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

Abstract

We demonstrate here 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 ITO sample cell using polarized optical microscopy and dielectric spectroscopic techniques. The cross-polarised optical textures clearly show that the doping of CDs (concentration ≥0.25wt%) in planar anchored 8CB, LC led to change its alignment from planar to vertical. Intrestingly, such induced vertical alignment remains stable throughout SmA and N phases of the 8CB, LC material. Moreover, the magnitude of real dielectric permittivity is found to increase with increasing concentration of CDs and exhibited vertical alignment values for composites (≥0.25wt%). The observance of short axis molecular relaxation for composites (≥0.25wt%) without the application of bias field affirmed again the induced vertical alignment. The accumulation of CDs at the substrate surface and their interaction with alignment and ITO layers could be attributed as an important factor for such induced vertical alignment. 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 as 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 SmA and N phases of thermotropic smectogenic liquid crystal material (8CB) by dopant CDs could pave the way forward for their applications in the flexible displays, biosensors, electro-optical memory, and other tunable photonic devices.