Band gap states in nanocrystalline WO₃thin films studied by soft x-ray spectroscopy and optical spectrophotometry

Abstract

Nanocrystalline tungsten trioxide (WO3) thin films prepared by DC magnetron sputtering have been studied using soft x-ray spectroscopy and optical spectrophotometry. Resonant inelastic x-ray scattering (RIXS) measurements reveal band gap states in sub-stoichiometric γ-WO3-x with x = 0.001-0.005. The energy positions of these states are in good agreement with recently reported density functional calculations. The results were compared with optical absorption measurements in the near infrared spectral region. An optical absorption peak at 0.74 eV is assigned to intervalence transfer of polarons between W sites. A less prominent peak at energies between 0.96 and 1.16 eV is assigned to electron excitation of oxygen vacancies. The latter results are supported by RIXS measurements, where an energy loss in this energy range was observed, and this suggests that electron transfer processes involving transitions from oxygen vacancy states can be observed in RIXS. Our results have implications for the interpretation of optical properties of WO3, and the optical transitions close to the band gap, which are important in photocatalytic and photoelectrochemical applications.

Critical Review—Photocurrent Spectroscopy in Corrosion and Passivity Studies: A Critical Assessment of the Use of Band Gap Value to Estimate the Oxide Film Composition

“…In the absence of stoichiometric deviations, the disordered structure of the a-SC induces the localization of electronic states close to the conduction and/or valence band edges, as described by the density of states (DOS) model proposed by Mott-Davis for amorphous semiconductors. [37][38][39] A further support to our approach to the estimate of the bowing parameter comes out from the data of Figure 3 pertaining to mixed (Y x Gd 1-x ) 2 O 3 oxides for which a good agreement between experimental and theoretical values is also observed by assuming for the two d-metal cations A = 1.35 and B = −1.5 (see Eq. 3b) and an electronegativity parameter of 1.20 and 1.27 for Y and Gd respectively, in quite good agreement with the Pauling scale and uncertainty limits.…”

“…37,38 This aspect must be taken into account owing to the fact that there are few techniques at our disposal able to correlate the gathered in-situ information on the solid state properties (E g , flatband potential, U FB , and internal photoemission threshold, E th ) of passive films and corrosion layers with their chemical composition, as a function of chemical and electrical variables employed in the experiments.…”

Influence of defect structure on colour tunability and magneto optical behaviour of WO3 nanoforms

“…For the sample W110, the peaks at 638 cm À1 and 946 cm À1 are assigned to the a g phonons of the WO 3 $H 2 O lattice, in agreement with previous reports. 48 These two peaks are associated with the stretching mode of the W(6+)-O(2) and W(6+)-O(3) respectively. For Tungstite, the W]O bond peak, is caused by structural water molecules.…”

Transparent Supercapacitors: From Optical Theories to Optoelectronics Applications

“…[49,50] Figure 2c compares the E g values of various SC materials that were taken from experimental results and theoretical calculations. Co(OH) 2 , [51][52][53] Co 3 O 4 , [54] GO, [55,56] ITO, [57] MnO 2 , [58,59] Ni(OH) 2 , [60] NiO, [61] PANi, [62][63][64] PEDOT:PSS, [65] Ppy, [66,67] RuO 2 , [68,69] SnO 2 , [47,70] WO 3 , [71,72] and ZnO. [73][74][75]…”