Effect of carbon dots on tuning molecular alignment, dielectric and electrical properties of a smectogenic cyanobiphenyl-based liquid crystal material

Abstract

Here, we demonstrate the effect of dispersing organosoluble carbon dots (CDs, ∼7-8 nm) on tuning the molecular alignment, dielectric and electrical properties of smectic A (SmA) and nematic (N) mesophases of a thermotropic smectogenic LC material, 4-octyl-4′-cyanobiphenyl (8CB) in a planar anchored indium tin oxide (ITO) sample cell using polarized optical microscopy and dielectric spectroscopic techniques. The cross-polarized optical textures clearly show that the doping of CDs (concentration ⩾0.25 wt%) in planar anchored 8CB liquid crtstal (LC) led to the changing of its alignment from planar to vertical. Interestingly, such an induced vertical alignment remains stable throughout the SmA and N phases of the 8CB LC material. Moreover, the magnitude of the real dielectric permittivity is found to increase with increasing concentration of CDs and exhibits vertical alignment values for composites (⩾0.25 wt%). The observance of short axis molecular relaxation for composites (⩾0.25 wt%) without the application of bias field confirms again the induced vertical alignment. The accumulation of CDs at the substrate surface and their interaction with the alignment and ITO layers can be attributed as an important factor for such induced vertical alignment. The electrical conductivity of 8CB is observed to increase significantly with the addition of CDs (i.e. an increment of up to two orders of magnitude in composites compared to pure 8CB) and attributed to the lowering of viscosity and change in molecular alignment. We certainly believe that such tunable molecular alignment throughout the SmA and N phases of thermotropic smectogenic LC material (8CB) by dopant CDs could pave the way for their applications in flexible displays, biosensors, electro-optical memory and other tunable photonic devices.