Self-build of C ₆₀-C ₇₀ molecular heterojunctions on highly oriented pyrolytic graphite

Building a novel heterojunction with hybrid structures can create new physical properties and enable unique functionality. We investigated the build of C_60-C₇₀ heterojunction between C₆₀ and C₇₀ molecular layers by Scanning Tunnelling Microscopy (STM). The C_60-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 C_60-C₇₀ molecular heterojunctions allow building a lateral/vertical van der Waals heterojunctions.