arXiv subjects

Osamu Takeuchi

Publications and source records attributed to Osamu Takeuchi.

5 recordsLinked to original sources

arXiv2026

Concerted Carrier-Barrier Dynamics in van der Waals Schottky Junctions Revealed by Time-Resolved Atomic Force Microscopy

Schottky junctions based on transition-metal dichalcogenides (TMDCs) have emerged as key building blocks for next-generation optoelectronic devices that demand ultrafast response and high sensitivity. However, the ultrafast, nanoscale carrier dynamics at these interfaces, crucial for device performance, have remained experimentally elusive. Here, we introduce optical pump-probe time-resolved atomic force microscopy to directly visualize, in real space, the nanosecond-scale modulation of the Schottky barrier potential at a van der Waals junction formed by point contact between WSe2 and a PtIr tip. Complementary analyses using transient absorption spectroscopy and light-modulated current-voltage characteristics together with model simulations reveal that time-resolved currents originate from the concerted temporal evolution of photoexcited carriers and the subsequent barrier response, processes that also define the rate-limiting steps of the photocurrent. Our results uncover the essential interfacial dynamics that underpin TMDC-based photodetectors and photovoltaic elements, while establishing a new measurement paradigm that complements and extends existing spectroscopic techniques. This approach provides direct access to nonequilibrium processes hidden at nanoscale interfaces, offering a powerful route to rational design of high-performance optoelectronic devices.

cond-mat.mes-hall↗

arXiv2024

Next-Generation Time-Resolved Scanning Probe Microscopy

Understanding the nanoscale carrier dynamics induced by light excitation is the key to unlocking futuristic devices and innovative functionalities in advanced materials. Optical pump-probe scanning tunneling microscopy (OPP-STM) has opened a window to these phenomena. However, mastering the combination of ultrafast pulsed lasers with STM requires high expertise and effort. We have shattered this barrier and developed a compact OPP-STM system accessible to all. This system precisely controls laser pulse timing electrically and enables stable laser irradiation on sample surfaces. Furthermore, by applying this technique to atomic force microscopy (AFM), we have captured time-resolved force signals with an exceptionally high signal-to-noise ratio. Originating from the dipole-dipole interactions, these signals provide insights into the carrier dynamics on sample surfaces, which are activated by photo-illumination. These technologies are promising as powerful tools for exploring a wide range of photoinduced phenomena in conductive and insulating materials.

cond-mat.mtrl-sci↗

arXiv2024

Time-resolved force microscopy using delay-time modulation method

We developed a time-resolved force microscopy technique by integrating atomic force microscopy using a tuning-fork-type cantilever with the delay time modulation method for optical pump-probe light. We successfully measured the dynamics of surface recombination and diffusion of photoexcited carriers in bulk WSe2, which is challenging owing to the effect of tunneling current in time-resolved scanning tunneling microscopy. The obtained results were comprehensively explained with the model based on the dipole-dipole interaction induced by photo illumination.

physics.app-ph↗

arXiv2023

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

Photoinduced carrier dynamics of nanostructures play a crucial role in developing novel functionalities in advanced materials. Optical pump-probe scanning tunneling microscopy (OPP-STM) represents distinctive capabilities of real-space imaging of such carrier dynamics with nanoscale spatial resolution. However, combining the advanced technology of ultrafast pulsed lasers with STM for stable time-resolved measurements has remained challenging. The recent OPP-STM system, whose laser-pulse timing is electrically controlled by external triggers, has significantly simplified this combination but limited its application due to nanosecond temporal resolution. Here we report an externally-triggerable OPP-STM system with a temporal resolution in the tens-picosecond range. We also realize the stable laser illumination of the tip-sample junction by placing a position-movable aspheric lens driven by piezo actuators directly on the STM stage and by employing an optical beam stabilization system. We demonstrate the OPP-STM measurements on GaAs(110) surfaces, observing carrier dynamics with a decay time of $\sim170$ ps and revealing local carrier dynamics at features including a step edge and a nanoscale defect. The stable OPP-STM measurements with the tens-picosecond resolution by the electrical control of laser pulses highlight the potential capabilities of this system for investigating nanoscale carrier dynamics of a wide range of functional materials.

physics.optics↗

arXiv2019

The Atomic and Electronic structure of 0° and 60° grain boundaries in MoS2

We have investigated atomic and electronic structure of grain boundaries in monolayer MoS2, where relative angles between two different grains are 0 and 60 degree. The grain boundaries with specific relative angle have been formed with chemical vapor deposition growth on graphite and hexagonal boron nitride flakes; van der Waals interlayer interaction between MoS2 and the flakes restricts the relative angle. Through scanning tunneling microscopy and spectroscopy measurements, we have found that the perfectly stitched structure between two different grains of MoS2 was realized in the case of the 0 degree grain boundary. We also found that even with the perfectly stitched structure, valence band maximum and conduction band minimum shows significant blue shift, which probably arise from lattice strain at the boundary.

cond-mat.mes-hall↗