Origin of mechanoluminescence from Mn-activated ZnAl₂O ₄: Triboelectricity-induced electroluminescence

Luminescence induced by friction, mechanoluminescence (ML) has been observed for ZnAl₂O₄: Mn²⁺ (ZAO:Mn) fabricated by systematically controlling the reducing temperature. The reducing treatment produced lattice defects under a reducing atmosphere. Those defects were associated with Zn and O vacancies through evaporation of ZnO in ZnAl ₂O₄, which was trapped with a large amount of carrier in the spinel. Results of dependence of ML intensity and integrated intensity for thermoluminescence on the reducing temperature showed that the trapped carrier plays an important role in producing the ML for ZAO:Mn. In addition, the ML for ZAO: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 ML for ZAO:Mn was caused by the effect of triboelectrification, but not piezoelectricity because ZnAl₂O 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 Mn²⁺ ions. Consequently, the Mn²⁺ ions are excited and release an emission band on the transition from ⁴T₁ to ⁶A₁. Evidence for these physical processes was corroborated from the finding that reduced ZAO:Mn showed highly efficient electroluminescence (EL). Therefore, it is inferred that the ML for ZAO:Mn is caused by triboelectricity-induced EL.