Methods for preparing emitting semiconductor nanocrystals (NCs) several nm in size have been greatly advanced over the last 20 years.[1–8] The high brightness and wavelength tunability of NCs with one excitation wavelength make them a new type of phosphor especially well-suited for biological tagging.[9, 10] These NCs usually require an appropriate matrix to prevent them from deteriorating. Sol–gel derived silica glass is one of the best candidates for such a matrix.[11, 12] Other than being used as a protection layer, glass matrices have been used to grow NCs by heating after the constituent ions of the NCs become oversaturated. The glass matrix acts as a distinctive reaction field, having solubility and diffusion constants as much as several orders of magnitude different from those of normal solutions.[13, 14]

In this communication, we report an anomalous photoluminescent (PL) behavior observed in silica-coated semiconductor NCs after heat treatment. There is a drastic increase in PL efficiency accompanied by a red-shift of the peak wavelength and a narrowing of the PL spectral width. The size of the NCs remains unchanged after this treatment. Since the prepared glass beads (heated silica-coated NCs) are more robust than the bare NCs, this phenomenon is applicable to several applications. Study of all the characterization available together with calculations led us to conclude that the phenomenon can be explained by the formation of small clusters (1 nm or less in size) in the vicinity of the NCs in the silica layers during heating.