Director, Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST)
Nano-structured liquid crystals
—Frontier of liquid crystal research—
Liquid crystal displays are now an indispensable part of TVs, cellular phones, digital cameras and so on. Since the early stage of its development, Dr. Yokoyama has taken a lead in the fundamental research involving the liquid crystal/substrate interface. Now, he has opened the door to a whole new world of liquid crystal science and technology.
In liquid crystal devices, such as displays, liquid crystal molecules are sandwiched between surface-treated glass plates in the proper alignment. The orientation of the liquid crystal molecules directly influences the performance of the devices. Demand for refinement and higher working speeds of the devices has made the understanding of the liquid crystal/substrate interface essential. Dr. Yokoyama says, “I was doing fundamental research on the interface, and my interest happened to match industrial needs.” In the early 80’s, he discovered an “interface-induced phase transition”, which is a different phase transition from that of bulk liquid crystals, at the liquid crystal/substrate interface. After that, he worked to characterize thermodynamically the interface. Since the scaling rule can be applied to liquid crystals, he was able to develop a method for measuring the electro-optical response of the interface without any influence from the bulk liquid crystals. This method has been used in the manufacturing of liquid crystal devices to characterize the substrate/liquid crystal interface prepared by orientation treatments such as rubbing and deposition.
“There are three approaches to the characterization of liquid crystal/substrate interfaces: the first is a macroscopic approach using thermodynamics, the second is a molecular theory-approach using statistical mechanics, and the third is a microscopic approach using the properties of the individual molecules,” says Dr. Yokoyama. After implementing a thermodynamic approach, his interest gradually shifted to the microscopic approach, and in the early 90’s, he devised a new method to determine the orientation of individual liquid crystal by measuring the electrostatic potential on a surface and developed the scanning Maxwell stress microscope by modifying an atomic force microscope (AFM). The scanning Maxwell stress microscope was designed to simultaneously investigate electrical properties, such as potentials and charges, and surface shapes at the atomic/molecular level, and it enabled Dr. Yokoyama to study the liquid crystal/substrate interface on the nano-scale.
In 1999, the “Yokoyama Nano-Structured Liquid Crystal Project”, the Japan Science and Technology Agency (JST), was launched. Liquid crystal molecules form nano-sized aggregates and these aggregates form a higher-order structure due to long-range interactions. The goal of the project has been to create liquid crystals with novel properties by controlling the structure of the nano-sized aggregates, which is the basic unit of its hierarchical structure. One of his group’s achievements is a “tristable memory liquid crystal device”. Dr. Yokoyama has elucidated the principles behind this type of device and experimentally confirmed that it worked. In conventional orientated liquid crystals, producing a memory device is not possible because the liquid crystal molecules are stable only in one orientation. However, liquid crystals with three metastable orientations can be prepared by creating a pattern with six-fold rotational symmetry on the surface of the substrates by AFM nano-rubbing. When an electric field is applied parallel to one of the three metastable orientations, the orientation is maintained without the electric field. This technology has opened the way to design an ultra-low power-consumption display, where the images never go away even when it is turned off.
“Nanotechnology is a fairly new field of technology, and it gives researchers opportunities to explore the most state-of-the-art scientific field. I hope young researchers will not be intimidated by this unknown field. I want them to pursue not only currently known research areas but also to create their own research areas in nanotechnology. If others say your research is unsuitable for nanotechnology, you should take it as a compliment and keep pursuing it,” says Dr. Yokoyama. He continued his research despite opinions that the area of liquid crystals had been fully explored. He adds, “Doing research is somewhat similar to archaeological excavation. Even if you make a finding, it is not the end. There are more underneath the finding.”
