# Nematic Liquid Crystal Composite Materials for DC and RF Switching
## 4 Field-Induced Conductivity Enhancement in Metal Nanoparticle and Liquid Crystal Composite  
In this section, we review the effects of metal-nanoparticle (i.e., Au and Ag) doping on the electrical conductivity behavior of LC matrices to compare with LC-CNT composites. Metal-nanoparticle doping has an impact on liquid crystal conductivity. The incorporation of Gold nanoparticles (GNPs) in a superstructure is an active field of research [85]. Works have been done to combine the self-assembly nature of liquid crystals and the excellent electrical/optical properties of gold nanoparticles to enhance the functionality of the composites. Some of these works are orientationally ordering gold nanorods [86], controlling light scattering electrically [87], field tuning plasmonic responses [88], light-driven molecular motor [89], etc. Doping a small number of GNPs in an LC matrix has a significant impact on its electrical conductivity. [PERSON] et al. observed nearly an increase of two orders of magnitude in the conductivity (both for \(\sigma_{\perp}\) and \(\sigma_{\parallel}\)) by incorporating 5.0 wt.% alkanethiol-capped GNPs in a 5 CB liquid crystal [49]. With temperature, both types of conductivity increase monotonically, except there is a small sudden decrease in \(\sigma_{\parallel}\) and a small sudden increase in \(\sigma_{\perp}\) at T\({}_{\text{NI}}\) as shown in Figure 16. The conductivity anisotropy \(\sigma_{\text{a}}\) (i.e., \(\sigma_{\parallel}/\)\(\sigma_{\perp}\)) decreases with the GNP doping concentration and approaches 1 for higher concentrations. This indicates that the nematic behavior diminishes with the addition of GNPs. The possible explanation of this conductivity characteristics is, in liquid crystals, GNPs behave like ionic additive. It is evident by that fact that the activation energy is always lower for a composite compared to the pure compounds. Therefore, the incorporation of GNPs enhances the ionic jumps between atomic sites. The authors concluded that orientational ordering has only a little effect on this conductivity increase as they did not notice a drastic change in the conductivity during the N-I transition. However, they expect that the positionally ordered phase of LC (like the 1-D ordered smectic phase or the 3-D ordered B phase) may have an orientational ordering effect on conductivity. Several works have been done to increase the conductivity of discotic liquid crystals by metal NP doping. [PERSON] et al. reported a moderate increase in conductivity by doping with 20-50 wt.% of GNPs [90; 91]. In their later work, by doping with 1.0 wt.% of 2.4 nm GNPs coated with a triphenylene derivate in columnar discotic HAT7, they observed an increase of 6 orders of magnitude in the conductivity [92]. However, none of these works mentioned the effect of an electric field. [PERSON] et al. demonstrated a field-induced increase in the conductivity of a discotic liquid crystal doped with only 1.0 wt.% of GNPs [93]. They use GNPs coated with a methylbenzene thiol dopant in HAT6 and Hexadecane liquid crystals. For HAT6 mixing, 1.0 wt.% of GNPs increased the conductivity by two orders of magnitude in all three phases (solid crystal, columnar liquid crystal, and isotropic liquid). Moreover, in the isotropic phase (above 100 \({}^{\circ}\)C), applying an electric field above a certain threshold increases the conductivity another three to four orders of magnitude as shown in Figure 17a. However, this change is irreversible; conductivity does not change further even if you reduce, turn off, or even reverse the field, as shown in Figure 17b. Furthermore, in a cooling operation (from isotropic to LC phase to crystal phase), the high conductivity value is sustained. Similarly, in the case of a 1% GNP doping in hexadecane (Crystalline to isotropic phase transition temperature 18 \({}^{\circ}\)C), similar orders of magnitude of an increase in conductivity was found, but the change disappeared (reversible) in the transition from the isotropic to crystalline phases (during cooling cycle).