Graphene membranes’ anomalous dynamics

The burgeoning exploration and development of the nanoworld is forcing scientists to reexamine and reframe many established aspects of our familiar macroscopic world. Among the tenets experiencing renewed interest is the nature of the thermal fluctuations that underlie the random walks of Brownian motion (see, for example, the Quick Study by Mark Raizen and Tongcang Li, Physics Today, January 2015, page 56 ). The University of Arkansas’s Paul Thibado and colleagues now report ultraprecise dynamical measurements on a freestanding, atomically thin sheet of graphene that show clear deviations from classical Brownian behavior. With its regular hexagonal lattice, monolayer graphene offers an exceptionally clean two-dimensional system for studying membrane behavior. Using a custom scanning tunneling microscope (STM), the researchers tracked the out-of-plane dynamics of an atom-sized region of the membrane with subnanometer, millisecond resolution for more than two and a half hours. Over that time, the region’s height exhibited Brownian excursions that spanned 10 nm, but they were punctuated with rare, large jumps—so-called Lévy walks (see the article by Joseph Klafter, Michael Shlesinger, and Gert Zumofen, Physics Today, February 1996, page 33 ). Molecular dynamics simulations revealed that the jumps arose from spontaneous mechanical buckling, which changed the region’s local curvature and height. The graphene studies may yield insights into the contributions thermal fluctuations make to the proper functioning of biomembranes. Moreover, the STM could be used to tune the graphene’s stochastic fluctuations, offering the tantalizing prospect of artificial nanoscale motors and engines. (M. L. Ackerman et al., Phys. Rev. Lett. 117, 126801, 2016, doi:10.1103/PhysRevLett.117.126801 .)