# Nematic Liquid Crystal Composite Materials for DC and RF Switching
## 4 Field-Induced Conductivity Enhancement in Metal Nanoparticle and Liquid Crystal Composite
In both cases (HAT6/GNP and Hexadecane/GNP), conductive pathways are formed upon applying a sufficiently large electric field between the electrodes of LC cell, which is evident form the observation of the aggregation of nanoparticles in their interdigitated electrode experiments [93]. Here, the concentration of GNPs is well below the percolation limit, so the formation of conductive pathway is only possible by the field-induced organization of nanoparticles [94; 95; 96]. A possible explanation is GNPs moves in the electric field by electrophoresis, or an even field can also polarize the particles. This induces the self-assembly of particles (i.e., GNPs) by dipole-dipole interactions. This happened specially in the isotropic phase as LC shows a low viscosity, which eases the movement of GNPs. During crystallization. HAT6 can preserve the high conductivity, but the hexadecane composite cannot, as mention earlier. This is because, in the case of HAT6 during the cooling operation, homogeneously aligned grain boundaries are formed perpendicular to the electrodes, whereas in the case of hexadecane, grain boundaries are formed in a random orientation which disrupts the conductive GNP chains. Their major conclusion is that the orientational ordering of the LC molecules do not have much of an effect on the conductivity increase (specially in isotropic phase); the interaction between the gold nanoparticle and electric field plays the vital role [93]. Further investigations may be required to confirm this reasoning. Although in an isotropic phase, LC molecules do not have any effect in the self-assembly of GNPs, a nanosphere can be organized by a liquid crystal matrix in the nematic phase. For example, [PERSON] et. al demonstrated the self-assembly of CdS nanocrystals (quantum dots, QDs) in a 5 CB liquid crystal matrix [97]. Even though Quantum dots (nanosphere) do not have anisotropy in electro-optic properties, their introduction may disrupt the uniform nematic alignment of the composite; thus, interns change its properties. It has been shown that the micrometer size spherical nanoparticle tends to be organized into a cylindrical chain directed to the global nematic director [98]. However, whether it is also applicable to a nanometer-sized particle was unknown. [PERSON] et al. dispersed 1.0 wt.% of CdS nanocrystals (diameter of 2.3 nm) in 5 CB. By dielectric measurement, they found that the nanocrystal organized into a highly anisotropic structure along the LC director. The one-dimensional chain of QDs follows a global LC director orientation upon applying electric fields as shown in Figure 18. Moreover, there is no evidence of large-scale agglomerations of QD at any temperature. Figure 16: The temperature dependence of electrical conductivity in LC-CNT composites (in the nematic to isotropic phases) (**a**) along (\(\sigma_{\parallel}\)) and (**b**) perpendicular (\(\sigma_{\perp}\)) to the director for the pure compound and LC-gold nanoparticle (GNP) composites with different doping concentrations [49]. The discussion above indicates the possibility of field-induced conductivity modulation in the nematic phase by incorporating metal nanoparticles in liquid crystal. [PERSON] et al. demonstrated field-induced conductivity modulation in a silver NP-doped liquid crystal composite [99]. They doped a 5 CB crystal with 0.5 wt.% of Ag nanosphere with a mean diameter of 10 nm. The equilibrium conductivity (no applied electric field or other stimuli) of the composite enhanced after the incorporation of nanoparticles was also found of SNPs [49; 100], as metal nanoparticle acts as ionic additives. By applying an AC voltage in the range of 1-20 V, they observed about an increase of two orders of magnitude in the conductivity at a lower frequency region. They also confirmed their observation by DC measurement. All the measurements are done under ambient temperature. Instead of spherical QDs, metal nanoroad can also be used to enhance the conductivity of liquid crystal. [PERSON] et.