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Computational Studies of the Electronic Structures of Copper-Doped CdSe Nanocrystals: Oxidation States, Jahn–Teller Distortions, Vibronic Bandshapes, and Singlet–Triplet Splittings

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journal contribution
posted on 16.02.2016, 00:00 by Heidi D. Nelson, Xiaosong Li, Daniel R. Gamelin
The electronic structures of copper-doped CdSe nanocrystals (NCs) are investigated using time-dependent density functional theory. Comparison of the electronic structures of Cu+- and Cu2+-doped NCs indicates that only the Cu+ ground state is consistent with the experimental absorption and photoluminescence (PL) spectra of copper-doped NCs, Cu2+-doped NCs being characterized by low-energy charge-transfer and d–d excited states that quench visible PL. In the luminescent metal-to-conduction-band charge-transfer (MLCBCT) excited state of the Cu+-doped CdSe NCs, the photogenerated hole is calculated to be localized at the copper dopant. Strong electron–phonon coupling in this MLCBCT excited state causes substantial geometric distortion along totally symmetric and Jahn–Teller nuclear coordinates, with a correspondingly large excited-state nuclear reorganization energy. This excited-state nuclear reorganization causes the broad PL band shape and large PL Stokes shift observed experimentally. Singlet and triplet MLCBCT excited-state configurations are also examined computationally. The sign and strength of the computed magnetic exchange coupling between the conduction-band electron’s spin and the copper-localized spin are both consistent with experimental results. These calculations yield fundamental insights into the electronic structures and photophysical properties of copper-doped semiconductor NCs relevant to their potential application as spectral conversion phosphors in lighting and solar technologies.