Correlation between Surface Chemistry, Density, and Band Gap in Nanocrystalline WO3 Thin Films

• Published in: ACS applied materials & interfaces Vol. 4; no. 3; pp. 1371 - 1377
• Main Authors: Vemuri, R.S, Engelhard, M.H, Ramana, C.V
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 01.03.2012
• Subjects: ARGON; ATOMIC FORCE MICROSCOPY; band gap; CHEMISTRY; DEPOSITION; Environmental Molecular Sciences Laboratory; INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; MATERIALS SCIENCE; MIXTURES; OPTICAL PROPERTIES; OXYGEN; REFLECTIVITY; SCANNING ELECTRON MICROSCOPY; spectrophotometry; SPUTTERING; SUBSTRATES; surface chemistry; THIN FILMS; TUNGSTEN; VALENCE; WO3 thin films; X-RAY PHOTOELECTRON SPECTROSCOPY; X-ray reflectivity; XPS density; spectrophotometry; XPS density; WO3 thin films; optical properties; surface chemistry; X-ray reflectivity; band gap
• ISSN: 1944-8244; 1944-8252
• DOI: 10.1021/am2016409

Nanocrystalline WO3 thin films were produced by sputter-deposition by varying the ratio of argon to oxygen in the reactive gas mixture during deposition. The surface chemistry, physical characteristics, and optical properties of nanocrystalline WO3 films were evaluated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray reflectivity (XRR), and spectrophotometric measurements. The effect of ultramicrostructure was significant on the optical properties of WO3 films. The XPS analyses indicate the formation of stoichiometric WO3 with tungsten existing in fully oxidized valence state (W6+). However, WO3 films grown at high oxygen concentration (>60%) in the sputtering gas mixture were over stoichiometric with excess oxygen. XRR simulations based on isotropic WO3 film–SiO2 interface–Si substrate modeling indicate that the density of WO3 films is sensitive to the oxygen content in the sputtering gas. The spectral transmission of the films increased with increasing oxygen. The band gap of these films increases from 2.78 to 3.25 eV with increasing oxygen. A direct correlation between the film density and band gap in nanocrystalline WO3 films is established on the basis of the observed results.