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

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 spec...

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Bibliographic Details
Published in:ACS applied materials & interfaces 2012-03, Vol.4 (3), p.1371-1377
Main Authors: Vemuri, R.S, Engelhard, M.H, Ramana, C.V
Format: Article
Language:English
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
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Summary: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.
ISSN:1944-8244
1944-8252
DOI:10.1021/am2016409