University of Missouri - Saint Louis

The Graduate School

Announcement

An oral examination in defense of the dissertation for the degree

Doctor of Philosophy

Jinfeng Wang
M.A. in Physics, July, 2004, Nankai University
B.A. in Optical Information Science and Technology, July, 2001,Shandong University

Characterization and Synthesis of Nanoscale Materials

Abstract

This dissertation focuses on the systematic study of techniques for characterization and synthesis of nanoscale materials. We have achieved several goals: (1) Various metal oxide nanostructures have been synthesized and characterized by scanning electron microscopy and transmission electron microscopy; (2) New methods for using lattice fringe visibility to determine the orientation between nanocrystals and a cylindrical support have been described; (3) Controllable zinc oxide nanostructures have been designed and synthesized successfully; (4)The systematic study of zinc oxide nanostructures provides new insight into the interface between Au catalyst and ZnO nanowires, as well as the importance of tin catalyst thickness to ZnO self assembly; (5) Many yet unexplained experimental structures are reported as well.

Firstly, using thermal evaporation and controlling experimental parameters, including source material temperature, substrate temperature, substrate material, gas flow rate, and catalyst, high number density uniform zinc oxide nanostructure growth has been achieved. Aligned zinc oxide nanowires, randomly oriented zinc oxide nanowires, zinc oxide container-shaped structures, and zinc oxide nanobelts have been synthesized with high yield.

Secondly, using a one parameter family of lattice fringe geometry curves, we show how to examine the epitaxial relationship between catalyst particles and a cylindrical support. Using digital darkfield techniques, this investigation can be automated.

Thirdly, the structure relationship between catalyst particles and zinc oxide nanowires have been investigated using scanning and high resolution scanning transmission electron microscopes. A vapor-solid-solid growth model involving a hexagonal array of aligned growth regions is proposed in zinc oxide nanowire formation. Evidence indicates in particular that gold catalyst particles remain solid during ZnO nanowire growth.

Finally, the effect of tin catalyst thickness on nanostructure formation has been investigated. The catalyst abundance on the substrate has a direct impact on its ability to absorb ZnO. The thicker coated substrates can absorb more source vapor, and form larger structures, than can thinner coated substrates.

Defense of Dissertation Committee