Kesarwani
et al
.,
Sci. Adv.
8
, eabm0100 (2022) 1 April 2022
SCIENCE ADVANCES
|
RESEARCH ARTICLE
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PHYSICS
Control of
trion-to-exciton conversion in
monolayer
WS
2
by orbital angular momentum of
light
Rahul Kesarwani
1
, Kristan Bryan
Simbulan
1
, Teng-De
Huang
1
, Yu-Fan
Chiang
1
, Nai-Chang
Yeh
2
*,
Yann-Wen Lan
1
*, Ting-Hua
Lu
1
*
Controlling the density of exciton and trion quasiparticles in monolayer two-dimensional (2D) materials at room
temperature by nondestructive techniques is highly desired for the development of future optoelectronic devices.
Here, the effects of different orbital angular momentum (OAM) lights on monolayer tungsten disulfide at both
room temperature and low temperatures are investigated, which reveal simultaneously enhanced exciton inten-
sity and suppressed trion intensity in the photoluminescence spectra with increasing topological charge of the
OAM light. In addition, the trion-to-exciton conversion efficiency is found to increase rapidly with the OAM light
at low laser power and decrease with increasing power. Moreover, the trion binding energy and the concentration
of unbound electrons are estimated, which shed light on how these quantities depend on OAM.
A phenomeno-
logical model is proposed to account for the experimental data. These findings pave a way toward manipulating
the exciton emission in 2D materials with OAM light for optoelectronic applications.
INTRODUCTION
Excitons are one of the most studied quasiparticles in modern quan
-
tum materials because of their technological relevance to advanced
electronic, optoelectronic, and photonics devices (
1
–
3
). Recently,
excitonic-based materials have been used in the development of high-
performance photodetectors (
4
), light-emitting diodes (
5
), excitonic
lasers (
6
), valleytronic transistors (
7
), and optical interconnectors (
8
).
Moreover, strong excitonic effects also contribute to nonlinear op-
tical excitations such as two-photon luminescence and high-order
harmonic generation (
9
). The wide range of applications associated
with exciton quasiparticle may be attributed to strong light-matter
interaction that enhances the photoluminescence (PL) and electrolu
-
minescence properties of the material (
10
). It is therefore desirable to
devise new techniques to control and enhance radiative excitons in ex
-
isting materials to improve the excitonic-based device applications.
An exciton is a neutral excitation that consists of an electron-hole
pair bound by Coulomb interaction in a semiconductor or an insu-
lator (
11
). Much research efforts have been made to improve the
excitonic behavior in composite materials, although more attention is
still needed to advance the development of excitonic devices (
12
,
13
).
Earlier, excitonic devices have been largely based on gallium arsenide
(GaAs) quantum dots or quantum wells because of their rich excitonic
properties. However, the weak exciton binding energy (<7 meV) in
these systems requires operation at low temperatures (near 4 K)
(
14
,
15
) and implies ultrashort radiative lifetimes, which are orders
of magnitude shorter than that of conventional semiconducting light
emitters (
16
). This drawback can be overcome by replacing the host
material for excitons with monolayers of transition metal dichalco-
genides (TMDs) such as WS
2
, WSe
2
, and MoS
2
(
17
,
18
). Because of
the reduced Coulomb screening effects in lower dimensions, these
monolayer TMD materials have much stronger exciton binding ener
-
gies (in the range of 0.2 to 0.4 eV) so that excitons may be trapped and
PL from radiative recombination of excitons can still be observed at
room temperature (RT) (
19
). In addition, these atomically thin TMD
materials (on the order of 0.6 nm) can be more easily fabricated by
various deposition techniques than GaAs-based quantum wells. These
attributes of TMDs are therefore promising for developing practical
excitonic devices. However, the exciton strength in the PL spectra of
TMD monolayers at RT is often substantially suppressed probably
because of various defects (e.g., vacancies, impurities, surface degra
-
dation, etc.) that behave as nonradiative exciton recombination sites
and reduce the quantum efficiency (
20
). In addition, defects in the mono
-
layers may result in reduced binding energies for the electron-hole
pairs and may contribute to excess doping that increases the Coulomb
screening effects (
21
). These natural defects in the monolayer TMDs
can give rise to the formation of impurity states near the Fermi level,
leading to a large number of free carriers that can be easily coupled
with excitons and form positive and negative trions (
22
). Hence, ideal
TMD-based excitonic devices that are operational at RT are still not
feasible so far (
23
). Various methods have been explored to reduce the
defects and to enhance the excitonic behavior in monolayer TMDs,
including chemical synthesis (
24
), gate voltage control (
25
), and mag
-
netic field (
26
).
The orbital angular momentum (OAM) of light with helical wave
-
fronts having a well-defined OAM ℓℏ per photon was first demon-
strated in 1992 by Allen
et al.
(
27
). When light carrying a finite OAM
illuminates on a sample, a net force is applied to the material as the
result of nonuniform field distributions (
28
,
29
). In addition, an OAM
is transferred to the material, leading to the manifestation of different
degrees of freedom of light (
30
,
31
). In our previous studies, we in-
vestigated the effect of OAM light on the excitonic properties of
monolayer MoS
2
and revealed an unusual light-like exciton band dis
-
persion of valley excitons (
32
). We expect that further application of
OAM to the studies of light-matter interaction can lead to more
findings, because this process can provide notable control of the ex-
citonic behaviors at RT without the need for any surface treatments
of the sample or the application of external electric/magnetic fields.
In this work, we present a new approach to enhancing the exci-
tonic behavior in monolayer tungsten disulfide (WS
2
) by applying
OAM of light, also known as twisted light (
28
). We find that the
excitonic behavior and the contribution of trions in WS
2
monolayer
1
Department of Physics, National Taiwan Normal University, Taipei, Taiwan.
2
Depart-
ment of Physics, California Institute of Technology, Pasadena, CA 91125, USA.
*Corresponding author. Email: ncyeh@caltech.edu (N.-C.Y.); ywlan@ntnu.edu.tw
(Y.-W.L.); thlu@ntnu.edu.tw (T.-H.L.)
Copyright © 2022
The Authors, some
rights reserved;
exclusive licensee
American Association
for the Advancement
of Science. No claim to
original U.S. Government
Works. Distributed
under a Creative
Commons Attribution
NonCommercial
License 4.0 (CC BY-NC).
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Kesarwani
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.,
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, eabm0100 (2022) 1 April 2022
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can be controlled at RT using different topological charges and powers
of OAM light and propose a phenomenological model to account
for the interaction of OAM light with monolayer TMDs. Our find-
ings further demonstrate that OAM of light can command the inter-
and intravalley transitions in TMD materials, which offers a pathway
to tuning optical devices.
RESULT AND DISCUSSION
Monolayer WS
2
flakes were grown on SiO
2
(300 nm)/Si substrate
via the chemical vapor deposition (CVD) technique (
33
). Twisted light
with different topological charges was generated by a spatial light
modulator (SLM) and integrated with an in-house PL and Raman setup,
as schematically shown in Fig. 1A. The OAM light was illuminated
onto the monolayer WS
2
sample to study the excitonic behavior from
the resulting PL spectra. The laser was a continuous wave, and the
excitation wavelength was 532 nm.
The horizontally polarized light was collimated to a phase-only
SLM of 1920 pixels by 1080 pixels with a pixel pitch of 8
m. The phase
patterns of different ℓ values were displayed on the SLM through a
computer-generated hologram, which converted the incident beam
into twisted light. The phase variations of twisted light with vari-
ous odd numbers of ℓ are shown in Fig. 1B. The twisted beam passed
through a beam reducer and was focused on the monolayer WS
2
flake
by a 50× long working distance objective lens. The optical micros-
copy image of the WS
2
flake is shown in Fig. 1C. The fabricated WS
2
flakes are monolayer, which have been confirmed by atomic force
microscopy (AFM) and Raman spectroscopy, as shown in fig. S1.
Figure 1 (D and E) is the spatially resolved PL and Raman intensity
maps of the monolayer WS
2
under the illumination of a Gaussian
beam (ℓ = 0), and the maps show that the PL and Raman (
E
2
g
1
) signals
were uniform throughout the sample. The excitonic behavior in the
PL spectra of WS
2
sample was studied and analyzed as a function
of different OAM ℓ values and different laser powers on the sam-
ple surface.
The formation of excitons is often accompanied by trions, espe-
cially under the high laser power excitation. Figure 2A shows the PL
spectra under different excitation laser powers from 100 to 800
W. While
the PL intensities associated with both neutral excitons and trions
increase steadily with increasing power, contributions from trions
apparently increased much faster with increasing power than exci-
tons. This phenomenon may be attributed to heating and strain-
induced formation of defect states under high laser power (
20
,
34
).
Given that the increase in laser power has different effects on the for
-
mation of trions and excitons within the monolayer WS
2
, an alterna
-
tive approach to better controlling the quasiparticles must be devised.
Figure 2B displays the PL spectra under different excitations of OAM
light from ℓ = 0 to 5 at a fixed power of 800
W.
The decreasing PL
intensity of WS
2
with the increasing value of the topological charge
of light (ℓ) may be attributed to the decreasing power density on the
sample due to the increasing OAM beam size with ℓ. Figure 2C de-
picts the deconvoluted PL spectrum with ℓ = 0 excitation to distin-
guish the exciton and trion peaks. The blue, red, and gray lines are
fitting curves signifying the neutral excitons, trions, and defect states,
respectively. Detailed analysis of the PL spectra taken at each con-
stant power, as exemplified in Fig. 2B and fig. S2, reveals that the trion
intensity steadily decreases with the increasing topological charge
of the OAM light. This demonstration of controlling the formation of
trions by OAM light presents a novel approach to achieving exciton-
rich monolayer WS
2
.
Fig. 1. Experimental setup and optical results of a CVD-grown monolayer WS
2
.
(
A
) Schematic of the PL and Raman spectroscopy setup for optical beam carrying
OAM.
CCD, charge-coupled device. ND filter, neutral density filter; HWP, half-wave plate. (
B
) Phase variations of the incident optical beam with different OAM values, ℓ.
(
C
) Optical microscopy image of an as-grown monolayer WS
2
flake on SiO
2
/Si substrate. (
D
and
E
) Intensity maps of the PL and Raman
(
E
2
g
1
) mode of the monolayer WS
2
sample shown in (C). a.u., arbitrary units.
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, eabm0100 (2022) 1 April 2022
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To mitigate the heating-induced formation of defect state, we in-
vestigated the PL spectra taken at 77 and 300 K under a lower laser
power (≤100
W) for different values of OAM (ℓ = 0 to 8) and ex-
tracted the dependence of trion and exciton intensities on the
OAM. Figure 3 (A and B) illustrates the deconvolution of PL spectra
that distinguishes the exciton and trion peaks under the fundamental
mode (ℓ = 0) and odd-numbered OAM light excitation at 300 and
77 K on a fixed laser power of 30
W, respectively. We note that the
full width at half maximum (FWHM) linewidths of all three peaks un-
der the fundamental mode (ℓ = 0) became narrower from 300 to 77 K.
Fig. 2. Power and OAM light dependence PL of the monolayer WS
2
.
(
A
) PL spectra of monolayer WS
2
excited by the fundamental mode of light ℓ
= 0 under different
laser powers from 100 to 800
W. (
B
) Control the trion intensity by different OAM lights (ℓ
= 0 to 5) at a fixed laser power of 800
W. (
C
) Theoretical fitting to the spectrum
taken with ℓ
= 0 and 800
W of power, which reveals contributions from different components of the neutral excitons (blue), trions (red), and defect states (gray).
Fig. 3. OAM light dependence PL of the monolayer WS
2
.
Deconvoluted PL spectra of a monolayer WS
2
for an odd number of topological charges of OAM light with a
laser power of 30
W at (
A
) 300
K and (
B
) 77
K.
The theoretical fitting to the spectrum represents the contributions from excitons (blue), trions (red), and defect states
(gray). (
C
) Comparison of the variations of the trion/exciton intensity ratio with the OAM value ℓ under a constant laser power of 30
W at 300
K (red squares) and 77
K
(blue squares). (
D
) Trion-to-exciton conversion efficiency at 300
K as a function of the laser power.
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For other OAM excitations, the deconvolution of PL spectra and
detailed fitting for different contributions and parameters are shown
in figs. S2 and S3 and tables S1 and S2, respectively. Although the de
-
convoluted third peak (gray color) in the PL spectra (Fig. 3, A and B)
at ~640 nm (1.937 eV) and ~605 nm (2.049 eV) for RT and 77 K,
respectively, was referred to as a defect peak in both cases, the na-
ture of the corresponding defects at RT and 77 K was different: At
RT, the defect peak at ~640 nm may be attributed to bound excitons
(
35
), while the low-temperature defect peak at ~605 nm was found
to be associated with the activation of shallow localized states (
36
).
We have assessed the dependence of exciton and trion intensities on
the OAM light and laser power by estimating the number of trions
converted into excitons with increase ℓ from the trion-to-exciton
intensity ratio. Figure 3C exhibits the trion-to-exciton intensity ra-
tio extracted from Fig. 3 (A and B) with different OAM light excitations.
This analysis implies the substantial transformation of trions into
excitons with increasing topological charge up to ℓ = 5, beyond
which the trion-to-exciton intensity ratio (
I
trion
/
I
exciton
) saturates. For
comparison of the cases with different laser powers, we also per-
formed similar experiments for the trion/exciton intensity ratio at
300 K and found that the trion/exciton intensity ratio increases with
increasing laser power for each fixed value of OAM, as shown in fig. S4.
After exponentially fitting the trion-to-exciton intensity ratio versus
topological charge ℓ for a given power using the relation (
I
trion
/
I
exciton
) =
c
0
exp. (−
/ℓ), with
c
0
being a numerical constant, the coefficient
that represents the efficiency of trion-to-exciton conversion with in
-
creasing OAM under a constant power can be estimated. We define
the conversion efficiency
as a coefficient that represents the ratio
of the decreasing trion population to the corresponding increas-
ing exciton population as a function of ℓ at a constant laser power.
Figure 3C depicts the (
I
trion
/
I
exciton
) versus ℓ dependence under a
constant power of 30
W at 300 and 77 K, while the comparison of
the
values at 300 K under different laser powers is provided in
Fig. 3D. It shows that the conversion efficiency is very high at low
laser power and decreases exponentially with increasing power, which
implies that the OAM light can suppress trions efficiently at low power.
Detailed analyses of the PL spectra of monolayer WS
2
for the other
laser powers (50, 70, and 100
W) at 77 and 300 K are given in figs. S5 to
S8. Furthermore, we have also conducted a control experiment that com
-
bines twisted light with electrostatic gating and found that OAM light
can effectively reduce the free electron concentration and suppresses
trion formation. The effect of gate voltage on the trion-to-exciton con
-
version efficiency of the WS
2
sample for 100-
W laser power at 300 K
is shown in figs. S9 and S10. We further note that we did not perform
further PL measurements at lower temperatures (<77 K) because the
trion-to-exciton intensity ratio taken at 77 K with a fixed laser power
was already nearly independent of the OAM value (ℓ) of light as
shown in Fig. 3C.
To gain further insights into the effect of OAM and laser power on
trions and excitons, we examined the FWHM linewidths of the PL
spectra, because the behavior of excitons and trions in the material, in
-
cluding intra- and intervalley trions, may be dependent on their inter
-
actions with different OAM values of light (
37
). Figure 4 (A and B)
shows the three-dimensional (3D) plot of the trion and exciton FWHM
at 300 K as a function of the topological charge of OAM light and the
laser power, respectively. As evidenced in Fig. 4A, the trion FWHM
linewidth decreases rapidly with increasing OAM and increases slightly
with laser power. The dependence on the OAM may be understood
in terms of the presence of both intra- and intervalley trions: When
monolayer WS
2
interacts with fundamental light (ℓ = 0), both intra-
valley (dominant) and intervalley (secondary) trions can be excited,
which result in a relatively large FWHM linewidth (
38
). On the other
hand, when monolayer WS
2
is excited by twisted light with ℓ ≠ 0, the
probability of forming intravalley trions is suppressed with increas-
ing ℓ, as manifested by the decreasing FWHM linewidth. In contrast,
the exciton FWHM linewidth does not exhibit substantial variations
with either the OAM or laser power, as shown in Fig. 4B. In particular,
we note that the OAM light evidently affects the trion properties
without disturbing the exciton characteristics in the monolayer WS
2
.
Detailed FWHM linewidth analyses of the exciton and trion states
extracted from the PL spectra at 77 K are given in fig. S11, where the
exciton FWHM is found to be nearly independent of either the OAM
or the laser power and the trion FWHM exhibits a slight decrease
with increasing OAM for the laser power at 30 and 50
W.
The enhancement of the density of exciton quasiparticles due to
the suppression of trions is highly dependent on the binding energy
of the trion and the concentration of unbound electrons at the Fermi
level (
39
,
40
). A higher value of trion binding energy corresponds to a
stronger Coulomb interaction between the exciton and an unbound
electron (
41
). The splitting between the exciton and trion energies is
predicted to be linearly dependent on the Fermi energy (
E
F
) and is
defined as (
42
)
Fig. 4. FWHM analysis of trion and exciton from deconvoluted PL spectra.
FWHM linewidths of the PL spectral intensities taken at 300
K are shown as a function of
the OAM of light and laser power after deconvolution for contributions from (
A
) trions and (
B
) excitons, respectively.
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E
X
o
−
E
X
−
=
E
X
−
b
+
E
F
(1)
where
E
X
−
b
is the trion binding energy,
E
Xo
and
E
X
−
are the energy
levels of excitons and trions, respectively. Because an exciton can be
considered as an ionized trion, (
E
Xo
−
E
X
−
) defines the minimum
energy required to remove one electron from the trion. Hence, the
density of trions is proportionally dependent on the unbound elec-
tron density at the defect/Fermi level. The native defect density in
the material corresponds to the unbound electron concentration
responsible for the formation of trions (
43
), and the defect concen-
tration may be derived from the mass action law, which is a theory
used to estimate the defect concentration in 2D materials optically
(
22
). Therefore, the density of unbound electrons associated with
defects at the Fermi level may be estimated accordingly (
44
).
Siviniant
et al.
(
45
) first proposed the mass action law for exci-
tons and trions, which is given by the following expression
n
Xo
n
e
−
─
n
X
−
=
K
(
T
) =
4
m
eff
K
B
T
─
ℏ
2
exp
(
−
E
X
−
b
─
K
B
T
)
(2)
where
n
Xo
,
n
X
−
, and
n
e
−
are the concentrations of excitons, trions,
and unbound electrons at the Fermi level of monolayer WS
2
, re-
spectively.
m
eff
is the reduced effective mass of monolayer WS
2
,
k
B
is
the Boltzmann’s constant,
T
is the temperature, ℏ is the reduced
Plank’s constant, and
E
X
−
b
is the binding energy of the trions. The c
on-
centration of trion/exciton ratio is directly evaluated from the area of trion
and exciton peak spectra in the PL spectra (
45
,
46
). The effective mass
of monolayer WS
2
is defined by (
47
)
m
eff
=
m
Xo
m
e
*
_
m
X
−
, where
m
e
*
= 0.31
m
e
is the effective electron mass,
m
h
*
= 0.42
m
e
is the effective hole
mass,
m
Xo
=
m
e
*
m
h
*
_
(
m
e
*
+
m
h
*
)
is the exciton mass, and
m
X
−
=
m
e
*
(
m
e
*
+
m
h
*
)
_
(2
m
e
*
+
m
h
*
)
is the
trion mass. The detailed calculation for the density of unbound elec-
trons at the Fermi/defect level of WS
2
is provided in note S1.
Using Eqs. 1 and 2 and assuming that
n
Xo
and
n
X
−
are propor-
tional to the PL spectral weight of the exciton and trion peaks, re-
spectively, we obtain estimated trion binding energies and unbound
electron concentrations of monolayer WS
2
at 300 K in Fig. 5 (A and B),
respectively, as functions of the OAM and laser power. In Fig. 5A, we
find that the trion binding energy depends on the topological charge
of the OAM light and the laser power. For low power (30
W), the
binding energy is around 24 meV for ℓ = 0 and varies only slightly
and nonmonotonically with increasing ℓ. When power increases
from 50 to 100
W, the trion binding energy increases from 31 to 36 meV
for the fundamental mode of light (ℓ = 0). A similar trend of in-
crease in the trion binding energy
E
X
−
b
with power is also found for
ℓ > 0. This increase in
E
X
−
b
with power for a given ℓ may be attributed
to the excess light–induced defect states in the material, which result
in more unbound electrons at the defect level and therefore higher
probabilities for the formation of trions. On the other hand,
E
X
−
b
de
-
creases with increasing ℓ from 1 to 8 for high powers of 50, 70, and
100
W, as shown in Fig. 5A. In particular, a substantial decrease
in
E
X
−
b
is found from 36 meV with ℓ = 0 to 27 meV with ℓ = 8 under
100-
W laser power. This steady decrease in
E
X
−
b
with increasing ℓ is
consistent with the rapid decrease in
I
trion
/
I
exciton
with ℓ in Fig. 3C. In
addition, the substantial decrease in
E
X
−
b
by 9 meV from ℓ = 0 to
ℓ = 8 for 100
W further confirmed the reduction of intravalley trions
(of a larger binding energy) and the enhancement of intervalley trions
(of a smaller binding energy) with increasing ℓ (
48
). Similarly, we
find in Fig. 5B that the concentration of unbound electrons at the
defect level decreases with increasing OAM of light. The estimated
unbound electron densities associated with intrinsic defects via the
mass action law agree well with previous reports that estimated the
defect concentration through different means (
49
,
50
).
When light having OAM illuminates on a sample, there is a net
force applying to the material and a torque acting on the electron
and atom according to the relation
〈
L
z
̇
〉
_
P
=
ℓ
_
B
, where
P
is the total
power in the beam,
〈
L
z
̇
〉
=
is the torque exerted upon an object,
and
B
is the angular frequency of the beam (
51
). It implies that the
torque acting on the object is directly proportional to the topologi-
cal charge of light at fixed power. The OAM light exerting a torque
on the atom and molecules can control the induced current in de-
vices (
28
–
31
), which implies that the OAM light can affect and trap
unbound electrons at a defect level in the WS
2
sample by applying a
torque. The concentration of unbound electrons at the defect level
decreases with increasing OAM of light as shown in Fig. 5B, which
may be attributed to OAM light–induced nonzero torque on elec-
trons at the defect level/conduction band (
51
), leading to the reduc-
tion of unbound electrons and the suppression of intravalley trions
with increasing ℓ. While the unbound electron concentration de-
creases rapidly with the OAM for ℓ ≤ 5, it becomes almost a con-
stant for higher values of OAM (ℓ > 5) even after increasing the laser
power. In addition, the concentration of unbound electrons for
ℓ > 0 increases slightly with increasing power of up to 50
W and
then saturates at higher powers. We further note that we have also
Fig. 5. Data analysis of the trion binding energy and the unbound electron concentration.
(
A
) Estimation of the binding energy
(
E
X
−
b
) of trions with different OAM
values and laser powers at 300
K. (
B
) Dependence of the unbound electron concentration (
n
e
−
) on the OAM and laser power at 300
K.
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investigated the PL spectra of several monolayer WS
2
and MoS
2
sam-
ples with different initial trion-to-exciton intensity ratios and found that
the characteristics of the trion and exciton dependence on the OAM
of light are all similar, which confirms the reliability of this method.
Proposed model for the mechanism of twisted
light-matter interaction
The band structure of pristine monolayer WS
2
at the band edges
K and K′ reveals a valence band splitting around 395 meV and a con-
duction band splitting around 17 meV as the result of spin-orbit coupling
(
40
,
52
). In the presence of excess defect-induced electron doping,
which is common among WS
2
, the density of unbound electrons de
-
pends on the density of defects in the material, and the excess electrons
associated with the defect level can be easily transferred to the conduc
-
tion band. Previous studies have shown that, at high temperatures,
unbound electrons are more likely to transfer from the defect level to
the conduction band and form trions through Coulomb interactions
with excitons (
53
). These trions may be divided into intravalley trions
and intervalley trions, depending on whether the electrons coupled with
the excitons are from the same or different valleys (
40
,
54
). Further
studies of trions in the TMD material have indicated that intravalley trions
have shorter lifetimes and larger FWHM than intervalley trion (
38
).
From our experimental results at RT, we have found that OAM
light on monolayer WS
2
can effectively suppress the formation of
trions and reduce both the FWHM of the PL peak and the binding
energy of trions. To describe the OAM light–matter interaction with
the electron-rich monolayer WS
2
film based on these experimental
results (
40
), we propose a phenomenological model as depicted by
the band structure diagrams in Fig. 6. Figure 6A shows that, under
the excitation of fundamental light (ℓ = 0), intravalley trions domi-
nate over intervalley trions, which is consistent with our experimental
findings of larger FWHM and binding energy of trions in Fig. 4A and
Fig. 5A for ℓ = 0. In contrast, as schematically illustrated in Fig. 6B,
under OAM light illumination (ℓ ≠ 0), free electrons are affected by
the OAM light–induced torque and become bound electrons, which
reduces the probability of trion formation. In addition, intervalley
trions become dominant over intravalley trions due to multipole in
-
teractions induced by OAM light, which is consistent with the ex-
perimental findings of smaller FWHM and binding energy of trions
in Fig. 4A and Fig. 5A for ℓ ≠ 0.
When light carrying a finite OAM (ℓ ≠ 0) interacts with a sus-
pended atom in the medium, a torque induced by the twisted light
is exerted onto the atom (
51
). The strength of the torque depends
on the value of the topological charge of the OAM and the power of
light. In the case of monolayer WS
2
, the illumination of OAM light
may induce two effects. First, a finite torque induced by the twisted
light provides an effective binding force acting on the unbound elec
-
trons in the conduction band, which affects the properties of charged
trions without disturbing the neutral excitons (
51
). Second, larger
topological charges can stimulate multipole transitions, thereby en-
hancing the intervalley transitions (
55
). Our observation of a sub-
stantial decrease in the trion binding energy from 36 meV for ℓ = 0
to 27 meV for ℓ = 8 under 100-
W laser power is strongly suggestive
of the intervalley trion formation at larger OAM (
48
). In addition,
the suppression of intravalley trions and the enhancement of neutral
excitons with increasing ℓ as shown in Fig. 3C further corroborates
the effects of OAM light. This interchangeable effect is also a well-
known fingerprint of a trion-exciton pair in a PL spectrum (
56
). On
Fig. 6. Phenomenological model for OAM light–matter interaction.
Schematic comparison of the interaction mechanism of monolayer WS
2
with excitation of (
A
) the
fundamental mode of light (ℓ
= 0) where intravalley excitons/trions dominate over intervalley excitons/trions and (
B
) the twisted light (ℓ
≠ 0) where intravalley excitons/trions
are suppressed and intervalley excitons/trions become dominant. Here, the red (blue) energy bands represent the spin-up (spin-down) bands, and
X
intra
T
1−
and
X
inter
T
2−
denote
the intra- and intervalley trions, respectively.
Downloaded from https://www.science.org at California Institute of Technology on April 04, 2022
Kesarwani
et al
.,
Sci. Adv.
8
, eabm0100 (2022) 1 April 2022
SCIENCE ADVANCES
|
RESEARCH ARTICLE
7 of 8
the other hand, at sufficiently low temperatures, the unbound elec-
trons are mostly frozen so that the torque from OAM light no lon
-
ger helps reduce the unbound electron concentration or the trion
density, which is consistent with our finding of negligible effects of OAM
light on the trion-to-exciton ratio. Thus, our proposed phenomeno
-
logical model provides a reasonable account for the effect of OAM
light on controlling the trion behavior in monolayer TMDs.
In summary, we have investigated the effect of different topological
charges of OAM light on the binding energies and FWHM of exci-
tons and trions in monolayer WS
2
at 77 and 300 K.
By systematically
increasing the topological charge of OAM light from 0 to 8 for each
value of the illuminating power from 30 to 800
W, we analyze the
evolution of the trion and exciton contributions to the PL spectra. It
is found that, for low laser power and at 300 K, the trion-to-exciton in
-
tensity ratio decreases rapidly with increasing OAM up to ℓ = 5 and
then saturates for ℓ ≥ 5, while no discernible trion-to-exciton con-
version is observable at low temperature (77 K). A feasible mechanism
for the suppression of trions with increasing OAM is the exertion of
nonzero torque, the unbound electrons in the monolayer WS
2
through
the OAM light–matter interaction: The finite torque of OAM light in
-
teracts with WS
2
and results in a reduced concentration of unbound
electrons at the defect level/conduction band, thereby reducing the
probability of trion formation. In addition, OAM light is found to
suppress intravalley trions and enhance intervalley trions. In contrast,
OAM light has little effect on neutral excitons. Therefore, a notable
enhancement of exciton emission can be achieved by applying OAM
light to suppress the overall trion formation, which offers a new
strategy to manipulate the PL spectra in monolayer TMDs for RT
photonic and optoelectronic applications.
MATERIALS AND METHODS
Sample preparation
SiO
2
/Si substrates (300-nm SiO
2
) were used for the CVD growth of WS
2
.
Before the growth, SiO
2
/Si substrates were cleaned in acetone and
isopropyl alcohol for 30 min to remove organic impurities, then soaked
in Nanostrip for 60 min, and lastly washed with deionized water and
dried with nitrogen gas. For synthesis of monolayer WS
2
, we used
WO
3
and S as precursors in an atmospheric pressure CVD system to
grow monolayer WS
2
on Si/SiO
2
substrates. Our setup includes the
following parts: a quartz tube with a diameter of 2.54 cm and a length
of 100 cm, a 2.54 cm–inner diameter horizontal split tube furnace
(Lindberg/Blue M), and two mass flow controllers calibrated for Ar
and H
2
, with stainless steel flanges at both ends connected to a chiller
water circulation system operating at 10°C.
In the first step of the pro-
cedure, 95 mg of WO
3
precursor mixed with 5 mg of potassium iodide
was placed in a quartz boat containing the SiO
2
/Si substrates set face-
down directly above the W source precursor, and the quartz boat was
then positioned at the center of the furnace. A second boat containing
100 mg of S (Alfa Aesar; 99.999+%) was placed upstream at 16 cm
away from the W source. Next, the system was pumped down to 3 ×
10
−2
torr to eliminate air and moisture. After the system reached the
base pressure, the Ar/H
2
(80/40 standard cubic centimeter per minute)
carrier gas was introduced until atmospheric pressure was achieved. The
furnace was then heated up with a ramp rate of 35°C/min to the growth
temperatures (750° to 850°C). The sulfur component melted at 150°C
was sent into the furnace at the growth temperature to grow WS
2
.
The sample growth procedure proceeded for 10 min, after which the
furnace was directly opened to RT to stop the reaction immediately (
33
).
Raman and
PL spectroscopy
The Raman and PL spectra were acquired using a Kymera 328i spec
-
trometer, and a backscattering configuration was performed using a
1200- and 150-line/mm grating, respectively. We used a continuous
wave laser (model: LASOS DPSSL series, GLK 3320 TS01) of 532 nm
as an excitation source. An Olympus 50× objective lens with a 0.50 nu
-
merical aperture was used for collecting light. Laser power was measured
with a Thorlabs optical power meter. For OAM light generation, the
SLM is a PLUTO phase only SLM by HOLOEYE.
The SLM uses a
panel (model: HED 6010 VIS) that operates optimally within the 420-
to 700-nm wavelength range. It has a resolution of 1920 by 1080, with
a reflectivity of around 65%, pixel pitch of 8
m, and fill factor of
87%. The active area is 1.78 cm diagonal at the reflective optical
mode. The cryogenic chamber made by Cryo Industries of America
Inc. was used for low-temperature PL measurement (77 K).
Atomic force microscopy
The AFM measurements were performed in a noncontact tapping mode,
using silicon tips with a Nanoview 1000 AFM (Utek Material).
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at https://science.org/doi/10.1126/
sciadv.abm0100
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Acknowledgments:
We thank W.-H.
Lin for developing the ML-WS
2
sample and useful
discussions.
Funding:
This work was supported by the Ministry of Science and Technology,
Taiwan under contract nos. MOST 109-2811-M-003-513, MOST 108-2112-M-003-010-MY3, and
MOST 109-2112-M-003-002. N.-C.Y. acknowledges joint support by the National Science
Foundation under the Physics Frontier Centers program for the Institute for Quantum
Information and Matter (IQIM) at the California Institute of Technology (award #1733907) and
the Army Research Office under the MURI program (award #W911NF-16-1-0472).
Author
contributions:
The experiments were performed by R.K., K.B.S., T.-D.H., and Y.-F.C.
The
numerical calculation were completed by R.K., and Y.-F.C. provided assistance in performing
the optical measurements. N.-C.Y., Y.-W.L., and T.-H.L. supervised this research. All authors
have read and approved the manuscript. All authors discussed the results and commented on
the manuscript.
Competing interests:
The authors declare that they have no competing
interests.
Data and materials availability:
All data needed to evaluate the conclusions in the
paper are present in the paper and/or the Supplementary Materials.
Submitted 20 August 2021
Accepted 10 February 2022
Published 1 April 2022
10.1126/sciadv.abm0100
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Control of trion-to-exciton conversion in monolayer WS
2
by orbital angular
momentum of light
Rahul KesarwaniKristan Bryan SimbulanTeng-De HuangYu-Fan ChiangNai-Chang YehYann-Wen LanTing-Hua Lu
Sci. Adv.
, 8 (
13),
eabm0100.
• DOI: 10.1126/sciadv.abm0100
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