"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 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
|Contact: Evan Lerner|
University of Pennsylvania