In the June 6 issue of Science, they show how molecular anchors made of simple hydrogen- and carbon-based compounds can provide a stable mooring for liquid crystals in patterns, allowing researchers to exploit the unique qualities of the liquid crystal for practical uses on both flat and curved surfaces, such as diffraction gratings and curved viewing screens. These anchored liquid crystals also promise to be a powerful tool in the study of the surfaces of materials. Liquid crystals constitute a state of matter, just like the more widely known states of liquid, solid and gas. In liquid crystal form, molecules are free to move about in space, but are ordered in such a way that the molecules, on average, point in the same direction. If molecules can be compared to soldiers, then the solid state would be the soldiers standing at attention (facing the same direction and frozen in position), the liquid crystal state would be the soldiers marching in formation (all in the same direction, but free to move), and the liquid state would be the soldiers wandering around at a carnival (free to move around at random).
Despite the high-tech nature of many liquid crystal uses, Abbott emphasizes that the liquid crystals themselves are not "exotic chemicals." The liquid crystal state is one that many types of materials may form, given the right conditions, appearing anywhere from red blood cells to detergent to alkylcyanobyphenyls, the workhorse of the liquid crystal industry.
Liquid crystals retain many of the optical qualities -- such as the ability to bend light and change its color -- as their more stationary solid crystal counterparts, but they also have the added advantage of a liquid's mobility, so they are easily moved around in an electric field. The combination of these qualities can make liquid crystals change colors when heated, important for such devices as thermometers, or can allow light through the liquid crystal when electricity is applied, creating things like liquid crystal displays, or LCDs. In a standard application, such as a computer screen, the surface is coated with a long-chain molecule, and then rubbed with cloth so that the crystals will face a preferred direction. Rubbing introduces all kinds of imperfections, such as scratches, dust, and static, but it has been very successful at allowing alignment of liquid crystals over a large area.
Abbott and Gupta were interested in getting away from the process of rubbing, and looking at how to align the crystals in different ways relative to the surface; in the past, the liquid crystals generally have been aligned in a single direction.
"The challenge was how to make a surface such that the molecules will spontaneously assume a certain pattern of orientations relative to the surface," Abbott says.
Abbott and Gupta found a way to make the liquid crystals line up by building a sandwich of two glass plates, one with a uniform orientation of anchors, and the other with a maze of anchoring docks. Rather than a physical connection like a boat's anchor, these molecular anchors function as berths to orient the liquid crystal molecules.
Metals, such as gold, have many free electrons flowing along their surfaces that cause electrically-sensitive liquid crystal molecules to orient themselves relative to the metal surface. Using an extremely thin layer of gold, 30 to 50 molecules thick, adhered to a glass plate, the researchers added another single layer of molecules called alkanethiols, ranging from four to 18 carbons long. Very few things stick strongly to the surface of gold except alkanethiols, and once the layer has formed (which happens spontaneously), it directs the orientation of liquid crystals applied on top of it.
To make the more intricate patterns required of a diffraction grating (used to direct the flow of light in many applications), the researchers applied the thiol with, as Gupta explains, a method comparable to inking a rubber stamp. First the researchers "ink" a tiny stamp with thiol solution, then they "stamp" it on the gold-coated surface. The thiol remains in pattern of stamp. By using different thiols, the researchers created grids that caused sections of liquid crystal flowing above the thiols to orient differently from neighboring sections.
Abbott, who describes himself as a surface scientist, is interested in the effects of liquid crystals from the standpoint of how they can be used to give information about what is going on at a surface. "What is most exciting to me," he says, "is to see how subtle the changes in surface structure can be, and how large the changes in liquid crystal behavior can be in response to them." He explains that the 30 or so molecules of gold on the surface, plus one molecule of thiol, directs the behavior of 100,000 molecules of liquid crystal above the surface.
Abbott does not expect that this technology will change the well-established methods of preparing thin layers of liquid crystal for portable computer screens or digital watches. Where it may have an impact is for specialty devices using a curved surface, such as optical lenses and screens that would be visible from many different angles, like movie screens on airplanes. Abbott also envisions someday having a blood test based on liquid crystals: for example, a glass plate coated with antibodies to viral proteins. A blood sample could be spread on it, then a layer of liquid crystals; if the blood had proteins in it, it would transmit light differently than if there were not proteins, and this could be immediately apparent to the naked eye.
About the anchoring process in general, Gupta says, "There are lots of issues to be worked out before this technology can be put into practice."
Their research was funded by the National Science Foundation, as part of the Center for Polymer Interfaces and Macromolecular Assemblies, a group consisting of UC Davis, Stanford University, and IBM Almaden. This is the first NSF-funded center that includes a corporate laboratory as an equal partner.
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