Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Yang Song

Abstract

Pressure plays a critical role in regulating the structures and properties of materials. Since Percy Bridgeman was recognized by the 1946 Nobel Prize for his contribution to high-pressure physics, high-pressure research as an interdisciplinary area has attracted extensive attentions. Nowadays, the high-pressure research involves broad frontier areas, such as chemistry, physics, biology, material and earth science. For instance, brand new classes of inorganic materials of unusual stoichiometries and crystal structures, with wide range of optical, mechanical, electronic and magnetic properties, have been produced at high pressure. Pressure-induced structural transformations between crystalline and amorphous materials, as well as among insulators, conductors and even superconductors, have been extensive documented. This Ph.D. work focused on investigating pressure-induced structural transformations in materials and understanding the involved chemistry, which was assembled into two parts.

In part I, two molecular systems, chlorocyclohexane (CCH) and azobenzene (AB), were investigated to examine the pressure effect on chemical conformations, stability of ring structures and reactivities using Raman spectroscopy. For CCH, pressure-induced conformational change as well as rich phase transformations were observed upon compression. Both AB and hydrazobenzene (HAB) underwent a phase transition at a similar pressure. However, origins of these phase transformations were drastically different. High-pressure structures of both molecules were examined based on spectroscopic data. Their distinctive high-pressure behaviors were analyzed and interpreted with the aid of ab initio molecular orbital calculations.

Part II focuses on pressure-induced structural transformations of one dimensional (1D) nanomaterials using vibrational spectroscopy and synchrotron X-ray diffraction. Individual studies were first carried out on BN nanotubes and GaN nanowires aiming at investigating their high-pressure behaviors. Then, systematic studies were conducted on TiO2 and ZnO nanowires focusing on the size- and morphology- effect on their high-pressure behaviors. These 1D nanomaterials behaved dramatically different from bulk counterparts and nanoparticles, in terms of phase transition pressure, phase transition sequence, and compressibility. In addition, morphology of each material before and after compression was examined by scanning electron microscope. Our studies provide more insight into the understanding of unique high-pressure behaviors of nanomaterials and show profound implications for producing controlled structures with new applications achieved by combined pressure-morphology tuning.

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