Alkanethiol Self-assembled Monolayers Research Articles

Self-Assembled Monolayers of Alkanethiols on Nickel Insert: Characterization of Friction and Analysis on Demolding Quality in Microinjection Molding.

When the part geometry scaling down from macro to microscale level, the size-induced surface effect becomes significant in the injection molding process. The adhesion between polymer and nickel (Ni) mold insert during the process can lead to defects in necking, warping and deformation of microstructure. In this study, the self-assembled monolayers (SAMs) with low surface energy were deposited on the Ni surface to reduce the adhesion and further improve the demolding quality of the microstructure. Results show that the alkyl mercaptan SAMs with chemical bonds and close alignment can be successfully deposited on the surface of Ni by the solution deposition method. The contact angle, surface free energy, and friction coefficient before and after anti-adhesion treatment on the surface of mold insert were measured. In addition, the anti-adhesion properties of different alkyl mercaptan materials and the correspondingly replication quality of microstructure parts after injection molding were analyzed. It is found that the Ni mold insert treated by the perfluorodecanethiol has the best wear resistance and still shows good reproducibility at the 100th demolding cycle.

Regenerable ZnO/GaAs Bulk Acoustic Wave Biosensor for Detection of Escherichia coli in "Complex" Biological Medium.

A regenerable bulk acoustic wave (BAW) biosensor is developed for the rapid, label-free and selective detection of Escherichia coli in liquid media. The geometry of the biosensor consists of a GaAs membrane coated with a thin film of piezoelectric ZnO on its top surface. A pair of electrodes deposited on the ZnO film allows the generation of BAWs by lateral field excitation. The back surface of the membrane is functionalized with alkanethiol self-assembled monolayers and antibodies against E. coli. The antibody immobilization was investigated as a function of the concentration of antibody suspensions, their pH and incubation time, designed to optimize the immunocapture of bacteria. The performance of the biosensor was evaluated by detection tests in different environments for bacterial suspensions ranging between 103 and 108 CFU/mL. A linear dependence between the frequency response and the logarithm of E. coli concentration was observed for suspensions ranging between 103 and 107 CFU/mL, with the limit of detection of the biosensor estimated at 103 CFU/mL. The 5-fold regeneration and excellent selectivity towards E. coli detected at 104 CFU/mL in a suspension tinted with Bacillus subtilis at 106 CFU/mL illustrate the biosensor potential for the attractive operation in complex biological media.

Fabrication and tribological properties of self-assembled monolayers of alkanethiols on nickel substrates

Self-assembled monolayers (SAMs) are considered as the potential lubricants and can be used to control the adhesion and the wear in micro-electromechanical system. In this study, alkanethiols SAMs of single component and mixed alkanethiols SAMs with different alkyl chain lengths were fabricated on the surfaces of nickel (Ni) substrates by the solution deposition method. The surface morphology and the wetting property of bare Ni substrate and different SAMs were characterized by the atomic force microscopy and water contact angle measurement. Moreover, a ball-on-plate tribometer was selected to measure the tribological properties of bare Ni substrate and different SAMs. The surface free energy of Ni Substrate decreased significantly after the surface modified by different alkanethiols molecules, which effectively reduced the surface adhesion of Ni substrate. The friction coefficients of Ni substrates coated with different SAMs were much lower than that of bare Ni substrate. In addition, the mixed alkanethiols SAMs had the greatest lubricity and wear resistance compared to other alkanethiols SAMs of single component.

Exploring End-Group Effect of Alkanethiol Self-Assembled Monolayers on Electrochemical Aptamer-Based Sensors in Biological Fluids

The continuous, real-time monitoring of specific analytes in situ in biological fluids would provide personalized, high-precision pharmacokinetic information for the goal of precision medicine. Due to their conformationally linked signaling mechanism, electrochemical aptamer-based (E-AB) sensors are promising candidates for accurate measurements in such complex media. They suffer, however, from severe baseline drift when interrogated continuously and in real-time manner. In response, here, we investigate a couple of self-assembled monolayers in the application of E-AB sensors, achieving the improvement of their baseline stability and simultaneous modulation of sensor performance, e.g., target affinity and specificity.

Synthesis and bioconjugation of alkanethiol-stabilized gold bipyramid nanoparticles

Gold bipyramid (GBP) nanoparticles are promising for a range of biomedical applications, including biosensing and surface-enhanced Raman spectroscopy, due to their favorable optical properties and ease of chemical functionalization. Here we report improved synthesis methods, including preparation of gold seed particles with an increased shelf life of ∼1 month, and preparation of GBPs with significantly shortened synthesis time (< 1 h). We also report methods for the functionalization and bioconjugation of the GBPs, including functionalization with alkanethiol self-assembled monolayers (SAMs) and bioconjugation with proteins via carbodiimide cross-linking. Binding of specific antibodies to the nanoparticle-bound proteins was subsequently observed via localized surface plasmon resonance sensing. Rabbit IgG and goat anti-Rabbit IgG antibodies were used as a model system for antibody-antigen interactions. As-synthesized, SAM-functionalized, and bioconjugated bipyramids were characterized using scanning electron microscopy, UV–vis spectroscopy, zeta potential, and dynamic light scattering.

High-resolution X-ray absorption spectroscopy of alkanethiolate self-assembled monolayers on Au(111) and Ag(111)

High-resolution C K-edge near-edge X-ray absorption fine structure (NEXAFS) spectra of non-substituted alkanethiolate self-assembled monolayers (SAMs) on Au(111) and Ag(111) were studied, with a particular emphasis on their fine structure and temperature dependence. Depending on the orientation of the light polarization with respect to the surface normal, the spectra were either dominated by the 'C−H' band, comprised of several σ*C−H / Rydberg resonances or exhibited several usually "hidden" features, including a sharp pre-edge resonance at ∼286.8 eV, related to the terminal methyl group of the SAM forming molecules and accompanied by a vibronic feature. The spectra exhibit pronounced reversible temperature dependence at going from cryogenic (60−70 K) to room temperature, including a shift of the 'C−H' band to lower energy by 250−300 meV, broadening and decrease in intensity of nearly all resonances, and disappearance of several "fine structure" features. This behavior was attributed to the nuclear motion effects, with the major impact of progressing orientational and conformational disorder in the SAMs, and correlated with the length of the of molecular backbone and specific character of the SAM-ambient interface.

From Order to Disorder of Alkanethiol Self-Assembled Monolayers on Complex Au (211), (221), and (311) Surfaces: Impact of the Substrate

We investigate the impact of the substrate on the structural properties and the morphology of alkanethiol self-assembled monolayers (SAMs) on gold, using first principles calculations and atomistic molecular dynamics simulations. We consider hexadecanethiols on Au(211), Au(221) and Au(311) surfaces which contain few-atom wide terraces separated by monoatomic steps similar to the complex Au surfaces used in experiments. The structure of the SAMs is probed via several structural properties including tilt angles, mean C atom heights from the surface, precession angles, gauche defects, gyration tensors and relative shape anisotropy. Comparing these properties to those of the well-studied SAMs on Au(111), we observe similarities but also striking differences. A clear order to disorder transition is observed by changing the substrate: well-ordered SAMs on (111) and (211) surfaces become mixed ordered-disordered structures on (311) and fully disordered on (221). The presence of steps on the Au surfaces also results in preferential tilt orientations with long-range order. Our results show that in addition to the expected grafting density dependence, the transition from order to disorder crucially depends on substrate morphology. The onset of ordering behavior is related to the atomic structure of the surface. The key parameter that affects long-range order is the energy for changing the dihedral angle between Au-S-$\mathrm{C^{(1)}-C^{(2)}}$ of the adsorbed alkanethiol.

Effect of Concentration, Chain Length, Hydrophobicity, and an External Electric Field on the Growth of Mixed Alkanethiol Self-Assembled Monolayers: A Molecular Dynamics Study.

Growing functionalized self-assembled monolayers (SAMs) with fewer defects and lower cost is the focus of ongoing investigations. In the present study, molecular dynamics simulations were performed to investigate the process of SAM formation on a gold substrate from mixed alkanethiolates in ethanol solution. Using the mixed-SAM system of 11-mercaptoundecanoic acid (MUA) with either 1-decanethiol (C9CH3) or 6-mercaptohexanol (C6OH) in a 3:7 ratio as the standard SAM model, we systematically investigated the effects of the concentration, chain length, functional group, and an external electric field on SAM growth. The results showed that the initial growth rate and surface coverage of the SAM are dependent on the ligand concentration. At a certain high concentration (about 1.2-1.5 times the minimum concentration), the final surface coverage is optimal. Reducing the chain length and increasing the proportion of hydrophobic diluting molecules are effective ways to improve the surface coverage, but the compositional ligands have to be changed, which may not be desirable for the functional requirements of SAMs. Furthermore, by investigating the behavior of the alkanethiolates and ethanol solvent under an applied external field, we find that a strong electric field with a proper field direction can facilitate the generation of defect-free monolayers. These findings will contribute to the understanding of mixed-SAM formation and provide insight into experimental design for efficient and effective SAM formation.

Reverse Size-Dependent Electrooxidation of Gold Nanoparticles Coated with Alkanethiol Self-Assembled Monolayers

The oxidation potential of weakly stabilized, citrate-coated Au and other spherical metal nanoparticles (NPs) shifts negative by an amount proportional to 1/radius due to a size-dependent negative ...

Modification of Alkanethiolate Self-Assembled Monolayers by Ultraviolet Light: The Effect of Wavelength

The effect of the wavelength upon the modification of the archetypical nonsubstituted alkanethiolate (AT) self-assembled monolayers (SAMs) on Au(111) by ultraviolet (UV) light was investigated. As ...

Interaction of Tris with DNA molecules and carboxylic groups on self-assembled monolayers of alkanethiols measured with surface plasmon resonance

Functional materials employing organic coatings on inorganic substrates are perceived as potential platforms for applications in a variety of fields. Therefore, the investigation of interactions of such systems with the microenvironment has become an important research direction in surface science. Herein, we study the interaction of one of the buffers most commonly used in biological studies, Tris buffer, with self-assembled monolayers (SAMs) of alkanethiols and short DNAs using a surface plasmon resonance (SPR) biosensor. We show that the interaction between Tris and carboxylic groups of SAMs is a complex multiphasic process. We demonstrate that Tris base binds to the protonated carboxylic groups. When those groups become deprotonated, Tris base dissociates and Tris acid is attracted, which results in the formation of a diffuse layer over the charged surface. In addition, we show that the interaction of Tris with the immobilized DNA molecules biases the determination of surface concentrations of the immobilized DNA molecules and thus also the determination of hybridization efficiencies.

Mass Fabrication of 3D Silicon Nano-/Microstructures by Fab-Free Process Using Tip-Based Lithography.

Methods for the mass fabrication of 3D silicon (Si) microstructures with a100nm resolution are developed using scanning probe lithography (SPL) combined with metal-assisted chemical etching (MACE). Protruding Si structures, including Si nanowires of over 10µm in length and atypical shaped Si nano- and micropillars, are obtained via the MACE of a patterned gold film (negative tone) on Si substrates by dip-pen nanolithography (DPN) with polymer or by nanoshaving alkanethiol self-assembled monolayers (SAMs). Furthermore, recessed Si structures with arbitrary patterning and channels less than 160nm wide and hundreds of nanometers in depth are obtained via the MACE of a patterned gold film (positive tone) on Si substrates by alkanethiol DPN. As an example of applications using protruded Si structures, nanoimprinting in an area of up to a centimeter is demonstrated through 1D and 2D SPL combined with MACE. Similarly, submicrometer polydimethylsiloxane (PDMS) stamps are employed over millimeter-scale areas for applications using recessed Si structures. In particular, the mass production of arbitrarily shaped Si microparticles at submicrometer resolution is developed using silicon-on-insulator substrates, as demonstrated using optical microresonators, surface-enhanced Raman scattering templates, and smart microparticles for fluorescence signal coding.

Superexchange Coupling-Induced Enhancements of Thermoelectric Performance in Saturated Molecules.

To develop thermoelectric devices, it is of the utmost importance to design organic building blocks to have efficient thermopower. Whereas conjugated and aromatic molecules with intrinsic narrow band gaps are attractive candidates to achieve efficient thermoelectric properties, saturated molecules are usually avoided owing to intrinsically poor thermopower. Here we demonstrate that thermopower of saturated molecules can be enhanced by superexchange coupling. Specifically, thermoelectric properties of large-area junctions that contain self-assembled monolayers of oligo(ethylene glycol) thiolates and alkanethiolates are compared. Through large-area thermopower measurements using a liquid metal top electrode, we show that the superexchange coupling enhances the Seebeck coefficient and counterintuitively leads to an increase in the Seebeck coefficient with increasing the length in a certain conformation. The improved thermoelectric performance is attributed to the superexchange-induced enhanced ability to mediate metal wave function in junctions. Our work offers new insights for improving the thermoelectric performance of nonconjugated, saturated molecules.

Interfacial Structure and Function of Nano-Structured Membranes of Newly Synthesized Phosphorylcholine Derivatives on Single Crystal Electrodes

Properties of modified surface for biosensing interface are recognized as important subjects to develop methodologies in bio-related analysis and techniques. Therefore, various surface modification materials to fabricate biosensing interface have been synthesized and studied on properties of modified surface of gold,1-4 carbon,5 silicon6 and some polymer7,8 substrates for application to biosensor and micro flow analytical system. The non-specific adsorption of proteins that occurs during analysis of biomolecules often has a serious detrimental effect on selectivity and detection limit, and research on surface modification materials has attracted great interest with a view to avoiding the interference caused by the non-specific adsorption of proteins. The properties of polyethylene glycol (PEG) such as flexibility, chemical stability, water solubility, and low cytotoxicity make it a versatile surface modification material. The most attractive property of PEG is its suppressive effect on the non-specific adsorption of proteins. While polymers consisting of the ethylene glycol moiety have been studied extensively, it has also been reported that self-assembled monolayers (SAMs) of oligoethylene glycol-alkane thiols suppress the non-specific adsorption of proteins in a similar way to surfaces modified with PEG. Our previous work showed that SAMs of oligoethylene glycol–alkane thiols effectively suppressed the non-specific adsorption of proteins allowing us to realize the high performance detection of proteins.1 On the other hand, phosphorylcholine derivatives are other promising candidates for suppressing the non-specific adsorption of proteins because the phosphorylcholine group is one of the primary lipid components of plasma membranes. Although SAMs of phosphorylcholine-alkane thiols were also reported to suppress the non-specific adsorption of proteins, there have been relatively few studies related to SAMs of phosphorylcholine-alkane thiols compared with those on SAMs of oligoethylene glycol–alkane thiols. Therefore, we are interested in the properties of phosphorylcholine SAMs. It was found that newly synthesized phosphorylcholine-alkane thiols bearing the oligoethylene glycol moiety, namely phosphorylcholine–oligoethylene glycol–alkane thiols, formed SAMs on gold surfaces and suppressed fibrinogen adsorption more effectively than the corresponding oligoethylene glycol–alkane thiols.2 The incorporation of oligoethylene glycol moieties in phosphorylcholine-alkane thiols enhanced the suppressive effect on concanavalin A adsorption but not on fibrinogen adsorption. Furthermore, we studied the newly synthesized zwitterion–alkane thiol SAMs which have phosphorylcholine, inverse phosphorylcholine and sulfobetaine terminal groups as shown in Figure 1, and repot here mainly the results on the interfacial structure and function of the phosphorylcholine-alkane thiol SAMs on a single crystal Au(111) electrode in solution. An electrochemical scanning tunneling microscope (EC-STM) was used to observe surface feature of phosphorylcholine-dodecanethiol (C12PC) SAMs on Au(111) in 0.1M HClO4, and mostly flat terrace surface with the surface roughness less than 0.3 nm was observed in a wide scan area, indicating that the molecules were closely packed. A typical high-resolution EC-STM images of C12PC SAM on Au(111) was presented in Figure 2. Bright spots aligned toward √3 directions were clearly observed with spacing of 0.5 nm, corresponding to individual C12PC molecules. The molecular arrangement was assigned to (√3 x 3√3)R30° structure with two C12PC molecules in the lattice. The surface coverage of the monolayer was 2/9, which is consistent with that from the charge of electrochemical reductive desorption process with an assumption of one-electron reaction. In the case of the phosphorylcholine–diethylene glycol–alkane thiols (C12O2PC) SAMs, the monolayer exhibited the double-striped (√3 x 3√3)R30° structure with a molecular coverage of 2/9, where every three C12O2PC molecular rows was missing. In comparison with C12PC and C12O2PC SAMs, the molecular arrangement of C12PC would be (√3 x 3√3)R30°-2C12PC missing row structure, which is the same as that of the C12O2PC SAM, because of bulkiness of phosphorylcholine head group. A detailed analysis of the surface structure will be discussed with the function suppressing the non-specific adsorption of proteins.References(1) Y. Sato, K. Yoshioka, M. Tanaka, T. Murakami, M. N.Ishida, N. Niwa, Chem. Commun., 4909 (2008).(2) M. Tanaka, T. Sawaguchi, Y. Sato, K. Yoshioka, O. Niwa, Tetrahedron Lett., 50, 4092 (2009).(3) S. Taira, D. Kaneko, K. Yokoyama, T. Sawaguchi, J. Molecular Imaging & Dynamics, S1-001, 1 (2012).(4) M. Tanaka, T. Sawaguchi, Y. Hirata, O. Niwa, K. Tawa, C. Sasakawa, K. Kuraoka, J. Colloid and Interface Sci., 497, 309 (2017).(5) M. Tanaka, T. Sawaguchi, Y. Sato, K. Yoshioka, O. Niwa, Langmuir, 27, 170 (2011).(6) M. Tanaka, T. Sawaguchi, M. Kuwahara, O. Niwa, Langmuir, 29, 6361 (2013).(7) M. Tanaka, Y. Hirata, T. Sawaguchi, S. Kurosawa, Arkivoc, ii, 330 (2018).(8) M. Tanaka, Y. Ogawa, Y. Hirata Y, T. Sawaguchi, S. Kurosawa, Sensors and Materials, 31, 33 (2019). Figure 1

Direct Measurement of Electron Transfer Rates between Iron(II) Phthalocyanine and Gold with Different Interface Structures

Electron transfer in a metal/molecule system has been extensively studied using various methods. In particular, the relation between the structure of metal/molecule junctions and the electron transfer property is of great interest in the research field of molecular devices. In this study, we measured electron transfer rates of iron(II) phthalocyanine/gold system with different junctions to study the contribution of the atomistic interfacial structures to the electron transfer characteristics.An Au single-crystal disk with (111) or (100) orientation was modified with self-assembled monolayers of imidazole-terminated alkanethiols with the alkyl chain of ten carbon atoms (ImC10SH). Then, iron(II) phthalocyanine (FePc) complexes were attached on top of the monolayer through the axial ligation of the central Fe ions with the imidazole groups. The Au-S bonding structure was measured using Surface-enhanced Raman spectroscopy (SERS).1) The electron transfer rate between Au and FePc was measured by analyzing the cyclic voltammogram (CV) of redox behavior for FePc in 0.1 M perchloric acid aqueous solution.SERS measurements showed that the Au-S bonding structures were different between Au(111) and Au(100). According to the peak position of nAu-S, the dominant Au-S structure was determined to be three-fold hollow geometry on Au(111) and two-fold bridge geometry on Au(100). Figure shows the scan rate dependence of CVs of FePc/ImC10SH/Au(100) and FePc/ImC10SH/Au(111). With increasing the scan rate, the peak separation between the oxidation and reduction peaks became larger in both cases. However, the magnitude of the separation was larger for Au(111) than for Au(100), suggesting that the electron transfer through the Au-S junction was slower at the hollow site on Au(111) than at the bridge site on Au(100). The electron transfer rate can be calculated from the relation between the peak separation and the scan rate. We will present detailed quantitative analysis of the electron transfer rate for different Au-S bond structures, alkyl chain lengths, etc.1) M. Inagaki, K. Motobayashi, K, Ikeda, J. Phys. Chem. Lett., 8, 4236 (2017)Figure. Scan rate dependence of CVs of FePc/ImC10SH/Au(100) and, FePc/ImC10SH/Au(111) in 0.1 M perchloric acid aqueous solution. Figure 1

In Situ Monitoring of Alkanethiol Self‐Assembly onto Zinc Selenide: The Role of Substrate Pretreatment and Its Implication in Bacterial Attachment

AbstractThe interface between pioneer sessile bacteria and a supporting substrate can be probed in situ and at the molecular scale by infrared spectroscopy in the attenuated total reflection mode (ATR‐FTIR). Here, a self‐assembled monolayer (SAM) of amino‐terminated alkanethiol is formed on the internal reflection element (IRE) composed of zinc selenide, and the attachment of model bacterium Lactobacillus rhamnosus GG (LGG) is subsequently studied. The impact of the beforehand surface preparation of the IRE on the SAM is studied on ZnSe substrates (i) cleaned by exposure to ozone/UV, (ii) acid cleaned, or (iii) coated with a thin gold film. The surface properties of the obtained substrates are analyzed by atomic force and electron microscopies, and elastic ion backscattering spectrometry. The kinetics of the formation and the organization of the formed SAMs are strongly surface dependent, as evidenced with ATR‐FTIR. Acid‐cleaned and gold‐coated IREs are the least and most favorable substrates for alkanethiol SAM formation, respectively. Regardless of differences in SAM properties, the average degree of LGG attachment is similar on all functionalized substrates. The molecular organization of LGG cells, however, is substrate‐dependent suggesting a possible effect of SAM organization on the bacteria–substrate interface.

Probing fibronectin adsorption on chemically defined surfaces by means of single molecule force microscopy

Atomic force microscope (AFM) based single molecule force spectroscopy (SMFS) and a quartz crystal microbalance (QCM) were respectively employed to probe interfacial characteristics of fibronectin fragment FNIII8–14 and full-length fibronectin (FN) on CH3–, OH–, COOH–, and NH2-terminated alkane-thiol self-assembled monolayers (SAMs). Force-distance curves acquired between hexahistidine-tagged FNIII8–14 immobilised on trisNTA-Ni2+ functionalized AFM cantilevers and the OH and COOH SAM surfaces were predominantly ‘loop-like’ (76% and 94% respectively), suggesting domain unfolding and preference for ‘end-on’ oriented binding, while those generated with NH2 and CH3 SAMs were largely ‘mixed type’ (81% and 86%, respectively) commensurate with unravelling and desorption, and ‘side-on’ binding. Time-dependent binding of FN to SAM-coated QCM crystals occurred in at least two phases: initial rapid coverage over the first 5 min; and variably diminishing adsorption thereafter (5–70 min). Loading profiles and the final hydrated surface concentrations reached (~ 950, ~ 1200, ~ 1400, ~ 1500 ng cm−2 for CH3, OH, COOH and NH2 SAMs) were consistent with: space-filling ‘side-on’ orientation and unfolding on CH3 SAM; greater numbers of FN molecules arranged ‘end-on’ on OH and especially COOH SAMs; and initial ‘side-on’ contact, followed by either (1) gradual tilting to a space-saving ‘end-on’ configuration, or (2) bi-/multi-layer adsorption on NH2 SAM.

Molecular Dynamics Simulations of Water Condensation on Surfaces with Tunable Wettability.

Water condensation plays a major role in a wide range of industrial applications. Over the past few years, many studies have shown interest in designing surfaces with enhanced water condensation and removal properties. It is well known that heterogeneous nucleation outperforms homogeneous nucleation in the condensation process. Because heterogeneous nucleation initiates on a surface at a small scale, it is highly desirable to characterize water-surface interactions at the molecular level. Molecular dynamics (MD) simulations can provide direct insight into heterogeneous nucleation and advance surface designs. Existing MD simulations of water condensation on surfaces were conducted by tuning the solid-water van der Waals interaction energy as a substitute for modeling surfaces with different wettabilities. However, this approach cannot reflect the real intermolecular interactions between the surface and water molecules. Here, we report MD simulations of water condensation on realistic surfaces of alkanethiol self-assembled monolayers with different head group chemistries. We show that decreasing surface hydrophobicity significantly increases the electrostatic forces between water molecules and the surface, thus increasing the water condensation rate. We observe a strong correlation between our rate of condensation results and the results from other surface characterization metrics, such as the interfacial thermal conductance, contact angle, and the molecular-scale wettability metric of Garde and co-workers. This work provides insight into the water condensation process at the molecular scale on surfaces with tunable wettability.

Interaction of the (2√3 × 3)rect. Adsorption-Site Basis and Alkyl-Chain Close Packing in Alkanethiol Self-Assembled Monolayers on Au(111): A Molecular Dynamics Study of Alkyl-Chain Conformation.

We show that the adsorption site basis of the (2√3 × 3)rect. phase of n-alkanethiol self-assembled monolayers plays a key role in determining the molecular conformation of the close-packed alkyl chains. Ten proposed reconstructed Au–S interfaces are used to explore the minimized energy alkyl-chain packing of n-decanethiol molecules using molecular dynamics with the all-atom description. In this comparative study, all models have the same alkyl-chain surface density of four molecules per unit cell; thus, differences are due to the headgroup spacing within the 4-molecule basis as opposed to the average surface density. We demonstrate for the first time the 4-molecule-basis twist structure driven by the packing of alkanethiol molecules in a large simulation box (100 molecules, 25 unit cells) using molecular dynamics. Our results validate the prediction put forward by Mar and Klein that to achieve the 4-molecule-basis twist symmetry observed by the experiment, the headgroups must deviate from the high-symmetry (√3 × √3)R30° sites. The key structural parameters: tilt, twist, and end-group height, as well as their spatial order, are compared with experimental results, which we show is a highly sensitive approach that can be used to vet proposed Au–S interfacial models.

Multiscale Modeling Reveals the Cause of Surface Stress Change on Microcantilevers Due to Alkanethiol SAM Adsorption.

Experimental results show that the adsorption of the self-assembled monolayers (SAMs) on a gold surface induces surface stress change that causes deformation of the underlying substrate. However, the exact mechanism of stress development is yet to be elucidated. In the present study, multiscale computational models based on molecular dynamics (MD) simulations are applied to study the mechanism governing surface stress change. Distinct mechanisms for adsorption-induced surface deformation, namely, interchain repulsion and thiol-gold interaction-driven gold surface reconstruction, are investigated. Two different interatomic potentials, embedded atom method and surface-embedded atom method (SEAM), are used in the MD simulations to study the reconstruction-induced surface stresses. Comparison of the predicted surface stress changes, resulting from MD and continuum mechanics-based models, with the observed experimental response indicates that a modified SEAM-based multiscale model can better capture the surface stress changes observed during alkanethiol SAM formation, and gold surface reconstruction is the primary factor behind the surface stress change. The interchain repulsions of SAM are found to have a minimal contribution. Also, both the simulations and experiments show that the surface stress change increases with the increase of surface coverage density and larger grain size.