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 a limited understanding of the 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 size, morphology, phase, and surface property of as-prepared nanorods during calcination at elevated temperatures. It reveals a complex dependence of phase on 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 a sphalerite phase ratio increasing to 30% and then decreasing to 0. Further, the ML performance is found to be primarily determined by 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 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.