Surface Stabilized Ferroelectric Liquid Crystal Displays

Figure 4.9 Schematic diagram of the bookshelf cell structure of the surface-stabilized ferroelectric liquid crystal display, (a) The director is along the direction Ai when the applied field is up. (b) The director is along the direction Ai when the applied field is down, (c) Directions of the polarizer and analyzer.

On a macroscopic scale, the spontaneous polarization vector in the optically active phase spirals about an axis perpendicular to the smectic layers (Fig. 20), and sums to zero. This macroscopic cancellation of the polarization vectors can be avoided if the helical structure is unwound by surface forces, by an applied field, or by pitch compensation with an oppositely handed dopant. The surface stabilized ferroelectric liquid crystal display utilizes this structure and uses coupling between the electric field and the spontaneous polarization of the smectic C phase. The device uses a smectic C liquid crystal material in the so-called bookshelf structure shown in Fig. 21a. This device structure was fabricated by shearing thin (about 2 i,m) layers of liquid crystal in the... 

Illustration of the Surface Stabilized Ferroelectric Liquid Crystal Displays (SSFLCD). 

The realization of this device geometry was first applied in 1980 in the surface-stabilized ferroelectric liquid crystal display and provided much faster switching times than the nematic devices of the time (<0.1 ms) however, the main drawback of the smectic device has been the stability of liquid crystal alignment within the pixels. Nematics are very fluid-like, and after a deformation, they rapidly revert to their previous uniform state of alignment (think about what happens when you press on your laptop screen). Smectics are much more viscous and unfortunately do not self-repair when deformed. 

H. Fume, Y. limura, Y. Miyamoto, H. Endoh, H. Fukuro, and Sh. Kobayashi, Fabrication of a zigzag defect-free surface-stabilized ferroelectric liquid crystal display using polyimide orientation ftlm. Japanese Journal of Applied Physics 37, 3417 (1998). 

The surface-stabilized ferroelectric liquid crystals in the smectic C (SmC ) phase are among the most interesting types of liquid-crystalline systems because of their potential applications in high-resolution flat panel displays and fast electro-optical devices [73-76]. Within this class of compounds, ferroelectric liquid-crystalline polymers (FLCPs) have gained theoretical and practical interest as systems which combine the properties of polymers and ferroelectric liquid crystals. This combination is achieved by attaching the ferroelectric mesogen to a main chain via a flexible spacer... 

So far, four display modes have been proposed in ferroelectric and antiferroelectric display applications, as shown in Figure 9.34. A bistable switching in surface stabilized ferroelectric liquid crystals (SSFLCs) has been manufactured as a passive matrix liquid crystal display (PM-LCD). The counterpart of AFLC is a tristable switching, which is also a promising candidate for PM-LCD. In addition to these PM-LCDs, active matrix displays (AM-LCDs) are also proposed in FLC and AFLC materials, i.e., deformed helix FLCD (DHFLC) and V-shaped LCD (VLCD). In this section, PM-AFLCD and AM-VLCD will be described. 

Figure 5.29 (a) Bookshelf geometry in a surface-stabilized ferroelectric liquid crystal (SSFLC) display showing two states of polarization. Here the surface acts to unwind the helix, (b) Chevron geometry in an FLC. This disturbs the switchable polarization in the bookshelf geometry... 

Work in other display areas has of course occurred. Through the seminal work of R. B. Meyer and the research of Clark and Lagerwall [55] on surface stabilized ferroelectric liquid crystal devices based on chiral smectic C liquid crystal materials, the potential for ferroelectric devices has been fully explored in recent years. With their faster switching capability, they are attractive, and the difficulties over addressing schemes and the manufacture of ferroelectric displays will perhaps soon be overcome to give the marketplace a further liquid crystal device. 

Soon after the initial discovery of ferro-electricity in chiral smectic LCs it was predicted that, if the helix of an SmC phase were suppressed by surface forces in very thin layers between two glass electrodes, then this would pin the molecules in their positions and allow switching between two energetically equivalent polarization directions, thereby giving rise to an electro-optic memory effect [22]. This is the basis of the electro-optic display device called the surface stabilized ferroelectric liquid crystal... 

It is our belief that block copolymers containing LC segments are materials with novel and unencountered properties which will offer great opportunities for developing high performance materials. Here we would like to give two examples. One example is a microphase stabilized ferroelectric liquid crystal (MSFLC) [109] for potential flat panel display applications, while the other is a material for stable, low surface energy [110] application. 

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. 

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