Photoluminescence of doped ZnS nanoparticles under hydrostatic pressure

The pressure dependence of the photoluminescence from ZnS : Mn ²⁺, ZnS : Cu²⁺, and ZnS : Eu²⁺ nanoparticles were investigated under hydrostatic pressure up to 6 GPa at room temperature. Both the orange emission from the ⁴T₁ - ⁶A ₁ transition of Mn²⁺ ions and the blue emission from the DA pair transition in the ZnS host were observed in the Mn-doped samples. The measured pressure coefficients are -34.3(8) meV/GPa for the Mn-related emission and -3(3) meV/GPa for the DA band, respectively. The emission corresponding to the 4f⁶5d¹ - 4f⁷ transition of Eu²⁺ ions and the emission related to the transition from the conduction band of ZnS to the t₂ level of Cu²⁺ ions were observed in the Eu- and Cu-doped samples, respectively. The pressure coefficient of the Eu-related emission was found to be 24.1(5) meV/GPa, while that of the Cu-related emission is 63.2(9) meV/GPa. The size dependence of the pressure coefficients for the Mn-related emission was also investigated. The Mn emission shifts to lower energies with increasing pressure and the shift rate (the absolute value of the pressure coefficient) is larger in the ZnS : Mn²⁺ nanoparticles than in bulk. Moreover, the absolute pressure coefficient increases with the decrease of the particle size. The pressure coefficients calculated based on the crystal field theory are in agreement with the experimental results.