Electrical, Optical, and Thermal Transport Properties of Oxygen-Deficient Amorphous WO_x (2.5 < x < 3) Films

Oxygen-deficient tungsten oxide (WO_x) is known as an active material for various future applications such as smart displays, photocatalysts, Li-ion battery, and so on. WO_x exhibits versatile properties depending on the valence state of W, which can vary from +6 to +4. Therefore, clarifying the relationship between x, the valence state of W ion, and the material properties of WO_x is crucial for discovering more unique device applications. In case of crystalline WO_x, since WO_x has many different phases, the valence state of W cannot be modulated continuously from +6 to +4. On the other hand, there is no phase boundary in amorphous (a-) WO_x, the valence state of W ion can be continuously modulated against x, and the effect of valence state of W on the material properties of WO_x can be thoroughly examined. Here, we report the electrical, optical, and thermal properties of a-WO_x films with several valence states of +6 (d⁰), +5 (d¹), and +4 (d²) for x ranging from 2.511 to 2.982. Although the +6 dominant films were electrical insulators with optical transparency in the visible region, we found that both optical transmissivity and electrical resistivity decreased drastically with increase in the +5 concentrations, which also enhances the thermal conductivity because heat can be carried by additional conduction electrons. As the +4 state became dominant in the film, the resistivity slightly increased, whereas the low visible transmission was maintained. These results suggest that the redox of tungsten between +6 and +5 is attributed to all versatile properties of a-WO_x, which would be of great use for developing unique devices in the future.