Lithography brings liquid crystals into line

17 Jun 2002

US scientists could have the answer to the long-standing problem of molecular alignment in liquid crystals.

Cast your mind back a couple of decades and, chances are, this was when you bought your first digital watch. This 1980s icon was made possible by scientists who exploited the optical properties of nematic liquid crystals. By using electric fields to align a crystal's molecules, scientists were able to control its birefringence or multiple refracting properties. Such methods have led to today's widespread use of nematics in electro-optical applications.

But for many years, controlling molecule orientation has troubled scientists. Previous methods that relied on molecular-scale anisotropy or surface roughness to guide molecule direction have lacked accuracy. US researchers, however, claim to have demonstrated a lithography-based technique that overcomes this problem, (Science 291 2576).

Baek-woon Lee and Noel Clark from the University of Colorado took a glass substrate and deposited a silicon-based monolayer. By covering the surface with a lithographic mask and blasting it with an ultraviolet light, they removed narrow sections of the monolayer to leave a striped monolayer-glass pattern. They then sandwiched liquid crystal between two such surfaces and discovered that the liquid-crystal molecules lined up with the monolayer-glass boundaries.

"Liquid-crystal alignment is achievable without molecular anisotropy or roughness," they said. "Lithographically dividing a surface into two molecularly smooth isotropic regions produces alignment that is governed by the boundary lines between them."

Lee and Clark have produced similar results using microcontact printing and are keen to take their research further. "We believe that any complex pattern of nematic alignment can be easily fabricated at micrometer resolution," they said. "But to make effective devices, comparable results must be obtained on indium tin oxide and other transparent conducting substrates."