By Zeeya Merali
Ink stamps have been used to print text and pictures for centuries. Now, engineers have adapted the technique to build pixels into the first full-colour 'quantum dot' display -- a feat that could eventually lead to televisions that are more energy-efficient and have sharper screen images than anything available today.
Engineers have been hoping to make improved television displays with the help of quantum dots -- semiconducting crystals billionths of a metre across -- for more than a decade. The dots could produce much crisper images than those in liquid-crystal displays, because quantum dots emit light at an extremely narrow, and finely tunable, range of wavelengths.
The colour of the light generated depends only on the size of the nanocrystal, says Byoung Lyong Choi, an electronic engineer at the Samsung Advanced Institute of Technology in Yongin, South Korea. Quantum dots also convert electrical power to light efficiently, making them ideal for use in energy-saving lighting and display devices.
Easier said than done
Attempts to commercialize the technology have been hampered because it is difficult to make large quantum-dot displays without compromising the quality of the image. The dots are usually layered onto the material used to make the display by spraying them onto the surface -- a technique similar to that of an ink-jet printer. But the dots must be prepared in an organic solvent, which "contaminates the display, reducing the brightness of the colours and the energy efficiency", says Choi.
Choi and his colleagues have now found a way to bypass this obstacle, by turning to a more old-fashioned printing technique -- details of which appear today in Nature Photonics. The team used a patterned silicon wafer as an 'ink stamp' to pick up strips of dots made from cadmium selenide, and press them down onto a glass substrate to create red, green and blue pixels without using a solvent.
The idea may sound simple, but getting it to work was not easy, Choi explains. "It took us three years to get the details right, such as changing the speed and the pressure of the stamp to get a 100% transfer."
The team has now produced a 10-centimetre full-colour display. The pixels ware brighter and more efficient than in quantum dot displays made by rival methods, says Choi. For example, "the maximum brightness of the red pixels is about 50% better," he says. The maximum power efficiency for the red pixels is about 70% better.
Around the bend
Bending the screen did not greatly affect the display's performance, which means that the displays can be rolled up for portability, or used to make flexible lighting, says Choi.
Paul O'Brien, an inorganic chemist who studies quantum dots at the University of Manchester, UK, commends the group's achievement. He notes that quantum dots are "robust", so their efficiency will not quickly degrade. "For televisions, where you want a long lifetime, quantum dots are appealing," he adds.
Seth Coe-Sullivan, the chief technology officer of QD Vision, a company in Watertown, Massachusetts, that produces devices with lighting based on quantum dots, notes that Choi and his team's method is cheap. "We all have our eyes on making large-screen televisions, and this fabrication technique seems to be cost-effective," he says.
But Coe-Sullivan adds that it may take some time to commercialize quantum-dot displays for big items. "I can imagine that we will have small cell-phone displays using this technology within around three years," he says. "For the rest, there may be more of a wait."