Applications, molecular electronics liquid crystal displays

We are all familiar with tire tliree states of matter gases, liquids and solids. In tire 19tli century the liquid crystal state was discovered [1 and 2] tliis can be considered as tire fourtli state of matter [3].The essential features and properties of liquid crystal phases and tlieir relation to molecular stmcture are discussed here. Liquid crystals are encountered in liquid crystal displays (LCDs) in digital watches and otlier electronic equipment. Such applications are also considered later in tliis section. Surfactants and lipids fonn various types of liquid crystal phase but this is discussed in section C2.3. This section focuses on low-molecular-weight liquid crystals, polymer liquid crystals being discussed in tire previous section. 

Organic materials are used in the existing electronics industries mainly for passive purposes insulating and structural support materials. There are, however, exceptions, such as photoresists, liquid crystal displays, and electrocopying. More challenging to many researchers in a diversity of fields is the application of organic conductors from the viewpoint of the fabrication of molecular electronics, to which this chapter is devoted. 

One type of material that has transformed electronic displays is neither a solid nor a liquid, but something intermediate between the two. Liquid crystals are substances that flow like viscous liquids, but their molecules lie in a moderately orderly array, like those in a crystal. They are examples of a mesophase, an intermediate state of matter with the fluidity of a liquid and some of the molecular order of a solid. Liquid crystalline materials are finding many applications in the electronics industry because they are responsive to changes in temperature and electric fields. 

Low molecular mass compounds capable of forming liquid crystals have been known since the late 1880s. They did not assume commercial importance until the late 1960s, however, when their properties were exploited in the design of electronic displays. Following the development of commercial applications for liquid crystals, polymers began to be studied for their potential in this application. 

In linear optical processes the physical properties of the liquid ciystal, such as its molecular structure, individual or collective molecular orientation, temperature, density, population of electronic levels, and so forth, are not affected by the optical fields. The direction, amplitude, intensity, and phase of the optical fields are affected in a unidirectional way (i.e., by the physical parameters of the liquid ciystal). The optical properties of liquid crystals may, of course, be controlled by some externally applied dc or low-frequency fields this gives rise to a variety of electro-optieal effects which are widely used in mai eleetro-optieal display and image-processing applications as discussed in previous chapters. 

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