Structure and electronic properties of pure and nitrogen doped nanocrystalline tungsten oxide thin films
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 WO 3 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 (25–500°C). The nanocrystalline t-WO 3 films exhibit a strong (001) texturing. The band gap narrowing from 3.25 to 2.92 eV with grain size occurs due to quantum confinement 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 between 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 WO 3 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 stabilization of WO3. The results provide a road map to phase-controlled synthesis of pure and Ndoped nanocrystalline WO3 films with desired properties.^
Chemistry, Inorganic|Engineering, Materials Science
Vemuri, Venkata Rama Sesha Ravi Kumar, "Structure and electronic properties of pure and nitrogen doped nanocrystalline tungsten oxide thin films" (2013). ETD Collection for University of Texas, El Paso. AAI3589591.