N-alkanethiolate Monolayers Research Articles

Enhanced thermal stability and structural ordering in short chain n-alkanethiol monolayers on gold probed by vibrational spectroscopy and EQCM

Monolayers of alkanethiols with varied chain lengths, CH3(CH2)nSH where n=3, 5, and 7, on gold substrates have been prepared by adsorption from (1) neat thiol, (2) millimolar thiol solution in alcohol (conventional method), and (3) potential-controlled adsorption. Reflection absorption infrared spectroscopy (RAIRS) and electrochemical quartz crystal microbalance (EQCM) have been used to characterize the integrity of the monolayers. Methylene and methyl stretching modes along with C-S stretching modes have been used as benchmarks to follow the order-disorder transitions in the monolayer structure, in the temperature range from 25 to 175 degrees C. Monolayers adsorbed from neat thiol show superior quality in terms of stability and structural arrangement. Short chain thiols with n=3, 5, and 7 do show substantial stability. The possibility of multilayer formation is ruled out by EQCM studies comparing the frequency and mass change associated with the monolayer desorption. Self-assembled monolayers (SAMs) adsorbed under potential control behave very similarly to the monolayers adsorbed from neat thiol as far as stability and structural orientation are concerned, irrespective of the chain length. The adsorption from neat thiol gets rid of the solvent-induced defects and arrests the propagation of defects under temperature constraints.

Alkanethiols on Platinum: Multicomponent Self-Assembled Monolayers

We have studied the formation of self-assembled monolayers (SAMs) of n-alkanethiols on platinum thin films using X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIRS), spectroscopic ellipsometry (SE), and contact angle (CA) measurements. Specifically, SAMs of 1-hexanethiol, 1-dodecanethiol, and 1-octadecanethiol were grown on polycrystalline Pt films, and the effects of Pt surface preparation, deposition conditions, and solvent treatments on the initial quality and stability of the monolayer in air were investigated. The SAMs prepared under ambient conditions on piranha-cleaned and UV/ozone-cleaned substrates were compared to monolayers formed on template-stripped Pt in an inert atmosphere. We found that alkanethiols deposited from 1 mM ethanolic solutions on piranha-cleaned Pt formed densely packed monolayers in which alkyl chains were oriented close to the surface normal. Stored in the laboratory ambient, these monolayers were unchanged over about 1 week but were largely oxidized in about 1 month. No evidence was found of molecules being weakly bound within the monolayer or having undergone C-S bond scission; however, three distinct sulfur states were observed for all samples in the XPS of the S 2p region. The lowest- and highest-binding-energy components are assigned to alkylthiolate and partially oxidized alkylthiolate species, respectively. The remaining S 2p component (approximately one-third of the sulfur layer), intermediate in binding energy between the other two components, is attributed to a chemisorbed species with a S binding configuration distinct from the majority alkylthiolate: for example, S bound to Pt bound to O, S with a different Pt coordination number, or S in an adsorbed disulfide.

Molecular Modeling of n-Alkanethiolate Monolayers on Gold: Adlayer Orientation Effects

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Odd−Even Variations in the Wettability of n-Alkanethiolate Monolayers on Gold by Water and Hexadecane: A Molecular Dynamics Simulation Study

Molecular dynamics (MD) simulations were performed to investigate odd-even chain length dependencies in the wetting properties of self-assembled monolayers (SAMs) of n-alkanethiols [CH3(CH2)n-1SH] on gold by water and hexadecane. Experimentally, the contact angle of hexadecane on the SAMs depends on whether n is odd or even, while contact angles for water show no odd-even dependence. Our MD simulations of this system included a microscopic droplet of either 256 water molecules or 60 hexadecane molecules localized on an n-alkanethiolate SAM on gold with either an even or odd chain length. Contact angles calculated for these nanoscopic droplets were consistent with experimentally observed macroscopic trends in wettability, namely, that hexadecane is sensitive to structural differences between odd- and even-chained SAMs while water is not. Structural properties for the SAMs (including features such as chain tilt, chain twist, and terminal methyl group tilt) were calculated during the MD simulations and used to generate IR spectra of these films that compared favorably with experimental spectra. MD simulations of SAMs in contact with slabs of water and hexadecane revealed that the effects of these solvents on the structure of the SAM was restricted to the chain terminus and had no effect on the inner structure of the SAM. The density profiles for water and hexadecane on the SAMs were different in that water displayed a significant depletion in its density at the liquid/SAM interface from its bulk value, while no such depletion occurred for hexadecane. This difference in contact may explain the lack of an odd-even variation in the wetting characteristics of water on these surfaces, because the water molecules are positioned further away from the surface and, therefore, are not sensitive to the structural differences in the average orientations for the terminal methyl groups in odd- and even-chained SAMs. In contrast, the differences in the wetting properties of hexadecane on the odd- and even-chained SAMs may reflect the closer proximity of these molecules to the SAM surface and a resulting greater sensitivity to the differences in the terminal methyl group orientations in the SAMs. SAM-solvent interaction energies were calculated during the MD simulations, yielding interaction energies that differed on the even- and odd-chained surfaces by approximately 10% for hexadecane and negligibly for water, in accord with estimates using experimental wetting results.

Cross-Step Place-Exchange of Oligo(phenylene−ethynylene) Molecules

We have observed nitro-functionalized oligo(phenylene-ethynylene) molecules exhibiting motion up and down Au{111} substrate monatomic step edges within host self-assembled monolayers of n-alkanethiols, independent of previously observed conductance switching. Single molecules have been imaged with scanning tunneling microscopy to place-exchange reversibly between the top and bottom of monatomic substrate step edges.

Photoinduced Electroreduction of Chlorophyllide on Alkanethiol-Coated Mercury

Monolayers of n-alkanethiols of chain length from C12 to C18 were self-assembled on a hanging mercury drop electrode, and a film of chlorophyllide (Chlide) was adsorbed on top of them. The reduction photocurrents following illumination of the Chlide film were measured over the potential range in which the Chlide is electroinactive in the dark, and their action spectra were determined. Plotting the derivative of the photocurrents with respect to the applied potential against potential yields bell-shaped curves that can be fitted to a Gaussian. The potential of the Gaussian maximum was used to determine the reorganization energy lambda for the Chlide electroreduction process. An increase in the thiol chain length causes lambda to decrease regularly and the photocurrent to decay exponentially with the monolayer thickness, with a decay constant beta of about 0.17 A(-1).

Effect of Metal−Metal Interactions on Electron Transfer: an STM Study of One-Dimensional Metal String Complexes

Linear metal string complexes of [MnL4(NCS)2] (Scheme 1, M = NiII, CoII, or CrII; n = 3 or 5; L = dpa- or tpda2- where dpa- is dipyridylamido anion and tpda2- is tripyridyldiamido dianion) are studied for the fundamental understanding of the effect of inter-nuclei interactions on electron transfer (ET). The metal strings are isolated within n-alkanethiol monolayers and the properties of ET through various metal strings are differentiated by scanning tunneling microscopy. Although very similar in physical dimension, their apparent heights against the same alkanethiol background are significantly different. The discrepancy is ascribed to electron localization−delocalization among the metal centers.

Characterization of the Molecular Orientation of Self-Assembled Monolayers of Alkanethiols on Obliquely Deposited Gold Films by Using Infrared−Visible Sum-Frequency Spectroscopy

This paper reports the use of infrared (IR)−visible sum-frequency spectroscopy to characterize the surface of gold films deposited by physical vapor deposition at an oblique angle of incidence and the orientation of the methyl end groups of self-assembled monolayers (SAMs) of n-alkanethiols formed on these gold films. The oblique deposition of the gold breaks the macroscopic azimuthal degeneracy that is characteristic of gold films grown without a preferred direction of deposition. By tuning the frequency of the incident IR light to either the symmetric or the asymmetric methyl stretch resonance and by recording the azimuthal dependence of the sum-frequency signal, the orientation of the methyl groups of the n-alkanethiols on the obliquely deposited gold films were shown to be distributed around a preferred in-plane direction coincident with the direction of the gold deposition. The angles between the normal of the sample and the methyl groups were determined to be 85 ± 2° and 44 ± 10° for the SAMs formed from pentadecanethiol and hexadecanethiol, respectively. In contrast, when using gold films that were not obliquely deposited, no preferred in-plane orientation of the methyl groups could be observed.

Chain-length-dependent change in the structure of self-assembled monolayers of n-alkanethiols on Au(111) probed by broad-bandwidth sum frequency generation spectroscopy

The structure of the self-assembled monolayers (SAMs) of n-alkanethiols [CH3(CH2)nSH, n=3–11, 13–15, 17] on Au(111) has been studied using broad-bandwidth sum frequency generation spectroscopy. Sum-frequency vibrational spectra show three pronounced CH3 vibrational modes for all alkanethiol investigated, indicating that the commonly accepted picture that the alkyl chain for the long-chain alkanethiol SAMs has the all-trans conformation applies even to the short chain SAMs. The chain-length dependence of the ratio of the intensity for the CH3 symmetric vibrational mode to that for the CH3 asymmetric mode clearly shows the odd–even effect due to the difference in the direction of methyl group for SAMs with odd and even n, also supporting that the alkyl chain of SAMs has the all-trans conformation. An analysis of the vibrational intensities with respect to the angle between the main axis of the methyl group and the surface normal reveals that the structure of the alkanethiol SAMs gradually changes with n.

Elaboration of Self-Assembled Monolayers of n-Alkanethiols on Nickel Polycrystalline Substrates: Time, Concentration, and Solvent Effects

Self-assembled monolayers (SAMs) of n-alkanethiols on mechanically polished polycrystalline nickel substrates are presented. The effects of several preparation conditions of the surface upon the metal/sulfur bonds and monolayer properties are evidenced. Of particular importance are the electrochemical reduction of the native oxide, the thiol concentration (10-3 versus 10-1 M), and the nature of the solvent (ethanol, acetonitrile, n-heptane, toluene, and carbon tetrachloride).

Surface-Monolayer-Controlled Molecular Alignment of Short n-Alkane Multilayers

C K-edge near-edge X-ray absorption fine structure studies have been performed on the molecular orientation of short n-alkane (C6H14) multilayers on n-alkanethiolate (C6H13S) monolayers. We have found that the molecular orientation of the n-alkane multilayers is in accordance with that of n-alkanethiolate monolayers on which the multilayers were formed. Monolayers of thiophene also have a significant effect on the alignment of the n-alkane multilayers condensed on the monolayers. The growth manner of the n-alkane films is discussed in terms of the energy balance between surface and interface free energies.

The Role of Buried Hydrogen Bonds in Self-Assembled Mixed Composition Thiols on Au{111}

We have investigated the role of internal functionality in self-assembled monolayers of a family of amide-containing alkanethiol molecules on Au{111} using scanning tunneling microscopy. In addition to van der Waals interactions that are present within n-alkanethiol self-assembled monolayers, hydrogen bonding between adjacent buried amide groups contributes to the stability of the amide-containing molecules on the surface and causes spontaneous phase separation upon coadsorption with an n-alkanethiol. A deposition solution concentration dependence study reveals that this is an observed trend across a range of examined solution compositions. Additionally, hydrogen bonding affects the packing structure of the amide-containing alkanethiol self-assembled monolayers. Although they adopt the same (√3×√3)R30° base lattice as n-alkanethiolate self-assembled monolayers, the amide-containing molecules form superlattice structures that are more linear than n-alkanethiol monolayers due to the hydrogen bonds they form. The internal functionality of monolayers can be used to control their formation and stability.

Coverage-Dependent Adsorption of CH3S and (CH3S)2 on Au(111): a Density Functional Theory Study

The origin of the superlattice present in the commensurate hexagonal structure of self-assembled monolayers of n-alkanethiols on gold and the question of whether the thiols are adsorbed onto the surface as dimers (disulfides) or monomers (thiolates) have been under debate for many years. Looking for a better understanding of the structural properties of these systems, we have performed a theoretical study of the molecular and dissociative adsorption of dimethyl disulfide on Au(111) as a function of coverage (0.25 ≤ ϑ ≤ 1), using gradient-corrected density functional (DFT) calculations with a slab geometry. For the dissociated state, our results indicate that the hcp hollow site is much less favorable than the fcc site. For the latter site, we find that, because of surface gold atom relaxation, the adsorption energy depends strongly on ϑ, changing from ∼18 kcal/mol at ϑ = 0.25 to ∼3 kcal/mol at ϑ = 1. For the bridge site, instead, the adsorption energy is a weak function of ϑ, and for all investigated coverages, this site is by far the most stable. According to our DFT approach, the adsorption of dimethyl disulfide is dissociative with a thermodynamic gain, at ϑ = 1, of ∼13 kcal/mol with respect to the adsorbed molecular state. We also find, however, that the energy of c(4 × 2) structures containing at least two inequivalent CH3S groups per unit cell (with a minimum S−S distance of ∼3.7 Å) is, within the accuracy of our approach, indistinguishable from the pure ( × ) hexagonal structure. Our results suggest that the full solution of this thorny problem will require, also for the shortest chains, an estimate of the energetic contribution of dispersion forces that are not included in the DFT calculations.

Structural effects on the electrochemical and spectroelectrochemical properties of asymmetric viologens on the Au electrode surface

Cyclic voltammetry and FTIR spectroscopy were used to study the structural effects on the electrochemical and spectral properties of the asymmetric alkyl viologen, N-ethyl-N′-octadecyl viologen (1) on bare and n-alkanethiol (CH3(CH2)nSH)-coated Au electrodes. The self-assembled monolayers (SAMs) of 1 on the Au electrode showed multiple redox peak for the first reduction of 1 in aqueous 0.1 M NH4PF6 solution. When the assembly of 1 was inserted into monolayers of n-alkanethiols of n>11, no redox response was observed in 0.1 M NH4PF6. On the other hand, when the alkyl chain length of thiol decreased from n=11 to 5, the reduction peak potential of 1 shifted to less negative potential. Moreover, the redox reaction of 1 on n-alkanethiol monolayers of n<11 was found to be very slow in the presence of PF6− ion, but to be fast in the presence of other supporting electrolytes (typically Cl−, SO42− and ClO4− ions). The Γox/Γred ratio increases when the alkyl chain length (m) of viologen decreased from m=18 to 16, 14 and 12. The monomer-dimer FTIR spectral features were observed for the SAM of 1 on the bare Au electrode in the presence of PF6− ion, whereas only monomer spectral features were observed on the n-hexanethiol (HT)-coated electrode. It is suggested that the redox moiety of 1 was less compact on a bare Au electrode and the entry of more water molecules into the monolayer strongly favors the dimerization. On the other hand, on the HT-coated electrode, the redox moiety of 1 was more compact than on the bare Au electrode and showed monomer spectral features.

Characterization of n-alkanethiolate monolayers adsorbed to electrochemically deposited gold nanocrystals on glassy carbon electrodes

In this work we combine electrochemical metal deposition with the self-assembly of thiolate monolayers in an effort to develop a novel method for attaching predefined functional groups to glassy carbon (GC) electrodes. Gold nanocrystals were electrochemically deposited on polished GC substrates by a potential step and subsequently modified by exposure to ethanolic solutions of n-alkanethiols. The structure of the resultant monolayers was then studied using electrochemical blocking analysis and infrared reflection absorption spectroscopy. Our analyses indicate that monolayers adsorbed to gold nanocrystals on GC adopt a more disordered structure relative to similar monolayers adsorbed to planar Au substrates. This is likely due to a high density of step sites on electrochemically deposited Au particles. Finally, we demonstrate our ability to employ a sequence of deposition and modification steps to introduce multiple functional groups on a single GC electrode.

Structure and growth of 4-methyl-4′-mercaptobiphenyl monolayers on Au(111): a surface diffraction study

In an attempt to quantify the role played by the rigidity of the molecular backbone on the self-assembly process, monolayers of 4-methyl-4′-mercaptobiphenyl assembled on Au(111) were characterized by grazing incidence X-ray diffraction and low-energy atomic beam diffraction. Two phases of different density were observed. In the low-density ‘striped’ phase, the diffraction pattern is consistent with a commensurate rectangular (8×2 3 ) surface lattice. Systematic absences and the intensity modulation in the diffraction pattern suggest that the structure can be composed by staggered molecular rows arranged in a head-to-head orientation with their molecular axes close to the surface. The diffraction pattern of the high-density phase can be described by a commensurate ( 3 × 3 )R30° surface lattice. The measured intensity variation along the (1, 1) Bragg rod is consistent with a tilt angle of at most 19° from the surface normal. Therefore, both similarities and differences with the diffraction patterns of the low-density phase and the high-density phase of the monolayers of n-alkanethiol on Au(111) have been detected. The thermal behavior of the monolayers of 4-methyl-4′-mercaptobiphenyl was also examined. Both phases are found to be thermally more stable than the corresponding phases of monolayers of n-alkanethiols. Finally, the growth behavior of the monolayers of 4-methyl-4′-mercaptobiphenyl was investigated and various growth protocols were tried. Compared with the case of n-alkanethiol monolayers, the high-density phase of the monolayers of 4-methyl-4′-mercaptobiphenyl is more difficult to prepare.

A grazing incidence surface X-ray absorption fine structure (GIXAFS) study of alkanethiols adsorbed on Au, Ag, and Cu

Self-assembled monolayers of n-alkanethiols, CH 3–(CH 2) x –SH, on Au, Ag, and Cu have been studied with GIXAFS at the sulfur K-edge. For both pentanethiol and decanethiol monolayers on Ag and Cu, the three-fold hollow site is found to be the most probable sulfur binding site. However, observations for octadecanethiol indicate that the three-fold hollow site is not the exclusive binding site. In addition, the possible existence of disulfide bonds on the metal surface (adsorbed dialkyldisulfides) is not supported by the data. Preliminary results from monolayers on Au are also reported.

Surface-Enhanced Electronic Raman Scattering from Self-Assembled Alkanethiol Monolayers on Gold Surfaces

A preliminary model is presented that describes the observed surface-enhanced electronic Raman scattering spectrum of self-assembled n-alkanethiol monolayers on roughened Au surfaces. At room temperature, a series of intense resonance Raman spectral lines, superimposed on the normal surface-enhanced Raman vibrational spectrum, are produced when monolayers of CH3(CH2)mSH (m = 8 to 17) are illuminated with laser radiation between 630 and 740 nm. We describe the effects of isotopic substitution, and of varying both the alkane chain length and the temperature of the monolayer films.

Effects of Film Crystallinity on the Protective Properties of Self-Assembled Monolayers of Alkanethiols on Copper

Packing and structure within ultrathin organic films can have a dramatic impact on bulk and interfacial properties, such as the ability of the film to provide corrosion resistance to the underlying substrates, especially to microelectronic devices. This communication provides a molecular- level examination of the effect of crystallinity on the protection provided to copper by crystalline self-assembled monolayers of n-alkanethiols. These ultrathin films, formed by a simple chemisorption process, are shown to be even more protective than thicker polymer films.

Molecular processes of adsorption and desorption of alkanethiol monolayers on Au(111)

The adsorption and desorption of n-alkanethiol monolayers on Au(111) have been studied under ultrahigh-vacuum condition by the use of scanning tunneling microscopy (STM), thermal desorption spectroscopy (TDS), and Auger electron spectroscopy (AES). Molecularly resolved STM observations for the alkanethiol monolayers have revealed that at least four different phases evolve during growth, which results in a multistep growth of the monolayer. The desorption species drastically changes at a critical coverage, which is accompanied by a structure change from a low-density flat-lying phase to a denser standing-up phase: While the latter phase bimolecularly desorbs as disulfides, the former phase unimolecularly desorbs as thiolate radicals. The coverage-dependent change of the desorption mode is explained in terms of the difference in the molecule-substrate bonding.