Supplementary Information

Ultrafast nanoscale exciton dynamics via laser-combined scanningtunneling microscopy in atomically thin materials

Hiroyuki Mogi1, Yusuke Arashida1, Ryusei Kikuchi1, Ryosuke Mizuno1, Jun
Wakabayashi1, Naoki Wada2, Yasumitsu Miyata2, Atsushi Taninaka1, Shoji Yoshida1,
Osamu Takeuchi1, and Hidemi Shigekawa1
1

Faculty of pure and applied sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan2

Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan1

Supplementary Figures

Supplementary Figure 1: Schematic of the newly developed TR-MPSTM. a, TR-MPSTM
system. BS (beam splitter), PBS (polarized beam splitter), OPO (optical parametric oscillator),
FR (Faraday rotator), EOM (electro-optical modulation element). b, Principle of newly designed
delay-time (DT) modulation system, which significantly increases SN ratio. M1, M2: mirror 1
and mirror 2. See Methods for details.
In time-resolved STM, a modulation method for lock-in detection is indispensable. We foundit necessary to shorten the measurement time to stably measure the weak signal of excitons andenable two-dimensional imaging. Therefore, we developed a new delay time modulation system
and increased the light intensity by 80 times and the signal-to-noise ratio.
Figure S1b shows the principle of our new delay-time modulation system (DT modulation
system) that modulates the light passing through Path 2 shown in the blue shaded area in Fig. S1a.
The figures in the upper and lower yellow arrows show the changes in polarized light (red arrows)
in the direction from PBS1 to PBS2 (forward) and the direction from PBS2 to PBS1 (backward),
respectively.
In PBS1, only linearly polarized light in the z-direction is reflected. The plane of polarization2

is rotated by 90 ° by the λ/2 wave plate and the Faraday rotator (FR) and is incident on the EOM.
When no high voltage (HV) is applied to the EOM (OFF), the light beam passes through PBS2
while maintaining the plane of polarization (Path 2-1). When HV is applied to the EOM (ON),the plane of polarization of the light beam is rotated 90 ° and reflected by PBS2 (Path 2-2). In
both paths, the mirrors (M1 and M2) placed perpendicular to the optical path reflect the two lightbeams and they merge at PBS2. As a result, the optical path length can be modulated by selecting
Path 2-1 with a short optical path length and Path 2-2 with a long optical path length dependingon the presence or absence of the applied voltage to the EOM. The optical path length difference
between Path 2-1 and Path 2-2 is 1.8 m (a time of ~ 6 ns). Since the plane of polarization rotatesin the backward direction when HV applied to the EOM is ON, polarization in the x-direction is
obtained after the light beam passes through the EOM regardless of whether or not HV is applied.
After that, since the rotation directions of the polarization planes induced by the λ/2 wave plateand FR are opposite, the polarization planes are not rotated, and the polarization in the x-direction
is maintained. Therefore, the light beam reflected by PBS1 and traveling in the forward direction
is now directed toward BS2, which is 90 ° different from that of the incident beam.
The light emitted from the delay-time modulation system merges with the light passed throughPath1 at BS2 to form a pulse-pair train with delay-time modulation, as shown in Supplementary
Fig. 1a. We used a DG645 digital delay/pulse generator (Stanford Research Systems, Inc.) and a
JK-Flip Flop in toggle mode to adjust the ON/OFF of the EOM driver (DPD-300-4.6-Al, Eksma
Optics) and control the frequency and phase of the delay time.

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Supplementary Figure 2: AFM image of the grain boundary (GB) shown in Fig. 3 a.
A large-scale AFM image of the GB shown in Fig. 3a taken in air before cleaning thesurface. Small dots are adsorbates deposited on the surface during the growth process.
They do not have a triangular form and could be removed with the cleaning method1 using
the STM tip in the present case. b. A magnified AFM image of the GB, showing acomplex structure similar to that reported in a previous paper2. Since the STM image
depends on the electronic structure and bias voltage, the AFM image was measured.
Detailed analysis of the relationship between the GB structure and the exciton
dynamics is attractive, which, however, is beyond the scope of this work. We leave thisissue for future studies. See Supplementary Fig. 7 for the modulation of the ripple
structure near the GB edge.

4Supplementary Figure 3: Tunnel current characteristics due to exciton dissociation. a,b, Light-modulated scanning tunneling spectroscopy (LM-STS) spectra obtained for various
setpoint currents Iset for WSe2 and WS2, respectively. To reduce the thermal expansion3,4, currentvoltage (I-V) measurement was performed while changing the light intensity as 50% (2.5 mW)→ 100 → 0 → 75 → 25 every 1 ms5. c, Schematic illustration of the band structures for various
STM tip-sample distances determined by Iset. d, e, Relationship between light intensity and tunnel
current. Using the IV curves shown in a and b, the current values for five light intensities at eachvoltage were obtained for each set current Iset. A denotes the value obtained by integrating thestraight blue line in the figure, which was extrapolated from the current values for the light5

intensities of 0% and 25% and B denotes the integrated value using the actual plot. f, g, B/A
obtained for WSe2 and WS2, respectively, for different Vs and Iset. h, Schottky barriers obtainedfor WSe2 and WS2.
We investigated the mechanism of exciton dissociation with a high binding energy by theelectric field generated by the bias voltage applied between the STM tip and the sample. The
tunnel current should depend on the strength of the electric field that induces exciton dissociationand the number of excitons, determined by the light intensity. Therefore, the dependences of thetunnel current on the bias voltage and light intensity were investigated. The linearity of the signal
intensity with the light intensity is also very important information in performing the opticalpump-probe method.
In each region of WS2 and WSe2, LM-STS measurement5 was performed under the feedback
conditions of Vs = -7 V, light source wavelength of 532 nm, and an energy intensity of 2.5 mW.
Figures S3a and S3b show typical results of LM-STS measured at locations in WSe2 and WS2
with different set currents (Iset) and results averaged over 256 measurements, respectively. In the
WS2 region, measurements were taken near the top of the ripples. Compared with the STM setup,
which measures conductive samples, the applied voltage here is very large (Vs = -7 V), which isthe field emission region in a general STM setup6. However, in this measurement, the bias voltage
is divided into the tunnel junction, the inside of the sample, and the electrode-sample interface
resistance. Therefore, the STM measurement is performed in the tunnel region rather than in thefield emission region.
If the set current Iset was low, a large gap was visible in the I-V curve. Therefore, it isconsidered that there is no influence of the free carriers generated by the light irradiation. However,from the results of the LM-STS shown in Figs. S3a and S3b, the gap voltage region of both WSe2
and WS2 becomes narrower as the set current increases and finally becomes gapless. That is, the6

I-V characteristics measured by conventional STM reflect the intrinsic local density of states ofthe material, but in this case, the gap voltage is strongly dependent on the probe-sample distance,
which depends on Iset. To clarify the reason for this, in this experiment, we consider the in-plane
tip induced band bending (TIBB) introduced by the voltage applied between the STM tip and thesample and the mechanism of carrier generation caused by charge separation in the area.
First, it is known that when a TMDC is irradiated with light, excitons with a very large binding
energy (~ 0.3 eV for WS2 and WSe2 on SiO2) are generated7, and an electric field above a certain
level is required to cause charge separation. Therefore, when the distance between the STM tip
and the sample is large and the electric field in the sample is weak, a nonlinearity appears, where
the current suddenly increases when Vs exceeds a certain value (threshold value). With theapproach of STM tip, as shown in Supplementary Figs. 3a and 3b, exciton dissociation occurs at
lower Vs, and a smaller gap voltage is observed. As shown in Supplementary Fig. 3c, thiscorresponds well with the decrease in the tunnel resistance and the increase in the electric field
introduced into the sample plane as the STM tip approaches the sample.
Furthermore, to confirm the relationship between the light intensity and the tunnel current,
the linearity of the current response with the light intensity was analyzed. Using the I-V curves
shown in Supplementary Figs. 3a and 3b, the current values for five light intensities at eachvoltage were obtained for each set current Iset. Supplementary Figs. 3d and 3e show the results
obtained for WS2 for Vs = 4 V when Iset = 50 and 80pA, respectively. In the figures, A denotes thevalue obtained by integrating the straight blue line in the figure, which was extrapolated from thecurrent values for the light intensities of 0% and 25% and B denotes the integrated value using
the actual plot. The values of B/A are 0.78 and 0.98 for Supplementary Figs. 3d and 3e,
respectively. B/A is considerably smaller than 1 when the saturation tendency is strong(Supplementary Fig. 3d) and near 1 when it is linear (Supplementary Fig. 3e). Supplementary7

Figs. 3f and 3g show the values of B/A obtained for different values of Vs and Iset. The
measurement points in the gap voltage region where almost no current flows were observed wereomitted because of the large error. When the distance between the STM tip and the sample waslarge, namely Iset was small or Vs was low, the saturation tendency was observed (B/A < 1). On
the other hand, as the distance between the STM tip and the sample decreased, namely Iset waslarge or Vs increased, B/A tended to approach 1 and a linear response was shown.
Under an ideal constant electric field, the photocurrent generated by charge separation will be
proportional to the number of incident photons (B/A = 1). Since the effective permittivity
increases as the number of excitons inside the single-layer sample increases owing to lightirradiation, the increase in the internal electric field is suppressed by screening. Therefore, whenthe electric field generated in the sample is weak, it is insufficient to dissociate excitons, and it isconsidered that B/A becomes saturated. It was found B/A tends to be more saturated for WS2 thanfor WSe2. In the ripple structure in the WS2 region, the distance from the substrate to the WS2 film
increases periodically, and the state of the film at a long distance from the substrate is close to
free standing. On the other hand, it has been reported that the absorbance tends to be higher in thefree-standing state than for the case of a fused silica substrate8.Therefore, this difference in B/A
is considered to be due to the higher exciton density in WS2, which is located farther from thesubstrate. That corresponds well with that exciton dissociation is difficult in WS2 than in WSe2.
According to these considerations, the measured current can be explained by the dissociation of
excitons induced under the STM tip.
When the STM tip and the sample were closest to each other, the voltage at which the current
became zero was -0.52 V for WSe2 and -0.60 V for WS2, as shown in Supplementary Figs. 3a and3b. This is thought to correspond to the open-circuit voltage generated by the built-in electric field,
where a Schottky barrier was created between the STM tip and the sample. Supplementary Fig.

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3h shows the band structure based on these values. For the bandgap values, those from theprevious paper were used9. For bulk samples, Schottky barriers are required for normal I-V
measurements10, but it has been shown that evaluation is possible in this way, even for singlelayer samples that do not have sufficient free carriers.
In the previous paper11, since the probe was brought into contact with the sample with a higher
voltage, a strong electric field was applied over the wide depletion layer region, as shown in Fig.
4 of ref. 11. Therefore, in ref. 11, excitons generated in that region were immediately dissociated,and phenomena such as pair annihilation of excitons were not observed.

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Supplementary Figure 4: Two dimensional dI/dV mapping results. a, dI/dV mapping
extracted for the area shown in Fig. 2c under the condition of Vs = + 5 V. b, One-dimensional
dI/dV map obtained from a. A straight line parallel to the interface was considered at intervals of
21 nm from the WS2/WSe2 interface in a, and dI/dV was averaged over the region of ± 15 nm
width from each straight line. This map summarizes the results obtained by repeating the same
operation over the area surrounded by the yellow lines in a.
The spatial mapping of the I-V curve was acquired on a grid in the same area as the STM
observation shown in Fig. 2c. Supplementary Fig. 3a shows the dI/dV map extracted under thecondition Vs = + 5 V to visualize the gap voltage width. The white dashed line shows the line of
the heterojunction interface. There is a clear difference in the dI/dV values between WSe2 andWS2 regions, but dI/dV is relatively uniform within each region. Supplementary Fig. 3b shows a
one-dimensional dI/dV map obtained by integrating and averaging the dI/dV curves in the
direction parallel to the straight line of the heterojunction interface (dashed line in the figure). The
gap voltage region is clear, and the gap in the WS2 region is larger than that in the WSe2 region.
This is consistent with the exciton dissociation being more difficult in WS2 than in WSe2, asshown in Figs. 3d to 3f. In the ripple structure in the WS2 region, the screening effect from thesubstrate was weakened and the binding energy increased. As shown in Supplementary Fig. 3b,

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the WSe2/WS2 interface is steep and is in good agreement with the STM image in Fig. 2c and theresults of previous research papers12. These data were derived from the I-V curves and define thevoltages at which excitons dissociate. Therefore, in an equilibrium state, the desired values can beobtained.

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Supplementary Figure 5: Schematic model to explain the TR-MP-STM signal. a, Delay timedependence of the dynamics of exciton density N(t, td) excited by a pulse pair of pump and probe
light. N(t, td) shapes for three different values of td are shown as examples. N(t, td) above thethreshold blue line is detected as tunnel current I(t, td). b, TR-MP-STM signal DI(td) based on themechanism shown in a, where DI(td) is the integral of I(t, td) for t over the detection time. Delaytime modulation technique shown in Fig. S1b is used for measurement. Under the following two
conditions, DI(td) decreases with td unlike the case of absorption bleaching4: (1) The effect ofabsorption bleaching is not so large. (2) The probability of dissociation depends on the density of
excitons and has a threshold.
In the previous paper11, laser excitation was carried out with a wavelength of 800 nm, close to
the absorption edge of WSe2. Namely, the change in carrier density at the conduction band edge
was probed, and the effect of absorption bleaching was large. On the other hand, in the present
case, the excitation was carried out with a wavelength of 633 nm, and the excitons generated to ahigher energy region in WSe2 were supposed to relax to the conduction band edge in the fs toseveral tens of fs range. In addition, the average laser intensity was set to about 1/100 comparedto that used in ref. 11. Therefore, the influence of absorption bleaching is considered to be small
in this case.

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Supplementary Figure 6: Two-dimensional I-V map obtained over a ripple area. a, Cross
section of a ripple structure. b, Two-dimensional map of I-V curves measured at each point along
the cross section. I-V curve rises at lower VS in the area where the ripple contacts with the substrate,
indicating that dissociation of excitons is easier there due to the effect of the substrate.

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Supplementary Figure 7: Analysis of the ripple structures near the GB. a, b, STM
topographic and currents Is images shown in Fig. 3a. c, Normalized cross-sections of the
topographic image along the eleven lines in a, namely, at eleven different distances from the edge.
The edge shape was obtained from the Is image, which corresponds to the line along Is = 2.8 pA(almost corresponding to the flexion point of each scan line around the edge). As shown in c, theripple structure is modulated near the GB edge, corresponding well with the change in the lifetime
shown in Fig. 6. As can be seen in Fig. 3a, Id was measured when the probe was in the area to theright of the right GB edge. Therefore, to reduce the influence of the structures in GB, thismeasurement was performed on the right side of the GB edge.

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