Optical properties of manganese-doped nanocrystals of ZnS

We report for the first time that doped nanocrystals of semiconductor can yield both high luminescent efficiencies and lifetime shortening at the same time. Nanocrystals of Mn-doped ZnS with sizes varying from 3.5 to 7.5 nm were prepared by a room temperature chemical process. These nanosized particles have an external photoluminescent quantum efficiency as high as 18% at room temperature and a luminescent decay at least 5 orders of magnitude faster than the corresponding ${\mathrm{Mn}}^{2+}$ radiative transition in the bulk crystals. Luminescent measurements show that the efficiency increases with decreasing size of the particles, as expected within the framework of an electron-hole localization theory. These results suggest that doped nanocrystals are indeed a new class of materials heretofore unknown.