Local dimerization and dedimerization of C60 molecules under a tip of scanning tunneling microscope: A first-principles study

The local dimerization and dedimerization of C-60 molecules in a C-60 thin film using a scanning tunneling microscopy (STM) [M. Nakaya et al. Adv. Mater. 22, 1622 (2010)] are promising techniques for realizing ultradense data storages. However, the detailed mechanism of the reversible topochemical reactions has not been clarified yet. Based on the density functional theory we explain the mechanism in terms of charging and electric-field effects on the molecules. The total-energy calculations reveal that when the C-60 molecules in the surface layer are negatively charged, the dimerization is promoted and inter-layer dimers composed of two C-60 molecules in different layers are formed dominantly over in-plane dimers. When the thin-film surface is positively charged or the inter-layer dimers are exposed to a strong electric field, a C-60 monomer pair becomes more stable than a C-60 dimer, and the dedimerization is promoted. These results predict competition between the dimerization and dedimerization of a negatively charged C-60 binary system in a strong electric field, which is indeed confirmed by our STM experiments. In addition, the dedimerization induced in the electric field is discussed from the viewpoints of the intermolecular donor-acceptor interaction and the charge-dipole relaxation of a C-60 binary system. (C) 2020 Elsevier Ltd. All rights reserved.