SAN FRANCISCO—Researchers from the University of Pennsylvania have demonstrated the ability to coat nanoscale particles of the semiconducting material cadmium selenide on flexible plastics, a breakthrough that could lead to high-performance electronics on flexible substrates.
The research team, led by David Kim, a doctoral student at Penn's department of materials science and engineering, used spin coating to deposit cadmium selenide nanocrystals dispersed in an ink-like liquid on a flexible plastic sheet. Their work was recently 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," Kagan said. "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, according to the researchers. While 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, opening up the possibility of using more flexible plastic foundations, according to the researchers.
Flexible circuit fabricated in the Penn University lab of Professor Cherie Kagan.
Credit: David Kim/Yuming Lai, Penn
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 30-nanometer 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.
"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."
Replacing rigid silicon wafers with flexible plastic substrates could open up new applications for electronics. Kim and his team claim that with the combination of flexibility, relatively simple fabrication processes and low power requirements, 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 said. Related stories: