Introduction to Liquid Crystals
Liquid crystals (LCs) are matter in a state that has properties between those of conventional liquid and those of solid crystal. Liquid crystal materials are unique in their properties and uses. As research into this field continues and as new applications are developed, liquid crystals will play an important role in modern technology. This tutorial provides an introduction to the science and applications of these materials. For instance, an LC may flow like a liquid, but its molecules may be orientated in a crystal-like way.
There are many different types of LC phases, which can be distinguished by their different optical properties (such as birefringence). When viewed under a microscope using a polarized light source, different liquid crystal phases will appear to have distinct textures. The contrasting areas in the textures correspond to domains where the LC molecules are oriented in different directions. Within a domain, however, the molecules are well ordered. LC materials may not always be in an LC phase (just as water may turn into ice or steam).
Liquid crystal materials were first discovered in 1888 by an Austrian botanist, F. Renitzer. However those liquid crystals were not suitable for any commercial usage, and it is only 25 years ago since the first material suitable for electronically driven displays, was developed. The first room-temperature nematic liquid crystal was observed in the late 1960s. Unfortunately this crystal had quite a short temperature range as it was affected by impurities. Occasionally in homologous series the temperature range could reach from –40 to +100 degrees Celsius. Unfortunately these mixtures were very unstable and they possessed a negative dielectric anisotropy not useful in the twist cell.
The major breakthrough came when cyanobiphenyl materials were discovered a few years later. The more stable phase had a large positive dielectric anisotropy as well as a strong birefringence nearly ideal for the twist cell. During the 1970s and 1980s several liquid crystal compounds and phases were discovered, primarily by the industry, but also in several research programs on liquid crystal materials in colleges and universities around the world.
The ferroelectric chiral smecic (FLC) phase was discovered in 1975 and proved to have a unique form of ferroelectricity. The first display based on the FLC phase was actually patented in 1980. Another example of new liquid crystal phases also discovered during this intense research period, are the forms of polymer dispersions. Also a new effect, the electroclinic effect, was discovered during this period and is now being carefully studied for possible future display applications. Lately several new materials has been discovered such as the retardation film which is extremely important for the supertwisted nematic (STN) and twisted nematic (TN) displays.
Most of this research is situated in Japan due to strong manufacturing capability and high research funds. The most frequently used liquid crystalline phase used today in display devices is the nematic phase. The phase is used both in the TN cell as well as in the active matrix (AM) TN cell. About 60 % of all nematic materials supplied by Merck-Japan, which is the biggest producer of such materials, goes to these applications. The active matrix TN cell is expected to grow substantially during the next 5 years as this technology dominates the manufacturing industry today. Other types of rapidly growing display types are the electrically controlled birefringence (ECB) and polymer-dispersed liquid crystals (PDLCs). Due to the relatively recent technology discovered in producing these cells, they have not yet reached a fully commercial usage.
Liquid crystal displays (LCD’s) offer several advantages over traditional cathode-ray tube displays. LCD’s are flat, and they use only a fraction of the power required by CRT’s. They are easier to read and more pleasant to work with for long periods of time than most ordinary video monitors. One should also now that there are several tradeoffs, such as limited view angel, brightness, and contrast, not to mention high manufacturing costs. As research continues, this limitations are slowly becoming less significant.
Today’s LCD’s come mostly in two flavors passive and active. The less expensive passive matrix displays trade off picture quality, view angel, and response time with power requirements and manufacturing costs. Active matrix displays have superior picture quality and viewing characteristics, but need more power to run and are much more expensive to fabricate.
Liquid crystal displays show great potential for the future and there are improvements to be made. The next question is what are the physical limits of this technology ? To answer this question we need to explain liquid crystal in general to determine what characteristics it has and what makes it so appropriate for use in displays. We need to examine in detail the two common kinds of liquid crystal displays passive and active matrix to see how each works.
Liquid crystal is a fourth “state” that certain kinds of matter can enter into under the right conditions. The molecules in solids exhibit both positional and orientational order, in other words the molecules are constrained to point only certain directions and to be only in certain positions with respect to each other. In liquids the molecules do not have any positional or orientational order, the direction the molecules point and positions are random.
The liquid crystal “phase” exists between the solid and liquid phase, the molecules in liquid crystal do not exhibit any positional order, but they do possess a certain degree of orientational order. The molecules do not all point the same direction all the time. They tend to point more in one direction over time than other directions. This direction is referred to as the director of the liquid crystal. The “amount” of order is measured by the order parameter of the liquid crystal. This order parameter is highly dependent on the temperature of the sample. See figure 3.1, a typical order vs. temperature relationship. Tc is the temperature of transition between the liquid crystal and the liquid states.