Doped semiconductor nanoparticles - a new class of luminescent materials?

Five years ago it was reported that nanocrystalline ZnS:Mn2+ can yield both high luminescence efficiencies and a spectacular lifetime shortening, which suggested that doped semiconductor nanoparticles form a new class of luminescent materials for various applications. From lifetime measurements and time-resolved spectroscopy we conclude that the Mn2+ emission of nanocrystalline ZnS:Mn2+ does in fact not show a spectacular shortening of the decay time upon decreasing particle size. The luminescence of nanocrystalline ZnS:Mn2+ indeed shows a short decay time component (approx. 100 ns), but also a long (ms) decay time is observed. The short decay time is ascribed to a defect-related emission of ZnS, and is not from the decay of the 4T1-6A1 transition of the Mn2+ impurity. The 4T1-6A1 transition of the Mn2+ has a 'normal' decay of about 1.9 ms. Based on our observations, we conclude that doped semiconductor nanoparticles do not form a new class of luminescent materials, combining a high efficiency with a short (ns) decay time.