Ligand-Mediated Energy-Level Modification in PbS Quantum Dots as Probed by Density of States (DOS) Spectra

Ligands are known to passivate the surface of semiconductor nanocrystals and consequently alter their Fermi energy and band positions. In this work, we have utilized scanning tunneling spectroscopy (STS) to record differential tunnel conductance spectra (dI/dV) of PbS nanocrystals; since dI/dV has a correspondence to the semiconductors’ density of states, their band edges with respect to the Fermi energy could be located directly from the dI/dV spectra. With a series of ligands in PbS quantum dots (QDs), ligand-dependent shift in Fermi energy has been observed. From the location of Fermi energy relative to the conduction and valence band edges, the STS measurements allowed determination of the semiconductor type, which could be correlated to the functional groups of the ligands. While thiol-based ligands introduced excess sulfur, resulting in lead-vacancies and thereby p-nature in PbS QDs, other ligands yielded n-type QDs due to their electron-donating nature. The p- and n-type nanocrystals, when cast sequentially to form pn- or np-homojunctions, resulted in current rectification due to type II band alignment at the interface.