Surface Fluorination for Controlling the PbS Quantum Dot Bandgap and Band Offset

Fully fluorinated perfluorocarbon ligands are shown to modify the energetics and dielectric environment of quantum dots (QDs), resulting in a large hypsochromic shift in the optical gap. The original oleic acid (OA) ligands on PbS QDs can be completely replaced with thiolate and carboxylate-based perfluorocarbons, e.g., -SCF₃ and CF₃(CF₂)₁₄COOH (pFA), respectively. Ultraviolet photoelectron spectroscopy indicates that the work function varies by >1.3 eV depending on the electronegativity of the surface ligand, while cyclic voltammetry shows that an OA:pFA ratio of ∼2:1 increases the oxidation potential by 0.18 eV in solution. The diminished reduction potential of the conduction band is confirmed by photoinduced electron transfer experiments. The short thiolate ligands, -SCF₃ and -SCH₃, enhance the electron-donating ability of PbS QDs up to 7-fold because of an increase in the permeability of the ligand shell. This work shows that electron-withdrawing halogens like fluorine and chlorine can control the bandgap and band offsets of nanocrystals for the future design and optimization of functional organic/inorganic hybrid nanostructures.