Enhancement of photoluminescence of ZnS:Mn nanocrystals modified by surfactants with phosphate or carboxyl groups via a reverse micelle method

A colloidal suspension of ZnS:Mn nanocrystals was prepared in sodium bis(2-ethylhexyl)suflosuccinate reverse micelles, and then modified by surfactants with phosphate or carboxyl groups. The photoluminescent intensity at 580 nm due to d–d transition of Mn2+ ions increases up to a factor of 6.3 and its quantum efficiency increases from 1.7% to 8.1% after modification. According to 31P nuclear magnetic resonance spectra, surfactants with phosphate groups adsorb on the surface of ZnS nanocrystal and 31P nucleus spins are relaxed rapidly by interaction with five unpaired 3d electrons of Mn2+, showing that phosphate groups are located in the vicinity of Mn2+. The excitation spectra for the emission due to phosphate or carboxyl groups are similar to those for the emission at 580 nm corresponding to the excitation of ZnS. Both excitation spectra shift in parallel with increasing the amount of surfactant to show the linear relationship. We, therefore, attribute the increase in quantum efficiency at 580 nm to additional energy transfer of ZnS→functional groups→Mn2+ as well as to the reduction of energy loss due to non-radiative transition by surface modification.