# Nematic Liquid Crystal Composite Materials for DC and RF Switching
## 1 Introduction  
A radio-frequency (RF) switch is a device that is used to route high-frequency signals (from hundreds of MHz to tens of GHz) through a transmission line. RF switches are extensively used in wireless communication technologies (i.e., cellular and satellite communications). We need a material which shows a large and sharp change in resistivity in response to external stimuli to realize a high-performance RF switch. A low power consumption and a simple synthesis and fabrication process are also highly desired in the perspective of commercial applications. Taking this into account in this article, we reviewed published works reporting the conductivity modulation of carbon nanotube-doped liquid crystal (LC-CNT) composites and related aspects to reveal the possibility ofthe composites in DC and RF switching applications. Liquid crystals (LCs) are widely used in display technologies, photonic devices, electro-optic modulators, and optical switches for a long time [1; 2; 3; 4; 5]. For LC devices, the interaction between anisotropic dipole moments with external stimuli (temperature, electric fields, magnetic fields, and EM waves) is a major field of interest. LC shows orientational ordering which can be controlled by external stimuli, which modulates its properties. Furthermore, to modify LC's equilibrium properties (no external stimuli for perturbation), an LC matrix is often doped with nanoparticles (NPs) [6; 7; 8]. For example, a small concentration of MgO and SiO\({}_{2}\) doping decreases the threshold voltage for field-induced reorientation [9; 10]. Metal nanoparticles can alter the elastic coefficient and rotational viscosity of the composite [11; 12]. Ferromagnetic and ferroelectric NP suspensions increase an LC's dielectric anisotropy [13]. Besides these NP doping, the incorporation of carbon nanotubes in liquid crystal is getting significant attention. Studies revealed that carbon nanotube (CNT) molecules can influence nematic ordering, which in turn alters the electrical properties of the LC matrix [14]. Moreover, an increase in LC alignments, an improvement in the electro-optic response for displays, a change in the nematic to isotropic transition temperature, and an alteration of the ion concentration are also observed in the LC-CNT composites [15; 16; 17; 18]. The CNT doping effects are more noticeable for conductivity modulation than a dielectric constant modulation [19; 20]. Figure 1 graphically summarizes the commonly used nanoparticles to enhance a liquid crystal's properties.  
In this paper, we reviewed published work demonstrating the effects of CNT doping on LC-CNT composite's electrical behavior. We focused our discussion on field-induced conductivity modulation. After a brief discussion on liquid crystals and CNTs, we reviewed the effect of CNT on the composite's equilibrium (no applied field), electrical conductivity, and field-induced switching. Further, we reviewed the possible origin and physical mechanisms responsible for conductivity modulation. This may help researchers find a better way to further enhance conductivity modulation. We also reviewed a metal nanoparticle- (i.e., Au and Ag) doped LC composite's conductivity behavior to compare with LC-CNT composites. In Section 5, we compare different composite systems to point out which composite could be most suitable for DC and RF switching applications. Finally, we investigated composite material's RF switching capabilities with a simple analytical model to support the central idea of this review article. We did an RF simulation as a proof of concept. For the simulations, we used values reported on the literature and extrapolated the values for design optimization. We evaluated the transmission characteristics' loss up to 10 GHz.