Monolayers Of Aromatic Thiols Research Articles

Friction of aromatic thiol monolayers on silver: SFA and AFM studies of adhesive and non-adhesive contacts

(a) Friction coefficients and (b) critical shear stresses of thiol monolayers on silver, measured with SFA (○) and AFM (red circles).

Enhancement of the Catalytic Activity of Fe Phthalocyanine for the Reduction of O2 Anchored to Au(111) via Conjugated Self-Assembled Monolayers of Aromatic Thiols As Compared to Cu Phthalocyanine

We have prepared self-assembled monolayers (SAMs) of 4-aminothiophenol (4-ATP) and 1-(4-mercaptophenyl)-2,6-diphenyl-4-(4-pyridyl)pyridinium tetrafluoroborate (MDPP) functionalized with iron phthalocyanine (FePc) and copper phthalocyanine (CuPc) adsorbed on gold (111) electrodes. The catalytic activity of these SAMs/MPc was examined for the reduction of O2 in aqueous solutions and compared to that of bare gold and with gold coated directly with preadsorbed MPc molecules. Scanning tunneling microscopy (STM) studies confirm the functionalization of the 4-ATP by MPc. STM images reveal that iron phthalocyanine molecules are chemically anchored to 4-aminothiophenol organic monolayers, probably having an “umbrella” type orientation with regards to the surface. The electrocatalytic studies carried out with Au/4-ATP/FePc and Au/MDPP/FePc electrodes show that the O2 reduction takes place by the transfer of 4-electron to give water in contrast to a 2-electron transfer process observed for the bare gold. The modifie...

Enhanced Protective Properties and Structural Order of Self-Assembled Monolayers of Aromatic Thiols on Copper in Contact with Acidic Aqueous Solution

In the present work we used different techniques to study self-assembled monolayers (SAMs) of two aromatic thiols, namely, benzenethiol (BT) and 2-naphthalenethiol (2-NT), and one alkylic thiol, 1-undecanethiol (1-UT), on polycrystalline copper, comparing their corrosion inhibition efficiency and their stability up to a week in H2SO4 0.5 M. Both electrochemical impedance spectroscopy (EIS) and linear polarization highlighted different aging trends for 1-UT on one side and aromatic thiols on the other. 1-UT was initially the best corrosion inhibitor among the three thiols, as expected from its larger thickness, but it degraded very rapidly. On the contrary, BT and 2-NT showed a noticeable increase of their protective properties during the first hours of exposure to the electrolyte leading to a superior performance over any 1-UT sample. Raman spectroscopy suggested this behavior to be related to an enhancement of the structural order of the aromatic layer. In addition, both XP spectroscopy and electrochemical measurements revealed BT and 2-NT layers to be stabler than 1-UT. In particular, BT layers exposed to H2SO4 0.5 M exhibited better protective properties, with respect to the freshly prepared samples, lasting for over 1 week of aging.

Enhanced catalytic activity of Fe phthalocyanines linked to Au(111) via conjugated self-assembled monolayers of aromatic thiols for O 2 reduction

We have obtained and characterized self assembled monolayers (SAMs) of 4-aminothiophenol (4-ATP) and 1-(4-mercaptophenyl)-2,6-diphenyl-4-(4-pyridyl)pyridinium tetrafluoroborate (MDPP) functionalized with iron phthalocyanine (FePc) adsorbed on gold (111) electrodes. The catalytic activity of these SAMs/FePc was examined for the reduction of O 2 in aqueous media (pH = 4) and compared with that of bare gold and of gold coated directly with FePc molecules. Scanning tunneling microscopy (STM) studies confirm the functionalization of the 4-ATP by FePc. The electrocatalytic studies carried out with Au/FePc, Au/4-ATP/FePc and Au/MDPP/FePc electrodes show that the O 2 reduction takes place by a 4-electron transfer to give water in contrast to a 2-electron-transfer process observed on the bare gold. The activity of the electrodes increases as follows: Au < Au/FePc < Au/4-ATP/FePc < Au/MDPP/FePc.

Copper protection by self-assembled monolayers of aromatic thiols in alkaline solutions

Copper corrosion in alkaline solutions is inhibited by the formation of self-assembled monolayers of aromatic thiols, made of either benzenethiol or 2-naphthalenethiol or 4-acetamidothiophenol. Electrochemical experiments, based on voltammetry and impedance spectroscopy, point out the much lower reactivity of copper surfaces towards oxidation, when covered by compact adlayers of the above molecules bonded through the S atom. The peculiar shape and peak position in the voltammetric reduction of residual oxides grown on modified metal surfaces suggest that they are due to Cu(I) suboxides, probably grown on reactive metal defects. XPS experiments have confirmed that the aromatic adlayers are still covering most of the Cu surface even after 1 h immersion in 0.5 M NaOH. The main changes in Auger and XP spectra indicate the formation of much less Cu(2)O in the protected samples than in the corresponding bare Cu aged in NaOH. From the experimental data the presence of defective copper oxides on modified Cu has been deduced.

Synchrotron PES and NEXAFS studies of self-assembled aromatic thiol monolayers on Au(1 1 1)

Self-assembled monolayers (SAMs) on various metal, semiconductor or insulator substrates can be easily modified with specific functional groups of interest and have promising applications in surface wetting (hydrophobic/hydrophilic modification), tribology, corrosion protection, sensor electrodes modification, molecular and biomolecular recognition, protein adsorption, cell adhesion, and molecular- or organic-electronic device fabrications. In this paper, we highlight recent progress in the development of SAMs on solid substrates as well as their practical applications, with particular emphasis on the characterization of self-assembled aromatic thiol monolayers with different functional groups on Au(1 1 1) using synchrotron-based photoemission spectroscopy and near-edge X-ray absorption fine structure measurements. The SAM-related molecular orientation, electronic structures, and chemical bonding are presented. Using copper(II) phthalocyanine as a model system, we address the applications of aromatic thiol SAMs in organic electronics devices, demonstrating the effective reduction of the hole injection barrier and manipulation of the molecular orientation of active organic thin films.

Tuning of Ag work functions by self-assembled monolayers of aromatic thiols for an efficient hole injection for solution processed triisopropylsilylethynyl pentacene organic thin film transistors

We demonstrate tuning of hole injection barriers in bottom contact triisopropylsilylethynyl pentacene (TIPS-pentacene) organic thin film transistors (OTFTs) by forming the self-assembled monolayers (SAMs) of thiophenol, 4-fluorothiophenol, or pentafluorothiophenol on the pristine Ag electrode. The work functions of SAM-treated Ag electrodes are measured by Kelvin probe method. The TIPS-pentacene OTFT devices were fabricated by a drop-cast method with a micropipette like an inkjet printing. The OTFTs with pentafluorothiophenol-Ag electrodes as source and drain exhibit carrier mobility of 0.17cm2∕Vs and on/off current ratio of 105 because of almost no hole injection barrier to TIPS pentacenes. The SAM-treated Ag electrodes are robust over repeated electrical scans of 100cycles.

Self-Assembled Monolayers of Aromatic Thiols Stabilized by Parallel-Displaced π−π Stacking Interactions

Parallel-displaced pi-pi stacking interactions have been known to be the dominant force in stabilizing the double helical structure of DNA and the tertiary structure of proteins. However, little is known about their roles in self-assembled monolayers of other large pi molecules such as aromatic thiols. Here we report on a systematic study of the self-assembled monolayers of four kinds of anthracene-based thiols, 9-mercaptoanthracene (MA), (4-mercaptophenyl) (9-anthryl) acetylene (MPAA), (4-mercaptophenyl) (10-nitro-9-anthryl) acetylene (MPNAA), and (4-mercaptophenyl) (10-carboxyl-9-anthryl) acetylene (MPCAA) on Au(111), in which a spacer and different functional groups (NO2 and COOH) are intentionally designed to introduce and thus allow the investigation of various intermolecular interactions, in addition to pi-pi interactions in the base molecules. We find that all molecules form long-range-ordered monolayers and, more interestingly, that these assembled monolayers exhibit essentially the same fundamental packing structure. On the basis of high-resolution scanning tunneling microscopy observations, we propose the space-filling models for the observed superstructures and demonstrate that all superstructures can be understood in terms of the parallel-displaced pi-pi stacking interactions, despite the presence of competing dipole-dipole and H-bonding interactions associated with these specially designed functional groups.

ToF-SIMS investigation of functional mixed aromatic thiol monolayers on gold

Self assembled mixed component aromatic layers on gold, prepared from solutions of 4′-(4-mercaptophenyl)-2,2′:6′,2″terpyridine and mercaptobenzene, have been investigated by means of ToF-SIMS. For such a system, static SIMS provides quantitative information in excellent agreement with XPS data. The mixed layer is formed with a quasi-constant composition, almost independent of the ratio of the constituents in the starting solution. This indicates the existence of a particularly stable structure. The intensity behaviour of clusters ions in static SIMS spectra suggests that the surface layer is a homogeneous mixture rather than phase separated. Multivariate analysis of SIMS spectra allows to discriminate samples with very similar surface composition, but prepared from different starting solutions, providing a fine control of surface composition.

ToF-SIMS investigation of functional mixed aromatic thiol monolayers on gold

ToF-SIMS investigation of functional mixed aromatic thiol monolayers on gold

Boundary Friction of Aromatic Self-Assembled Monolayers: Comparison of Systems with One or Both Sliding Surfaces Covered with a Thiol Monolayer

The boundary friction of three aromatic thiol monolayers with different packing densities on gold was studied with friction force microscopy. The friction of each monolayer was measured against unfunctionalized silicon tips and thiol-functionalized gold-covered tips. The experiments were done in ethanol to significantly reduce the tip−substrate adhesion and therefore the dependence of the friction force on contact area. This allowed a direct, quantitative comparison of load- and velocity-dependent friction with tips of different radii. In close-packed systems, the friction force was lower when both surfaces were covered with a monolayer, and plateaus in the friction force vs velocity curves appeared at higher velocity, suggesting a more fluidlike sliding. A transition in the monolayers at high loads was found to be dependent on the tip radius and appeared at higher average pressure for more close-packed systems but did not depend on whether the system contained one or two monolayers.

Scanning tunneling spectroscopy on π-conjugated phenyl-based oligomers: A simple physical model

The current-voltage (I-V) characteristics obtained by scanning tunneling spectroscopy (STS) on uncoated gold substrates and gold substrates coated with self-assembled monolayers of aromatic thiols are shown to be in good quantitative agreement with the Landauer-B\"uttiker approach to the heterojunction conductance. The self-assembled monolayers consist of one and three chained phenyl rings, forming an azomethine oligomer; both monomer and trimer are chemisorbed on gold via sulphur while terminated from the opposite side by an amine head group. It is found that if the electrostatic energy of the applied potential is ramped within the HOMO-LUMO energy gap (highest occupied molecular orbital-lowest unoccupied molecular orbital) the particular form of the apparent I-V relations reflect an interplay of (i) the WKB factor that accounts for the through-air tunneling between the STM tip and the coated/uncoated substrate, (ii) resonancelike structure of the tip-apex local density of states (LDOS), and (iii) the relative position and intensity of the HOMO and LUMO level peaks in the molecular LDOS. The latter two factors give rise to asymmetry of I-V characteristics. The tunneling spectroscopy measurements and their theoretical evaluation indicate that the STM tip probes a molecular carbon atom rather than the uppermost nitrogen, possibly due to plowing through the molecules. The findings of the work are supported by a close similarity of measured and calculated $I(V)$ curves, which is obtained on different but genetically related samples, at different setpoints of the tunnel current, and with essentially one and the same set of parameters (inferred from experiments and semi-empirical self-consistent modeling of the molecular electronic structure).

Structure, reactivity and microrheology in self-assembled monolayers

Self-assembled monolayers of thiols on gold surfaces provide convenient systems for modeling adsorption and adhesion. A transverse shear mode (TSM) device was used to determine how octadecanethiol, octadecylamine and tetradecanol mechanically couple to gold. An order—disorder transition was detected for the first time in the chemisorbed thiol monolayer and thin film. No such transition is observed for the physisorbed amine or alcohol films, although melt transitions are observed for both chemisorbed and physisorbed films. These results illustrate both an important property of self-assembling systems, and also the utility of the TSM as a probe of adhesive interactions, interfacial shear and viscosity of very thin films. Surface-enhanced Raman spectra of thiol and disulfide monolayers show definitively that aromatic thiols adsorb on gold with cleavage of the SH bond, and that the SS bond in symmetric aromatic disulfides also cleaves upon adsorption. Aromatic thiol monolayers are apparently more stable than their alkyl counterparts towards oxidation. The first deliberate preparations of oxidized thiol monolayers (sulfinates, sulfonates) are described. The relative adsorptivities of thiolates, sulfinates and sulfonates are explained by frontier orbital theory. Charge transfer interactions are shown to be more important that Coulombic attractions in determining the relative adsorptivities, and ultimately the stability of the monolayers. These experimental and theoretical results contribute to our fundamental understanding of adhesive interactions, and provide a rational basis for designing new adsorbates and adhesives with prescribed surface interactions.