Control of anisotropic self-assembly of gold nanoparticles coated with mesogens

The self-assembly was investigated of a series of gold nanoparticles (GNP) that were covered with laterally attached mesogenic molecules (ligands) and different amounts of alkylthiols (co-ligands). The presence of mesogens induces long-range ordered packing of GNPs. Here we report three different ordered liquid crystalline phases built around the GNPs, two of which have not been observed previously in thermotropic liquid crystals. These are the 3D simple hexagonal packing and the face-centred cubic (FCC) packing of spheres, i.e. GNPs in this case. It has been found that the volume fraction of co-ligands in such systems is critical for the anisotropy of the self-assembly of GNPs; either too small or too large a co-ligand volume fraction causes the formation of one of the two nearly or fully isotropic packing types mentioned above. In these two structures the rod-like mesogens have no preferred orientation. At intermediate co-ligand fractions highly anisotropic strings of GNPs are obtained, packed in a 2D hexagonal lattice, with the mesogens surrounding the strings and lying parallel to their axis. By matching the experimental X-ray diffraction intensities with those calculated from models, the mean deviation of the 2 nm GNPs from their ideal lattice position was found to decrease from 0.84 nm for the material with no co-ligands to 0.44 nm for the complex with dodecylthiol; this increase in order is attributed to the soft alkyls evening out the particle non-uniformities. The results help us to understand and control the way organic molecules affect the organization of nanoparticles, and facilitate the design of new hybrid materials through self-assembly.