Calcination-Induced Transformation of ZnS:Mn2+ Nanorods to Microparticles for Enhanced Mechanoluminescence

Mechanoluminescent (ML) materials with reduced dimensions have the potential to advance the development of optical microdevices. However, their progress has been hindered by the limited understanding of ML properties at the length scale of nano- to micrometers. This study aims to optimize the ML performance of Mn2+-doped wurtzite ZnS by examining the evolution of the size, morphology, phase, and surface property of as-prepared nanorods during calcination at elevated temperatures. It reveals a complex dependence of the phase on the surface-capping ligands, calcination temperature, and doping level. Along with a dimensional increase from nano- to micrometers, consistent wurtzite to sphalerite conversion is observed for calcining samples with doping ratios below 0.92 at. %, while those with higher doping ratios exhibit a wurtzite–sphalerite–wurtzite transition upon calcination from low to high temperatures with the sphalerite phase ratio increasing to 30% and then decreasing to 0. Further, the ML performance is found to be primarily determined by the phase composition with the highest intensity observed in ZnS microparticles doped with 1.0 at. % Mn2+ and calcined at 1000 °C. In contrast, photoluminescence is less sensitive to phase components but is significantly affected by the optical absorption from carbon species generated by the pyrolysis of surface ligands. Heavily doped samples (3.0 at. % Mn2+) display no noticeable wurtzite-to-sphalerite phase transition but exhibit a positive dependence of ML on the particle size when the latter varies from 100 nm to 2 μm upon calcination from 100 to 1000 °C with ML starting to appear at the size of ∼200 nm. The microparticles with tailored ML properties are expected to contribute to the miniaturization of advanced optical devices such as mechano-optical sensors and actuators.