Self-assembled Monolayers Chain Research Articles

Probing charge transfer through antifouling polymer brushes by electrochemical methods: The impact of supporting self-assembled monolayer chain length

Ultrathin surface-tethered polymer brushes represent attractive platforms for a wide range of sensing applications in strategically vital areas such as medicine, forensics, or security. The recent trends in such developments towards “real world conditions” highlighted the role of zwitterionic poly(carboxybetaine) (pCB) brushes which provide excellent antifouling properties combined with bio-functionalization capacity. Highly dense pCB brushes are usually prepared by the “grafting from” polymerization triggered by initiators on self-assembled monolayers (SAMs). Here, multi-methodological experimental studies are pursued to elucidate the impact of the alkanethiolate SAM chain length (C6, C8 and C11) on structural and functional properties of antifouling poly(carboxybetaine methacrylamide) (pCBMAA) brush. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a custom-made 3D printed cell employing [Ru(NH3)6]3+/2+ redox probe were used to investigate penetrability of SAM/pCBMAA bilayers for small molecules and interfacial charge transfer characteristics. The biofouling resistance of pCBMAA brushes was characterized by surface plasmon resonance; ellipsometry and FT-IRRAS spectroscopy were used to determine swelling and relative density of the brushes synthesized from initiator-bearing SAMs with varied carbon chain length. The SAM length was found to have a substantial impact on all studied characteristics; the highest value of charge transfer resistance (Rct) was observed for denser pCBMAA on longer-chain (C11) SAM when compared to shorter (C8/C6) SAMs. The observed high value of Rct for C11 implies a limitation for the analytical performance of electrochemical sensing methods. At the same time, the pCBMAA brushes on C11 SAM exhibited the best bio-fouling resistance among inspected systems. This demonstrates that proper selection of supporting structures for brushes is critical in the design of these assemblies for biosensing applications.

Weak interaction engineering on thermal transport in metal-organic semiconductor nanocomposites with self-assembled monolayers

The engineering of weak interactions between metals and organic semiconductors is crucial in regulating energy carrier transport within organic semiconductor electronics. In this study, we introduce self-assembled monolayers (SAMs) to improve thermal transport in metal-organic semiconductor nanocomposites. SAM molecules with different terminal groups (-CH3, -COOH, and -C6H5) and chain lengths are utilized to modify the surface of Au nanoparticles with multiple core diameters. The introduction of SAMs significantly boosts the thermal transport of metal-organic semiconductor nanocomposites, which can be attributed to improved vibrational coupling and stronger nonbonding interactions. Furthermore, longer SAM chains broaden the area of van der Waals regions and further strengthen the intermolecular interaction across the interface, reducing the interfacial thermal transport barrier. Additionally, quantum chemistry calculation and evaluations of interfacial adhesion energy indicate that the π-π stacking interaction between TPD and SAM(-C6H5) molecules forms the best interfacial combination of nanoparticles and TPD molecules, resulting the greatest thermal transport characteristics. Finally, free energy calculations demonstrate that the variation of nanoparticle surface topology with the increase of the SAM chain length induces more obvious free energy fluctuation, which leads to tighter connections between TPD and SAM molecules under weak interaction.

Molecular Structure Effect of a Self-Assembled Monolayer on Thermal Resistance across an Interface.

Understanding heat transfer across an interface is essential to a variety of applications, including thermal energy storage systems. Recent studies have shown that introducing a self-assembled monolayer (SAM) can decrease thermal resistance between solid and fluid. However, the effects of the molecular structure of SAM on interfacial thermal resistance (ITR) are still unclear. Using the gold–SAM/PEG system as a model, we performed nonequilibrium molecular dynamics simulations to calculate the ITR between the PEG and gold. We found that increasing the SAM angle value from 100° to 150° could decrease ITR from 140.85 × 10−9 to 113.79 × 10−9 m2 K/W owing to penetration of PEG into SAM chains, which promoted thermal transport across the interface. Moreover, a strong dependence of ITR on bond strength was also observed. When the SAM bond strength increased from 2 to 640 , ITR first decreased from 106.88 × 10−9 to 102.69 × 10−9 m2 K/W and then increased to 123.02 × 10−9 m2 K/W until reaching a steady state. The minimum ITR was obtained when the bond strength of SAM was close to that of PEG melt. The matching vibrational spectra facilitated the thermal transport between SAM chains and PEG. This work provides helpful information regarding the optimized design of SAM to enhance interfacial thermal transport.

Chemical and structural characterization of EUV photoresists as a function of depth by standing-wave x-ray photoelectron spectroscopy

The success in the miniaturization of the electronic device constituents depends mostly on the photolithographic techniques. Recently, to achieve patterning at the sub-10-nm node, extreme ultraviolet (EUV) lithography has been introduced into high volume production. Continued scaling of EUV via increased numerical aperture to achieve nodes at 3-nm and below requires the development of fundamentally new patterning materials and new characterization methods. Current EUV-resist film thicknesses are in the 20- to 40-nm range, and further thickness reduction is required for the next generation. Therefore, interfaces become exceedingly important, and the properties of the resist film would be dominated by top and bottom interfacial effects. X-ray photoelectron spectroscopy (XPS) combined with standing-wave excitation (SW-XPS), a fairly new method in the EUV lithography field, previously had been largely applied in multilayers and superlattices for characterizing the composition and electronic structure of buried layers and interfaces as a function of depth. We applied the SW-XPS method to organic/inorganic photoresists to provide depth-selective information on their structural and chemical conditions of as a function of temperature, EUV exposure, different underlayers, and other fundamental parameters. As a first attempt, we perform an SW-XPS feasibility study on self-assembled monolayer (SAM) films after exposure to an electron beam. By SW-XPS, we determined that the interface between the Al2O3 underlayer and the SAMs is smooth, with a mean roughness of about 0.2 nm. Moreover, we determined that the SAM chains are, on average, tilted by ∼30 deg off the sample normal. The SW-XPS results also suggest that the SAM is not a perfectly aligned and uniform monolayer, with some areas having thickness higher than a single monolayer. We demonstrated that SW-XPS can provide useful information on ultrathin materials with high potential for being used as a characterization method of organic/inorganic photoresists.

Electrochemical and surface enhanced infrared absorption spectroscopy studies of TEMPO self-assembled monolayers

The electron transfer kinetics of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) monolayers have been studied using both a chronocoloumetric method and surface sensitive infrared spectroelectrochemistry (attenuated total reflection surface enhanced infrared absorption spectroscopy, ATR-SEIRAS). Redox active self-assembled monolayers (SAMs) are constructed by covalently attaching 4-amino-TEMPO to preformed monolayers of different chain length alkanoic acid thiol monolayers. The density of TEMPO groups on the resulting Cn-TEMPO (n = 6, 8, 11, 12, 16) SAMs is approximately 20% of a full monolayer coverage and the resulting SAMs exhibit very little kinetic heterogeneity. Standard heterogeneous rate constants are measured as a function of SAM chain length. Tafel plots constructed for n = 12 show asymmetry between the forward (oxidation of the radical to the nitrosonium cation) and reverse (reduction to the TEMPO radical) reactions. The reorganization energy of the nitrosonium cation is estimated to be 1.0 ± 0.1 eV for the C16-TEMPO SAM. ATR-SEIRAS studies reveal that the oxidation of the TEMPO radical leads to a conformational change (chair-chair flip) in the piperdinyl ring. The nitrosonium conformer pushes the N=O+ center away from the electrode and into a polar environment. Conformational differences in the reduced and oxidized species are cited as the reason for kinetic asymmetry in the TEMPO/TEMPO+ redox system. Time-resolved ATR-SEIRAS is used to extract the standard rate constant for C12-TEMPO on gold island films deposited on an indium tin oxide (ITO) surface. Standard rate constants for SAMs formed on Au@ITO are found to be higher than those formed on gold beads due to greater kinetic heterogeneity caused by the larger roughness of the gold islands.

Improving thermal conduction across cathode/electrolyte interfaces in solid-state lithium-ion batteries by hierarchical hydrogen-bond network

Effective thermal management is an important issue to ensure safety and performance of lithium-ion batteries. Fast heat removal is highly desired but has been obstructed by the high thermal resistance across cathode/electrolyte interface. In this study, self-assembled monolayers (SAMs) are used as the vibrational mediator to tune interfacial thermal conductance between an electrode, lithium cobalt oxide (LCO), and a solid state electrolyte, polyethylene oxide (PEO). Embedded at the LCO/PEO interface, SAMs are specially designed to form hierarchical hydrogen-bond (H-bond) network with PEO. Molecular dynamics simulations demonstrate that all SAM-decorated interfaces show enhanced thermal conductance and dominated by H-bonds types. The incorporation of poly(acrylic acid) (PAA) SAM drastically enhances interfacial thermal conductance by approximately 211.69%, largely due to the formation of a strong H-bond, -COOH···:O, between PAA and PEO. Even with weaker H-bonds such as -OH···:O, it still outperforms the pristine interface as well as interfaces decorated with non-H-bonded SAMs, e.g. PE. Such improvement is attributed to the unique hierarchical H-bond network at the interface, which removes discontinuities in temperature field, straighten SAM chains, make materials strongly adhere, and couple the vibrational modes of materials. The study is expected to guide surface engineering for more effective thermal management in lithium-ion batteries.

An atom probe analysis of self-assembled monolayers: A novel approach to investigate mixed and unmixed self-assembled monolayers (SAMs) on gold

Atom probe tomography was used to analyze self-assembled monolayer of octanethiol and (1H,1H,2H,2H) perfluoro decanethiol (PFDT) in a mixed and unmixed state on a gold surface. The first part of this report, we have analyzed the two SAMs with a direct comparison, whereas in the second part, a binary mixture of these two SAMs (with a 1:1 ratio) are analyzed. Au was taken as a metal substrate in both cases. It has been observed that, although having similar structure and belongs from the same SAMs group, the sequence of evaporation of both chains appeared significantly different molecular fractions in the first case, while in the mixed phase condition the two SAMs chains were arranged in a patchy domain by forming a small nanoscale cluster on the gold surface. Primarily, this study gives an overall understanding of evaporation sequence and surface morphology of the SAMs in an unmixed and mixed condition on the gold substrate, which may lead to the further design process in various applications.

Nucleation and adhesion of ultra-thin copper films on amino-terminated self-assembled monolayers

In this work, we report on the effect of amino-terminated self-assembled monolayers (SAMs) on the growth and adhesion of copper on a dielectric surface in ultra-high vacuum. The nucleation and adhesion of copper is studied for a range of self-assembled monolayers both with and without nitrogen containing terminal groups, and as a function chain length using X-ray photoelectron spectroscopy, dynamic water contact angle, sheet resistance, and adhesion testing measurements.In-situ X-ray photoelectron spectroscopy studies of ultra-thin copper films show that the presence of nitrogen significantly improves the nucleation of copper to the surface, particularly those coated with long chain SAMs. However, upon thermal annealing short chain amino-terminated SAMs retain much of the deposited copper while significant desorption occurs for longer chains.Results consistent with these observations are obtained during conventional tape test measurements to determine adhesion. As such, for CMOS interconnect applications which require copper trenches with a nano-scale cross section, short chain SAMs offer excellent nucleation and adhesion, as well as the potential to act as a pore-sealant for low-k materials, without impacting significantly on the cross-sectional area of the copper lines.

The Effect of an Antigalvanic Reduction of Silver on Gold for the Stability of a Self-Assembled Alkanethiol Monolayer and Chemical Lift-Off Lithography

The effects of silver antigalvanic reduction (Ag AGR) on the structure of alkanethiol self-assembled monolayers (SAMs) on gold with regard to the ordered structure and surface coverage were studied using electrochemical methods. In this paper, we report the structure changes as well as the chemical lift-off lithography (CLL) performance of SAMs dependent on chain lengths, namely, 1-butanethiol, 6-mercapto-1-hexanol, and 11-mercapto-1-undecanol. Cyclic voltammetry of the redox probe and linear sweep voltammetry of the reductive desorption demonstrated that the structure of SAMs with Ag AGR pretreatment are more ordered, resulting from higher surface coverage and stronger molecular interaction. Meanwhile, the influence of Ag AGR on the CLL efficiency behaved differently depending on the SAM chain lengths in both electrochemistry and AFM imaging. In general, the formation of alkanethiol SAMs on Ag AGR/Au has enabled a facile method, which possesses a great potential on the quality control of the SAMs system.

New self assembly monolayer onto SiGe as a selective biosensor for single-strand DNA

An important step of fabrication of selective DNA probe/receptor is the functionalization of semiconducting surfaces with a self-assembled monolayer (SAM) with an appropriate surface termination to interact with DNA. In this work, we studied an immobilization of single-strand DNA (ssDNA) onto self-assembly monolayer of both the heavily exploited aminopropyltriethoxy silane (APTES) and n-(2-aminoethyl)-11-aminoundecyltrimethoxysilane (NAATS), a bi-functional amino silane onto the surface of SiGe substrate. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) techniques were employed to characterize the silanization of SAMs on SiGe surface. The immobilization process was confirmed using fluorescence intensity measurements. The coverage values of APTES and NAATS SAMs were estimated to be 71.3 and 83 XPS% at 78°, respectively from the C/Ge ratio. Comparing the performance towards both specific and non-specific ssDNA complements for both silanes, it was observed that a higher selectivity and sensitivity was obtained by using NAATS SAM. This work highlighted the importance of SAM chain length by comparing one short alkyl chains, APTES with that long-chain counterpart, NAATS.

InGaZnO Thin-Film Transistors Modified by Self-Assembled Monolayer With Different Alkyl Chain Length

InGaZnO (IGZO) thin-film transistors (TFTs) modified by self-assembled monolayers (SAMs) based on triethoxysilane (TES) with three different alkyl chain lengths were fabricated and the relationship between the SAM chain length and the TFT electrical properties was investigated. The mobility increased and the hysteresis of transfer curves was reduced after SAM-modification, owing to less adsorption/ desorption effect on the IGZO surface. IGZO-TFTs modified by TES with longer alkyl chain lengths exhibited better electrical performance, which was attributed to the formation of a more hydrophobic and higher ordered monolayer due to the cohesive interaction between the molecules.

Molecular simulations of mixed self-assembled monolayer coated gold nanoparticles in water.

Molecular dynamics simulations have been employed to study the hydration of a series of nanoparticles, each of which was coated with a mixed self-assembled monolayer (SAM) comprising methyl- and hydroxy-terminated alkane thiol chains. The mixing ratio of those chains are different for each nanoparticle. The simulations focused on the wetting behavior of the SAM-coated gold nanoparticles and the distribution and structure of their interfacial water molecules. The interactions of the mixed SAM-coated gold nanoparticles with water were analyzed by evaluating the radial distribution function, hydrogen bonds, the dipole orientations of the water molecules, and the water residence time in the interfacial region. The wettability of the mixed SAM-coated gold nanoparticles improved as the concentration of terminal hydroxy moieties was increased. The distribution and dynamics of the interfacial water molecules were found to be influenced by the mixing ratio of the terminal moieties of the SAM chains. The results of our simulations suggest that the surface interactions of the mixed SAM-coated gold nanoparticles with the aqueous medium can be modulated by systematically altering the mixing ratio of the terminal methyl and hydroxy moieties. This work may lead to new biological and technological applications and inspire the development of novel biomimetic materials. Graphical Abstract Mixed SAM-coated gold nanoparticles.

Construction of Binding Group Terminated Self-Assembled Monolayers for Multilayer Formation

Introduction. The self-assembly technique is one of the best methods to arrange the molecule in order onto the solid substrate surface and the alkylthiol self-assembled monolayers (SAMs) on gold and alkyl SAMs on silicon have been widely studied because of their high stability, high packing density, and high orientation [1,2]. Thus, they are expected to apply to the modern nanotechnology such as sensors, wetting control, corrosion inhibition, and biomolecular and molecular electric devices [1,2]. Moreover, using the binding group terminated SAM as a substrate, multilayer with various functionalities can be constructed. In order to control the multilayer thickness and functionality, we have to construct highly oriented and well-packed SAMs with binding group as a terminal group [3]. In this study, we constructed four kinds of binding group terminated SAMs such as 4-mercaptobenzoic acid (4-MBA) and 3-mercaptopropionic acid (3-MPA) SAMs on Au(111) and 2-methoxy ethyl (2-ME) and 11-ethoxy undecyl (11-EU) SAMs on Si(111) by various preparation conditions and their characteristics were estimated by electrochemical measurements for the reductive desorption, angle-resolved X-ray photoelectron spectroscopy (ARXPS), attenuated total reflection - Fourier transformed infrared (ATR-FTIR) spectroscopy, and contact angle measurements. Experimentals. After Au(111) single crystal disks were annealed by a gas burner and quenched with ultrapure water, they were immersed in ethanol solution containing 1 mM 4-MBA or 1 mM 3-MPA for certain periods at room temperature. From the results of the electrochemical measurements for the reductive desorption and of ARXP specta, density, stability, and orientation of the constructed 4-MBA and 3-MPA SAMs were investigated in order to find the optimum preparation conditions of the densely-packed and well-oriented SAMs on Au(111). After etching of n-typed Si(111) prisms, 2-ME and 11-EU SAMs were prepared on Si(111) by dipping Si prism into liquid state of 2-ME or 11-EU under UV light irradiation and nitrogen atmosphere for several hours. From the results of the ATR-FTIR spectra and contact angle measurement, density, stability, and orientation of the constructed 2-ME and 11-EU SAMs were investigated in order to find the optimum preparation conditions of the densely-packed and well-oriented SAMs on Si(111). Results and Discussion. Linear sweep voltammograms (LSVs) measured in a deaerated 0.1 M KOH at Au(111) electrodes modified with the 4-MBA SAMs, which were prepared for several dipping periods, showed cathodic peaks due to the reductive desorption of 4-MBA, around -0.60 V. With increasing of the dipping period, the peak potential shifted to negative, namely stability increased, and the surface coverage of 4-MBA, namely density, increased. However, full width of half maximum (fwhm) of the cathodic peak, which shows homogeneity of SAM’s orientation, decreased until the dipping period of 1 h and then increased with increasing of the dipping period. This result indicated that repulsion interaction between carboxylate groups should increase with increasing of the dipping period, namely density. For 5 h dipping, the surface coverage was saturated to 0.52 nmol cm-2. From the XP spectrum of the 4-MBA SAM, we confirmed that the 4-MBA SAM prepared by the dipping period of 5 h is highly stable. As compared with 4-MBA SAM on Au(111) prepared for the same dipping period, 3-MPA SAM on Au(111) was less stable, less densely-packed, and less highly oriented, because van der Waals interaction between 4-MBA molecules, namely hydrophobic interaction between phenyl groups, should be stronger than that between 3-MPA, namely hydrophobic interaction between propyl chains. Modification of metal complex molecules such as metallo-porphyrin and metallo-phthalocyanine on the 4-MBA SAM modified Au(111) is now under investigation. On the other hand, 2-ME and 11-EU SAMs on Si(111) was relatively stable, because C-Si bonding energy is higher than S-Au bonding energy. As compared with 2-ME SAM on Si(111) for the same preparation period, 11-EU SAM was more stable, more densely-packed, and more highly oriented on Ai(111), because van der Waals interaction between 11-EU, namely hydrophobic interaction between longer chains, should be higher than those between shorter chains. Now we are trying to find preparation condition to construct more stable, densely-packed, and highly oriented 2-ME SAM on Si(111) because shorter chain SAM is easy to use as a substrate for multilayer formation. References. [1] A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly, Academic Press, New York (1990).[2] T. Kondo, R. Yamada, and K. Uosaki, in Organized Organic Ultrathin Films – Fundamentals and Applications, Wiley-VCH, Chap 2 (2012).[3] T. Arisaka, B. Zhang, N. Aoki, and T. Kondo, 224th ECS Meeting, San Francisco, #44 (2013).

A simulation study on the thermal and wetting behavior of alkane thiol SAM on gold (111) surface

Molecular dynamics simulations have been performed to investigate the structural, thermal and wetting properties of self-assembled monolayer (SAM) of alkane thiol on gold surface. The specific heat capacity of the gold SAM surface was found to linearly increase with the temperature in the range 100–300K. It was found to drop down at 400K and this decrease might be attributed to the disorder of the SAM chains. Hydration of gold SAM surface for two different terminal groups, namely methyl (hydrophobic), and hydroxy (hydrophilic) was studied at room temperature. The difference in their wetting behavior and the structure of their interfacial water were examined from the estimation of the z density profile, radial distribution function, hydrogen bonds and orientation of water dipoles in the interfacial region. The present simulation results suggest that the wetting behavior of the gold SAM surface can be modified by altering the terminal functional group of the SAM chains.

Vascular endothelial growth factor attachment to hydroxyapatite via self-assembled monolayers promotes angiogenic activity of endothelial cells

Currently, tissue engineered constructs for critical sized bone defects are non-vascularized. There are many strategies used in order to promote vascularization, including delivery of growth factors such as vascular endothelial growth factor (VEGF). In this study, hydroxyapatite (HA) was coated with self-assembled monolayers (SAMs). The SAMs were in turn used to covalently bind VEGF to the surface of HA. The different SAM chain length ratios (phosphonoundecanoic acid (11-PUDA):16-phosphonohexadecanoic acid (16-PHDA) utilized in this study were 0:100, 25:75, 50:50, 75:25, and 100:0. Surfaces were characterized by contact angle (CA) and atomic force microscopy, and an in vitro VEGF release study was performed. It was observed that CA and root-mean-squared roughness were not significantly affected by the addition of SAMs, but that CA was significantly lowered with the addition of VEGF. VEGF release profiles of bound VEGF groups all demonstrated less initial burst release than adsorbed control, indicating that VEGF was retained on the HA surface when bound by SAMs. An in vitro study using human aortic endothelial cells (HAECs) demonstrated that bound VEGF increased metabolic activity and caused sustained production of angiopoietin-2, an angiogenic marker, over 28days. In conclusion, SAMs provide a feasible option for growth factor delivery from HA surfaces, enhancing angiogenic activity of HAECs in vitro.

Nanotribology of self-assembled monolayer with a probe tip investigated using molecular dynamics simulations

The nanotribology of an alkanethiol self-assembled monolayer (SAM) under tilt contact with a scanning probe tip is studied using molecular dynamics (MD) simulations. The tilt contact is described in terms of the tilt angle and the magnitude of the specimen–tip separation. The effects of tilt angle and magnitude of the specimen–tip separation on the normal force, friction force, friction coefficient, shear strength of the tip–SAM junction, and self-recovery characteristics are evaluated during the scanning probe tip process at a temperature of 300K. The simulation results clearly show that the magnitudes and periods of the normal force and friction force increase with decreasing magnitude of the specimen–tip separation due to a large change of the tilt angle of the SAM chains during the deformation and recovery stages. For scanning and indentation processes, the effect of the tilt angle of the probe tip on the normal force is more significant than that on the friction force for the SAM. The behaviors of interfacial contact forces, friction coefficient, and shear strength strongly depend on the number of interacting atoms and the contact area, which increases with decreasing magnitude of the specimen–tip separation and increasing tilt angle of the probe tip. The self-recovery of SAM is significantly affected by the magnitude of the specimen–tip separation; the recovery ability of SAM is worse for magnitude of the specimen–tip separation below −0.9nm with a large tilt angle of the probe tip.

Single Molecule Dynamics on Hydrophobic Self-Assembled Monolayers

The interactions between adsorbate molecules and hydrophobic surfaces are of significant interest due to their importance in a variety of biological and separation processes. However, it is challenging to extrapolate macroscopic ensemble-averaged force measurements to molecular-level phenomena. Using total internal reflection fluorescence microscopy to image individual molecules at hydrophobic solid-aqueous interfaces, we directly observed dynamic behavior associated with the interactions between fluorescently labeled dodecanoic acid (our probe molecules) and self-assembled monolayers (SAM) comprising n-alkyltriethoxysilanes with systematically increasing chain length (from n = 4-18). In all cases, we observed at least two characteristic surface residence times and two diffusive modes, suggesting the presence of multiple distinct adsorbed populations. In general, the mean surface residence time increased and the mobility decreased with increasing SAM chain length, consistent with stronger probe-surface interactions. However, these trends were not primarily due to changes in characteristic residence times or diffusion coefficients associated with the individual populations but rather to a dramatic increase in the fraction associated with the long-lived slow-moving population(s) on long-chain SAMs. In particular, on longer (16-18 carbon) alkylsilane monolayers, the probe molecule exhibited far fewer desorption-mediated "flights" than on short (4-6 carbon) monolayers. Additionally, probes on the longer chain surfaces were much more likely to exhibit extended surface residence times as opposed to short transient surface visits.

Study of Insulating Properties of Alkanethiol Self-Assembled Monolayers Formed Under Prolonged Incubation Using Electrochemical Impedance Spectroscopy

The electrochemical interfacial properties of a well-ordered self-assembled monolayer (SAM) of 1-undecanethiol (UDT) on evaporated gold surface have been investigated by electrochemical impedance spectroscopy (EIS) in electrolytes without a redox couple. Using a constant-phase element (CPE) series resistance model, prolonged incubation times (up to 120 h) show decreasing monolayer capacitance approaching the theoretical value for 1-undecanethiol. Using the CPE exponent α as a measure of ideality, it was found that the monolayer approaches an ideal dielectric (α = 0.992) under prolonged incubation, which is attributed to the reduction of pinholes and defects in the monolayer during coalescence and annealing of SAM chains. The SAMs behave as insulators until a critical potential, Vc, is exceeded in both cathodic and anodic regimes, where electrolyte ions are believed to penetrate the monolayers. Using a Randles circuit model for these cases, the variation of the capacitance and charge transfer resistance with applied dc potential shows decreased permeability to ionic species with prolonged incubation time. The EIS data show that UDT (methylene chain length n = 10), incubated for 120 h, forms a monolayer whose critical voltage range extends from −0.3 to 0.5 V versus Ag/AgCl, previously attained only for alkanethiol at n = 15. At low frequencies where ion diffusion occurs, almost pure capacitive phase (−89 deg) was attained with lengthy incubation.

Characterizing the Molecular Order of Phosphonic Acid Self-Assembled Monolayers on Indium Tin Oxide Surfaces

Self-assembled monolayers (SAMs) of alkanephosphonic acids with chain lengths between 8 and 18 carbon units were formed on thin films of indium tin oxide (ITO) sputter-deposited on silicon substrates with 400 nm thermally grown SiO(2). The silicon substrates, while not intended for use in near-IR or visible optics applications, do provide smooth surfaces that permit systematic engineering of grain size and surface roughness as a function of the sputter pressure. Argon sputter pressures from 4 to 20 mTorr show systematic changes in surface morphology ranging from smooth, micrometer-sized grain structures to <50 nm grains with 3× higher surface roughness. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy experiments are conducted for alkanephosphonic acids deposited on these wide range of ITO surfaces to evaluate the effects of these morphological features on monolayer ordering. Results indicate that long-chain SAMs are more highly ordered, and have a smaller tilt angle, than short-chain SAMs. Surprisingly, the 1-octadecyl phosphonic acids maintain their order as the lateral grain dimensions of the ITO surface shrink to ∼50 nm. It is only when the ITO surface roughness becomes greater than the SAM chain length (∼15 Å) that SAMs are observed to become relatively disordered.

Surface Barrier Properties of Self-Assembled Monolayers as Deduced by Sum Frequency Generation Spectroscopy and Electrochemistry

This paper presents a model on the electrochemical behaviors of n-alkanethiol self-assembled monolayers (SAMs) on gold electrodes with different chain length (C10, C12, C16, and C18) using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and sum frequency generation spectroscopy (SFG). The SAMs studied reached their maximum density with a deposition time of ≈72 h according to the kinetics results shown by EIS Bode phase plots. For shorter chain SAMs, C10 and C12 monolayers, the results from CV, EIS, and SFG were consistent with each other. Significant amounts of thiols were removed by reductive desorption of the films for C10 and C12 SAMs, as indicated by decreases in the electrochemical waves and SFG peaks. Surprising differences were observed for longer-chain SAMs made from C16 and C18 monolayers. The CV results suggested that a large amount of the C16 and C18 monolayers chemically desorbed from the gold surface, while the EIS and SFG results illustrated that C16 and C18 layers sti...