Molecular rotors with a fixed off-center rotation axis have been observed for single tetra-tert-butyl zinc phthalocyanine molecules on an Au(111) surface by a scanning tunneling microscope at LN 2 temperature. Experiments and first-principles calculations reveal that we introduce gold adatoms at the surface as the stable contact of the molecule to the surface. An off-center rotation axis is formed by a chemical bonding between a nitrogen atom of the molecule and a gold adatom at the surface, which gives them a welldefined contact while the molecules can have rotation-favorable configurations. Furthermore, these singlemolecule rotors self-assemble into large scale ordered arrays on Au(111) surfaces. A fixed rotation axis off center is an important step towards the eventual fabrication of molecular motors or generators. DOI: 10.1103/PhysRevLett.101.197209 PACS numbers: 85.85.+j, 63.20.dk, 68.37.Ef, 82.37.Gk The motion of single atoms or molecules plays an important role in nanoscale engineering at the single atomic or molecular scale [1,2]. The controllability of molecular motion is critical for molecular motors [3], which may convert external energy into orchestrated motion at the molecular level [4,5]. For molecular rotors [6] a high level of control over the rotation axis and, equivalently, selfassembly on a very large scale, are the key ingredients for their integration into complex molecular machines. Previously, the reported molecular rotors on surfaces had no fixed axis on the surface [7][8][9][10]. In addition, the studies mainly focused on single molecules, while it is desirable, for eventual applications, that individual molecular rotors self-assemble into large scale ordered arrays while keeping their original functions. Here we show, using scanning tunneling microscopy (STM) [7][8][9][10][11][12], that single tetratert-butyl zinc phthalocyanine (ðt-BuÞ 4 -ZnPc) molecules on the reconstructed Au(111) surface possess a welldefined rotation axis fixed on the surface, and also, that these single-molecule rotors form large scale ordered arrays due to the reconstruction of the gold surface.Our experiments were conducted with an Omicron MBE-LTSTM system with a base pressure below 3:0 Â 10 À10 mbar. An atomically clean Au(111) surface was prepared by repeated cycles of Ar þ sputtering and subsequent annealing, and then dosed with a minute quantity of ðt-BuÞ 4 -ZnPc molecules. In our experiments, the STM tip is grounded, and the bias voltage refers to the sample voltage. All STM measurements in this study are conducted at 78 K if not specified. First-principles calculations were carried out based on density functional theory (DFT), a Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) for exchange-correlation energy [13], projector augmented waves (PAW) [14], and a plane wave basis set as implemented in the Vienna ab initio simulation package (VASP) [15]. A cð5 Â 8Þ supercell was employed to model the isolated molecule on the gold surface. Because of numerical limitations and the size of t...

Binding configurations, interface electronic structures, and magnetic properties of 3d-transition metal phthalocyanine (MPc, where M=Mn, Fe, Co, Ni, Cu or Zn) molecular systems on Au(111) substrate, are systematically investigated with first-principles density functional theory (DFT) calculations using PW91 exchange-correlation functional. We also calculated the corresponding properties of free standing molecules and did comparisons between these two cases. It is found that MnPc, FePc and CoPc have a stronger binding than that of NiPc, CuPc and ZnPc.For the magnetic property of the MPc molecules, it is not affected after the molecular adsorption, except for CoPc. In addition, for the adsorption properties of FePc on Au(111), we find out that the low adsorption energy and small energy differences between different configurations allow the FePc molecules to diffuse on Au (111) substrate easily at certain temperatures.

We report for the first time a new methodology to determine molecular configurations of a large molecular complex in a dynamical process on a metal surface by combining time-resolved tunneling spectroscopy (I-t) and density functional theory calculation (DFT). Two examples, ðt-BuÞ 4 -ZnPc and FePc, representing molecular rotation and lateral diffusion on Au(111) surfaces, respectively, were applied to demonstrate our method. Through analysis of statistical occupation time for each configuration, the molecular configuration numbers and energy differences between different configurations of these molecular systems could be unambiguously determined. These experimental results are further compared with DFT calculation to determine corresponding molecular configurations. Importantly, through the spatial I-t mapping, valuable insights of molecular surface diffusion paths are obtained.

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