Molecular insight into intrinsic-trap-mediated emission from atomically precise copper-based chalcogenide models

Luminescent Cu-doped semiconductor nanocrystals have long played a pivotal role in the advancement of lighting and display technologies. The luminescence observed in colloidal copper-based I-III-VI nanocrystals is attributed to defect emission arising from donor-acceptor pair recombination of excited charge carriers. However, a detailed atomic-level exploration of how distinct chemical components precisely influence the defect position has remained challenging, primarily due to inherent local structural imprecision of the traditional I-III-VI nanocrystals. In this study, we have prepared a set of copper-containing I-III-VI metal chalcogenide nanoclusters, 1-CuInS, 1-CuGaS, and 2-CuGaS, serving as unique models to address the aforementioned issues. Interestingly, despite possessing an identical crystalline structure, 1-CuInS and 1-CuGaS exhibit significantly different photoluminescence behaviors. For comparsion, 1-CuGaS and 2-CuGaS, which share the same second building units but differ in structural configuration, demonstrate similar luminescence performance. More importantly, we found that the green emission observed in 1-CuInS likely stems from the radiative recombination of electrons migrating from shallow delocalized traps to copper-localized holes. In contrast, the red emission observed in both 1-CuGaS and 2-CuGaS is presumably due to the recombination of electrons originating from deeply localized traps with copper-localized holes. This disparity in trap sites appears to be highly dependent on the presence of trivalent metal ions (In3+ and Ga3+) within the clusters, and the hypothesis is further substantiated through photoluminescence characterization of 1-CuInGaS containing both In3+ and Ga3+ ions simultaneously. Furthermore, we have explored the impact of introducing Cd ions into 1-CuInS, which can alter the position of shallow delocalized traps and thereby fine-tune the luminescence properties. Our findings shed light on the intricate interplay of chemical composition and defect states in copper-containing I-III-VI nanoclusters, offering valuable insights into the optoelectronic properties of copper-based semiconductor nanocrystals.