Self-Assembled Superlattices of Gold Nanoparticles in a Discotic Liquid Crystal

Abstract

There is considerable interest in the inclusion of functionalised nanoparticles in discotic liquid crystals as they exhibit interesting self-assembled structures and electronic properties. In this view, we have carried out magnetic susceptibility, DC conductivity and small angle X-ray diffraction studies (SAXS) on hexanethiol covered gold nanoparticles (GNPs) in a discotic liquid crystal namely hexahexylthiotripheneylene (HHTT). Our SAXS pattern for the pure HHTT in the helical columnar liquid crystalline phase (H) showed a sharp peak corresponding to the intercolumnar spacing between the discotic columns. On the other hand, our SAXS pattern for the GNPs:HHTT (1:4 wt%) in the H phase of the composite showed a broad peak. The broad peak exhibits shoulders indicating additional peaks and they can be indexed to 2D hexagonal superlattices. Our room temperature transmission electron microscope (TEM) image of the pure hexanethiol covered gold nanoparticles showed predominantly 1.2 nm diameter GNPs along with a small number of 2.6 nm and 4.6 nm GNPs. From our studies, we infer that the smaller 1.2 nm GNPs randomly occupy positions in the liquid crystalline matrix within the columns as well as in between the columns. The bigger nanoparticles (2.6 nm and 4.6 nm) form 2D intercalated hexagonal structure with the disc molecules. We find about two to three orders of increase in the DC conductivity in the isotropic and mesophases with respect to those of the pure HHTT. We attribute this increase mainly to the insertion of 1.2 nm GNPs in the columns. We also find further increase in the conductivity by some three orders of magnitude in the crystalline phase of the composite with respect to its higher temperature phases. Magnetic susceptibility studies also showed a discontinuous change near the crystallization temperature. Enhanced core-core ordering in the crystalline phase of the composite with respect to its higher temperature phases may be responsible for these observations.