The two darlings of carbon nanomaterials, carbon nanotubes and graphene, increasingly are joining forces even as they are having their obituaries read while still hardly out of the lab. We’ve seen them being used in hybrid energy storage applications and for supercapacitors.
Now researchers at the Michigan Technological University (MTU) have combined these two nanomaterials to tackle a far more difficult application field: electronics. Specifically, the researchers have created digital switches by making a sandwich of carbon nanotubes and graphene.
The carbon nanotubes in question are of the boron nitride variety, which serve as pretty effective insulators. Strictly speaking, boron nitride is a semiconductor, however, its band gap is so wide it effectively serves as an insulator. The boron nitride nanotubes’ large band gap makes up for the fact that graphene doesn’t have any band gap.
The trick for the researchers was to find a way to bring these two materials together so that their attractive electronic properties could be leveraged in one device that would allow the speedy movement of electrons and then completely shut down that movement.
In research published in the journal Scientific Reports, the MTU researchers exfoliated the graphene and modified its surface so that it was made of small holes. The researchers then coaxed the carbon nanotubes to grow out of these holes.
“When we put these two aliens together, we create something better,” said Yoke Khin Yap, a professor at MTU, in a press release. “When we put them together, you form a band gap mismatch—that creates a so-called ‘potential barrier’ that stops electrons.”
The electrons are able to move smoothly along the graphene but when they come up to the hair-like sprouts of boron nitride carbon nanotubes they are slowed way down. It is at these points of where the graphene meets the carbon nanotubes, known as heterojunctions, that digital switches are made possible.
“Imagine the electrons are like cars driving across a smooth track,” Yap said in the release. “They circle around and around, but then they come to a staircase and are forced to stop.”
The researchers claim that the switch ratio—how fast the material can turn on and off the flow of electrons—is very high, measuring several orders of magnitude faster than current graphene switches.
In addition to having a relatively high switch ratio compared to current graphene switches, the hybrid material also avoids the issue of electron scattering in which electrons are dispersed in directions you don’t want them to go. The hybrid material gets around this issue because the two materials that make it up have the same atomic arrangement pattern.
“You want to control the direction of the electrons,” Yap added. “This is difficult in high speed environments, and the electron scattering reduces the number and speed of electrons.”