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, Tennodai 1-1-1, Tsukuba, Japan
2Department of Physics, Tokyo Metropolitan University, Minamiosawa1-1, Tokyo, Japan

mogi.hiroyuki.fp@u.tsukuba.ac.jp, https://dora.bk.tsukuba.ac.jp/

Photoresponse of WSe2/MoSe2 in-plane heterostructureprobed by a laser-combined multiprobe spectroscopy
In recent years, transition metal dichalcogenides (TMDC) family such as MoS2 and WSe2 has attracted muchattention due to their remarkable optoelectronic properties. These monolayers have direct bandgaps in a visible
range and ultrafast photo response was recently reported in MoS2 monolayer [1]. Studying band arrignment,
optical/electrical characteristics, and transient carrier dynamics is absolutely necessary. For such purposes,
scanning tunneling microscopy (STM) has been used to evaluate the band structure, and time-resolved
photocurrent measurement has been used to observe ultrafast carrier dynamics with a prototype of multiterminal device structure prepared by electrode fabrication techniques such as lithography. However, the wiring
process requires physical and chemical treatments of sample. In the case of small (~μm) device structures suchas a monolayer TMDC island formed on a insulative material, the probe access is rather difficult because of its
limited observable area. In addition, each measurment should be done in an identical clean environment
because such kind of thin material is sensitive to the adhesives and impurities. An effective approach is to apply
multiprobe (MP) STM techniques, in which multiple tips are used to measure, for example, transportcharacteristics between two desired points, repeatably. However, the simultaneous use of MP techniques withlaser technology has not been realized because of the complexities and the handling difficulties.
Here, we present a microscopy method that we have developed by combining MP techniques with opticalmethods such as Light Modulated scanning tunneling spectroscopy (LM-STS) [2] and Optical Pump-Probe(OPP) STM [3]. To achieve the experiment, we integrated optical zoom lens (VH-Z100T, WD=24mm, KeyenceCo., Ltd.) above the MP-STM system, which is used for monitoring probe/sample arrangement as well as forlaser positioning and focusing with ~µm precision[4].
In this experiment, we observed the field-effect transistor (FET) characteristics and photocarriers response in amonolayer island of a WSe2/MoSe2 in-plane heterostructure grown on a SiO2/Si substrate (Fig.1a). The FET
characteristics and photocurrent were measured in the experimental setup shown in Fig. 1b. Two contact
atomic force (c-AFM) tips coated with PtIr5 were placed in contact with the sample in the MoSe2 or WSe2 area toact as the source and drain. As shown in Fig.1c, the spatial distribution of photocurrent (Iph) vs drain-source
voltage (Vds) was measured across the MoSe2/WSe2 interface by changing the focusing spot potision(continuous-wave light of 532 nm, ~16 µW). As a result, we were able to visualize the band structure including
the schottkey barrier of probe-sample interface and band shift on MoSe2/WSe2 interface as shown in Fig.1d.
Next, we show the positional dependence of time-resolved measurement realized by the OPP-MP
spectroscopy we have developed. The dynamics of photocarriers we have observed in the experiments byusing femtosecond pulsed laser with a pulse width of ~150 fs. The probes were arranged across theMoSe2/WSe2 interface, and the laser spot position was set on each probe apex to measure the MoSe2 or WSe2
region independently. In the WSe2 region, four wide range lifetimes were obtained by fitting the signals withexponential functions (τ1 = ~24 ps, τ2 = ~200 ps, τ3 = ~20 ns, and τ4 = 500 ns <). On the other hand, in theMoSe2 region, we obtained two nanosecond components of lifetimes. This difference may come from the effectof the deep-level trap states in the case of MoSe2. Details will be discussed in the presentation.

By combining scanning MP microscopy with optical methods such as the Lihgt modulated spectroscopy andOPP methods, we have succeeded in developing a microscopy method useful for measuring electronicstructures and photoinduced carrier dynamics in microscopic structures. This technique is expected to play animportant role in the researches to develop advanced functional devices with microscopic structures.
References[1] H. Wang, et al Nat. Commun., 6, 8831 (2015).
[2] S. Yoshida, Y. Kanitani, O. Takeuchi, and H. Shigekawa, Appl. Phys. Lett. 92 (2008), 10.
[3] H. Mogi, W. Zihan, R. Kikuchi, C.Y. Yoon, S. Yoshida, O. Takeuchi,and H. Shigekawa, Appl. Phys. Express 12(2019), 025005.
[4] H. Mogi, W. Zihan, T. Banba, Y. Takaguchi, N. Endo, S. Yoshida, A. Taninaka, H. Oigawa, Y. Miyata, O.
Takeuchi, and H. Shigekawa Appl. Phys. Express 12 (2019), 045002

FiguresFigure 1: Monolayer MoSe2/WSe2 heterostructure sample, b Multiprobe photocurrent measurement schematic, c
Obtained mapping of Iph- Vds characteristics, and d estimated band structures.

Figure 2: The carrier dynamics measurement by Optical pump-probe multiprobe technique.
Schematics and the results in the cases of irradiating WSe2 region (a,b), and MoSe2 region (c, d)