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Nanophotonics 2024; 13(7): 1069– 1077
Research Article
Dong Hyeon Kim, Chanwoo Lee, Sung Hyuk Kim, Byeong Geun Jeong, Seok Joon Yun,
Hyeong Chan Suh, Dongki Lee, Ki Kang Kim and Mun Seok Jeong*
Probing the multi-disordered nanoscale alloy
at the interface of lateral heterostructure
of MoS–WS
https://doi.org/10.1515/nanoph-2023-0826
Received November 20, 2023; accepted December 22, 2023;
published online January 19, 2024
Abstract:Transition metal dichalcogenide (TMDs) het-
erostructure, particularly the lateral heterostructure of two
dierent TMDs, is gaining attention as ultrathin photonic
devices based on the charge transfer (CT) excitons gener-
ated at the junction. However, the characteristics of the
interface of the lateral heterostructure, determining the
electronic band structure and alignment at the heterojunc-
tion region, have rarely been studied due to the limited
spatial resolution of nondestructive analysis systems. In
this study, we investigated the confined phonons resulting
from the phonon-disorder scattering process involving mul-
tiple disorders at the lateral heterostructure interface of
MoS2–WS2to prove the consequences of disorder-mediated
deformation in the band structure. Moreover, we directly
observed variations in the metal composition of the multi-
disordered nanoscale alloy Mo1−xWxS2, consisting of atomic
vacancies, crystal edges, and distinct nanocrystallites. Our
findings through tip-enhanced Raman spectroscopy (TERS)
imply that a tens of nanometer area of continuous TMDs
Dong Hyeon Kim, Chanwoo Lee, and Sung Hyuk Kim contributed equally
to this work.
*Corresponding author: Mun Seok Jeong, Department of Physics,
Hanyang University, Seoul 04763, Korea, E-mail: mjeong@hanyang.ac.kr.
https://orcid.org/0000-0002-7019-8089
Dong Hyeon Kim and Sung Hyuk Kim, Department of Physics, Hanyang
University, Seoul 04763, Korea; and Department of Energy Science,
Sungkyunkwan University, Suwon 16419, Korea
Chanwoo Lee,Byeong Geun Jeong and Ki Kang Kim, Department of
Energy Science, Sungkyunkwan University, Suwon 16419, Korea
Seok Joon Yun, Department of Semiconductor, University of Ulsan, Ulsan
44610, Republic of Korea
Hyeong Chan Suh, Department of Physics, Hanyang University, Seoul
04763, Korea
Dongki Lee, Department of Nanotechnology and Advanced Materials
Engineering, Sejong University, Seoul 05006, Korea
alloy forms the multi-disordered interface of the lateral het-
erostructure. The results of this study could present the way
for the evaluation of the TMDs lateral heterostructure for
excitonic applications.
Keywords: tip-enhanced Raman spectroscopy (TERS);
molybdenum disulfide (MoS2); tungsten disulfide (WS2);
multi-disorder; nanoscale alloy
1 Introduction
The widespread investigation of two-dimensional (2D) lay-
ered transition metal dichalcogenides (TMDs) has garnered
significant attention due to their exceptional physical prop-
erties [1]–[5]. The distinctive features arising from the direct
band gap of monolayer TMDs within the visible to near-
infrared range oer opportunities for modulating band o-
set and band gap [6]. Particularly noteworthy are the unique
behaviors of quasi-particles resulting from the quantum
confinement eect in the vertical direction, which positions
TMDs as pivotal in advancing next-generation quantum
engineering applications such as optoelectronics, spintron-
ics, and valleytronics [7]–[12].
Semiconductor heterojunctions, consisting of two
dierent materials, play a crucial role in actively
controlling charge carrier behaviors. Through van der
Waals stacking of two dissimilar TMDs [13], researchers
have fabricated vertical heterostructure semiconductors
with type-II band alignment (staggered), enabling the
modulation of carrier flow at the interface through an
internal field [14],[15]. Vertical heterostructure TMDs
can generate charge transfer (CT) excitons [16],[17]in
the out-of-plane direction, which can be manipulated
with bias voltage. Furthermore, lateral heterostructure
TMDs have been extensively studied, leveraging covalently
bonded edge contacts in the in-plane structure to easily
control the electronic band structure and alignment
with no dielectric gap [6],[17]. However, the interfaces
of lateral heterostructure TMDs exhibit multiple disorders,
Open Access. ©2024 the author(s), published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License.
1070 —D. H. Kim et al.: Disordered nanoscale alloy of lateral heterostructure of TMDs
including atomic vacancies, substitutions, nanocrystallites,
nanoscale alloys, etc., which disrupt the application of
heterostructure of a semiconductor due to the inducing
of distortion at the electronic band structure [6],[18],
[19]. To investigate the interface nature of lateral
heterostructure TMDs, several studies have employed
confocal microscopy-based photoluminescence (PL) and
Raman spectroscopies. Despite the spatially resolved
spectroscopic information, the optical diraction limit
constrains the exploration of heterojunction interface
characteristics at the nanoscale. While recent studies have
reported on exciton and phonon behavior at the nanoscale
using near-field scanning optical microscopy (NSOM) and
tip-enhanced Raman spectroscopy (TERS) [20]–[24], there
is a lack of research on alloy composition changes with
various disorders at the nanoscale.
In this study, we utilized scanning tunneling
microscopy (STM)-based TERS to investigate the interface
nature of the lateral heterostructure of MoS2–WS2.TERS
measurements at 10 nm intervals made it possible
to directly observe alloy composition changes and
disorder-related phonon properties in the nanoscale
heterojunction region. The multispectral information
obtained through TERS, reflecting multi-disordered
continuous transition metal composition changes, provides
valuable insights for understanding and applying the
interfacial phenomena of the lateral heterostructure
of MoS2–WS2based on the clue to figure out the local
electronic band structure for excitonic applications.
2 Methods
2.1 Synthesis of monolayer MoS–WSlateral
heterostructure
The monolayer MoS2–WS2lateral heterostructure was synthesized by
an atmospheric chemical vapor deposition (CVD) process. In order to
synthesize the heterostructure, the precursor solution was prepared
by mixing four dierent chemical solutions of Wprecursor, Mo pre-
cursor, promoter, and medium solution. First, the Wprecursor was
fabricated by dissolving 0.1 g of ammonium metatungstate hydrate
((NH4)6H2W12O40 ·xH2O, Sigma–Aldrich) in 10 ml of DI water. Second,
the Mo precursor was fabricated by dissolving 0.1 g of the ammonium
heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O, Sigma–Aldrich) in
10 ml of DI water. Third, the promoter was fabricated by dissolving
0.1 g of sodium hydroxide (NaOH, Sigma–Aldrich) in 30 ml of DI water.
Lastly, the medium solution of iodixanol solution (Sigma–Aldrich) was
used to mix the metal precursors with the promoter. The four dierent
solutions were mixed in a ratio of 1 (W): 1 (Mo): 3 (NaOH): 0.5 (iodixanol),
then was spin-coated on a SiO2/Si substrate with 3000 rpm for 1 min.
The two-zone CVD furnace was used to control the temperature of the S
and the substrate zone independently. The pure S and the precursor
coated substrate were introduced into the upstream S zone and the
downstream substrate zone, respectively. The temperature of the S zone
is elevated to 220 ◦Cat50◦Cmin
−1while the substrate zone was ramped
to 800 ◦Cat100◦Cmin
−1with flowing N2(500 sccm) and H2(5 sccm)
gases. After 10 min of growth, both furnaces are opened and naturally
cooled to room temperature.
2.2 TERS tip fabrication using electrochemical etching
The gold nano tip utilized for TERS measurements was produced
through electrochemical etching [25],[26]. In this process, a gold wire
(diameter of 250 μm, purity of 99.95 %, Nilaco) served as the anode and
was connected to a wave function generator. The generator applied a
square-wave voltage ranging from a minimum of −25 mV to a max-
imum of 3.5 V, with a frequency and duty cycle of 300 Hz and 20 %,
respectively. Acting as the cathode in the etching process, a ring-shaped
platinum wire (with a diameter of 200 μm, purity of 99.98 %, Nilaco)
was immersed in an etchant comprising a 37 % HCl solution and 99.5 %
anhydrous ethanol. Following the self-terminating etching process, the
resulting TERS gold nano tip underwent rinsing with acetone, ethanol,
DI water, and IPA solutions.
2.3 STM-based TERS measurements
The TERS system (NTEGRA Spectra, NT-MDT) is composed of both
scanning tunneling microscopy (STM) and a confocal Raman scatter-
ing system. Preceding the TERS scanning process, STM imaging was
employed to scrutinize the interface of the lateral heterostructure
MoS2–WS2monolayer. The STM images were acquired under specific
scanning conditions, with a tunneling current of approximately 6 nA
and a bias voltage of 0.1 V, within an ambient environment. The TERS
scanning procedure utilized an excitation laser with a wavelength of
632.8 nm and an objective lens possessing a numerical aperture (NA)
of 0.7 (Mitutoyo). Multispectral TERS spectra were acquired through
a spectrometer featuring 1800 grooves/mm grating, blazed at 500 nm,
and a CCD (Andor) cooled to a temperature of −80 ◦C. The same gold
nano tip was used for all STM and TERS measurements.
3 Results and discussion
3.1 STM and TERS characterization
of monolayer lateral heterostructure
MoS–WS
The monolayer lateral heterostructure of MoS2–WS2by CVD
process has been synthesized to investigate the interfa-
cial nature of lateral heterostructure TMDs. As shown in
Figure 1a, the Mo, W, and S atoms are covalently bonded,
which can generate an intrinsic p-n heterojunction as an in-
plane structure [8],[19]. The synthesis conditions and tran-
sition metal reactivity dierences of each TMDs induced
the laterally separated structure of MoS2–WS2.Thewet-
transferred synthesized lateral heterostructure semicon-
ductors on a flat Au substrate were prepared to perform
the STM measurements of the heterostructure interface. The
angle between the nanotip and a normal of the prepared
sample was precisely controlled to prevent tip drift issues
D. H. Kim et al.: Disordered nanoscale alloy of lateral heterostructure of TMDs —1071
Figure 1: STM-TERS measurement for the lateral heterostructure TMD. (a) Schematic illustration of the lateral heterostructure of MoS2–WS2.
(b) Schematic of STM-based TERS system. (c) Optical microscope image of a sample. (d– f) STM images for the interface of the lateral heterostructure
MoS2–WS2. The white dashed squares in (c– e) indicate the STM imaging area of the fore figures, respectively. The white dashed arrow in (f) indicates
the TERS scanning range and direction. The green dotted lines in (d– f) indicate the interface in the lateral heterostructure.
during STM imaging and TERS scanning and to increase
the degree of Raman signal enhancement. To control the
position of the nanotip, the tunneling current was measured
between the wired nano tip and Au substrate by applying a
bias voltage [27]–[29]. Firstly, we measured the wide-area
STM topography, which includes the heterostructure inter-
face, and the scanning area is marked with a white dashed
square in the optical microscope image (Figure 1c). In order
to visualize the interface of the lateral heterostructure in
detail, it was magnified that the area of the interfacial region
identified with OM image to 2 μm×2μm by STM imaging.
Also, we conducted TERS line trace measurement from WS2
to MoS2region for 1um with 10 nm interval to investigate
minutely the lattice vibration characteristics in the region
of lateral heterostructure interface. The white dashed
arrow in Figure 1f indicates the TERS line trace region and
direction.
Figure 2a shows the raw TERS spectra with PL
background acquired by TERS line trace measurement.
As the 632.8 nm (∼1.96 eV) laser excitation source well
fitted the resonance conditions with the ∼1.94 eV of A
excitonic absorption for WS2(∼1.87 eV for MoS2), the strong
resonance eect (semi-resonance eect) could enormously
enhance the Raman scattering signal, and it leads the TERS
intensity and SNR dierences between WS2and MoS2
regions [30]–[33]. In order to figure out the variation of
resonance Raman spectra at a glance, we normalized the
PL (the tail of the WS2PL) and plasmon (from the gap-mode
localized surface plasmon resonance, LSPR) background
signal subtracted from the TERS spectra (Figure 2b).
Figure 2c and d are the representative TERS spectra of each
TMDs material, and well-known phonon modes, including
first-order modes, are marked. For convenience, we used
red and blue colors to represent the MoS2and WS2phonon
modes, respectively, based on the band gap. From the
frequency dierence between E′(Γ)andA′
1(Γ)ofMoS
2and
WS2, it is confirmed that both semiconductor materials
are monolayer (Figure S1, Supplementary Materials). In
addition, the first-order Raman scattering modes from
the center of the Brillouin zone around 354, 385, 405, and
417 cm−1in Figure 2b show drastic changes in both Raman
intensity and phonon mode frequency near the interface of
the lateral heterostructure.
3.2 Multi-disordered interface with confined
phonon
Although the TERS spectra in Figure 2b show significant
fluctuations along the tip displacement, especially in
the interface region of the lateral heterostructure, it
is dicult to distinguish the change in phonon modes
in detail because numerous phonon modes were
observed due to the strong resonance eect of the
tip-enhanced resonance Raman scattering (TERRS).
The resonance Raman scattering process allowed
the observation of the second-order Raman scattering
signals, which it is unable to measure with non-resonant
conditions. Thus, the strong LSPR owing to the nanocavity
1072 —D. H. Kim et al.: Disordered nanoscale alloy of lateral heterostructure of TMDs
Figure 2: TERS scanning across the interface in the lateral heterojunction. (a) TERS multispectral line trace along the white dashed arrow in Figure 1f.
(b) Normalized TERS spectra of (a) with background subtraction. Signature TERS spectra for monolayer (c) MoS2and (d) WS2. The representative
phonon modes of each material are marked.
between the Au nanotip and the mirror image of the
nanotip in the Au substrate greatly enhanced the weak
Raman scattering signal (Figure S2, Supplementary
Materials). In order to investigate the details of phonon
modes, we assigned the deconvoluted spectra of the
TERS spectrum from the interfacial region along the
tip displacement from 170 nm to 310 nm, which shows
drastic variation in the TERS spectra. Including the first-
and second-order Raman scattering modes, nineteen
phonon modes are convoluted in the range of 280 cm−1
to 480 cm−1.Figure 3a briefly presents the deconvolution
result of the phonon modes of the Mo1−xWxS2alloy with a
representative TERS spectrum. The deconvoluted spectra
were summarized in Table S1 (Supplementary Materials)
with the corresponding phonon mode and frequency
from the experimental results and previous reports. The
numbers marked on each spectrum in Figure 3a are
matched with the peak number in Table S1, and nineteen
deconvoluted spectra contain information related to the
multi-disorders, such as atomic vacancies, substitutions,
line defects, alloys, and nanocrystallites [30],[34]–[36].
Also, not only the phonon modes that could be observed by
far-field Raman spectroscopy but also forbidden phonon
modes are accompanied in the deconvolution process.
For the sake of convenience, the deconvoluted spectra in
Figure 3a were categorized into four. As mentioned above,
the phonon modes with red color and blue color indicate the
vibrational modes of monolayer MoS2and the monolayer
WS2, respectively. First, the first-order Raman modes
originating from the center of the Brillouin zone, Γ-point,
wereobserved(peak1,7,9,12,14,19)[36]–[38]. These
fundamental phonon modes are usually used to identify
the existence of respective TMDs and the layer number
of TMDs. Second, the strong resonance eect between the
laser excitation source and the A excitonic absorption could
lead to the observation of the combination of longitudinal
acoustic (LA) and transverse acoustic (TA) phonon modes
at the M point of the Brillouin zone (peak 4), and the
overtone of the LA mode at the M point of the Brillouin
zone (peak 6) [38]–[40]. We also observed the longitudinal
optical (LO) phonon, the M point mediated by the disorders,
which may originate from the double resonance process
of MoS2(peak 8) [35],[38]. Third, the commonly reported
disorders and atomic vacancy-mediated phonon modes
were also observed. The sulfur vacancies of WS2led to
symmetry breaking and allowed us to observe the Dmode
and D′mode at the interface of the lateral heterostructure,
which shows the same results with our previous research
(peak 13, 16) [30],[34],[41]. Besides, for the MoS2,the
vacancy of Mo, S, and MoS6related phonon modes were
calculated by density functional theory (DFT) before,
there is only rare observation by the peak broadening
and appearance of the shoulder of these phonon modes
with conventional methods. However, we observed both
separated to easily distinguishable vacancy-related modes
and shoulders from the other phonon modes from the TERS
spectra near the interface region (peak 11, 15, 17) [42]–[46].
Finally, we observed the evidence of the presence of the
nanocrystallites of MoS2and WS2(peaks 2, 3, 5, 18) [35],[36].
Thus, the combination of LA and TA modes at the M point of
D. H. Kim et al.: Disordered nanoscale alloy of lateral heterostructure of TMDs —1073
Figure 3: Diverse phonon modes at the interface of the lateral heterostructure with different origins. (a) Representative TERS spectrum of Mo1−xWxS2
at the interface of the lateral heterostructure. The gray spectrum indicates the convoluted spectrum of nineteen deconvoluted spectra. (b) Normalized
TERS spectra extracted from Figure 2b. Tip displacements are marked with the gray arrow. (c) Schematic illustration of lateral heterostructure interface
with multi-disorder. The dashed circles and highlighted triangles indicate the atomic vacancy and the nanocrystallites, respectively.
MoS2could be induced by the crystal-edge-related phonon
(peak 10) [47]. These phonon modes change drastically along
the tip displacement in the interfacial region (Figure 3b)
because the observed Raman signals are the information of
an ensemble of electric fields from the vicinity of tip-apex
due to the high spatial resolution of TERS. Thus, both the
phonon frequency and intensity could be easily influenced
by the disorder density and nanocrystallite domain size.
Furthermore, Figure 3c is a brief sketch of the concept of
the multi-disordered interface of lateral heterostructure
that could help catch the deconvoluted results at a
glance.
For the most part, the origin of multi-disorder-related
phonon modes can be explained by the phonon confine-
ment eect. The phonon confinement eect is a kind
of momentum conservation rule for the phonon-disorder
scattering process, which could induce the relaxation of
the Raman selection rule through the phonon weighting
function. When the momentum conservation is required,
q≅0, where qis the momentum wave vector of the lattice
vibration, the first-order Raman scattering can be intro-
duced that can induce the zone center (Γpoint)-related
phonon from ideal crystalline materials without disorder.
However, where the disorders are induced in the crys-
talline materials internally and/or externally, crystal struc-
ture would be broken and produced various nanostruc-
tures that contain multi-disorder such as atomic vacancy,
nanocrystallites, and alloy. So, the disordered crystal with a
finite phonon correlation length (Lc) dierent from the pris-
tine crystal (Lc≅∞) could show the relaxation of the prin-
cipal selection rule (q≅0) for the Raman scattering process
and introduce the lattice vibration away from the center
of the Brillouin zone. In other words, the multi-disorders
induced the selection-rule breaking to observe the forbid-
den phonons. The following phonon weighting function
and the phonon confinement model by Richter–Wang– Ley
(RWL model) can explain the relaxation process in detail
[34]–[36],
1074 —D. H. Kim et al.: Disordered nanoscale alloy of lateral heterostructure of TMDs
W(r,Lc)=exp(−ar2∕L2
c)(1)
I(𝜔)∝∫|C(q)|2
(𝜔−𝜔(q))2+(Γ0∕2)2dq(2)
I(𝜔)=∫exp(−q2L2
c∕2𝛼)
(𝜔−𝜔(q))2+(Γ0∕2)22𝜋qdq(3)
where 𝛼is an alterable confinement coecient represent-
ing an attenuation of the lattice vibration amplitude, I(𝜔)
is the intensity of the first-order Raman mode of the spe-
cific system, C(q) is the Fourier coecient of the weighting
function W(r,Lc), 𝜔(q)is the phonon dispersion curve in
the infinite domain caused by disorders, and Γ0is the width
of the phonon peaks. The integral means the integration
over the whole range of the Brillouin zone. For Eq. (3), not
only q≅0 phonons but also q≠0 phonons can be involved
in the Raman scattering process for the phonon confine-
ment model, the phonon confined interface with atomic
vacancies and nanostructures with nanocrystallites with
some domains, Lc. In addition, it is complicated to “see” the
nature of these tiny nanostructures without the powerful
TERRS.
In Figure 4, we plotted the tendency of the prominent
Raman scattering signals of each semiconductor along the
tip displacement. The green dashed lines indicate the lat-
eral heterostructure interface, as confirmed by the OM and
STMimages(Figure 1c–f, respectively). In accordance with
the interface line, Figure 4a–cshow the decrease of the
normalized TERS intensity of WS2in domain A, and Figure
4e–hshow the increase of the normalized TERS intensity
of MoS2in domain B. Note that, it is dierent that deduced
line with the normalized TERS line profile with the interface
line seen by the OM and STM images. Near the spot with
tip displacement ≈200 nm, the TERS signal shows drastic
changes, which means that the actual interface is shifted
from the center. Due to the formation of the multi-disorders
with crystal alloy, the interface of the lateral heterostructure
is not atomically sharp like OM and STM images and intro-
duced the invasion on both sides and exhibited the dier-
ence of location with around 250 nm between the interface
line of microscopic image and TERS line profile.
Also, Figure 4c and d, which shows a consistent inten-
sity tendency along the tip displacement, imply the possi-
bility of the presence of alloy Mo1−xWxS2. Since the nor-
malized TERS intensity and WS2PL background suddenly
decreased at the position with 200 nm, the signal decreas-
ing region (200 nm ≤tip displacement <500 nm) could be
considered as defective region or continuous alloy formed
region. However, it suggests that the gradual signal dimin-
ishing has not been induced with only disorders by means
of our high spatial resolution. Moreover, the Raman inten-
sity in Figure 4c depends on the dierence between the
laser energy and band gap of the semiconductor, which
determines the strength of the resonance Raman scattering
process, which can also lead to PL intensity change [30],
[39],[40]. In other words, there is the possibility with that
the formation of continuous alloy and several disorders that
cause the noise on signals.
3.3 Variations of nanoscale alloy
composition at lateral heterostructure
To probe the variations of the alloy characteristics
in the nanoscale, we investigated the atomic ratio
Figure 4: Tip-enhanced optical signal variation along the TERS line trace. Tip-enhanced optical signal line profiles along the white dashed arrow
in Figure 1f correspond to (a–c) WS2phonon modes (blue solid line), (d) photoluminescence background (black solid line) and (e–h) MoS2phonon
modes (red bold line). The domains A and B in (a) indicate the WS2and MoS2regions, respectively.
D. H. Kim et al.: Disordered nanoscale alloy of lateral heterostructure of TMDs —1075
between Mo and W, the crystal alloy compositions,
by the deconvoluted TERS spectra in Figure 3a and b
(Figure S3, Supplementary Materials)intheMo
1−xWxS2
region with drastic change in phonon frequency. Figure 5a
clearly shows the intensity changes of the first-order Raman
scattering signals along the tip displacement from 170 nm
to 320 nm. As shown in Figure 5b and c, which demonstrate
the Raman spectra of L1 and L2, the TERS intensity on
two dierent A′
1modes of MoS2and WS2were reversed.
Figure 5d and e shows the deconvolution results along
the tip displacement that E′and A′
1modes frequency of
MoS2at the region, which is considered as alloy formed.
According to the tip travels from 170 nm to 260 nm position,
the frequency of both first-order Raman modes gradually
shifted to close to each other due to the composition
changes of Mo1−xWxS2. Over the 260 nm position, most of
the peak frequencies maintained the last frequency values
of E′,andA′
1, respectively. It implies that the termination
of changes in alloy composition, material change from
WS2to MoS2, and emergence of intact monolayer MoS2
(𝜔A′1−𝜔E′<20 cm−1). One of the anomalies is that the
E′mode frequency in the region highlighted in red in
Figure 5d shows a frequency shift at the localized 40 nm
area. Since the A′
1mode frequency did not show any
changes after the 260 nm position, which proved the
formation of intact MoS2,theabruptE′mode frequency
change could be explained by the variations of vacancy
density, nanocrystallites domain sizes which directly aect
to the phonon frequency of the heterogeneous interface
region. Moreover, we set up the function, y=ax +b,for
this research that is fit to the model of Chen et al. which
could be used to calculate the composition value of Walong
the tip displacement in the nanoscale (see details in the
Supplementary Materials)[48]. As shown in Figure 5f,theW
composition xconverged continuously from 0.2 to 0 within
tens of nanometers. In other words, via spectroscopic
analysis, we have demonstrated the nanoscale crystal alloy
and the gradual variation in Wcomposition xwithin the
vicinity of the lateral heterostructure interface, within
a range of several tens of nanometers. By using TERRS,
with high spatial resolution and ultra-high sensitivity
on signal, we can explore the nanoscopic nature of the
heterogeneous materials, and it can apply in material
science and engineering as well as the conventional
spectroscopic studies.
Figure 5: Probing the composition variation of alloy in nanoscale. (a) Normalized TERS spectra of Mo1−xWxS2region with respect to the tip
displacement from 180 nm to 320 nm. The black dashed lines L1 and L2 indicate the normalized TERS spectra (b) and (c), respectively. Assigned phonon
modes with red and blue colors indicate the vibration modes of MoS2and WS2, respectively. The graph of variation of (d) E′and (e) A′
1mode peak
position of MoS2along the tip displacement in Mo1−xWxS2region. All the peak position and error bar values are plotted from deconvolution results.
(f) The graph of change in calculated Wcomposition xon the basis of (e) in alloy region.
1076 —D. H. Kim et al.: Disordered nanoscale alloy of lateral heterostructure of TMDs
4 Conclusions
In this research, we have carried out an STM-based TERS
experiment to investigate the interfacial nature of the lat-
eral heterostructure of the MoS2–WS2monolayer. TERRS
line trace analysis across the heterogeneous interface
revealed the multi-disorders that are hard to prevent at the
TMDs lateral heterostructure. The disorder-induced crystal
structure exhibited numerous phonon modes due to the
phonon-disorder scattering process based on the phonon
confinement eect. Our results provided the spectroscopic
evidence of the presence of disorder complexes containing
atomic vacancies, crystal edge, and nanocrystallites at the
heterojunction interface. In addition, the variation of Lc,
which could be easily influenced by the disorder concen-
tration and domain sizes, is attributed to the phonon fre-
quency changes along the tip displacement. Furthermore,
we obtained the gradual composition change of Mo1−xWxS2
using spectroscopical information, within the range of tens
of nanometers, based on the atomic ratio of the crystal
alloy that was spatially resolved. In conclusion, our research
on a nondestructive nanoscale imaging system for probing
lattice vibrational characteristics at the interface of lateral
heterostructure holds promise for advancing the applica-
tions and assessment of excitonic functionalities based on
bandgap modulation in TMDs lateral heterostructure.
Research funding: M.S.J. acknowledges support from the
National Research Foundation of Korea (NRF) grant funded
by the Korean government’s Ministry of Science and
ICT (MSIT)(NRF-2022R1A2C2091945), the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MSIT) (No. RS-2023-00260527), and the chal-
lengeable Future Defense Technology Research and Devel-
opment Program through the Agency For Defense Devel-
opment (ADD) funded by the Defense Acquisition Pro-
gram Administration (DAPA) in 2023 (No. 915019201). K.K.K.
acknowledges support from the Basic Science Research
(2022R1A2C2091475) and Next-generation Intelligence Semi-
conductor Program (2022M3F3A2A01072215) through the
National Research Foundation of Korea (NRF), which is
funded by the Ministry of Science, ICT & Future Planning,
Institute for Basic Science (IBS-R011-D1) and Advanced Facil-
ity Center for Quantum Technology.
Author contributions: All authors have accepted responsi-
bility for the entire content of this manuscript and approved
its submission.
Conflict of interest: Authors state no conflicts of interest.
Informed consent: Informed consent was obtained from all
individuals included in this study.
Ethical approval: The conducted research is not related to
either human or animals use.
Data availability: Data sharing is not applicable to this arti-
cle as no datasets were generated or analyzed during the
current study.
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Supplementary Material: This article contains supplementary material
(https://doi.org/10.1515/nanoph-2023- 0826).
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