jp9b12026_si_001.pdf (1.08 MB)
Influence of Discrete Defects on Observed Acoustic–Phonon Dynamics in Layered Materials Probed with Ultrafast Electron Microscopy
journal contribution
posted on 2020-02-25, 21:16 authored by Spencer
A. Reisbick, Yichao Zhang, David J. FlanniganThe structural anisotropy
of layered materials leads to disparate
lattice responses along different crystallographic directions following
femtosecond photoexcitation. Ultrafast scattering methods are well-suited
to resolving such responses, though probe size and specimen structure
and morphology must be considered when interpreting results. Here
we use ultrafast electron microscopy (UEM) imaging and diffraction
to study the influence of individual multilayer terraces and few-layer
step-edges on acoustic-phonon dynamics in 1T-TaS2 and 2H-MoS2. In TaS2, we find that a multilayer terrace produces
distinct, localized responses arising from thickness-dependent c-axis phonon dynamics. Convolution of the responses is
demonstrated with ultrafast selected-area diffraction by limiting
the probe size and training it on the region of interest. This results
in a reciprocal-space frequency response that is a convolution of
the spatially separated behaviors. Sensitivity of phonon dynamics
to few-layer step-edges in MoS2 and the capability of UEM
imaging to resolve the influence of such defects are also demonstrated.
Spatial frequency maps from the UEM image series reveal regions separated
by a four-layer step-edge having 60.0 GHz and 63.3 GHz oscillation
frequencies, again linked to c-axis phonon propagation.
As with ultrafast diffraction, signal convolution is demonstrated
by continuous increase of the size of the selected region of interest
used in the analysis.
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Keywords
terraceaxis phonon dynamicsaxis phonon propagationreciprocal-space frequency responseimagingregionSpatial frequency mapsfew-layer step-edgesconvolutionLayered Materials ProbedUltrafast Electron Microscopy2 H-MoS 21 T-TaS 2influenceUEM image seriesultrafast selected-area diffractionuse ultrafast electron microscopy63.3 GHz oscillation frequenciesprobe size
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