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

Area of Science:

• Materials Science
• Nanotechnology
• Quantum Chemistry

Background:

• Lead sulfide (PbS) colloidal quantum dots (CQDs) offer tunable near-infrared bandgaps, making them suitable for photovoltaics, light-emitting devices, and photodetectors.
• Optimizing these devices necessitates a thorough understanding of their electronic structure.
• Existing methods like ultraviolet photoelectron spectroscopy (UPS) are limited to surface analysis and cannot probe core electronic states.

Purpose of the Study:

• To comprehensively determine the band structures of ligand-functionalized PbS CQDs.
• To investigate the depth-dependent electronic properties from the surface to the core.
• To elucidate the factors influencing the surface and core electronic characteristics.

Main Methods:

• Combined ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) for simultaneous band structure determination.
• Utilized gas cluster ion beam (GCIB) sputtering for depth profiling analysis.
• Analyzed the electronic states of PbS CQDs before and after surface treatment.

Main Results:

• Initially, PbS CQDs exhibit a weak n-type surface due to oxygen compensation of n-type iodine ligands, while the sulfur-rich core shows a strong p-type structure.
• After removing surface oxygen, the iodine-passivated surface becomes n+-type, and the core remains weakly p-type.
• Ensemble-averaged measurements revealed a distinct n+-p transition at the surface of the PbS CQDs.

Conclusions:

• The observed surface sharp n+-p transition is attributed to the interplay between iodide-induced electron donation and oxygen-induced compensation at the surface.
• The sulfur-rich core composition further contributes to the observed electronic structure.
• Understanding this transition is critical for advancing PbS CQD-based device performance.