Polycrystalline WO3−x Thin Films Obtained by Reactive DC Sputtering at Room Temperature

Abstract

Tungsten oxide thin films have applications in various energy-related devices owing to their versatile semiconductor properties, which depend on the oxygen content and crystalline state. The concentration of electrons increases with intrinsic defects such as oxygen vacancies, which create new absorption bands that give rise to colored films. Disorders in the crystal structure produce additional changes in the electrical and optical characteristics. Here, WO3−x thin films are prepared on unheated glass substrates by reactive DC sputtering from a pure metal target, using the discharge power and the oxygen-to-argon pressure ratio as control parameters. A transition from amorphous to polycrystalline state is obtained by increasing the sputtering power and adjusting the oxygen content. The surface roughness is higher and the bandgap energy is lower for polycrystalline layers than for amorphous ones. Moreover, the electrical conductivity and sub-bandgap absorption increase as the oxygen content decreases.

Synthesis of Ultrathin Alloy (Mo, V)-Tungsten-Oxide Nanowires: Implications for Electrochromic and Supercapacitor Applications

“…This subband (Urbach tail) is influenced by defects and disorders of the substoichiometric nature and the addition of cations obtained in our alloyed TMO. 51,52 It can be seen directly by the more moderate increase when the amount of the added cation increases (Figure S4a). Furthermore, the Urbach energy can be extracted from the UV−vis absorbance by eq 3: 51,52…”

“…This subband (Urbach tail) is influenced by defects and disorders of the substoichiometric nature and the addition of cations obtained in our alloyed TMO. , It can be seen directly by the more moderate increase when the amount of the added cation increases (Figure S4a). Furthermore, the Urbach energy can be extracted from the UV–vis absorbance by eq : , where α, α 0 , E u , and hν are the absorption coefficient, a constant, the Urbach energy, and the incident photons’ energy (Planck constant times frequency), respectively. Assuming the absorbance ( A ) is proportional to α, the unsurprising value of E u = 0.26 eV is calculated for W–O (Figure S4f), typical of substoichiometric tungsten-oxide, and may be ascribed to oxygen vacancy states, as was also observed in its large LSPR .…”

“…Assuming the absorbance (A) is proportional to α, the unsurprising value of E u = 0.26 eV is calculated for W−O (Figure S4f), typical of substoichiometric tungsten-oxide, and may be ascribed to oxygen vacancy states, as was also observed in its large LSPR. 51 The increasing Urbach energy when cations are incorporated reflects on the addition of localized states as a result of Mo/V alloying (Figure S4). 52 Although their optical properties are distinct, the alloyed Mo (or V)−W−O preserves the octahedral symmetry of the untreated W−O.…”

“…Furthermore, the Urbach energy can be extracted from the UV–vis absorbance by eq : , ln ( α ) = ln ( α 0 ) + 1 E normalu · h v where α, α 0 , E u , and hν are the absorption coefficient, a constant, the Urbach energy, and the incident photons’ energy (Planck constant times frequency), respectively. Assuming the absorbance ( A ) is proportional to α, the unsurprising value of E u = 0.26 eV is calculated for W–O (Figure S4f), typical of substoichiometric tungsten-oxide, and may be ascribed to oxygen vacancy states, as was also observed in its large LSPR . The increasing Urbach energy when cations are incorporated reflects on the addition of localized states as a result of Mo/V alloying (Figure S4).…”

“…The orange color of the W–V–O results from absorbance at 400–600 nm and can be attributed to the addition of V 5+ cations with d–d localized transition. , The addition of V 4+ should result in a strong absorption below 300 nm but is not observed in our system due to the strong absorbance of the W–O at its band edge. ,, The absorbance near 400 nm and its widening can be ascribed to the extent of the absorption edge, i.e., the Urbach tail. This subband (Urbach tail) is influenced by defects and disorders of the substoichiometric nature and the addition of cations obtained in our alloyed TMO. , It can be seen directly by the more moderate increase when the amount of the added cation increases (Figure S4a). Furthermore, the Urbach energy can be extracted from the UV–vis absorbance by eq : , where α, α 0 , E u , and hν are the absorption coefficient, a constant, the Urbach energy, and the incident photons’ energy (Planck constant times frequency), respectively.…”