Researchers question the future of graphene based electronics
1 min read
Researchers at the National Institute of Standards and Technology (NIST) claim creating graphene based devices may be more challenging than previously thought. New measurements indicate that layering graphene on a substrate transforms its bustling speedway into 'steep hills and valleys' - making it considerably harder for electrons to move around freely.
In an article published in Nature Physics, scientist Joseph Stroscio, pictured, stated that graphene's ideal properties were only available when isolated from the environment. "To get the most out of graphene, we have to understand fully how its properties change when put in real world conditions, such as part of a device where it is in contact with other kinds of materials," he said.
To perform the experiment, the NIST group made its own semiconductor chip 'sandwich', comprised of alternating conducting, semiconducting and insulating layers and structures. The team used a single atomic sheet of graphene and another conductor separated by an insulating layer. When the bottom conductor was charged, it induced an equal and opposite charge in the graphene.
Examined under a scanning tunnelling microscope (stm), which is sensitive to the charged state of the graphene, the high electron mobility was expected to make the graphene look like a featureless plane. But, according to researcher Nikolai Zhitenev, variations in the electrical potential of the insulating substrate interrupted the orbits of the electrons in the graphene, creating wells where the electrons pooled and reduced their mobility.
This effect was especially pronounced when the group exposed the substrate mounted graphene to high magnetic fields. Then the electrons, already made sluggish by the substrate interactions, lacked the energy to scale the mountains of resistance and settled into isolated pockets of 'quantum dots', nanometer scale regions that confine electrical charges in all directions.
However, the researchers also found that direct access to the graphene with a scanned probe made it possible to investigate the physics of other substrate interactions on a nanoscopic scale, something which is less possible in conventional semiconductor devices where the important transport layers are buried below the surface.
"Usually, we cannot study insulators at atomic scale," said Stroscio. "The stm works with a closed loop system that keeps a constant tunnelling current by adjusting the tip sample distance. On an insulator there is no current available, so the system will keep pushing the tip closer to the substrate until it eventually crashes into the surface. The graphene lets us get close enough to these substrate materials to study their electrical properties, but not so close that we damage the substrate and instrument."
