Unveiling molecular alignment, dielectric and electrical conductivity of an unaligned 4-octyl-4′-cyanobiphenyl liquid crystal doped with carbon dots

Abstract

We report the achievement of vertical alignment in the smectic A (SmA) and nematic (N) phases of a 4-octyl-4′-cyanobiphenyl (8CB) LC doped with carbon dots (CDs, ∼2.8±0.72 nm) at concentrations ≤ 0.3 wt%. The composites were filled in indium tin oxide (ITO) sample cells without alignment layer (i.e., ITOWAL cells) and characterized using polarizing optical microscope and dielectric spectroscopic techniques. Optical textures displayed the attainment of induced vertical alignment (dark optical texture) even for the lowest concentration of 0.03 wt% used in the experiments. The enhanced dark state was observed with increasing concentration of up to 0.3 wt%. This was also verified by measuring the real dielectric permittivity (εʹ) of the composites. For instance, at 25ºC the value of εʹ for 0.03 wt% composite was 12.6 which increased to 14.8 for 0.3 wt% composite demonstrating the better vertical alignment. The appearance of short axis molecular relaxation in the dielectric loss (ε ̋) of all composites was another confirmation of induced vertical alignment. Measurements at various temperatures show that the induced vertical alignment remains stable throughout the SmA and N phases of 8CB. Additionally, bias voltage studies were conducted to assess the implication of an aligning voltage on εʹ and ε ̋. Electrical conductivity was calculated from ε ̋ and for the highest concentration, an increase of two orders of magnitude in the dc conductivity (σ_dc) compared to a polyimide alignment layer vertically aligned pure 8CB sample. The conductivity exhibited Arrhenius behavior for all composites. This work demonstrates that CDs induced vertical alignment in both SmA, and N phases could replace the conventional alignment techniques required to obtain the vertical alignment of LC materials. Moreover, these composites could pave the way forward for the fabrication of sensors and other optical devices due to their thermally stable alignment.