Researchers at Tohoku University have developed a super flexible liquid crystal (LC) device, in which two ultra-thin plastic substrates are firmly bonded by polymer wall spacers.
The team, led by Professor Hideo Fujikake and Associate Professor Takahiro Ishinabe of the School of Engineering, hopes the new organic materials will help make electronic displays and devices more flexible, increasing their portability and all round versatility. New usage concepts with flexibility and high quality display could offer endless possibilities in near-future information services.
Previous attempts to create a flexible display using an organic light-emitting diode (OLED) device with a thin plastic substrate were said to be promising, but unstable. The plastic substrates are poor gas-barriers for oxygen and water vapor, and the OLED materials can seriously be damaged by their gasses. As for flexible OLEDs, there has also been no device fabrication technology established so far for large-area, high-resolution and low-cost displays.
To overcome these challenges, Fujikake’s research team decided to try making existing LC displays flexible by replacing the conventional thick glass substrates, which are both rigid and heavy, with the plastic substrates, because LC materials do not deteriorate even for poor gas barrier of flexible substrates.
Flexible LC displays have many advantages, such as established production methods for large-area displays. The material itself, which is inexpensive, can be mass produced and shows little quality degradation over time.
However, in conventional flexible LC displays, one important problem remains. The gap of plastic substrates (100 ?m thick) sandwiching an LC layer becomes non-uniformed when the LC device is bent, causing the display image to be distorted.
In their study, Fujikake’s team developed a super-flexible LC device by bonding two ultra-thin transparent polyimide substrates (10 ?m thick approximately) together, using robust polymer wall spacers.
|The structure of super-flexible LC device is created by ultra-thin plastic substrates bonded by polymer wall spacers.|
The ultra-thin transparent substrate is made using the coating and debonding processes of a polyimide solution supplied by Mitsui Chemicals. The result is a flexible sheet, similar to food-wrapping cling film.
|The ultra-thin polyimide film (left) was formed by coating and debonding processes, and the roll-up resistance (right) was tested for developing super-flexible LC devices.|
The substrate has the attractive features of heat resistance, and the ability to form fine pixel structures, including transparent electrodes and colour filters. The refractive index anisotropy is extremely small, making wide viewing angles and high contrast ratio possible.
The polymer wall spacers bonding substrates are formed by irradiating a twisted-alignment LC layer including monomer component with patterned ultra-violet light through single thin substrate. While the substrate gap is more variable as the substrate thickness is decreased, the stabilization of ultra-thin substrates becomes possible by small pitch polymer walls.
The research team also demonstrated that the device uniformity is kept without breaking spacers even after a roll-up test to a curvature radius of 3mm for rollable and foldable applications.
The above research results show that LC displays with large-area, high-resolution and excellent stability can be as flexible as OLED displays. The super-flexible LC technology is applicable to mobile information terminals, wearable devices, in-vehicle displays and large digital signage.
Moving forward, the team plans to form image pixels and soften the peripheral components of polarizing films, and a thin light-guide sheet for backlight.
Scientists have created the world’s thinnest lens, one two-thousandth the thickness of a human hair, opening the door to flexible computer displays and a revolution in miniature cameras.
Lead researcher Dr Yuerui (Larry) Lu from ANU Research School of Engineering said the discovery hinged on the remarkable potential of the molybdenum disulphide crystal.
“This type of material is the perfect candidate for future flexible displays,” said Dr Lu, leader of Nano-Electro-Mechanical System (NEMS) Laboratory in the ANU Research School of Engineering.
“We will also be able to use arrays of micro lenses to mimic the compound eyes of insects.”
The 6.3-nanometre lens outshines previous ultra-thin flat lenses, made from 50-nanometre thick gold nano-bar arrays, known as a metamaterial.
“Molybdenum disulphide is an amazing crystal,” said Dr Lu
“It survives at high temperatures, is a lubricant, a good semiconductor and can emit photons too.
“The capability of manipulating the flow of light in atomic scale opens an exciting avenue towards unprecedented miniaturisation of optical components and the integration of advanced optical functionalities.”
Molybdenum disulphide is in a class of materials known as chalcogenide glasses that have flexible electronic characteristics that have made them popular for high-technology components.
Dr Lu’s team created their lens from a crystal 6.3-nanometres thick – 9 atomic layers – which they had peeled off a larger piece of molybdenum disulphide with sticky tape.
They then created a 10-micron radius lens, using a focussed ion beam to shave off the layers atom by atom, until they had the dome shape of the lens.
The team discovered that single layers of molybdenum disulphide, 0.7 nanometres thick, had remarkable optical properties, appearing to a light beam to be 50 times thicker, at 38 nanometres. This property, known as optical path length, determines the phase of the light and governs interference and diffraction of light as it propagates.
“At the beginning we couldn’t imagine why molybdenum disulphide had such surprising properties,” said Dr Lu.
Collaborator Assistant Professor Zongfu Yu at the University of Wisconsin, Madison, developed a simulation and showed that light was bouncing back and forth many times inside the high refractive index crystal layers before passing through.
Molybdenum disulphide crystal’s refractive index, the property that quantifies the strength of a material’s effect on light, has a high value of 5.5. For comparison, diamond, whose high refractive index causes its sparkle, is only 2.4, and water’s refractive index is 1.3.
Learn more: World’s thinnest lens to revolutionise cameras