Investigating Phonon Behavior in Heterogeneous Nanoscale Systems with Ultrafast Electron Microscopy

Abstract

In photoexcited systems, the interplay in space and time of thermal carriers such as phonons and electrons involve many overlapping and coupled processes that can change as a function of material properties as well as extrinsic boundary conditions and imperfections. In order to gain understanding of the behavior of thermal carriers on the nanoscale, studies of the above-gap excitation of semiconductors were performed using an ultrafast electron microscope. Additionally, considerable effort was put into method and instrument development, including a method of in situ thermometry using precise reciprocal lattice vector measurements to determine specimen temperature within 10 °C accuracy over a range of 350 °C. The effect of nanoscale imperfections on the mechanical, multi-mode excitation in silicon, as well as the necessity of multimodal imaging techniques were also investigated. Mechanical oscillations in the 5-10 MHz regime were observed in imaging and diffraction modalities, but higher-frequency oscillations, up to 30 MHz, were observable only in the highly-spatially-resolved imaging mode. These results stressed the effects of nanoscale heterogeneity on mechanical-mode excitation, as well as the potential for crucially missing data as a result of using techniques that rely upon ensemble averaging. In addition, the first-ever direct visualization of the generation, travel, and decay of individual acoustic phonons in the vicinity of atomic-scale defects was performed on germanium and tungsten diselenide. Cross-propagating acoustic wavefront imaging and diffraction studies were performed on the transition metal dichalcogenide tantalum disulfide, which validated the importance of a multi-modal approach and suggested potential optoacoustic switching mechanisms. Further study of photoexcitation of germanium led to the discovery of the time-dependent dispersion of the S1 Lamb mode, starting at hypersonic speeds (35,000 m/s) and decaying to the speed of sound (5,300 m/s) over hundreds of picoseconds. The invariance of the observed phonon velocity or frequency as a function of photon energy suggests that, here, the specimen morphology plays a determining role in the energy dissipation timeline following dense-plasma generation.