Coverage-dependent adsorption geometry of octithiophene on Au(111)

The adsorption behavior of α-octithiophene (8T) on the Au(111) surface as a function of 8T coverage has been studied with low-temperature scanning tunneling microscopy, high resolution electron energy loss spectroscopy as well as with angle-resolved two-photon photoemission and ultraviolet photoemission spectroscopy. In the sub-monolayer regime 8T adopts a flat-lying adsorption geometry. Upon reaching the monolayer coverage the orientation of 8T molecules changes towards a tilted configuration, with the long molecular axis parallel to the surface plane, facilitating attractive intermolecular π-π-interactions. The photoemission intensity from the highest occupied molecular orbitals (HOMO and HOMO - 1) possesses a strong dependence on the adsorption geometry due to the direction of the involved transition dipole moment for the respective photoemission process. The change in molecular orientation as a function of coverage in the first molecular layer mirrors the delicate balance between intermolecular and molecule/substrate interactions. Fine tuning of these interactions opens up the possibility to control the molecular structure and accordingly the desirable functionality.     

Influence of molecular geometry on the adsorption orientation for oligophenylene-ethynylenes on Au(111)

The adsorption structures formed from a class of oligophenylene-ethynylenes on Au(111) under ultrahigh vacuum conditions is compared based on high-resolution scanning tunneling microscopy (STM) measurements. The molecules consist of three or four benzene rings connected by ethynylene spokes and are all functionalized identically with an aldehyde, a hydroxyl, and a bulky tert-butyl group. Compounds with the conjugated spokes placed in the para, meta, and threefold configurations were previously found to exclusively form molecular layers with flat-lying adsorption geometries. In contrast, the associated compound with spokes in the ortho configuration surprisingly differs in its adsorption by forming only structures with an upright adsorption orientation. The packing density for the structures formed by the compound with the ortho configuration is less dense than that in conventional self-assembled monolayers while still keeping the conducting backbone in an upright orientation. These structures are thus interesting from the perspective of performing single-molecule conduction measurements on the oligophenylene-ethynylene backbones.     

In situ STM imaging of polyethylene glycol adsorbed on an Au(111) electrode in pH 3

In situ scanning tunneling microscopy (STM) is used to examine the electrified interface of Au(111) immersed in a pH 3 sulfate medium containing polyethylene glycol (PEG) with an average molecular weight of 6000. The cyclic voltammograms thus obtained show two sharp peaks at − 0.35 and − 0.38 V (vs. reversible hydrogen electrode), which correlates with the STM observation of a highly ordered (2 × 2 √ 3)rect structure and the adsorption of PEGs. X-ray photoelectron spectroscopy is used to examine the film formed at − 0.4 V, revealing prominent C–C and C–O–C structures, thereby supporting the view of adsorption and reduction PEGs on the Au(111) electrode. STM imaging at the initial stage of PEG's adsorption reveals winding linear segments 0.6 nm wide and 20–40 nm long, implying helical conformations of PEGs. The PEG film dissolves and yields a high density of nanoclusters, as the potential is switched stepwise from − 0.4 to 0.9 V.     

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.     

Decorated Ru/Au(111) and Os/Au(111) surfaces: An in situ STM and electrochemistry study

The structure and reactivity of bimetallic electrodes obtained by spontaneous deposition of Ru and Os on Au(111) single-crystal surfaces are studied. In situ electrochemical STM and cyclic voltammetry are used to characterize a wide range of surface morphologies thus produced. The STM results on Ru/Au(111) demonstrate a pronounced step decoration, while a random distribution of Ru nuclei, quite uniform in size, occurs on terraces. Osmium deposits show a slight preference for deposition on steps, but it also occurs readily on terraces. However, many of the Os islands grow into multilayer heights. The coverage of the Au(111) by the deposited Ru or Os islands for a particular solution concentration depends on the deposition time. Nanostructures of Ru and Os are tested for catalytic behavior and correlated to CO oxidation activity as measured by CO stripping voltammetry. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1385–1392. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Characterization of self-assembled monolayers of thiols on Au(111)

Self-assembled monolayers (SAMs) of n-butanethiol, n-dodecanethiol and their equimolar mixture on Au(111) were prepared and characterized by ellipsometry, contact angle measurement, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Results revealed that these SAMs are oriented ultrathin films with the thickness of nanometer scale, and the SAMs were influenced by the molecular chain length, the lattice orientation and cleanliness of the substrates. The surface of the longer chain SAM is hydrophobic. The thicknesses of three SAMs of n-butanethiol, n-dodecanethiol and their mixture revealed by ellipsometry and XPS are about 0.59 - 0.67nm, 1.60- 1.69 nm and 1.23 - 1.32nm, respectively. AFM images further demonstrated that the SAM formed by the mixture has some microdomains with two different thicknesses.     

In situ STM study of potential-driven transitions in the film of a cationic surfactant adsorbed on a Au(111) electrode surface

Electrochemical scanning tunneling microscopy (EC-STM) has been employed to study the structure of a film formed by cationic surfactant N-decyl-N,N,N-trimethylammonium triflate (DeTATf) adsorbed on the Au(111) electrode surface. The film is disordered at potentials corresponding to either large negative charge densities or to positive charge densities. At small negative charge densities, an ordered adlayer of flat-lying DeTATf molecules is formed. High-resolution images of this adlayer reveal that the triflate anion is coadsorbed with the N-decyl-N,N,N-trimethylammonium cation, effectively forming an ion pair at the electrode surface. This is a significant result because it explains why this surfactant behaves like a zwitterionic surfactant at the metal/solution interface.     

STM vizualization of thiol-containing peptide dendrimers on Au(111)

Peptide dendrimers assembled by solid-phase peptide synthesis using a branching diamino acid at every 2(nd) or 3(rd) position provide readily accessible synthetic model systems for proteins and enzymes. They adopt a globular shape by topology rather than by folding. Peptide dendrimers of 2(nd) and 3(rd) generation functionalized with a cysteine or cystine residue in the core were adsorbed on Au(111) surface and imaged by STM at air, under UHV, or in solution. The dendrimers appear as globular features with dimensions suggesting an extended flattened conformation, forming both single globules and ordered arrays on the surface. These images represent the first direct visualization of peptide dendrimer enzyme models.     

In situ STM study of potential-dependent height change of a tetrathiafulvalene derivative embedded in alkanethiol self-assembled monolayers on Au(111)

Electroactive tetrathiafulvalene thiol islands embedded in an n-alkanethiol SAM matrix were studied under potential control using in situ scanning tunneling microscopy. Unlike previously studied stochastic switching, the apparent height of the island could be intentionally controlled in the present system by choosing the appropriate potential and the island size. The dependence of the height change on the potential and the size is explained as structural change of the island induced by the charging effect of the electroactive moiety.     

In situ STM study of Au(111)/Os bimetallic surfaces: spontaneous deposition and electrochemical dissolution

We provide an electrochemical and structural characterization by in situ STM of Au(111)/Os electrodes prepared by spontaneous deposition of Os on Au(111). Surfaces with Os coverage values up to the saturation coverage were examined, from 10%. Using comparisons to previous work on Au(111)/Ru, Pt(111)/Ru, and Pt(111)/Os, we find that we may now generalize that Os deposits spontaneously faster than Ru and has a greater tendency to form 3-D structures. Additionally, the Au(111) substrate shows preferential step and near-step decoration in both cases, although it is less pronounced for Os than Ru. We also investigated the incremental dissolution of the Os from Au(111), to better understand electrochemical dissolution processes in general and to better control the Os deposit structure. The application of controlled electrochemical treatments (cyclic voltammetry up to increasingly positive values) significantly increased the dispersion of the Os deposit by generating smaller, more widely spaced islands. Upon voltammetry up to 0.75 V, the Au(111)/Os surface showed evidence of alloying and the formation of 3-D structures suggestive of strong Os-Os (oxidized) species interactions. The CO stripping results show the Au(111)/Os is not particularly effective for this reaction, but such results help to complete the overall picture of NM-NM catalytic combinations. Although the Au(111)/Os system itself is not catalytically active, the electrochemical manipulation of the deposit structure demonstrated here may be applied to other noble metal/noble metal (NM/NM) catalytic substrates to find optimal deposit morphologies.     

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.     

Methanethiolate adsorption site on Au(111): a combined STM/DFT study at the single-molecule level

The chemisorptive bonding of methanethiolate (CH(3)S) on the Au(111) surface has been investigated at a single-molecule level using low-temperature scanning tunneling microscopy (LT-STM) and density functional theory (DFT). The CH(3)S species were produced by STM-tip-induced dissociation of methanethiol (CH(3)SH) or dimethyl disulfide (CH(3)SSCH(3)) at 5 K. The adsorption site of an isolated CH(3)S species was assigned by comparing the experimental and calculated STM images. We conclude that the S-headgroup of chemisorbed CH(3)S adsorbs on the 2-fold coordinated bridge site between two Au atoms, consistent with theoretical predictions for CH(3)S on the nondefective Au(111) surface. Our assignment is also supported by the freezing of the tip-induced rotational dynamics of a single CH(3)SH molecule upon conversion to CH(3)S via deprotonation.     

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…     

Dispersion and STM Characterization of Au-CdSe Nanohybrids on Au(111)

We report the dispersion and scanning tunneling microscopy (STM) characterization of iso-lated Au-CdSe nanohybrids on atomically flat Au(111) through surface modifications. The top terminal groups of spacer molecules self-assembled on the surface are found critical for locking the nanohybrids into a well-separated state. The STM results indicate that both thiol and carboxylic terminals are effective in this aspect by making strong interaction with the Au portions of the nanohybrids. An argon ion sputtering technique is also proposed to clean up organic contaminants on the surface for improved STM imaging of individual Au-CdSe nanohybrids. These observations help to enrich technical approaches to dispersing individual nanostructures on the surface and provide opportunities to explore their local electroluminescent and energy transfer properties at the nanoscale.     

Stress in self-assembled monolayers: omega-biphenyl alkane thiols on Au(111)

Self-assembled monolayers of omega-(4'-methylbiphenyl-4-yl) alkane thiols CH3(C6H4)2(CH2)(n)SH (BPn, n = 2, 3, and 5) on Au(111) substrates, prepared at room and elevated temperatures, were studied using scanning tunneling microscopy. In contrast to the biphenyl thiol analogues with n = 0 or 1, ordered domains of large size are formed which exhibit small, periodic height variations on a length scale of several nanometers. These are attributed to solitons (or domain walls), resulting from structural mismatch between the molecular adlayer and the gold substrate. The implications of these results for the design of aromatic thiols to cope with stress and yield low-defect density self-assembled monolayers are discussed.     

In situ STM investigation of spinodal decomposition and surface alloying during underpotential deposition of Cd on Au(111) from an ionic liquid

The electrodeposition and anodic dissolution of Cd on Au(111) in an acidic chloroaluminate ionic liquid (MBIC-AlCl(3), 42 : 58 mol%) have been investigated by cyclic voltammetry and in situ STM. In the Cd underpotential deposition region, various nanostructures can be distinguished. At a potential of 0.95 V vs. Al/Al(iii), a transformation from a well ordered AlCl(4)(-) adlayer to a ( radical3 x radical19) superstructure, presumably due to Cd-AlCl(4)(-) coadsorption, is observed. Reducing the potential to 0.45 V, surface alloying of Cd and Au occurs, which is evidenced for the first time by typical spinodal structures occurring both during deposition and dissolution of the surface alloy layer having a hexagonal structure. At still lower potentials below 0.21 V, a layer-by-layer growth of bulk Cd sets in.     

Schiff碱N-aete-N在Au(111)上自组装单分子膜的电化学及STM研究

利用电化学技术及扫描隧道显微镜(STM),于0.1mol/LHClO₄溶液中研究了Schiff碱N-aete-N在单晶Au(111)面上所形成的自组装单分子膜(SAMs)的电化学性质及结构.N-aete-N在Au(111)电极表面的吸附抑制了金的阳极氧化,同时使固/液界面双层电容明显降低.观察到N-aete-NSAMs的高分辨STM图像.N-aete-N分子在Au(111)表面上以(6×7)结构单胞呈二维有序排列,其表面浓度为5.5×10^-11mol/cm².     

Methylthiolate on Au(111): adsorption and desorption kinetics

Low energy electron diffraction, Auger electron spectroscopy, X-ray photoelectron spectroscopy and line of sight mass spectrometry have been used to study the adsorption and desorption of dimethyldisulfide (DMDS) on Au(111). At 300 K adsorption is dissociative, forming a chemisorbed adlayer of methylthiolate with a 1/3 ML, (sq rt 3 x sq rt 3)R30 degrees, structure. At 100 K adsorption is molecular, with dissociation to form the 1/3 ML (sq rt 3 x sq rt 3)R30 degrees methylthiolate structure occurring at 138-160 K. A physisorbed DMDS layer, with a coverage of 1/6 ML of DMDS, forms on top of the (sq rt 3 x sq rt 3)R30 degrees chemisorbed MT surface for T < or = 180 K, with multilayers forming for T < or = 150 K. In temperature programmed desorption, multilayers of DMDS desorbed with zero order kinetics and an activation energy of 41 kJ mol(-1); the physisorbed layer desorbed with first order kinetics, exhibiting repulsive lateral interactions with an activation energy which varied from 63 kJ mol(-1) (theta = 0) to 51 kJ mol(-1) (theta = 1); the chemisorbed methylthiolate layer desorbed associatively as DMDS via the physisorbed layer, the activation energy for the reaction, 2 methylthiolate --> physisorbed DMDS, exhibiting repulsive lateral interactions with an activation energy which varied from 65 kJ mol(-1) (theta = 0) to 61 kJ mol(-1) (theta = 1). The physisorbed disulfide layer explains the pre-cursor state adsorption kinetics observed in sticking probability measurement, while its relatively facile formation provides a mechanism by which thiolate self-assembled monolayers can become mobile at room temperature.     

Decoupling surface reconstruction and perchlorate adsorption on Au(111)

On Au(111) electrodes, the investigation of ClO₄⁻ adsorption is hampered by a simultaneous surface reconstruction. We demonstrate that these two processes can be decoupled in cyclic voltammograms by a proper choice of the scan rate and of the initial potential. Our approach allowed the establishment of a relation between potentials of zero charge for the reconstructed and unreconstructed Au(111) surfaces.     

In situ friction study of Ag Underpotential deposition (UPD) on Au(111) in aqueous electrolyte

The electrodeposition of silver on Au(111) was investigated using lateral force microscopy (LFM) in Ag⁺ containing sulfuric acid. Friction force images show that adsorbed sulfate forms structure ( on Au(111) prior to Ag underpotential deposition (UPD) and structure ( ) on a complete monolayer or bilayer of Ag. Variation of friction with normal load shows a non-monotonous dependence, which is caused by increasing penetration of the tip into the sulfate adlayer. In addition, the friction force is influenced by the varying coverage and mobility of Ag atoms on the surface. Before Ag coverage reaches the critical value, the deposited silver atoms may be mobile enough to be dragged by the movement of AFM tip. Possible penetration of the tip into the UPD layer at very high loads is discussed as a model for self-healing wear. However, when the coverage of Ag is close to 1, the deposited Ag atoms are tight enough to resist the influence of the AFM tip and the tip penetrates only into the sulfate adlayer.