Self-build of C60-C70 molecular heterojunctions on highly oriented pyrolytic graphite

Abstract

Building a novel heterojunction with hybrid structures can create new physical properties and enable unique functionality. We investigated the build of C60-C70 heterojunction between C60 and C70 molecular layers by Scanning Tunnelling Microscopy (STM). The C60-C70 organic molecular heterojunction was successfully built by layer-by-layer growth method on the HOPG surface using ultra-high vacuum STM. Both molecules can rotate freely within their corresponding molecular layers at room temperature. When C70 molecules are deposited onto a preformed first layer of C60, the C70 molecules assemble into a close-packed second layer with a lattice-matched to the underlying C60 layer. The C70 molecules in the second layer adopt a standing up configuration with its long molecular axis perpendicular to the substrate which is accompanied by the elimination of the energy component associated with the rotation around the short molecular axis. C70 and C60 molecules at layers margins become mobile due to annealing processes and can jump up and form a new packing layer on its origin layer, C70 molecules remain as isolated molecules standing with their long axis perpendicular to the layers. The C60-C70 molecular heterojunctions allow building a lateral/vertical van der Waals heterojunctions.