Dynamic tailorable local luminescence patterns on single upconversion fluoride microcrystals via in situ oxidation through laser irradiation

Abstract

Upconversion (UC) micro/nanomaterials have enabled many promising applications owing to their unique near-infrared to visible emission characteristics. However, some practical applications are limited by the static and invariant emission colours and patterns of UC materials. Here, we demonstrated a novel dynamic UC luminescence pattern and luminescent heterogeneous spatial distribution mediated by laser irradiation. Interestingly, the distinct red dumbbell pattern was submerged gradually by moving the green stripes from the excitation position at one end of a microtube to the other end along the length direction when elevating the power density of excitation light. The underlying reason for the dynamic luminescence patterns obtained by power density controlling was explored, and a mechanism based on an in situ oxidation reaction on NaYbF₄:Ho³⁺ UC microcrystals was revealed. The oxidation reaction between O₂ and NaYbF₄:Ho³⁺ microcrystals was induced by increasing excitation power density and irradiation duration leading to a lattice-mismatched NaYbF₄ and YbOF heterojunction. This unique heterojunction architecture not only modifies the profiles and colours of the luminescent patterns, but also separates the processes of light–light conversion and light–heat conversion in time and space. These studies have provided a deeper insight into the local in situ oxidation mechanisms of fluoride crystals and offered a novel route toward the tuning of UC luminescence using a heterojunction.