The First Molecular Wheel: A Theoretical Investigation
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The First Molecular Wheel: A Theoretical Investigation Gustavo Brunetto1, Fernando Sato2, Xavier Bouju3, and Douglas S. Galvao1 Applied Physics Department, State University of Campinas, 13083-859, Campinas, São Paulo, Brazil. 2 Physics Department, Federal University of Juiz de Fora, Juiz de Fora, 36036-330, Minas Gerais, Brazil. 3 Nanosciences Group, Centre d’élaboration de materiaux et d’études structurales, CEMESCNRS, PO Box 94347, F-31055 Toulouse Cedex 4, France.
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ABSTRACT Recently, the first molecular nanowheel was synthesized and characterized from Scanning Tunneling Microscope (STM) experiments. It was demonstrated that a specifically designed hydrocarbon molecule (C44H24) could roll on a copper substrate along the [110] surface direction. In this work we report a preliminary theoretical analysis of the isolated molecule and of its rolling processes on different Cu surfaces. We have used ab initio and classical molecular dynamics methods. The simulations showed that the rolling mechanism is only possible for the [110] surface. In this case, the spatial separation among rows of copper atoms is enough to ‘trap’ the molecule and to create the necessary torque to roll it. Other surface orientations ([111] and [100]) are too smooth and cannot provide the necessary torque for the rolling process. INTRODUCTION Recently [1], it was reported the first experimental evidences of a possible molecular rolling (nanowheel) over a metallic surface from scanning tunneling microscope (STM) experiments. The molecule used has a simple geometry, composed of two triptycene groups, like two wheels connected by carbons with single and double alternating bonds forming the central axis (Figures 1 and 2). The signal profiles acquired by the STM during the molecular manipulations were interpreted as evidences of the rolling process, in a rotational movement similar to the observed one of a macroscopic wheel. In this work we report a preliminary theoretical study of this molecule and of its rolling processes over different metallic surfaces, that is Cu(111), Cu(110), and Cu(100). THEORY Due to the size of the system, the study of the dynamics of the nanowheel (molecular dynamics (MD) simulations) over different Cu substrates is only possible using classical methods. The use of fully quantum methods would be cost prohibitive. In the present work we used the well-known UFF [2] molecular force field. UFF is available in the Materials Studio package [3]. As the Cu surfaces do not reconstruct and no significant charge transfer between the molecule and the substrate is expected, MD simulations were carried out using frozen substrates. This approach has proven effective in the study of similar systems [4]. The molecular wheel dimer is similar to already studied molecules-machines like Landers. It was demonstrated that such hydrocarbon molecules are physisorbed on the copper surface [10]. In order to test the reliability of the molecular geometries obtained with UFF we have also carried out ab initio DFT (GGA-PBE with van der Waals correction
