Quantum dots: eco-friendly version wanted

Quantum dots: eco-friendly version wanted

By eeNews Europe

Today’s flat panel TVs can a offer breathtakingly realistic image quality. Not only because they have extremely high resolutions but also because their colour reproduction is so sophisticated. Responsible for the natural colours are tiny crystals, arranged in layers only a few atoms thick – the quantum dots. These nano particles, deployed in the backlighting section of LC displays, do not only cause a firework of colours; instead they also have another interesting property. “A large advantage of quantum dots lies in the fact that it is possible to modify their optical properties by varying their size,” explains Armin Wedel from Fraunhofer Institute for applied polymer research (IAP) in Potsdam (Germany). “Thus, it is not necessary to produce three different materials for the basic colours blue, green and red. Instead, you can work on a single feedstock.” This helps reducing cost and time.

Since several years, the researchers in Fraunhofer IAP are developing quantum dots for customers active in a variety of industries. For each application they create customised nano particles through chemical synthesis. During this process, initially very small particles are created that emit blue light. From a size of some two nanometres on, the colour changes to green. The largest quantum dots emit light in the red part of the spectrum. Currently, Wedel and his team develop quantum dots for an LC display backlight unit on behalf of Dutch NDF Special Light Products B. V. Towards this end, the Fraunhofer researchers devise crystals for a range of colours and embed them in plastic materials. These materials then are processed to films, and these films then are integrated into the displays as converter foil.

This task confronted the researchers with a new challenge: Currently the EU Commission is discussing the ban of environmentally harmful cadmium in consumer goods by 2017. So far, this material was considered the ideal starting matter because cadmium-based quantum dots offer a very high spectral bandwidth of only 20 to 25 nanometres which results in very good colour definition and excellent handling properties. Therefore, researchers around the globe are looking feverishly for suitable materials with similar properties. The IAP team believes it is on a promising way. “In cooperation with NDF we are testing quantum dots based on indium phosphide”, says Wedel. The IAP researchers have already been able to reach a spectral definition of 40 nanometres. At first sight this does not appear to be far from the quality achievable with cadmium-based quantum dots. However, there is still a large potential for improvements. “We regard this as a first milestone but we are working to make it even better”, Wedel says.

The effort could pay out: Not only the manufacturers of colour displays are in need of the tiny crystals. Quantum dots could also be used to drive the efficiency of solar cells higher or be used in bioanalytics. For such peculiar applications, the optical properties of the quantum dots need to be adapted to the respective requirements.

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