Fullerene adlayers on various single-crystal metal surfaces prepared by transfer from L films

Fullerene adlayers prepared by the simple Langmuir-Blodgett (LB) method onto various well-defined single-crystal metal surfaces were investigated by in situ scanning tunneling microscopy (STM). The surface morphologies of fullerene adsorbed onto metal surfaces depended largely on the adsorbate-substrate interactions, which are governed by the types of surfaces. Too weak adsorption of C60 molecules onto iodine-modified Au(111) (I/Au(111)) allows surface migration of the molecules, and then, STM cannot visualize the C60 molecules. Stronger and appropriate adsorption onto bare Au(111) leads to highly ordered arrays relatively easily due to the limited surface migration of C60. On iodine-modified Pt(111) (I/Pt(111)) and bare Pt(111) surfaces, which have stronger adsorption, randomly adsorbed molecular adlayers were observed. Although C60 molecules on Au(111) were visualized as a featureless ball due to the maintenance of the rapid rotational motion (perturbation) of C60 on the surface at room temperature, those on I/Pt(111) revealed the intramolecular structures, thus indicating that the perturbation motion of molecules on the surface was prohibited.     

Two-dimensional connective nanostructures of electrodeposited Zn on Au (111) induced by spinodal decomposition

Phase formation of surface alloying by spinodal decomposition has been studied at an electrified interface. For this aim Zn was electrodeposited on Au(111) from the ionic liquid AlCl(3)-MBIC (58:42) containing 1 mM Zn(II) at different potentials in the underpotential range corresponding to submonolayer up to monolayer coverage. Structure evolution was observed by in situ electrochemical scanning tunneling microscopy (STM) at different times after starting the deposition via potential jumps and at temperatures of 298 and 323 K. Spinodal or labyrinth two-dimensional structures predominate at middle coverage, both in deposition and in dissolution experiments. They are characterized by a length scale of typically 5 nm which has been determined from the power spectral density of STM images. Structure formation and surface alloying are governed by slow kinetics with a rate constant k with activation energy of 120 meV and preexponential factor of 0.17 s(-1). The evolution of the structural features is described by a continuum model and is found to be in good agreement with the STM observations. From the experimental and model calculation results we conclude that the two-dimensional phase formation in the Zn on Au(111) system is dominated by surface alloying. The phase separation of a Zn-rich and a Zn-Au alloy phase is governed by two-dimensional spinodal decomposition.     

Ordered arrays of semi-crown ligands on an Au(111) electrode surface: in situ STM study

Self-organized systems have attracted much at-tention due to their potential applications in nano- technology as a bottom-up?approach for the con-struction of molecule-scale devices and nanostruc-tures[1—4]. Beyond the self-assembly of small molecu-lar building blocks, Schnherr et al. recently suc-ceeded in arranging the rosette supramolecular nanos-tructures in two dimensions on HOPG[5,6]. Moreover, interest has tremendously increased in the su-pramolecular structures via coordination-dr…     

In situ STM study of zinc electrodeposition on Au(111) from the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate

The electrodeposition of Zn on Au(111) was investigated with cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) in the air and water stable ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIm]TfO) with a Zn(TfO)₂ concentration of 0.2 M. It has been found that the structure [EMIm]TfO/Au(111) is very complex. Furthermore, the addition of Zn(TfO)₂ changes the interfacial structure significantly. The first STM-probed Zn islands appear at +0.3 V, and their growth leads to the formation of a thin zinc layer. A bulk deposition of Zn is obtained with in situ STM at ?0.1 V. Furthermore, in situ STM reveals that the deposition of Zn is accompanied by the formation of Au-Zn surface alloys.     

Ordered arrays of semi-crown ligands on an Au(111) electrode surface: <Emphasis Type="Italic">in situ</Emphasis> STM study

In situ scanning tunneling microscopy (STM) and cyclic voltammetry were employed to investigate the adsorption structures of three semi-crown ligands on an Au(111) surface under the potential control. It is found that all the molecules formed ordered arrays in 0.1 mol/L HClO₄ solution, although their geometric structures are complex and asymmetric. The driving force was supposed to come from the balance between intermolecular and molecule-substrate interactions. High resolution STM images revealed internal molecular structures, orientations and packing arrangements in the ordered adlayers. The results are useful for preparing ordered arrays of transition metal-mediated nanostructures.     

In situ STM imaging and direct electrochemistry of Pyrococcus furiosus ferredoxin assembled on thiolate-modified Au111 surfaces

We have addressed here electron transfer (ET) of Pyrococcus furiosus ferredoxin (PfFd, 7.5 kDa) in both homogeneous solution using edge plane graphite (EPG) electrodes and in the adsorbed state by electrochemistry on surface-modified single-crystal Au111 electrodes, This has been supported by surface microscopic structures of PfFd monolayers, as revealed by scanning tunneling microscopy under potential control (in situ STM). Direct ET between PfFd in phosphate buffer solution, pH 7.9, and EPG electrodes is observed in the presence of promoters. Neomycin gives rise to a pair of redox peaks with a formal potential of ca -430 mV (vs SCE), corresponding to [3Fe-4S]1+/0. The presence of an additional promoter, which can be propionic acid, alanine, or cysteine, induces a second pair of redox peaks at approximately -900 mV (vs SCE) arising from [3Fe-4S]0/1-. A robust neomycin-PfFd complex was detected by mass spectrometry. The results clearly favor an ET mechanism in which the promoting effect of small organic molecules is through formation of promoter-protein complexes. The interaction of PfFd with small organic molecules in homogeneous solution offers clues to confine the protein on the electrode surface modified by the same functional group monolayer and to address diffusionless direct electrochemistry, as well as surface microstructures of the protein monolayer. PfFd molecules were found to assemble on either mercaptopropionic acid (MPA) or cysteine-modified Au111 surfaces in stable monolayers or submonolayers. Highly ordered (2 radical 3 x 5)R30 degrees cluster structures with six MPA molecules in each cluster were found by in situ STM. Individual PfFd molecules on the MPA layer are well resolved by in situ STM. Under Ar protection reversible cyclic voltammograms were obtained on PfFd-MPA/Au111 and PfFd-cysteine/Au111 electrodes with redox potentials of -220 and -201 mV (vs SCE), respectively, corresponding to the [Fe3S4]1+/0 couple. These values are shifted positively by 200 mV relative to homogeneous solution due to interactions between the promoting layers and the protein molecules. Possible mechanisms for such interactions and their ET patterns are discussed.     

Structural Transformation of Surface-Confined Porphyrin Networks by Addition of Co Atoms

The self-assembly of a nickel-porphyrin derivative (Ni-DPPyP) containing two pyridyl coordinating sites and two pentyl chains at trans meso positions was studied with scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) on Au(111). Deposition of Ni-DPPyP onto Au(111) gave rise to a close-packed network for coverages smaller or equal to one monolayer as revealed by STM and LEED. The molecular arrangement of this two-dimensional network is stabilized via hydrogen bonds formed between the pyridyl's nitrogen and hydrogen atoms from the pyrrole groups of neighboring molecules. Subsequent deposition of cobalt atoms onto the close-packed network and post-deposition annealing at 423 K led to the formation of a Co-coordinated hexagonal porous network. As confirmed by XPS measurements, the porous network is stabilized by metal-ligand interactions between one cobalt atom and three pyridyl ligands, each pyridyl ligand coming from a different Ni-DPPyP molecule.     

Supramolecular Cl⋅⋅⋅H and O⋅⋅⋅H Interactions in Self‐Assembled 1,5‐Dichloroanthraquinone Layers on Au(111)

The role of halogen bonds in self‐assembled networks for systems with Br and I ligands has recently been studied with scanning tunneling microscopy (STM), which provides physical insight at the atomic scale. Here, we study the supramolecular interactions of 1,5‐dichloroanthraquinone molecules on Au(111), including Cl ligands, by using STM. Two different molecular structures of chevron and square networks are observed, and their molecular models are proposed. Both molecular structures are stabilized by intermolecular Cl???H and O???H hydrogen bonds with marginal contributions from Cl‐related halogen bonds, as revealed by density functional theory calculations. Our study shows that, in contrast to Br‐ and I‐related halogen bonds, Cl‐related halogen bonds weakly contribute to the molecular structure due to a modest positive potential (σ hole) of the Cl ligands.     

Identifying Multinuclear Organometallic Intermediates in On‐Surface [2+2] Cycloaddition Reactions

We investigate the on‐surface [2+2] cycloaddition reaction of 2,3,6,7,10,11‐hexabromotriphenylene (HBTP) on Ag(111), Cu(111), Au(111), and Cu‐dosed Au(111) surfaces using STM and DFT simulation focusing on the organometallic intermediates. The fully debrominated HBTP molecules form an organo‐silver framework on Ag(111) and an organo‐copper framework on Cu(111), both incorporating multinuclear metal adatom clusters. The organo‐silver framework is converted into porous covalent networks via [2+2] cycloaddition above 240 °C. In contrast, the organo‐copper framework is very stable and does not undergo [2+2] cycloaddition even at 300 °C. On Au(111), no organo‐gold intermediate of [2+2] cycloaddition is observed. After loading Cu onto Au(111), the partially debrominated HBTP molecules bind to Cu adatom dimers to form multinuclear organo‐copper complexes at 100 °C which undergo [2+2] cycloaddition at 140 °C. This study shows that the choice of surface can direct the reaction pathway.     

Giant porphyrin disks: control of their self-assembly at liquid-solid interfaces through metal-ligand interactions

The synthesis and self-assembly behaviour of porphyrin dodecamers 1H(2) and Zn-1, which consist of twelve porphyrins that are covalently attached to a central aromatic core, is described. According to STM, 1D and 2D NMR studies, and molecular modelling calculations, the porphyrin dodecamers have a yo-yo-shaped structure. Their large pi surface, in combination with their disk-like shape, allows them to form self-assembled structures, which in the case of Zn-1 can be tuned by adding bidentate ligands. The self-assembly of the molecules at the liquid-solid interface of 1-phenyloctane with highly oriented pyrolytic graphite or Au(111) was imaged by using STM. The porphyrin disks in the self-assembled arrays have an edge-on orientation on the surface. The addition of bidentate axial ligands to the Zn-1 molecules in the arrays allows their intermolecular distance to be precisely controlled.     

Au(111)表面Verdazyl自由基的构象转换

Pure organic radical molecules on metal surfaces are of great significance in exploration of the electron spin behavior. However, only a few of them are investigated in surface studies due to their poor thermal stability. The adsorption and conformational switching of two verdazyl radical molecules, namely, 1, 5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2, 3-d]thiophen-2-yl)-6-oxoverdazyl (B2P) and 1, 5-biisopropyl-3-(benzo[b]benzo[4,5]thieno[2, 3-d]thiophen-4-yl)-6-oxoverdazyl (B4P), are studied by scanning tunneling microscopy (STM) and density functional theory (DFT). The adsorbed B2P molecules on Au(111) form dimers, trimers and tetramers without any ordered assembly structure in which two distinct appearances of B2P in STM images are observed and assigned to be its "P" and "T" conformations. The "P" conformation molecules appear in the STM image with a large elliptical protrusion and two small ones of equal size, while the "T" ones appear with a large protrusion and two small ones of different size. Likewise, the B4P molecules on Au(111) form dimers at low coverage, strip structure at medium coverage and assembled structure at high coverage which also consists of above-mentioned two conformations. Both B2P molecules and B4P molecules are held together by weak intermolecular interaction rather than chemical bond. STM tip induced conformational switching of both verdayzl radicals is observed at the bias voltage of +2.0 V. The "T" conformation of B2P can be switched to the "P" while the "P" conformation of B4P can be switched to the "T" one. For both molecules, such a conformational switching is irreversible. The DFT calculations with Perdew-Burke-Ernzerhof version exchange-correlation functional are used to optimize the model structure and simulate the STM images. STM images of several possible molecular conformations with different isopropyl orientation and different tilt angle between verdazyl radical and Au(111) surface are simulated. For conformations with different isopropyl orientation, the STM simulated images are similar, while different tilt angles of verdazyl radical lead to significantly different STM simulated images. Combined STM experiments and DFT simulations reveal that the conformational switching originates from the change of tilting angle between the verdazyl radical and Au(111) surface. The tilt angles in "P" and "T" conformations are 0° and 50°, respectively. In this study, two different adsorption conformations of verdazyl radicals on the Au(111) surface are presented and their exact adsorption structures are identified. This study provides a possible way to study the relationship between the electron spin and configuration conversion of pure organic radical molecules and a reference for designing more conformational switchable radical molecules that can be employed as interesting molecular switches.     

Effect of lattice-gas atoms on the adsorption behaviour of thioether molecules

Using STM topographic imaging and spectroscopy, we have investigated the adsorption of two thioether molecules, 1,2-bis(phenylthio)benzene and (bis(3-phenylthio)-phenyl)sulfane, on noble and transition metal surfaces. The two substrates show nearly antipodal behaviour. Whereas complexes with one or two protruding centres are observed on Au(111), only flat and uniform ad-structures are found on NiAl(110). The difference is ascribed to the possibility of the thioethers to form metal-organic complexes by coordinating lattice-gas atoms on the Au(111), while only the pristine molecules adsorb on the alloy surface. The metal coordination in the first case is driven by the formation of strong Au-S bonds and enables the formation of characteristic monomer, dimer and chain-like structures of the thioethers, using the Au atoms as linkers. A similar mechanism is not available on the NiAl, because no lattice gas develops at this surface at room temperature. Our work demonstrates how surface properties, i.e. the availability of mobile ad-species, determine the interaction of organic molecules with metallic substrates.     

Effect of adlayer structure on the host-guest recognition between calcium and crown-ether-substituted phthalocyanine arrays on Au single-crystal surfaces.

Adlayers of 15-crown-5-ether-substituted cobalt(II) phthalocyanine (CoCRPc) were prepared by immersion of either Au(111) or Au(100) substrate into benzene-ethanol (9:1 v/v) mixed solutions containing CoCRPc. In situ STM imaging was carried out after transferring the CoCRPc-modified Au crystals into aqueous HClO(4) solution. The packing arrangement of the CoCRPc array on Au(111) was determined to be p(8 x 4 radical 3R - 30 degrees ), and the internal structure was clearly observed by high-resolution STM. Two adlayer structures of CoCRPc, (8 x 9) and (4 radical 5 x 4 radical 5)R26.7 degrees, were found on the Au(100)-(1 x 1) terrace. In the presence of 1 mM Ca(2+), two Ca(2+) ions were trapped in two diagonally located 15-crown-5-ether moieties of each CoCRPc molecule on Au(111), whereas encapsulation of Ca(2+) ions was not seen in the CoCRPc arrays on the Au(100)-(1 x 1) surface. The present study demonstrates that the relationship between crown moieties of CRPc and the underlying Au lattice is important in the trapping of Ca(2+) ions in crown rings.     

Formation of heterocaryotic and homonuclear bridged–dimeric complexes on surface

The formation of coordinated dimeric complexes bridged by axial ligands on surface is observed with the help of a 1,3,5-tris(10-carboxydecyloxy)benzene(TCDB) template through scanning tunneling microscopy(STM). STM images of molecular adlayers of zinc tetraphenylporphyrin(Zn TPP), zinc phthalocyanine(Zn Pc), and their mixture are reported. Zn TPP and Zn Pc can spontaneously form highly an ordered structure with a 1:1 molar ratio, which is different from that of individual Zn Pc. The coordinated bimolecular complexes bridged with axial ligands, simply as Zn Pc–DPP–Zn TPP and Zn Pc–DPE–Zn Pc, are presented and the corresponding surface structures are compared. Zn Pc and Zn TPP can be connected by an axial ligand DPP and formed assembled structures out of surface. Two types of arrays with entirely new structure are obtained for the Zn Pc–DPE–Zn Pc complex. These bridged hybrid complexes provide an example of design of self-organized crystals on the basis of coordination through non-covalent interactions.     

Epitaxial electrodeposition of ZnO on Au(111) from alkaline solution: exploiting amphoterism in Zn(II)

The amphoteric nature of ZnO is used to produce the material from strongly alkaline solution. The solution pH is lowered globally to produce ZnO powder, and it is lowered locally at a Au(111) surface to produce epitaxial films. ZnO powder is precipitated from a solution of 10 mM Zn(II) in 0.25 M NaOH by simply adding 1 M HNO3 to the solution. For the film electrodeposition, the local pH at the electrode surface is decreased by electrochemically oxidizing the ascorbate dianion. The chemically precipitated ZnO powder grows with a sea urchin-like nanostructure, whereas the electrodeposited films have a columnar structure. ZnO electrodeposited onto a Au(111) single crystal has a ZnO(0001)[1011]//Au(111)[110] orientation relationship.     

Surface Trapping and STM Observation of Conformational Isomers of a Bis(Terpyridine) Ligand from Metallosupramolecular Grids

Tetranuclear Co‐grid complexes incorporating bis‐tridentate ligands, namely 4,6‐bis(2,2′‐bipyrid‐6‐yl)‐2‐phenylpyrimidine, were electrosprayed onto a Au(111) substrate under an ultrahigh vacuum. Fragmentation occurs leaving the ligands in four different conformations. Most ligands are found to form H‐bonded dimers. The most abundant conformer of the ligand on the Au surface is an asymmetric form, which has not been observed before. The present results indicate that the fragmentation of coordination compounds during the deposition process, in principle, allows for surface trapping, identification, and investigation of high energy, out‐of‐equilibrium conformations of the ligand molecules at low temperatures, which otherwise would not be observable.     

Potential-induced phase transition of low-index Au single crystal surfaces in propylene carbonate solution

In situ scanning tunneling microscopy (STM) was employed to examine the surface structures of Au(111), Au(100), and Au(110) single crystals in propylene carbonate (PC) containing tetrabutylammonium perchlorate (TBAP). All three electrodes exhibited potential-induced phase transition between the reconstructed and unreconstructed (1 × 1) structures at negative and positive potentials, respectively. The potential-induced phase transition of the Au electrode surfaces is attributed to the interaction of the TBA cation and the perchlorate anion at the electrode surface, which is similar to that which takes place in aqueous solutions. In addition to static atomic structures, dynamic processes of both the reconstruction and the lifting of the reconstruction were investigated by means of in situ STM. The lifting of reconstructed Au(111)-(√3 × 22) on Au(111) to the (1 × 1) structure is completed within 1 min at a positive potential. The diffusion of Au atoms on the Au(100) plane in the PC solution proceeds more rapidly than that in the aqueous solution, suggesting that the PC solvent plays an important role in accelerating the diffusion of Au atoms.     

Ordered molecular assemblies of substituted bis(phthalocyaninato) rare earth complexes on Au(111): in situ scanning tunneling microscopy and electrochemical studies

Substituted bis(phthalocyaninato) rare earth complexes ML2 (M = Y and Ce; L = [Pc(OC8H17)8]2, where Pc = phthalocyaninato) were adsorbed onto single crystalline Au(111) electrodes from benzene saturated with either YL2 or CeL2 complex at room temperature. In situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV) were used to examine the structures and the redox reactions of these admolecules on Au(111) electrodes in 0.1 mol dm(-3) HClO4. The CVs obtained with YL2- and CeL2-coated Au(111) electrodes respectively contained two and three pairs of redox peaks between 0 and 1.0 V (versus reversible hydrogen electrode). STM molecular resolution revealed that YL2 and CeL2 admolecules were imaged as spherical protrusions separated by 2.3 nm, which suggests that they were oriented with their molecular planes parallel to the unreconstructed Au(111)-(1 x 1). Both molecules when adsorbing from approximately micromolar benzene dosing solutions produced mainly ordered arrays characterized as (8 x 5 radical3)rect (theta = 0.0125). The redox reactions occurring between 0.2 and 1.0 V caused no change in the adlayer, but they were desorbed or oxidized at the negative and positive potential limits. The processes of adsorption and desorption at the negative potentials were reversible to the modulation of potential. Electrochemical impedance spectroscopy (EIS) and CV measurements showed that YL2 and CeL2 adlayers could block the adsorption of perchlorate anions and mediating electron transfer at the Au(111) electrode, leading to the enhancement of charge transfer for the ferro/ferricyanide redox couple.     

Time-dependent organization and wettability of decanethiol self-assembled monolayer on Au(111) investigated with STM

A detailed study on the time-dependent organization of a decanethiol self-assembled monolayer (SAM) at a designed solution concentration onto a Au(111) surface has been performed with scanning tunneling microscopy (STM). The SAMs were prepared by immersing Au(111) into an ethanol solution containing 1 microM decanethiol with different immersion times. STM images revealed the formation process and adlayer structure of the SAMs. It was found that the molecules self-organized into adlayers from random separation to a well-defined structure. From 10 s, small domains with ordered molecular organization appeared, although random molecules could be observed on Au(111) at the very initial stage. At 30 s, the SAM consisted of uniform short stripes. Each stripe consisted of sets of decanethiol mainly containing eight molecules. With the immersion time increasing, the length of the stripes increased. At 5 min, the alkyl chains overlapped each other between the adjacent stripes, indicating the start of a stacked process. After immersing Au(111) in decanethiol solution for 3 days, a densely packed adlayer with a (radical 3 x radical 3)R30 degrees structure was observed. The formation process and structure of decanethiol SAMs are well related to sample preparation conditions. The wettability of the decanethiolate SAM-modified Au(111) surface was also investigated.     

Adlayers of C60-C60 and C60-C70 fullerene dimers formed on au(111) in benzene solutions studied by STM and LEED

Scanning tunneling microscopy (STM) and low-energy electron diffraction were used to reveal the structures of ordered adlayers of [2+2]-type C60-C60 fullerene dimer (C120) and C60-C70 cross-dimer (C130) formed on Au(111) by immersingit in abenzene solution containing C120 or C130 molecules. High-resolution STM images clearly showed the packing arrangements and the electronic structures of C120 and C130 on the Au(111) surface in ultrahigh vacuum. The (2 square root3 x 4square root3)R30 degrees, (2square root3 x 5square root3)R30 degrees, and (7 x 7) structures were found for the C120 adlayer on the Au(111) surface, whereas C130 molecules were closely packed on the surface. Each C60 or C70 monomer cage was discerned in the STM image of a C130 molecule.