Octadecanethiol Self-assembled Monolayers Research Articles

Electrochemical insights: octadecanethiol self-assembled monolayers on leaded brass for enhanced anti-corrosion protection in non-complexing media—unraveling the impact of halide ions

Electrochemical insights: octadecanethiol self-assembled monolayers on leaded brass for enhanced anti-corrosion protection in non-complexing media—unraveling the impact of halide ions

Novel approach to passivation of InAs/GaSb type II superlattice photodetectors

The innovative two-step passivation by octadecanethiol (ODT) self-assembled monolayers (SAMs) and the following silicon dioxide (SiO2) deposition was used for the type-II InAs/GaSb superlattice photodetector. To understand the mechanism of passivation, the (100) GaSb surface covered with the ODT and, for comparison, with the biphenyl thiol (BPT), was characterized by the atomic force microscopy, Raman spectroscopy and contact angle analysis. The results of the study indicated the presence of the homogeneous both the ODT and the BPT monolayers; however, the ODT SAMs were more stable. Therefore, the ODT-based wet treatment was used in the two-step passivation resulting in a reduction of the dark current by one order of magnitude for passivated detector compared with an unpassivated device.

Phase-Sensitive Sum-Frequency Generation Measurements Using a Femtosecond Nonlinear Interferometer.

Phase-sensitive sum-frequency spectroscopy is a unique tool to interrogate the vibrational structure of interfaces. A precise understanding of the interfacial structure often relies on accurately determining the phase of χ(2), which has recently been demonstrated using a nonlinear interferometer in conjunction with a frequency-scanning picosecond laser system. Here, we implement nonlinear interferometry using a femtosecond laser system for broadband sum-frequency generation. The phase of the vibrational response from a self-assembled monolayer of octadecanethiol on gold is determined using the nonlinear femtosecond interferometer. The results are compared to those obtained using the more traditional heterodyne-detected phase measurements. Both methods give a similar phase spectrum and phase uncertainty. We also discuss the origin of the phase uncertainties and provide guidelines for further improvement.

Comprehensive Stability Improvement of Silver Nanowire Networks via Self-Assembled Mercapto Inhibitors.

Instability of silver nanowire (AgNW) has been regarded as a major obstacle to its practical applications in optoelectrical devices as transparent electrodes. Physical barrier layers such as polymer, metal, and graphene have been generally employed to improve the stability of AgNW in previous study. Herein, we first report self-assembled organothiols as an inhibitor for AgNW to achieve an overall enhancement in antioxidation, antisulfidation, thermal stability, and anti-electromigration. The self-assembled monolayers (SAMs) of phenyl azoles, methoxy silane, and methyl alkane were formed on the surface of AgNW via Ag-S covalent bond as barrier layers which provided protective effects against corrosives (e.g., O2, H2S). In particular, the decoration of 2-mercaptobenzimidazole (MBI) offered the best resistance to H2S and excellent stability under a high-temperature and high-humidity environment (85 °C and 85 RH %) for 4 months. Moreover, different SAMs exhibited a stabilizing or destabilizing effect on Plateau-Rayleigh instability of AgNW, which realized the tunability of degradation temperature of AgNWs, for example, increasing by ≥100 °C with MBI SAM or decreasing by ∼50 °C with octadecanethiol SAM compared with pristine AgNWs. Notably, the MBI-decorated AgNWs could survive at 400 °C which is by far the highest bearing temperature for solution-processed AgNW film. As a result, a transparent heater made of the MBI-AgNWs exhibited superior heating characteristics (higher working temperature and durability), as compared with the pristine AgNW-based heater. Our findings on the organothiols decoration not only provide a new paradigm in overall stability improvement of NW of noble metals but also show the potential in morphology controllability of metal NW.

Waveguide-integrated mid-infrared plasmonics with high-efficiency coupling for ultracompact surface-enhanced infrared absorption spectroscopy.

Waveguide-integrated plasmonics is a growing field with many innovative concepts and demonstrated devices in the visible and near-infrared. Here, we extend this body of work to the mid-infrared for the application of surface-enhanced infrared absorption (SEIRA), a spectroscopic method to probe molecular vibrations in small volumes and thin films. Built atop a silicon-on-insulator (SOI) waveguide platform, two key plasmonic structures useful for SEIRA are examined using computational modeling: gold nanorods and coaxial nanoapertures. We find resonance dips of 90% in near diffraction-limited areas due to arrays of our structures and up to 50% from a single resonator. Each of the structures is evaluated using the simulated SEIRA signal from poly(methyl methacrylate) and an octadecanethiol self-assembled monolayer. The platforms we present allow for a compact, on-chip SEIRA sensing system with highly efficient waveguide coupling in the mid-IR.

Surface-Induced Heterogeneity Analysis of an Alkanethiol Monolayer on Microcrystalline Copper Surface Using Sum Frequency Generation Imaging Microscopy

An octadecanethiol (ODT) self-assembled monolayer on microcrystalline copper is investigated by sum frequency generation (SFG) imaging microscopy and electron backscattering diffraction (EBSD). The strong SFG signal contrast across the domain boundary indicates the existence of grain structures on copper surface, which is further verified by EBSD measurements. The nonresonant contribution of SFG response shows anisotropy with respect to the in-plane rotation of the sample relative to the surface normal and varies in each crystal domain area. The resonant contribution of the monolayer, such as the amplitude ratio of CH3-sym/CH3-asym, is azimuthally nearly isotropic. Since the zzz tensor component of nonlinear susceptibility dominates, the resonant part of SFG spectra on the metal surface does not show any anisotropy. Further, a strong correlation between the local metal structures with the top monolayer packing behaviors is identified based on the statistical distribution analysis. Using the methyl group a...

Surface passivation of (100) GaSb using self-assembled monolayers of long-chain octadecanethiol

The passivation of (100) GaSb surface was investigated by means of the long-chain octadecanethiol (ODT) self-assembled monolayer (SAM). The properties of ODT SAM on (100) GaSb were characterized by the atomic force microscopy using Kelvin probe force microscopy mode and X-ray photoelectron spectroscopy. The chemical treatment of 10mM ODT-C2H5OH has been applied to the passivation of a type-II superlattice InAs/GaSb photodetector. The electrical measurements indicate that the current density was reduced by one order of magnitude as compared to an unpassivated photodetector.

Quantum dot monolayer for surface plasmon resonance signal enhancement and DNA hybridization detection

We report results of studies relating to the fabrication of a surface plasmon resonance (SPR)-based nucleic acid sensor for quantification of DNA sequence specific to chronic myelogeneous leukemia (CML). The SPR disk surface has been modified with octadecanethiol self-assembled monolayer followed by deposition of the tri-n-octylphosphine oxide capped cadmium selenide quantum dots (QD) Langmuir monolayer. The deposition is performed via Langmuir–Blodgett (LB) technique. For the sensor chip preparation, covalent immobilization of the thiol-terminated DNA probe sequence (pDNA) using displacement reaction is accomplished. This integrated SPR chip has been used to detect target complementary DNA concentration by monitoring the change in coupling angle via hybridization. It is revealed that this biosensor exhibits high sensitivity (0.7859m0pM−1) towards complementary DNA and can be used to detect it in the concentration range, 180pM to 5pM with detection limit as 4.21pM. The results of kinetic studies yield the values of hybridization and dissociation rate constants as 9.6×104M−1s−1 and 2.3×10−2s−1, respectively, with the equilibrium constant for hybridization as 4.2×106M−1.

Density Analysis on Self-Assembled Monolayers (SAMs) of Functionalized Alkanethiol on Gold over Polyethylene Terephthalate (PET) Substrate

In this work, an equivalent circuit from electrochemical impedance spectroscopy (EIS) has been designed and executed for qualitative analyzing of leaky behavior of self‐assembled monolayers (SAMs) of octadecanethiol [CH3(CH2)17‐SH] on gold (Au) suface. Polyethylene terephthalate (PET), the cost‐effective substrate has been utilized for metalization of thin Au layer with subsequent formation of octadecanethiol SAM on it with systematic variation of adsorption time. The SAM layer shows its highly dependency on the adsorption time of the surfactants of octadecanethiol with the metal surface.

Interaction of Graphene with Self Assembled Monolayers

SummaryThe adsorption of graphene onto solid surface is vital for the numerous device applications. In the present work, we studied interaction of graphene with chemically modified surfaces. We formed the self‐assembled monolayers (SAM) of octadecanethiol (ODT), mercapto undecanoic acid (MUA) and biphenyl dithiol (BPD) onto the gold substrates. The nature of the surfaces obtained for ODT and MUA were hydrophobic and hydrophilic, respectively. The SAM of BPD yields the surface with phenyl rings having delocalized π‐electrons. Due to hydrophobic nature of the graphene, it adsorbs onto the SAMs of ODT and BPD. The surface morphology of the graphene onto SAMs was studied using atomic force microscope. Due to π−π stacking interaction between the graphene and BPD molecules, graphene prefers planar adsorption onto the SAM of BPD. The hydrophobic interaction between the graphene and ODT leads to random and non‐uniform adsorption of graphene onto the SAM of ODT. Surface plasmon resonance (SPR) phenomenon is extremely sensitive technique to detect minute change in dielectrics at the given interface. The adsorbed layers of the organic materials were studied using a SPR instrument.

Measurement of optical anisotropy in ultrathin films using surface plasmon resonance

The optical phenomenon, surface plasmon resonance (SPR) is employed for the measurement of optical anisotropy in the ultrathin films fabricated through Langmuir–Blodgett (LB) and self-assembled monolayer (SAM) techniques onto 50 nm gold film supported on BK7 glass substrates. The resonance angle (RA) is measured using a home built setup in Kretschmann configuration. The LB films and SAM can provide a single layer of highly ordered and organized molecules on the two dimensional surface. If the film forming molecules are anisotropic, their organization in the LB films and SAM can yield an anisotropic film due to tilt of the molecules with respect to the surface normal. The SPR spectra are recorded for the two orthogonal directions of the film with respect to the plane of incidence. The spectra are simulated by modeling the Fresnel's reflection from 4-layers viz., prism, gold, ultrathin films and air; and the real and imaginary parts of refractive index are estimated. Our study shows the metallic and dielectric nature of the LB films of bundles of single walled carbon nanotubes (SWCNTs) when the long axis of SWCNTs are aligned parallel and perpendicular to plane of incidence, respectively. The optical anisotropy was estimated from the change in real part of refractive index (Δnr) of the ultrathin films measured in the orthogonal directions. In addition, we have also studied such optical anisotropy in the LB film of cadmium-stearate and self-assembled monolayer of octadecanethiol.

Grain Structures and Boundaries on Microcrystalline Copper Covered with an Octadecanethiol Monolayer Revealed by Sum Frequency Generation Microscopy.

An octadecanethiol (ODT) self-assembled monolayer on microcrystalline copper was investigated by sum frequency generation (SFG) imaging microscopy. The crystal grain and grain boundaries of the copper surface were mapped in the SFG image based on the strong brightness contrast of the SFG signal across the boundary. Local SFG spectra reveal significant difference with each other as well as the average SFG spectra, indicating the heterogeneity of the copper surface resulting from copper grains with distinct crystallographic facets and orientations. It is demonstrated that the SFG signal of crystalline domain areas contains azimuthal anisotropy with respect to the plane of incidence. In addition, the statistical orientation analyses of amplitude ratio of CH3-sym/CH3-asym and corresponding contour maps imply that the orientation of ODT molecules is affected by the underlying copper.

Corrosion Inhibition of Two Brass Alloys by Octadecanethiol in Humidified Air with Formic Acid

Self-assembled monolayers of octadecanethiol (ODT) have previously shown to provide excellent corrosion inhibition on copper exposed to humidified air containing formic acid, mimicking indoor atmospheric corrosion. ODT layers are, however, much less efficient corrosion inhibitors for zinc. In this work, we elucidate the possibility of using ODT monolayers to inhibit corrosion of brass. Based on a quantitative analysis of corrosion products, we found that ODT provides equally good corrosion inhibition of single-phase Cu20Zn as of pure copper, retarding the transportation of corrosion stimulators to the brass surface. On double-phase Cu40Zn, however, local galvanic effects led to less efficient corrosion inhibition and more corrosion products than on Cu20Zn.

Inhibition of Interfacial Oxidative Degradation During SiOxPlasma Polymer Barrier Film Deposition on Model Organic Substrates

Interfacial processes during the initial stages of SiOx-like plasma-polymer barrier coating deposition were investigated by means of polarization modulation infrared reflection-absorption spectroscopy, and the resulting effect on defect densities were studied by cyclic voltammetry. Octadecanethiol self-assembled monolayers on Au-film coated wafers served as sensor layers to investigate interface chemistry during the plasma deposition. Both the spectroscopic and electrochemical data revealed that a thin SiOCH interlayer could reduce oxidative degradation of the SAM during subsequent deposition of the SiOx barrier film from an oxygen-rich plasma phase. The present electrochemical investigation confirmed effective inhibition of interfacial oxidative degradation processes of an aliphatic polymer in the presence of a SiOCH interfacial layer.

Metastable Molecular Metal–Semiconductor Junctions

We demonstrate herein how to mechanically modulate the electrical properties of metastable molecular junctions, i.e., mercury–silicon junctions modified with “mobile” octadecanethiolate (C18) self-assembled monolayers (SAMs). By enlarging the mercury drop contact or changing its shape, the current density–voltage response of these molecular junctions vary remarkably from rectifying (off) to ohmic (on). More importantly, such switching behavior is reversible and reproducible when the shape of the mercury drop is changed from spherical to elliptical and vice versa (by pressing and releasing the mercury drop). Evaluation of the rectification ratio and effective barrier height of these molecular junctions enables determination of the threshold surface area of the mercury contact for the modulated electrical switching.

In Situ Vibrational Study of the Reductive Desorption of Alkanethiol Monolayers on Gold by Sum Frequency Generation Spectroscopy

In situ sum frequency generation vibrational spectroscopy (SFG) was used to monitor the reductive desorption of decanethiol (DT) and octadecanethiol (ODT) self-assembled monolayers (SAMs) grown on evaporated gold on silica. At negative potentials, the alkyl chains of both monolayers became disordered as monitored by the appearance of methylene symmetric stretching modes in the collected spectra. The increased tilting of the terminal methyl groups on the chains of the DT monolayer further support this observation. The disappearance of the methyl C–H stretching vibrational modes at the reductive potential suggests that DT molecules diffused away from the surface after reduction. ODT molecules, on the other hand, retained their two-dimensional structure near the gold surface, implied by the strong methyl vibrational modes at the reductive potential. After four reductive cycles, a large portion of the DT SAM was fully desorbed, while the ODT monolayer existed as a combination of physisorbed and chemisorbed mo...

Adsorption of β-amyloid oligomers on octadecanethiol monolayers

Hypothesisβ-Amyloid oligomers of different aggregation and physiological functions exhibit distinct adsorption behavior allowing them to be discriminated in preparations. ExperimentsTwo forms of amyloid oligomers, small 1–4nm and large 5–10nm were formulated using synthetic 42 amino acids β-amyloid peptide. Forms differ in their size and physiological function. A systematic study of adsorption of these amyloid species on self-assembled monolayers of octadecanethiol on gold was performed. Structural changes upon adsorption of oligomers were interrogated by the reflection absorption infrared spectroscopy. FindingsThe amount of adsorbed peptide material, as detected by surface plasmon resonance spectroscopy, is similar in case of both small and large oligomers. However, adsorption of small oligomers leads to a transformation from beta sheet rich to beta sheet depleted secondary structure. These changes were accompanied by the unique morphology patterns detectable by atomic force microscopy (AFM), while the quartz microbalance with dissipation indicated a formation of a compact adsorbate layer in case of small oligomers. These effects may be integrated and utilized in bioanalytical systems for sensing and detection of Alzheimer’s disease related peptide forms in artificial, and possibly, real preparations.

Fabrication of Octadecyl and Octadecanethiolate Self-Assembled Monolayers on Oxide-Free Si(111) with a One-Cell Process

Self-assembled monolayers (SAM) of 1-octadecene (ODE) and 1-octadecanethiol (ODT) were deposited on an oxide-free Si(111) surface with a one-cell method. The etching and SAM deposition of Si(111) were performed in one cell containing immiscible solutions in two layers: an aqueous solution of NH4F and a toluene solution of organic SAM precursors (ODE and ODT). To remove surface Si oxides, the Si(111) surface was initially etched in the lower layer of NH4F solution. The Si as etched was subsequently moved directly to the upper solution of the precursors for deposition of the SAM under illumination of white light. This one-cell approach avoids the Si surface, as etched, coming in contact with the atmosphere, so eliminating oxidation and contamination. The ODE and ODT SAM were characterized with measurements with an atomic force microscope (AFM) and X-ray photoelectron spectra (XPS). The resulting ODE SAM was more stable than the ODT SAM and exhibited satisfactory resistance to oxidation under the ambient atmosphere. The ODT SAM prepared with this one-cell method exhibited a resistance to atmospheric oxidation better than with a two-cell method.

Combined in Situ Quartz Crystal Microbalance with Dissipation Monitoring, Indirect Nanoplasmonic Sensing, and Vibrational Sum Frequency Spectroscopic Monitoring of Alkanethiol-Protected Copper Corrosion

In this study, we have applied three techniques to simultaneously and in situ study the initial stage of corrosion of copper protected by a self-assembled monolayer of octadecanethiol (ODT). We combined quartz crystal microbalance with dissipation monitoring (QCM-D), indirect nanoplasmonic sensing (INPS), and vibrational sum frequency spectroscopy (VSFS) and obtained complementary information about mass uptake and optical and spectroscopic changes taking place during the initial corrosion phase. All three techniques are very sensitive to the formation of a corrosion film (thickness in the range 0-0.41 nm) under mildly corrosive conditions (dry air, <0.5% relative humidity). The three techniques yield information about the viscoelasticity of the corrosion film (QCM-D), the homogeneity of the corrosion reaction on the surface (INPS), and the stability of the ODT protection layer (VSFS). Furthermore, by also studying the corrosion process in humid air (ca. 70% relative humidity), we illustrate how the combination of these techniques can be used to differentiate between simultaneously occurring processes, such as water adsorption and corrosion product formation.

Sum Frequency Generation Vibrational Spectroscopy of Cholesterol in Hybrid Bilayer Membranes

The assignment of the vibrational spectrum of cholesterol is surprisingly incomplete for such a fundamental molecule. To improve our understanding, a new investigation of the spectra of cholesterol in the C-H stretching region has been undertaken using the surface specific technique of Sum Frequency Generation (SFG) vibrational spectroscopy and the complementary technique of Reflection Absorption Infrared Spectroscopy (RAIRS). They were used to record the spectra of monolayers of cholesterol in hybrid bilayer membranes (HBMs). In addition to cholesterol, spectra were recorded of HBMs comprising monolayers of the partially deuterated cholesterol isotopologues d6-cholesterol and d7-cholesterol, and the cholesterol analogues cholestanol and androstanol to aid assignment of the spectra. Monolayers of each of the five molecules were used to form the distal leaflet of HBMs with the proximal leaflet consisting of a monolayer of deuterated mercaptoundecanoic acid (d-MUA) self-assembled on a gold substrate. Although cholesterol has five methyl groups and eleven methylene groups, by using molecules in which certain groups were either deuterated or entirely absent, it was possible to assign vibrational bands to specific sets of methyl or methylene groups either in the alkyl chain or sterol ring system of the molecule. Analysis of the spectra showed that the alkyl chains of cholesterol are orientated away from the substrate, which is opposite to their orientation in HBMs when the proximal leaflet is a hydrophobic self-assembled monolayer of octadecane thiol (ODT) adsorbed on gold. Additionally it was shown that in the d-MUA HBM, the α-face of the cholesterol ring is inclined toward the layer of air above the film, and the β-face is inclined toward the gold substrate.