Atomic structures of metallic and semiconducting nanocrystals by coherent electron diffraction

Electrons can interact with matters much more strongly compared to other radiation probes such as X-ray and neutron. Therefore, in principle, electron beam can record diffraction patterns from individual nanocrystals of a few nanometers in diameter, which has never been achieved before with any other structural probes. In this thesis, I will demonstrate recording coherent electron diffraction patterns from individual metallic and semiconducting nanocrystals, and discuss three applications of these diffraction patterns for studying the atomic structures of nanocrystals. These applications are so far difficult to achieve via direct imaging.;The first application of single-particle diffraction patterns is diffractive imaging. In diffractive imaging, diffraction patterns are inverted to form a real space image. Through diffractive imaging, diffraction-limited resolution can readily be achieved without aberration correction. In the thesis, the effects of various experimental factors such as dynamical scattering, noise and beam convergence will be investigated via simulations. Examples of achieving sub-A imaging resolution from CdS quantum dots via diffractive imaging will be shown.;The second application of single-particle diffraction patterns is to study the surface structures of nanocrystals. Au nanocrystals were studied in this thesis. By modeling coherent diffraction intensity around the Bragg reflections recorded in the diffraction patterns, we found that the surface bond lengths of the nanocrystals are contracted relative to the interior atoms. The surface relaxation is also found to be coordination dependent, where large out-of-plane bond length contractions are found for the edge and {100} atoms, while a much smaller contraction is found for the {111} atoms.;The third application of the single-particle diffraction technique is to measure the temperature-dependent structures of individual nanocrystals. Again, Au nanocrystals were studied for this experiment. Experimentally, we recorded diffraction patterns from the same Au nanocrystals at different temperatures. From the diffraction data, we found that the coefficient of thermal expansion (CTE) of TiO2-supported Au nanocrystals is size dependent: the CTE is positive for nanocrystals larger than 7 nanometers in diameter and negative for the smaller nanocrystals. From the diffraction intensity, we also extracted through modeling the mean square displacement of the surface atoms of the Au nanocrystals as a function of temperature, and found that surface atoms have a significantly larger vibration amplitude than the interior atoms.