Defect-mediated phonon dynamics in TaS2 and WSe2.

Daniel R Cremons,David J Flannigan,Dayne A Plemmons

doi:10.1063/1.4982817

Daniel R Cremons, David J Flannigan + Show 1 more

Open Access

https://doi.org/10.1063/1.4982817

Copy DOI

Abstract

We report correlative crystallographic and morphological studies of defect-dependent phonon dynamics in single flakes of 1T-TaS2 and 2H-WSe2 using selected-area diffraction and bright-field imaging in an ultrafast electron microscope. In both materials, we observe in-plane speed-of-sound acoustic-phonon wave trains, the dynamics of which (i.e., emergence, propagation, and interference) are strongly dependent upon discrete interfacial features (e.g., vacuum/crystal and crystal/crystal interfaces). In TaS2, we observe cross-propagating in-plane acoustic-phonon wave trains of differing frequencies that undergo coherent interference approximately 200 ps after initial emergence from distinct interfacial regions. With ultrafast bright-field imaging, the properties of the interfering wave trains are observed to correspond to the beat frequency of the individual oscillations, while intensity oscillations of Bragg spots generated from selected areas within the region of interest match well with the real-space dynamics. In WSe2, distinct acoustic-phonon dynamics are observed emanating and propagating away from structurally dissimilar morphological discontinuities (vacuum/crystal interface and crystal terrace), and results of ultrafast selected-area diffraction reveal thickness-dependent phonon frequencies. The overall observed dynamics are well-described using finite element analysis and time-dependent linear-elastic continuum mechanics.

Highlights

IntroductionThe pioneering work by Zewail and co-workers on the development of femtosecond (fs) transmission electron microscopy (TEM) (i.e., ultrafast electron microscopy, UEM) has led to many contributions to the understanding of ultrafast structural and electronic dynamics in materials. because UEM is essentially an expansion of conventional TEM into the ultrafast temporal regime (with current instruments comprised of commercially available technologies), fs structural studies can be conducted in both real and reciprocal space with this approach. As is common with dedicated ultrafast electron diffraction instruments, UEM parallel-beam diffraction has been used to probe reciprocal-space dynamics of structurally ordered materials, with the results being interpreted within the context of the (spatially averaged) photoinduced response of interplanar spacings, variation in atomic-vibration amplitudes and reciprocal-lattice orientations, and changes in unit-cell symmetries. Owing to the illumination system and scan coils, UEM has the added capability of the formation of nanoscale probes ideally suited for elucidating dynamics within specific specimen regions of interest via convergent-beam and scanning UEM.21–24Fax: þ1 (612) 626-7246.2329-7778/2017/4(4)/044019/16VC Author(s) 2017.044019-2 Cremons, Plemmons, and FlanniganStruct
We report correlative crystallographic and morphological studies of defectdependent phonon dynamics in single flakes of 1T-TaS2 and 2H-WSe2 using selected-area diffraction and bright-field imaging in an ultrafast electron microscope
We have described the direct visualization of in-plane acoustic-phonon dynamics at discrete interfaces in single flakes of TaS2 and WSe2

Summary

Introduction

The pioneering work by Zewail and co-workers on the development of femtosecond (fs) transmission electron microscopy (TEM) (i.e., ultrafast electron microscopy, UEM) has led to many contributions to the understanding of ultrafast structural and electronic dynamics in materials. because UEM is essentially an expansion of conventional TEM into the ultrafast temporal regime (with current instruments comprised of commercially available technologies), fs structural studies can be conducted in both real and reciprocal space with this approach. As is common with dedicated ultrafast electron diffraction instruments, UEM parallel-beam diffraction has been used to probe reciprocal-space dynamics of structurally ordered materials, with the results being interpreted within the context of the (spatially averaged) photoinduced response of interplanar spacings, variation in atomic-vibration amplitudes and reciprocal-lattice orientations, and changes in unit-cell symmetries. Owing to the illumination system and scan coils, UEM has the added capability of the formation of nanoscale probes ideally suited for elucidating dynamics within specific specimen regions of interest via convergent-beam and scanning UEM.21–24Fax: þ1 (612) 626-7246.2329-7778/2017/4(4)/044019/16VC Author(s) 2017.044019-2 Cremons, Plemmons, and FlanniganStruct. The pioneering work by Zewail and co-workers on the development of femtosecond (fs) transmission electron microscopy (TEM) (i.e., ultrafast electron microscopy, UEM) has led to many contributions to the understanding of ultrafast structural and electronic dynamics in materials.. Because UEM is essentially an expansion of conventional TEM into the ultrafast temporal regime (with current instruments comprised of commercially available technologies), fs structural studies can be conducted in both real and reciprocal space with this approach.. As is common with dedicated ultrafast electron diffraction instruments, UEM parallel-beam diffraction has been used to probe reciprocal-space dynamics of structurally ordered materials, with the results being interpreted within the context of the (spatially averaged) photoinduced response of interplanar spacings, variation in atomic-vibration amplitudes and reciprocal-lattice orientations, and changes in unit-cell symmetries..

Methods

Results

Conclusion