Rajratan Basu

Nematic anchoring on carbon nanotubes

A dilute suspension of carbon nanotubes (CNTs) in a nematic liquid crystal (LC) does not disturb the LC director. Due to a strong LC-CNT anchoring energy and structural symmetry matching, CNT long axis follows the director field, possessing enhanced dielectric anisotropy of the LC media. This strong anchoring energy stabilizes local pseudonematic domains, resulting in nonzero dielectric anisotropy in the isotropic phase. These anisotropic domains respond to external electric fields and show intrinsic frequency response. The presence of these domains makes the isotropic phase electric field-responsive, giving rise to a large dielectric hysteresis effect.

Effects of ferroelectric nanoparticles on ion transport in a liquid crystal

A small quantity of BaTiO3 ferroelectric nanoparticles (FNPs) of 50 nm diameter was doped in a nematic liquid crystal (LC), and the free ion concentration was found to be significantly reduced in the LC + FNP hybrid compared to that of the pure LC. The strong electric fields, due to the permanent dipole moment of the FNPs, trapped some mobile ions, reducing the free ion concentration in the LC media. The reduction of free ions was found to have coherent impacts on the LCs conductivity, rotational viscosity, and electric field-induced nematic switching.

Dielectric hysteresis, relaxation dynamics, and nonvolatile memory effect in carbon nanotube dispersed liquid crystal

Self-organizing nematic liquid crystals (LCs) impart their orientational order onto dispersed carbon nanotubes (CNTs) and obtain CNT-self-assembly on a macroscopic dimension. The nanotube-long axis, being coupled to the nematic director, enables orientational manipulation via the LC nematic reorientation. Electric-field-induced director rotation of a nematic LC+CNT system is of potential interest due to its possible application as a nanoelectromechanical system. Electric field and temperature dependence of dielectric properties of a LC+CNT composite system have been investigated to understand the principles governing CNT assembly mediated by the LC. In the LC+CNT nematic phase, the dielectric relaxation on removing the applied field follows a single-exponential decay, exhibiting a faster decay response than the pure LC above a threshold field. The observed dielectric behaviors on field cycling in the nematic phase for the composite indicates an electromechanical hysteresis effect of the director field due...

Carbon nanotube-induced chirality in an achiral liquid crystal

A small quantity of carbon nanotubes was dispersed in an achiral liquid crystal (LC), and the mixture was found to exhibit a weak degree of chirality. The induced chirality in the LC was probed by means of the electroclinic effect in the LC’s smectic-A phase, which showed significant pretransitional behavior on approaching the smectic-A–smectic-C transition temperature from above. The results suggest that there is a net chirality associated with the carbon nanotubes, which is transmitted into the LC.

Effect of carbon nanotubes on the field-induced nematic switching

A small quantity of carbon nanotubes (CNT) was doped in a nematic liquid crystal (LC), and the LC + CNT hybrid was found to exhibit a faster field-induced nematic switching compared to that of the pure LC. The field-induced switching time was probed by means of the electro-optic response of the samples. The hybrid system also revealed a reduced rotational viscosity and an enhanced dielectric anisotropy. The results suggest that the hybrid system undergoes a faster field-induced switching, as the CNTs favorably alter the rotational viscosity and the dielectric anisotropy of the nematic matrix.

Carbon nanotube-induced macroscopic helical twist in an achiral nematic liquid crystal

An achiral nematic liquid crystal was doped with a small quantity of carbon nanotubes having a net chirality, and the mixture was found to exhibit an average mechanical twist over macroscopic dimensions. The nanotube-induced chiral pitch length P was determined as a function of the average nanotube concentration by measuring the radii of curvature of reverse twist disclination lines in 90° nematic twist cells. The results suggest that the nanotubes’ spatial concentration can vary significantly across the cell and that at high average concentration, the nanotubes undergo aggregation, resulting in an apparent saturation of P−1 at high concentrations. The macroscopic helical twisting power of the nanotubes has been estimated from the results.

Effects of graphene on electro-optic switching and spontaneous polarization of a ferroelectric liquid crystal

A small quantity of graphene flakes was doped in a ferroelectric liquid crystal (FLC), and the field-induced ferroelectric electro-optic switching was found to be significantly faster in the FLC + graphene hybrid than that of the pure FLC. Further studies revealed that the suspended graphene flakes enhanced the FLCs spontaneous polarization by improving smectic-C ordering resulting from the π–π electron stacking, and reduced rotation viscosity by trapping some of the free ions of the FLC media. These effects coherently impacted the FLC-switching phenomenon, enabling the FLC molecules to switch faster on reversing an external electric field.

Chiral induction in thioester and oxoester liquid crystals by dispersed carbon nanotubes

Multi-walled carbon nanotubes were dispersed at low concentrations into various achiral liquid crystals having either a thioester or oxoester linkage group in the core. The presence of the carbon nanotubes resulted in chiral signatures being observed in the liquid crystals, including an electroclinic effect (a rotation of the liquid crystal director perpendicular to, and linear in, an applied electric field) in both the nematic and smectic A phases, and a macroscopic helical twist of the liquid crystal director in the nematic phase. For both experiments the chiral signatures for the thioester liquid crystals were found to be an order of magnitude larger than those of the oxoesters. We speculate that the much larger strength of the thioesters chiral properties is a result of stronger non-covalent interactions between the liquid crystal molecule and carbon nanotube.

Nano-electromechanical rotation of graphene and giant enhancement in dielectric anisotropy in a liquid crystal

A nematic liquid crystal (LC) is doped with dilute concentrations of pristine monolayer graphene (GP) flakes, and the LC + GP hybrids are found to exhibit a dramatic increase in the dielectric anisotropy. Electric field-dependent conductance studies reveal that the graphene flakes follow the nematic director that mechanically rotates on increasing an applied electric field. Further studies show that the π–π electron stacking, between the graphenes honeycomb structure and the LCs benzene rings, stabilizes pseudo-nematic domains that collectively amplify the dielectric anisotropy by improving the orientational order parameter in the nematic phase. These anisotropic domains interact with the external electric field, resulting in a nonzero dielectric anisotropy in the isotropic phase as well. The enhancement in dielectric anisotropy, due to the LC–graphene coupling, is found to have subsequent positive impacts on the LCs orientational threshold field and elasticity that allows the nematic director to respond quicker on switching the electric field off.

Dielectric response of multiwalled carbon nanotubes as a function of applied ac-electric fields

The complex dielectric constant (e∗) is reported for multiwalled carbon nanotubes (MWCNTs) up to 105 Hz as a function of ac-electric field amplitudes Erot (in phase and same frequency as the measurement) and Eac (different phase and fixed frequency with respect to the measurement). A slow relaxation process (mode 1) is observed, which shifts to higher frequency with increasing Erot but is independent of Eac. A fast relaxation process (mode 2) is also observed, which is independent of Erot but shifts to higher frequency with increasing Eac (opposite to that of mode 1). An ac-conductivity analysis of MWCNT reveals insights on how Erot and Eac influence the dissipation.