Electrochemical Deposition onto Self‐Assembled Monolayers: New Insights into Micro‐ and Nanofabrication

Pattern transfer with high resolution is a frontier topic in the emerging field of nanotechnologies. Electrochemical molding is a possible route for nanopatterning metal, alloys and oxide surfaces with high resolution in a simple and inexpensive way. This method involves electrodeposition onto a conducting master covered by a self‐assembled alkanethiolate monolayer (SAMs). This molecular film enables direct surface–relief pattern transfer from the conducting master to the inner face of the electrodeposit, and also allows an easy release of the electrodeposited film due their excellent anti‐adherent properties. Replicas of the original conductive master can be also obtained by a simple two‐step procedure. SAM quality and stability under electrodeposition conditions combined with the formation of smooth electrodeposits are crucial to obtain high‐quality pattern transfer with sub‐50 nm resolution.     

Exchange of Methyl‐ and Azobenzene‐Terminated Alkanethiols on Polycrystalline Gold Studied by Tip‐Enhanced Raman Mapping

Mixed thiol self‐assembled monolayers (SAMs) presenting methyl and azobenzene head groups were prepared by chemical substitution from the original single‐component n‐decanethiol or [4‐(phenylazo)phenoxy]hexane‐1‐thiol SAMs on polycrystalline gold substrates. Static contact‐angle measurements were carried out to confirm a change in the hydrophobicity of the functionalized surfaces following the exchange reaction. The mixed SAMs presented contact‐angle values between those of the more hydrophobic n‐decanethiol and the more hydrophilic [4‐(phenylazo)phenoxy]hexane‐1‐thiol single‐component SAMs. By means of tip‐enhanced Raman spectroscopy (TERS) mapping experiments, it was possible to highlight that molecular replacement takes place easily and first at grain boundaries: for two different mixed SAM compositions, TERS point‐by‐point maps with <50 nm step sizes showed different spectral signatures in correspondence to the grain boundaries. An example of the substitution extending beyond grain boundaries and affecting flat areas of the gold surface is also shown.     

Attachment of Amine‐ and Maleimide‐Containing Ferrocene Derivatives onto Self‐Assembled Alkanethiol and Alkanedithiol Monolayers: Voltammetric Evaluation of Cross‐Linking Efficiencies and Surface Coverage of Electroactive Groups

Ferrocene derivatives containing primary amines and maleimide groups were attached covalently onto N‐hydrosuccinimidyl (NHS)‐terminated alkanethiol self‐assembled monolayers (SAMs) and SAMs of alkanedithiol. The surface coverage and efficiencies of the two cross‐linking reactions were evaluated with cyclic voltammetry. All the ferrocene derivatives attached onto the alkanethiol or alkanedithiol SAMs exhibit reversible redox waves. The surface coverage of the aminated ferrocene groups was compared to that of N‐hydrosuccinimidyl (NHS)‐terminated alkanethiol SAM. The covalent attachment of β‐ferrocenylethylamine onto a 11,11′‐dithio‐bis(succinimidylundecanoate) SAM yielded an efficiency as high as 63.1%. The cross‐linking efficiency of this reaction was found to increase with the nucleophilicity of the amino groups. SAMs of longer alkyl chains favor the attachment of a greater number of ferrocene derivatives. As for the Michael‐type electrophilic addition between the sulfhydryl groups of the alkanedithiol SAMs and the ferrocenyl maleimide, the cross‐linking efficiencies were found to range from 6.5% to 25.7%, depending on the alkanedithiol chain length. The difference in the efficiencies between the two types of cross‐linking reactions might be partially attributable to the steric hindrance imposed by the SAMs and the relative sizes of the functional groups.     

Robust Self‐Assembled Monolayers of RuII and OsII Polypyridines on Gold Surfaces: Exploring New Potentials

Functional monolayers : Ru^II and Os^II bis‐terpyridine complexes have been attached through a piperazine‐supported dithiocarbamate to a gold substrate (see picture). The robust tether, and the favourable reduction in oxidation potential induced by the electron‐rich piperazine result in self‐assembled monolayers with excellent reversible redox behaviour and exceptional stability.

     

Comparative study of self‐assembly of a range of monofunctional aliphatic molecules on magnetron‐sputtered aluminium

The adsorption of a range of organic molecules from toluene onto the oxidized surface of magnetron‐sputtered aluminium metal is studied using sessile drop water contact angle measurements. Molecules with different head group functionalities and various chain lengths are considered, including alkyl carboxylic acids, alkyl phosphonic acids, alkyl amines, alkyl trimethoxysilanes, alkyl trichlorosilanes and epoxy alkanes. Alkyl phosphonic and carboxylic acids are identified as readily forming the most well‐packed monolayers on the aluminium surface, whereas the others adsorb less well and the chlorosilanes polymerize as a result of combination with moisture to form a thick deposit. The high‐adsorption‐density monolayers of alkyl phosphonic and carboxylic acids were studied using polarization modulation infrared reflection–absorption spectroscopy (PM‐IRRAS) and x‐ray photoelectron spectroscopy (XPS): PM‐IRRAS reveals relatively poorer ordering of the C₁₀ alkyl carboxylic acid monolayer compared with that formed from the phosphonic acid, and XPS data suggest that this is likely to relate to a lower ability to displace preadsorbed volatile organic compounds. Copyright © 2004 John Wiley & Sons, Ltd.     

Self‐Assembled Monolayers of Alkoxy‐Substituted Octadehydrodibenzo[12]annulenes on a Graphite Surface: Attempts at peri‐Benzopolyacene Formation by On‐Surface Polymerization

Self‐assembled monolayers of a series of tetraalkoxy‐substituted octadehydrodibenzo[12]annulene (DBA) derivatives 1 c – g possessing butadiyne linkages were studied at the 1,2,4‐trichlorobenzene (TCB) or 1‐phenyloctane/graphite interface by scanning tunneling microscopy (STM). The purpose of this research is not only to investigate the structural variation of two‐dimensional (2D) monolayers, but also to assess a possibility for peri‐benzopolyacene formation by two‐dimensionally controlled polymerization on a surface. As a result, the formation of three structures, porous, linear, and lamella structures, were observed by changing the alkyl chain length and the solute concentration. The formation of multilayers of the lamella structure was often observed for all compounds. The selection of molecular networks is basically ascribed to intermolecular and molecule–substrate interactions per unit area and network density. The selective appearance of the linear structure of 1 d is attributed to favorable epitaxial registry matching between the substrate lattice and the overlayer lattice. Even though the closest interatomic distance between the diacetylenic units of the DBAs in the lamella structure (≈0.6 nm) is slightly larger compared to the typical distances necessary for topochemical polymerization, the reactivity toward external stimuli (electronic‐pulse irradiation from an STM tip and UV irradiation) was investigated. Unfortunately, no evidence for polymerization of the DBAs on the surface was observed. The present results indicate the necessity for further designing a suitable system for the on‐surface construction of structurally novel conjugated polymers, which are otherwise difficult to prepare.     

Oriented immobilization of a fully active monolayer of histidine-tagged recombinant laccase on modified gold electrodes

The formation of a dense monolayer of histidine-tagged recombinant laccase on gold electrodes by using a short thiol-NTA linker is described, as well as a kinetic analysis of the process by cyclic voltammetry. From a detailed analysis of the catalytic reduction of dioxygen by laccase in the presence of a one-electron redox mediator it can be concluded that the immobilized enzyme remains as active as in homogeneous solution.     

Magnetic and Electrochemical Properties of a TEMPO‐Substituted Disulfide Diradical in Solution,in the Crystal,and on a Surface

A study of the magnetic and electrochemical properties of a TEMPO‐substituted disulfide diradical in three different environments was carried out: in solution, in the crystal, and as a self‐assembled monolayer (SAM) on an Au(111) substrate, and the relationship between them was explored. In solution, this flexible diradical shows a strong spin‐exchange interaction between the two nitroxide functions that depends on the temperature and solvent. Structural, dynamic, and thermodynamic information has been extracted from the EPR spectra of this dinitroxide. The magnetic interactions in the crystal include intra‐ and intermolecular contributions, which have been studied separately and shown to be antiferromagnetic in both cases. Finally, we demonstrate that both the magnetic and electrochemical properties are preserved upon chemisorption of the diradical on a gold surface. The resulting SAM displayed anisotropic magnetic properties, and angle‐resolved EPR spectra of the monocrystal allowed a rough determination of the orientation of the molecules in the SAM.     

Polyvinyl‐Locked versus Free Quaterthiophene: Effect of Spatial Constraints on the Electronic Properties of n‐Hexylquaterthiophene

A soluble, low‐weight fraction of poly(α‐vinyl,ω‐n‐hexyl‐quaterthiophene), PT4Hex, having n‐hexylquaterthiophenes as side‐chain groups, is prepared by free‐radical polymerization of α‐vinyl,ω‐n‐hexyl‐quaterthiophene and the corresponding properties compared to those of free di‐n‐hexylquaterthiophene (T4Hex). Optical analysis (absorption and emission) and X‐ray diffraction data indicate that in the polyvinyl‐locked architecture the quaterthiophene pendants adopt a cofacial arrangement with a mutual distance close enough for π–π orbitals to overlap (～4 Å). As a consequence of the close chain packing, a shift of the reduction potential of about 0.5 V toward less negative values with respect to free T4Hex, is found for PT4Hex films. Due to its enhanced electron affinity, PT4Hex displays an electron‐acceptor behavior when blended with alkylated and silylated quaterthiophenes acting as donors.     

Nanoadhesion on Rigid Methyl‐Terminated Biphenyl Thiol Monolayers: A High‐Rate Dynamic Force Spectroscopy Study

Nanoadhesion on a self‐assembled monolayer of 4‐methyl‐4′‐mercaptobiphenyl is measured using a modified atomic force microscope. The dependence of the adhesion force on the loading rate is analyzed with the Dudko–Hummer–Szabo model, and the kinetic and interaction potential parameters for a single terminal group are extracted. The energy and location of the activation barrier suggest that the adhesion is dominated by van der Waals dispersion forces. The humidity effect on the nanoadhesion is also studied. The results are compared with previously measured values for methyl‐terminated alkane thiols and the influence of the thiol rigidity on the adhesion force is discussed.     

An Electrically Driven and Readable Molecular Monolayer Switch Based on a Solid Electrolyte

The potential application of molecular switches as active elements in information storage has been demonstrated through numerous works. Importantly, such switching capabilities have also been reported for self‐assembled monolayers (SAMs). SAMs of electroactive molecules have recently been exploited as electrochemical switches. Typically, the state of these switches could be read out through their optical and/or magnetic response. These output reading processes are difficult to integrate into devices, and furthermore, there is a need to use liquid environments for switching the redox‐active molecular systems. In this work, both of these challenges were overcome by using an ionic gel as the electrolyte medium, which led to an unprecedented solid‐state device based on a single molecular layer. Moreover, electrochemical impedance has been successfully exploited as the output of the system.     

Electrochemistry as an Attractive and Effective Tool for the Synthesis and Immobilization of Porphyrins on an Electrode Surface

Magnesium(II) 10‐phenyl‐5,15‐p‐ditolylporphyrin is easily and cleanly transformed by electrolysis. A nitro group is first introduced at the free meso position by anodic substitution. Hydrogenation into the amine is then carried out electrocatalytically under ambient conditions with water as a hydrogen supplier. The synthesized porphyrin under the nickel(II) form can be covalently grafted onto a platinum electrode by electrochemical reduction of the diazonium cation, generated in situ by a reaction of the nickel(II) aminoporphyrin with sodium nitrite and trifluoroacetic acid. The electrosynthesized thin film gives an electrochemical response typical of a porphyrin material. Films grown under our conditions have a maximum surface coverage of approximately 5×10^?10 mol cm^?2. The modified electrode exhibits a reproducible electrochemical behavior and a good level of stability over potential cycling and exposition to air.     

Structure of 4‐biphenylthiolate on Au nanoparticle surfaces studied by UV‐Vis absorption spectroscopy,transmission electron microscopy and surface‐enhanced Raman scattering

Structure of 4‐biphenylthiolate on Au nanoparticle surfaces has been studied by UV‐Vis absorption spectroscopy, transmission electron microscopy and surface‐enhanced Raman scattering (SERS). 4‐Biphenylthiolate is found to have a standing geometry on Au from the presence of the benzene ring CH stretching band identified at ～3060 cm^?1. The ν_8a band at 1597 cm^?1 in the ordinary Raman spectrum was found to split clearly into two features at 1599 and 1585 cm^?1. This result suggests that orientation of the phenyl rings in 4‐biphenylthiolate may be quite different and should not lie in the same plane on Au nanoparticle surfaces. On the basis of the electromagnetic enhancement factor, the dihedral angle could be estimated with a reported value of the tilt angle. Copyright © 2004 John Wiley & Sons, Ltd.