Size and shape-dependent electron-hole relaxation dynamics in US nanocrystals

The dynamics of photoluminescence (PL) in CdS nanocrystals (NCs) with various sizes and shapes dispersed in water-in-oil microemulsions (reverse micelles) are studied with picosecond-resolved time correlated single photon counting (TCSPC), streak camera and nanosecond resolved gated PL spectroscopic techniques. The time resolved PL-spectra exhibit several discrete features, which are not pronounced in steady state PL spectra. In quantum dots (QD), slower carrier recombination dynamics are observed in smaller sized NCs. The relaxation of the excited state carriers in a quantum rod (QR) is found to be slower at early time and faster in the longer time scales compared to those in QD with similar volume. The signature of thermoluminescence (TL) of CdS-QR in low temperature (77 K) is clearly evident from our studies. We also study the effect of the shape of the NCs on their PL anisotropy. The steady state and time resolved PL anisotropy of a QR show significantly different behavior compared to those of a QD with similar volume. The results of the NCs are important in the context of designing better, more effective physical and biological PL probes. (c) 2006 Elsevier B.V. All rights reserved.