arXiv subjects

Ding-Shyue Yang

Publications and source records attributed to Ding-Shyue Yang.

6 recordsLinked to original sources

arXiv2026

Carrier-coupled ultrafast structural dynamics and interlayer energy transport of supported transition metal dichalcogenide heterostructures

Understanding the electronic coupling and energy flow across layered two-dimensional heterostructures (HSs) is crucial to the exploitation of carrier and phonon transports as well as thermal management in next-generation optoelectronic devices. By using reflection ultrafast electron diffraction, we directly examine photoinduced out-of-plane structural dynamics of supported MoS2/WS2 bilayer HSs and their individual monolayers. Experimental evidence reveals the launch of ultrafast carrier-coupled intralayer atomic motions due to interlayer charge transfer across the van der Waals (vdW) heterojunctions that is absent for individual monolayers. Such a notable carrier-lattice correlation is in addition to the electronic coupling manifested in the enhanced optical absorption for HSs. Also, different pathways of energy flow as a result of carrier-phonon coupling and phonon scattering are reported with the corresponding characteristic times. On longer timescales, relaxation of thermalized atomic motions can be sufficiently described by a thermal transport model. A higher thermal boundary conductance (TBC) across MoS2/WS2 HSs is obtained compared to those at the monolayer-substrate interfaces; however, the similar TBC values suggest comparable couplings of phonons across vdW contacts. These results further shed light on the optical, phonon, and interfacial thermal properties of vertically-stacked vdW HSs.

cond-mat.mes-hall↗

arXiv2025

Direct Observation of Energy Transport Dynamics and High Thermal Conductance across Single Solid-Molecule Junctions

Interfaces play a crucial role in energy transport at the nanoscale. However, direct experimental observations of interfacial thermal conductance across molecular junctions have remained challenging due to the high spatiotemporal resolution required for probing. Here, we report dynamic energy transport processes across multi-component molecular junctions observed at the atomic level by employing reflection ultrafast electron diffraction. A clear temporal sequence of energy transfer is revealed at early times following photoexcitation of Au(111) surfaces chemically bonded with self-assembled monolayers (SAMs) of alkanethiols: from the gold surface layer (SL) to the head groups of a SAM and then to the CH2-CH2 methylene lattice. Remarkably, the structural dynamics of the gold SL differ significantly from those of clean gold. Furthermore, the methylene lattice dynamics exhibit chain-length insensitivity but with a length-dependent retention time to reach full thermalization. Quantitatively, we find an agreement in the increased out-of-plane atomic motions between the surface gold atoms and the SAM methylene units, signifying the nature of motion-based coupling for interfacial energy transport. High interfacial thermal conductance (~300 MW/m^2/K) and high thermal conductivity for the methylene lattice (~60 W/m/K) are obtained under the condition of impulsive heating, which provide strong support for previous room-temperature theoretical predictions and also the ballistic nature of intrachain heat transfer. This time- and spatially-resolved experimental approach will enable future quantitative assessment of interfacial energy transport across solid-molecule junctions.

physics.chem-ph↗

arXiv2024

Impacts of Hot Electron Diffusion, Electron-Phonon Coupling, and Surface Atoms on Metal Surface Dynamics Revealed by Reflection Ultrafast Electron Diffraction

Metals exhibit nonequilibrium electron and lattice subsystems at transient times following femtosecond laser excitation. In the past four decades, various optical spectroscopy and time-resolved diffraction methods have been used to study electron-phonon coupling and the effects of underlying dynamical processes. Here, we take advantage of the surface specificity of reflection ultrafast electron diffraction (UED) to examine the structural dynamics of photoexcited metal surfaces, which are apparently slower in recovery than predicted by thermal diffusion from the profile of absorbed energy. Fast diffusion of hot electrons is found to critically reduce surface excitation and affect the temporal dependence of the increased atomic motions on not only the ultrashort but sub-nanosecond times. Whereas the two-temperature model with the accepted physical constants of platinum can reproduce the observed surface lattice dynamics, gold is found to exhibit appreciably larger-than-expected dynamic vibrational amplitudes of surface atoms while keeping the commonly used electron-phonon coupling constant. Such surface behavioral difference at transient times can be understood in the context of the different strengths of binding to surface atoms for the two metals. In addition, with the quantitative agreements between diffraction and theoretical results, we provide convincing evidence that surface structural dynamics can be reliably obtained by reflection UED even in the presence of laser-induced transient electric fields.

physics.chem-ph↗

arXiv2023

Cross-Examination of Photoinitiated Carrier and Structural Dynamics of Black Phosphorus at Elevated Fluences

Revived attention in black phosphorus (bP) has been tremendous in the past decade. While many photoinitiated experiments have been conducted, a cross-examination of bP's photocarrier and structural dynamics is still lacking. In this report, we provide such analysis by examining time-resolved data acquired using optical transient reflectivity and reflection ultrafast electron diffraction, two complementary methods under the same experimental conditions. At elevated excitation fluences, we find that more than 90% of the photoinjected carriers are annihilated within the first picosecond (ps) and transfer their energy to phonons in a nonthermal, anisotropic fashion. Electronically, the remaining carrier density around the band edges induces a significant interaction that leads to an interlayer lattice contraction in a few ps but soon diminishes as a result of the continuing loss of carriers. Structurally, phonon-phonon scattering redistributes the energy in the lattice and results in the generation of out-of-plane coherent acoustic phonons and thermal lattice expansion. Their onset times at ~6 ps are found to be in good agreement. Later, a thermalized quasi-equilibrium state is reached following a period of about 40-50 ps. Hence, we propose a picture with five temporal regimes for bP's photodynamics.

cond-mat.mtrl-sci↗

arXiv2022

Strong Coupling of Self-Trapped Excitons to Acoustic Phonons in Bismuth Perovskite $\textrm{Cs}_{3}\textrm{Bi}_{2}\textrm{I}_{9}$

To assess the potential optoelectronic applications of metal-halide perovskites, it is critical to have a detailed understanding of the nature, strength, and dynamics of the interactions between carriers and the polar lattices. Here, we report the electronic and structural dynamics of bismuth-based perovskite $\textrm{Cs}_{3}\textrm{Bi}_{2}\textrm{I}_{9}$ revealed by transient reflectivity and ultrafast electron diffraction. A cross-examination of these experimental results combined with theoretical analyses allows the identification of the major carrier-phonon coupling mechanism and the associated time scales. It is found that carriers photoinjected into $\textrm{Cs}_{3}\textrm{Bi}_{2}\textrm{I}_{9}$ form self-trapped excitons on an ultrafast time scale. However, they retain most of their energy and their coupling to Fröhlich-type optical phonons is limited at early times. Instead, the long-lived excitons exert an electronic stress via deformation potential and develop a prominent, sustaining strain field as coherent acoustic phonons in 10 ps. From sub-ps to ns and beyond, a similar extent of the atomic displacements is found throughout the different stages of structural distortions, from limited local modulations to a coherent strain field to the Debye-Waller random atomic motions on longer times. The current results suggest the potential use of bismuth-based perovskites for applications other than photovoltaics to take advantage of carriers' stronger self-trapping and long lifetime.

cond-mat.mtrl-sci↗

arXiv2022

Ultrafast carrier-coupled interlayer contraction, coherent intralayer motions, and phonon thermalization dynamics of black phosphorus

Black phosphorus (BP) exhibits highly anisotropic properties and dynamical behavior that are unique even among two-dimensional and van der Waals (vdW) layered materials. Here, we show that an interlayer lattice contraction and concerted, symmetric intralayer vibrations take place concurrently within few picoseconds following the photoinjection and relaxation of carriers, using ultrafast electron diffraction in the reflection geometry to probe the out-of-plane motions. A strong coupling between the photocarriers and BP's puckered structure, with the alignment of the electronic band structure, is at work for such directional atomic motions without a photoinduced phase transition. Three temporal regimes can be identified for the phonon thermalization dynamics where a quasi-equilibrium without anisotropy is reached in about 50 ps, followed by propagation of coherent acoustic phonons and heat diffusion into the bulk. The early-time out-of-plane dynamics reported here have important implications for single- and few-layer BP and other vdW materials with strong electronic-lattice correlations.

cond-mat.mes-hall↗