Electrons in Atomically Thin Carbon Sheets Behave like Massless Particles

G a single, one-atom-thick sheet of carbon atoms arranged in a honeycomb lattice, is the twodimensional building block for carbon materials of every other dimensionality. It can be stacked into 3D graphite, rolled into 1D nanotubes, or wrapped into 0D buckyballs. But for decades scientists presumed that a single 2D graphene sheet could not exist in its free state; they reasoned that its planar structure would be thermodynamically unstable and possibly curl into carbon soot. The University of Manchester’s Andre Geim and colleagues there and at the Institute for Microelectronics Technology in Chernogolovka, Russia, put that presumption to rest a few months ago by isolating single graphene sheets.1 Their method is astonishingly simple: Use adhesive tape to peel off weakly bound layers from a graphite crystal and then gently rub those fresh layers against an oxidized silicon surface. The trick was to find the relatively rare monolayer flakes among the macroscopic shavings. Although the flakes are transparent under an optical microscope, the different thicknesses leave telltale interference patterns on the SiO2, much like colored fringes on an oily puddle. The patterns told the researchers where to hunt for single monolayers using atomic force microscopy. The work confirmed that graphene is remarkable—stable, chemically inert, and crystalline under ambient conditions. Its honeycomb lattice, pictured in figure 1 with each carbon atom connected to its neighbors through strong covalent bonds, explains graphene’s strength and rigidity. And it can carry huge current densities— about 108 A/cm2, roughly two orders of magnitude greater than copper. Microelectronics engineers are paying attention. In semiconductor heterostructures used to make FET devices, for instance, it takes million-dollar epitaxy machines and exquisite care to tie up dangling surface bonds and eliminate impurities in quantum wells. The preparation minimizes the scattering of electrons against interfaces and defects to ensure the largest electron mean-free paths in the device. But in graphene, just 1 Å thick, scientists have a material that is relatively defect free and whose electrons have a respectable mean-free path naturally, without materials manipulation and processing. Graphene can hardly be more low tech, and yet it still exhibits high conductivities. “It’s really counterintuitive and remains to be understood,” comments Geim, “but the electron wavefunction appears to localize only parallel to the sheet and does not interact with the outside world, even a few angstroms away.”