Browsing by Subject "ultrafast electron microscopy"
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Item Photo-induced Structural Dynamics of Molybdenum Disulfide(2017-05) McKenna, AlyssaIn this dissertation, the photo-induced structural dynamics of molybdenum disulfide (MoS2) have been imaged over seven orders of magnitude in time – picoseconds (ps, 10-12 seconds) to tens of microseconds (10-5 seconds) – and in real space with up to 2 nanometer (nm, 10-9 meter) spatial resolution. Molybdenum disulfide, a heavily studied layered material due to its tunable electronic bandgap, is of interest for a variety of device applications and due to the tunability of many of its properties with strain (e.g., thermal properties and electronic bandgap). Ultrafast electron microscopy (UEM) combines the nanometer spatial resolution of transmission electron microscopy with the femtosecond temporal resolution of ultrafast pump-probe spectroscopy and was used to directly image the photo-excited dynamics in a freestanding, micrometer-size, multilayer MoS2 flake viewed along the [001] direction. Following photo-excitation with a 515-nm, 700-femtosecond laser pulse, individual wave trains were observed to emerge from nanoscale morphological features, particularly the vacuum-specimen interface. The strain waves propagated in-plane along or near contrast features at approximately the speed of sound (7 nm per ps) and at frequencies in the tens of gigahertz (GHz), which identified them as Lamb modes (quasi-shear plate waves). Using the dispersion relation for 50-nm thick MoS2 plate and the cut-off frequencies of an anisotropic plate, the waves were identified as second-order anti-symmetric or symmetric Lamb modes, which are differentiated by their symmetry with respect to the mid-plane of the specimen. The change in contrast was attributed to the tilting of planes into or out of the diffraction condition for anti-symmetric waves or a change in in-plane inter-planar spacing for the longitudinal wave coupled to the symmetric waves. The strain waves were then imaged on the nanosecond (ns, 10-9 seconds) timescales and found to undergo phonon-phonon scattering. This scattering resulted in the damping of the high-GHz oscillations and the emergence of incoherent oscillations with frequencies on the order of single GHz. On microsecond timescales, three megahertz (MHz, 106 hertz), whole-flake mechanical resonances were observed. These mechanical modes were found to have microsecond lifetimes and were simulated using finite element modeling. This work provides insight into the photoexcitation of high-velocity elastic strain waves and the subsequent mode-coupling processes and whole-flake oscillations. Work to understand the factors needed for high resolution ultrafast electron imaging is also presented. High-resolution ultrafast imaging first requires that the specimen drift is low in the requisite acquisition time at high magnifications. Thus, average specimen drift rates and the required acquisition times at these magnifications were measured. To produce the high temporal resolution in UEM, electron packets rather than a continuous beam of electrons are produced via the photoelectric effect upon irradiation with laser pulses. Next, the stability in the number of electrons in the packets was probed. The lattice fringes of tungsten disulfide nanoparticles (6 Å spacing) were successfully imaged.Item Static and Dynamic Electron Microscopy Investigations at the Atomic and Ultrafast Scales(2016-05) Suri, Pranav KumarAdvancements in the electron microscopy capabilities – aberration-corrected imaging, monochromatic spectroscopy, direct-electron detectors – have enabled routine visualization of atomic-scale processes with millisecond temporal resolutions in this decade. This, combined with progress in the transmission electron microscopy (TEM) specimen holder technology and nanofabrication techniques, allows comprehensive experiments on a wide range of materials in various phases via in situ methods. The development of ultrafast (sub-nanosecond) time-resolved TEM with ultrafast electron microscopy (UEM) has further pushed the envelope of in situ TEM to sub-nanosecond temporal resolution while maintaining sub-nanometer spatial resolution. A plethora of materials phenomena – including electron-phonon coupling, phonon transport, first-order phase transitions, bond rotation, plasmon dynamics, melting, and dopant atoms arrangement – are not yet clearly understood and could be benefitted with the current in situ TEM capabilities having atomic-level and ultrafast precision. Better understanding of these phenomena and intrinsic material dynamics (e.g. how phonons propagate in a material, what time-scales are involved in a first-order phase transition, how fast a material melts, where dopant atoms sit in a crystal) in new-generation and technologically important materials (e.g. two-dimensional layered materials, semiconductor and magnetic devices, rare-earth-element-free permanent magnets, unconventional superconductors) could bring a paradigm shift in their electronic, structural, magnetic, thermal and optical applications. Present research efforts, employing cutting-edge static and dynamic in situ electron microscopy resources at the University of Minnesota, are directed towards understanding the atomic-scale crystallographic structural transition and phonon transport in an iron-pnictide parent compound LaFeAsO, studying the mechanical stability of fast moving hard-drive heads in heat-assisted magnetic recording (HAMR) technology, exploring the possibility of ductile ceramics in magnesium oxide (MgO) nanomaterials, and revealing the atomic-structure of newly discovered rare-earth-element-free iron nitride (FeN) magnetic materials. Via atomic-resolution imaging and electron diffraction coupled with in situ TEM cooling on LaFeAsO, it was found that additional effects not related to the structural transition, namely dynamical scattering and electron channeling, can give signatures reminiscent of those typically associated with the symmetry change. UEM studies on LaFeAsO revealed direct, real-space imaging of the emergence and evolution of acoustic phonons and resolved dispersion behavior during propagation and scattering. Via UEM bright-field imaging, megahertz vibrational frequencies were observed upon laser-illumination in TEM specimens made out of HAMR devices which could be detrimental to their long-term thermal and structural reliability. Compression testing of 100-350 nm single-crystal MgO nanocubes shows size-dependent stresses and engineering strains of 4-13.8 GPa and 0.046-0.221 respectively at the first signs of yield accompanied by an absence of brittle fracture, which is a significant increase in plasticity of a brittle ceramic material. Atomic-scale characterization of FeN phases show that it is possible to detect interstitial locations of low atomic-number nitrogen atoms in iron crystal and hints at a development of novel routes (without involving rare-earth elements) for bulk permanent magnet synthesis.