Detecting elusive surface atoms with atomic force microscopy

In his famous 1959 speech titled “There is plenty of room at the bottom” (1), Nobel prize winning physicist Richard P. Feynman predicts that matter will be controlled and manipulated at the atomic scale in the foreseeable future. Furthermore, he proclaims that “it should be possible to see... individual atoms.” In many ways, Feynman's vision was realized with the invention of the scanning tunneling microscope (STM) by Gerd Binnig and Heinrich Rohrer in 1982, for which they were awarded the Nobel prize in physics (2). With the STM, quantum mechanical tunneling of electrons from a sharp metal tip to a conductive substrate is used to detect individual surface atoms with atomic resolution. The ability to “see” atoms in real space immediately solved many controversies in surface science including the complicated 7 × 7 reconstruction of the Si(111) surface (3).

One complication with the STM is that the tunneling current is a function of not only the surface topography but also the local electronic structure. On the graphite surface, there are two different types of carbon atoms in the basal plane, as distinguished by the presence (α) or absence (β) of a carbon atom in the plane immediately below the surface. Coupling of 2p_z orbitals from α carbon atoms of adjacent layers lowers their bonding energies. Thus, the 2p_z orbitals from β carbon atoms form the highest occupied (and lowest unoccupied) orbitals where the Fermi level crosses. These local electronic structure variations imply that the STM can only detect every other atom on the graphite surface. Consequently, an alternative imaging mechanism is required to detect the “hidden” α atoms on the graphite surface. The article by Hembacher et al. (4) in this issue of PNAS outlines a novel scanning probe microscope that solves the hidden atom problem. Their …