Nanotubes and silicon: unexpected ingredients in a new optical device

“A lot of dis­cov­eries in the lab­o­ra­tory are purely acci­dental,” said Swastik Kar, an assis­tant pro­fessor of physics in the Col­lege of Sci­ence.

He and Yung Joon Jung, an asso­ciate pro­fessor of mechan­ical and indus­trial engi­neering, have received a three-year, $309,000 National Sci­ence Foun­da­tion grant to explore a phe­nom­enon they dis­cov­ered entirely by chance, which could afford a new gen­er­a­tion of extremely effi­cient electronics.

Kar’s exper­tise is in the physics of graphene, which is a sheet of carbon atoms, one-atom-thick. Because of its struc­ture, graphene is a supe­rior thermal and elec­tric con­ductor. Jung’s work focuses on the mechanics of carbon nan­otubes, or nanometer-sized rolled-up sheets of graphene.

“The two mate­rials are closely related in many ways,” said Kar.

Last year, the Provost’s office awarded Jung and Kar a Tier 1 Inter­dis­ci­pli­nary Seed Grant to develop new opto­elec­tronic and solar devices using both graphene and carbon nanotubes.

But their inves­ti­ga­tions weren’t going as planned. Shining light on the devices gen­er­ated a com­pletely unex­pected behavior.

“We kept get­ting a weird kind of response,” said Kar. Added Jung: “We thought there must be some­thing wrong.”

So they decided to take a step back and remove the graphene from the equa­tion, leaving a layer of carbon nan­otubes over a sil­icon sub­strate. Lo and behold, Kar said, the results remained the same.

The weird responses had nothing to do with the graphene, but rather, were related to an unex­pected prop­erty at the inter­face between the carbon nan­otubes and the silicon.

Past research by other groups has shown that inter­ac­tions between sil­icon atoms and carbon nan­otubes can turn light into elec­trical cur­rent. “That is pretty much the basis of all pho­to­di­odes and solar cells,” said Kar. “If such a device is held in dark­ness, there’s little or no cur­rent at all. You shine light and cur­rent flows — it’s called photocurrent.”

One of the optoelectric devices made of silicon and carbon nanotubes. Photo by Mary Knox Merrill.

But dif­ferent from con­ven­tional diodes, the pho­tocur­rent in Kar and Jung’s devices can be con­trolled by applying a voltage. “A few volts can change the pho­tocur­rent by up to four orders of mag­ni­tude. That is what makes it a very sen­si­tive pho­to­switch,” Kar said. “The pho­tocur­rent grows almost expo­nen­tially, resulting in large pho­tocur­rents for rel­a­tively small light inten­si­ties.” They believe the behavior comes from the highly orga­nized carbon nan­otube archi­tec­tures unique to Jung’s lab.

Since only small amounts of light are required, the phe­nom­enon could be useful for low power opto­elec­tronics. A dig­ital camera using the pho­to­switch, for example, could pro­duce crisp images in very low light. If the behavior can work in the infrared spec­trum, it could mean more effi­cient night-vision technologies.

But before it can be used in any prac­tical appli­ca­tion, the team must first under­stand the under­lying physics of the phenomenon.

Kar’s lab will quan­tify the behavior, map­ping the phys­ical prop­er­ties of these curious devices while a col­lab­o­rator in South Korea will use his exper­tise in the­o­ret­ical physics to ana­lyze the results computationally.

Ulti­mately, they hope the inter­dis­ci­pli­nary approach will afford a better under­standing of the behavior on an atomic level.

“That is the beauty of nanoscience and nan­otech­nology research,” said Jung. “By col­lab­o­rating with people from dif­ferent back­grounds you can accom­plish great things.”