Copper's Role in the Photoluminescence of AgCu InS Nanocrystals, from Copper-Doped AgInS ( x ∼ 0) to CuInS ( x = 1).

A series of AgCu InS nanocrystals (NCs) spanning from 0 ≤ x ≤ ∼1 was synthesized by partial cation exchange to identify copper's contributions to the electronic structure and spectroscopic properties of these NCs. Discrete midgap states appear above the valence band upon doping AgInS NCs with Cu (small x). Density functional theory calculations confirm that these midgap states are associated with the 3d valence orbitals of the Cu impurities. With increasing x, these impurity d levels gradually evolve to become the valence-band edge of CuInS NCs, but the highest-occupied orbital's description does not change significantly across the entire range of x. In contrast with this gradual evolution, AgCu InS NC photoluminescence shifts rapidly with initial additions of Cu (small x) but then becomes independent of x beyond x > ∼0.20, all the way to CuInS ( x = 1.00). Data analysis suggests small but detectable hole delocalization in the luminescent excited state of CuInS NCs, estimated by Monte Carlo simulations to involve at most about four copper ions. These results provide unique insights into the luminescent excited states of these materials and they reinforce the description of CuInS NCs as "heavily copper-doped NCs" in which photogenerated holes are rapidly localized in copper 3d-based orbitals.