We studied the coupling and impact of ligands on the QD optical and electrical properties.  We demonstrated that the bandedge energies can be shifted by over 2 eV for a QD absorber with 1 eV bandgap.  We also demonstrated that the addition of ligands causes the optical absorption of the QD/ligand complex to increase due to electronic coupling between the QD and ligands.  The coupling increases for smaller ligand optical gaps.  We utilize the enhanced absorbance of the QD/ligand to construct ligand adsorption isotherms. We model these isotherms with a 2-d square lattice model, which allows us to extract differences in trends of binding free energies and nearest neighbor coupling. As expected oleate binds more strongly than any of the functionalized cinnamates, but the binding preference is mitigated by dipole-dipole interactions for both large positive and negative dipoles. We explain this trend in binding free energy as a function of dipole moment via a collective electrostatic interaction with the lattice. For cinnamic acids with electron withdrawing molecular dipoles (negative dipoles), the isotherms show behavior associated with strong nearest neighbor association that causes the ligand exchange reaction to display a phase transition from all oleate coverage to all cinnamate coverage as more cinnamate is added, with a sharpness dictated by the ligand dipole moment: more negative dipole moments leads to sharper order-disorder phase transitions than those observed with positive dipole moments, as a function of ligand addition.  Using these observations, we prepared PbS QD with Janus-shell ligands.

We developed a facile method to prepare n and p-doped PbSe QDs via a post-synthetic cation exchange technique. Quantitative XRD analysis suggests a substitutional doping mechanism, with the lattice parameters decreasing upon either Ag⁺ or In³⁺ incorporation. A significant bleach of the first excitonic transition is observed, which is coupled with the appearance of a size-dependent intraband absorption in the NIR, indicating a successful introduction of electron/hole impurities dopants. We also observe a decrease of PLQY and a faster exciton decay with higher cation incorporation. Spectroelectrochemical measurements show a characteristic n-type behavior, which agrees with the substitutional doping mechanism of In³⁺ in PbSe. We proposed a model whereby the majority of the added cations remains at the QD surface and do not interact with the PbSe QD core states.  Small amounts of excess cations diffuse into the lattice and establish equilibrium between surface-bound and lattice-incorporated cation dopants.