Constructing an Array of Anchored Single-Molecule Rotors on Gold Surfaces

Abstract

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...

Rotational and constitutional dynamics of caged supramolecules

“…2 D and E). In agreement with earlier STM observations of single rotating molecules (10,(29)(30)(31), this imaging is ascribed to a unit rapidly rotating in arbitrary directions while residing on the average at the preferred surface orientations. With the present system, the envelope of the intracavity molecular features accordingly corresponds exactly to the superposition of the two distin- guishable rotator orientations (compare Fig.…”

Visualization and thermodynamic encoding of single-molecule partition function projections

“…Thus, rapid intrapore thermal motions including transient occupation of all accessible states occurs, with hopping rates largely exceeding that of the fast-scanning direction (typically 4 Hz). Accordingly, the imaging process renders an intrapore corrugation pattern with intensity maxima retracing the preferred configurations 33,[45][46][47] .…”

Identifying Multiple Configurations of Complex Molecules in Dynamical Processes: Time Resolved Tunneling Spectroscopy and Density Functional Theory Calculation

“…Furthermore, studying molecular configurations in a dynamic process represents more scientific importance but significant technological challenges. Recently, a little progress has been achieved on the dynamical process (including action spectroscopy) of molecular systems [7][8][9][10][11][12][13][14][15], but mainly for small and simple molecules; however, study on a large molecular complex has been lacking due to insufficient experimental techniques and the intimidating requirement of computation [16]. While the I-t spectroscopy has been successfully applied for investigating motion of single atoms and small molecules [10,12,13], simple extension of such a method to a larger molecule is not straightforward.…”

“…This molecule was found to rotate at 80 K without lateral diffusion [ Fig. 1(b) shows a typical STM image], but maintained fully stationarity at 5 K [8]. The I-t spectroscopy measurement was performed at one fixed place, and typical tunneling currents were obtained by frequencycounting statistical analysis used for small molecules in a former study [12].…”

“…While the large tip apex typically leads to poor topographic resolution due to tip convolution [as evident in Fig. 1(b)] [8], its advantage in our current study is that different molecular configurations could be collected during the I-t spectroscopy measure-…”