Preparation of silica-encapsulated ZnSe nanocrystals by mixed surfactant microemulsions

Silica-encapsulated ZnSe nanocrystals (NCs) have been prepared by employing sodium (2-ethylhexyl)sulfonate (AOT)/water/cyclohexane microemulsions containing ZnSe quantum dots with polyoxyethylenenonylphenylether (NP5)/water/cyclohexane microemulsions containing tetraethylorthosilicate (TEOS). Size tunable silica nanoparticles were achieved by using various water-to-surfactant ratios, W ([H2O]/[surfactant]). In order to characterize as-synthesized nanocrystals, photoluminescence (PL) spectroscopy, UV-visible spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) were employed. Cubic zinc blende quantum dots (QDs) (2.5 nm in diameter) were successfully encapsulated by silica nanoparticles (14.3-28.9 nm in diameter). The enhanced photoluminescence efficiency about 10% has been observed when compared with the results from the similar work using AOT microemulsions. The entire size of the silica-encapsulated nanocrystals increased with increasing W, then decreased slightly when free water exists in the core of the NP5/AOT microemulsion. On the other hand, the ZnSe NCs in the silica did not change their size during the synthesis. In addition, the possible mechanisms for growth of silica particles and the kinetics of silica particles formation were discussed.