Correlation with surface defects and maiden observation of disorder activated phonon in cubic ZnS QDs

Defects and deformation potential in quantum dots (QDs) were found to control the Raman modes however the disorder activated phonon (DAP) mode could not be seen in the cubic phase ZnS. With a maiden observation of a DAP mode the crucial role of surface defects, in particular, elemental 'S' is reported. The DAP mode was seen with significant intensity at 153 cm^-1 along with the LO mode at 347 cm^-1 for the cubic ZnS. ZnS nanoparticles (NPs) of 3 to 5 nm were synthesized to understand origin of the DAP mode and its correlation with defects. The strongest DAP mode was observed in ZnS QDs of 3 nm size which showed the highest surface defects, in particular, the elemental type sulfur as revealed by the photoluminescence study. With increase in crystal size, the bulk-like property set in with the appearance of a weak DAP mode. Further, the reason behind the unclear observation of the mode in a cubic ZnS crystal near room temperature and effects of unaltered surface defects were investigated by the chemical functionalization with oleic acid and the heat treatment studies. The results revealed existence of a strong correlation between surface defects and synthesis conditions for observation of a DAP peak in cubic ZnS NPs.