Structure transformation from Sierpinski triangles to chains assisted by gas molecules.

Reversible transformations between fractals and periodic structures are of fundamental importance for understanding the formation mechanism of fractals. Currently, it is still a challenge to controllably achieve such a transformation. We investigate the effect of CO and CO2 molecules on Sierpinski triangles (STs) assembled from Fe atoms and 4,4″-dicyano-1,1':3',1″-terphenyl (C3PC) molecules on Au surfaces. Using scanning tunneling microscopy, we discover that the gas molecules induce a transition from STs into 1D chains. Based on density functional theory modeling, we propose that the atomistic mechanism involves the transformation of a stable 3-fold coordination Fe(C3PC)3 motif to Fe(C3PC)4 with an axially bonded CO molecule. CO2 causes the structural transformation through a molecular catassembly process.

Observation of Biradical Spin Coupling through Hydrogen Bonds.

Investigation of intermolecular electron spin interaction is of fundamental importance in both science and technology. Here, radical pairs of all-trans retinoic acid molecules on Au(111) are created using an ultralow temperature scanning tunneling microscope. Antiferromagnetic coupling between two radicals is identified by magnetic-field-dependent spectroscopy. The measured exchange energies are from 0.1 to 1.0 meV. The biradical spin coupling is mediated through O─H⋯O hydrogen bonds, as elucidated from analysis combining density functional theory calculation and a modern version of valence bond theory.

Packing Biomolecules into Sierpinski Triangles with Global Organizational Chirality.

Fractals are found in nature and play important roles in biological functions. However, it is challenging to controllably prepare biomolecule fractals. In this study, a series of Sierpinski triangles with global organizational chirality is successfully constructed by the coassembly of l-tryptophan and 1,3-bi(4-pyridyl)benzene molecules on Ag(111). The chirality is switched when replacing l-tryptophan by d-tryptophan. The fractal structures are characterized by low-temperature scanning tunneling microscopy at the single-molecule level. Density functional theory calculations reveal that intermolecular hydrogen bonds stabilize the Sierpinski triangles.

On-surface preparation of coordinated lanthanide-transition-metal clusters.

The study of lanthanide (Ln)-transition-metal (TM) heterometallic clusters which play key roles in various high-tech applications is a rapid growing field of research. Despite the achievement of numerous Ln-TM cluster compounds comprising one Ln atom, the synthesis of Ln-TM clusters containing multiple Ln atoms remains challenging. Here, we present the preparation and self-assembly of a series of Au-bridged heterometallic clusters containing multiple cerium (Ce) atoms via on-surface coordination. By employing different pyridine and nitrile ligands, the ordered coordination assemblies of clusters containing 2, 3 and 4 Ce atoms bridged by Au adatoms are achieved on Au(111) and Au(100), as revealed by scanning tunneling microscopy. Density functional theory calculations uncover the indispensable role of the bridging Au adatoms in constructing the multi-Ce-containing clusters by connecting the Ce atoms via unsupported Ce-Au bonds. These findings demonstrate on-surface coordination as an efficient strategy for preparation and organization of the multi-Ln-containing heterometallic clusters.

Tuning rotation axes of single molecular rotors by a combination of single-atom manipulation and single-molecule chemistry.

Defining the axis of a molecular rotation is vital for the bottom-up design of molecular rotors. The rotation of tin-phthalocyanine molecules on the Ag(111) surface is studied by scanning tunneling microscopy and atomic/molecular manipulation at 4 K. Tin-phthalocyanine acts as a molecular rotor that binds to Ag adatoms and the substrate. Four different rotation axes are constructed at positions from the center to the periphery of the molecule. Furthermore, using the asymmetric appearance of the modified molecule, the rotation direction of the molecules is identified. This work provides a new approach for designing molecular rotors or motors with definable rotation radii and functions.

Effects of the molecule-electrode interface on the low-bias conductance of Cu-H2-Cu single-molecule junctions.

The atomic structure and electronic transport properties of a single hydrogen molecule connected to both symmetric and asymmetric Cu electrodes are investigated by using the non-equilibrium Green's function formalism combined with the density functional theory. Our calculations show that in symmetric Cu-H2-Cu junctions, the low-bias conductance drops rapidly upon stretching, while asymmetric ones present a low-bias conductance spanning the 0.2-0.3 G0 interval for a wide range of electrode separations. This is in good agreement with experiments on Cu atomic contacts in a hydrogen environment. Furthermore, the distribution of the calculated vibrational energies of the two hydrogen atoms in the asymmetric Cu-H2-Cu junction is also consistent with experiments. These findings provide clear evidence for the formation of asymmetric Cu-H2-Cu molecular junctions in breaking Cu atomic contacts in the presence of hydrogen and are also helpful for the design of molecular devices with Cu electrodes.

Cu-metalated carbyne acting as a promising molecular wire.

The atomic structure and electronic transport properties of Cu-metalated carbyne are investigated by using the non-equilibrium Green's function formalism combined with density functional theory. Our calculations show that the incorporation of Cu atom in carbyne improves its robustness against Peierls distortion, thus to make Cu-metalated carbyne behave as a one-dimensional metal. When a finite Cu-metalated carbyne chain is connected to two (111)-oriented platinum electrodes, nearly linear current-voltage characteristics are obtained for both the atop and adatom binding sites. This is due to the efficient electronic coupling between the Cu-metalated carbyne chain and the Pt electrodes, demonstrating the promising applications of Cu-metalated carbyne chains as molecular wires in future electronic devices.

Controlling Molecular Growth between Fractals and Crystals on Surfaces.

Recent studies demonstrate that simple functional molecules, which usually form two-dimensional (2D) crystal structures when adsorbed on solid substrates, are also able to self-assemble into ordered openwork fractal aggregates. To direct and control the growth of such fractal supramolecules, it is necessary to explore the conditions under which both fractal and crystalline patterns develop and coexist. In this contribution, we study the coexistence of Sierpinski triangle (ST) fractals and 2D molecular crystals that were formed by 4,4″-dihydroxy-1,1':3',1″-terphenyl molecules on Au(111) in ultrahigh vacuum. Growth competition between the STs and 2D crystals was realized by tuning substrate and molecular surface coverage and changing the functional groups of the molecular building block. Density functional theory calculations and Monte Carlo simulations are used to characterize the process. Both experimental and theoretical results demonstrate the possibility of steering the surface self-assembly to generate fractal and nonfractal structures made up of the same molecular building block.

ZnO Hierarchical Nanostructure Photoanode in a CdS Quantum Dot-Sensitized Solar Cell.

A hierarchical array of ZnO nanocones covered with ZnO nanospikes was hydrothermally fabricated and employed as the photoanode in a CdS quantum dot-sensitized solar cell (QDSSC). This QDSSC outperformed the QDSSC based on a simple ZnO nanocone photoanode in all the four principal photovoltaic parameters. Using the hierarchical photoanode dramatically increased the short circuit current density and also slightly raised the open circuit voltage and the fill factor. As a result, the conversion efficiency of the QDSSC based on the hierarchical photoanode was more than twice that of the QDSSC based on the simple ZnO nanocone photoanode. This improvement is attributable to both the enlarged specific area of the photoanode and the reduction in the recombination of the photoexcited electrons.

Nanogap based graphene coated AFM tips with high spatial resolution, conductivity and durability.

After one decade of analyzing the intrinsic properties of graphene, interest into the development of graphene-based devices and micro electromechanical systems is increasing. Here, we fabricate graphene-coated atomic force microscope tips by growing the graphene on copper foil and transferring it onto the apex of a commercially available AFM tip. The resulting tip exhibits surprising enhanced resolution in nanoscale electrical measurements. By means of topographic AFM maps and statistical analyses we determine that this superior performance may be related to the presence of a nanogap between the graphene and the tip apex, which reduces the tip radius and tip-sample contact area. In addition, the graphene-coated tips show a low tip-sample interaction, high conductivity and long life times. The novel fabrication-friendly tip could improve the quality and reliability of AFM experiments, while reducing the cost of AFM-based research.

Helicity-dependent single-walled carbon nanotube alignment on graphite for helical angle and handedness recognition.

Aligned single-walled carbon nanotube arrays provide a great potential for the carbon-based nanodevices and circuit integration. Aligning single-walled carbon nanotubes with selected helicities and identifying their helical structures remain a daunting issue. The widely used gas-directed and surface-directed growth modes generally suffer the drawbacks of mixed and unknown helicities of the aligned single-walled carbon nanotubes. Here we develop a rational approach to anchor the single-walled carbon nanotubes on graphite surfaces, on which the orientation of each single-walled carbon nanotube sensitively depends on its helical angle and handedness. This approach can be exploited to conveniently measure both the helical angle and handedness of the single-walled carbon nanotube simultaneously at a low cost. In addition, by combining with the resonant Raman spectroscopy, the (n,m) index of anchored single-walled carbon nanotube can be further determined from the (d,θ) plot, and the assigned (n,m) values by this approach are validated by both the electronic transition energy Eii measurement and nanodevice application.

Half-metallic properties of single-walled polymeric manganese phthalocyanine nanotubes.

We present a theoretical study of the electronic and magnetic properties of single-walled manganese phthalocyanine (MnPc) nanotubes which can be thought of as rolled-up ribbons of the two-dimensional (2D) polymeric MnPc sheet. Our density functional theory calculations show that all of the MnPc nanotubes investigated here are half-metals with 100% spin polarization around the Fermi level. Following the increase of the tube diameter, the number of spin-down energy bands of MnPc nanotubes is always increased while the spin-up band gap of MnPc nanotubes approaches that of the 2D MnPc sheet in an oscillatory manner. Because the half-metallic character of MnPc nanotubes is deeply rooted in the distribution of electrons in the energy bands dominated by the Mn 3d atomic orbitals, adsorption of CO molecules on the Mn ions leads to a redistribution of electrons in the Mn 3d orbitals and thus can tune precisely the spin state and electronic transport properties of MnPc nanotubes, demonstrating promising applications of MnPc nanotubes in future molecular spintronics and single-molecule sensors.

Effects of the covalent linker groups on the spin transport properties of single nickelocene molecules attached to single-walled carbon nanotubes.

The understanding of how the spin moment of a magnetic molecule transfers to a carbon nanotube, when the molecule is attached to it, is crucial for designing novel supramolecular spin devices. Here we explore such an issue by modeling the spin transport of a single-walled carbon nanotube grafted with one nickelocene molecule. In particular we investigate how the electron transport becomes spin-polarized depending on the specific linking group bonding nickelocene to the nanotube. We consider as linkers both aziridine and pyrrolidine rings and the amide group. Our calculations show that, at variance with aziridine, both pyrrolidine and amide, do alter the sp(2) character of the binding site of the nanotube and thus affect the transmission around the Fermi level. However, only aziridine allows transferring the spin polarization of the nickelocene to the nanotube, whose conductance at the Fermi level becomes spin-polarized. This suggests the superiority of aziridine as a linker for grafting magnetic molecules onto carbon nanotubes with efficient spin filtering functionality.

Spin transport properties of single metallocene molecules attached to single-walled carbon nanotubes via nickel adatoms.

The spin-dependent transport properties of single ferrocene, cobaltocene, and nickelocene molecules attached to the sidewall of a (4,4) armchair single-walled carbon nanotube via a Ni adatom are investigated by using a self-consistent ab initio approach that combines the non-equilibrium Green's function formalism with the spin density functional theory. Our calculations show that the Ni adatom not only binds strongly to the sidewall of the nanotube, but also maintains the spin degeneracy and affects little the transmission around the Fermi level. When the Ni adatom further binds to a metallocene molecule, its density of states is modulated by that of the molecule and electron scattering takes place in the nanotube. In particular, we find that for both cobaltocene and nickelocene the transport across the nanotube becomes spin-polarized. This demonstrates that metallocene molecules and carbon nanotubes can become a promising materials platform for applications in molecular spintronics.

Spin transport properties of 3d transition metal(II) phthalocyanines in contact with single-walled carbon nanotube electrodes.

The spin transport properties of a series of 3d transition metal(ii) phthalocyanines (MPc, M = Mn, Fe, Co, Ni, Cu and Zn) sandwiched between two semi-infinite armchair single-walled carbon nanotube electrodes are investigated by using a self-consistent ab initio approach that combines the non-equilibrium Green's function formalism with spin density functional theory. Our calculations show that among the six molecules only MnPc and FePc can act as nearly perfect spin filters and at the same time have a large transmission around the Fermi level. This is dominated by the highest occupied molecular orbital (HOMO) of the corresponding MPc molecule. In contrast to the other four MPc molecules, whose HOMO is the a(1u) orbital located over the Pc ring, the HOMO of MnPc and FePc is a doubly degenerate pi-type orbital composed of the 3d(xz) and 3d(yz) atomic orbitals of the metal center. The spin polarization of MnPc and FePc is independent of the size of the SWCNT electrodes and can be tuned by chemisorption at the metal center, demonstrating that MPc and carbon nanotubes are a promising materials platform for applications in molecular spintronics.

Electronic transport calculations for the conductance of Pt-1,4-phenylene diisocyanide-Pt molecular junctions.

The low-bias transport properties of a single 1,4-phenylene diisocyanide (PDI) molecule connected to two platinum (Pt) electrodes are investigated using a self-consistent ab initio approach that combines the non-equilibrium Green's function formalism with density functional theory. Our calculations demonstrate that the zero-bias conductance of an asymmetric Pt-PDI-Pt junction, where the PDI molecule is attached to the atop site at one Pt(111) electrode and to a Pt adatom at the other, is 2.6 x 10( - 2)G(0), in good agreement with the experimental value (3 x 10( - 2)G(0)) measured with break junctions. Although the highest occupied and the lowest unoccupied molecule orbitals in PDI are both pi-type, delocalized along the entire molecule, their electronic coupling with the highly conducting states of the Pt electrode is blocked at the atop site, leading to the small transmission. This indicates that more efficient electronic contacts are needed to fabricate molecular devices with a high conductance using Pt electrodes and aromatic isocyanides such as PDI.

Tuning the magneto-transport properties of nickel-cyclopentadienyl multidecker clusters by molecule-electrode coupling manipulation.

Spin transport in a series of organometallic multidecker clusters made of alternating nickel atoms and cyclopentadienyl (Cp) rings is investigated by using first-principles quantum transport simulations. The magnetic moment of finite NinCp(n+1) clusters in the gas phase is a periodic function of the number of NiCp monomers, n, regardless of the cluster termination and despite the fact that the band structure of the infinite [NiCp]infinity chain is nonmagnetic. In contrast, when the clusters are sandwiched between gold electrodes, their spin polarization is found to strongly depend on the molecule-electrode coupling. On the one hand, a substantial magnetic moment and a large spin polarization can be detected for NiCp2 and Ni4Cp5 with both weak and modest molecule-electrode coupling. On the other hand, when the coupling of the clusters is strong and mediated by Ni adatoms, the spin polarization of all NinCp(n+1) (n = 1-4) clusters is destroyed, although their low-bias conductance is large. This demonstrates that the magnetism and the spin-transport properties of fragile molecular magnets, such as NinCp(n+1), can be tuned in a controllable way by changing the contact geometry.

Spin filter effect of manganese phthalocyanine contacted with single-walled carbon nanotube electrodes.

We present a theoretical study of the spin transport through a manganese phthalocyanine (MnPc) molecule sandwiched between two semi-infinite armchair single-walled carbon nanotube (SWCNT) electrodes. Ab initio modeling is performed by combing the nonequilibrium Green's function formalism with spin density functional theory. Our calculations show that MnPc not only can act as a nearly perfect spin filter, but also has a large transmission around the Fermi level, which is dominated by the highest occupied molecule orbital (HOMO). The HOMO of MnPc is found to be a singly filled doubly degenerate molecular orbital, where the electrodes' Fermi level can easily pin. The spin filter effect of MnPc is very robust regardless of whether the open ends of the SWCNT electrodes are terminated by hydrogen, fluorine, or carbon dimers, demonstrating its promising applications in future molecular spintronics.

The spin filter effect of iron-cyclopentadienyl multidecker clusters: the role of the electrode band structure and the coupling strength.

We present a theoretical study of spin transport in a series of organometallic iron-cyclopentadienyl, Fe(n)Cp(n+1), multidecker clusters sandwiched between either gold or platinum electrodes. Ab initio modeling is performed by combining the non-equilibrium Green's function formalism with spin density functional theory. Due to the intrinsic bonding nature, the low-bias conductance of the Fe(n)Cp(n+1) clusters contacted to gold electrodes is relatively small even for strong cluster-electrode coupling. However, a nearly 100% spin polarization of the transmitted electrons can be achieved for the Fe(n)Cp(n+1) (n>2) clusters. In contrast, the Fe(n)Cp(n+1) (n>2) clusters attached to platinum electrodes through Pt adatoms not only can act as nearly perfect spin filters but also show a much larger transmission around the Fermi level, demonstrating their promising applications in future molecular spintronics.