Decanethiol Monolayers Research Articles

Molecular-Scale Characterization of the Reaction of Ozone with Decanethiol Monolayers on Au(111)

The chemical reaction of ozone with decanethiol monolayers on Au(111) was characterized using X-ray photoelectron spectroscopy and scanning tunneling microscopy. Our results show that exposure to ozone results in oxidation of the thiol terminus. The reaction initiates at the domain boundary network of the alkanethiol monolayer and propagates into the domains. Above a threshold surface oxygen content, the monolayer converts to a two-dimensional fluid that can subsequently recrystallize to a commensurate monolayer of partially oxidized thiol. Further exposure to ozone results in conversion of the monolayer to a fluid phase and a 10% to 30% expansion of the Au lattice at the Au−thiol interface with concomitant formation of Au islands. Our results demonstrate that crystallographic defects in monolayer films can play an important role in their chemical reactions.

Surface-Confined Metallodendrimers: Isolated Nanosize Molecules

The incorporation of dialkyl sulfide side chains in metallodendrimers is a simple method for their insertion into a monolayer of decanethiol formed by self-assembly on a gold surface. The dendrimer binds through the sulfide group to a defect in the monolayer on the gold surface (see picture). The surface concentration of the isolated dendrimer adsorbate can be regulated by the adsorption time (for example, 55 adsorbates on a surface of 200x200 nm(2) after 20 h).

Solvent Effect on the Structure of the Self-Assembled Monolayer of Alkanethiol

Abstract Structures of self-assembled monolayer (SAM) of decanethiol on Au(111) surface prepared in ethanol, toluene, DMF and heptane solution were examined by scanning tunneling microscopy (STM) and electrochemical reductive desorption measurement in 0.5 M KOH aqueous solution. While no significant difference was detected in the reductive desorption measurement except for the SAM prepared in heptane, the STM observation showed that domain size and pit size were strongly affected by solvent.

The 6-(10-Mercaptodecoxyl)quinoline Self-Assembled Monolayer on Gold: Spectroscopy and Wettability Investigation

A self-assembled monolayer (SAM) with an exposed basic group has been generated by the adsorption of a novel quinoline derivative 6-(10-mercaptodecoxyl)quinoline (MDQ) on a gold surface. The structure and surface properties of the SAMs of MDQ were characterized by ellipsometry, reflection absorption FTIR (RA-IR), and contact angle titration. It was found that the quinoline moiety of MDQ forms an ordered layer on the top of the film and the alkyl chain exhibits a packing density similar to that of a monolayer of decanethiol. The molecular orientation of MDQ in SAMs was evaluated by RA-IR spectroscopy which indicates that the alkyl chain exhibits a tilting angle of 24 ± 4° with respect to the surface normal and a twisting angle of 50 ± 5° around its axis. Contact angle titration in the pH range between 2 and 12 illustrates that the surface of SAM of MDQ is less hydrophilic for acidic solution than for basic solution, which is in contrast with the previous reports on SAMs that basic groups are located at the ends of the molecules.

Growth kinetics of decanethiol monolayers self-assembled on Au(111) by molecular beam deposition: An atomic beam diffraction study

A low-energy atom diffraction study of the self-assembly of decanethiol on Au(111) is presented that includes both the growth kinetics and the adsorption energetics for this model system of soft/hard interfaces. Initial dosing on bare gold very rapidly forms the known low-density, lying-down, striped monolayer with a sticking coefficient close to unity. Further dosing results in an intermediate state that lacks long-range order. After an initial ‘induction time’, the final, dense, standing-up c(4×2) phase grows from this intermediate state with a growth rate that is about 500 times slower than that of the striped phase. Our data are consistent with the X-ray results of Eberhardt et al. (A. Eberhardt, P. Fenter, P. Eisenberger, Surf. Sci., 397 (1998) L285) where the growth rate of the c(4×2) phase was studied and found to depend linearly on the pressure at low pressures and high temperatures, changing to a quadratic dependence at higher pressures and lower temperatures. At still higher pressures and lower crystal temperatures, we find that the growth rate pressure dependence weakens substantially, becoming less than linear. In order to understand this behavior, an investigation of the adsorption energetics of thiols on top of the monolayer surface at different points during the growth is made. These results show that in the low- T, high- P regime, the c(4×2) phase grows under a full cover of molecules physisorbed on top of the (decaying) striped phase. This explains the weakening in the pressure dependence mentioned above since, at the limit of growth under a thick overlayer, the growth rate is expected to become pressure-independent.

Defects in Microcontact-Printed and Solution-Grown Self-Assembled Monolayers

The structure of microcontact-printed self-assembled monolayers is significant for a number of technological applications as well as for a fundamental understanding of the self-assembly process. The structural differences between printed and solution-deposited monolayers of alkanethiol on Au(111) have been examined utilizing grazing incidence X-ray diffraction and atomic force microscopy. Although the local structures are found to be similar, the domain structure and therefore macroscopic packing of the molecules are found to vary. Surprisingly, printed decanethiol monolayers appear to be significantly better ordered translationally than those formed by standard solution techniques.

Electrochemical Stability of Self-Assembled Monolayers on Gold

Most applications of functionalized self-assembled monolayers (SAMs) are based on changes in electrochemical properties. Such applications require electrochemical stability. This study reveals that there is only a limited potential window from −0.8 to +0.4 VMSE where self-assembled monolayers of sulfur-containing adsorbates (thiols and alkyl sulfides) on gold are electrochemically stable. A sensitive parameter describes the quality/stability of the monolayers (viz. the charge-transfer resistance). The substitution pattern of the adsorbates influences the window. Highly ordered monolayers, like that of tetrasulfide-based cavitand (10), are more resistant to extreme potentials. The stability window from −0.9 to +0.5 VMSE for this sulfide-based monolayer is comparable to that for a decanethiol monolayer.

Calixarene Monolayers as Quartz Crystal Microbalance Sensing Elements in Aqueous Solution

We have examined p-tert-butylcalix[4]arenetetrathiolate (BCAT) monolayers for their potential use as molecular recognition elements for in situ aqueous chemical sensors. Spectroscopic and wetting studies of BCAT monolayers on Au{111} reveal that the calixarene molecules exist in monolayers, preferentially oriented with their phenyl rings parallel to the surface normal axis. Using quartz crystal microbalance (QCM) sensors with gold-coated electrodes, the chemical specificity of monolayers and thin films to a variety of aromatic and aliphatic analytes in aqueous solution was examined. The response of BCAT sensors was compared to the responses of p-tert-butylcalix[4]arene (BCA)- and decanethiolate (DT)-coated QCM electrodes. BCAT is very selective for alkylbenzenes, much more so than either its spray-coated thin-film analogue, BCA, or the highly ordered DT monolayer. From these measurements, the factors behind molecular differentiation in each film are explored. Drawing upon these findings, the roles of cavitation and film order in molecular recognition for calixarene films are discussed.

On the structure and evolution of the buried S/Au interface in self-assembled monolayers: X-ray standing wave results

We describe a structural study of the S/Au interface for decanethiol monolayers (C10) on a Au(111) surface using the technique of X-ray standing waves (XSWs). The XSW results for full-coverage monolayers are inconsistent with any model incorporating a single sulfur adsorption site, such as the widely assumed threefold hollow site on the Au(111) surface. Instead, the XSW results reveal two distinct sulfur head group sites, each with a distinct lateral and vertical location with respect to the underlying gold lattice. We discuss structural models that are consistent with these results. We have also studied the evolution of the structure versus coverage with XSW and X-ray photoelectron spectroscopy (XPS) and have determined that the local S/Au interface structure of the “lying down” striped phase at low coverages (0.27 ML, 1 ML=4.62×10 14 molecules cm −2) is indistinguishable from that of the “standing up” c(4×2) phase at saturation (1 ML). Some important implications concerning the structure and growth of these monolayers are discussed.

Adsorption mechanisms, structures, and growth regimes of an archetypal self-assembling system: Decanethiol on Au(111)

We present a study using several techniques of the growth of decanethiol monolayers deposited on single-crystal gold surfaces. Through independent measurements of coverage, energetics, and structure as a function of the growth rate and temperature, we provide a quantitative, in-depth description of the molecular processes by which these aliphatic molecules ``self-assemble'' into highly ordered structures in the absence of a solvent. We find that the multiple-energy scales present in these systems produce distinct adsorption mechanisms, structures, and growth regimes, indicating a complexity that is likely to be a general characteristic of this broad class of self-assembling systems.

Development of environmental scanning electron microscopy electron beam profile imaging with self‐assembled monolayers and secondary ion mass spectroscopy

AbstractA method for demonstrating the scattering of the primary electron beam in the presence of a gas has been developed. A self‐assembled decanethiol monolayer is damaged by primary beam electrons. The damaged portion of the mono‐layer is exchanged with another thiol‐containing molecule by immersion in solution. The resulting film is imaged using a secondary ion mass spectrometer. Three‐dimensional reconstruction of the data yields a representation of scattered electrons in the gaseous environment of the environmental scanning electron microscope.

Influence of Solvent on the Interfacial Structure of Self-Assembled Alkanethiol Monolayers

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) is used to study the structure of octadecanethiol, tetradecanethiol, and decanethiol monolayers in the presence of air, deuterium oxide, and acetonitrile-d3. It is found that the structure of the monolayer is dependent on both the adjacent environment and the initial structural order of the monolayer. In the D2O solutions, spectral changes are similar for the three monolayers and are characteristic of interaction predominantly between the solvent and the terminal portion of the monolayers. This correlates with electrochemical capacitance measurements that indicate little permeation of the aqueous electrolyte in the monolayer, even with the largely disordered decanethiol monolayer. With acetonitrile solutions, the three monolayers have different behaviors. The octadecanethiol monolayer changes little from the ex situ structure, indicating little interaction between the solvent and the monolayer. Tetradecanethiol and decanethiol, however, have a significant increase in the relative intensity of the asymmetric methyl vibration, compared to the ex situ value, characteristic of a restructuring of the monolayer. These spectral data suggest that solvation of the increasingly defective structure of the shorter alkanethiol monolayers occurs and is responsible for the structural behavior, consistent with electrochemical results.

Evidence for Nondestructive Adsorption of Dialkyl Sulfides on Gold

The nondestructive adsorption of long chain dialkyl sulfides on gold is reported. Self-assembled monolayers of decanethiol, didecyl disulfide, and didecyl sulfide were prepared and characterized by XPS, grazing-angle FT-IR, wettability studies, and TOF-SIMS. The first three techniques provide evidence that well-packed monolayers are formed. The positive SIMS spectrum of didecyl sulfide showed fragments that originate from intact dialkyl sulfide attached to gold without C−S bond cleavage. Our results underline the generally accepted nondestructive adsorption process of dialkyl sulfides and contradict the recently reported cleavage of one of the alkyl sulfur bonds of sulfides upon adsorption.

Electrodeposition of Cu Nanoparticles on Decanethiol-Covered Au(111) Surfaces: An in Situ STM Investigation

Copper electrodeposition has been followed by in situ electrochemical STM on Au(111) electrodes covered by complete decanethiol monolayers. It has been found that Cu nanoparticles (2−5 nm) were formed at potentials comprising the underpotential deposition (UPD) region on clean gold. The nanoparticle clusters appear to follow a nucleation and sudden growth process as their maximal size is attained instantaneously on the time scale of the STM imaging process. Nanoparticle heights correspond to one atomic layer of Cu. The distribution density of the Cu deposits reaches a maximal value at potentials within the UPD window, as no new formation of clusters nor growth of already existing clusters is seen at potentials well into the bulk deposition potential region. Bulk deposition of isolated Cu nodules is finally seen at potentials 200 mV negative of the Nernstian potential for Cu reduction, probably resulting from thiol film breakdown. Moreover, nanoparticles remain on the Au surface at potentials as high as 10...

Potential-Dependent Rectified Electron Transfer at a Meldola′s Blue Incorporated Decanethiol-Modified Electrode as a Model of Biological Membrane

Meldola′s blue was immobilized into a self-assembled decanethiol monolayer modified on a gold electrode to provide a biological membrane model for electron transport having a molecular gate. Cyclic voltammograms of ferricyanide at this modified electrode showed only its reduction current at potentials where Meldola′s blue was reduced, but not at the redox potential of ferricyanide itself and no reaction was observed for ferrocyanide, indicating the direction of electron flow was controlled through the functionalized monolayer. Similar electrochemical responses were also observed for both octacyanotungustate and octacyanomolybdate. The cathodic peak currents observed in metal cyanide solutions at the modified electrode decreased in the order of Fe(CN)63− > W(CN)83− > Mo(CN)83− at a given pH. From the analysis of the voltammograms using the microelectrode assembly model, the potential-dependent rectified electron flow was explained in terms of a gate function of Meldola′s blue in the monolayer, and the apparent electron transfer rate constant, k0app, and the apparent diffusion coefficient, Dapp, of metal cyanide ions in the monolayer were also estimated.