Controlling the assembly of CdS nanorods via solvent and acidity

We report control over the phase behavior of CdS nanorods via the solvent and acidity. CdS nanorods were synthesized using alkane phosphonic acid ligands, which were replaced after synthesis by a series of aromatic ligands. Change of ligand enabled us to cast films from different solvents. By replacing toluene with ethanol or water the rod-rod interactions dominate over rod-substrate interactions, thereby favoring simple hexagonal ordering (2D). When dispersed in water, a net electrostatic charge on the nanorods could be induced by deprotonating the ligands at high pH. This net charge favors 2D nematic ordering over homeotropic ordering of the nanorods on a substrate. A calculation of the van der Waals and electrostatic interactions is presented that explains the observed influence of solvent and pH.