Formation of periodic stripe patterns in nematic liquid crystals doped with functionalized gold nanoparticles

Mixtures of nematic liquid crystals (LCs) were produced by doping small quantities of gold nanoparticles coated with non-chiral hexane- (Au1), dodecane- (Au2) or chiral Naproxen-functionalized dodecane thiolates (Au3, Au4). Circular dichroism (CD) spectroscopy confirmed the optical activity for both Naproxen-functionalized gold nanoclusters. The small CD measured for Au1 and Au2 as well as the weak CD above 400 nm measured for Au3 and Au4 is attributed to scattering artifacts of dense particles aggregating in solution. For all mixtures, characterization of the nanoparticle doped nematic phase by polarized optical microscopy revealed the formation of uniform stripe textures or patterns separated by areas of homeotropic alignment due to a spatial separation of particle-rich and particle-poor domains. Similar characteristic textures were also observed for mixtures of the chiral nematic phase produced by doping either only the Naproxen-functionalized thiol 3b or Naproxen and additionally dodecane thiolate-protected gold nanoparticles Au2. On the basis of these findings, observed for the first time for alkane thiolate-capped gold nanoclusters doped into nematic LCs, two different scenarios are suggested. In the first scenario, the optically active gold nanoparticles Au3 and Au4 transfer chirality to the non-chiral nematic LC host. In the second scenario, all functionalized gold nanoclusters Au1-Au4 form topological defects resulting in chain-like particle aggregates, separated by areas of homeotropic alignment due to particles residing at the LC-glass interface.