Synthesis and characterization of surface-modified colloidal CdTe quantum dots

The controlled synthesis of quantized colloidal CdTe nanocrystals (in aqueous solutions) with narrow size distributions and stabilized against rapid oxidation was achieved by capping the quantum dot particles with 3-mercapto-1,2-propanediol. Nanocrystals (i.e., quantum dots) with mean diameters of 20, 25, 35, and 40 Å were produced. Optical absorption spectra showed strong excitonic peaks at the smallest, size; the absorption coefficient was shown to follow an inverse cube dependence on particle diameter, while the extinction coefficient per particle remained constant. The quantum yield for photoluminescence increased with decreasing particle size and reached 20% at 20 Å. The valence band edges of the CdTe quantum dots were determined by pulse radiolysis experiments (hole injection from oxidizing radicals); the bandgaps were estimated from pulse radiolysis data (redox potentials of hole and electron injecting radicals) and from the optical spectra. The dependence of the CdTe bandgap on quantum dot size was found to be much weaker than predicted by the effective mass approximation; this result is consistent with recently published theoretical calculations by several groups.