By Will Soutter
National Institute of Standards and Technology(NIST) researchers have discovered that when graphene is subjected to mechanical strain, forming it into a drumhead, the effect of magnetic fields are mimicked, creating a quantum dot, which is an exotic semiconductor type having a large number of potential applications in electronic devices. The June 22, 2012 issue of Science has published the results.
NIST researchers showed that straining graphene membrane creates pseudomagnetic fields that confines the graphene's electrons and creates quantized quantum dot-like energy levels. The background is a false color image of the graphene drumheads made from a single layer of graphene over 1 micron-sized pits etched in a silicon dioxide substrate. Credit: N. Klimov and T. Li, NIST/UMD
Graphene is a single carbon atom layer formed in the shape of a honeycomb lattice. The material has the potential to conduct electricity at room temperature with minimal resistance. Graphene is a key candidate for a wide range of applications from high-speed transistors to flexible displays.
As the electron speed moving through graphene is reduced due to substrates, a University of Maryland postdoctoral NIST researcher suspended graphene over shallow holes in a silicon dioxide substrate making an array of graphene drumheads. An innovative scanning probe microscope, built and designed at NIST was used to measure the properties of graphene.
Research on the drumheads showed that graphene rose to meet the microscope tip due to van der Waals force, which is a weak electrical force that forms an attraction between objects very near to each other.
Nikolai stated that their instrument showed that graphene was bubble-shaped clamped at the edges; however simulations done by University of Maryland colleagues proved that graphene’s highest point only was being detected. The shape, they said was more like a circus tent. The drumhead strain could be adjusted by means of the conducting plate on which the substrate and graphene were mounted to form a countervale attraction and pull down the drumhead. The graphene could be pulled into or out of the hole below it.
It was also shown that the material’s electrical properties changed by changing the degree of strain. This technique of forming semiconducting areas such as quantum dots in graphene by shape modification can offer scientists band gap and high speed critical to computing and other applications.
NIST fellow Joseph Stroscio stated that in order to make a graphene quantum dot, it was necessary to cut a nano graphene piece. It is possible to achieve the same thing with strain-induced pseudomagnetic fields.
The research work was done in partnership with the Korea Research Institute of Standards and Science and the University of Maryland, College Park.