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

Abstract

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 scanning tunneling 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.

Externally-triggerable optical pump-probe scanning tunneling microscopy with a time resolution of tens-picosecond

“…First, the temporal resolution in the tens-picosecond range will greatly expand the range of applications. For example, the picosecond range exciton dynamics in transition metal dichalcogenides [34] can be investigated using this system. Second, the optical system has significantly improved the stability of the laser illumination of the tunnel junction.…”

“…For example, small island structures on an insulating substrate, which are inaccessible to a single probe STM, can be observed by using one tip as an electrode, while conducting STM measurements using the other tip. Such OPP-multiprobe techniques have already been developed and applied for investigating a monolayer island of a WSe 2 /MoSe 2 heterostructure [47], monolayer and bilayer WSe 2 islands grown on SiO 2 substrates [48], and exciton dynamics in an in-plane WS 2 /WSe 2 heterostructure [34]. The placement of a position-movable aspheric lens directly on the multiprobe stage will similarly improve the stability of laser spot on the sample surface, allowing us to conduct time-resolved mapping experiments.…”

“…The recent OPP-STM systems, whose laser-pulse timing is electrically controlled by external triggers, have significantly improved both the ease of use and the stability of the optical system [23,32,33], but the temporal resolutions have been limited to the nanosecond range. For example, recent time-resolved observation of exciton dynamics in transition metal dichalcogenides has revealed the lifetime of exciton in several tens picosecond ranges [34]. It is thus indispensable to improve the temporal resolution higher than the nanosecond range.…”

Recent Strategies for the Synthesis of Phase-Pure Ultrathin 1T/1T′ Transition Metal Dichalcogenide Nanosheets

“…For Group VIIB, ReS 2 and ReSe 2 are semiconducting and have highly anisotropic symmetry. , Structural phases are periodically related to the d -band electrons of the transition metal. , One TMD nanosheet polytype could be directly grown using chemical vapor deposition (CVD), , solid state reaction, − vdW epitaxy, ,, atomic layer deposition (ALD), − or wet chemical synthesis, , and transformed from another polytype, ,, as shown in Figure . To elucidate TMD structures, various characterization tools have been studied and developed, including optical microscopy, , atomic force microscopy (AFM), , Raman spectroscopy, − transmission electron microscopy (TEM), − photoluminescence (PL) spectroscopy, − scanning tunneling microscopy/spectroscopy (STM/STS), − and angle-resolved photoemission spectroscopy (ARPES). − The diversity of chemical compositions and structural phases is a striking feature of TMDs, and the rapid development of TMD synthesis and structural characterization methods enables a wide range of TMD applications in, for example, electronic devices, , biosensors, − photodetectors, laser emitters, − and energy storage. − …”

Tip-Induced and Electrical Control of the Photoluminescence Yield of Monolayer WS2

“…Nevertheless, we believe that the observation of a threshold bias voltage for the onset of long-range PL quenching is a strong indication that long-range PL quenching is governed by local charge carrier injection. Finally, the use of an STM tip as a nanoscale and scanning electrode in the wide-field PL microscopy of semiconductors opens up prospects for the study of the exciton and charge carrier interactions in more intricate TMD-based systems: e.g., strain and defect-engineered TMD layers or van der Waals heterostructures. , …”