Spectroscopic insight into molecular fluctuations and phase stability of nematic composites containing gold nanoparticles or carbon nanotubes.

A general model for interactions between nanoparticle dopants and nematic liquid crystals suffers from a lack of experimental data on nanoparticle-host interactions. This dielectric spectroscopy study intends to fill this gap by addressing the impact of gold nanoparticles, carbon nanotubes and toluene molecules on the molecular fluctuation dynamics in the nematic liquid crystal mixture E7. By correlating phase transition temperatures and rotational fluctuation frequencies, we show that the presence of nanoparticles or organic solvent molecules in the nematic host generally destabilizes the nematic state. We also report a clearly different magnitude of destabilization of the nematic state for toluene compared to nanoparticle dopants: while the presence of toluene increases the rate of molecular fluctuations by effectively diluting the host phase, nanoparticle dopants barely affect the molecular fluctuation dynamics. A corresponding trend for the decrease of phase transition temperatures confirms that small organic molecules reduce the strength of intermolecular interactions between host molecules to a significantly larger extent than nanoparticle dopants. We identify the diverse distribution of toluene or nanoparticles in the liquid crystal host phase to play a key role for the resulting effects of doping. The results of our experimental study will help to validate recent theoretical approaches on molecular dynamics in nematic composites and offer a substantial contribution towards stable liquid crystal nanodispersions with tailored properties for plasmonic or electronic applications.