CdSe nanocrystals show unique size-dependent optical properties due to the spatial confinement of carriers and the large number of surface atoms [1]. Over the years, many methods for the synthesis of CdSe nanocrystals have been implemented. These methods can be divided into two categorises. The first is an organometallic synthesis based on the high-temperature thermolysis of the precursors or on a dehalosilylation reaction. The other method is the synthesis in an aqueous medium using polyphosphates or thiol as capping agents [2]. In this work, the CdSe nanocrystals were prepared by a new method at room temperature (25 °C) using aqueous solution of cadmium acetate, sodium seleno sulfite, and tri-sodium citrate at a pH of about 7.0. Tri-sodium citrate was used as stabilizing agent. The absorption spectrum shows well defined absorption edge at wavelength of about 512 nm. This peak, which is blue shifted as compared to the bulk CdSe (Eg=1.8 eV) is assigned to the S0→ S1 electronic transition for the first excitonic state of CdSe nanocrystals with band gap (Eg) value of 2.6 eV. The XRD patterns of the CdSe nanocrystals exhibit three distinguished peaks at the angles of 25.6º, 42.7º and 50.8º corresponding to diffraction of the (111), (220) and (311) planes of the cubic phase, respectively. The average nanocrystallites size of CdSe powder is estimated to be about 4.5 nm, which is consistent with one observed (5 nm) from the TEM image.
