WS2/Au(I) heterostructures, which is attributed
to the additional “hot spots” effect between two adjacent Au
prisms within each dimer structure, as well as the larger Au
amount in the dimer arrays. The near-field intensity
distribution at the Au/SiO2 interface is roughly similar to
that at the Au/WS2 interface, as shown in Figure S5.
The enhanced PL signals of WS2 provide a solid base to
study the valley polarization modulation by resonant plasmonic
effect. Pure WS2 generally possesses a low valley polarization,
especially under nonresonant excitation at room temperature.
The loss of valley polarization is mainly induced by intervalley
1
scattering according to the relation: DVP ∝ 1 + 2Γ / Γ ,35
ν
0
where DVP stands for the degree of valley polarization,
recombination rate (Γ0), and intervalley scattering rate (Γν)
that dominantly depend on the intrinsic properties of WS2,
excitation energy, and ambient temperature. Intrinsic valley
polarizations of monolayer and bilayer WS2 are measured as
3.0 and 6.6% in our experiments, respectively, corresponding
to an intervalley scattering rates of Γν ≈ 16.17 Γ0 and 7.07 Γ0.
The competition between Γ0 and Γν is crucial to determine the
valley helicity.35 When introducing the Au arrays, the behaviors
of valley excitons can be described by the steady-state
equations35
dn+
= P+ − n+(Γatot+ + Γ0tot) + (n− − n+)Γv = 0
dt
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Figure 3. Total decay rates (a, b) and radiative efficiency (c, d) for σ± dipoles in monolayer and bilayer WS2/Au heterostructures. (e, f) Schematic
illustrations of the modulation of valley excited PL process without and with Au(II) under σ+ excitation.
dn−
= P − − n−(Γatot− + Γ0tot) − (n− − n+)Γv = 0
dt
observed under LCP and RCP excitations, as shown in Figures
S6 and S7 for monolayer and bilayer WS2, respectively.
Despite the promotion and chirality of the exciton decay, the
presence of Au structures may also depolarize the valley
polarization of the detected far-field PL signals during the
excitation and emission processes. The excitation process
involves a conversion from far field to near field, and
electromagnetic field intensity distributions at 532 nm are
simulated to analyze the near-field components ER and EL, as
shown in Figures 4 and S8, where EL = 1/2(Ex − iEy) and ER =
(2)
where n± stands for the valley population, P± represents the
valley pumping rate, and Γtot
a± is the Au-modulated decay rate of
excitons on K and K′ valleys.
To study the plasmonic enhancement in the exciton decay
process, 112 pairs of σ+ and σ− dipoles are employed in the
simulations of total decay rates and radiation efficiency in
WS2/Au systems, as shown in Figure 3a−d. The total decay
tot
rates (Γtot
a− and Γa+ ) of Au(II) are significantly larger than that
of Au(I), especially for coupling with bilayer WS2, which
indicates the enhancement of exciton generation. The radiation
efficiency (ηa− and ηa+) of Au(I) is almost consistent under σ−
and σ+ excitons at the emission wavelength (620 nm for
monolayer and 635 nm for bilayer), while it is different for
Au(II). This chiral radiation efficiency results from the
asymmetrical structure of Au(II). Figure 3e,f presents the
basic mechanism for increasing and tailoring the valley
polarization in the energy band framework. When the K(K′)
valley is coupled with RCP (LCP) light, the σ+(σ−) excitons
are generated and occupy in the lowest energy level (Figure
3e). Some σ+(σ−) excitons scatter into K′(K) valley in the
channel of phonon scattering, and the pseudo-spin state is
changed to σ−(σ+). When the Au plasmonic structures are
introduced, the polarized light will strongly undergo collective
oscillation with localized surface plasmons, which will enhance
the electromagnetic field intensity and radiation efficiency of
excitons through the Purcell effect (Figure 3f). At the same
time, the chiral radiation efficiency further provides a
possibility to manipulate the valley polarization of WS2.
Quantitative evaluation is performed in bilayer WS2/Au as
an instance. Considering the emission wavelength of bilayer
tot
tot
WS2 (635 nm), for the Au(I) structure, Γtot
a+ ≈ 6.3 Γ0 , Γa− ≈
tot
tot
5.3 Γ0 , and ηa−/ηa+ ≈ 1.8:2, where Γ0 is the total decay rate of
excitons without Au modulation. In consequence, the degree of
valley polarization for the exciton transition will reach 30.8%
under σ− excitation and 34.9% under σ+ excitation. For the
tot
tot
Au(II) structure, Γtot
and ηa−/ηa+ ≈
a+ ≈ Γa− ≈ 11.6 Γ0
0.048:0.060. The degree of valley polarization will reach 38.0
and 55.3% under σ− and σ+ excitations, respectively. Given the
above predictions, distinctly elevated valley polarization with
tailorable chirality can be theoretically expected in WS2, with
the assistance of Au plasmonic structures, especially the Au(II)
arrays. Chiral near-field intensity distributions are further
Figure 4. Near-field intensity distribution (a−d) EL and (e−h) ER in
bilayer WS2/Au heterostructures, with LCP and RCP excitation at
532 nm (log scale).
1/2(Ex + iEy), respectively. Although the single prism of Au(I)
is achiral in its configuration, the staggered arrangement of the
units can also produce some general chirality into the near-field
components for Au(I) arrays. And the chirality of the near-field
components in Au(II) is even more pronounced for both
global arrays and each dimer-prism unit. Different from the
primitive WS2, pure circularly polarized excitation in WS2/Au
heterostructures can generate excitons in both the K and K′
valleys, producing a pumping rate of
P+
P−
∼
ER 2
|.
EL
Taking into
account this depolarization mechanism, the degree of valley
polarization is decreased to 27.9% (33.0%) under σ− excitation
and 31.6% (48.0%) under σ+ excitation for bilayer WS2/Au(I)
(bilayer WS2/Au(II)). Moreover, during the emission process,
the presence of Au structures will scatter the light from near
field to far field. Modulation of the circularly polarized
emission from σ+ and σ− dipole pairs will bring about
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additional depolarization effect, which should also be
concerned when estimating the final valley polarization.
Therefore, compared with the above values, the practical
polarization for detected PL signals may be slightly suppressed,
as well as that of the chirality.
The increase of the radiation decay rate indicates a reduction
of the carrier lifetime, which is further investigated through the
time-resolved photoluminescence (TRPL) measurements, as
shown in Figure S9. For instance, the carrier lifetimes of
pristine bilayer WS2 and bilayer WS2/Au(I) and Au(II)
hybrids are measured as 283, 243, and 221 ps, respectively.
The reduced exciton lifetimes after coupling with the Au
plasmonic structures are consistent with enhanced exciton
radiation recombination processes of WS2.
Derived from the framework of the theoretical design, higher
valley polarizations can be achieved under σ+ excitation.
Accordingly, the room temperature valley-polarized PL spectra
of monolayer and bilayer WS2 are taken under an RCP light
with 532 nm nonresonant excitation, as shown in Figure 5. The
temperature and resonance excitation were always necessary to
obtain a relatively high polarization in WS211,13,14 because the
intervalley scattering and spin relaxation can be effectively
suppressed. Therefore, combining the plasmonic coupling
effect and the low temperature or resonance excitation can be
expected to further improve the optical response and valley
polarization of WS2. Given this, a 589 nm excitation closer to
resonant pumping is further employed to study the valleypolarized PL spectra of monolayer and bilayer WS2, as shown
in Figure S10. Considering the neutral exciton emission, the
valley polarization of the monolayer (bilayer) WS2 is increased
from 11.0% (15.4%) to 23.8% (45.1%) after coupling with Au
structures. Although large valley polarizations can be obtained
under low temperature or resonance excitation, the test
conditions and environment are relatively harsh. However,
we achieve considerable enhancement of the valley polarization
under nonresonant excitation at room temperature, which has
a more crucial significance in practical applications. In addition
to the valley and spin indexes, the bilayer WS2 also has an
additional index called layer polarization, which can provide
information on the location of charge carriers. The stronger
enhancement of the valley polarization in bilayer WS2 just
demonstrates this layer polarization, which is aroused from not
only the interlayer interactions but also the higher responsivity
to the chiral plasmonic effect.
Research Article
■
CONCLUSIONS
In summary, two kinds of Au plasmonic arrays with achiral
single prisms and chiral dimer prisms are designed for the
study of enhanced optical response and valley polarization in
monolayer and bilayer WS2. Raman and PL spectra of WS2/Au
heterostructures exhibit marked and uniform enhancement
effect, which is more prominent for the WS2/Au(II) system.
Near-field intensity distribution evidences the formation of
localized surface plasmon resonance, and thus is responsible
for the promoted optical response. Resonant enhancement on
the valley polarization is further investigated by analyzing the
increased electromagnetic field intensity, exciton decay rate,
and radiation efficiency. The value is not only layer-dependent
but also associated with the chirality of Au periodic structures.
Possible depolarization effects are also assessed in the
excitation and emission processes. After a good theoretical
design, a tunable valley polarization up to 30.0% is achieved in
bilayer WS2, with the plasmonic coupling with Au(II) arrays.
Our work provides an important way for enhancing the optical
signals of 2D materials and offers a potential platform to
exploit the manipulation of valley degree of freedom in the
plasmonic-based optoelectronic and valleytronic applications.
Figure 5. (a−c) Valley-polarized PL spectra of monolayer WS2 on
SiO2/Si, Au(I), and Au(II) arrays, respectively. (d−f) Valley-polarized
PL spectra of bilayer WS2 on SiO2/Si, Au(I), and Au(II) arrays,
respectively. The degrees of polarization distributions are denoted by
the blue circles. The spectra are measured under a 532 nm
nonresonant excitation at room temperature.
■
generated PL spectra are mixed with the RCP and LCP
components, as denoted by the black and red curves. The
profile and position of all of the valley-polarized peaks are
consistent with previous results in Figure 1, with the degree of
polarization distribution denoted by the blue circles.
Considering the neutral exciton emission, monolayer WS2
exhibits a really low valley polarization of 3.0% on SiO2/Si
surface (Figure 5a) and attain higher values of 7.7 and 13.3%,
respectively, as coupling with Au(I) and Au(II) arrays (Figure
5b,c). Bilayer WS2 possesses an intrinsic higher valley
polarization of 6.6% (Figure 5d), and the value is increased
to 12.0 and 30.0%, respectively (Figure 5e,f). The enhancement of valley polarizations is well consistent with the
theoretical analysis, and the magnitudes are reasonably a little
lower than the predicted values. In the previous reports, low
THEORETICAL AND EXPERIMENTAL SECTION
Fabrication of Plasmonic Coupling Heterostructures. Nanosphere lithography method is employed to construct Au plasmonic
arrays. Single-layer polystyrene (PS) nanospheres with a diameter of
530 nm are fabricated on a SiO2/Si substrate using the self-assembly
technology to form hexagonal closed packages, as shown in Figure S1.
Then, Au atoms are thermally evaporated perpendicularly onto the
triangle gaps of the nanospheres, forming Au(I) arrays. To obtain the
Au(II) arrays, angle-resolved nanosphere lithography (AR NSL) is
adopted.40 By dissolving the nanospheres away with the tetrahydrofuran (THF) solution and thermal annealing at 200 °C, Au(I) and
Au(II) arrays are successfully achieved. Monolayer WS2 is synthesized
on the sapphire structure using the typical CVD growth process with
sulfur (S) (Aladdin, 99.99%) and tungsten trioxide (WO3) (Alfa
Aesar, 99.9%) precursors.43 As-grown WS2 is transferred through the
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Zhiming Wu − Department of Physics, OSED, Fujian
Provincial Key Laboratory of Semiconductor Materials and
Applications, Xiamen University, Xiamen 361005, P. R.
China; orcid.org/0000-0003-2310-5270;
Email: zmwu@xmu.edu.cn
polymethyl methacrylate (PMMA)-assisted wet-transfer method onto
the top of Au(I) and Au(II) prism arrays. Thermal annealing is then
performed with 100 sccm argon and 5 sccm hydrogen to remove the
organic residual and achieve better contact between Au and WS2
films.
Characterizations. Surface topography of all of the samples is
observed through an FEI Quanta-600 FEG Environmental SEM with
a beam voltage of 6 kV. The thickness of the Au(I) and Au(II) arrays
is measured using an SPA400 Nanonavi AFM. Extinction spectra of
Au(I) and Au(II) arrays are characterized by a UV/vis spectrophotometer (PerkinElmer, Lambda 850). Stoichiometry and bonding
states of WS2 are confirmed by a Quantum 2000 X-ray photoelectron
spectroscopy (XPS) (Figure S3). Raman and PL signals are pumped
using a 532 nm excitation laser and detected through a Horiba
LabRam HR Evolution confocal spectrometer with a 100× objective
lens. The TRPL spectra were recorded on a microscope spectrometer
(HORIBA, MicOS) using a 325 nm pulsed semiconductor laser. For
the valley polarization measurements, a linear polarizer and a quarterwave plate are placed in the excitation path to produce the left- and
right-handed circularly polarized excitation lights. The polarization of
emission lights is analyzed by the quarter-wave plate and another
broad-band linear polarizer before the spectrometer system. All of the
characterizations are carried out at room temperature.
FDTD Simulations. Commercial FDTD simulation software
package (FDTD Solutions, Lumerical Solutions, Inc.) is employed
to perform the near-field electromagnetic wave calculations.44
According to the periodicity of the structures, only one unit is
considered in the simulations, where the periodic boundary
conditions along the x and y axes are adopted. Plane waves are
incident perpendicularly along the negative direction of the z-axis, and
the reflection, transmission, and extinction are monitored using a
group of power monitors. To perform circularly polarized excitation,
two plane waves polarized perpendicular to each other on the X−Y
plane with a ±90° phase offset. A total of 112 pairs of dipoles with a
phase difference of ±90° are distributed in the simulation area with
random phases and position to calculate the chiral emission.
Electromagnetic fields and decay rates are simulated within the
power monitors and transmission box, respectively. The refractive
indexes and extinction coefficients of Si, SiO2, and Au are obtained
from Palik’s Handbook of Optical Constants,45 and the dielectric
functions of monolayer and bilayer WS2 are deduced from the firstprinciples simulations.
■
Authors
Gaohong Liu − Department of Physics, OSED, Fujian
Provincial Key Laboratory of Semiconductor Materials and
Applications, Xiamen University, Xiamen 361005, P. R.
China
Xuanli Zheng − Department of Physics, OSED, Fujian
Provincial Key Laboratory of Semiconductor Materials and
Applications, Xiamen University, Xiamen 361005, P. R.
China
Haiyang Liu − Department of Physics, OSED, Fujian
Provincial Key Laboratory of Semiconductor Materials and
Applications, Xiamen University, Xiamen 361005, P. R.
China
Jun Yin − Department of Physics, OSED, Fujian Provincial
Key Laboratory of Semiconductor Materials and
Applications, Xiamen University, Xiamen 361005, P. R.
China; orcid.org/0000-0003-4551-3515
Congming Ke − Department of Physics, OSED, Fujian
Provincial Key Laboratory of Semiconductor Materials and
Applications, Xiamen University, Xiamen 361005, P. R.
China
Weihuang Yang − Key Laboratory of RF Circuits and System
of Ministry of Education, Hangzhou Dianzi University,
Hangzhou 310018, P. R. China
Xu Li − Department of Physics, OSED, Fujian Provincial Key
Laboratory of Semiconductor Materials and Applications,
Xiamen University, Xiamen 361005, P. R. China
Chunmiao Zhang − Department of Physics, OSED, Fujian
Provincial Key Laboratory of Semiconductor Materials and
Applications, Xiamen University, Xiamen 361005, P. R.
China
Junyong Kang − Department of Physics, OSED, Fujian
Provincial Key Laboratory of Semiconductor Materials and
Applications, Xiamen University, Xiamen 361005, P. R.
China
ASSOCIATED CONTENT
* Supporting Information
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsami.1c06622.
■
Research Article
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsami.1c06622
Schematic diagrams of the preparation process of the PS
nanospheres arrays and the Au single- and dimer-prism
arrays, morphologies of WS2/Au heterostructures 2D PL
mapping, XPS of as-grown monolayer WS2, near-field
distribution of the monolayer and bilayer WS2/Au
hybrids under linearly polarized excitation, near-field
distribution in monolayer WS2/Au heterostructures
under circularly polarized excitation, near-field distribution in bilayer WS2/Au heterostructures under circularly
polarized excitation, and near-field distribution EL and
ER in monolayer WS2/Au heterostructures (PDF)
Author Contributions
G.L., X.Z., and H.L. contributed equally to this work. The
manuscript was written through contributions of all authors.
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors gratefully acknowledge the National Science Fund
for Excellent Young Scholars (grant no. 62022068), the
National Natural Science Foundation of China (grant nos.
61874092, 61774128, 61974123, 61674124, and 61804129),
Science and Technology Project of Fujian Province of China
(grant nos. 2018I0017 and 2019H0002), and Science and
Technology Key Project of Xiamen (grant no.
3502ZCQ20191001).
AUTHOR INFORMATION
Corresponding Authors
Yaping Wu − Department of Physics, OSED, Fujian Provincial
Key Laboratory of Semiconductor Materials and
Applications, Xiamen University, Xiamen 361005, P. R.
China; orcid.org/0000-0001-9325-2212; Email: ypwu@
xmu.edu.cn
35102
https://doi.org/10.1021/acsami.1c06622
ACS Appl. Mater. Interfaces 2021, 13, 35097−35104
ACS Applied Materials & Interfaces
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www.acsami.org
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ABBREVIATIONS USED
TMDCs, transition-metal dichalcogenides
vdW, van der Waals
LCP, left-handed circularly polarized
RCP, right-handed circularly polarized
PL, photoluminescence
2D, two-dimensional
Ga NPs, gallium hemispherical nanoparticals
CVD, chemical vapor deposition
SERS, surface-enhanced Raman scattering
FDTD, finite-difference time domain
SEM, scanning electron microscopy
AFM, atomic force microscopy
DVP, degree of valley polarization
PS, polystyrene
AR NSL, angle-resolved nanosphere lithography
THF, tetrahydrofuran
S, sulfur
WO3, tungsten trioxide
PMMA, polymethyl methacrylate
XPS, X-ray photoelectron spectroscopy
TRPL, time-resolved photoluminescence
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Research Article
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