Pan Xia1 MingLee Tang1

1, University of California, Riverside, Riverside, California, United States

Band offsets of semiconductors is critical for their functionality in numerous optoelectronic applications such as photovoltaics, photoelectrochemistry and light emitting diodes. Here, it is the first time that around 0.2 nm R-CF3 and 2 nm CF3(CF2)14-R (R is the binding group) perfluorocarbon ligands have been applied as capping ligand for lead sulfide (PbS) quantum dots(QD). We show energy of band edges of lead sulfide (PbS) colloidal semiconductor nanocrystals, specifically quantum dots (QDs), can be tuned in a big range through surface chemistry engineering via a simple and robust ligand exchange in organic solution. Not only dipole moment, but also electronegativity of ligand affect the absolute band energy shifts of PbS QD. It shows that dielectric constant of ligand shell can tune the bandgap of semiconductor QD.

Furthermore, strongly electronic coupled 3-D, ordered QD arrays, superlattices will be fabricated and characterized with field-effect transistor measurements. As fluorinated materials have low surface energy and low steric hindrance, it is hypothesized that the superlattice of PbS QD has low permeability to moisture/ air as the organic ligand shell forming a kinetic barrier on QD surface. Moreover, different length of fluorinated ligands will be applied as QD surface ligand to tune electronic coupling of QD and investigate the effect of ligand length and dipole moment in the carrier mobility. An increased QD packing density and conductivity is expected due to wavefunction overlap of neighboring QDs, resulting from the shorter length of those fluorinated ligands, ~ 0.1 nm, compared to the original ligands on QD surface, ~2 nm. Finally, a significant enhancement of charge transfer efficiency may be obtained with air-stable and band energy tunable PbS QD thin films.