Bright Blue Emitting Cu-doped Cs2ZnCl4 Colloidal Nanocrystals

We report here the synthesis of undoped and Cu-doped Cs₂ZnCl₄ nanocrystals (NCs), in which we could tune the concentration of Cu from 0.7% to 7.5%. According to electron paramagnetic resonance analysis, in 0.7% and 2.1% Cu-doped NCs the Cu ions were present in the +1 oxidation state only, while in NCs at higher Cu concentrations we could detect Cu(II) ions. The undoped Cs₂ZnCl₄ NCs were non emissive, while the Cu-doped samples had a bright intra-gap photoluminescence (PL) at 2.6eV mediated by band-edge absorption. The PL quantum yield was maximum (~55%) for the samples with low Cu concentration (≤ 2.1%) and it systematically decreased when further increasing the concentration of Cu, reaching 15% for the NCs with the highest doping level (7.5%). Density functional theory calculations indicated that the PL emission could be ascribed only to Cu(I) ions: these ions introduce intra-gap states that promote the formation of self-trapped excitons, through which an efficient emission takes place.