Giant upconversion enhancement of Er³⁺ ions doped nanosheets via heterojunction engineering

Even though there have been significant advancements in the development of rare earth (RE) doped upconversion (UC) materials, the challenge remains to develop highly efficient UC phosphors due to scientific interest and application need. Here, we report that heterojunction engineering leads to the significant UC emission enhancement of Er³⁺ ions, challenging the long-standing notion that luminescence of RE ions is nearly independent of photocarrier transfer. It shows that via an in-situ construction strategy, Bi₂O₃ is placed on Bi₃O₄Br:Er³⁺ nanosheets to compose type Ⅱ heterojunction, allowing photogenerated holes to transfer from matrix semiconductor to the Bi₂O₃. Thus, under excitation by 1550 nm laser, the outer heterojunction prolongs the decay time of Er³⁺ and improves the integral intensity of visible UC emission nearly 25.0 times. The results of experiment and theory calculation indicate that under near-infrared light irradiation, photogenerated-hole transferring via heterojunction suppresses the recombination of excited electrons occupying intermediate energy states in Er³⁺, enabling energy reabsorption processes that trigger subsequent transitions to higher-lying levels. This finding of the work provides an insight into the influence of carrier migration on the photoluminescence of lanthanide ions, which will be helpful for the development of optoelectronic devices related to RE ions.