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

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.