Effect of alkyl chain length on the dielectric and electro-optical properties of graphene quantum dots doped cyanobiphenyl based nematic liquid crystals composites

Abstract

The effects of alkyl chain length and doping of biocompatible/eco-friendly graphene quantum dots (GQDs) at 0.5 wt% on the dielectric and electro-optical properties of cyanobiphenyl (nCB, n = 5, 7) nematic liquid crystals (NLCs) are investigated using optical polarising microscope, electro-optical and dielectric spectroscopic techniques. Our results revealed that GQDs modulated the dielectric and electro-optical properties of nCB. Optical textural studies confirmed the uniform dispersion of GQDs in both NLCs (5CB and 7CB). The threshold voltage evaluated from voltage-dependent optical transmission was found to decrease by 16 % and 6 % in GQDs-5CB and GQDs-7CB composites compared to pure NLCs, respectively. The increment in birefringence by 2 % and 1.3 % and order parameter by 4 % and 0.6 % in GQDs composites with 5CB and 7CB, respectively was observed. Both transverse and longitudinal components of the dielectric permittivity (εˈ⊥ and εˈ∥) showed positive increments of 12 % and 5.4 % in GQDs-5CB composite as compared to pure 5CB. However, the increment was found to be 8.9 % and 4.1 % in GQDs-7CB composite. The GQDs-7CB composite exhibited fewer noticeable changes due to the stronger interaction between the molecules, as compared to GQDs-5CB composite. Our results showed that dispersion of GQDs and the number of carbon atoms ‘n’ in the alkyl chain of CB’s constituent molecules affects their properties, such as display parameters and mesophase stability of 5CB and 7CB homologues. Such GQDs-NLCs composites would be certainly useful in the tunable electrical and electro-optical devices.