Graphene is a 2D honeycomb lattice of carbon that was first isolated in 2004. It boasts a wealth of fascinating electronic properties, many of which come from the fact that electrons in the material travel through it at extremely high speeds for relatively long distances without hitting anything. This means that electronic devices, like transistors, made from graphene could be faster than any that exist today.

The team, led by Sagar Bhandari and Robert Westervelt at Harvard, studied a sample made from a hBN−graphene−hBN sandwich with two narrow contacts along each side, separated by 2.0 µm, and large source and drain contacts at either end. Hexagonal boron nitride (hBN) is an excellent substrate for graphene, because the two materials have very similar lattice constants. The hBN-graphene-hBN sample was placed on a heavily doped Si substrate, which acts as a back-gate, covered by an insulating layer of SiO2.

Scanning gate microscopy images electron flow

Bhandari and colleagues observed the semicircular flow of electrons in graphene between the two narrow contacts when the diameter of the cyclotron orbit is equal to the contact spacing. They obtained their result by placing a scanning gate microscope (SGM) tip just above the graphene sheet. The charge on the tip creates an image charge in the conducting graphene directly below the tip that scatters the electron flow, casting a shadow “downstream”. By measuring the change in electron flow as the tip is scanned above the graphene, the researchers obtain an image of the cyclotron orbit.

“Imaging electron trajectories in this way allows us to directly see and understand how electrons move though graphene,” explains Westervelt. “The technique will be a very valuable tool for designing new ‘ballistic electronics’.”

Graphene electrons behave like quantum waves

“Although the above description treats electrons as if they were classical particles, they actually behave as quantum waves,” he says. “Indeed, we previously used scanning gate microscopy to image interference fringes for electrons in a GaAs two-dimensional electron gas, spaced by half the Fermi wavelength. We now plan to do similar experiments to image electron waves in graphene and other atomic-layer materials.

“In collaboration with Philip Kim’s group, also at Harvard, we are imaging electron flow though narrow channels and constrictions in bilayer graphene in an effort to develop quantum point contacts that are only half a wavelength wide,” he tells nanotechweb.org. “We also hope to use the SGM tip to create graphene quantum dots that hold an integer number of electrons.

"By combining Kim’s team’s expertise in device design and fabrication with my group’s imaging technique, we hope to make important advances in the field of atomic layer devices," he adds.

The research is detailed in Nano Letters DOI: 10.1021/acs.nanolett.5b04609.