Uncovering temperature-induced changes in bandgap and electronic heterogeneity in transition metal oxides through optical absorption spectroscopy: A review

The progress made in transition metal oxides has resulted in the identification of numerous potential applications in nanoelectronics, photonics, sensing, energy storage, and optoelectronics. Optical absorption spectroscopy is an invaluable technique for examining the changes in bandgap as a function of temperature and evaluating the degree of disorder in transition metal oxides. In this review, experimental data obtained in our previous investigation is taken as a reference and systematically explained from other literature. Variations of temperature were studied to understand the optical properties, including the band gap (E_g), Urbach focus (E₀), and Urbach energy (E_U). It has been noted that an increase in temperature causes a reduction in the energy band gap (E_g) and leads to a significant anomaly around the transition point (T_P). Moreover, close to T_P, E_u exhibits metastability, suggesting that its value at temperature T is not constant but varies over time (t). To assess the potential influence of intergranular strain on the Barium Titanate (BaTiO₃), a study that investigates the variation of temperature behaviour in the case of both pellet and powder samples using Raman and optical techniques. This approach aids in the identification of structural and thermal disorders in transition metal oxides, thereby enhancing our understanding of the crucial role played by intergranular strain at the grain boundaries. The temporal variation of E_U, which also exhibits the impact on the observed metastability, may be accurately described by the stretched exponential function using the density of states. First-principles calculations were used to comprehend the observed alteration in the Energy gap (E_g) and Urbach Energy (E_U) caused by the structural and thermal changes. This review suggests that optical investigations can serve as a highly responsive method for detecting disorder or heterogeneity within the oxide samples.