Enhanced Electro-Optical Properties of Liquid Crystals Devices by Doping with Ferroelectric Nanoparticles

Abstract

Over the past few decades, liquid crystal (LC) displays (LCDs) have been at the leading edge of their field. Many scientists and manufacturers have devoted studies to improve the performance of LCD characteristics such as a fast response, high contrast ratio, and wide viewing angle. Therefore the in-plane switching (IPS) mode (Oh-e et al., 1995) was developed for wide viewing angle; vertical alignment (VA) mode for a higher contrast ratio; and ferroelectric (Clark et al., 1980; Meyer et al., 1975) and antiferroelectric LC (Chandani et al., 1988, 1989) for faster responses, instead of the common nematic LC and twist nematic (TN) modes. Recent studies of liquid crystals doped with nanoparticles have given rise to a number of novel practical applications and pointed the way toward innovative improvement of the physical and electro-optical properties of liquid crystal by means of chemical synthesis (Kobayashi & Toshima, 2007). Enhancement of the electro-optical properties of liquid crystal is dependent on the size, type, concentration, and intrinsic characteristics of the nanoparticles used for doping. The nanoparticles should share similar attributes to the liquid crystal molecules and be of a size that would not significantly disrupt the order of the liquid crystal. Low doping concentrations (<3% by weight) are usually chosen to yield a more stable and even distribution in the liquid crystal, which lowers the interaction forces between particles. Commonly used doping nanoparticles include ferromagnetic nanoparticles, metallic nanoparticles, inorganic nanoparticles, and ferroelectric nanoparticles. In the case of ferromagnetic nanoparticles, the large permanent magnetic moments couple with the LC direction, leading to improvements in their magnetic properties. This is known as ferronematics (Brochard & Gennes, 1970). In the case of metallic nanoparticles, due to the surface plasmon resonance and depolarization of the electric field, the metallic nanoparticles can enhance the memory effect of the ferroelectric liquid crystal (FLC) (Kaur et al., 2007) and dielectric properties of nematic liquid crystal (NLC) (Miyama et al., 2004; Shiraki et al.,2004). In the case of inorganic nanoparticles, their intrinsic structures can affect the vertical alignment without the need for an alignment layer (Jeng et al., 2007). Due to the large permanent dipole moments, ferroelectric nanoparticles induce realignment of neighboring liquid crystal molecules, thereby increasing the order parameter and lowering the threshold voltage (Reznikov et al., 2003).