Effects of structure on photoluminescence characteristics of Mn2+-doped ZnS quantum dots for anti-counterfeiting ink application

Abstract

The zinc-blende-to-wurtzite transition of thioglycolic acid (TGA)-capped ZnS:Mn2+ QDs prepared at 80 °C in air has been observed with the increasing TGA/Zn2+ ratios. Effects of structure on the photoluminescence characteristics of ZnS:Mn2+ QDs were studied in this paper. The relative PL intensity ratio of the Mn2+ orange-red emission and the ZnS blue emission (IOE/IBE) and color purity (CP) (CP ≈ 65.6%) were maximum with the optimal molar ratio of TGA/Zn2+ = 3, which indicate the efficient transmission of excitation energy from the ZnS host to the Mn2+ impurity. Furthermore, the investigated result shows that the energy transfer in the cubic ZnS host lattice is more efficient than that of the hexagonal ZnS and the color correlated temperature (CCT) values increase with increasing the TGA/Zn2+ ratio. The crystal field strength (Dq) values of ZnS:Mn2+ QDs decrease from 603 cm−1 to 455 cm−1 with the increasing of the TGA/Zn2+ ratio from 0 to 6, respectively. The ZnS:Mn2+ QDs were applied in anti-counterfeiting ink, which was written on a glass sheet. The image was then excited by normal light and 385 nm UV light excitation, which emitted orange-red color with the UV light excitation. These investigated results can be particularly suitable for security ink applications or optoelectronic devices. Moreover, the zinc-blende-to-wurtzite phase transition in this study will be of great importance for the family of ZnS-based doped quantum dots.