title: Electronic and optical properties of nanocrystalline WO₃ thin films studied by optical spectroscopy and density functional calculations.
abstract: The optical and electronic properties of nanocrystalline WO3 thin films prepared by reactive dc magnetron sputtering at different total pressures (Ptot) were studied by optical spectroscopy and density functional theory (DFT) calculations. Monoclinic films prepared at low Ptot 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 Eg ≈ 3.1 eV, which increase with increasing Ptot by 0.1 eV, and correlate with the structural modifications of the films. The electronic structures of triclinic δ-WO3, and monoclinic γ- and ε-WO3 were calculated using the Green function with screened Coulomb interaction (GW approach), and the local density approximation. The δ-WO3 and γ-WO3 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 Eg in ε-WO3 is higher than in the δ-WO3 and γ-WO3 phases, which provides an explanation for the Ptot dependence of the optical data.
authors.affiliation: Division of Solid State Physics, Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, PO Box 534, SE-75121 Uppsala, Sweden. Malin.Johansson@angstrom.uu.se
authors.forename: Malin B
authors.initials: MB
authors.lastname: Johansson
authors.forename: Gustavo
authors.initials: G
authors.lastname: Baldissera
authors.forename: Iryna
authors.initials: I
authors.lastname: Valyukh
authors.forename: Clas
authors.initials: C
authors.lastname: Persson
authors.forename: Hans
authors.initials: H
authors.lastname: Arwin
authors.forename: Gunnar A
authors.initials: GA
authors.lastname: Niklasson
authors.forename: Lars
authors.initials: L
authors.lastname: Osterlund
mesh_terms: D004583:Electrons; D053758:Nanoparticles / Q000737:chemistry*; D055620:Optical Phenomena; D010087:Oxides / Q000737:chemistry*; D011789:Quantum Theory; D013057:Spectrum Analysis; D014414:Tungsten / Q000737:chemistry*
chemical_list: D010087:Oxides; C511604:tungsten oxide; D014414:Tungsten