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The defect structure for Cu(2+) in CdSe nanocrystals is theoretically studied by analyzing the spin Hamiltonian parameters of this impurity center. This center is ascribed to Cu(2+) occupying the octahedral interstitial site, rather than the tetrahedral substitutional Cd(2+) site proposed by previous work. The Cu(2+) center exhibits slight terragonal elongation distortion (characterized by the elongation parameter rho approximate to 0.03) due to the Jahn-Teller effect. The theoretical spin Hamiltonian parameters and optical transition show good agreement with the experimental data. The above unusual defect structure (occupation and symmetry) for Cu(2+) in CdSe nanocrystals is discussed, as compared with the conventional trigonally distorted tetrahedral Cu(2+) centers in bulk II-VI semiconductors. (C) 2009 Elsevier B.V. All rights reserved.