This diode arrangement is used for energy harvesting, but here it’s used to isolate the graphene-induced current from the battery powering the electrically isolated STM. At a tip-sample distance of 2 nm or less, tunneling electrons dominate the current; for larger distances, displacement current dominates.
Obviously, this work involves intense and deep physics, and is explained in their paper “Fluctuation-induced current from freestanding graphene” published in APS Physical Review E. It’s behind a paywall but is also posted here (there’s also a short simplistic video animation here).
Frankly, there’s some leap of faith required here despite the full analysis in published paper, especially since Prof. Thibado also promotes their research with optimistic statements such as “An energy-harvesting circuit based on graphene could be incorporated into a chip to provide clean, limitless, low-voltage power for small devices or sensors.” But you never know—and should “never say never” when it comes to physics and technology advances.
MIT’s Graphene Discovery
In an unrelated development, a team at the Materials Research Laboratory at the Massachusetts Institute of Technology (MIT) has conceived of a way to harvest RF energy ranging from microwaves to the terahertz band. The analysis looks at the physics and presumed limitations of the quantum-mechanical behavior of graphene, and ways to overcome them. They found that by combining graphene with another material—in this case, boron nitride—the electrons in graphene should skew their motion toward a common direction, thus yielding current flow.
While previous experimental technologies have been able to convert terahertz waves into dc, they could do so only at ultracold temperatures, which obviously limits their practical applications. Instead, lead researcher Hiroki Isobe began an investigation to see if a material’s own electrons could be induced at a quantum-mechanical level to flow in one direction, in order to steer incoming electromagnetic-energy waves into a dc current. The material used would have to be free of impurities so that the electrons in it would flow without scattering off irregularities in the material, and graphene was an attractive material.
But that was only the starting point. To direct graphene’s electrons to flow in one direction required “breaking” the material’s inherent symmetry. Thus, the electrons would feel an equal force in all directions, meaning that any incoming energy would scatter randomly. Others had experimented with graphene by placing it atop a layer of boron nitride such that the forces between graphene’s electrons were knocked out of balance—electrons closer to boron felt one force while those electrons closer to nitrogen experienced a different pull.
This “skew scattering” can result in useful current flow. The research team envisioned a terahertz rectifier consisting of a small square of graphene sitting on top of a layer of boron nitride. It would be sandwiched within an antenna that collects and concentrates ambient terahertz radiation, boosting its signal enough to convert it into a dc current (Fig. 3).