Luminescence quantum efficiency of nanocrystalline ZnS : Mn2+. 1. Surface passivation and Mn2+ concentration

The luminescence quantum efficiency of nanocrystalline ZnS:Mn2+ is studied to provide a better understanding on how the quantum efficiency is influenc ed by the Mn2+ concentration, the nature of the passivating polymer, and th e synthesis conditions. The results show an increase of the luminescence qu antum efficiency with the Mn2+ concentration in the nanocrystals for very l ow Mn2+ concentrations. Between 0.3 and 1.5 at. % Mn2+ the increase in quan tum efficiency levels off, to reach an almost constant level between 1.5 an d 5.6 at. % Mn2+. Up to a concentration of 5.6 at. %, no concentration quen ching is observed. The influence of the nature of the passivating polymer i s investigated by comparing the luminescence quantum efficiencies for nanop articles coated with poly(vinylbutyral) (PVB), poly(vinyl alcohol) (PVA), m ethacrylic acid (MA), or sodium polyphosphate (PP) or without a passivating polymer. For the presently used synthesis method (in water), the highest q uantum efficiencies (around 4%) are obtained for nanocrystalline ZnS:Mn2+ c apped with PP. Nanoparticles synthesized in a nitrogen atmosphere have high er quantum yields than nanoparticles made in ambient air. In general, large variations in luminescence properties are observed due to unintentional va riations in the synthesis conditions. For research on the luminescence prop erties and quantum efficiencies of nanocrystalline ZnS:Mn2+, it is very imp ortant to check the reproducibility of results, to standardize synthesis co nditions, and to measure absolute quantum efficiencies rather than relative changes in luminescence intensity.