Enhancing the upconversion of Er³⁺ incorporated BaTiO₃ by introducing oxygen vacancies

Lanthanide-incorporated crystals display the phenomenon of upconversion (UC), wherein near-infrared (NIR) light is converted into ultraviolet-visible (UV-Vis) emission with a narrow bandwidth. This unique photophysical property renders lanthanide UC materials highly promising for diverse applications. However, the limited quantum efficiency (∼3%) hinders the broader utilization of UC materials. Consequently, numerous studies have focused on overcoming this low efficiency. Notably, it has been observed that manipulation of the site symmetry in UC materials significantly enhances their UC efficiency. In this study, we investigate the UC enhancement of Er³⁺ incorporated BaTiO₃ (E-BT) crystals through the introduction of oxygen vacancies (O_V). The O_V were created using a post-heat treatment method, and the annealing time was varied to control the quantity of O_V. An optimal annealing time of 6 hours was determined for efficient O_V generation, beyond which the O_V content decreased. Remarkably, E-BT crystals with O_V exhibited up to three-fold greater UC compared to those without O_V. This outcome suggests that O_V induce symmetry changes in the E-BT crystal structure. Furthermore, the degree of UC enhancement in E-BT was found to be proportional to the amount of O_V present.