Dislocations in Motion: Engineering Mechanoluminescence via Pressure-Driven Phase Transitions
Mechanoluminescent (ML) materials that emit light under mechanical stress are attracting growing attention for their potential in next-generation sensing, display, and energy-harvesting technologies. Among these, Mn/Cu-doped zinc sulfide (ZnS) has emerged as a leading candidate due to its bright emission, low activation threshold, and remarkable self-recovery over thousands of cycles. Despite these advantages, the fundamental mechanisms governing ML remain unresolved, with ongoing debate between two primary models: the piezoelectric effect and the triboelectric effect. The piezoelectric effect is generally associated with scenarios where ML materials emit light directly under external pressure and exhibit self-recoverable performance, whereas the triboelectric effect dominates when emission occurs at the interface of layered materials. Previous research on ZnS-based ML systems has focused on phase transitions from the sphalerite phase to the wurtzite phase, as the latter is widely recognized as critical for ML activity. A recent study published in Advanced Materials introduces a transformative approach that not only enhances ML performance but also provides new insights into its underlying mechanism. The authors demonstrate that controllable phase transitions can be used to tune and optimize ML properties in ZnS. For the first time, they report a gradual and reversible transition between hexagonal wurtzite (wt-ZnS) and cubic sphalerite (sp-ZnS) phases at room temperature using low uniaxial pressure in the range of 0–30 MPa, a striking departure from the extreme conditions traditionally required for such transformations. Applying modest pressure with a standard tableting machine converts wt-ZnS into sp-ZnS without the need for high-temperature or high-pressure environments, while subsequent thermal annealing reverses the process, enabling reversible structural cycling. High-resolution transmission electron microscopy reveals that this phase transition is mediated by dislocations: a 1/3⟨1120⟩ screw dislocation decomposes into two 30° Shockley partial dislocations, 1/3⟨1010⟩ and 1/3⟨0110⟩, whose glide converts portions of the wt-ZnS lattice into the sp-ZnS structure. These dislocation-driven rearrangements introduce stacking faults and localized lattice distortions that exert a profound influence on luminescence behavior. Under dynamic loading, charged dislocations facilitate carrier transport toward Mn2+ luminescent centers by altering the energy transfer pathways. Regions undergoing phase transition exhibit stronger mechanoluminescence due to an enhanced local piezoelectric field compared to areas without phase transition. The study further uncovers strikingly different responses for Mn and Cu dopants under pressure. For Mn-doped ZnS, ML intensity increases by up to 2.7 times at moderate pressures around 10 MPa, an enhancement attributed to stronger local piezoelectric fields and more efficient carrier transport. In contrast, Cu-doped ZnS exhibits pronounced ML quenching under similar conditions, underscoring the fundamentally different energy transfer pathways associated with these dopants. Pressure-thermal cycling enables tunable ML and photoluminescence (PL) properties, opening new opportunities for adaptive optical devices. The manuscript also reports on the ML behavior of Mn, Cu co-doped ZnS, which exhibits similar pressure-responsive characteristics to Cu-doped ZnS, but with the emission predominantly originating from Mn2+ centers. The authors propose a dislocation-mediated ML mechanism in which charged dislocations generate local electric fields that alter carrier migration and complexation. Under dynamic loading, the piezoelectric field in ZnS separates carriers, which become trapped at defect levels and subsequently recombine at luminescent centers, leading to light emission.