Observation of Donor-Bound Excitons in n-Type-Doped Two-Dimensional Semiconductor Nanocrystals

The optical and electronic properties of semiconductor nanocrystals are customizable by introducing electronic states into their band gap. In II-VI semiconductors, heterovalent p-type dopants such as Cu and Ag create localized acceptor states near the valence band, forming acceptor-bound excitons that have been extensively studied. In contrast, donor-bound excitons have remained elusive, as n-type dopants (e.g., indium) typically enhance electron transport properties without influencing radiative recombination dynamics. Here, we present direct experimental evidence of an emissive donor state in indium-doped CdSe nanoplatelets, synthesized via a colloidal method. Using a combination of spectroscopic techniques, we observe donor-bound exciton emission characterized by a lifetime 2 orders of magnitude shorter and a bandwidth twice as narrow as those of Ag/Cu-based acceptor states. Notably, the donor-bound exciton emission remains invariant with dopant concentration, unlike acceptor-bound excitons, which show concentration-dependent spectral shifts, as supported by band structure calculations. These results demonstrate the formation of a three-level electronic system enabled by n-type doping, revealing previously inaccessible photophysical behavior in colloidal semiconductor nanocrystals, with implications for optoelectronic device engineering.