N-alkanethiolate Self-assembled Monolayers Research Articles

Charge Transport and Rectification in Arrays of SAM-Based Tunneling Junctions

This paper describes a method of fabrication that generates small arrays of tunneling junctions based on self-assembled monolayers (SAMs); these junctions have liquid-metal top-electrodes stabilized in microchannels and ultraflat (template-stripped) bottom-electrodes. The yield of junctions generated using this method is high (70-90%). The junctions examined incorporated SAMs of alkanethiolates having ferrocene termini (11-(ferrocenyl)-1-undecanethiol, SC(11)Fc); these junctions rectify currents with large rectification ratios (R), the majority of which fall within the range of 90-180. These values are larger than expected (theory predicts R <or= 20) and are larger than previous experimental measurements. SAMs of n-alkanethiolates without the Fc groups (SC(n-1)CH(3), with n = 12, 14, 16, or 18) do not rectify (R ranged from 1.0 to 5.0). These arrays enable the measurement of the electrical characteristics of the junctions as a function of chemical structure, voltage, and temperature over the range of 110-293 K, with statistically large numbers of data (N = 300-800). The mechanism of rectification with Fc-terminated SAMs seems to be charge transport processes that change with the polarity of bias: from tunneling (at one bias) to hopping combined with tunneling (at the opposite bias).

Surface Dipole Layer Potential Induced IR Absorption Enhancement in n-Alkanethiol SAMs on GaAs(001)

The work function of n-alkanethiol self-assembled monolayers (SAMs) prepared on the GaAs(001) surface was measured using the Kelvin probe technique yielding the SAM 2D dipole layer potential (DLP). Direct n-dependent proportionality between the DLP values and the C-H stretching mode infrared (IR) absorption intensities was observed, which supports a correspondence of reported IR enhancements with the electrostatic properties of the interface. X-ray photoelectron spectroscopy is also used to verify the work function measurements. In addition, the principal components of the refractive index tensor are shown to be n-invariant in the ordered SAM phase. Our results suggest that a local field correction to the transition dipole moment accounts for the observed increase in IR activity through an increase to the electronic polarizability.

Tribology of Monolayer Films: Comparison between n-Alkanethiols on Gold and n-Alkyl Trichlorosilanes on Silicon

This Article presents a quantitative comparison of the frictional performance for monolayers derived from n-alkanethiolates on gold and n-alkyl trichlorosilanes on silicon. Monolayers were characterized by pin-on-disk tribometry, contact angle analysis, ellipsometry, and electrochemical impedance spectroscopy (EIS). Pin-on-disk microtribometry provided frictional analysis at applied normal loads from 10 to 1000 mN at a speed of 0.1 mm/s. At low loads (10 mN), methyl-terminated n-alkanethiolate self-assembled monolayers (SAMs) exhibited a 3-fold improvement in coefficient of friction over SAMs with hydroxyl- or carboxylic-acid-terminated surfaces. For monolayers prepared from both n-alkanethiols on gold and n-alkyl trichlorosilanes on silicon, a critical chain length of at least eight carbons is required for beneficial tribological performance at an applied load of 9.8 mN. Evidence for disruption of chemisorbed alkanethiolate SAMs with chain lengths n <or= 12 is shown through EIS analysis of tribology wear tracks. The direct comparison between the tribological stability of alkanethiolate and silane monolayers shows that monolayers prepared from n-octadecyl dimethylchlorosilane and n-octadecyl trichlorosilane withstood normal loads at least 30 times larger than those that damaged octadecanethiolate SAMs. Collectively, our results show that the tribological properties of monolayer films are dependent on their internal stabilities, which are influenced by cohesive chain interactions (van der Waals) and the adsorbate-substrate bond.

Characterization of self-assembled monolayers by using a near-field microwave scanning microprobe

A near-field microwave scanning microprobe (NSMM) technique has been used to investigate the material properties of n-alkanethiol self-assembled monolayers (SAMs) on a gold (Au) surface. We demonstrate that near-field microwave probing technique can achieve the noncontact detection of the thickness of SAMs by measuring the microwave reflection coefficient S 11 at an operating frequency near 5.3 GHz. We also directly image the patterned SAMs by NSMM. The thickness (chain length) of SAMs is determined from the visualized microwave reflection coefficient changes on the Au surface with high sensitivity. This nano-scale measurement of SAMs has a great potential for investigating the surface profile with high sensitivity.

Molecular Dynamics Simulation of Oxygen Transport through ω-Alkoxy-n-alkanethiolate Self-Assembled Monolayers on Gold and Copper

We have used molecular dynamics (MD) simulations to investigate the influences of the position of the ethereal oxygen on the ability of omega-alkoxy-n-alkanethiolate self-assembled monolayers (SAMs) to act as barrier films against through-film oxygen transport as relevant to the uses of these films in corrosion inhibition. Our MD simulations reveal that when the ether linkage is too close to the metal surface or to the chain ends, the free-energy barrier of SAMs toward oxygen diffusion was approximately 5 kJ/mol less than for a non-ether-containing n-alkanethiolate SAM having the same chain length. MD simulations show that SAMs having an ether linkage near a chain end contain a highly disordered terminal region. As a result, the SAMs allow a more rapid transport of oxygen across these monolayers than through n-alkanethiolate SAMs of similar length lacking the ether unit. Additionally, SAMs with the ether linkage close to the metal surface undergo a structural transition to an alternating flipped structure that is less crystalline compared to that of an n-alkanethiolate SAM. Together, these factors diminish the barrier properties of the omega-alkoxy-n-alkanethiolate SAMs below those for their unsubstituted analogues.

Molecular Dynamics Simulation of Oxygen Transport through n-Alkanethiolate Self-Assembled Monolayers on Gold and Copper

Molecular dynamics (MD) simulations were performed to investigate the influence of molecular structure on the ability of n-alkanethiolate self-assembled monolayers (SAMs) on gold and copper to act as barrier films against through-film oxygen transport as relevant to the uses of these films in corrosion inhibition. Specific explorations focused on the effects of the packing density of the adsorbates, their chain length, and the size of the diffusing species. The MD simulations showed that the resistances offered by these monolayers against transport of small molecules strongly depend on the penetrant size and that the free volume alignment within these films has a large impact on the diffusivities of oxygen through them. The MD simulations revealed that the barrier for transport through these films increase by approximately 0.3 to 0.6 kJ/mol for increases in their thickness of only roughly an angstrom. MD simulations show the existence of a middle section in these SAMs for n >or= 12 that was more crystalline and dense than the rest of the monolayer. The barrier resistances offered by these films toward oxygen transport, as calculated by the MD simulations, were a function of the crystallinity, density, and thickness of the middle section of the SAMs.

On the Interpretation of Multiple Waves in Cyclic Voltammograms of Self-Assembled Monolayers of n-Alkane Thiols on Gold

A homologue series of n-alkane thiols (CH3(CH2) m SH, MCm, m between 8 and 22) adsorbed on Au/mica substrates was studied in an alkaline environment using cyclic voltammetry (CV) and second harmonic generation (SHG). The known split of the reductive desorption signal in the cyclic voltammogram for thiols with chain length of m &gt; 12 disappears again for m = 22. Simultaneous recording of SHG and CV data allowed correlation of thiol coverage with charge which was found to be linear, irrespective of the shape of the desorption signal. This evidences that the double peak structure of the desorption signal does not arise from the separation of faradaic and capacitive processes. The same is found for the double wave of the readsorption signal. The transient appearance of a split desorption signal for the one electron process of thiol desorption in the range of 12 &lt; m &lt; 22 is suggested to originate from intermolecular interactions.

Scanning Electrochemical Microscopy. 59. Effect of Defects and Structure on Electron Transfer through Self-Assembled Monolayers

Electron transfer (ET) rate kinetics through n-alkanethiol self-assembled monolayers (SAMs) of alkanethiols of different chain lengths [Me(CH2)nSH; n=8, 10, 11, 15] on Au and Hg surfaces and ferrocene (Fc)-terminated SAMs (poly-norbornylogous and HS(CH2)12CONHCH2Fc) on Au were studied using cyclic voltammetry and scanning electrochemical microscopy (SECM). The SECM results allow determination of the ET kinetics of solution-phase Ru(NH3)63+/2+ through the alkanethiol SAMs on Au and Hg. A model using the potential dependence of the measured rate constants is proposed to compensate for the pinhole contribution. Extrapolated values of koML for Ru(NH3)63+/2+ using the model follow the expected exponential decay (beta is 0.9) for different chain lengths. For a Fc-terminated poly-norbornyl SAM, the standard rate constant of direct tunneling (ko is 189+/-31 s(-1)) is in the same order as the ko value of HS(CH2)12CONHCH2Fc. In blocking and Fc SAMs, the rates of ET are demonstrated to follow Butler-Volmer kinetics with transfer coefficients alpha of 0.5. Lower values of alpha are treated as a result of the pinhole contribution. The normalized rates of ET are 3 orders of magnitude higher for Fc-terminated than for blocking monolayers. Scanning electron microscopy imaging of Pd nanoparticles electrochemically deposited in pinholes of blocking SAMs was used to confirm the presence of pinholes.

Achieving Precision and Reproducibility for Writing Patterns of n‐alkanethiol Self‐assembled Monolayers with Automated Nanografting

Nanografting is a high-precision approach for scanning probe lithography, which provides unique advantages and capabilities for rapidly writing arrays of nanopatterns of thiol self-assembled monolayers (SAMs). Nanografting is accomplished by force- induced displacement of molecules of a matrix SAM, followed immediately by the self-assembly of n-alkanethiol ink molecules from solution. The feedback loop used to control the atomic force microscope tip position and displacement enables exquisite control of forces applied to the surface, ranging from pico to nanonewtons. To achieve high-resolution writing at the nanoscale, the writing speed, direction, and applied force need to be optimized. There are strategies for programing the tip translation, which will improve the uniformity, alignment, and geometries of nanopatterns written using open-loop feedback control. This article addresses the mechanics of automated nanografting and demonstrates results for various writing strategies when nanografting patterns of n-alkanethiol SAMs.

Modification of copper with self-assembled organic coatings

Self-assembled monolayers of n-alkanethiols (CH 3(CH 2) n−1 SH; n = 12 and 18) were prepared on a copper surface, and the quality and protection efficiency of resulting coatings against aqueous corrosion of copper were investigated. A combination of physical (scanning electron microscopy, energy dispersive X-ray spectroscopy and contact angle measurements) and electrochemical (cyclic voltammetry and impedance spectrometry) methods was used to correlate the structure of the coatings with their barrier properties during exposures in an aqueous solution. Impedance results reveal that the coatings behave almost like an ideal dielectric, with a resistance of several MΩ cm 2 and a coating capacitance that agree well with the value calculated according to the theory of dielectrics. Protection efficiency of chemisorbed alkanethiol coatings determined from dc and ac measurements were above 99%.

Monolayer-Directed Electrodeposition of Oxide Thin Films: Surface Morphology versus Chemical Modification

In this paper we describe the general role that surface morphology and chemical nature play in guiding the cathodic deposition of oxide thin films onto electrode surfaces. By use of a “featureless” stamp for microcontact printing (μCP), the pregrooved microfeatures (“mountains” and “valleys” representing the track trails) of recordable compact disc (CD-R) gold substrates can be selectively modified with OH- or CH3-terminated self-assembled monolayers (SAMs). For comparison, “flat” gold substrates were patterned with the above SAMs in parallel “microstrips” that are analogous to the CD-R substrate (but no height differences). Electrochemical deposition of zirconia thin films showed that, on the CD-R substrates, surface morphology (height difference) dominates over the blocking effects of the SAMs; that is, deposition occurred primarily on the mountains despite these sites being modified with organic monolayers. For flat gold substrates it was found that n-alkanethiolate SAMs block deposition in modified ar...

Alkanethiol Reductive Desorption from Self-Assembled Monolayers on Gold, Platinum, and Palladium Substrates

The reductive desorption of n-alkanethiolate (RS-) self-assembled monolayers (SAMs) on gold, platinum, and palladium substrates was investigated. The kinetics of this process were too slow to measure directly in the case of RS−Pt and RS−Pd SAMs. Thus, the faradaic response of potassium ferricyanide after partial or complete desorption was determined and used to quantify the relative electrochemically accessible surface area. Both the type of underlying metal substrate and the chain length of the n-alkanethiol were varied. The relative alkanethiol binding affinity and effective kinetics of binding for each of the substrates were discussed in light of these observations.

Molecular Self-Assembly at Bare Semiconductor Surfaces: Characterization of a Homologous Series of n-Alkanethiolate Monolayers on GaAs(001)

Structural trends for a homologous series of n-alkanethiolate self-assembled monolayers (SAMs), C(n)H(2n+1)S- with 12 < or = n < or = 19, on GaAs(001), studied by a combination of grazing incidence X-ray diffraction and infrared spectroscopy, along with ancillary probes, show an overall decay in organization with decreasing n, with the largest changes occurring below n = 15-16. The long-chain monolayers form a mosaic structure with < or =10 nm domains of molecules organized in an incommensurate pseudo-hcp arrangement with nearest neighbor distances of 4.70 and 5.02 A, a 21.2 A(2) area per chain, two chains per subcell in a herringbone packing with a chain tilt angle of 14 degrees , and preferential domain alignment along the substrate [110]([110]) step edge direction. In contrast, for n < 14 no evidence of translational ordering is seen and the alkyl chains exhibit a loss of conformational ordering and coverage relative to the n > 16 cases. A 4'-methyl-biphenyl-4-thiolate companion SAM shows evidence for ordered structures but with lattice parameters close to those expected for a structure commensurate with the intrinsic GaAs(001) square lattice. These trends are explained on the basis of competitions between lattice, interfacial, and intermolecular forces controlling the nanoscale structures of the SAMs. Overall these results provide an important aspect of understanding the effects of SAM formation on surface properties such as electronic and chemical passivation.

Characterization of n-alkanethiol self-assembled monolayers on mercury by impedance spectroscopy and potentiometric measurements

Abstract Long chain n-alkanethiol self-assembled monolayers (SAMs) on a hanging mercury drop electrode were studied in detail for the first time by impedance spectroscopy (IS) and potentiometry. IS reveals the detailed behavior of the monolayers in the absence and presence of two one-electron redox couple: hexaamineruthenium(III), Ru ( NH 3 ) 6 3 + and tris(4,4′-bipyridine)cobalt(III), Co ( bpy ) 3 3 + . The pinholes-free layers behave as almost ideal capacitors and are permeable to some extent to redox species, depending on their hydrophobicity. Interestingly, Co ( bpy ) 3 3 + that exhibits sluggish kinetics on a bare Hg electrode reveals a more facile kinetics than Ru ( NH 3 ) 6 3 + on an n-octadecanethiol/Hg interface. Potentiometric measurements recorded upon immersing the bare and SAM modified Hg electrode into a solution containing a mixture of the oxidized and reduced forms of the redox couples provide invaluable information on the charge transfer across the monolayer and its ohmic resistance. While Ru ( NH 3 ) 6 3 + cannot freely penetrate into the monolayer and therefore establishes a potential difference across the monolayer; penetration of Co ( bpy ) 3 2 + into the film causes the Fermi level of the Hg surface to attain the Nernst potential of the solution. Finally, we find that increasing the length of the alkane chain of the thiols increases linearly the ohmic resistance of the layer.

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.

Effect of chain length on the adhesion behaviour of n-alkanethiol self-assembled monolayers on Au(111): An atomic force microscopy study

The effect of chain length on the adhesion behaviour of n-alkanethiols CH3(CH2)nSH, wheren = 5, 6, 7, 9, 10, 11, 14 and 15 has been followed by carrying out pull-off force measurement using atomic force microscopy (AFM). The self-assembled monolayers on Au(111) surface has been characterized by reflection absorption infra-red spectroscopy (RAIRS) and contact mode AFM. It is observed that the work of adhesion is independent of thiol chain length though the standard deviation is high for short chain length thiol-based monolayers. This may be attributed to the relatively more deformable nature of the short chain thiol films due to their heterogeneity in the monolayer structure than the long chain ones. This, in turn, increases the contact area/volume between the AFM tip and the monolayer, and hence the force of adhesion. However, in the presence of water, the force of adhesion is lower than that observed in air reflecting the effects of capillary forces/polar components associated with the surface energy.

Investigation of carboxylic-functionalized and n-alkanethiol self-assembled monolayers on gold and their application as pH-sensitive probes using scanning electrochemical microscopy

We investigated the insulating properties of n-alkanethiol self-assembled monolayers (SAMs) of varying chain lengths [CH 3(CH 2) n SH; n = 7, 9, 11, 15] on polycrystalline gold electrodes using scanning electrochemical microscopy (SECM) and cyclic voltammetry. On the basis of SECM approach curves we examined the local ET through monolayers with increasing chain length in different redox mediators. We were able to distinguish the monolayers because of their different insulating properties and in addition, the status of SAM formation after immersion times of 2 h and 24 h, respectively, could be observed. Cyclic voltammetric measurements confirmed the SECM results and were in good agreement with other experimental data in the literature. High-resolution SECM images of hexadecanethiol SAM micropatterns down to 4 μm in diameter formed by microcontact printing (μCP) were obtained in the feedback mode. Furthermore, we studied the ET and the pH-dependent behavior of mercaptoundecanoic acid monolayers on gold at varying pH and in different redox mediator solutions to test their application as pH-sensors. An additional influence on the ET could be established based on Coulomb/ionic interactions between the charged monolayer and the redox mediator at changing pH. Therefore, we present a new approach for designing pH-sensitive SECM probes using 11-mercaptoundecanoic acid-coated 10 μm-diameter gold ultramicroelectrodes (HOOC–C 11SH/Au UMEs) in aqueous solutions containing hexacyanoferrate. Voltammetric measurements at HOOC–C 11SH/Au UMEs at different pH values enabled us to estimate the degree of dissociation of the carboxylic-terminated monolayers.

Truly Quantitative XPS Characterization of Organic Monolayers on Silicon: Study of Alkyl and Alkoxy Monolayers on H−Si(111)

The quantitative characterization of the chemical composition (bonding at grafted and ungrafted sites, surface coverage) is a key issue for the application of silicon-organic monolayer hybrid interfaces. The primary purpose of this article is to demonstrate that X-ray photoelectron spectroscopy (XPS) requires to be truly quantitative to deal with two main questions. The first one is accounting for X-ray photodiffraction (XPD), a well-known phenomenon that is responsible for azimuthal variations of the XPS signal intensity. A simple procedure is proposed to account for XPD in angle-resolved measurements. The second critical point concerns the choice of photoelectron attenuation lengths (AL). This article demonstrates that n-alkanethiol self-assembled monolayers on Au(111) can be used as a reference system to derive the effective monolayer thickness on silicon substrates and that one may use the empirical relationship established by Laibinis and co-workers to calculate the relevant ALs (Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017). A self-consistent approach is presented to justify the above assertions and to give a complete compositional description of alkyl and alkoxy monolayers directly grafted on atomically flat H-Si(111) surfaces. Direct evidences are provided that a Si-C and a Si-O-C linkage is formed, respectively, after reaction with decene and decanol and that the ungrafted sites remain saturated with H atoms. Moreover, the quantitative spectra analysis of satellite peaks at fixed polar angle and three independent angle-resolved Si2p and C1s spectra all give the same surface coverage very close to its theoretical limit.

Establishing the Molecular Basis for Molecular Electronics

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