An Urbach study of the degree of structural disorder in CdTe nanocrystals in glass matrix

Abstract

CdTe nanocrystals embedded in matrixes such as, glass, plastic and, aqueous solution have been investigated extensively due to their potential applications in nonlinear optical devices, biotechnology and medicine. When the size of bulk semiconductor is reduced to nanometer scales, the crystal gains new optical and electronic properties. Energy levels are quantized as in atoms and the energy difference between levels widens with the inverse of the size squared. CdTe nanocrystals were grown in glass by solid-state phase precipitation at the above glass transition temperature. A size dependent blue shift in the asymptotic absorption edge was observed in optical absorption spectra. The crystalline composition was studied by Resonant Raman spectroscopy. The width of the optical absorption tail at long wavelengths depends on the spread of electron energy levels within the structure, called the Urbach energy, which was calculated depending on size. We observe that the Urbach tail narrows as the surface to volume ratio of the nanocrystals decreases.