May 31 2019
In 2004, a super thin material, graphene, was discovered by scientists, which is no less than a 100 times stronger than steel and the best known conductor of electricity and heat.
Graphene, a super tough wonder material, hasn’t made it into electronics yet because it doesn’t conduct an electric current on its own. Researchers used a laser technique to permanently stress graphene into a structure that allows the flow of electric current. (Image credit: Purdue University image/Gary Cheng)
This means that graphene could accelerate the arrival of faster electronics than is possible currently with silicon.
But to really be beneficial, graphene would need to hold an electric current that switches on and off, similar to what silicon does in the form of numerous transistors on a computer chip. This switching produces strings of 0s and 1s that a computer uses for processing data.
Purdue University scientists, in partnership with the University of Michigan and the Huazhong University of Science and Technology, demonstrate how a laser method could permanently stress graphene into forming a structure that permits the flow of electric current.
This structure is referred to as a “band gap.” Electrons have to jump across this gap so as to turn into conduction electrons, which makes them capable of conveying electric current. But graphene does not inherently possess a band gap.
Purdue scientists developed and broadened the band gap in graphene to a record 2.1 electronvolts. To work as a semiconductor such as silicon, the band gap would need to be at least the earlier record of 0.5 electronvolts.
“This is the first time that an effort has achieved such high band gaps without affecting graphene itself, such as through chemical doping. We have purely strained the material,” said Gary Cheng, professor of industrial engineering at Purdue, whose lab has examined numerous ways to make graphene more beneficial for commercial applications.
The existence of a band gap allows semiconductor materials to swap between insulating or conducting an electric current, based on whether their electrons are forced across the band gap or not.
Exceeding 0.5 electronvolts unravels even more possibilities for graphene in next-generation electronic devices, the scientists say. Their research findings have been published in an issue of Advanced Materials.
“Researchers in the past opened the band gap by simply stretching graphene, but stretching alone doesn’t widen the band gap very much. You need to permanently change the shape of graphene to keep the band gap open,” Cheng said.
Cheng and his collaborators not only maintained the band gap in an open state in the graphene, but also made it to where the gap width could be adjusted from zero to 2.1 electronvolts, giving researchers and manufacturers the choice to merely use specific properties of graphene based on what they want the material to achieve.
The scientists made the band gap structure permanent in graphene using a method known as laser shock imprinting, which Cheng created in 2014 along with researchers from Harvard University, the Madrid Institute for Advanced Studies, and the University of California, San Diego.
For this research, the team used a laser to produce shockwave impulses that entered an underlying sheet of graphene. The shock of the laser causes the graphene to strain onto a trench-like mold – permanently shaping it. Fine-tuning the laser power modifies the band gap.
While still not close to incorporating graphene into semiconducting devices, the method offers more flexibility in exploiting the material’s magnetic, optical, and thermal properties, Cheng said.
The research was supported by numerous entities, including the National Science Foundation (Grant numbers CMMI-0547636 and CMMI 0928752) and the National Research Council Senior Research Associateship.