Mid-Gap States and Normal vs Inverted Bonding in Luminescent Cu~+- and Ag~+-Doped CdSe Nanocrystals

摘要

Mid-gap luminescence in copper (Cu~+)-doped semiconductor nanocrystals (NCs) involves recombination of delocalized conduction-band electrons with copper-localized holes. Silver (Ag~+)-doped semiconductor NCs show similar mid-gap luminescence at slightly (~0.3 eV) higher energy, suggesting a similar luminescence mechanism, but this suggestion appears inconsistent with the large difference between Ag~+ and Cu~+ ionization energies (~1.5 eV), which should make hole trapping by Ag~+ highly unfavorable. Here, Ag~+-doped CdSe NCs (Ag~+:CdSe) are studied using time-resolved variable-temperature photoluminescence (PL) spectroscopy, magnetic circularly polarized luminescence (MCPL) spectroscopy, and time-dependent density functional theory (TD-DFT) to address this apparent paradox In addition to confirming that Ag~+:CdSe and Cu~+:CdSe NCs display similar broad PL with large Stokes shifts, we demonstrate that both also show very similar temperature-dependent PL lifetimes and magneto-luminescence. Electronic-structure calculations further predict that both dopants generate similar localized mid-gap states. Despite these strong similarities, we conclude that these materials possess significandy different electronic structures. Specifically, whereas photogenerated holes in Cu~+:CdSe NCs localize primarily in Cu(3d) orbitals, formally oxidizing Cu~+ to Cu~(2+), in Ag~+:CdSe NCs they localize primarily in 4p orbitals of the four neighboring Se~(2-) ligands, and Ag~+ is not oxidized. This difference reflects a shift from "normal" to "inverted" bonding going from Cu~+ to Ag~+. The spectroscopic similarities are explained by the fact that, in both materials, photogenerated holes are localized primarily within covalent [MSe_4] dopant clusters (M = Ag~+, Cu~+). These findings reconcile the similar spectroscopies of Ag~+- and Cu~+-doped semiconductor NCs with the vastly different ionization potentials of their Ag~+ and Cu~+ dopants.