Electronically coupled nanocrystal superlattice films by in situ ligand exchange at the liquid-air interface

The ability to remove long, insulating ligands from nanocrystal (NC) surfaces without deteriorating the structural integrity of NC films is critical to realizing their electronic and optoelectronic applications. Here we report a nondestructive ligand-exchange approach based on in situ chemical treatment of NCs floating at the liquid-air interface, enabling strongly coupled NC superlattice films that can be directly transferred to arbitrary substrates for device applications. Ligand-exchange-induced structural defects such as cracks and degraded NC ordering that are commonly observed using previous methods are largely prevented by performing ligand exchange at the liquid-air interface. The significantly reduced interparticle spacing arising from ligand replacement leads to highly conductive NC superlattice films, the electrical conductivities and carrier mobilities of which are 1 order of magnitude higher than those of the same NC films subject to substrate-supported exchange using previously reported procedures. The in situ, free-floating exchange approach presented here opens the door for electronically coupled NC superlattices that hold great promise for high-performance, flexible electronic and optoelectronic devices.