Long-lived Mn2+ emission in nanocrystalline ZnS:Mn2+

Recently, it has been reported that doped semiconductor nanoparticles can yield both high luminescence efficiencies and a spectacular lifetime shortening, which suggests 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 does not show a spectacular shortening of the decay time upon decreasing particle size as reported earlier. The luminescence of nanocrystalline ZnS:Mn2+ indeed has a short decay time (similar to 100 ns), but also shows a long ms range decay time. The short decay time is ascribed to a defect-related emission of ZnS, and is not from the decay of the T-4(1)-(6)A(1) transition of the Mn2+ impurity as suggested by other authors. The T-4(1)-(6)A(1), 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. [S0163-1829(98)52038-6].