Sunday, 2 December 2012

Flexible, Low-Voltage Circuits Made Using Nanocrystals

ScienceDaily (Nov. 26, 2012) — Electronic circuits are typically integrated in rigid silicon wafers, but flexibility opens up a wide range of applications. In a world where electronics are becoming more pervasive, flexibility is a highly desirable trait, but finding materials with the right mix of performance and manufacturing cost remains a challenge.
Flexible circuit fabricated in the Kagan lab. (Credit: David Kim and Yuming Lai)
Now a team of researchers from the University of Pennsylvania has shown that nanoscale particles, or nanocrystals, of the semiconductor cadmium selenide can be "printed" or "coated" on flexible plastics to form high-performance electronics.
The research was led by David Kim, a doctoral student in the Department of Materials Science and Engineering in Penn's School of Engineering and Applied Science; Yuming Lai, a doctoral student in the Engineering School's Department of Electrical and Systems Engineering; and professor Cherie Kagan, who has appointments in both departments as well as in the School of Arts and Sciences' Department of Chemistry. Benjamin Diroll, a doctoral student in chemistry, and Penn Integrates Knowledge Professor Christopher Murray of Materials Science and of Chemistry also collaborated on the research.
Their work 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," Kagan said. "Here, we show that these cadmium selenide nanocrystal devices can move electrons 22 times faster than in amorphous silicon."
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.
"The more complex circuits are like buildings with multiple floors," Kagan said. "The gold acts like staircases that the electrons can use to travel between those floors."
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," Lai said. "We can also show amplifiers, which amplify the signal amplitude in analog 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 said. "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."
With the combination of flexibility, relatively simple fabrication processes and low power requirements, 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 any more," Kim said.
The research was supported by the U.S. Department of Energy and the National Science Foundation.
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