Revelation of Core-Surface p-n Junction Structures for PbS Quantum Dots

PbS colloidal quantum dots (CQDs) with tunable near-infrared bandgaps are promising for applications in photovoltaics, light-emitting devices, and photodetectors. Device optimization requires a clear understanding of the electronic structure. Photoelectron spectroscopy probes the occupied states of ligand-functionalized PbS CQDs. However, ultraviolet photoelectron spectroscopy (UPS) probes only the electronic structure at the CQD surface and cannot access core electronic states. To obtain comprehensive surface and core information, we combined UPS and X-ray photoelectron spectroscopy (XPS) to simultaneously determine band structures and employed a gas cluster ion beam (GCIB) sputtering for depth profiling. The CQDs are initially covered by iodine and oxygen; the oxygen acts as an acceptor that partially compensates the heavily n-type iodine ligands, resulting in a weak n-type surface, whereas the sulfur-enriched core exhibits a strong p-type structure. After removal of surface oxygen, the surface of the iodine-passivated quantum dots exhibits n⁺-type, and the core demonstrates weak p-type. These ensemble-averaged measurements reveal a surface sharp n⁺-p transition in PbS CQDs, arising from competition between iodide-induced electron donation and oxygen-induced compensation at the surface together with the sulfur-rich core composition.