Two 'braking mechanisms' for tin phthalocyanine molecular rotors on dipolar iron oxide surfaces.

Manipulation of artificial molecular rotors/motors is a key issue in the field of molecular nanomachines. Here we assemble non-planar SnPc molecules on an FeO film to form two kinds of rotors with different apparent morphologies, rotational speeds and stabilities. Both kinds of rotors can switch to each other via external field stimulation and the switch depends on the polarity of the applied bias voltage. Furthermore, we reveal that the molecular fragment has a great influence on the motions of molecules. Combining scanning tunneling microscopy and DFT calculations, two braking mechanisms are addressed for molecular rotors. One is the transformation of adsorption configurations under the external electric field stimulus that enables the molecular rotor to stop/restart its rotation. The other is the introduction of embedded molecular fragments that act as a brake pad and can stop the molecular rotation. We find that the rotation can be recovered by separating the molecule from the fragments. Our study suggests a good system for manipulating molecular rotors' properties in nanophysics and has important value for the design of controllable molecular machines.

Effect of porosity on thickness uniformity of MgF2 films on spherical substrates.

Simulation based on Knudsen's law shows that film thickness uniformity above 99% can be realized on spherical substrates with optimized profiles of shadowing masks. However, a type of optical thickness nonuniformity is revealed when the masks are applied for thickness correction of MgF2 films experimentally. The optical thickness nonuniformity depends on steepness of the spherical surfaces and reaches 5% approximately for surfaces with CA/RoC = 1.22. Porosity of the MgF2 film is superimposed on Knudsen's law to interpret the optical thickness nonuniformity. For theoretical simulation, the influence of porosity on optical thickness distribution is characterized by a new parameter that describes nonlinear dependence of deposition rate on cosine function of molecular injection angles in Knudsen's law. Utilizing the optimized deposition model, optical thickness uniformity of MgF2 films approaching to or above 99% has been achieved for surfaces of different steepness in a single coating run.

Highly ordered molecular rotor matrix on a nanopatterned template: titanyl phthalocyanine molecules on FeO/Pt(111).

Molecular rotors, motors and gears play important roles in artificial molecular machines, in which rotor and motor matrices are highly desirable for large-scale bottom-up fabrication of molecular machines. Here we demonstrate the fabrication of a highly ordered molecular rotor matrix by depositing nonplanar dipolar titanyl phthalocyanine (TiOPc, C32H16N8OTi) molecules on a Moiré patterned dipolar FeO/Pt(111) substrate. TiOPc molecules with O atoms pointing outwards from the substrate (upward) or towards the substrate (downward) are alternatively adsorbed on the fcc sites by strong lateral confinement. The adsorbed molecules, i.e. two kinds of molecular rotors, show different scanning tunneling microscopy images, thermal stabilities and rotational characteristics. Density functional theory calculations clarify that TiOPc molecules anchoring upwards with high adsorption energies correspond to low-rotational-rate rotors, while those anchoring downwards with low adsorption energies correspond to high-rotational-rate rotors. A robust rotor matrix fully occupied by low-rate rotors is fabricated by depositing molecules on the substrate at elevated temperature. Such a paradigm opens up a promising route to fabricate functional molecular rotor matrices, driven motor matrices and even gear groups on solid substrates.

Fe on Sb(111): Potential Two-Dimensional Ferromagnetic Superstructures.

It is highly desirable to fabricate two-dimensional ferromagnetic membranes based on orthodox magnetic elements because of their inherent magnetic properties. In this work, we report on two superstructures including a honeycomb-like lattice and identical nanocluster arrays formed by depositing Fe on Sb(111). Combined with first-principles calculations, both detailed atomic structures have been clarified. The honeycomb structure consists of a single layered Fe-Sb phase, and the cluster phase is assigned as a (3 × 3) Fe3Sb7 superlattice. Both structural phases exhibit high magnetic moments localized on d bands of Fe. Our results provide a method to fabricate 2D magnetic superstructures possessing great potential in the realization of the Haldane model, spintronics applications, and single atom catalysis.

Ionic compound mediated rearrangement of 3, 4, 9, 10-perylene tetracarboxylic dianhydride molecules on Ag(100) surface.

Tailoring of the assembly structure of organic molecular monolayer is of great importance to improve the performance of molecular devices. In this work, a typical ionic compound, namely KCl, was used to mediate the rearrangement of 3, 4, 9, 10-perylene tetracarboxylic dianhydride (PTCDA) monolayer on Ag(100). Combined scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) results indicate that both molecule and molecular superlattice would rotate after the dosing of KCl. The density functional theory calculation shows that KCl would exist in the form of molecules rather than ions on Ag(100) and demonstrates that experimentally observed structural transition induced by KCl molecules is energetically favored.

Molecular orbital imaging of cobalt phthalocyanine on native oxidized copper layers using STM.

To observe molecular orbitals using scanning tunneling microscopy, well-ordered oxidized layers on Cu(001) were fabricated to screen the individual adsorbed cobalt phthalocyanine (CoPc) molecules from the electronic influence of the metal surface. Scanning tunneling microscope images of the molecule on this oxidized layer show similarities to the orbital distribution of the free molecule. The good match between the differential conductance mapping images and the calculated charge distribution at energy levels corresponding to the frontier orbitals of CoPc provides more evidence of the screening of the oxidized layer from interactions between the metal surface and supported molecules.

Molecular orientation and lattice ordering of C60 molecules on the polar FeO/Pt(111) surface.

C(60) molecules assemble into close packing layer under the domination of the intermolecular interaction when deposited onto Pt(111)-supported FeO layer kept at 400 K. From corresponding high resolution scanning tunneling microscopy (STM) image, a kind of C(60) molecular orientational ordering stabilized by the intermolecular interaction is revealed as C(60)/FeO(111)-(√133 × âˆš133) R17.5° structure and determined from the commensurability between the C(60) nearest-neighbor distance and the lattice of the underlying oxygen layer. Moreover, due to the inhomogeneously distributed work function of the underlying FeO layer, the C(60) molecular electronic state is periodically modulated resulting in a bright-dim STM contrast. In addition, one coincidence lattice ordering is determined as 8 × 8 superstructure with respect to the C(60) primitive cell, which overlays a 3 × 3 moiré cell of the underlying FeO layer.

Molecular orientations and interfacial structure of C60 on Pt(111).

Molecular orientations and assembled structures of C(60) molecules on Pt(111) have been characterized by low-temperature scanning tunneling microscopy for coverage between 0.1 ML and 1.5 ML. At room temperature, C(60) molecules preferentially decorate the steps and nucleate into single layer islands (SLIs) with hexagonal close-packed structures upon increasing coverage. C(60) islands comprise two differently oriented C(60)∕Pt(111)-(√13 × âˆš13) R13.9° phases, in which five types of molecular orientation of C(60) carbon cage configurations are clearly identified by the high-resolution scanning tunneling microscopy image. Further annealing treatment leads to more uniform molecular orientation without apparent aggregation of C(60) SLIs. As coverage increases above 1 ML, domains corresponding to (2√3 × 2√3) R30° superstructure appear. To explain the above transformation, an interfacial reconstruction model is proposed according to the detailed study of the molecular adsorption structures in different domains.

Coverage-dependent structures of cobalt-phthalocyanine molecules adsorbed on Cu(001) surface.

The morphologies, self-assembly structures, and stability of cobalt-phthalocyanines (CoPc) molecules adsorbed on Cu(001) with coverage ranging from 0.2 monolayer (ML) to 1.6 ML are investigated by ultrahigh-vacuum low-temperature scanning tunneling microscopy (UHV LT-STM) at liquid nitrogen temperature. Upon increasing the deposition of CoPc molecules various structures, such as isolated adsorption, quasi-hexagonal structure, square root(29) x square root(29) structure, are well characterized by the corresponding high-resolution STM images. The CoPc-CoPc intermolecular interaction and CoPc-substrate interfacial interaction dominate the structural evolutions. For the coverage higher than 1 ML, CoPc molecules preferentially locate on top of the molecules underneath and organize into square root(58) x square root(58) structure. As more and more CoPc molecules adsorb on the first layer, in some square root(58) x square root(58) regions molecular insertion leads to the formation of the square root(29) x square root(29) domain to effectively decrease the energy of the whole system.

The initial stage of the dissociative adsorption and the surface electronic state evolution of NH3 on Si(111)-(7 × 7).

Adsorption of NH3 molecules on Si(111)-(7 × 7) has been studied by scanning tunneling microscopy. We find that the dissociative adsorption is site-selective and exhibits two adsorption structures resulting from different reaction channels: [Formula: see text] and [Formula: see text]. To explain the dissociation processes, an adsorption model for these reactions is given. Furthermore, the evolution of the local electronic structures is investigated by means of atomically resolved scanning tunneling spectroscopy to clarify the effect of different fragments on the surface states. Finally, we discuss the adsorption position of H atoms from the NH3 dissociation.