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Ultrafast nanoscale exciton dynamics via laser-combined
scanning tunneling microscopy in atomically thin materials
Hiroyuki Mogi1, Yusuke Arashida1, Ryusei Kikuchi1, Ryosuke Mizuno1, Jun Wakabayashi1, Naoki Wada2, Yasumitsu Miyata
Atsushi Taninaka1, Shoji Yoshida1, Osamu Takeuchi1 and Hidemi Shigekawa 1 ✉2,1234567890():,;

Long-term stable excitons, bound pairs of electrons and holes, in van der Waals materials were found to be handled at room
temperature, paving the way to develop optoelectronic/photonic devices for future high-speed communication technology. To
miniaturize and integrate such functions to achieve highly efﬁcient excitonic devices, understanding and manipulation of exciton
dynamics in the nanoscale structures is absolutely essential. Since the spatial resolution of the optical techniques, which have been
mainly used in the research of this ﬁeld, is limited to μm to several tens of nm, the introduction of new technology is strongly
desired. Here, we demonstrate a method to probe and visualize neutral excitons using the laser-combined multiprobe scanningtunneling microscopy (STM) system. Ultrafast dynamics of excitons in the nanostructures produced in an in-plane WS2/WSe2
heterostructure, such as dynamics in the ps region of many-body effects under high density and spatial variation in the effect of
local defects on lifetime, was successfully revealed with a spatial resolution of 1 nm order. This method is expected to accelerate
research on exciton dynamics and the development of applications directly based on the experimental results of nanoscale.
npj 2D Materials and Applications (2022)6:72 ; https://doi.org/10.1038/s41699-022-00345-1

INTRODUCTION
To develop the high speed (terabits per second) opticalcommunication technology, which has become indispensable inmodern society, the development of information transmission
methods that do not use charge transfer with large transmission
losses is essential. Thus, the use of excitons, electrically neutral
quasi-particles formed by bound electrons and holes, has
attracted considerable attention. However, the binding energyof excitons generated in a conventional three-dimensional
semiconductor is in the range of meV to 10 meV orders, andexcitons are easily dissociated at room temperature. Therefore,such devices can be realized only in a low-temperature environment, which has been the most important problem to be solved
for the application of excitons1.
In recent years, transition metal dichalcogenides (TMDCs),
which are two-dimensional materials in the form of atomic layers,
have been attracting considerable attention because of their highcarrier mobility, clear bandgap, and strong light-matter interactions2. In particular, excitons generated in TMDCs by lightirradiation have high binding energy (up to 100 meV order) due
to the conﬁnement effect derived from the two-dimensional
structure and the reduced screening effect3. Therefore, twodimensional excitons exist stably even at room temperature4 andcan move long distance in the atomic layer. Furthermore, exciton
resonance causes strong emission and absorption at the same
wavelength, which makes spatial energy transfer via photons easy
and is particularly promising for application in photonic integrated
circuit materials5.
However, the optical properties are strongly dependent on the
exciton structure. Multibody effects such as the exciton-exciton
annihilation (EEA) process through collisions also appear strongly6.
In addition, for example, exciton funneling is caused by thenarrowing of the bandgap due to in-plane stress7,8. The
experimental results correspond well with the theoretical

calculations. However, when actually making a device, various
local distortions and defects are formed, and each of them has adifferent effect on the dynamics of excitons.
Since the exciton radius is on the order of 1 nm9, it is necessary
to clarify its dynamics on the nm scale to understand and utilize its
characteristics. Despite that, so far, research on exciton dynamics
has mainly been actively carried out by photoluminescence (PL)10,
pump-probe methods11, photoelectron emission microscopy12
and scanning near-ﬁeld optical microscope13. With such measurement methods, the spatial resolution is limited to μm to several
tens of nm, and it is difﬁcult to evaluate the local exciton dynamics
while observing the nanoscale structure on the order of 1 nm.
Scanning tunneling microscopy (STM) has the atomic-levelspatial resolution, and recently, a high temporal resolution has
also been given14–16, making STM one of the promisingtechniques for the purpose. However, since tunnel current is theprobe of STM, neutral excitons have never been considered to bea target.
Here, we demonstrate a method to probe and visualize neutral
exciton dynamics using laser-combined STM. Ultrafast dynamics of
excitons in the nanostructures produced in atomically thinmaterials, such as dynamics in the ps region of many-body effects
under high density and the spatial variation in the effect of ripples
and grain boundaries (GBs) on lifetime, was successfully revealedwith a spatial resolution of 1 nm order.
RESULTS AND DISCUSSION
Laser-combined STM system for probing exciton dynamics
Excitons are dissociated by an electric ﬁeld. In STM, the bias
voltage applied between the probe and the sample causes theelectric ﬁeld immediately below the STM tip in the area of several
nanometers. Therefore, by dissociating excitons using this electric
ﬁeld, it becomes possible to realize the visualization of neutral

1
Faculty of pure and applied sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan. 2Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397,
Japan. ✉email: hidemi@ims.tsukuba.ac.jp

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1234567890():,;

2

Fig. 1 Schematic illustrations of exciton dynamics in two-dimensional materials. a Time-resolved STM (TR-STM) setup for measuring carrier
dynamics in TMDCs. b Band structure showing the mechanism to detect excitons by STM. The direction of the band bending depends on thebias voltage applied between the STM tip and the sample. c Schematic structure of the time-resolved-multiprobe STM system we havedeveloped. BS beam splitter, PBS polarized beam splitter, OPO optical parametric oscillator, FR Faraday rotator, EOM electro-opticalmodulation element (see Supplementary Fig. 1 for details).

excitons with a spatial resolution higher than that of opticalmethods (Fig. 1a, b).
Figure 1c shows the time-resolved multiprobe STM (TR-MP-STM)
we have developed. Since STM requires the conductivity of thesample, it is difﬁcult to measure a small sample placed on an
insulator substrate with a single probe. In multiprobe STM (MPSTM), by using two contact AFM probes as electrodes, measurement can be performed without pretreatment, such as electrodefabrication, and the gate voltage can be adjusted along with thevoltage between the source and the drain, enabling measurementin the operating environment. In this experiment, we used the
STM mechanism with a new modulation technique as a probe tomeasure the dynamics of excitons at the nanoscale and visualize
them two-dimensionally (see Methods and Supplementary Fig. 1).
npj 2D Materials and Applications (2022) 72

Sample preparation for nanoscale analysis and measurement
setup
To evaluate multibody effects such as the EEA process throughcollisions, the effect of the nanoscale potential, and the spatial
resolution of the system, as a sample, a single-layer WS2/WSe2 inplane heterostructure island was prepared on a SiO2/p-Si (SiO2
thickness: 285 nm) substrate by chemical vapor deposition17. After
growing a threefold-symmetric WS2 structure, a WSe2 structure
was grown around it, by controlling the growth conditions18.
Triangular-shaped structures, indicating the existence of secondlayer islands19, were not observed over the area chosen forexperiments. Similarly to the technique reported in a previous
paper20, the areas used for measurements were cleaned byscanning the STM tip to remove adsorbents.
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Fig. 2 Single-layer WS2/WSe2 in-plane heterostructure sample prepared on a Si/SiO2 substrate. a Optical microscope image of the sample
setup, and a schematic illustration of the sample showing the structure of the area surrounded by the square frame in the optical image. After
growing a threefold-symmetric WS2 structure, a WSe2 structure is grown around it18. In the schematic illustration, the Au electrode on WS2 istransparent. The pale-yellow areas above and below the Au electrodes are Au thin ﬁlms of different thicknesses. The left and right sides of the
SiO2/Si regions are shadows due to the Au thin ﬁlms on the outside. b Schematic of the measurement setup. c, STM topography image
acquired at the WSe2/WS2 heterointerface, which position is shown in a with a green square. d Line proﬁles along AB and CD in c. The ripple
structure is clearly shown in the WS2 area. e Fluorescence spectrum measured in each region of WSe2 and WS2 (excitation wavelength 532 nm,
spot diameter 2 μm, excitation power 2.5 mW). Measured positions are marked in a with a red (WSe2) and a blue (WS2) circles, respectively.
The spectrum for WS2 (blue curve) can be divided into two peaks (pink and light blue curves) with different intensity by ﬁtting.

Figure 2a shows an optical microscope image of the sample anda schematic illustration of the structure of the area surrounded by
the square frame in the image. As shown in the schematic
diagram, an island with a structure having an angular change of~7.6° in the middle of the WS2/WSe2 interface was selected. Thisangular change is thought to be due to the conﬂuence of islands
that grew in different plane directions during the growth of WS2.
The existence of a GB was conﬁrmed, as shown by the red line inthe ﬁgure. (see Supplementary Fig. 2 for more detail). Using thisGB, detailed experiments were conducted to study the effects ofsuch local structures on exciton dynamics.
Reﬂecting the ambipolar ﬁeld effect transistor (FET) characteristics of the WS2/WSe2 structure, it is an off state when the gatesource voltage is 0 V under the dark condition. Since excitons are
unstable in conventional three-dimensional semiconductors such
as GaAs, the dynamics of photoexcited carriers were measured byobserving the current change due to photoexcitation. However, in
TMDCs, since excitons are stable, it is necessary to separate theirdynamics from those in dark current upon photoexcitation. In
addition, to prevent complexity due to the presence of trions, etc.,
the measurement was performed under a gate-source voltage of
0 V, resulting in almost no free carriers. In the presence of free
carriers, the band bending is reduced due to the effect of surface
photovoltage, and the material bandgap should appear in I-V
curves. However, as shown in Supplementary Fig. 3, such
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phenomena were not observed, supporting that the effect offree carriers are not dominant in this case.
Figure 2c, d respectively show the STM topography image
acquired at the WS2/WSe2 heterointerface and the line proﬁlealong AB and CD in the image. The WSe2 region is relatively ﬂatwith a height roughness of ~0.3 nm, which corresponds to theroughness of the SiO2/Si substrate. On the other hand, ripple
structures with various amplitudes in the range of 0.3–1.2 nm wereobserved in the WS2 region. It has been reported that such ripple
structures are spontaneously/artiﬁcially formed in a TMDC single
layer21,22 (see Methods). It has also been reported that themodulation of the electronic state structure by stress greatly
affects the excitonic characteristics23,24.
First, to conﬁrm the state of the sample, the ﬂuorescence
spectrum was measured in each region of WS2/WSe2 as shown in
Fig. 2e. As a result of Lorentz function ﬁtting, the peak center andfull width at half maximum (FWHM) were obtained to be 1.74 and0.085 eV for WSe2 (black line), and 1.97 and 0.10 eV for WS2 (blueline), respectively. The peak center positions were roughly inagreement with previously reported results25. The ﬂuorescenceintensity of WS2 was about eight times higher than that of WSe2.
On the SiO2 substrate, the dielectric constant of the sample
ﬂuctuates due to substrate roughness and the impurity distribution. Excitons gather at the local minimum potential area
generated by the interaction with the substrate. Therefore, at
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Fig. 3 Evaluation using grain boundaries in the WS2 region. a STM topographic image and images of currents Is and Id ﬂowing into of thesource and drain electrodes on the sample (Vs = −6 V, Iset = 5 pA), respectively. Each image consists of 128 horizontal line scans. The light spot
size was about 20 μm and the area was illuminated almost entirely. No apparent overlayers with a triangular shape were not observed in theilluminated area. Red triangles indicate the left side edge. See Supplementary Fig. 2 about the GB structures. b Schematic diagram of therelationship between the STM tip position and the current obtained at the source and drain electrodes. c Steplike change and the spatial
derivative obtained from the plot of Is in a. The values were averaged over a ±15 nm area from the interface. d, e Electric ﬁeld and exciton
binding energy dependences of S for WS2 and WSe2, respectively. f Cross-sections along the broken lines shown in d (Eb = 0.28 eV) ande (Eb = 0.31 eV).

higher density regions, EEA in the non-radiative process is actively
generated, suppressing PL26.
In the WSe2 area, WSe2 contacts the substrate over the entire
interface, but in the WS2 area, the contact points are reduced dueto the ripple structure. Therefore, WSe2 has a greater inﬂuence
from the substrate, and PL is greatly suppressed. These are ingood agreement with the observed difference in PL intensity27.
Since such ripples spatially modulate the stress, the bandgap
should be modulated accordingly. We also carried out ﬁtting
analysis on the PL spectra. As shown in Fig. 2e, the WSe2 signal
remained in one peak, which directly shows the effect of trions is
not dominant, while the WS2 signal was separated into two peaks(pink and light blue). Considering each peak as a signal from thesuspended and contact areas, the results correspond well with theprevious ones27. The energy position of the contact component(pink peak) depends on the effect of the substrate. So far, exciton
manipulation and PL enhancement by using a relatively large
ripple structure of μm order created mechanically have been
proposed28. Therefore, in this experiment, the PL enhancement in
WS2 may occur even with ripples on the order of 10 nm22.
However, it is difﬁcult to detect complex exciton dynamics innanostructures with a high spatial resolution by optical methods,
which is an important new possibility of our method.
Analytical model of exciton measurement by MP-STM
First, we investigate the mechanism of exciton dissociation with ahigh binding energy by the electric ﬁeld generated by the bias
voltage applied between the STM tip and the sample (seenpj 2D Materials and Applications (2022) 72

Supplementary Fig. 3 and its explanations). In a weak electric ﬁeld,
the photocurrent is determined by the dissociation rate of thedensest ground-state excitons29,30. Therefore, it is expected thatthe origin of the tunnel current It can be explained by themechanism of excitons dissociation under the STM tip. We
assumed the following simple model:



(1)
It ðtÞ / N  1  exp Γ tpass ¼ N  S:
Here N (t) is the exciton density at a certain time t, Γ(t) is theexciton dissociation rate, tpass is the time required for excitons topass through the region under the STM tip where the electric ﬁeld
is applied, S is the dissociation probability (see Methods). Onedimensionally, tpass can be considered to be tpass = w/v, where v isthe average exciton velocity and w is the width of the region in
the sample where the electric ﬁeld is applied by the STM tip. Also,v can be derived from the following equation for the diffusion
coefﬁcient D31:
 2 
v τ
(2)
D¼
:
2
Here, τ is the momentum scattering time, which is ~ 100 fs at 80
K31. D is affected by various factors (board, temperature, exciton
density, etc.). Calculation, using D = 0.5 cm2/s for WSe2/hBN under
weak excitation by continuous light32, gives v ~ 3.16 × 104 m/s.
On the other hand, to obtain w, an evaluation using GBs in the
WS2 region was performed. Figure 3a shows an STM topographic
image and images of currents Is and Id ﬂowing into the source anddrain electrodes on the sample, respectively (Vs = −6 V,
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Iset = 5 pA). Figure 3b shows a schematic diagram of the relationship between the STM tip position and the current obtained at thesource and drain electrodes. Assuming that the GB region consistsof a thin, one-dimensional, high-resistance structure, when the
distance between the STM tip and the GB is sufﬁciently large, thephotocarriers that separate immediately below the STM tip ﬂow to
only one of the easily accessible electrodes. In contrast, when the
STM tip approaches the GB, a strong electric ﬁeld induced by the
STM tip is applied over the GB region. Therefore, even when the
STM tip is on the source (drain) electrode side of the GB, exciton
dissociation occurs on the GB, and the drain (source) current is
detected.
In the STM topographic image shown in Fig. 3a, the GB appears
to be wide, but as can be seen from the Is and Id images, theamount of current clearly changes on the right side, indicatingthat the right edge has a high resistance and becomes the turning
point. The three images in Fig. 3a were obtained by 128 horizontal
line scans of the STM tip from bottom to top. Figure 3c shows the
cross-section (steplike dashed line) obtained from the Is image in
Fig. 3a and its spatial derivative (red line). To obtain the crosssection, we averaged the vales of 128 lines with the following
manner. The right edge of the GB, turning point, is not straight but
has a zigzag shape as shown in the Is image. Therefore, the 128
lines were ﬁrst adjusted so that the locations with Is = 2.8 pA,
which almost correspond to the ﬂexion point of each line aroundthe edge, matched each other. Then, the values of Is for the 128
lines were averaged to obtain the cross-section shown in Fig. 3c.
As a result of estimating the transition width, an FWHM of
2.5 nm was obtained, indicating that the spatial resolution of oursystem is in the 1 nm order. tpass ~80 fs was obtained as the timerequired for excitons to pass through this region with a velocity ofv ~ 3.16 × 104 m/s.
To obtain the exciton dissociation rate Γ, we referred to thedissociation rate of ground-state excitons under a weak in-planeelectric ﬁeld (<50 V/μm) obtained by Høgni et al. using weak-ﬁeld
asymptotic theory29. When a TMDC membrane is sandwiched bydielectrics placed on its top and bottom, the binding energy of
excitons is changed by dielectric screening. The relationshipbetween Γ and the binding energy of excitons is as follows.
μΓ  2 W0 ðE Þjg0 j2ε
W0 ð E Þ ¼ k

 2 2k12

pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
4k
2k 3
k ¼ 2jEb j
exp 
E
3E

(3)

(4)

Here μ is the reduced mass of excitons, which was taken fromref. 33. ε = (εa + εb)/2 is the average relative permittivity of theupper side (εa) and lower side (εb) of the TMDC membrane. W0depends on the in-plane electric ﬁeld and is called the ﬁeld factor,
Eb is the scaled binding energy of a ground-state exciton, and E is
the in-plane electric ﬁeld. g0, called the asymptotic coefﬁcient, isderived from an unperturbed ground state at the distance ofinﬁnity. The exciton binding energy and the electric ﬁeld
dependences of S (=(1 − exp (−Γ tpass)), derived from Eqs. (1) to(3) using tpass ~80 fs and Γ, calculated for WSe2 and WS2 are shownin Fig. 3d, e, respectively.
Exciton dissociation tends to be more difﬁcult in WS2 than in
WSe2, higher electric ﬁeld is necessary for the former to dissociate
the excitons with the same binding energy, as shown in Fig. 3d, e,
which is considered to be due to the sensitivity of the material
dependence of g0 in Eq. (3). Therefore, it changes sensitively withthe material (see Supplementary Fig. 4 for details).
Assuming ε = 2.4 (εa = 1 as vacuum, εb = 3.8 as SiO2), thebinding energies Eb of the excitons of WSe2 and WS2 are obtained
as 0.28 eV and 0.31 eV29, respectively. Figure 3f shows crosssections along the broken lines shown in Fig. 3d, e corresponding
to those values. From the intersections of the broken lines and thehorizontal axis in the ﬁgure, the threshold electric ﬁelds are
obtained as 35 (WSe2) and 40 MV/m (WS2). These values are ingood agreement with those in the previous paper30. Assumingthat these electric ﬁelds are applied to the region directly under
the STM tip (~2.5/2 = 1.25 nm from Fig. 3c), the threshold voltages
for dissociating excitons on the nanoscale are directly analyzed as~44 mV (WSe2) and ~50 mV (WS2), respectively.
Conﬁrmation of EEA process by TR-MP-STM measurement
Next, to investigate exciton dynamics in detail, time-resolved
measurement was performed using the optical pump-probe MPSTM system shown in Fig. 1 and Supplementary Fig. 1. First, it wasinvestigated whether the EEA effect could be measured as theprocess of decay of excitons. The measurement was performed on
the WSe2 side, which was not affected by the ripple structure andshowed a strong EEA effect.
Figure 4a shows schematic illustrations of the EEA dynamics. In
this sample and the temperature condition, the exciton lifetime
due to direct recombination is of ns order3. Therefore, to suppress
the effect of long-lived components on the time-resolved
spectrum, for example, the process by which residual excitons
are further excited34, experiments were conducted with the laser
repetition rate of frep = 1 MHz here. The measurement conditions

Fig. 4 Exciton-exciton annihilation dynamics. a Schematic illustrations of exciton-exciton annihilation dynamics. b TR-MP-STM signals
obtained in the WSe2 region for the ﬂuences of 6, 12, and 24 μJ/cm2. Vs = +6 V, Iset = 1 pA, and frep = 1 MHz. c Mapping of S for changes in
exciton density and exciton binding energy. d Cross sections along the three lines in c. The measurements were performed at the same placeas the PL measurement, indicated by the red triangle in Fig. 2a. The heterostructure interface was not exposed to light.
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Fig. 5 TR-MP-STM measurement on ripple structure formed in WS2 region. a Schematic of nanoscale ripple structure observed in Fig. 2c.
Vs = −3.5 V, Iset = 40 pA, frep = 80 MHz, ﬂuence 13 μJ/cm2 and center wavelength 633 nm. b STM image taken over a ripple area in WS2 and thetime-resolved signal acquired at the corresponding tip position on a color scale. c TR-MP-STM signal obtained along the red and blue lines inb. d Cross section along the green line in the STM image (upper ﬁgure in b), and the variation in the time constant for the corresponding
position (lower ﬁgure in b). e Schematic diagram of the principle of spatially changing exciton dynamics. The error bars in d are the errors
obtained with the ﬁtting shown in c.

were Vs = +6 V and Iset = 1 pA, and, as shown in Fig. 4b, themeasurement was performed for ~160 ps while changing the lightintensity (6, 12, and 24 μJ/cm2). It is known that dynamics due tothe EEA process shown in Fig. 4a 6,26 are produced in this timescale, namely, rapidly changing components were observed on
the ps scale, corresponding to the EEA process measured by the
optical pump-probe method.
Since TR-STM combined with the optical pump-probe method
uses the absorption bleaching mechanism, the average current
decreases with decreasing delay time35. Therefore, in this setup, as
the delay time approaches zero, the time-resolved signal
ΔI ¼ hIðtd Þi  hIð1Þi

(5)

is expected to increase in the negative direction. However, as
shown in Fig. 4b, the obtained time-resolved signal ΔI increased in
the positive direction as the delay time approached zero. To
understand the results, a new mechanism is required to explain
the increase in ΔI with decreasing delay time, and the higher
exciton density, different from the absorption bleachingmechanism.
One possibility is to consider the following two points in thiscase: (1) The effect of absorption bleaching is not so large. (2) The
npj 2D Materials and Applications (2022) 72

probability of dissociation depends on the density of excitons andhas a threshold. As a result, I (td) decreases with td (seeSupplementary Fig. 5 for details). It has been reported that thebinding energy of excitons is reduced by the screening effect of
the excitons themselves photogenerated in a single-layer WS2
sample36. On the other hand, the value of S derived from Eq. (1)also increases signiﬁcantly with decreasing exciton binding
energy. Using the relationship between the change in exciton
binding energy with changing exciton density36, the change in S
with changing binding energy shown in Fig. 3d can be expressedas shown in Fig. 4c. Figure 4d shows the relationship along thethree lines in Fig. 4c. As the exciton density increases, S increases
non-linearly, and when the delay time is short and theinstantaneous exciton density is large, a larger current due toexciton dissociation can be obtained. The exciton density shown
here is in good agreement with the order of the excitation
intensity used in this experiment.
We analyze the experimental results by the EEA mechanism26,37,38. The rate equation can be written asdNðtÞ1¼ γNðtÞ2  NðtÞ:
dt
τ

(6)

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Fig. 6 TR-MP-STM measurement results obtained at a grain boundary. a STM image corresponding to the area shown in Fig. 3a and TR-MPSTM measurement results obtained over the surface for the delay times of 1, 10, 100, 500, 1000, and 1950 ps. 1950 ps is the limit of the delay
time of our system used here. Here, the values in the image obtained for 1950 ps was subtracted from the other images. The measurement
conditions were Vs = −3 V, Iset = 10 pA, ﬂuence 13 μJ/cm2 and center wavelength 633 nm, and the excitation energy was adjusted to the peakof the exciton resonance spectrum of WS2. b TR-MP-STM spectra obtained in the white square shown in a. x indicates the distance from theboundary. The 56 spectra for each x were averaged. Lifetimes obtained by ﬁtting are shown in each ﬁgure. c Lifetime as a function of x. As canbe seen in Fig. 3a, Id was measured when the probe was in the area to the right of the right GB edge. Therefore, to reduce the inﬂuence of thestructures in GB, this measurement was performed on the right side of the GB edge. The error bars in b are the standard deviation of the 56point values in the y direction at each x point in the white square shown in a. The error bars in c are the ﬁtting errors obtained in b.

where, N (t) is the exciton density, N(0) = N0, γ is the EEA rate, andτ is the exciton recombination lifetime.
For times shorter than τ, Eq. (6) can be written asdNðtÞ γNðtÞ2 ðt  τ Þ;
dt

(7)

giving 1
1:N ðt Þ  γ t þ
N0 γ

(8)

In the region of t << τ, the exponential decay appears as astraight line. Therefore, we introduced the following function andanalyzed the signals shown in Fig. 4b (td > 2 ps was consideredhere).
N ðt Þ ¼

Aþ Bt þ Ct þ t0(9)

By ﬁtting the spectra for 6, 12, 24 μJ/cm2 in Fig. 4b withparameters A, B, and C, t0 was obtained as 14.6 ± 1.0, 10.8 ± 0.5,
and 4.58 ± 0.29 ps, respectively. On the other hand, the lightabsorption rate of WSe2 at a wavelength of 600 nm was estimatedto be ~3% was applied39. Thus, N0 was obtained as 5.5 × 1011,
1.1 × 1012, and 2.2 × 1012 cm−2 for light intensities of 6, 12, and24 μJ/cm2, respectively. It has been reported that the ps-scale EEA
process predominates in the exciton density range of1011–1013 cm−2 in a TMDC atomic layer on a thermally oxidized
SiO2 substrate26. Using these values, γ was determined to be
0.10 ± 0.02 cm2s−1, in good agreement with 0.12 cm2s−1 obtainedby the time-resolved PL of WS2/SiO226.
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This is the ﬁrst ever detection of exciton dynamics using themechanism of exciton dissociation in the nm order region below
the STM tip using TR-MP-STM. The spatial resolution of the timeresolved STM (~2.5 nm) greatly exceeds that of conventional
ultrahigh-speed time-resolved measurement methods such as the
optical pump-probe method11, time-resolved PL10, time-resolved
PEEM12, and scanning near-ﬁeld optical microscope13 methods(several tens of nm).
Time-resolved STM measurement on ripple structure
It was shown that the dynamics of the EEA process in the highdensity state, which is the characteristic of exciton dynamics in
low-dimensional systems, can be measured by TR-MP-STM. Next,
to show the high usefulness of this method, TR-MP-STM
measurements were performed on the nanostructures, ripples
and GBs, which are characteristic of this sample.
Figure 5a shows a schematic diagram of the nanoscale ripple
structure observed in Fig. 2c. Figure 5b shows an STM image takenover a ripple area in WS2 (top ﬁgure), and the time-resolved
signals acquired at the corresponding tip positions with a color
scale (bottom ﬁgure). While the STM tip was moved onedimensionally across the ripple, it was stopped every 2.6 nm,
and a delay time scan was performed ﬁve times for 2 ns to obtainthe time-resolved spectrum at each STM tip position. The STM
conditions were Vs = −3.5 V and Iset = 40 pA.
Figure 5c shows the data obtained at typical points shown by
the red (top area) and blue (bottom area) dashed lines in Fig. 5b.
In the time scale larger than 100 ps, the EEA process due to thehigh-density state immediately after excitation is approximately
completed, and the recombination relaxations of thermalized
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exciton and exciton migration mainly occur. In fact, singlecomponent exponential ﬁttings matched the data well in thisregion, and the lifetimes were 2.41 ± 0.12 and 0.50 ± 0.01 ns forthe top and bottom regions, respectively. The drive limit of themechanical delay stage of our present system is 2 ns. In thisexperiment, we reduced the light intensity by 1/100 compared tothat used in ref. 40 to reduce the free carriers’ inﬂuence andinvestigate the excitons’ dynamics. Therefore, to increase thesignal-to-noise ratio, we introduced a new modulation method
instead of thinning out the pulsed light using a Pockels cell as in
the case of the previous paper (details are shown in Supplementary Fig. 1). Although ﬁtting looks good, we need to use anotherapproach, including the previous method40, when discussing
longer lifetimes in more detail. Since that is not the case here, weleave this issue for future works.
Figure 5d shows the cross section along the green line in the
STM image in Fig. 5b and the relaxation time constant obtained ateach location. If the ﬁtting error exceeded 50% of the extracted
value, it was removed from the data. It was clariﬁed that the top
region of the ripple structure has a long lifetime of ns order, andthe bottom region has a short lifetime of 100 ps order. In addition,
the exciton lifetime tended to be particularly long in the top
region of the ripples with a small width and height appearing at
x < 42 nm.
Next, the origin of these phenomena will be discussed. Figure
5e shows a schematic diagram of the principle of spatiallychanging exciton dynamics. There are three possible effects of thesubstrate. (1) Fluctuations in exciton density: Excitons accumulate
where the bandgap is locally reduced by the roughness of thesubstrate26. (2) Screening: The radius of excitons increases,
thereby increasing the distance over which two excitonsinteract41. (3) Interaction of the substrate with phonons42. (1)and (2) affect the fast decay component due to the EEA effect. In
fact, as shown in Fig. 5c, the EEA effect can be seen even in the
low current (low density) region at the bottom (blue line). On theother hand, the slow decay component after the EEA effect has
decreased is expected to have a shorter lifetime if the effect of (3)is present. This was conﬁrmed as being due to the difference inlifetime measured at the top and bottom locations of the ripple.
From the viewpoint of STM measurement, it is considered thatthe binding energy of excitons becomes smaller, and dissociation
due to an electric ﬁeld becomes easier. Therefore, the measured
current becomes larger in the bottom region. In fact, consistent
results were obtained (Supplementary Fig. 6).
Furthermore, the exciton lifetime depends on the size of theripple, as shown in Fig. 5d, as described above. The reason for thelong exciton lifetimes in the top region of the ripples with small
width and height is considered to be as follows. The following
equation expresses the maximum accumulated stress for aripple28.
ϵ ¼ π2 th=ð1  σ Þλ2(10)Here, σ is Poisson’s ratio (WS2: 0.18), t is the ﬁlm thickness, h is the
ripple height, and λ is the ripple width. The maximum stress isinversely proportional to the ripple width. In addition, it has beenreported that the band structure changes from a direct to anindirect transition type43. Smaller ripples may have a longer
lifetime owing to similar effects.
TR-MP-STM measurement at a grain boundary
The effect of defect levels existing in the GBs was investigated.
The defects in GBs are considered to enhance the recombination
over a region of about 5 nm from the GB44,45. This phenomenon
cannot be conﬁrmed with the resolution of PL (~several tens ofnm), and we attempted to conﬁrm it with TR-MP-STM. In theregion of WS2 containing a GB shown in Fig. 3a, TR-MP-STM maps
were obtained for delay times of 1, 10, 100, 500, 1000, and 1950 ps
npj 2D Materials and Applications (2022) 72

together with a topographic image of the region with frep of
80 MHz. 1950 ps is the limit of the delay time of our system used
here. The results are shown in Fig. 6a, where the values in theimage obtained for 1950 ps was subtracted from the other images.
The time-resolved measurements in Fig. 6a were performed
over the entire surface in the ﬁgure at each pixel of 2 nm
(x-direction) × 0.3 nm (y-direction). In the area surrounded by thewhite square around the boundary of 10 nm (x-direction from theboundary) × 16.8 nm (y-direction), 5 (=10/2) × 56 (=16.8/0.3)
time-resolved spectra were measured for x = 0, 2, 4, 6, and 8 nm.
The 56 spectra for each x were averaged, and the results areshown in Fig. 6b. Fitting was performed by a single exponentialfunction using all the data before averaging.
As shown in Fig. 6a, the obtained time-resolved signal was not
uniform, and ﬂuctuations were observed on a scale of about
10 nm. This is because the exciton binding energy changes nonuniformly with the change in the dielectric constant, which isrelated to the roughness of SiO2 used as the substrate and theresidual impurities induced by the resist treatment. Figure 6csummarizes the results shown in Fig. 6b. The lifetime changes overthe 8 nm, with the spatial resolution of 1 nm order, was observed,
for the ﬁrst time. Similar effects of GB have been reported inprevious papers44,46. The defects in GBs are considered to enhance
exciton recombination over a region of about 5 nm from the GBs.
The band structure was observed to be modulated continuously
over ~5 nm around a grain boundary (GB). As can be seen in Fig.
3a, Id was measured when the probe was in the area to the right of
the right GB edge. Therefore, to reduce the inﬂuence of such
structures in GB, the measurements were performed on the rightside of the GB edge.
There is another possibility in the present case. The effect of theripple structure was conﬁrmed, as shown in Fig. 5. Since the edgeof the GB is zigzag and disordered, the modulation of the ripple
structure may have an inﬂuence on the exciton dynamics (seeSupplementary Fig. 2 for more details). To analyze that, wemeasured the cross-sections around the GB edge, which issummarized in Supplementary Fig. 7. As the results show, theripple structure is distorted near the GB, supporting its effect on
the lifetime of the excitons. Since the GB edge has a disorderedstructure with a narrow width, it was difﬁcult to stably measure I-V
curves under laser irradiation in the 10 nm region. To clarify themechanism in more detail, further study is needed, which wewould like to leave for a future work.
We have developed a time-resolved multiprobe STM (TR-MPSTM) method by combining multiprobe STM with a new delay
time modulation approach of pump-probe optics. First, weclariﬁed the principle of STM measurement to probe neutral
excitons. Then, using this system, we succeeded in evaluating the
spatiotemporal dynamics of excitons in nanostructures such as
ripples and GBs in monoatomic-layer semiconductors with aspatial resolution of 1 nm order. In the WSe2 region, the rate of the
EEA process induced by collisions between excitons wasdetermined to be 0.10 ± 0.02 cm2s−1. Subsequently, exciton
dynamics in the nanoscale ripple structures was revealed. It wasalso directly observed that the exciton dynamics is modulated by
local defects. Exciton recombination lifetime changed near a GB
over a region about 8 nm, which agreed well with the modulationof the ripple structure near the GB edge.
This method is expected to accelerate research on exciton
dynamics and the development of applications directly based on
the analysis of experimental results of nanoscale.
METHODS
Newly developed delay time modulation method
A schematic diagram of the TR-MP-STM system is shown in
Supplementary Fig. 1a. The wavelength of Ti: sapphire laser pulse
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H. Mogi et al.

9(pulse time width ~150 fs, wavelength 830 nm, repetition rate
80 MHz) is converted by an optical parametric oscillator (OPO)(~400 fs, 633 nm). The beamline is divided into two by an
unpolarized half mirror, and a delay time scan of up to ~2 ns
can be performed by a mechanical delay stage mounted on one ofthe optical paths (Path 1). In the other optical path (Path 2), an
EOM and a Faraday rotator are used to change the polarization.
Delay-time modulation is achieved by switching between two
optical paths (Path 2-1, Path 2-2). Supplementary Fig. 1b shows
the principle of new delay time modulation shown in the blue
shaded area in Supplementary Fig. 1a. The newly developed
MPSTM part for time-resolved measurement also has its own
novel structure (See Supplementary information for more details).
Explanation of Eq. (1)While excitons pass through the electric ﬁeld (t = 0 − tpass), they
are dissociated and measured as tunnel current. Since thedissociation rate is Γ, the rate equation at which n excitons in
the electric ﬁeld dissociate is shown as dn/dt = −Γ·n, and theprobability that one exciton dissociates during tpass becomes(1 − exp (−Γ·tpass)). Therefore, the tunnel current It obtained bydissociating the N excitons diffused immediately below the probeevery second is proportional to N · S = N · (1 − exp (−Γ·tpass)).
Multiprobe STM for time-resolved measurement
As shown on the left of Fig. 1a and Supplementary Fig. 1a, an
optical zoom lens was placed on the multiprobe measuring
unit40,47. This system can mount up to four STM/AFM probes, andthe position of the probe tip can be determined with μm accuracy
by observing the optical image using a CCD camera. Therefore, it
is possible to access the desired location of the isolated twodimensional sample on the insulating substrate and perform STM/
AFM measurement. In addition, a half mirror is installed inside thezoom lens, and the laser can be focused at the desired position on
the sample through the objective lens, making it an ideal system
for TR-STM measurements. To easily perform preliminary experiments such as STM observation and light-modulated scanningtunneling spectroscopy (LM-STS) measurement48, the optical pathcan be switched by the ﬂip mirror in front of the zoom lens so that
continuous laser light is used instead of pulsed light.
In this experiment, instead of using AFM probes in directcontact with the sample as before40, two Au electrodes wereprepared on the sample. Then, two Pt-coated conductive
cantilevers (spring constant 0.2 Nm−1) were mechanically brought
into contact with the Au electrode and used to apply the sample
bias voltage Vs. In this state, one STM tip was made approach thesample and performed various measurements. In addition, to
combine STM with photoexcitation, we developed a holder with
an elaborate structure to prevent the measured part from being inthe shadow of the probe. The time-resolved signal ΔI was
detected by a lock-in ampliﬁer using the periodic change in thevoltage used for EOM modulation as a reference. All experiments
were performed under ultrahigh vacuum (~10−7 Pa) at atemperature of 85 K.
Besides, the time-resolved data are obtained immediately
below the STM tip because excitons are dissociated by the bias
voltage applied between the STM tip and sample. Since STM has
an atomic-level resolution, we can observe and examine the area
we choose for experiments before measurement. In the ripple
measurement in Fig. 5, for example, excitons are conﬁned to eachregion. Therefore, the result of the time-resolved measurement isobtained without the inﬂuence from the adjacent structure.
Origin of ripple structure in WSe2 region
The stress generated at the heterointerface in the WSe2/WS2 plane
due to the lattice constant mismatch should generate tensile
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stress on WS2, which does not correspond to this ripple
structure49. It has been reported that a ripple structure wasformed when the GBs in single-layer MoS2 on a sapphire substrate
were irradiated with light. However, in that case, the ripple period
coincided with the step period of the substrate, and theinteraction between the MoS2 layer and the substrate step wasconsidered to be the cause22. On the other hand, in this case, thesubstrate was SiO2/Si and unrelated to the step period. According
to a previous report of calculation, the ripple structure is formed
spontaneously50, which have occurred in this study, However, thedetails of its formation are beyond the scope of this paper.
DATA AVAILABILITY
The data that support the ﬁndings of this study are available from the corresponding
author upon reasonable request.

Received: 1 April 2022; Accepted: 27 September 2022;

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ACKNOWLEDGEMENTS
We acknowledge the ﬁnancial supports of a Grant-in-Aid for Scientiﬁc Research fromJapan Society for the Promotion of Science (17H06088, 20H00341), Japan Core
Research Evolutional Science and Technology (CREST) (JPMJCR1875, JPMJCR16F3),
and JST, PRESTO (JPMJPR22AA), Japan.

AUTHOR CONTRIBUTIONS
H.M. developed the system, and took data with R.K., R.M., and J.W. N.W. and Y.M.
prepared samples. Y.A., S.Y., and O.T. provided technical advice. H.S. organized andsupervised the project and edited the paper with H.M.

COMPETING INTERESTS
The authors declare no competing interests.

ADDITIONAL INFORMATION
Supplementary information The online version contains supplementary material
available at https://doi.org/10.1038/s41699-022-00345-1.
Correspondence and requests for materials should be addressed to Hidemi
Shigekawa.
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