Division of Solid State Physics, Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, PO Box 534, SE-75121 Uppsala, Sweden.
J Phys Condens Matter. 2013 May 22;25(20):205502. doi: 10.1088/0953-8984/25/20/205502. Epub 2013 Apr 25.
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.
采用直流磁控反应溅射法在不同总压强（Ptot）下制备了纳米 WO3 薄膜，并通过光谱光学和密度泛函理论（DFT）计算研究了其光学和电子性能。在低 Ptot 下制备的单斜晶薄膜由于极化子的存在，在近红外区域显示出吸收，这归因于应变薄膜结构。对光学数据的分析得出带隙能量 Eg ≈ 3.1 eV，其随 Ptot 的增加以 0.1 eV 的幅度增加，并与薄膜的结构变化相关。使用格林函数与屏蔽库仑相互作用（GW 方法）和局域密度近似计算了三方晶系 δ-WO3、单斜晶系 γ-和 ε-WO3 的电子结构。发现 δ-WO3 和 γ-WO3 相具有非常相似的电子性质，价带和导带的色散较弱，与直接带隙一致。对态密度的分析表明，带边缘附近的光吸收由禁带跃迁（>3 eV）和允许跃迁（>3.8 eV）组成。计算表明，ε-WO3 的 Eg 高于 δ-WO3 和 γ-WO3 相，这为光学数据的 Ptot 依赖性提供了解释。
