Crystal clear

Tuesday 30 June 1998

LCD technology goes further with new findings in liquid crystal behaviour

liquid crystal displays (lcds) are ubiquitous in modern electronic devices. From cellular phones to laptop computers and now televisions, they form an integral part of most consumer electronics. Given their importance, a lot of research is going on to improve the existing technology and have qualitatively better displays for the next generation of devices. Two recent developments might bring us closer to high performance displays for use in televisions and desktop monitors. G P Bryan-Brown and his colleagues at the Defence Evaluation and Research Agency in the uk have reported the development of a lcd with a new way of manipulating the crystal orientation. R A M Hikmet and H Kemperman of Phillips Research at Eindhoven, the Netherlands, have developed switchable mirrors from liquid crystal gels.Both these developments, though not related, could revolutionise the manufacture and use of lcds (Nature, Vol 392, No 6674 and 6675).

Liquid crystals, their very name an oxymoron, blend structures and proper ties of the normally disparate liquid and crystalline solid states. Liquids can flow while solids cannot. On the other hand, crystalline solids possess special symmetry properties that liquids lack. Between the crystalline solid at low temperatures and the ordinary liquid state at high temperatures lies an intermediate state, the liquid crystal. Liquid crystals, which are normally elongated organic molecules, share with liquids the ability to flow but also display symmetries inherited from crystalline solids. They can also move easily under the influence of electric fields.

The performance of high quality displays depends crucially on how well the orientation of the liquid crystal material can be controlled. lcd consists of a liquid crystal layer sandwiched between two substrate surfaces. In most displays today, a mechanism called the twisted nematic switching is used. In this, an electric field is used to turn the display black and white, hence forming an image. The disadvantage is that the image brightness depends on the viewing angle, as anyone who has used a laptop will know too well. To get around this problem, the Hitachi company of Japan introduced a new switching mechanism some years ago. However, it had low illumination, was slow to respond and was significantly more expensive to replace the conventional displays.

Byran-Brown and his team have found a way out. They have used a grooved upper substrate, which is then treated with a surfactant to orient the liquid crystal molecules. The lower substrate is then oriented parallel to the grooves above. The liquid crystal is itself of a type that aligns perpendicular to the electric field, making it different from the conventional material where the alignment is parallel. The result is that the behaviour of the liquid crystal cell is highly sensitive to the groove pattern and also has an optical electrical response that is much less dependent on the viewing angle. This makes it ideal for high performance applications such as wide-screen televisions. Though a lot of work will be needed before the applications hit the market, the device is a definite improvement over the expensive and slow Hitachi display.

The researchers at Phillips Research have worked on another aspect of lcds, namely their use as switchable reflectors. For this, they used a cholestric liquid crystal phase. Normal liquid crystal materials are in what is called a nematic phase. Here, the molecule positions are disordered in all directions but their orientations are alike.This phase can be changed to the cholestric liquid crystal phase by doping it with chiral molecules (molecules that have a handedness). The researchers created cross-linked cholestric gels by using photopolymerisation in a patterned manner of liquid crystal monoacrylate and diacrylate mixture in the presence of other liquid crystal molecules. The result is that they have successfully made image recordings that become visible on application of an electric field. This could prove to be very useful in manufacture of optical devices such as lenses, shutters, reflectors and filters.

Both these developments have tremendous commercial potential if further research proves the viability of mass production. But one cannot be too sure because the gap between demonstrating a device or an effect in the laboratory does not necessarily mean that it would be a commercial success, as evidenced by Hitachi's development of switchable displays. Nevertheless, the researchers are hopeful that their work has the potential of revolutionising the field of lcds.

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