Quantum dots already in trouble

Despite manufacturers already experimenting with quantum dots, legislative change in the EU could stop a boom in uptake before it ever started.

It helps to have the word ‘quantum’ in a technology’s name when you want mainstream potential, but in reality ‘quantum dots’ have somewhat high jacked the term. Though the nanocrystals made of semiconductor materials are small enough to exhibit quantum mechanical properties – at just a few atoms thick – the properties are far more predictable than much within the strange world of quantum physics. And it is this mastery of the properties that is the really exciting engineers.

Researchers have studied applications for quantum dots in transistors, solar cells, and lasers, but perhaps the most promises – and wide spread application – is to improve existing light-emitting diode (LED) design and subsequent screen displays. The electronic characteristics of a quantum dot are closely related to its size and shape. Quantum dots have electron energy states that can be tuned to precise colours, such as the rich reds and greens. In addition, as quantum dots naturally produce monochromatic light, they can be more efficient than light sources that need to be colour filtered.

“One big advantage of quantum dots is that their optical properties can be selectively modified by changing their size,” explains Dr Armin Wedel of the Fraunhofer Institute for Applied Polymer Research IAP in Potsdam, Germany. “This means you no longer have to manufacture three separate materials for the colours red, green and blue. Now it is possible to do the job with just one. This saves both time and money.”

For some time, TV manufacturers have expressed concern about the cost of making large screen sets using organic light emitting diodes (OLEDs). They now believe the quantum dot can get them close to the current colour saturation but at a fraction of the price.

Over the last several years, Fraunhofer IAP researchers in Potsdam have been developing quantum dots for customers in a wide range of industry sectors. It manufactures the nanoparticles using chemical synthesis and customises them for each application.

This initially results in very small particles that radiate blue light. At sizes above approximately 2nm the colour changes to green. The largest of the quantum particles at 7nm emits within the red spectral range. These quantum dots will improve the colour rendering and colour realism of the displays as crystals are manufactured for the different emission colours and are embedded in plastics. These plastics are subsequently processed into films and built into the display as a conversion film.

However, researchers are already facing a new challenge as the EU Commission is currently considering a ban on cadmium in consumer goods by 2017, due to its damaging effect on the environment.

The problem is cadmium is considered the ideal material for manufacturing these crystals. Cadmium-based quantum dots can achieve a narrowband spectrum sharpness of just 20 to 25nm. It means display manufacturers around the world are locked in to an arms race to find suitable replacement materials with similar characteristics.

“We are testing quantum dots based on indium phosphide,” says Dr Wedel. His team has already managed to achieve a spectral sharpness of 40nm. While in all respects this is close, it does not yield the colour fidelity needed. “We see this as a good first milestone, but we are still striving for further improvement,” says Wedel. “We’re in a good position thanks to our extensive experience in manufacturing quantum dots to meet specific customer requirements.”

Justin Cunningham

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What about the cadmium free dots developed in the UK by Nanoco, and about to go into mass production in a factory built by Dow Chemicals as part of a licensing agreement. Surely cadmium free will give a far higher prospect of success?

Comment Richard Lock, 10/12/2015

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