Browsing by Subject "transmission electron microscopy"

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    An in-situ analytical scanning and transmission electron microscopy investigation of structure-property relationships in electronic materials
    (2014-08) Wagner, Andrew
    As electronic and mechanical devices are scaled downward in size and upward in complexity, macroscopic principles no longer apply. Synthesis of three-dimensionally confined structures exhibit quantum confinement effects allowing, for example, silicon nanoparticles to luminesce. The reduction in size of classically brittle materials reveals a ductile-to-brittle transition. Such a transition, attributed to a reduction in defects, increases elasticity. In the case of silicon, elastic deformation can improve electronic carrier mobility by over 50%, a vital attribute of modern integrated circuits. The scalability of such principles and the changing atomistic processes which contribute to them presents a vitally important field of research. Beginning with the direct observation of dislocations and lattice planes in the 1950s, the transmission electron microscope has been a powerful tool in materials science. More recently, as nanoscale technologies have proliferated modern life, their unique ability to spatially resolve nano- and atomic-scale structures has become a critical component of materials research and characterization. Signals produced by an incident beam of high-energy electrons enables researchers to both image and chemically analyze materials at the atomic scale. Coherently and elastically-scattered electrons can be collected to produce atomic-scale images of a crystalline sample. New specimen stages have enabled routine investigation of samples heated up to 1000 °C and cooled to liquid nitrogen temperatures. MEMS-based transducers allow for sub-nm scale mechanical testing and ultrathin membranes allow study of liquids and gases. Investigation of a myriad of previously "unseeable" processes can now be observed within the TEM, and sometimes something new is found within the old. High-temperature annealing of pure a Si:H films leads to crystallization of the film. Such films provide higher carrier mobility compared to amorphous films, offering improved photovoltaic performance. The annealing process, however, requires exceptionally high temperature (> 600 °C) and time (tens of hours), limiting throughput and costing energy. In an effort to fabricate polycrystalline solar cells at lower cost, large (~30 nm) silicon nanocrystals were incorporated into hydrogenated amorphous silicon (a Si:H) thin films. When annealed, the embedded nanocrystals were expected to act as heterogeneous nucleation sites and crystallize the surrounding amorphous matrix. When observed in the TEM, an additional and unexpected event was observed. At the boundary between the nanocrystal and amorphous matrix, nanocavities were observed to form. Continued annealing resulted in movement of the cavities away from the nanocrystal while leaving behind a crystalline tail. The origins and fundamental mechanisms of this phenomenon were examined by in-situ heating TEM and ex-situ crystallographic TEM techniques. We demonstrate a mechanism of solid-phase crystallization (SPC) enabled by nanoscale cavities formed at the interface between an hydrogenated amorphous silicon film and embedded 30 nm to 40 nm Si nanocrystals. The nanocavities, 10 nm to 25 nm across, have the unique property of an internal surface that is part amorphous and part crystalline, enabling capillarity-driven diffusion from the amorphous to the crystalline domain. The nanocavities propagate rapidly through the amorphous phase, up to five times faster than the SPC growth rate, while "pulling behind" a crystalline tail. It is shown that twin boundaries exposed on the crystalline surface accelerate crystal growth and influence the direction of nanocavity propagation. HASH(0x7febe3ca40b8) The mechanical properties and mechanisms of plasticity in these same silicon nanocubes have also been investigated. The strain-dependent mechanical properties and the underlying mechanisms governing the elastic-plastic response are explored in detail. Elastic strains approaching 7% and flow stresses of 11 GPa were observed, significantly higher than that observed in other nanoscale volumes of Si. In-situ imaging revealed the formation of 5 nm dislocation embryos at 7% strain, giving way at 20% strain to continuous nucleation of leading partial dislocations with {111}-habit at the embryo surface.
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    Real-Space Imaging of Picosecond Structural Dynamics in Metals with Ultrafast Transmission Electron Microscopy
    (2021-01) Gnabasik, Ryan
    Probing the first picoseconds of structural response after photoexcitation in metal systems allows us to construct a timeline of how the absorbed energy evolves in space and time. To do this, we need a technique that is sensitive to structural changes at nanometer-picosecond spatiotemporal resolutions. Often techniques that fit these requirements operate in reciprocal space (e.g., ultrafast electron and x-ray diffraction) which are spot-size limited resulting in ensemble averaging over interesting defect and microstructure interactions. Here we use ultrafast transmission electron microscopy (UEM) to image the structural dynamics of two metal materials systems, the iron pnictide LaFeAsO and plasmonic gold nanorods, after ultrafast photoexcitation. First, plasmonic gold nanorods offer a viable route for coupling light into structures smaller than its wavelength. However, there is a convolution of optical and structural effects that prevent a complete understanding of how plasmonic structures behave, specifically in assemblies. The goal of this work is to use imaging in UEM to elucidate the structural response of a non-trivial assembly of gold nanorods to serve as complimentary structural information that when combined with optical models can form a complete picture. Investigations reveal early incoherent-to-coherent structural dynamics from an increase in thermal diffuse scattering followed by a roughly 10 ps delay believed to be phonon-phonon coupling ending with the launch and propagation of strain waves at the speed of sound. Because we are imaging, the response from individual nanorods in the assembly can be resolved as well as the potential effect on neighboring rods. This work has shown that we can construct a timeline for the first 100 picoseconds after ultrafast excitation from the electron-phonon coupling to the whole nanorod acoustic modes. Second, the iron pnictide class of materials display unconventional superconductivity when doped. Mechanistically different than the cuprates, the iron pnictide superconductivity has yet to be fully understood. It is believed that the nearly simultaneous structural and magnetic phase transitions of the parent compound offer clues to understanding the superconductivity mechanism. We use imaging in UEM to explore the structural dynamics of the parent compound. After photoexcitation, hypersonic Lamb modes emanate from the crystal-vacuum interface towards the bulk then interact with microscale crystal boundaries (e.g. step-edges, interfaces) that lead to reflection and wave interference. In addition, we present some initial exploration of probing the structural phase transition; showcasing the differences between the tetragonal and orthorhombic phases while demonstrating the capability of UEM to detect changes in elastic constants. The observations from these experiments show the strength of UEM to capture real-space, structural dynamics not easily accessible with reciprocal space techniques to provide complimentary information to the materials science and physics community.
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    Structure determination of zeolite nanosheets
    (2012) Zhang, Xueyi; Tsapatsis, Michael
    MFI and MWW zeolite nanosheets are building units for state-of-the-art zeolite thin films for gas separation. In this study, the structures of exfoliated MFI and MWW zeolite nanosheets were determined using a combination of experimental and simulation methods. Based on characterization results from atomic force microscopy and transmission electron microscopy, the structures and thicknesses of the exfoliated zeolite nanosheets were proposed. After optimization with Car-Parrinello molecular dynamics, X-ray diffraction patterns and electron diffraction patterns are simulated from these structures. The agreement between experimental and simulated characterization data suggested that the proposed structures should represent the actual structures of the exfoliated zeolite nanosheets. The methods used in this study can be extended to determining structures of other zeolite nanostructures.