Electronic and optical properties of nanocrystalline WO sub(3) thin films studied by optical spectroscopy and density functional calculations

The optical and electronic properties of nanocrystalline WO sub(3) thin films prepared by reactive dc magnetron sputtering at different total pressures (P sub(tot)) were studied by optical spectroscopy and density functional theory (DFT) calculations. Monoclinic films prepared at low P sub(tot) show...

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Published inJournal of physics. Condensed matter Vol. 25; no. 20; pp. 1 - 11
Main Authors Johansson, Malin B, Baldissera, Gustavo, Valyukh, Iryna, Persson, Clas, Arwin, Hans, Niklasson, Gunnar A, Osterlund, Lars
Format Journal Article
LanguageEnglish
Published 22.05.2013
Subjects
Density
Electronic properties
Mathematical analysis
Nanocrystals
Optical properties
Spectroscopy
Thin films
Tungsten oxides
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Summary:The optical and electronic properties of nanocrystalline WO sub(3) thin films prepared by reactive dc magnetron sputtering at different total pressures (P sub(tot)) were studied by optical spectroscopy and density functional theory (DFT) calculations. Monoclinic films prepared at low P sub(tot) show absorption in the near infrared due to polarons, which is attributed to a strained film structure. Analysis of the optical data yields band-gap energies E sub(g) [asymptotically =] 3.1 eV, which increase with increasing P sub(tot) by 0.1 eV, and correlate with the structural modifications of the films. The electronic structures of triclinic delta -WO sub(3), and monoclinic gamma - and [varepsilon]-WO sub(3) were calculated using the Green function with screened Coulomb interaction (GW approach), and the local density approximation. The delta -WO sub(3) and gamma -WO sub(3) phases are found to have very similar electronic properties, with weak dispersion of the valence and conduction bands, consistent with a direct band-gap. Analysis of the joint density of states shows that the optical absorption around the band edge is composed of contributions from forbidden transitions (> 3 eV) and allowed transitions (> 3.8 eV). The calculations show that E sub(g) in delta -WO sub(3) is higher than in the delta -WO sub(3) and gamma -WO sub(3) phases, which provides an explanation for the P sub(tot) dependence of the optical data.
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ISSN:0953-8984
1361-648X
DOI:10.1088/0953-8984/25/20/205502