rXXXX American Chemical Society A dx.doi.org/10.1021/nl2028409 | Nano Lett. XXXX, XXX, 000–000 LETTER pubs.acs.org/NanoLett A Molecular Switch Based on Current-Driven Rotation of an Encapsulated Cluster within a Fullerene Cage Tian Huang, † Jin Zhao, †,§,^ Min Feng, † Alexey A. Popov, ‡ Shangfeng Yang, ‡,§,|| Lothar Dunsch, ‡ and Hrvoje Petek* ,† † Department of Physics and Astronomy and Petersen Institute of NanoScience and Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States § Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China (USTC), Hefei 230026, China ^ Department of Physics, University of Science and Technology of China (USTC), Hefei 230026, China ‡ Department of Electrochemistry and Conducting Polymers, Leibniz-Institute for Solid State and Materials Research (IFW), Dresden, Germany ) Department of Materials Science and Engineering, University of Science and Technology of China (USTC), Hefei 230026, China b S Supporting Information M olecular materials present rich possibilities for construction of molecular switching devices based on structural isomer- ization stimulated by electrons or light. 15 The primary switch- ing events in vision and bacterial photosynthesis rely on a con- formational change through cistrans isomerization within a protein framework to trigger quantal transduction. 5 Photobiology has inspired studies of single-molecule switching in azobenzene 6 and its derivatives, 7,8 and more distantly in molecules such as rotaxane, 9 catenane, 10 phenylene ethynylene oligomers, 11 and porphyrin derivatives, 12 all of which undergo significant structur- al changes. Switches based on tautomerization 13,14 of NH bonds minimize structural changes, but are vulnerable to chemi- cal perturbations to their functional groups. By contrast to bare molecules, the inner space of fullerenes provides a more protected environment akin to a Faraday cage. Indeed, monatomic endohedral fullerenes have been imaged by low-temperature scanning tunneling microscopy (STM) and the internal structure of ionic bonding of metal atoms to the fullerene cage has been described. 15,16 For some molecules, the internal atom/cluster motion is known to occur even below room temperature for moieties with a close match between the ionic bond length and the cage radius. 17,18 The potentially facile motion of the encapsulated moiety through electronic or optical stimulation, such as has recently been described in the case of charge-transfer excitation of the Li@C 60 molecule, presents an attractive target for realization as a single molecule switch. 19 We describe the conductance switching of single Sc 3 N@C 80 molecules in a junction formed by an atomically ordered Cu- (110)-(2  1)-O substrate and atomically sharp LT-STM tip. The switching involves hierarchical, STM bias voltage-dependent, multiple-axis rotation of the encapsulated planar equilateral- triangle Sc 3 N cluster within the icosahedral C 80 cage. We demon- strate that the dominant low-bias switching mechanism occurs through excitation of the ScN stretching vibrations by the inelastic scattering of tunneling electrons, which consequently Received: August 16, 2011 Revised: November 4, 2011 ABSTRACT: By scanning tunneling microscopy imaging and electronic structure theory, we investigate a single-molecule switch based on tunneling electron-driven rotation of a triangular Sc 3 N cluster within an icosahedral C 80 fullerene cage among three pairs of enantiomorphic configurations. Bias-dependent action spectra and modeling implicate the antisymmetric stretch vibration of Sc 3 N cluster as the gateway for energy transfer from the tunneling electrons into the cluster rotation. Hierarchical switching of conductivity among multiple stationary states while maintaining a constant molecular shape, offers an advantage for the integration of endohedral fullerene-based single-molecule switches into multiple logic state molecular devices. KEYWORDS: Endohedral fullerene, enantiomerization, conductance switching, vibrational and electronic excitation, molecular machine, STM