CdS Nanoclusters: Synthesis, Characterization, Size Dependent Oscillator Strength, Temperature Shift of the Excitonic Transition Energy, and Reversible Absorbance Shift

Improved synthetic routes and size-selective precipitation have enabled the preparation of almost monodisperse CdS clusters. Six samples of 1-thioglycerol stabilized clusters with diameters of approximately 13, 14, 16, 19, 23, and 39 Å have been prepared as fully redispersible powders and were characterized by elemental analysis, powder X-ray diffraction, electron microscopy, thermogravimetric analysis, and UV-vis spectroscopy. Small-angle X-ray scattering was used to determine the mean cluster size. The well-structured UV-vis spectra reveal that the size-dependent shift of the 1s-1s excitonic transition is in agreement with the tight-binding theory and the pseudopotential theory. Moreover, as expected by quantum mechanical calculations the oscillator strength of the transition increases proportional to 1/r3. UV-vis spectra taken at various temperatures between 4 and 295 K have shown that the temperature shift of the excitonic transition energy becomes stronger with decreasing particle size. Strong, reversible absorbance shifts were observed, upon transferring the clusters from their solutions onto quartz plates and vice versa.