Date of Award

2013-01-01

Degree Name

Doctor of Philosophy

Department

Material Science and Engineering

Advisor(s)

Ramana V. Chintalapalle

Abstract

Tungsten oxide (WO3) is a multifunctional material which has applications in electronics, sensors, optoelectronics, and energy-related technologies. Recently, electronic structure modification of WO3 to design novel photocatalysts has garnered significant attention. However, a fundamental understanding of nitrogen induced changes in the structure, morphology, surface/interface chemistry, and electronic properties of WO3 is a prerequisite to producing materials with the desired functionality and performance. Also, understanding the effect of thermodynamic and processing variables is highly desirable in order to derive the structure-property relationships in the W-O/W-O-N material system. The present work was, therefore, focused on studying the effects of processing parameters on the microstructure, optical properties, electrical conductivity, and electronic structures of pure and nitrogen-doped (N-doped) WO3 films grown by sputter deposition. Efforts were made to understand the properties and phenomena of pure and N-doped WO3 at reduced dimensionality (i.e., nanoscale dimensions).

The results and analyses indicate that the growth temperature (Ts) has a significant effect on the microstructure of WO3 films. The grain size increases from 9 to 50 nm coupled with a phase transformation in the following sequence: amorphous (a) to monoclinic (m) to tetragonal (t) with increasing Ts (25-500oC). The nanocrystalline t-WO3 films exhibit a strong (001) texturing. The band gap narrowing from 3.25 to 2.92 eV with grain size occurs due to quantum con-finement effects. Correlated with the structure and optical properties, electrical conductivity also increases.

Physical properties such as thickness, grain size, and density are also sensitive to oxygen/nitrogen partial pressure during W-O/W-O-N sample fabrications. A direct relationship be-tween film density and band gap is evident in nanocrystalline t-WO3 films grown at various oxygen pressures. It is observed that nitrogen doping significantly influences the structure-property relationships. Crystallographic analysis revealed that excess nitrogen trapped in the WO3 crystal lattice induces a t-m phase transformation. The unique approach adopted in this work indicates a structure-dependent optical band gap variation leading to the lowest optical band gap (~2.14 eV) at 0.7 at.% of N incorporation into t-WO3 films. The results clearly provide evidence to tune the electronic structure and properties with controlled N doping coupled with specific phase stabili-zation of WO3. The results provide a road map to phase-controlled synthesis of pure and N-doped nanocrystalline WO3 films with desired properties.

Language

en

Provenance

Received from ProQuest

File Size

149 pages

File Format

application/pdf

Rights Holder

Vemnkata Rama Sesha Ravi Kumar Vemuri

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