# 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: In-Plane Switching  
Up to this point, we have discussed the effect of an out-of-plane applied electric field on an LC composite's electrical behavior. The in-plane field can also be used to modulate the LC composite property. The benefit of using an in-plane field is that it will ease the fabrication process and experimental testing setup. In 1992, an in-plane switching LCD was demonstrated by [PERSON] et al. [66]. In the IPS (In-plane switching) mode of operation, two electrodes are placed on the same substrate in contrast to being put on the either side of the cell as in the vertical alignment mode or twisted nematic mode [67]. When a horizontal electric field is applied to an IPS cell, positive LC molecules rotate in plane to align themselves to the field directions. Figure 13 shows the field pattern for the IPS and FFS (fringe field switching) modes of in-plane switching. The threshold voltage of transition is inversely proportion to the cell gap [68]. [PERSON] et al. showed that when the distance between electrodes is smaller than the width of the electrode and cell gap, the generated frringing field rotates the LC molecules above the bottom substrate [69]. By this way, electro-optical properties can be modified greatly. [PERSON] et al. investigated the CNT doping effect on the electro-optical properties of LC cells subjected to in-plane field [37]. They found a smaller retardation in the LC-CNT composite, resulting in a lower optical transmittance. In addition to that, they found a decrease in the decay response time compared to an undoped cell. [PERSON] et al. investigated the electrical conductivity behavior of an LC-CNT composite by applying an in-plane electric field [70]. For the switching medium, they used E7 and 0.0005 to 0.01 wt.% SWCNT mixture. For testing, they fabricated gold electrodes as shown in Figure 14a. The measurement was done on an air ambient in contrast to a traditional sandwiched cell. They found the nonlinear Current-Voltage characteristic (I-V characteristic), which is a signature of field-induced  
Figure 12: The memory efficiency M as a function of the weight concentration of carbon nanotubes for the different liquid crystals mixtures: (1) p-ethoxy-benzylidene-p-n-butylaniline- (EBBA) based series and (2) MLC6608-based series [64]. Figure 13: In-plane electrode configurations in an LC Cell: The black rectangle shows the metal electrodes, and the blue lines are equipotential lines generated by interdigital electrodes for in-plane switching (IPS) and fringe field switching (FFS) cells. conductivity modulation. In addition to that, they found an increase in conductivity on a subsequent sweep of the applied voltages. This indicates that the conductivity varies both with the operation time (how long the field is applied) and field strength. To verify the operational time dependence effect, they applied a fixed voltage of 7.5 V (1.5 \(\times\) 10\({}^{5}\) V/m) to the sample for a longer amount of time (say, for 30 min) and they found an abrupt increase in the current in the first few minutes. Subsequently, upon the removal of the electric field, the current goes to zero immediately. Here, the conductivity modulation is occurred by CNT reorientation via nematic LC molecules [15], and the magnitude of saturation currents depended on the CNT concentrations. In all these experiments, the samples were subjected to an electric field on the order of 10\({}^{5}\) V/m, which is enough to make a Freedericksz transition, which is also observed by [PERSON] et al. for traditional cells [15]. Although CNT lengths are much smaller than the cell gap, multiple CNTs can bundle together to make a percolation path between electrodes. When the sample is exposed to a sufficient electric field for a longer time, the number of conduction paths increases by aligning the CNTs along the field. Polarizing microscopy shows that relatively small LC molecules can change orientation (relaxation) after the field is removed.