Tailored Band Edge Positions by Fractional Ligand Replacement of Nonconductive Colloidal Quantum Dot Films

The tunable band edge position of colloidal quantum dot (CQD) films is a key part of efficient optoelectronic device design of various forms such as photovoltaics, light-emitting diodes, and photodetectors. An accurate estimation of shifts in the band edge position of CQD layers is still considered challenging, especially when the CQD films are nonconductive. Here, we investigate the effect of nonconductive CQD films on photoelectron spectroscopy (PES) and photoelectron yield spectroscopy (PYS). We demonstrate control of systematic band edge positions by fractional ligand replacement of nonconductive CQD film characterized with photoelectron yield spectroscopy and density functional theory calculations. As-synthesized CQDs with insulating oleate ligands were fractionally replaced with trans-3,5-difluorocinnamic acid molecules in a nonpolar solution. The fractionally replaced surface-bound ligands are quantitatively analyzed using 1H and 19F nuclear magnetic resonance (NMR) spectroscopy. We found that the energy levels of nonconducting CQD films shift linearly as a function of the number of bound trans-3,5-difluorocinnamates with specific dipole moments while retaining hydrophobic wettability.