US researchers create flexible, low voltage circuits using nanocrystals

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Researchers from the University of Pennsylvania have shown that nanoscale particles of the semiconductor cadmium selenide can be 'printed' onto flexible plastics to form high performance electronics.

Published in Nature Communications, the work shows that cadmium selenide nanocrystal devices can move electrons 22 times faster than in amorphous silicon. The nanocrystals can also be deposited at room temperature, opening up the possibility of using more flexible plastic foundations. "There have been a lot of electron transport studies on cadmium selenide, but until recently we haven't been able to get good performance out of them," said doctoral student David Kim. "The new aspect of our research was that we used ligands that we can translate very easily onto the flexible plastic; other ligands are so caustic that the plastic actually melts." As the nanocrystals are dispersed in an ink like liquid, multiple types of deposition techniques can be used to make circuits. The researchers used spincoating, but claim the nanocrystals could be applied through dipping, spraying or ink jet printing as well. On a flexible plastic sheet, a bottom layer of electrodes was patterned using a shadow mask — essentially a stencil — to mark off one level of the circuit. The researchers then used the stencil to define small regions of conducting gold to make the electrical connections to upper levels that would form the circuit. An insulating aluminum oxide layer was introduced and a 30nm layer of nanocrystals was coated from solution. Finally, electrodes on the top level were deposited through shadow masks to ultimately form the circuits. Using this process, the researchers built three kinds of low voltage circuits to test the nanocrystals' performance: an inverter, an amplifier and a ring oscillator. With the combination of flexibility, relatively simple fabrication processes and low power requirements, the team believes these cadmium selenide nanocrystal circuits could pave the way for new kinds of devices and pervasive sensors, which could have biomedical or security applications. "This research also opens up the possibility of using other kinds of nanocrystals, as we've shown the materials aspect is not a limitation anymore," Kim concluded.