Highly Ordered Nanorod Assemblies Extending over Device Scale Areas and in Controlled Multilayers by Electrophoretic Deposition

Here we describe the formation of vertically aligned nanorod assemblies over several multilayers using CdS and CdSe nanorods by electrophoretic deposition. The presence of both charge and dipole on the rods allows both field driven deposition and orientational order to form close packed arrays where each rod is vertically aligned. Comparing assembly formation in electrophoresis to spontaneous assembly in solution gives important insights into nanorod organization by these different mechanisms. We show the influence of ligand environment on net charge (zeta potential) and its influence on assembly formation in CdSe nanorods that have long chain alkyl ligands (low charge) or pyridine ligands (high charge). The experimental observations show that highly charged rods deposit too quickly to allow close-packing to occur with perpendicular alignment only occurring with a lower net charge. This is supported by simulation predicting a lower energy configuration with a preference for perpendicular alignment as the charge state decreases. The resolute order that is retained over device scale areas and over several multilayers combined with inherent scalability of electrophoretic deposition makes this approach highly attractive for large scale nanorod integration in electronic, photonic, or photovoltaic devices.