Synthesis of Zinc Blende-Phased CdSe Nanocrystals with Size-Tunable Optical Properties and Adjustable Valence Band Positions

CdSe nanocrystals with average sizes of 15, 24, and 32 nm have been synthesized from an aqueous solution of Na2SeSO3, HCl, and cadmium nitrate at 15, 45, and 70 degrees C, respectively, for about 1 h. Mixing aqueous CdCl2, HNO3, and Na2SeSO3 at 22 degrees C for 4 h yields 44 nm CdSe nanocrystals. X-ray and electron diffraction analyses indicate the possession of a zinc blende crystal structure for all the samples. Despite the large particle dimensions, their absorption band red-shifts significantly from 520 to 570 nm with increasing particle sizes, and band gap values decrease from 2.03 eV for 15 nm particles to 1.68 eV for 44 nm crystals. Although these nanocrystals are not emissive, introduction of the cetyltrimethylammonium chloride surfactant during crystal growth can restore their photoluminescence attributed to the improved crystal quality, and the similarly sized CdSe nanocrystals have an emission band red-shifting from 544 nm for 15 nm particles to 583 nm for 47 nm crystals. A band diagram was constructed for these CdSe nanocrystals using information from Mott-Schottky plots. While they have close conduction band positions, the notable size-related band gap variation means that their valence band energies differ considerably with implications of electrochemical and photocatalytic properties. The 44 nm CdSe particles also show the smallest electrochemical charge-transfer resistance.