Figure 4. Band edge shifts of ligand/QD complexes.

(a) Experimentally measured R-CA⁻/QD (solid squares) and 4-R-S⁻/QD (open circles) ionization energies as a function of calculated ligand dipole. The variation of ionization energy measurements made for duplicate samples was lower than the instrumental noise; therefore, the uncertainty of the data is less than the width of the data markers. The brown and grey open circles are data taken from Bent et al.¹¹ that correspond to 4-NO₂-S⁻ and 4-F-S⁻ capped PbS QDs, respectively. The blue dashed line is a fit to the data that includes the points associated with non-fluorinated ligands, and the red dashed line is a fit to the data that only includes the points associated with fluorinated ligands. (b) Calculated band edges renormalized by the number of ligands as computed for the three different isolated ligand/QD structural models with varying surface coverage as a function of the projected ligand dipole. (c) The energy of the highest occupied molecular orbital for isolated QDs (filled squares) and the square lattice of monolayer of QDs (open squares). The lines are guides to the eye and the larger data points represent the proposed physically accurate QD film environment—either isolated (closed squares) or inter-digitized monolayer (open squares)—for each R-CA⁻/QD sample based on inter-QD ligand shell electrostatic arguments as described in the text. (d) Side and (e) top view of a 3 × 3 repetition of the unit cell for an example R-CA⁻/QD monolayer.