Origin of mechanoluminescence from Mn-activatedZnAl2O4: Triboelectricity-induced electroluminescence

Luminescence induced by friction, mechanoluminescence (ML) has been observed for ${\text{ZnAl}}_{2}{\mathrm{O}}_{4}:$ ${\text{Mn}}^{2+}$ $(\text{ZAO}:\text{Mn})$ fabricated by systematically controlling the reducing temperature. The reducing treatment produced lattice defects under a reducing atmosphere. Those defects were associated with $\text{Zn}$ and $\mathrm{O}$ vacancies through evaporation of $\text{ZnO}$ in ${\text{ZnAl}}_{2}{\mathrm{O}}_{4}$, which was trapped with a large amount of carrier in the spinel. Results of dependence of $\text{ML}$ intensity and integrated intensity for thermoluminescence on the reducing temperature showed that the trapped carrier plays an important role in producing the $\text{ML}$ for $\text{ZAO}:\text{Mn}$. In addition, the $\text{ML}$ for $\text{ZAO}:\text{Mn}$ was strongly dependent upon the friction rod material; it was closely related to the surface voltage generated in the vicinity of the frictional surface. These results suggest that the $\text{ML}$ for $\text{ZAO}:\text{Mn}$ was caused by the effect of triboelectrification, but not piezoelectricity because ${\text{ZnAl}}_{2}{\mathrm{O}}_{4}$ has a centrosymmetric structure $(Fd3m)$. Therefore, the carrier that is trapped in the spinel can be excited by the local electric field derived from friction between the two dissimilar materials, where the excited carrier is accelerated toward the luminescent center of the ${\text{Mn}}^{2+}$ ions. Consequently, the ${\text{Mn}}^{2+}$ ions are excited and release an emission band on the transition from $^{4}T_{1}$ to $^{6}A_{1}$. Evidence for these physical processes was corroborated from the finding that reduced $\text{ZAO}:\text{Mn}$ showed highly efficient electroluminescence (EL). Therefore, it is inferred that the $\text{ML}$ for $\text{ZAO}:\text{Mn}$ is caused by triboelectricity-induced $\text{EL}$.