Self-assembly of extended polycyclic aromatic hydrocarbons on Cu111

We present a low-temperature scanning tunneling microscopy study on the self-assembly of extended polycyclic aromatic hydrocarbons with different symmetries on the Cu111 surface. All molecules show a commensurate monolayer structure, with significant structural differences with respect to the unit cell of the molecular lattice and the orientational ordering. We find that the molecular lattice and the molecular orientation are largely dominated by molecule-substrate interactions, whereas molecule-molecule interactions determine the molecular packing density via steric repulsion. Moreover, we show that the structure of the monolayer is transferred to the second layer via molecule-molecule interaction.     

Mechanochemistry Induced Using Force Exerted by a Functionalized Microscope Tip

Atomic‐scale mechanochemistry is realized from force exerted by a C₆₀‐functionalized scanning tunneling microscope tip. Two conformers of tin phthalocyanine can be prepared on coinage‐metal surfaces. A transition between these conformers is induced on Cu(111) and Ag(100). Density‐functional calculations reveal details of this reaction. Because of the large energy barrier of the reaction and the strong interaction of SnPc with Cu(111), the process cannot be achieved by electrical means.     

Observations of image contrast and dimerization of decacyclene by low temperature scanning tunneling microscopy

Low temperature scanning tunneling microscopy studies revealed both monomer and dimer forms of decacyclene (DC) on atomically clean Cu(100) and Cu(111). The observed image contrast in DC is strongly bias dependent and also influenced by tip modifications. Alternatively, dimers appear solely as protrusions and are nearly bias independent. We provide evidence of both dimer formation and dissociation and suggest that two DC molecules stack by aligning their molecular planes in a parallel fashion with respect to the surface. Dimers and their surface-dependent properties demonstrate the interplay between surface-molecule and molecule-molecule interactions.     

Photophysical properties of [60]fullerenes and phthalocyanines embedded in ordered mesoporous silica films annealed at various temperatures

The photophysical properties of fullerene and/or phthalocyanine dyes embedded in ordered mesoporous silica films and the influence of annealing temperature on the nature of the immobilized dye molecules has been investigated using photoluminescence (PL) and diffuse reflectance (DR) studies. The PL and DR studies show that fullerene (C60) and/or zinc phthalocyanine (ZnPc) molecules incorporated into transparent mesoporous silica films, via either sol-gel or grafting routes, exist predominantly in monomeric form. Careful choice of annealing temperature, between 25 and 225 degrees C, can further enhance monomeric dispersion. For C60-containing films, monomeric dispersion of fullerene was observed for annealing temperatures up to 175 degrees C for sol-gel derived films and 225 degrees C for grafted films. Both sol-gel and grafted ZnPc-containing films showed evidence of monodispersed phthalocyanine for annealing temperatures up to 225 degrees C. In general, annealing temperatures in the range 125-175 degrees C were found to yield optimal monodispersion of the dye molecules. When both C60 and ZnPc were incorporated into the silica films, no evidence of interaction between the dyes, i.e., charge-transfer transitions or the formation of fullerene/phthalocyanine charge-transfer complexes, was observed. This suggests that embedded fullerene and phthalocyanine molecules may be used for the preparation of solid-state optical limiters, based on reverse saturable absorption, where monomeric dispersion of the dye molecules is important.     

Functionalization, self-assembly, and photoswitching quenching for azobenzene derivatives adsorbed on Au(111)

We have used scanning tunneling microscopy to investigate the structure and photoswitching behavior of azobenzene molecules functionalized with bulky spacer groups and adsorbed onto Au(111). We find that positioning tert-butyl "legs" in a canted arrangement on the azobenzene phenyl rings quenches photoisomerizability of the molecule on Au(111). Addition of cyano groups at the para positions changes the molecular self-assembly significantly, but does not alter the quenched photoisomerizability. This behavior likely arises from a combination of molecule-surface interactions, molecule-molecule interactions, and alteration of azobenzene electronic structure resulting from the position-specific addition of tert-butyl groups.     

Synthesis of the axially substituted titanium Pc-C60 dyad with a convenient method

[structure: see text] We successfully synthesized the axially substituted titanium Pc-C(60) dyad with a convenient method that improves on the traditional asymmetrical phthalocyanine routine to covalently linked phthalocyanines with other functional molecules. The intramolecular photoinduced process between phthalocyanine donor and fullerene acceptor was preliminarily studied.     

Molecular interface effect of heterogeneous molecular junctions consisting of nonfluorinated and fluorinated phthalocyanines

We report the tunneling behavior of homogeneous and heterogeneous molecular junctions using p-type molecules of iron phthalocyanine (FePc), phthalocyanine (H(2)Pc), and copper(II) octaalkoxyl substituted phthalocyanine (CuPcOC8) and n-type molecule of copper hexadecafluorophthalocyanine (F(16)CuPc). The molecular films formed on the electrode surfaces were inspected by X-ray photoelectron spectroscopy (XPS). The measured characteristic tunneling curves of single-component phthalocyanines revealed comparable energy gaps for homogeneous tunneling junctions using the photoemission method. In contrast, for the heterogeneous tunnel junctions of mixed phthalocyanines including fluorinated phthalocyanine a distinctive offset of the energy gaps to the positive bias voltage direction can be clearly identified. It is suggested that the substitution of phthalocyanines and surface affinity of phthalocyanines could contribute to the controlled phase separation within the heterogeneous tunneling junctions. The apparent shift of the tunneling spectra is attributed to the existence of an internal electric field originated with the phase separation of the binary mixture of p-type and n-type phthalocyanines within the tunneling junction.     

Characterizing and Manipulating Individual Molecules by Scanning Tunneling Microscopy

Scanning tunneling microscopy (STM) can provide us the special means to characterize the locally physical and chemical properties of individual molecules, and even help us to manipulate the individual molecules for constructing new molecule-scale devices. Here we have adopted two new types of STM techniques to characterize the encapsulated metal atom inside a fullerene cage, and to construct a molecule-device with strong Kondo effect, respectively. The spatially dI/dV mapping spectra were used to unveil the energy-resolved metal-cage hybrid states of individual Dy@C82 molecule, and the important information about the spatial position of Dy atom inside the cage and the Dy-cage interaction was revealed. The high-voltage pulse by STM tip is controlled to induce the dehydrogenation of Co phthalocyanine molecule and change its adsorption configuration on Au(111) surface, so as to recover Kondo effect that disappears in the case of intact adsorbed molecule.     

Electronic superstructure of lead phthalocyanine on lead islands

Lead phthalocyanine (PbPc) deposited on lead islands on Ag(111) forms two-dimensional crystals of densely packed molecules. For particular orientations, the molecular arrays exhibit an electronic superstructure with an extraordinarily large unit cell. The molecules induce a shift of quantum well states, which are confined to the Pb islands. Patterns formed by PbPc on Ag(111) are drastically different.     

Supramolecular pattern of fullerene on 2D bimolecular "chessboard" consisting of bottom-up assembly of porphyrin and phthalocyanine molecules

Two-component adlayers consisting of zinc(II) phthalocyanine (ZnPc) and a metalloporphyrin, such as zinc(II) octaethylporphyrin (ZnOEP) or zinc(II) tetraphenylporphyrin (ZnTPP), were prepared by immersing either an Au(111) or Au(100) substrate in a benzene solution containing those molecules. The bimolecular adlayers thus prepared were investigated in 0.1 M HClO4 by cyclic voltammetry (CV) and electrochemical scanning tunneling microscopy (EC-STM). A supramolecularly organized "chessboard" structure was formed for the ZnPc and ZnOEP bimolecular array on Au(111), while characteristic nanohexagons were found in the ZnTPP and ZnOEP bimolecular adlayer. EC-STM revealed that the surface mobility and the molecular re-organization of ZnPc and ZnOEP on Au(111) were tunable by manipulating the electrode potential, whereas the ZnTPP and ZnOEP bimolecular array was independent of the electrode potential. A "bottom-up" hybrid assembly of fullerene molecules was formed successfully on an alternate array of bimolecular ZnPc and ZnOEP molecules. The bimolecular "chessboard" served as a template to form the supramolecular assembly of C60 by selective trapping in the open spaces. A supramolecular organization of ZnPc and ZnOEP was also found on the reconstructed Au(100)-(hex) surface. A highly ordered, compositionally disordered but alternate array of ZnPc and ZnOEP was formed on the reconstructed Au(100)-(hex) surface, indicating that the bimolecular adlayer structure is dependent on the atomic arrangement of underlying Au in the formation of supramolecular nanostructures composed of those molecules. On the bimolecular array consisting of ZnPc and ZnOEP on the Au(100)-(hex), no highly ordered supramolecular assembly of C60 was found, suggesting that the supramolecular assembly of C60 molecules is strongly dependent upon the bimolecular packing arrangement of ZnPc and ZnOEP.     

Adsorption of ammonia on multilayer iron phthalocyanine

The adsorption of ammonia on multilayers of well-ordered, flat-lying iron phthalocyanine (FePc) molecules on a Au(111) support was investigated by x-ray photoelectron spectroscopy. We find that the electron-donating ammonia molecules coordinate to the metal centers of iron phthlalocyanine. The coordination of ammonia induces changes of the electronic structure of the iron phthalocyanine layer, which, in particular, lead to a modification of the FePc valence electron spin.     

Length-dependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols: effect of metal work function and applied bias on tunneling efficiency and contact resistance

Nanoscopic tunnel junctions were formed by contacting Au-, Pt-, or Ag-coated atomic force microscopy (AFM) tips to self-assembled monolayers (SAMs) of alkanethiol or alkanedithiol molecules on polycrystalline Au, Pt, or Ag substrates. Current-voltage traces exhibited sigmoidal behavior and an exponential attenuation with molecular length, characteristic of nonresonant tunneling. The length-dependent decay parameter, beta, was found to be approximately 1.1 per carbon atom (C(-1)) or 0.88 A(-)(1) and was independent of applied bias (over a voltage range of +/-1.5 V) and electrode work function. In contrast, the contact resistance, R(0), extrapolated from resistance versus molecular length plots showed a notable decrease with both applied bias and increasing electrode work function. The doubly bound alkanedithiol junctions were observed to have a contact resistance approximately 1 to 2 orders of magnitude lower than the singly bound alkanethiol junctions. However, both alkanethiol and dithiol junctions exhibited the same length dependence (beta value). The resistance versus length data were also used to calculate transmission values for each type of contact (e.g., Au-S-C, Au/CH(3), etc.) and the transmission per C-C bond (T(C)(-)()(C)).     

利用给体-π-受体分子模板在Au(111)电极表面构筑富勒烯带状结构(英文)

富勒烯与功能分子之间的电荷作用已经被广泛应用于功能性器件的构筑中.这些功能器件的性能与电极表面的薄膜排布结构有着密切关系.因此,研究电极表面的富勒烯和功能分子的组装结构对这些器件的构筑和功能的发挥有着重要意义.本文利用电化学扫描隧道显微镜技术,在HClO4溶液中系统研究了C60分子与有机电子给体-π-受体分子C16H33O-I3CNQ[Z-β-(5-hexadecyloxy-1,3,3-trimethyl-2-indolium)-α-cyano-4-styryl dicyanomethanide]在Au(111)电极表面的二维组装结构.研究发现:C16H33O-I3CNQ分子在Au(111)电极表面组装形成具有短程有序性的条陇状结构;而C60分子在C16H33O-I3CNQ模板之上组装形成了带状结构.C60分子带状结构的形成方向受到了C16H33O-I3CNQ分子中电子给体-π-受体部分排列结构的影响.C60分子与C16H33O-I3CNQ分子之间的π-π堆积作用和电荷转移作用对这种带状结构的形成有着密切关系.这一结果为利用富勒烯和功能分子之间的作用构筑功能器件提供了一种新的制备方法.     

Chemical speciation of adsorbed glycine on metal surfaces

Experimental studies have reported that glycine is adsorbed on the Cu(110) and Cu(100) surfaces in its deprotonated form at room temperature, but in its zwitterionic form on Pd(111) and Pt(111). In contrast, recent density functional theory (DFT) calculations indicated that the deprotonated molecules are thermodynamically favored on Cu(110), Cu(100), and Pd(111). To explore the source of this disagreement, we have tested three possible hypotheses. Using DFT calculations, we first show that the kinetic barrier for the deprotonation reaction of glycine on Pd(111) is larger than on Cu(110) or Cu(100). We then report that the presence of excess hydrogen would have little influence on the experimentally observed results, especially for Pd(111). Lastly, we perform Monte Carlo simulations to demonstrate that the aggregates of zwitterionic species on Pt(111) are energetically preferred to those of neutral species. Our results strongly suggest that the formation of aggregates with relatively large numbers of adsorbed molecules is favored under experimentally relevant conditions and that the adsorbate-adsorbate interactions in these aggregates stabilize the zwitterionic species.     

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.     

Ammonia adsorption on iron phthalocyanine on Au(111): influence on adsorbate-substrate coupling and molecular spin

The adsorption of ammonia on Au(111)-supported monolayers of iron phthalocyanine has been investigated by x-ray photoelectron spectroscopy, x-ray absorption spectroscopy, and density functional theory calculations. The ammonia-induced changes of the x-ray photoemission lines show that a dative bond is formed between ammonia and the iron center of the phthalocyanine molecules, and that the local spin on the iron atom is quenched. This is confirmed by density functional theory, which also shows that the bond between the iron center of the metalorganic complex and the Au(111) substrate is weakened upon adsorption of ammonia. The experimental results further show that additional adsorption sites exist for ammonia on the iron phthalocyanine monolayer.     

Probing enantioselectivity on chirally modified Cu(110), Cu(100), and Cu(111) surfaces

Temperature programmed desorption methods have been used to probe the enantioselectivity of achiral Cu(100), Cu(110), and Cu(111) single crystal surfaces modified by chiral organic molecules including amino acids, alcohols, alkoxides, and amino-alcohols. The following combinations of chiral probes and chiral modifiers on Cu surfaces were included in this study: propylene oxide (PO) on L-alanine modified Cu(110), PO on L-alaninol modified Cu(111), PO on 2-butanol modified Cu(111), PO on 2-butoxide modified Cu(100), PO on 2-butoxide modified Cu(111), R-3-methylcyclohexanone (R-3-MCHO) on 2-butoxide modified Cu(100), and R-3-MCHO on 2-butoxide modified Cu(111). In contrast with the fact that these and other chiral probe/modifier systems have exhibited enantioselectivity on Pd(111) and Pt(111) surfaces, none of these probe/modifier/Cu systems exhibit enantioselectivity at either low or high modifier coverages. The nature of the underlying substrate plays a significant role in the mechanism of hydrogen-bonding interactions and could be critical to observing enantioselectivity. While hydrogen-bonding interactions between modifier and probe molecule are believed to induce enantioselectivity on Pd surfaces (Gao, F.; Wang, Y.; Burkholder, L.; Tysoe, W. T. J. Am. Chem. Soc. 2007, 129, 15240-15249), such critical interactions may be missing on Cu surfaces where hydrogen-bonding interactions are believed to occur between adjacent modifier molecules, enabling them to form clusters or islands.     

Electron Transport through Single π‐Conjugated Molecules Bridging between Metal Electrodes

Understanding electron transport through a single molecule bridging between metal electrodes is a central issue in the field of molecular electronics. This review covers the fabrication and electron‐transport properties of single π‐conjugated molecule junctions, which include benzene, fullerene, and π‐stacked molecules. The metal/molecule interface plays a decisive role in determining the stability and conductivity of single‐molecule junctions. The effect of the metal–molecule contact on the conductance of the single π‐conjugated molecule junction is reviewed. The characterization of the single benzene molecule junction is also discussed using inelastic electron tunneling spectroscopy and shot noise. Finally, electron transport through the π‐stacked system using π‐stacked aromatic molecules enclosed within self‐assembled coordination cages is reviewed. The electron transport in the π‐stacked systems is found to be efficient at the single‐molecule level, thus providing insight into the design of conductive materials.     

Molecular photoelectric switch using a mixed SAM of organic [60]fullerene and [70]fullerene doped with a single iron atom

We describe a photoswitch fabricated on indium tin oxide (ITO) as a self-assembled monolayer (SAM) of two fullerene molecules, a purely organic [60]fullerene that generates an anodic current and a [70]fullerene doped with a single iron atom. This device generates a bidirectional photocurrent upon irradiation at 340 and 490 nm. The new [70]fullerene iron complex bearing three rigid carboxylic acid legs, Fe[C(70)(C(6)H(4)C(6)H(4)COOH)(3)]Cp, generates only a cathodic current upon photoexcitation between 350 and 700 nm, whereas the organic [60]fullerene absorbs at wavelengths shorter than 500 nm. The quantum efficiency of the photocurrent generation by the mixed SAM is comparable to that of a single-component SAM, indicating that the individual diode molecules on ITO generate photocurrents independently with little cross talk.     

单原子分散的Au/Cu(111)表面合金的表面结构与吸附性质

Au-Cu双金属合金纳米颗粒对包括CO氧化和CO2还原等在内的多个反应有较好的催化活性,然而关于其表面性质的研究却相当匮乏。在此工作中,我们通过对低覆盖度的Au/Cu(111)和Cu/Au(111)双金属薄膜退火,制备出了单原子级分散的Au/Cu(111)和Cu/Au(111)合金化表面,并利用高分辨扫描隧道显微镜(STM)和扫描隧道谱(STS)进一步研究了掺杂原子的电子性质及其对CO吸附行为的影响。研究发现,分散在Cu(111)表面的表层和次表层Au单原子在STM上表现出不同衬度。在-0.5 e V附近,前者表现出相较于Cu(111)明显增强的电子态密度,而后者则明显减弱。吸附实验表明表层Au单原子对CO的吸附能力并没有得到增强,甚至会减弱其周围Cu原子的吸附能力。与Au在Cu(111)表面较好的分散相反,Cu原子倾向于钻入Au(111)的次表层,并且形成多原子聚集体。且Cu原子受Au(111)衬底吸电子作用的影响,其对CO的吸附能力明显减弱。这个研究结果揭示了合金表面的微观结构与性质的关联,为进一步阐明Au-Cu双金属催化剂的表面反应机理提供参考。