Dynamic Optical Tuning of Interlayer Interactions in the Transition Metal Dichalcogenides

Abstract

Modulation of weak interlayer interactions between quasi-two-dimensional atomic planes in the transition metal dichalcogenides (TMDCs) provides avenues for tuning their functional properties. Here we show that above-gap optical excitation in the TMDCs leads to an unexpected large-amplitude, ultrafast compressive force between the two-dimensional layers, as probed by in situ measurements of the atomic layer spacing at femtosecond time resolution. We show that this compressive response arises from a dynamic modulation of the interlayer van der Waals interaction and that this represents the dominant light-induced stress at low excitation densities. A simple analytic model predicts the magnitude and carrier density dependence of the measured strains. This work establishes a new method for dynamic, nonequilibrium tuning of correlation-driven dispersive interactions and of the optomechanical functionality of TMDC quasi-two-dimensional materials.

The lifetime of interlayer breathing modes of few-layer 2H-MoSe2 membranes

“…The reason for this should probably be ascribed to the experimental challenges associated with direct quantitative characterization of processes leading to phonon decay and coherence loss, as well as the complexity of modelling them. [32][33][34][35][36][37] Since TMDCs exhibit strong correlations between electronic states and lattice vibrations, 29,38 which naturally affect a whole range of fundamental properties of these materials, e.g. thermal transport, carrier mobility, light emission, among others, [1][2][3][4][5][6][7][8][9][10][11] having a good characterization of the phonon modes together with their damping (decay) rates is thus crucial to understand the possible decoherence channels that exist in these 2D-TMDC nanostructures, and are hence essential for the conception of electronic and optoelectronic devices.…”

Anisotropic structural dynamics of monolayer crystals revealed by femtosecond surface X-ray scattering

“…used to characterize in-plane structural dynamics of monolayer 2D materials 22,23 , while the out-of-plane structural dynamics of a monolayer crystal was only inferred 23 . Comparing with ultrafast electron diffraction, ultrafast x-ray diffraction has been an indispensable tool for studying structural dynamics in thin films, providing evidence on non-thermal structural response in transition metal dichacogenides 24 and electron-phonon coupling in superconducting FeSe 14 . In particular, ultrafast surface x-ray surface scattering can provide direct and quantitative measurements of structural changes by recording diffraction profile with non-zero momentum transfer along the out-of-plane direction, in contrast to techniques that only measure in-plane diffraction peaks or surface sensitive optical probes close to Brillouin zone center 25 .…”

Atomic-scale imaging of ultrafast materials dynamics

“…The color bar is in units of raw-image counts. 8 microscopy by ten orders of magnitude compared to fast digital detectors, opening the way to a wide range of ultrafast atomic-scale studies on, for example, elastic responses, transport processes, phase-change behaviors, and the roles of defects on nonequilibrium phenomena.…”

“…By varying the arrival time of the pump and probe pulses at the specimen, one can capture the time-varying response, with the temporal resolution set by the probing pulse duration (Figure 1). 8,9 From the electron-scattering perspective, major technological advances have occurred during the last 10 years, with adaptation of conventional instruments for ultrafast studies, refinement of pulse-compression schemes to mitigate the deleterious effects of space-charge broadening arising from electron-electron repulsion, and extension to relativistic energies enabling the generation of bright, sub-100-fs-duration electron pulses. [10][11][12] Accordingly, one can retain coherence lengths and achieve spatial resolutions that allow for atomicscale processes to be visualized on ultrafast time scales.…”