Tunable electronic properties by ligand coverage control in PbS nanocrystal assemblies.

It is well-known that controlling electronic properties of nanocrystal (NC) assemblies can be achieved by the usage of various types of ligands on the NC surface. However, the ligand coverage, which could also tune electronic properties, is always ignored. It is due to the lack of accurate evaluation methods of ligand amounts on the surface of NCs and the difficulty of ligand binding control at the nanoscale. Here, we demonstrate a precise ligand (oleic acid) coverage control of PbS NCs through a modified liquid/air assembly technique. In this way, both the assembly structures and electronic properties can be tuned by ligand coverage at the same time. In particular, the medium oleic acid coverage (2.1 ligand per nm), which forms a square lattice of PbS NCs, shows the best electronic properties, especially when it is compared with the full coverage (2.4 ligand per nm) and the sparse coverage (0.7 ligand per nm) of oleic acid on the NC surface. This ligand coverage controlled electronic properties of NC films will give new insights for finely tuning the properties of electronic devices based on NCs.