Emergent piezoelectricity as the driving force for mechanoluminescence in crystals of atomically precise copper nanoclusters.

Mechanoluminescence (ML) is a phenomenon of force-response self-luminescence, which was observed long ago. However, the underlying mechanism of ML remains ambiguous due to the complexity of light-emitting processes. Atomically precise metal nanoclusters are expected to serve as ideal model systems for unraveling ML mechanisms in specific nano-systems. Herein, we report the observation of mechanoluminescent crystals of metal nanoclusters. Cu7 clusters, which crystallize in a non-centrosymmetric trigonal space group, exhibit bright green ML when subjected to mechanical stress. Through piezoelectric experiments and theoretical calculations, we demonstrate that piezoelectricity arising from mechanical deformation of the crystal leads to an asymmetric distribution of internal dipoles within the crystal. Furthermore, crystal fracture under external force induces charge separation. Such alterations modulate the transition dipole moments and induce localized electronic transitions through strain-induced charge asymmetry, thus triggering stress-dependent luminescence. The findings provide insights into the mechanism of ML in a nanocluster system at the atomic level.