Doped semiconductor nanoparticles - a new class of luminescent materials?

\u3cp\u3eFive years ago it was reported that nanocrystalline ZnS:Mn\u3csup\u3e2+\u3c/sup\u3e 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 Mn\u3csup\u3e2+\u3c/sup\u3e emission of nanocrystalline ZnS:Mn\u3csup\u3e2+\u3c/sup\u3e does in fact not show a spectacular shortening of the decay time upon decreasing particle size. The luminescence of nanocrystalline ZnS:Mn\u3csup\u3e2+\u3c/sup\u3e 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 \u3csup\u3e4\u3c/sup\u3eT\u3csub\u3e1\u3c/sub\u3e-\u3csup\u3e6\u3c/sup\u3eA\u3csub\u3e1\u3c/sub\u3e transition of the Mn\u3csup\u3e2+\u3c/sup\u3e impurity. The \u3csup\u3e4\u3c/sup\u3eT\u3csub\u3e1\u3c/sub\u3e-\u3csup\u3e6\u3c/sup\u3eA\u3csub\u3e1\u3c/sub\u3e transition of the Mn\u3csup\u3e2+\u3c/sup\u3e 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.\u3c/p\u3