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Development of laser-combined scanning multiprobe spectroscopy andapplication to analysis of WSe2/MoSe2 in-plane heterostructure
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LETTER

Applied Physics Express 12, 045002 (2019)https://doi.org/10.7567/1882-0786/ab09b9

Development of laser-combined scanning multiprobe spectroscopy andapplication to analysis of WSe2/MoSe2 in-plane heterostructure
Hiroyuki Mogi1†, Zi-Han Wang1†, Takafumi Bamba1, Yuhei Takaguchi2, Takahiko Endo2, Shoji Yoshida1, Atsushi Taninaka1,
Haruhiro Oigawa1, Yasumitsu Miyata2, Osamu Takeuchi1, and Hidemi Shigekawa1*
1Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, 305-8573, Japan
Department of Physics, Tokyo Metropolitan University, Tokyo 192-0397, Japan2*

E-mail: hidemi@ims.tsukuba.ac.jp
These two authors contributed equally to this work.

†

Received February 6, 2019; accepted February 22, 2019; published online March 13, 2019
By combining scanning multiprobe (MP) microscopy with optical methods such as light-modulated spectroscopy (LMS) and optical pump-probe(OPP) method, we have succeeded in developing a microscopy method for measuring electronic structures and photoinduced carrier dynamics inmicroscopic structures. We demonstrated its performance by analyzing the electronic structures in a monolayer island of a WSe2/MoSe2 in-plane
heterostructure grown on a SiO2/Si substrate. By observing the field-effect transistor characteristics and photocurrent mapping over theheterostructure by LMS, we were able to visualize the band structure. Positional dependence of carrier dynamics was also successfully probed by
OPP-MP spectroscopy. © 2019 The Japan Society of Applied Physics

In current material science and its applications to thedevelopment of device technologies, the use of the lowdimensional functions realized in materials such as
topological insulators1) and mono-to-multilayer transition
metal dichalcogenides (TMDCs)2) has been attracting considerable attention. To advance such study, the in-depth
understanding and control of local electronic structures andtheir dynamics in microscopic structures are essential.3) For
such purposes, for example, scanning tunneling microscopy(STM) has been used to evaluate the band structure,4–6) andby forming metal electrodes using techniques such aslithography, time-resolved photocurrent measurement7)
which is an application of the optical pump-probe (OPP)
method8) has been used to observe ultrafast carrier dynamics.
In the case of small device structures such as a microscopic
TMDC island formed on a insulative material, wired electrodes can be prepared to enable conductive measurement,
which, however, requires physical and chemical treatments ofsamples.9) A possible approach is to apply multiprobe (MP)STM techniques, in which multiple tips are used to measure,for example, transport characteristics between two desired
points.10)
To obtain information on photoinduced phenomena, optical technologies must be introduced into the MP techniques.
We have been developing laser-combined STM and related
techniques, light-modulated scanning tunneling spectroscopy(LM-STS)11) and OPP-STM,12,13) and have been applying
them in the ﬁeld of nanoscale science.
In LM-STS, by simultaneously measuring the relationshipbetween tunnel current It and bias voltage V (I–V curvemeasurement) while alternately turning the photoillumination
on and off using a chopper, one can obtain and visualize
information such as the local band structure,11) carriertransport in a p–n junction,14) photocarrier generation in asolar cell structure,15) and surface photovoltage (SPV) effecton a phase transition.16) In OPP-STM, on the other hand,
femtosecond temporal resolution and STM spatial resolution
were simultaneously realized in a microscopy by combining
ultrashort pulse technology, such as OPP method, with STM.

The sample below the STM tip was photoilluminated by atrain of ultrashort laser pulse pairs (pump and probe pulses)
with a certain delay time td, and the tunnel current measured
as a function of the delay time It(td) gives local information of
the ultrafast carrier dynamics while simultaneously providing
an STM image of the local structure.13,17,18) A nonequilibrium carrier distribution, for example, was generated withultrashort laser pulses and its relaxation processes wereprobed using the STM tunneling current. Using OPP-STM,
one can achieve for example, direct measurement of the holecapture rate at a single impurity state in GaAs,19) andvisualization of the ultrafast carrier dynamics in a pin
structure.20)
LM-STS and OPP-STM are useful techniques; however,
only one STM tip has been used to probe electronic structures
and their dynamics immediately below the STM tip. Therefore,
in the case of small structures such as TMDC island structures
formed on an insulative material, measurement cannot be
carried out because of the low conductivity of such structures,and multiple probes are necessary to form a circuit. For further
advances to observe dynamics such as the transport of current
and spin in small structures, MP microscopy that enables
probing correlations of the dynamics between different locations at desired locations is a key requirement. However, thesimultaneous use of MP techniques with laser technology has
not been realized because of the difﬁculties that arise when the
two advanced technologies are combined.
Here, we present a microscopy method that we havedeveloped by combining MP techniques with optical methods
such as LM-STS and OPP-STM. By observing the ﬁeld-effecttransistor (FET) characteristics and photocarriers in a monolayer island of a WSe2/MoSe2 in-plane heterostructure grown
on a SiO2 substrate, we were able to visualize the band
structure. Positional dependent carrier dynamics was also
successfully probed by OPP-MP spectroscopy.
Figure 1 shows a schematic illustration of our newly
developed laser-combined MP system. Laser-combined STM
and MP-STM are well-realized technologies as measurement
methods, but there are some additional issues to be resolved

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Fig. 1. (Color online) Schematic illustration of the developed MP spectroscopy system. The system consists of two synchronized lasers with a pulse width of∼150 fs, which can be changed to a continuous-wave laser with a chopper when experiments such as LM-STS are carried out. A high-resolution optical
microscope (∼1 μm spatial resolution) with an optical access port was attached outside the main UHV chamber of the MP system, which was oriented
perpendicular to the sample plate. This particular design not only provides a way of checking the locations of the sample surface and tips but also has excellent
laser focusability. The STM/ c-AFM tip was attached diagonally so that its apex could be more clearly seen from above. The delay time between the pump andprobe pulses was modulated between td and td + Δtd at ∼1 kHz for lock-in detection. M, mirror; H.M., half mirror; F.M., ﬂip mirror; P.P., pulse picker; B.E.,
beam expander; I, current signal; Δtd; difference between the two delay times used for modulation; Vs, sample bias voltage; fmod, delay time modulation
frequency. Conductive AFM(c-AFM) cantilevers coated with PtIr5 (0.2 N m−1) were used in this experiment. Contact was detected by the change in thecapacitance between the Si gate and cantilever. Si(111)-7 × 7 image was taken to show the performance of this system. Since resistance of the target sample on
this paper was more than GΩ, it was difﬁcult to stably scan the STM tip to obtain an image with a good quality.

to combine them. Two typical examples are as follows: (1) inordinary OPP-STM using one probe, light is irradiated to thetip of the probe through one of two windows on the side of
the vacuum chamber at symmetrical positions with respect tothe sample. In addition to the measurement of tunnel current,
the reﬂected light is extracted through the other window for
OPP measurement so that both results can be compared.
However, when combined with a MP system, incident lightmay be blocked by the complicated shape of the probe heads.
(2) Moreover, in this arrangement, it is necessary to conﬁrm
the irradiation light spot on the sample by placing the zoom
lens at a position far from the sample (for example, at a
distance of 180 mm, NA = 0.083). Therefore, it becomes
more difﬁcult to precisely control the light spot position. In
addition, in an ordinary MP system, scanning electron
microscopy (SEM) is used to evaluate samples and thepositioning of probes. However, some samples are damagedby the high-energy electron beam irradiation during theobservation, and the deposition of contaminants such ashydrocarbons may occur. Furthermore, when the stiff probes
are mechanically brought into contact with the sample, theprobes and the sample may be deformed.
To solve these problems, we combined a long-focus
optical microscope system (working distance: 24 mm,
NA = 0.42) for positioning the probe and light irradiation.

Although the spatial resolution of an optical microscope(∼1 μm) was inferior to that of a SEM, in addition to solving
the above problems, it became possible to irradiate light fromdirectly above the probe via a half mirror mounted inside theoptical microscope and the spot size was reduced to ∼1 μm.
The vibration noise of the probe head section was reducedby isolating the spring vibration using an eddy current
damper. However, as the spot size became smaller, when
the probe stage shook against the light beam, it directly led to
the ﬂuctuation of light intensity, and it was found thatenhancement of the vibration isolation performance wasnecessary. Therefore, the number of magnets in the eddy
current damper was increased, and the air spring isolation
stage supporting the entire device was changed from a
passive type to an active type to enhance the vibration
isolation performance. As a result, the ﬂuctuation of theprobe stage at the spring suspension could be reduced toabout 100 nm order. When mechanical contact was necessary, conductive atomic force microscopy (c-AFM) cantilevers were used instead of stiff STM probes to avoid thedeformation of the probe tip and sample. We have developed
a probe holder for the compatible use of an STM tip and a cAFM cantilever, which also does not block the incident light.
The STM tip was used to obtain atomic resolution whenrequired in the experiment.

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Fig. 2. (Color online) FET characteristic measurement. (a) Optical image of WSe2/MoSe2 in-plane heterostructure formed on SiO2/Si substrate. (b)Schematic illustration of measurement setup. Ids, drain–source current; Vds, drain–source voltage; Vgs, gate–source voltage. (c) Ids as a function of Vgs obtained
for the Vds kept at 0.5 V (left) and 20 V (right). (d) Band structures of WSe2 and MoSe2 estimated from the results shown in (c). Ev, energy of the valence bandedge; Ec, Energy of the conduction band edge; EF, Fermi energy. The ﬁgures illustrate the mutual relationship of band structures on relative scales (not actual
values).

Next, to demonstrate the performance of the newly developed laser-combined MP system, three experiments, (1)analysis of the FET characteristics and photocarrier mapping,(2) light-modulated spectroscopy (LMS), and (3) time-resolved
spectroscopy with an ultrashort pulse laser, were carried out toanalyze the electronic structures in a monolayer island of a
WSe2/MoSe2 in-plane heterostructure. Figure 2(a) shows an
optical image of the sample. Isolated WSe2/MoSe2 islands
grown on a SiO2/Si substrate by halide-assisted chemical vapor
deposition21) were observed, in which a triangular MoSe2
single layer was surrounded by a WSe2 single layer. In somecases [e.g., upper right area of Fig. 2(a)], another shape wasformed owing to different growth conditions.22)
First, by observation of the FET characteristics andphotocurrent mapping over the heterostructure by LMS, theband structure of the WSe2/MoSe2 in-plane heterostructure
was analyzed. The FET characteristics were measured in theexperimental setup shown in Fig. 2(b). Two c-AFM tips
coated with PtIr5 were placed in contact with the sample in
the MoSe2 or WSe2 area to act as the source and drain. Then,
while maintaining the voltage applied between the two tips
Vds at a certain value, the source–drain current between the
two tips (Ids) was measured as a function of the gate voltage
Vgs applied between the Si substrate and the source tip.
Figure 2(c) shows the results obtained. A larger current
was observed in the MoSe2 region. In the MoSe2 region, thecurrent decreased when Vgs was negative, indicating that the
MoSe2 sample has n-type characteristics. On the other hand,in the WSe2 region, the rise in current was similar at both
positive and negative voltages, indicating the characteristics
of an undoped sample. The slight shift with respect to origin
suggests that the sample is slightly n-type. Although bothmaterials were undoped, the MoSe2 region, which grew ﬁrst,
was supposed to be more defective. In general, p-typecharacteristics are difﬁcult to obtain for these materials,23)
and when Se atoms are desorbed, n-type characteristics are
expected to be introduced.24,25) Based on these results,
Schottky barrier at MoSe2 did not suppress the current
compared with the barrier at the WSe2, possible band
structures for the MoSe2 (band gap: 1.6 eV) and WSe2
(band gap: 1.7 eV) regions26) with PtIr5 coated tips are shownin Fig. 2(d). The steepness of the band bending depends on
the amount of doping (almost undoped in the WSe2 region).
Next, to investigate the electronic structures of the
MoSe2/WSe2 interface and the Schottky barriers formed
between the tip and sample, LMS measurement was carriedout. Figure 3(a) shows a schematic illustration of themeasurement setup and the light spot used. Tips 1 and 2
were located in the MoSe2 and WSe2 regions, respectively.
LMS measurement was carried out at the nine positions
shown in Fig. 3(b) using continuous-wave light of 532 nm.
While the value of Vg was maintained at +40 V, Ids was

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Fig. 3. (Color online) LMS measurement. (a) Schematic illustration of measurement setup and an optical image of the laser spot used. (b) Optical image ofthe two tips located on a WSe2/MoSe2 in-plane heterostructure island. (c) Results obtained by LMS along the green line shown in (b): Iph (color scale) as afunction of Vds and the laser spot position shown in (b). Graph in the lower part indicates the cross sections along 1–3 which correspond to the positions shownin (b), where second positions from the edges were chosen for 1 and 3 to avoid the effect of shadow by tips. The inset shows a large scale feature of spectrum 1.
Graph on the right side indicates the cross section averaged in the rectangular area A in the center color image. (d) Possible band structure of the WSe2/MoSe2
in-plane heterostructure estimated from the results shown in Figs. 2 and 3(c). The ﬁgures illustrate the mutual relationship of band structures on relative scales(not actual values).

measured as a function of Vds by the lock-in detection methodwith the light intensity modulated at 320 Hz. The differencein the current ΔIds (=Iph, photocurrent) between the conditions with and without illumination shows the effect ofcharge separation caused by the in-plane electric ﬁeld
including the built-in ﬁeld at the interface and Schottky
barrier.27)
Figure 3(c) shows the results obtained three-dimensionally.
The positions indicated by the blue, green, and red arrows in
Figs. 3(b) and 3(c) correspond to that of Tip 1, interface andTip 2. The cross section in the lower part shows the Iph–Vds
curves measured at the positions 1–3. The blue, green, andred lines correspond to the cross sections along the lines 1–3,
indicated by arrows with the same color. The direction of Iph
reversed with the polarity of Vds for the curve 3 (red line),
indicating the similar change in the band bending caused by
the Schottky barrier at the position 3 (Tip 2 position). The
charge separation of excitons occurred in the depletion layer,
and current was detected because (1) the Schottky barrier
formed between Tip 1 and MoSe2 did not interrupt theelectron ﬂow apparently [see Fig. 2(c)] for Vds  0, and (2)the barrier at the hetero interface for holes was weakened for
Vds < 0, as shown in Fig. 3(d).
For the curves 1 and 2, Iph was almost 0 in the negative
bias region. Iph similarly increased with the Vds to +4 V,
however, Iph was slightly larger for the curve 2 aroundVds = 0, which is considered to be due to the existence of thebuilt-in potential at the interface. To conﬁrm this point, Iph
was averaged over the rectangular area A and shown in the
cross section on the right side of Fig. 3(c). There is a clear
peak around the interface, indicating the existence of thebuilt-in potential.9) The increase in Iph in the WSe2 region
compared to that in the MoSe2 region also suggests theexistence of the inner potential. Figure 3(d) shows possible
band structures of the WSe2/MoSe2 in-plane heterostructure(Vds = 0, positive, negative) visualized based on the results
analyzed above.
Finally, we show the positional dependence of timeresolved measurement realized by the OPP-MP spectroscopy
we have developed. The dynamics of photocarriers we haveobserved statically in the experiments above can be locallyprobed. Figure 4(a) shows the experimental setup. Four

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Fig. 4. (Color online) Results of OPP-MP spectroscopy. (a) Optical image of the experimental setup. (b)–(d) Schematic illustrations of the four experimentswith different relations between the two tip positions, laser spot positions (at Tip 1 apex or Tip 2 apex), Vds, and Vgs, with the conditions expressed as follows in
the form (Tip 1 position, Tip 2 position, laser spot position, Vds, Vgs): (b)-i (in MoSe2 ∼ 10 μm from interface, in WSe2 ∼ 500 nm from interface, at Tip 2 apex,+20 V, +160 V), (b)-ii (in MoSe2 ∼ 10 μm from interface, in WSe2 ∼ 500 nm from interface, at Tip 2 apex, −20 V, +160 V), (c) (in WSe2, in WSe2, at Tip 2
apex, +20 V, 0 V), (d) (in MoSe2 ∼ 10 μm from interface, in WSe2 ∼ 500 nm from interface, at Tip 1 apex, +20 V, −160 V). (e)–(g) Time-resolved spectra
obtained for the experimental setups shown in (b)-i, (c), and (d). The spectra obtained in three different delay time ranges are shown in (e) to show details. Here,Vgs was adjusted to control the band bending at the laser spot position. The laser power was 16 mW, and the spot size was about 1.8 μm. The ﬁgures illustrate
the mutual relationship of band structures on relative scales (not actual values).

experiments were carried out with different relations between
the two tip positions, laser spot positions, Vds, and Vgs, which
are schematically shown in Figs. 4(b)–4(d). The ﬁgures

illustrate the mutual relationship on relative scales of band
structures between tips and sample under different experimental conditions.

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A pulse train with an arbitrary repetition period was
generated by the pulse picking method with a Pockels cell. It
is possible to generate pulse pairs (pump and probe pulses) witha certain delay time by adjusting the synchronization systemand the timing of pulse picking on each optical axis. When thesample surface is photoilluminated, the transient photocurrent
is generated via charge separation due to the in-plane electricﬁeld. Then, when a probe pulse arrives during the relaxation
processes such as recombination, change in photovoltage, andtrapping,19,28,29) an additional photocurrent is induced andchanges the total magnitude of current depending on td,20)
providing time-resolves spectra. The photon energy of 1.55 eV(800 nm) was larger than the absorption edges of the excitonsin the MoSe2 and WSe2 (∼1.0 eV).30)
First, experiments were carried out by placing Tip 1 and Tip
2 in the MoSe2 and WSe2 regions, respectively, as shown in
Fig. 4(b). Figure 4(e) shows typical time-resolved spectra
obtained in the condition shown in Fig. 4(b)-i in three different
time scales to show their details. Four lifetimes were obtainedby ﬁtting the signals with exponential functions (τ1 = 24 ± 5 ps,
τ2 = 203 ± 34/181 ± 12 ps, τ3 = 20 ± 5 ns, and τ4 > 500 ns).
The slight differences among the lifetimes obtained in different
time scales were within the ﬁtting error. When the polarity of
Vds was reversed, as shown in Fig. 4(b)-ii, no signal wasobserved, which is considered to be due to the ﬂattening theband structure and the charge separation is reduced. These
results indicate that the photoinduced signal is generated by not
only charge separation but also the change in the band bending,
namely photovoltage below the Tip 2 in the WSe2 region,similar to the detection via SPV in the tip-induced band
bending region observed in previous works.31)
Next, the origin of the lifetimes were considered. Since thecharge separation process with the effect of the interface isvery fast, <50 fs in the case of WS2/MoS2,32) even if the fast
process exists, it is difﬁcult to probe it in this experimental
system with the jitter of the two lasers of ∼500 fs. However,to analyze the origin of the observed processes, we carriedout time-resolved experiment for a pure monolayer WSe2
sample similarly formed on a SiO2/Si substrate. Figures 4(c)and 4(f) show the schematic of the experimental setup andobtained result. The lifetimes of the two fast components, τ1and τ2 [picosecond order; Fig. 4(f)] were also obtained whenboth probes were placed in a WSe2 region. Namely, theselifetimes may be attributed to the defect-related recombination processes proposed for the case of MoS2.29)
On the other hand, when Tip 1 was placed in the MoSe2
region and irradiated with the polarity of Vds reversed, as
shown in Fig. 4(d), only the slow components (28 ± 12 ns,
130 ± 19 ns), were observed as shown in Fig. 4(h). The ﬁrst
one (28 ± 12 ns) similar to τ3 (=20 ± 5 ns, in WSe2, 16 ± 4 ns
in pure WSe2) is close to the value observed in MoS2 (10 ns
order) which is considered to be the bolometric process.33)
The other components (τ4: 500 ns < in WSe2, 130 ± 19 ns in
MoSe2) may have been due to the effect of the deep-level trap
states considered in the case of MoSe2.34) It also became
difﬁcult to observe the short lifetime components of τ1 and τ2
in the WSe2 region with the aging of the sample. Thereproposed various mechanisms,29,33,34) and to understand themechanism in more detail, it is necessary to investigate, forexample, the effects of the excitation wavelength, gate
voltage dependence, and sample growth conditions, as

related to the local structures. The microscopy method wehave developed is useful for carrying out various experiments
for such purposes.
In conclusion, by combining scanning MP microscopy
with optical methods such as the LMS and OPP methods, wehave succeeded in developing a microscopy method usefulfor measuring electronic structures and photoinduced carrier
dynamics in microscopic structures. We demonstrated its
performance by analyzing the electronic structures in amonolayer island of a WSe2/MoSe2 in-plane heterostructure.
By observing the FET characteristics and photocurrent
mapping over the heterostructure by LMS, we were able tovisualize the band structure. The positional dependence of thecarrier dynamics was also successfully probed by OPP-MP
spectroscopy. Another advantage of this method is that we
can observe phenomena with dynamics having wide range of
lifetimes from femtosecond to microsecond order or larger.
This method is expected to play an important role in the
researches to develop advanced functional devices withmicroscopic structures.
Acknowledgments Supports from Japan Society for the Promotion of
Science (Grants-in-Aid for Scientiﬁc Research: 17H06088) and CREST are
acknowledged.

1) X. L. Qi and S. C. Zhang, Rev. Mod. Phys. 83, 1057 (2011).
2) K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, Phys. Rev. Lett. 105,
136805 (2010).
3) W. Choi, N. Choudhary, G. H. Han, J. Park, D. Akinwande, and Y. H. Lee,
Mater. Today 20, 116 (2017).
4) Z. Alpichshev, J. G. Analytis, J. H. Chu, I. R. Fisher, Y. L. Chen, Z. X. Shen,
A. Fang, and A. Kapitulnik, Phys. Rev. Lett. 104, 016401 (2010).
5) S. Yoshida, Y. Kobayashi, R. Sakurada, S. Mori, Y. Miyata, H. Mogi,
T. Koyama, O. Takeuchi, and H. Shigekawa, Sci. Rep. 5, 14808 (2015).
6) H. Mogi, Y. Kobayashi, A. Taninaka, R. Sakurada, T. Takeuchi, S. Yoshida,
O. Takeuchi, Y. Miyata, and H. Shigekawa, Jpn. J. Appl. Phys. 56, 08LB06(2017).
7) M. Massicotte, P. Schmidt, F. Vialla, K. G. Schädler, A. Reserbat-Plantey,
K. Watanabe, T. Taniguchi, K. J. Tielrooij, and F. H. L. Koppens, Nat.
Nanotechnol. 11, 42 (2016).
8) Q. Cui, F. Ceballos, N. Kumar, and H. Zhao, ACS Nano 8, 2970 (2014).
9) Y. Gong et al., Nano Lett. 15, 6135 (2015).
10) M. Kolmer, P. Olszowski, R. Zuzak, S. Godlewski, C. Joachim, andM. Szymonski, J. Phys.: Condens. Matter 29, 444004 (2017).
11) S. Yoshida, Y. Kanitani, O. Takeuchi, and H. Shigekawa, Appl. Phys. Lett.
92, 102105 (2008).
12) Y. Terada, S. Yoshida, O. Takeuchi, and H. Shigekawa, Nat. Photonics 4,869 (2010).
13) H. Mogi, Z. H. Wang, R. Kikuchi, C. H. Yoon, S. Yoshida, O. Takeuchi,
and H. Shigekawa, Appl. Phys. Express 12, 025005 (2018).
14) S. Yoshida, Y. Kanitani, R. Oshima, Y. Okada, O. Takeuchi, and
H. Shigekawa, Phys. Rev. Lett. 98, 026802 (2007).
15) O. Takeuchi, N. Takeuchi, T. Ochiai, H. Kato, S. Yoshida, and
H. Shigekawa, Appl. Phys. Express 7, 021602 (2014).
16) Y. Terada, S. Yoshida, A. Okubo, K. Kanazawa, M. Xu, O. Takeuchi, and
H. Shigekawa, Nano Lett. 8, 3577 (2008).
17) M. Yokota, S. Yoshida, Y. Mera, O. Takeuchi, H. Oigawa, and
H. Shigekawa, Nanoscale 5, 9170 (2013).
18) S. Yoshida, Y. Terada, M. Yokota, O. Takeuchi, H. Oigawa, and
H. Shigekawa, Eur. Phys. J. Spec. Top. 222, 1161 (2013).
19) S. Yoshida, M. Yokota, O. Takeuchi, H. Oigawa, Y. Mera, and
H. Shigekawa, Appl. Phys. Express 6, 032401 (2013).
20) S. Yoshida, Y. Terada, R. Oshima, O. Takeuchi, and H. Shigekawa,
Nanoscale 4, 757 (2012).
21) S. Li, S. Wang, D. M. Tang, W. Zhao, H. Xu, L. Chu, Y. Bando, D. Golberg,
and G. Eda, Appl. Mater. Today 1, 60 (2015).
22) Y. Li, F. Wang, D. Tang, J. Wei, Y. Li, Y. Xing, and K. Zhang, Mater. Lett.
216, 261 (2018).
23) M. Tosun, S. Chuang, H. Fang, A. B. Sachid, M. Hettick, and Y. Lin, ACS
Nano 8, 4948 (2014).
24) Y. Guo, Y. Ji, H. Dong, L. Wang, and Y. Li, AIP Adv. 9, 025202 (2019).

045002-6© 2019 The Japan Society of Applied Physics

Appl. Phys. Express 12, 045002 (2019)

H. Mogi et al.

25) M. Tosun, L. Chan, M. Amani, T. Roy, G. H. Ahn, P. Taheri, C. Carraro,J. W. Ager, R. Maboudian, and A. Javey, ACS Nano 10, 6853 (2016).
26) J. Kang, S. Tongay, J. Zhou, J. Li, and J. Wu, Appl. Phys. Lett. 102, 012111(2013).
27) Y. Yi, C. Wu, H. Liu, J. Zeng, H. He, and J. Wang, Nanoscale 7, 15711(2015).
28) D. Kozawa et al., Nat. Commun. 5, 4543 (2014).
29) H. Wang, C. Zhang, W. Chan, S. Tiwari, and F. Rana, Nat. Commun. 6,8831 (2015).

30) H. L. Liu, C. C. Shen, S. H. Su, C. L. Hsu, M. Y. Li, and L. J. Li, Appl.
Phys. Lett. 105, 201905 (2014).
31) S. Yoshida, Y. Terada, M. Yokota, O. Takeuchi, Y. Mera, and
H. Shigekawa, Appl. Phys. Express 6, 016601 (2013).
32) X. Hong, J. Kim, S.-F. Shi, Y. Zhang, C. Jin, Y. Sun, S. Tongay, J. Wu,
Y. Zhang, and F. Wang, Nat. Nanotechnol. 9, 682 (2014).
33) E. Parzinger, M. Hetzl, U. Wurstbauer, and A. W. Holleitner, npj 2D Mater.
Appl. 1, 40 (2017).
34) K. Chen et al., npj 2D Mater. Appl. 1, 15 (2017).

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