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...

A series of new organic semiconductors for organic thin‐film transistors (OTFTs) using dithieno[3,2‐b:2′,3′‐d]thiophene as the core are synthesized. Their electronic and optical properties are investigated using scanning electron microscopy (SEM), X‐ray diffraction (XRD), UV‐vis and photoluminescence spectroscopies, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC). The compounds exhibit an excellent field‐effect performance with a high mobility of 0.42 cm2 V–1 s–1 and an on/off ratio of 5 × 106. XRD patterns reveal these films, grown by vacuum deposition, to be highly crystalline, and SEM reveals well‐interconnected, microcrystalline domains in these films at room temperature. TGA and DSC demonstrate that the phenyl‐substituted compounds possess excellent thermal stability. Furthermore, weekly shelf‐life tests (under ambient conditions) of the OTFTs based on the phenyl‐substituted compounds show that the mobility for the bis(diphenyl)‐substituted thiophene was almost unchanged for more than two months, indicating a high environmental stability.

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