Nanoscale Coherent-Acoustic-Phonon Dynamics in Molybdenum Disulfide Using Ultrafast Electron Microscopy
2021-06
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Nanoscale Coherent-Acoustic-Phonon Dynamics in Molybdenum Disulfide Using Ultrafast Electron Microscopy
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2021-06
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In this dissertation, photoexcited, defect-mediated anisotropic acoustic-phonon dynamics in molybdenum disulfide (MoS2) have been directly imaged on the nanometer-picosecond spatiotemporal scales in real space. MoS2 is a prototypical material system of transition metal dichalcogenides extensively studied due to the exceptional tunability of many properties (e.g., electronic band structure) via layer number and strain, and thus attracts interests in a broad range of device applications. Defects have been demonstrated to impact local properties and dynamics in a spatial range from a single atom to hundreds of nanometers. The combined nanometer spatial resolution and femtosecond temporal resolution of ultrafast electron microscopy (UEM) enables direct visualization of photoinduced anisotropic acoustic-phonon dynamics localized at nanoscale defects in freestanding, multilayer MoS2.Excitation of the phonon dynamics is achieved via uniformly illuminating the specimens with 515-nm, 300-fs laser pulses. Propagation of the waves locally deforms the crystal lattice, leading to a modulation of the local Bragg conditions. Visualization of the waves is thus achieved by monitoring modulation of diffraction contrast features (e.g., bend contours). When viewed along the [0001] direction, photoexcited individual phonon wave trains were observed to be emitted from crystal-crystal interfaces (e.g., step edge and terrace) and propagate along a single wave vector perpendicular to the interfaces at frequencies in the tens of gigahertz (GHz) range and at approximately the in-plane speed of sound (7 nm/ps). When viewed along a high-index zone axis (i.e., large angle between specimen normal and the incident electron wave vector), the observed diffraction-contrast dynamics exhibits no propagations but only oscillations. Such specimen configuration allows for projection of c-axis wave dynamics onto the image plane. The c-axis phonon velocity was extracted from the oscillation frequency and specimen thickness, consistent with c-axis speed of sound (2.9 nm/ps). The onset of the c-axis dynamics occurs a few picoseconds earlier than that of the in-plane dynamics, suggesting photoinduced modulation of interlayer spacing leading to launching of in-plane compression waves.
In addition to serving as launching sites of phonon wave trains, step edges can mediate localized dynamics that are distinct from that observed in pristine regions. The c-axis phonon modes exhibit dephased oscillations at an individual step edge owing to different specimen thicknesses, and even one-unit-cell of thickness difference manifests a few GHz of frequency shift. Step edges have also been demonstrated to induce new relaxation states extending to several hundred nanometers. Subsequently, a frequency reduction in the c-axis phonon oscillation (i.e., phonon softening) at an individual, nanoscale step edge was observed, indicative of associated softening of the elastic constant. This is consistent with results obtained with finite element modeling.
In the process of preparing ultrathin, pristine specimens for studying photoinduced structural dynamics in mono- and bilayer MoS2, substrate-directed spontaneous strain patterning was observed in the term of twelve-fold zone-axis patterns and six-fold centroidal Voronoi tessellation patterns. Vertical deformation of up to 35 nm for several bilayer MoS2 crystals were measured with atomic force microscopy. The formation mechanism of such pattern was elucidated with atomistic simulations.
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University of Minnesota Ph.D. dissertation. 2021. Major: Material Science and Engineering. Advisor: David Flannigan. 1 computer file (PDF); 202 pages.
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Zhang, Yichao. (2021). Nanoscale Coherent-Acoustic-Phonon Dynamics in Molybdenum Disulfide Using Ultrafast Electron Microscopy. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/226663.
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