Nematic display technology

The subject of liquid crystals has now grown to become an exciting interdisciplinary field of research with important practical applications. This book presents a systematic and self-contained treatment of the physics of the different types of thermotropic liquid crystals - the three classical types, nematic, cholesteric and smectic, composed of rod-shaped molecules, and the newly discovered discotic type composed of disc-shaped molecules. The coverage includes a description of the structures of these four main types and their polymorphic modifications, their thermodynamical, optical and mechanical properties and their behaviour under external fields. The basic principles underlying the major applications of liquid crystals in display technology (for example, the twisted and supertwisted nematic devices, the surface stabilized ferroelectric device, etc.) and in thermography are also discussed. 

As we look to the 1980 s a display gap still remains. We show in this paper that the device responsible for the commercial success of LCDs to date, the twisted nematic (TN) LCD, is limited to flat panel displays with less than about 10 independently addressable picture elements (pels). Thus, there is no display technology available today that will provide a low power, flat panel display with high information content (10 to 10 pels). 

The present research and development on LC display technology is conducted primarily in industrial labs. Academic research focuses mainly on more exciting and explorative topics that can not only stimulate fundamental scientific interest, but offer tremendous potential for innovative applications beyond the realm of displays, for example, new materials and attractive properties, and new uses in optics, nano/micromanipulation, novel composites, and biotechnology [7]. Future applications depend on the increase of complexity and functionality in LC materials and phases. The past three decades have seen the discovery of complex LC molecules with a variety of new shapes for instance, disc shape (Fig. 6.1b) [8], bent-core shape (Fig. 6.1c) [9], H shape (Fig. 6.1d) [10-13], board shape (Fig. 6.1e) [14,15], T shape (Fig. 6.1f) [16], cone shape (Fig. 6.1g) [17], and semicircular shape (Fig. 6.1h) [18]. The shapes of the molecules are not exactly associated with the types of mesophases formed. Like rod-shaped molecules, each complex shape is likely to organize a nematic, Sm, Col, and 3D-ordered mesophases [19,20]. The incorporation of functionality, amphiphilicity, and nano-segregation into these molecular shapes offers different ways to increase the complexity of LC phases. 

The most important multicolour display technology in current use involves liquid crystal displays (LCDs), which overtook CRTs in popularity around the mid-2000s. Liquid crystals (LCs), commonly referred to as the fourth state of matter, are materials that are intermediate in character between the crystalline solid and liquid states.Unlike normal isotropic liquids in which the molecules essentially adopt a randomised orientation, liquid crystals show some time-averaged positional orientation of the molecules. In this sense, they resemble solid crystalline materials, although they retain most of the properties of liquids, notably the ability to flow. They are formed most commonly from molecules with rod-like geometry, which are referred to as calamitic. These molecules may orientate in various ways to form different types of LC phases [mesophases). There are three main mesophase types smectic, nematic and chiral nematic. In the smectic mesophase, as illustrated in Figure 11.1(a), the molecules are arranged in raft-like layers with their molecular axes parallel. These layers can pass over each other as the material flows. In the... 

H. Mori, The wide view film for enhancing the field of view of LCDs, J. Display Technology 1,179 (2005). R. A. Soref, Transverse field effect in nematic liquid crystals, Appl Phys. Lett. 22, 165 (1973). 

One of the most common LC phases is the nematic, where the molecules have no positional order, but they have long-range orientational order. Thus, the molecules flow and their center of mass positions are randomly distributed as in a liquid. Most nematics are uniaxial the molecules are orientationally ordered about a common axis defined as the director and represented by the unit vector n (Table 2). It is very important to stress that nematics can be easily aligned by an external magnetic or electric field. Aligned nematics have the optical properties of uniaxial crystals and this makes them extremely useful in liquid crystal display technology. 

More advanced LCDs take advantage of so-called super-twisted nematic displays, in which the director rotates 270° between the two polarizers. This leads to a crisper distinction between on and off states. Color LCDs simply include appropriate dyes to make red, green, and blue (RGB) pixels—simple in principle but a fairly complex technology. 

Liquid crystals are materials that flow like liquids but are ordered in the fluid state. They are typically made up of rigid molecules with a large aspect ratio and functional groups that are responsive to electromagnetic fields. Nematic Uquid crystals have orientational order but no positional order (i.e., the centers of mass are randomly distributed, but the molecules retain a preferred orientation). Because the order is responsive to electromagnetic fields, these materials have found extensive use in display technology. 

Such a 90° twist cell is of particular importance for electro-optic displays as it forms the basis of the twisted nematic display [23], which is the dominant technology both for panels of a low to medium information content and for use over an active backplane in complex displays. The important features of the rotation of light by the twisted layer may be summarized as follows ... 

Chiral nematic liquid crystals, as the name suggests, are optically active variants of nematic liquid-crystalline compounds the incorporation of a chiral centre imparts properties which are unique to the chiral nematic phase and are responsible for their utilisation in a variety of differing display technologies and other related applications. The term cholesteric liquid crystal was originally used to describe this phase, and originates from the structural nature of the earliest chiral nematic liquid crystals which were derivatives of cholesterol [1,2], Nowadays, the term chiral nematic is used primarily because the materials are clearly derived from nematic type liquid crystals [3, 4], Despite these differences in definition, the terms cholesteric and chiral nematic phase are interchangeable and it is common to find references to either term in the literature. 

Biaxial thermotropic nematic L.C.s would be of great importance in L.C. display technology. Less than a decade ago, such L.C.s were suggested. The biaxiality of the phases was confirmed using NMR spectroscopy of deuterated probe molecules. An alternate technique that is based on the second order quadrupole shift detectable in Xe NMR spectra of dissolved xenon has been proposed. The method has many advantages, such as the NMR spectra are taken from a static sample and the Xe quadrupole coupling tensor is extremely sensitive to the symmetry of the phase. ... 

The static dielectric permittivity is an important parameter that characterises the response of a medium to the application of an electric field. Its value is determined by the distribution of the electric charges in molecules (polar and non-polar compounds) as well as by the intermolecular interactions (for example anisotropy of the medium and intermolecular correlations). In nematics the dielectric permittivity is a tensorial quantity. The value and the sign of the dielectric anisotropy play an important role in the application of nematics in display technologies. 

With the help of Fig. 3.18, we are now in a position to illustrate briefly how a twisted nematic display operates. For more details on this and other effects related to liquid crystal displays the reader is referred to the review articles by Schadt [242] and Scheuble [243]. This type of device, based on the experiments involving electric fields carried out by Schadt and Helfrich [241], was patented for technological applications by these authors [125] in 1970 and by Fergason [84] in 1971. Fergason also went on to develop other patents on twisted nematic displays, most notably one issued in 1973 [85]. 

Many technological applications of liquid crystals, as in electro-optic display devices, are based on multicomponent mixtures. Such systems offer a route to the desired material properties which cannot be achieved simultaneously for single component systems. Mixtures also tend to exhibit a richer phase behaviour than pure systems with features such as re-entrant nematic phases [3] and nematic-nematic transitions possible. In this section, we describe simulations which have been used to study mixtures of thermotropic calamitic mesogens. 

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