Researchers have demonstrated an intriguing way to accelerate nanoscale graphene structures to supersonic speeds, and have showed how to engineer materials to maintain these speeds over relatively long distances, in an article published in Scientific Reports.
The main insight of the research, which was carried out at Tsinghua University’s Center for Nano and Micro Mechanics (CNMM) in Beijing in collaboration with the LCN, is that a phenomenon called superlubricity can be tuned to allow graphene structures to move with almost no friction across the surface of graphite or other graphitic materials. Normally, the regular, crystalline structure of graphene would “lock in” to the practically identical structure of the surface of graphite, resulting in very high friction.
“By tailoring the underlying surface, for example creating a nanoribbon or stretching the surface slightly, our simulations show that nanoscale graphene structures can slide essentially indefinitely in a state of superlubricity, without sticking due to any lock-in effect”, says lead scientist, Yilun Liu, currently at the International Center for Applied Mechanics of Xi’an Jiaotong University. Dr. Liu used so-called molecular dynamics simulations to reveal this unique phenomenon.
“Until a few years ago, superlubricity was a theoretical phenomenon that could only be observed at very low speeds and under extreme conditions of ultra-high vacuum. But recent work at CNMM[1,2] has shown experimentally that for suitably engineered microstructures, high-speed superlubricity at up to 25m/s is a practical and reproducible phenomenon, even under ambient conditions”, says Quanshui Zheng, the director of CNMM. “In this new work, we show how profound this phenomenon could be at the nanoscale, for future high-speed, low-dissipation nanomechanical devices.”
The “slingshot” mechanism by which a nanoscale graphene structure can be accelerated to supersonic speeds is deceptively simple. At the nanoscale, the inertia of objects is extremely small. But their “surface free energy” becomes very important. This energy, which is related to the surface tension that keeps small objects floating on water, can be stored up in a tiny square of graphene by sliding it slightly off the edge of a larger piece of graphite (see Fig. a below).
Releasing the graphene from this state, it can quickly accelerate to over 400m/s as it moves back onto the graphite, to lower the surface free energy. The speed of sound is about 340 m/s.
“Scientists have long been aware that small objects like molecules travel fast at room temperature,” says co-author, Francois Grey, an Honorary Professor at LCN currently at New York University’s Center for Urban Science and Progress, “but normally that sort of motion is thermally induced, and so essentially random. Here we’re showing ways to accelerate and guide nanoscale objects across surfaces at very high speeds, with an absolute minimum of friction”.
The “frictional waveguides” that the authors propose in their article overcome friction by using surface free energy to stop graphitic structures from rotating into a high-friction lock-in state. “It’s like putting a nanoscale train on rails”, says Dr. Grey. Potential applications for this sort of technology include high frequency mechanical resonators  as well as futuristic reversible computing devices, which perform calculations while producing virtually no heat . Dr. Grey adds “we’re a long way from being able to build such nanomechanical devices in practice, but these results provide a good guide for how to get there.”
Fig. a: Speed of a square of graphene released at 400m/s, as it moves along a “frictional waveguide” made of a graphene nanoribbon (red curve) versus the case where it moves unconstrained on a graphite surface. In the latter case, the square is rapidly slowed down by “frictional scattering”, due to lock-in events. Inset shows slight jiggling of the high-speed graphene square as it bounces off the edges of the nanoribbon, in order to minimize surface free energy.