Abnormal Thermal Properties of ZnS:Mn2+ Nanophosphor

Red Emission from Copper-Vacancy Color Centers in Zinc Sulfide Colloidal Nanocrystals

“…Previous reports of blueshifted R-Cu emission upon increasing temperature from bulk ZnS:Cu were attributed to changing occupation in the vibrational levels of a highly localized center . However, that explanation is not consistent with the saturation in the blueshift at high temperatures, which we clearly observe and which also appears to occur in their measurement around 200 K. Characteristic defect PL in bulk and nanocrystalline ZnS:Mn also exhibits a blueshift upon increasing temperature with magnitudes between 25 and 80 meV, which has been attributed to crystal field variations due to lattice expansion. , The crystal field explanation would also not predict saturating behavior, and accordingly the temperature-dependent blueshift of ZnS:Mn defect PL does not saturate, in contrast to the R-Cu observations. As an additional indication that crystal field effects cannot sufficiently explain the measured R-Cu blueshift, we calculate only a 16 ± 0.01 meV increase in the energy separation between Cu Zn and V S levels of the neutral complex upon performing DFT computations with different ZnS lattice constants, corresponding to 0 and 300 K based on the ZnS thermal expansion coefficient .…”

“…As an additional indication that crystal field effects cannot sufficiently explain the measured R-Cu blueshift, we calculate only a 16 ± 0.01 meV increase in the energy separation between Cu Zn and V S levels of the neutral complex upon performing DFT computations with different ZnS lattice constants, corresponding to 0 and 300 K based on the ZnS thermal expansion coefficient. 38 It is worth noting that in nanocrystalline ZnS:Mn, NTQ has also been reported between 50 and 300 K, with positive thermal quenching resuming above 300 K. 37 The authors attributed the NTQ to the thermal depopulation of localized trap states created by lattice defects, organic impurities, and surface defects, which are all expected to be more prevalent in NCs compared to bulk materials. None of the above alternative models for the R-Cu blueshift and NTQ can explain the clear presence of two distinct emission peaks with different radiative lifetimes at low temperature, as we observe in Figure 6d, nor do they capture the correspondence in Figure 6e between the NTQ regime and the blueshift occurring at approximately the same temperature.…”