Direct Observation of Near-Surface Band Bending in Plasmonic Metal Oxide Nanocrystals

Band bending at semiconductor surfaces and interfaces is a key factor governing charge transport, carrier dynamics, and device performance; yet, direct experimental methods for probing band bending in semiconductor nanocrystals (NCs) have been limited. Here, we introduce an energy-dependent X-ray photoelectron spectroscopy (ED-XPS) approach as a generalizable technique to directly observe band bending in degenerately doped NCs. Experiments and simulations reveal that angle-dependent XPS lacks depth sensitivity for NCs due to their curvature, whereas ED-XPS reliably resolves depth-dependent shifts in core-level binding energies, allowing for quantification of band bending trends. Applying this method to a variable doping series of tin-doped indium oxide NCs, we observe a direct correlation between Sn doping and band bending, which supports previously hypothesized descriptions of a surface depletion layer originating from band bending. This ED-XPS approach extends to investigate doped cubic-shaped NCs, demonstrating applicability across different morphologies and doping compositions. We show that this technique can also be used to study how postsynthetic chemical surface modification affects band bending. Using two different surface modification chemistries, dipolar ligands and redox-active phosphorus species, we observe changes in band bending influenced by either tuning the built-in potential or reactively removing surface hydroxyls and donating free electrons, respectively. These results establish ED-XPS as a powerful technique to directly observe band bending for a wide range of NCs with different morphologies, chemical doping, or surface chemistry, providing new opportunities to engineer semiconductor nanomaterials for optoelectronic, catalytic, and sensing applications.