Alternating Current Measurements in Scanning Electrochemical Microscopy,Part 2: Detection of Adsorbates

A scanning electrochemical microscope (SECM) in ac mode is used for the characterisation of the adsorption process during the hydrogen evolution reaction (HER) in sulfuric acid solution. It is shown that this technique allows quantitative analysis of the adsorption process, and measurements of the differential capacitance with the frequency as parameter are obtained. The time constant for relaxation of adsorbed hydrogen (H_ads) is approximately 2 Hz, and analysis of the Nyquist plot allows direct evaluation of the charge involved. In addition, the direct comparison of the usual electrochemical impedance data and ac‐SECM results obtained simultaneously permits characterisation of processes occurring at the surface and in solution.     

拓宽具有原子分辨率的ECSTM研究至多晶电极表面

Electrochemical Scanning Tunneling Microscopy (ECSTM) has been extended to characterizc polycrystalline silver electrode surfaces in iodide solution. Potential-dependent ordered and disordered structures of the silver electrode as well as the iodine adsorption layer have been observed to coexist on polycrystalline silver electrode surfaces, for the first time. A very special column arrangement of the iodine adsorption layer, similar to the so called "missing row" type of structure has been observed. Some columns of the iodine adsorption layer roll over from one place to another along with the time and changing potential. A proposed model has been given to better describe the structure. The highly corrugated and loose surface structure of the polycrystalline surface are responsible for this special phenomenon.     

Hydrophilicity and Microsolvation of an Organic Molecule Resolved on the Sub‐molecular Level by Scanning Tunneling Microscopy

Low‐temperature scanning tunneling microscopy was used to follow the formation of a solvation shell around an adsorbed functionalized azo dye from the attachment of the first water molecule to a fully solvated molecule. Specific functional groups bind initially one water molecule each, which act as anchor points for additional water molecules. Further water attachment occurs in areas close to these functional groups even when the functional groups themselves are already saturated. In contrast, water molecules surround the hydrophobic parts of the molecule only when the two‐dimensional solvation shell closes around them. This study thus traces hydrophilic and hydrophobic properties of an organic molecule down to a sub‐molecular length scale.     

Some Unusual Features of Highly Oriented Pyrolytic Graphite Observed by Electrochemical Scanning Tunneling Microscopy

HOPGisoneofthemostcommonlyusedsubstratesinSTMforitiseasilycleavedtoalargeatondcallyflatareaonthebasalplane.ExceptforO.246nmperiodicityrelatedregularatomicstructure,thedefectsandothervarious5e,t,,,,nativetothebasa1planeofHOPGcancoverasmuchasl-lO%ofthesurface.'Ofthesefeatures,stepsarethe'mostconunonlyobserved.'ThewidespreaduseofthissubstratehasledtothediscoveryofsomecrystalimperfectionsnativetothebasalplaneofHOPG',includingstackingfaults,graphitestrandsandfibers,brokenorflakedIayersandsup…     

Non‐destructive Patterning of Carbon Electrodes by Using the Direct Mode of Scanning Electrochemical Microscopy

Patterning of glassy carbon surfaces grafted with a layer of nitrophenyl moieties was achieved by using the direct mode of scanning electrochemical microscopy (SECM) to locally reduce the nitro groups to hydroxylamine and amino functionalities. SECM and atomic force microscopy (AFM) revealed that potentiostatic pulses applied to the working electrode lead to local destruction of the glassy carbon surface, most likely caused by etchants generated at the positioned SECM tip used as the counter electrode. By applying galvanostatic pulses, and thus, limiting the current during structuring, corrosion of the carbon surface was substantially suppressed. After galvanostatic patterning, unambiguous proof of the formation of the anticipated amino moieties was possible by modulation of the pH value during the feedback mode of SECM imaging. This patterning strategy is suitable for the further bio‐modification of microstructured surfaces. Alkaline phosphatase, as a model enzyme, was locally bound to the modified areas, thus showing that the technique can be used for the development of protein microarrays.     

Reaction Intermediates on TiO2(110) Identified by Time-Lapse Scanning Tunneling Microscopy

The present account describes first time examples of scanning tunneling microscope (STM) visualization of reaction intermediates on a metal oxide surface. The topographic response of reactant-adsorbed TiO₂(110) surfaces to a temperature increment or to a pressure increment was monitored as a sequence of STM images. Acetates thermally decomposing to ketene were resolved in a temperature-jump STM observation. The kinetics of the acetate consumption was determined on the number of acetates resolved in the microscope images and agreed with the macroscopic rate law of ketene production. A pressure-jump study revealed how a chemisorbed carboxylate (RCOO^-) is exchanged by another carboxylate (R'COO^-) supplied from the ambient vapor phase. An impinging R'COOH was immobilized on the top of a RCOO^- and then squeezed itself into the monolayer of the RCOO^-. One of the carboxylates in the squeezed state returned to the vapor phase via the immobilized state.     

Trapping Nanostructures on Surfaces through Weak Interactions

The assembly of imidazole‐functionalized phenanthroline‐strapped zinc porphyrins (ZnPorphen) with alkyl or polyethylene glycol (PEG) side chains was studied in solution and by AFM after casting on highly oriented pyrolytic graphite (HOPG) or mica. The nature of the solvent and its evaporation time influenced the morphology of the objects observed. On HOPG, short rods of about 100 nm were observed after fast evaporation of solutions of the alkyl derivatives in CHCl₃, THF, or pyridine, whereas islands of aligned rows of longer wires were obtained from methylcyclohexane (MCH). Slow evaporation of MCH led to a three‐dimensional assembly. The PEG porphyrin assembled into short wires on HOPG or fibers on mica after slow evaporation of solutions in THF. This study shows the role of surface–molecule interactions in the interfacial assembly of ZnPorphen derivatives and contributes to understanding the parameters that control their noncovalent assembly into molecular wires on a surface.     

Atomic-Scale Insights into Electrode Surface Dynamics by High-Speed Scanning Probe Microscopy

Atomic-scale processes at electrode surfaces in liquid electrolytes are central elemental steps of electrochemical reactions. Detailed insights into the structure of these interfaces can be obtained with in situ scanning tunnelling and atomic force microscopy. By increasing the time resolution of these methods into the millisecond range, highly dynamic processes at electrode surfaces become directly observable. This review gives an overview of in situ studies with video-rate scanning probe microscopy techniques. Firstly, quantitative investigations into the dynamic behaviour of individual adsorbed atoms and molecules are described. These reveal a complex dependence of adsorbate surface diffusion on potential and co-adsorbed species and provide data on adsorbate–adsorbate and adsorbate–substrate interactions in a liquid environment. Secondly, results on collective dynamic phenomena are discussed, such as molecular self-assembly, the dynamics of nanoscale structures, nucleation and growth, and surface restructuring due to phase-formation processes.     

Microstructuring of Polystyrene Surfaces with Nonsolvent Sessile Droplets

Herein, we study the microstructuring of toluene‐vapor‐softened polystyrene surfaces with nonsolvent sessile droplets. Arrays of microvessels are obtained by depositing non‐evaporating droplets of ethylene glycol/water on the original polystyrene surfaces and subsequently exposing them to saturated toluene vapor. The droplets act as a mask on the polymer, thereby impeding the toluene vapor to diffuse and soften the polystyrene surface below them. Alternatively, the formation of microcraters at random positions—with an average depth‐to‐width aspect ratio of 0.5 and a diameter as small as 1.5 μm—is achieved by condensing water droplets on a softened polystyrene surface. The cross‐sections of the microvessels and the contact angle of the sessile water droplets suggest that the structures are formed by the combined action of the Laplace pressure at the bottom of the droplet and the surface tension acting at the three‐phase contact line of the droplets. As a support, the rim height and the depth of the microvessels are fitted with an elastic theory to provide Young’s modulus of the softened polystyrene surface.     

Identifying Terminal Assembly Propensity of Amyloidal Peptides by Scanning Tunneling Microscopy

The abnormal accumulation of beta-amyloids (Aβ) in brain is considered as a key initiating cause for Alzheimer's disease (AD) due to their richness in plaques and self-aggregate propensity. In recent studies, N-terminally extended Aβ peptides (NTE-Aβ) with the N-terminus originating prior to the canonical β-secretase cleavage site were found in humans and suggested to have possible relevance to AD. However, the effects of the extended N-terminus on the amyloidegenic structure and aggregation propensity have not been fully elucidated. Herein, we characterized the assembly structures of Aβ1-42, Aβ(−5)-42, Aβ(−10)–42 and Aβ(−15)-42 with both normal and reversed sequences on highly oriented pyrolytic graphite (HOPG) surfaces with scanning tunneling microscopy (STM). The molecularly resolved surface-mediated peptide assemblies enable identification of amyloidegenic fragments. The observations reveal that the assembly propensity of the C-terminal strand of Aβ1-42 is highly conserved and insensitive to N-terminal extensions. In contrast, different assembly structures of the N-terminal strand of Aβ variants can be observed with possible assignment of varied amyloidegenic fragments in the extended N-termini, which may contribute to the varied aggregation propensities of Aβ42 species.     

Identification of Core Segment of Amyloidal Peptide Mediated by Chaperone Molecules by using Scanning Tunneling Microscopy

We illustrate in this work that pristine assemblies of amyloidal peptides can be obtained by perturbations of reduced scanning bias, and show a broad distribution in peptide length. In contrast, the chaperone‐mediated peptide co‐assembly presents ordered lamellar structures with a homogeneous distribution in length, which could be attributed to the core segment of the peptide. The efforts are beneficial for gaining insight into the aggregation propensity of peptides and inter‐peptide interactions.     

Oligomeric Tectonics: Supramolecular Assembly of Double‐Stranded Oligobisnorbornene through π–π Stacking

Self‐assembly at the molecular level in solutions or on a surface is a subject of current interest. Herein we describe the tailoring of oligobisnorbornene 1 , which represents an innovative concept of a preorganized building block on the tens of nanometer scale. The rodlike 1 has vinyl and styrenyl end groups. Scanning tunneling microscopy (STM) reveals that the oligomers aggregate anisotropically along the long axis and form a one‐dimensional assembly in which, remarkably, no interstitial gap appears between neighboring oligomers. Dynamic light‐scattering (DLS) measurements indicate that the assembly develops in solution. With a shear treatment for dropcast films, a unidirectionally ordered domain with a defect density less than 0.5 % can be prepared. Simulation results by molecular dynamics suggest that there may be multiple interactions such as π–π stacking and dipolar attractions taking place between the termini of the oligomers. To demonstrate the importance of double bonds in the oligomeric backbones and termini towards the tectonic assembly, a hydrogenated analogue was synthesized; π–π interactions are thus less significant and the film morphology is completely different from that of 1 . This work extends the concept of molecular tectonics to preorganized oligomers and opens up a new avenue of nanopatterning toward nanodevices.     

Scanning Probe Microscopy beyond Imaging: A General Tool for Quantitative Analysis

A simple, fast and general approach for quantitative analysis of scanning probe microscopy (SPM) images is reported. As a proof of concept it is used to determine with a high degree of precision the value of observables such as 1) the height, 2) the flowing current and 3) the corresponding surface potential (SP) of flat nanostructures such as gold electrodes, organic semiconductor architectures and graphenic sheets. Despite histogram analysis, or frequency count (Fc), being the most common mathematical tool used to analyse SPM images, the analytical approach is still lacking. By using the mathematical relationship between Fc and the collected data, the proposed method allows quantitative information on observable values close to the noise level to be gained. For instance, the thickness of nanostructures deposited on very rough substrates can be quantified, and this makes it possible to distinguish the contribution of an adsorbed nanostructure from that of the underlying substrate. Being non‐numerical, this versatile analytical approach is a useful and general tool for quantitative analysis of the Fc that enables all signals acquired and recorded by an SPM data array to be studied with high precision.