Competitive Adsorption and Surface Alloying: Underpotential Deposition of Sn on Sulfate-Covered Cu(111)

We report an in situ scanning tunneling microscopy study on Sn underpotential deposition (UPD) on sulfate-covered Cu(111) electrode surfaces, which represents a system of anion adsorption in an extreme situation in terms of anion−anion and anion−substrate interactions. Owing to the strong sulfate adsorption, the UPD is initiated with “parasitical” adsorption of Sn adatoms almost exclusively along the periphery of the sulfate adlayer, as manifested in the appearance of brightened Moiré structure modulation at the terrace edge. A Sn-induced local enhancement of tunneling is offered to explain the brightening of the Moiré structure. The initially deposited Sn adatoms displace the sulfate ions from the edge and push themselves toward the interior region of the terrace, maintaining the brightened Moiré pattern at the forefront sites. Consequently, a region free of sulfate is provided for further Sn deposition. In addition to terrace edge reshaping, surface alloying is confirmed by the course-dependent anodic stripping, in which the Sn-covered region of the terrace (excluding the forefront sites) becomes fragmented. The surface alloying is favored, in view of strain relief, by the expanded topmost layer of the reconstructed Cu(111). The overall deposition process may be complete within a time window of several minutes. The present work reveals that when anion−substrate and anion−anion interactions are sufficiently strong and comparable to adatom−substrate interaction, the formation of metal adlayer is severely restricted and novel features of UPD are displayed

Electrochemical Growth of Three-Dimensional Nanostripe Architecture of Antimony on Cu(100)

We report the electrochemical growth of a well-oriented 3D nanostripe architecture of Sb on Cu(100), the system of which has a large crystallographic misfit but still shows thickness-independent growth features. A coincidence between Sb and Cu(100) occurs with 5 × dSb[110] = 6 × dCu[01̄0] and dSb[11̄0] = 2 × dCu[001]. Two observed factors are credited to the formation of periodically separated 2D arrays of ∼1-nm-wide nanostripes in the nanostructure; they are the high number of coincidences in Sb[11̄0] as well as the discontinuity of coincidence lattices in Sb[110] formed to release the residual misfit. The vertical alignment of the nanostripes is achieved following the layer-by-layer epitaxy of nanostripes, leading to the formation of a thick nanostripe film with regular straw-mat-type stacking

Supramolecular Aggregation of Inorganic Molecules at Au(111) Electrodes under a Strong Ionic Atmosphere

Neutral inorganic molecules are generally weak in surface adsorption and intermolecular interactions. Self-assembly of such types of molecule would provide valuable information about various interactions. At electrochemical interfaces, the relative strength of these interactions may be modified through control of electrode potential and electrolyte, which may lead to the discovery of new structures and new phenomena. However, studies of this nature are as yet lacking. In this work, we consider the covalent-bound semimetal compound molecules, XCl3 (X = Sb, Bi), as model systems of neutral inorganic molecules to investigate their self-assembly at electrochemical interfaces under a high ionic atmosphere. To fulfill such investigations, in situ STM and cyclic voltammetry are employed, and comparative experiments are performed on Au(111) in ionic liquids as well as aqueous solutions with high ionic strength. In the room temperature ionic liquid of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4), potential-dependent partial charge transfer between the Au surface and XCl3 molecules creates a molecule−surface interaction and provides the driving force for adsorption of the molecules. Supramolecular aggregations of adsorbed XCl3 are promoted through chlorine-based short-range intermolecular correlation under crystallographic constraint, while repulsive Coulombic interactions created between the partially charged aggregations facilitate their long-range ordering. For SbCl3 molecules, hexagonally arranged 6- or 7-member clusters are formed at 0.08 to −0.2 V (vs Pt), which assemble into a secondary (√31 × √31)R8.9° structure. For BiCl3 molecules, both the 6-membered hexagonal and 3-membered trigonal clusters are formed in the narrow potential range −0.3 to −0.35 V, and are also arranged into an ordered secondary structure. Comparative studies were performed with SbCl3 in concentrated aqueous solutions containing 2 M HCl to simulate the strong ionic strength of the ionic liquid. Almost identical 6-/7-member clusters and long-range (√31 × √31)R8.9° structure are observed at −0.1 V, demonstrating the crucial role of strong ionic strength in such supramolecular aggregations. However, such supramolecular structures are modified and eventually destroyed as ionic strength is further increased by addition of NaClO4 up to 6 M. The destructive changes of the supramolecular structures are attributed to the alteration of ion distribution in the double layer from cation-rich to anion-rich at increasing NaClO4 concentration. This modifies and eventually breaks the balance of intermolecular and molecule−electrolyte interactions. Finally, the dynamic behavior of the SbCl3 assembly is investigated down to molecular level. It has been demonstrated that the initial stage of assembly follows a two-dimensional nucleation and growth mechanism and has a potential-dependent rate that is closely related to the surface mobility of the SbCl3 clusters. There is a probability that clusters can escape from an existing assembly domain or insert into a vacancy in such a domain while they can also relax with central or ring members in a dynamic fashion. These phenomena indirectly reflect the dynamic properties of cations from electrolytes at the interface. The rich information contained in the self-assembly behavior of SbCl3 and BiCl3 demonstrates that neutral inorganic molecules can be employed for fundamental studies of a variety of interesting issues, especially the interplay of various interfacial interactions

Investigation of DNA Orientation on Gold by EC-STM

The immobilization of thiol-derivatized DNA on a Au (111) single crystal surface by self-assembly has been investigated by electrochemical scanning tunneling microscopy (EC-STM). Continuous potential-dependent orientation changes of double-stranded oligodeoxynucleotides (ODN) have been observed in a certain potential range from 200 to 600 mV (versus SCE). It is suggested that the DNA duplexes stand straight on the gold surface at potentials negative of the potential of zero charge (pzc) and then lay down on the surface when the potential shifts positively. These results are in agreement with the expectation based on the Coulombic interaction consideration between negatively charged DNA helices and gold surface. As the applied potential shifts positively, the surface charge changes from negative to positive, that is, the Coulombic force between negatively charged DNA helices and gold surfaces changes from repulsion to attraction. However, for the single-stranded oligodeoxynucleotides, no distinct changes in the surface structure were observed with the applied potential

Self-Assembly of a Rh(I) Complex on Au(111) Surfaces and Its Electrocatalytic Activity toward the Hydrogen Evolution Reaction

The self-assembly of a Wilkinson type of catalyst molecule, trans-RhCl(CO)(PPh3)2, on Au(111) surfaces and its electrocatalytic properties toward the hydrogen evolution reaction (HER) are investigated by employing scanning tunneling microscopy (STM), cyclic voltammetry (CV), and X-ray photoelectron spectroscopy (XPS). The self-assembled monolayers of RhCl(CO)(PPh3)2 are prepared from either dichloromethane or aqueous solutions, but the ordered structures are observed only in atmospheric conditions after solvents evaporate. In the electrolyte solutions, disordered yet uniformly sized spherical clusters of individual molecules are observed as a result of the conformational change of the molecule by the solvation effect of water. The immobilized Rh(I) molecular clusters are electrochemically stable in a wide potential window and exhibit remarkable electrocatalytic activity toward HER in perchloric acid solutions. Several comparative experiments involving similar types of immobilized complexes containing Ru(I) and Ir(I) centers and solution species of RhCl(CO)(PPh3)2 are performed. However, none of them are found to be electroactive to HER. The Tafel slope of HER on the Rh(I) complex modified Au(111) electrode in 0.1 M HClO4 is determined to be −0.061 V, which is almost in the middle of those on bare Au(111) (−0.093 V) and Rh covered (θRh ≈ 0.3) Au(111) (−0.034 V) electrodes. XPS measurements reveal a valence change of Rh(I) to Rh(0), and an oxidative addition and reductive elimination mechanism is suggested for the enhancement of HER

Resolving Fine Structures of the Electric Double Layer of Electrochemical Interfaces in Ionic Liquids with an AFM Tip Modification Strategy

We report enhanced force detection selectivity based on Coulombic interactions through AFM tip modification for probing fine structures of the electric double layer (EDL) in ionic liquids. When AFM tips anchored with alkylthiol molecular layers having end groups with different charge states (e.g., −CH₃, −COO^–, and −NH₃⁺) are employed, Coulombic interactions between the tip and a specified layering structure are intensified or diminished depending on the polarities of the tip and the layering species. Systematic potential-dependent measurements of force curves with careful inspection of layered features and thickness analysis allows the fine structure of the EDL at the Au(111)–OMIPF₆ interface to be resolved at the subionic level. The enhanced force detection selectivity provides a basis for thoroughly understanding the EDL in ionic liquids

Ionic Liquid Based Approach for Single-Molecule Electronics with Cobalt Contacts

An electrochemical method is presented for fabricating cobalt thin films for single-molecule electrical transport measurements. These films are electroplated in an aqueous electrolyte, but the crucial stages of electrochemical reduction to remove surface oxide and adsorption of alkane­(di)­thiol target molecules under electrochemical control to form self-assembled monolayers which protect the oxide-free cobalt surface are carried out in an ionic liquid. This approach yields monolayers on Co that are of comparable quality to those formed on Au by standard self-assembly protocols, as assessed by electrochemical methods and surface infrared spectroscopy. Using an adapted scanning tunneling microscopy (STM) method, we have determined the single-molecule conductance of cobalt/1,8-octanedithiol/cobalt junctions by employing a monolayer on cobalt and a cobalt STM tip in an ionic liquid environment and have compared the results with those of experiments using gold electrodes as a control. These cobalt substrates could therefore have future application in organic spintronic devices such as magnetic tunnel junctions

Self-Assembly of a Rh(I) Complex on Au(111) Surfaces and Its Electrocatalytic Activity toward the Hydrogen Evolution Reaction

The self-assembly of a Wilkinson type of catalyst molecule, trans-RhCl(CO)(PPh3)2, on Au(111) surfaces and its electrocatalytic properties toward the hydrogen evolution reaction (HER) are investigated by employing scanning tunneling microscopy (STM), cyclic voltammetry (CV), and X-ray photoelectron spectroscopy (XPS). The self-assembled monolayers of RhCl(CO)(PPh3)2 are prepared from either dichloromethane or aqueous solutions, but the ordered structures are observed only in atmospheric conditions after solvents evaporate. In the electrolyte solutions, disordered yet uniformly sized spherical clusters of individual molecules are observed as a result of the conformational change of the molecule by the solvation effect of water. The immobilized Rh(I) molecular clusters are electrochemically stable in a wide potential window and exhibit remarkable electrocatalytic activity toward HER in perchloric acid solutions. Several comparative experiments involving similar types of immobilized complexes containing Ru(I) and Ir(I) centers and solution species of RhCl(CO)(PPh3)2 are performed. However, none of them are found to be electroactive to HER. The Tafel slope of HER on the Rh(I) complex modified Au(111) electrode in 0.1 M HClO4 is determined to be −0.061 V, which is almost in the middle of those on bare Au(111) (−0.093 V) and Rh covered (θRh ≈ 0.3) Au(111) (−0.034 V) electrodes. XPS measurements reveal a valence change of Rh(I) to Rh(0), and an oxidative addition and reductive elimination mechanism is suggested for the enhancement of HER

Growth and Shape-Ordering of Iron Nanostructures on Au Single Crystalline Electrodes in an Ionic Liquid: A Paradigm of Magnetostatic Coupling

Fe electrodeposition on Au(111) and Au(100) in BMIBF4 ionic liquid is found to form hitherto unreported shape-ordered nanoscale morphologies of pseudorods and pseudosquare rings, respectively, both composed of grains of 4−7 nm. The manner of growth of the square rings is a ring-on-ring structure with enlarging side length and slightly protruding four corners. The generality of the growth mechanism is verified by the formation of almost exactly the same shape-ordered Fe nanostructures on Pt, i.e., pseudorod structure on Pt(111) and pseudosquare rings Pt(100). These structures are explained within the framework of magnetostatic interactions of spontaneously magnetized grains under crystallographic constraint of the substrate surface, which result in an antiparallel arrangement in magnetization of the grains at pseudorods and magnetic flux closure at the pseudosquare rings. The closed magnetic flux further leads to magnetic field-enhanced growth at the four corners and the outer peripheries of the pseudosquare rings. The observed shape-ordering of the Fe thin film serves as a paradigm of magnetostatic coupling, in which the roles of ionic liquid as surfactant and magnetic media may not be underestimated. The present work adds a new dimension to electrodeposition in ionic liquid, by which new magnetic film structures may be expected

Single-Molecule Force Spectroscopic Studies on Intra- and Intermolecular Interactions of G‑Quadruplex Aptamer with Target Shp2 Protein

With widespread applications in biosensors, diagnostics, and therapeutics, much investigation has been made in the structure of the G-quadruplexes and mechanism of their interactions with protein targets. However, in view of AFM based single-molecule force spectroscopic (SMFS) studies of G-quadruplex systems, only bimolecular approaches have been employed. In this article, we present an improved dual-labeling approach for surface immobilization of G-quadruplex DNA apatmers for investigation of intramolecular interaction from an integral unimolecular G-quadruplex system. The melting force of HJ24 G-quadruplex aptamer in the presence of K⁺ has been successfully measured. It has been found that dynamic equilibrium exists between unfolding and folding structures of the HJ24 aptamer even in pure water. We also investigated the interactions between the HJ24 aptamer and its target protein (Shp2) under the same solution condition. The HJ24/Shp2 unbinding force in the absence of K⁺, 42.0 pN, is about 50% smaller than that in the presence of K⁺, 61.7 pN. The great reduction in force in the absence of K⁺ suggests that the stability of G-quadruplex secondary structure is important for a stable HJ24/Shp2 binding. The methodology developed and demonstrated in this work is applicable for studying the stability of secondary structures of other unimolecular G-quadruplex aptamers and their interactions with target proteins