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Browsing by Author "Cremons, Daniel"

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    Investigating Phonon Behavior in Heterogeneous Nanoscale Systems with Ultrafast Electron Microscopy
    (2017-07) Cremons, Daniel
    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.