Nanocrystals enable faster, more flexible electronics

Researchers from the University of Pennsylvania have shown that cadmium selenide nanocrystals can be printed on flexible plastics to form high-performance electronics.

The work, led by doctoral student David Kim, was published in the journal Nature Communications.

‘We have a performance benchmark in amorphous silicon, which is the material that runs the display in your laptop, among other devices,’ said team member Prof Cherie Kagan in a statement. ’Here, we show that these cadmium selenide nanocrystal devices can move electrons 22 times faster than in amorphous silicon.’

Besides speed, another advantage cadmium selenide nanocrystals have over amorphous silicon is the temperature at which they are deposited. Whereas amorphous silicon uses a process that operates at several hundred degrees, cadmium selenide nanocrystals can be deposited at room temperature and annealed at mild temperatures.

Another innovation that allowed the researchers to use flexible plastic was their choice of ligands, the chemical chains that extend from the nanocrystals’ surfaces and help facilitate conductivity as they are packed together into a film. 

‘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,’ Kim said. ‘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.’

Because the nanocrystals are dispersed in an ink-like liquid, multiple types of deposition techniques can be used to make circuits. In their study, the researchers used spincoating, where centrifugal force pulls a thin layer of the solution over a surface, but 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 to mark off one level of the circuit. The researchers are then said to have 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 aluminium 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 circuits to test the nanocrystals’ performance for circuit applications: an inverter, an amplifier and a ring oscillator.

‘An inverter is the fundamental building block for more complex circuits,’ said Yuming Lai, a doctoral student in the Engineering School’s Department of Electrical and Systems Engineering. ‘We can also show amplifiers, which amplify the signal amplitude in analogue circuits, and ring oscillators, where “on” and “off” signals are properly propagating over multiple stages in digital circuits.’

‘And all of these circuits operate with a couple of volts,’ Kagan added. ‘If you want electronics for portable devices that are going to work with batteries, they have to operate at low voltage or they won’t be useful.’

Benjamin Diroll, a chemistry doctoral student and Prof Christopher Murray also collaborated on the research.