More than 40 years ago, the author was eager to study semiconductor technology as it was important in state-of-art electronics at that time, and joined Professor Masanobu Wada's laboratory at the Department of Electronics of Tohoku University because he was an authority on semiconductor electronics. In those days, Professor Wada predicted the progress of semiconductor technology and an information society, and the necessity of low-power, thin, and lightweight displays compatible with semiconductors. In the mean time, the liquid crystal display was proposed by Heilmeier et al.1) and Professor Wada thought that this was the material he desired. When the author went to graduate school after one year of research on semiconductors, he was recommended by Professor Wada to study liquid crystal. This was his first encounter with liquid crystal.
Just after the author started to study liquid crystal, he found that there were no experimental materials, instruments, literature on liquid crystal or colleagues to discuss it with in the laboratory, and he realised what a serious choice he had made. Anyway, he had to start a fundamental study of chemistry to synthesize liquid crystal material, and visited the library of the Department of Chemistry in Tohoku University every day. It took about half a year to master the fundamental knowledge required to synthesize chemical compounds. Then, the author decided to synthesize p-methoxybenzylidene-p'-n-butylaniline (MBBA) as shown in Fig. 1, which was a room-temperature liquid crystal recently discoverd.2) This was again a serious choice because it was necessary to use the difficult process of the Friedel–Crafts reaction3) at high temperatures and pressures to synthesize p-n-butylaniline, which is a composition material of MBBA.
After various difficulties related to synthesis and purification, extremely pure MBBA was successfully obtained. This took about one year from starting to study the synthesis of liquid crystal.
After a while, the author started to study the electronic and optical properties of liquid crystal and he found that the liquid crystal molecule aligned sometimes parallel and sometimes perpendicular to the glass substrates,4) as shown in Fig. 2, and these alignments strongly affected the electrooptical properties.5,6) Therefore, he decided to investigate the mechanism of the molecular alignment because it was predicted that the control of alignment would become one of the most important fundamental technologies of liquid crystal displays (LCDs).
The author and several supervised students have successfully clarified the molecular alignment mechanism: The microstructure of the substrate surface, such as the orientation state of the organic molecules coated or adsorbed on the surface, had a dominant effect on the molecular alignment of liquid crystal,7) as shown in Fig. 3, which was based on the molecular interaction between the liquid crystal and the surface.8,9) Furthermore, a method of measuring the surface alignment force (anchoring strength)10,11) and surface order parameter of liquid crystal12) was established. In addition, it was found that there was a strong correlation among the surface microstructure, anchoring strength, and surface order parameter.12–15) However, the author will skip the details as space is limited, and move the subject to LCDs.
With the progress of technology of integrated circuits (ICs), calculators became a pocket size from a table top type in 1972, and were widely used. However, the consumption of batteries was intense and the reduction of power consumption became an important subject. In the meantime, Sharp Corp. mass-produced a significantly low power calculator using LCD and CMOS-IC in 1973,16) as shown in Fig. 4, and it spread rapidly. The LCD was well suited for CMOS technology, and their combination produced new products one after another from the 1970s through the 1980s, by taking advantage of the features such as low voltage, low current and high integration (Table I). The products included an electronic typewriter and a word processor equipped with LCDs of several lines character display. Thereafter, a laptop computer began to be developed. For this application, the LCD with scanning line numbers of about 60 was used at first. Then, it became the main subject of LCD research to increase this number up to about 500.
Table I. Appearance of the new products by the collaboration of the LCD and semiconductor from the 1970s through the 1980s.
|Calculator||Numeric: 5–10 digits|
|Electronic typewriter||Alphabet: 1–2 lines|
|Word processor||Chinese character: 1–5 lines|
|Personal computer||Scanning line number: 60–120 lines|
The direct matrix drive LCDs used for the character displays in those days, had a severe problem in that the contrast decreased with the increase in scanning line number,17,18) as shown in Fig. 5, and this dilemma was thought to be inevitable. Rumors began that this was the limit of LCDs, and that there would be no future for them. The liquid crystal sessions of every society declined rapidly. The author was also advised by some professors and his peers that he should quit liquid crystal research as soon as possible because organic materials as well as liquid materials had never been used for the active device of electronics. Then, he looked for new areas of research other than liquid crystal for several years. However, one day, he noticed that he had been observing liquid crystal with the eyes of an outsider, and that the future of the liquid crystal depended on his effort. Therefore, he decided to stay in the liquid crystal research field.
In the mean time the contrast of LCDs with large scanning line numbers was significantly improved by the discovery of the supertwisted birefringent effect (SBE),19,20) supertwisted nematic liquid crystal (STN),21) double-STN,22,23) as shown in Fig. 6, and film-compensated STN.24–28) In addition, an amorphous silicon thin-film transistor was devised29) and it promoted the development of an active matrix LCD,30–33) which drastically solved the above-mentioned fundamental problem of LCDs.
On the other hand, the author worried about what kind of research he should do as a researcher of a university with very poor manpower and research funds compared with companies. Then, he decided to challenge a prospective theme that would become one of the most important themes in ten years, even though it would be very difficult, and fixed his attention on realizing full-color LCDs.
There had been many methods proposed to obtain color LCDs as follows.
- Guest–host mode34)
- Selective reflection of cholesteric liquid crystal35)
- Birefringence mode such as deformation of vertical alignment phase (DAP)36,37) and electrically controlled birefringence (ECB)38,39)
- DAP + cholesteric liquid crystal40)
- Twisted nematic cell (TN-cell)41) + color polarizers42,43)
- TN-cell + retardation films43,44)
- Optical rotatory dispersion of chiral nematic liquid crystal45,46)
- CRT + liquid crystal shutter (field sequential)47–50)
- Flat CRT + LCD (field sequential)51)
However, it seemed that none of them were practical. Then, almost all possible ideas to realize color LCDs were examined, and a solution to use the-three-primary-color mixture system was finally reached. There are two systems, as shown in Fig. 7; the additive mixture type using red, green, and blue, which are called the primary colors of light, and the subtractive mixture type using cyan, magenta, and yellow, which are called the primary colors of paints. Since the liquid crystal does not emit light, the author thought that it was natural to use the subtractive mixture type the same as paints. In this case, the structure shown in Fig. 850,52) was obtained using a guest–host-type LCD34,53–55) with dichroic dyes of three primary colors, and its trial production was successfully carried out.50,52) However, it was predicted that this type of LCD would not be practical for future high-resolution color LCDs, because color parallax occurred in oblique observation.
Then, the author continued to look for another solution to this problem, and finally arrived at the idea to change the arrangement of color elements, as shown in Fig. 9; namely, the three primary colors were arranged laterally.56,57) Only by this change, the color mixture became of the additive type and the primary colors changed to red, green, and blue [Fig. 9(b)]. Figure 10 shows the actual structure of this color LCD using the additive type mixture. It was composed of a usual black-and-white LCD and color filters formed inside the LCD cell to prevent the above mentioned parallax.56,57) By this method, the full color LCD with the possibility of high-resolution display was achieved successfully. However, when the author first presented this proposal, there were several severe objections. They were summarized as the following two problems. One of them was poor color purity, as was known in usual nonemissive materials such as printed papers. The other one was the relatively high power consumption due to the back light. The first problem of the color purity was considered to be solved as it was related to the spectrum and would be controllable, while the second problem of power consumption was an inevitable subject at that time.
Nevertheless, this method was adopted by many researchers of LCD,30–33,58,59) and has recently been widely used for liquid crystal televisions, notebook PCs, display monitors, and smartphones, as shown in Fig. 11, in which the first problem of color purity has been solved, as was expected. However, the second problem of power consumption has not yet been solved sufficiently. Therefore, the author decided to search for a solution to this problem and has been continuing this research.
7.1. Field sequential color LCD without color filter
To solve the problem of the power consumption of the color LCD, the color filter was removed; namely, a monochrome LCD and back lights of red, green, and blue were used, as shown in Fig. 12.60–63) The three color back lights were turned on sequentially and the color image of LCD was changed quickly according to the back light color. This method has the advantages of several times higher brightness (or lower back light energy) and three times higher resolution than the current color filter type. In this method, it was necessary to use an LCD with a response time shorter than 5 ms, which is 10 to 50 times faster than the conventional LCD. The author and coworkers fortunately developed the optically compensated bend cell (OCB-cell), as shown in Fig. 13.64–66) This LCD had bend alignment of liquid crystal molecules, which was first proposed by Bos et al.67) However, the bend alignment was unstable and he used this device as an alternative switching device of retardation to obtain multicolor operation of the oscilloscope.47) To solve the instability of the bend alignment, the authors64,65) theoretically analyzed the free energy as a function of voltage and found that the bend alignment became stable by using bias voltage. Then, the retardation of the bend-aligned liquid crystal cell was three-dimensionally compensated by using biaxial retardation films, as shown in Fig. 13. As a result, the OCB-cell obtained a very wide viewing angle and an extremely fast response (1–5 ms).
This field sequential color LCD was successively researched and developed as the Local Collective Research Development Program of Aomori Prefecture supported by Japan Science and Technology Agency (JST), and has been test-fabricated successfully,68–72) as shown in Fig. 14. Although it has not yet been put into practice, it is expected as the next-generation LCD.
7.2. Reflective color LCD without back light
For the further-low-power LCD, a reflective color LCD without a back light73) has been devised and developed, as shown in Fig. 15(a). To increase brightness, the diffusing angle of the reflected light was limited within a certain range. Such a property was obtained by using a metal reflector with a suitable roughness, which was designed by theoretical calculation.74,75) As a result, a significantly high brightness of the reflected light was obtained, while the LCD became metallic silver, different from the desired paper-like display. Then, this characteristic was analyzed, and the reason for this was found to be the strong dependence of the reflection intensity as a function of angle, as shown by curve A in Fig. 16, which was quite different from that of white paper (see curve B in Fig. 16). The authors tried to find the surface microstructure that provides constant reflectance like paper but within ±30° from the specular direction by theoretical calculation, as shown by curve C in Fig. 16. Then, the solution was found to be a parabolic structure.76,77) The size of the parabolic structure should be changed randomly with keeping a similar figure, as shown in Fig. 15(b), to prevent diffraction that occurs in the case of the periodic structure.
Subsequently, in cooperation with Sharp Corp. and other companies, the reflective color LCD was actually developed,78) and several years later, it was put into practical use as personal digital assistants (PDAs), portable game machines, and mobile phones. It induced a new business of small LCDs with high added value, and has progressed to the recent small LCDs with ultrahigh resolution for smartphones and tablets.
Finally, the future progress of the man-machine interface is discussed. Humans obtain ambient information through the five senses. Among them, the sense of vision is dominant and more than 85% of the ambient information is input as visual information. This is related to the processing speed of the five human senses, as shown in Fig. 17
Researchers examined the spiral “twist-bend” structure (right) formed by boomerang-shaped liquid crystal molecules (left and center) measuring 3 nanometers in length, using a pioneering X-ray technique at Berkeley Lab’s Advanced Light Source. A better understanding of this spiral form, discovered in 2013, could lead to new applications for liquid crystals and improved liquid-crystal display screens. (Credit: Zosia Rostomian/Berkeley Lab; Physical Review Letters, DOI: 10.1103/PhysRevLett.116.147803; Journal of Materials Chemistry C, DOI: 10.1039/C4TC01927J)
Liquid crystals, discovered more than 125 years ago, are at work behind the screens of TV and computer monitors, clocks, watches and most other electronics displays, and scientists are still discovering new twists—and bends—in their molecular makeup.
Liquid crystals are an exotic state of matter that flows like a fluid but in which the molecules may be oriented in a crystal-like way. At the microscopic scale, liquid crystals come in several different configurations, including a naturally spiraling “twist-bend” molecular arrangement, discovered in 2013, that has excited a flurry of new research.
Now, using a pioneering X-ray technique developed at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), a research team has recorded the first direct measurements confirming a tightly wound spiral molecular arrangement that could help unravel the mysteries of its formation and possibly improve liquid-crystal display (LCD) performance, such as the speed at which they selectively switch light on or off in tiny screen areas.
The findings could also help explain how so-called “chiral” structure—molecules can exhibit wildly different properties based on their left- or right-handedness (chirality), which is of interest in biology, materials science and chemistry—can form from organic molecules that do not exhibit such handedness.
“This newly discovered ‘twist-bend’ phase of liquid crystals is one of the hottest topics in liquid crystal research,” said Chenhui Zhu, a research scientist at Berkeley Lab’s Advanced Light Source (ALS), where the X-ray studies were performed.
“Now, we have provided the first definitive evidence for the twist-bend structure. The determination of this structure will without question advance our understanding of its properties, such as its response to temperature and to stress, which may help improve how we operate the current generation of LCDs.”
Zhu was the lead author on a related research paper published in the April 7 edition of Physical Review Letters.
While there are now several competing screen technologies to standard LCDs, the standard LCD market is still huge, representing more than one-third of the revenue in the electronic display market. The overall display market is expected to top $150 billion in revenue this year.
The individual molecules in the structure determined at Berkeley Lab are constructed like flexible, nanoscale boomerangs, just a few nanometers, or billionths of a meter, in length and with rigid ends and flexible middles. In the twist-bend phase, the spiraling structure they form resembles a bunch of snakes lined up and then wound snugly around the length of an invisible pole.
Zhu tuned low-energy or “soft” X-rays at the ALS to examine carbon atoms in the liquid crystal molecules, which provided details about the molecular orientation of their chemical bonds and the structure they formed. The technique he used for the study is known as soft X-ray scattering. The spiraling, helical molecular arrangement of the liquid crystal samples would have been undetectable by conventional X-ray scattering techniques.
The measurements show that the liquid crystals complete a 360-degree twist-bend over a distance of just 8 nanometers at room temperature, which Zhu said is an “amazingly short” distance given that each molecule is 3 nanometers long, and such a strongly coiled structure is very rare.
The driving force for the formation of the tight spiral in the twist-bend arrangement is still unclear, and the structure exhibits unusual optical properties that also warrant further study, Zhu said.
Researchers found that the spiral “pitch,” or width of one complete spiral turn, becomes a little longer with increasing temperature, and the spiral abruptly disappears at sufficiently high temperature as the material adopts a different configuration.
“Currently, this experiment can’t be done anywhere else,” Zhu said. “We are the first team to use this soft X-ray scattering technique to study this liquid-crystal phase.”
Standard LCDs often use nematic liquid crystals, a phase of liquid crystals that naturally align in the same direction—like a group of compass needles that are parallel to one another, pointing in one direction.
In these standard LCD devices, rod-like liquid crystal molecules are sandwiched between specially treated plates of glass that cause the molecules to “lie down” rather than point toward the glass. The glass is typically treated to induce a 90-degree twist in the molecular arrangement, so that the molecules closest to one glass plate are perpendicular to those closest to the other glass plate.
It’s like a series of compass needles made to face north at the top, smoothly reorienting to the northeast in the middle, and pointing east at the bottom. This molecularly twisted state is then electrically distorted to allow polarized light to pass through at varying brightness, for example, or to block light (by straightening the twist completely).
Future experiments will explore how the spirals depend on molecular shape and respond to variations in temperature, electric field, ultraviolet light, and stress, Zhu added.
He also hopes to explore similar spiraling structures, such as a liquid crystal phase known as the helical nanofilament, which shows promise for solar energy applications. Studies of DNA, synthetic proteins, and amyloid fibrils such as those associated with Alzheimer’s disease, might help explain the role of handedness in how organic molecules self-assemble.
With brighter, more laser-like X-ray sources and faster X-ray detectors, it may be possible to see details in how the spiraling twist-bend structure forms and fluctuates in real time in materials, Zhu also said.
“I am hoping our ongoing experiments can provide unique information to benefit other theories and experiments in this field,” he noted.
Other team members include Anthony Young, Cheng Wang, and Alexander Hexemer at Berkeley Lab, and Michael Tuchband, Min Shuai, Alyssa Scarbrough, David Walba, Joseph Maclennan, and Noel Clark at the University of Colorado Boulder.
Soft X-ray scattering measurements were conducted at Beamline 11.0.1 at the Advanced Light Source, a DOE Office of Science User Facility at Berkeley Lab. The work was supported by the DOE Office of Basic Energy Sciences and the National Science Foundation.
Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit www.lbl.gov.
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Updated: April 25, 2016