# Nematic Liquid Crystal Composite Materials for DC and RF Switching
## 3 Electrical Conductivity of Liquid Crystal and Carbon Nanotube Composites
### Field Induced Conductivity Modulation in Isotropic Phase: \(\pi\)-\(\pi\) Staking  
The conductivity modulation of a LC-CNT dispersion is also possible in the isotropic phase beside the nematic phase as described above. [PERSON] et al. demonstrated an insulator to conductor transition in an MWCNTs/5 CB composite in an isotropic phase [57]. In the nematic phase, LC molecules are subjected to a torque proportional to \(\Delta\)eE\({}^{2}\) for an applied electric field, but in the isotropic phase, no interaction occurs as \(\Delta\)e= 0. However, LC molecules of a specific molecular structure can interact with CNTs. [PERSON] et al. observed a molecular interaction of 5 CB with SWCNTs by Raman spectroscopy [58]. They suggested that the interaction occurs between the LC phenyl rings to the hexagonal structured CNT by \(\pi\)-staking. Using optical microscopy, they observed a stable nematic phase even above T\({}_{\text{NI}}\) around the CNT bundles in the 5 CB-CNT dispersion. On the other hand, an isotropic phase is prevalent in the dispersion where CNT is absent. Furthermore, on the reverse process, an isotropic to nematic (I-N) transition nucleates at the CNT bundles in the cooling phase. These are clear evidences of a strong LC molecule interaction with CNTs. Using the Density Functional Theory (DFT), [PERSON] et al. investigated the anchoring of an LC molecule on CNT [59]. The anchoring of LC molecules on CNT occurs by \(\pi\)-\(\pi\) stacking, maximizing the hexagon-hexagon interaction. A similar \(\pi\)-\(\pi\) interaction is also observed for CNT and graphene composites [60; 61]. The typical anchoring energy is 2 eV [59], which is stronger than the Van der Waals force. This anchoring induces a short-range orientational order (both in the nematic and isotropic phases) of LC molecules around the CNT wall known as pseudo nematic domain (PND), as shown in the Figure 9. PND shows dielectric anisotropy (\(\Delta\)e\(\
eq\) 0) for an LC host matrix. Here, the LC molecules are oriented along the long tube axis. In the nematic phase, the effect of PNDs are overshadowed by the long-range order of LC itself. However, its effect is prominent in the isotropic phase (T = 40 \({}^{\circ}\)C for 5 CB). Consequently, an electric field-induced orientation is possible in this case. PND can sustain in the isotropic phase even in a higher temperature as the thermal energy is not strong enough (0.01 eV) to break the LC molecules anchoring. [PERSON] et al. investigated the field-induced resistivity modulation for pure 5 CB, MWCNT in ethanol, and the 5 CB/0.005 wt.% MWCNT composite at a temperature greater than T\({}_{\text{NI}}\) (Figure 10) [57]. Pure 5 CB and MWCNT in ethanol does not show any modulation, whereas a 5 CB-CNT composite shows a drop in the resistivity of more than three orders of magnitude a threshold value of 7.26 V. In this case, randomly oriented PNDs align along the field, and several PND-capped CNTs made loosely connected conductive networks between the metal electrodes. The space between the cell electrode and CNT concentration has a strong influence on the conductive path (CNT wires) formation. [PERSON] et al. observed that the threshold voltage (V\({}_{\text{th}}\)) increases with cell separation. On the contrary, the threshold voltage was expected to decrease with CNT concentrations as the probability of forming a connected CNT network will increase [57]. However, in an experimental observation, they found an anomalous behavior. V\({}_{\text{th}}\) decreased for a CNT concentration ranging from 0.0018 to 0.005 wt.%. After crossing this range, V\({}_{\text{th}}\) started to increase since, above a critical concentration--in this case, it is 0.005 wt.% --CNT started to aggregate. It was also observed for other LC-CNT composites. Furthermore, in addition to the conductivity modulation, Joule heating was observed using a polarizing electron microscope as CNT wires made a high conducting path to draw huge amounts of currents.