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Wafer Thin Flexible Integrated Circuits

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 that can be bent, flexed, and even folded, without losing their capability to computate. This is another tool in the increasing range we have at our disposal to work towards true augmented reality computing – where computing devices literally do replace newspapers, magazines, and potentially even books and our clothing.


A finished three-layer, functional circuit

“We have a performance benchmark in amorphous silicon, which is the material that runs the display in your laptop, among other devices,” professor Cherie Kagan of both the School of Engineering and Applied Science, and the Department of Electrical and Systems Engineering. “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, opening up the possibility of using more flexible plastic foundations.

They were also extremely careful in their choice of ligands, the chemical chains that extend from the nanocrystals’ surfaces and helps facilitate conductivity as they are packed together into a film. They had to be careful to find one that melted into the plastic, binding to its surface, but not melting so much that he plastic dissolves around it.

“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,” stated David Kim, the doctoral student who led the study. “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.”

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.”

As a result, the finished product works rather like diagrams on sheets of transparent film. As you layer each page on top of the one below, connection points are made, and your diagram begins to take on a three dimensional shape. Exactly the same is occurring here, save that functional circuits rather than diagram are created. Another nice bonus is that because of this layering process, the circuits share one other attribute in common with the hypothetical printed sheets – each layer can be inkjet printed onto it surface, before being placed above the one below. This radically decreases the manufacturing costs when making them in bulk.

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,” Yuming Lai, a second doctoral student involved in the process, stated. “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.

References

Penn Researchers Make Flexible, Low-voltage Circuits Using Nanocrystals
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