Acid Self-assembled Monolayer Research Articles

Understanding the role of carboxylic acid surfactants in the growth inhibition effect during area-selective atomic layer deposition: the case of ZnO growth on Cu and Cu2O.

Herein, we report a detailed adsorption process of acetic acid (AA) as a model for the head group of carboxylic acid self-assembled monolayers on Cu and Cu2O (111) surfaces and the effect of diethyl zinc (DEZ) on its adsorption geometry on Cu2O (111) using quantum chemical calculations. The most stable adsorption configurations were obtained considering electrostatic potential compatibility from the molecule and surface. Overall, the adsorption behavior revealed bidentate binding as the most stable configuration. Weak van der Waals interactions are key in AA adsorption on Cu (111), while in Cu2O (111), coordination and hydrogen bonds dominated the interaction. AA adsorption geometry on Cu2O revealed that DEZ has no significative impact on the carbonyl-chemisorbed AA and bidentate adsorption modes. These results highlight the significance of the different adsorption modes for achieving area-selective deposition using atomic layer deposition and soft removal SAM molecules.

Simple, rapid and enzyme-free assay for potentiometric determination of L-cysteine based on meso-2,3-dimercaptosuccinic acid self-assembled gold electrode

As a sulfur containing α-amino acid in nature, L-cysteine plays a crucial role in a variety of metabolic pathways with many important physiological functions. However, it is still a great challenge to construct a simple and rapid approach for L-cysteine up to now. In this paper, a simple, rapid and enzyme-free method was developed for potentiometric determination of L-cysteine by using meso-2,3-dimercaptosuccinic acid self-assembled monolayer on gold electrode. The surface morphology and property of self-assembled membrane with and without L-cysteine combined were characterized by means of scanning electron microscopy, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry, respectively. The recognition mechanism may be attributed to formation of hydrogen bonds between meso-2,3-dimercaptosuccinic acid and the target molecule of L-cysteine, producing a double electric layer at the sensing interface, that the membrane potential decreases with increasing concentration of L-cysteine. The assay results showed that good linear potential response to L-cysteine in Tris-HCl buffer solution (pH=7.0) was obtained in a range of 1.0 × 10-8 to 1.0 × 10-3 mol/L, with a slope of −57.00 ± 0.63 mV/-pc (25 °C) and a detection limit of 7.9 × 10-9 mol/L. The electrode possessed fast response time (45 s), good reproducibility, and strong anti-interference performance. Moreover, the electrode can be well applied to the detection of L-cysteine in pig serums with a recovery rate of 94.3 ∼ 106.3 %, indicating a promising application prospect.

Tuning Hole-Injection in Organic-Light Emitting Diodes with Self-Assembled Monolayers.

Improving hole injection through the surface modification of indium tin oxide (ITO) with self-assembled monolayers (SAMs) is a promising method for modulating the carrier injection in organic light-emitting diodes (OLEDs). However, developing SAMs with the required characteristics remains a daunting challenge. Herein, we functionalize ITO with various phosphonic acid SAMs and evaluate the SAM-modified anodes in terms of their work function (WF), molecular distribution, coverage, and electrical conductivity. We fabricate and characterize green phosphorescent SAM-based OLEDs and compared their performance against devices based on the conventional poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) hole-injection layer. We find that the usage of [2-(3,6-diiodo-9H-carbazol-9-yl)ethyl]phosphonic acid (I-2PACz) SAM yields devices with superior performance characteristics, including a maximum luminance of ∼57,300 cd m-2 and external quantum efficiency of up to ∼17%. This improvement is attributed to synergistic factors, including the deep WF of ITO/I-2PACz (5.47 eV), the formation of larger I-2PACz molecular clusters, and the intrinsic I-2PACz dipole, that collectively enhance hole-injection.

Hydrogenated graphene systems: A novel growth and hydrogenation process

Octadecylphosphonic acid self-assembled monolayers were used as a combined carbon and hydrogen source to grow graphene films on sapphire substrates via hot filament chemical vapor deposition. The functionalized substrates were sealed with a thin Cu film and heated to 950°C under Ar flow. After synthesis, the Cu was etched away. The graphene samples then underwent a hydrogenation treatment in the same reactor setup, exposed to a CH4/H2 gas mixture at 820°C for 2 hours. The structure and properties of the graphene films before and after hydrogenation were characterized. Raman spectroscopy was employed to probe the defect-related bands and C-H bonding. X-ray diffraction provided insights into the crystalline structure and interlayer spacing. The ferromagnetic response was measured using a PPMS system across a range of temperatures and magnetic fields. XPS was used to assess the chemical composition and bonding. This multi-step process enabled a detailed evaluation of the novel synthesis protocol and its effects on the resulting hydrogenated graphene material.

Improving the performance parameters of organic field-effect transistors via alkyl chain length of boronic acid self-assembled monolayers

Interface modification is a promising technique for enhancing electrical parameters of Organic Field Effect Transistor (OFETs). In OFETs, self-assembled monolayer molecules are widely used for treatment dielectric/semiconductor interface layer. Modification of dielectric/semiconductor layer with SAM molecules ensures a variety of potential applications. Boronic acids with four different alkyl chain lengths (Cn-BA; n = 8, 10, 12, 14) molecules were used in this study to treat the Al2O3 dielectric surface in dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) based OFETs. Treated with SAMs improve the mobility of Al2O3 surfaces for linear and saturation regime and threshold voltages shifted from positive direction. The morphological and electrical characterizations were performed for fabricated OFET. The results show that alkyl-boronic acids SAM molecules open a new perspective for further optoelectronic applications due to its application for oxide surfaces and controllability.

Optimization of shell-isolated nanoparticle-enhanced Raman spectroscopy experiments with silver core-silica shell nanoparticles

The synthesized silver nanoparticles (60–70 nm) coated with a thin (5–6 nm) shell of silicon dioxide were applied for the analysis of popular 4-mercaptobenzoic acid (4-MBA) self-assembled monolayer by the shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). The detailed synthesis, purification of the nanoparticles, and optimizing washing procedure are provided. The obtained results are proved by HR-TEM, UV-Vis, and SHINERS data. The report opens up a broad possibility of using core-shell nanoparticles in SHINERS experiments, avoiding lengthy purification procedures while maintaining particle stability and Raman signal intensity.

Multifunctional Aluminum Pre-treatments from End-Functionalized Phosphonic Acid Self-Assembled Monolayers.

Aluminum alloys are used in advanced engineering applications as they possess a combination of favorable properties, including high strength, lightweightness, good corrosion resistance, machineability, and recyclability. Such applications often require forming the sheets into the final components, which is typically aided by an oil-based lubricant, followed by joining them using adhesives, which is hampered by residual lubricant. In this work, aluminum surfaces were modified with different self-assembled monolayers (SAMs), with the goal of significantly reducing the amount of lubricant while simultaneously improving friction properties, forming, and bonding performance. Our results show that SAMs terminated with hydrophilic and nucleophilic end groups give rise to high-energy surfaces with wetting properties that are stable over time. These surfaces showed significantly improved surface wetting by the lubricant, which in turn resulted in an improved forming performance at reduced lubricant coat weights. Moreover, the nucleophilic SAM termination provided outstanding performance in adhesive bonding tests under corrosive conditions.

Stabilized hole-selective layer for high-performance inverted p-i-n perovskite solar cells.

P-i-n geometry perovskite solar cells (PSCs) offer simplified fabrication, greater amenability to charge extraction layers, and low-temperature processing over n-i-p counterparts. Self-assembled monolayers (SAMs) can enhance the performance of p-i-n PSCs but ultrathin SAMs can be thermally unstable. We report a thermally robust hole-selective layer comprised of nickel oxide (NiOx) nanoparticle film with a surface-anchored (4-(3,11-dimethoxy-7H-dibenzo[c,g]carbazol-7-yl)butyl)phosphonic acid (MeO-4PADBC) SAM that can improve and stabilize the NiOx/perovskite interface. The energetic alignment and favorable contact and binding between NiOx/MeO-4PADBC and perovskite reduced the voltage deficit of PSCs with various perovskite compositions and led to strong interface toughening effects under thermal stress. The resulting 1.53-electron-volt devices achieved 25.6% certified power conversion efficiency and maintained >90% of their initial efficiency after continuously operating at 65 degrees Celsius for 1200 hours under 1-sun illumination.

Thermal Stability and Orthogonal Functionalization of Organophosphonate Self-Assembled Monolayers as Potential Liners for Cu Interconnect.

In this study, we investigated the thermal stabilities of butylphosphonic acid (BPA) and aminopropyltriethoxysilane (APTES) self-assembled monolayers (SAM) on a Si substrate. The thermal desorption and the thermal cleavage of the BPA and APTES SAM film on the Si substrate were studied by X-ray photoelectron spectroscopy (XPS) upon thermal treatment from 50 to 550 °C. XPS analyses show that the onset of the thermal desorption of the APTES monolayer occurs at 250 °C and the APTES SAM completely decomposed at 400 °C. Conversely, BPA SAM on Si shows that the onset of thermal desorption occurs at 350 °C, and the BPA SAM completely desorbed at approximately 500 °C. Our study revealed that the organophosphonate SAM is a more stable SAM in modifying the dielectric sidewalls of a Cu interconnect when compared to organosilane SAM. To overcome the spontaneous reaction of the organophosphonate film on the metal substrate, a simple orthogonal functionalization method using thiolate SAM as a sacrificial layer was also demonstrated in this study.

Soft removal of stearic acid self-assembled monolayer for area-selective atomic layer deposition

Stearic acid (SA) self-assembled monolayer (SAM) is proposed as a soft-removal passivating agent for area-selective atomic layer deposition (AS-ALD) applications. First, the SAM formation kinetics and its thermal stability were evaluated on a copper surface. We observed a highly packed, hydrophobic, and ordered monolayer. The transition temperature from order to disorder was observed above 160 °C. Next, the SA/copper was evaluated as a non-growth surface for ZnO ALD and monitored by X-ray Photoelectron Spectroscopy (XPS). The optimal ALD parameters were found at 70 °C, dosing 25 ms of diethyl zinc precursor and water, while purge was 10 s for each. In these conditions, the attenuation growth remained for at least 40 ALD cycles; despite zinc deposition after the first three cycles. Finally, the removal of the SA monolayer was investigated with water, acetone, and ethanol at room temperature. According to infrared and water contact angle, ethanol was the most effective, showing a complete removal after 30 min immersion time (without sonication or agitation). We conclude that SA SAM is an efficient removable copper passivating agent for area-selective atomic layer deposition without exposing samples to harsh conditions, like etching or acid treatments.

Molecular engineering of technologically relevant surfaces - carboxylic acids on naturally oxidized aluminum

Aluminum is a versatile, yet inexpensive metal that, when covered by oxide layer, might be employed as a gate dielectric in OFET devices, which carrier mobility could be enhanced by functionalization of dielectric by aliphatic carboxylic acid self-assembled monolayer (SAM). Although the organization of such SAMs plays an important role for the OFET performance, no systematical analysis of the impact of the preparation conditions on the quality of the carboxylate SAMs on aluminum oxide has been conducted so far, to the best of our knowledge. In the current work, we analyze, by the example of heptadecanoic acid SAM, the influence of the three crucial factors (i.e., type of solvent, immersion time, and incubation temperature) on the quality of the monolayer prepared by the standard immersion procedure. Several complementary techniques are combined to demonstrate a critical role of solvent choice in terms of monolayer order, orientation, thickness, and wettability. The results indicate in particular that the high solubility hinders the creation of high-quality SAMs, probably due to the solvent molecules incorporation into the monolayer. As a result of this study, we propose and rationalize a set of parameters enabling to obtain densely packed and well-ordered carboxylate SAMs on naturally oxidized aluminum.

An ultra-sensitive SPR immunosensor for quantitative determination of human cartilage oligomeric matrix protein biomarker

This paper reports the development of a novel surface plasmon resonance (SPR) immunosensor for ultra-sensitive quantitative determination of human articular cartilage oligomeric matrix protein (COMP), a major component of the extracellular matrix and an exploratory biomarker. Capture antibodies against human COMP (anti-COMP16F12) were covalently immobilized on an 11-mercaptoundecanoic acid (11-MUA) self-assembled monolayer (SAM)-coated SPR sensor disk and a dual sandwich-type signal amplification strategy using biotinylated detection antibodies against COMP (anti-COMP17C10-biot) and streptavidin-conjugated quantum dots (SAv‒QDs) were used for the development of an immunosensor. The binding of high-mass SAv‒QDs via biotin–streptavidin interaction to the surface of the immunosensor resulted in a drastic increase in the sensitivity. The developed immunosensor was able to detect concentrations of COMP in a range from 2.80 to 680.54 fM with a limit of detection (LOD) and a limit of quantification (LOQ) of 0.15 and 0.50 fM, respectively. The immunosensor exhibited good repeatability (relative standard deviation (RSD) 8.05%) and reproducibility (RSD 9.88%) as well as excellent operational stability (2.14 % decrease in SPR signal after 13 days). In addition, the analysis of secretomes of human knee articular cartilage explants from patients with osteoarthritis revealed that the immunosensor has good accuracy (analytical error less than 5 %). These results indicate that the immunosensor developed may be suitable for quantitative determination of COMP derived from articular cartilage and other synovial joint tissues in clinical studies.

Tripeptide Self-Assembled Monolayers as Biocompatible Surfaces for Cytochrome c Electrochemistry.

Biocompatible tripeptide self-assembled monolayers (SAMs) are designed with a carboxylate group on the terminal amino acid (glutamate, aspartate, or amino adipate) to electrostatically attract the lysine groups around the heme crevice in horse heart cytochrome c (cyt c), creating an electroactive protein/tripeptide/Au interfacial structure. Exposing the peptide/Au electrode to cyt c resulted in an 11 ± 3 pmol/cm2 electroactive protein surface coverage. Topographical images of the interfacial structure are obtained down to single-protein resolution by atomic force microscopy. Uniform protein monolayer assemblies are formed on the Au electrode with no major surface roughness changes. The cyt c/peptide/Au electrode systems were examined electrochemically to probe surface charge effects on the redox thermodynamics and kinetics of cyt c. Neutralization of protein surface charge due to adsorption on anionic COOH-terminated SAMs was found to change the formal potential, as determined by cyclic voltammetry. The cyt c/peptide/Au electrodes exhibit formal potentials shifted to more positive values, have a surface carboxylic acid pKa of 6 or higher, and produce effective cyt c surface charges (Zox) of -6 to -14. The Marcus theory is utilized to determine the protein electron transfer rates, which are ∼5 times faster for cyt c/tripeptide/Au compared to cyt c/11-mercaptoundecanoic acid SAMs of similar chain lengths.

Factors Affecting the Hydrophobic Property of Stearic Acid Self-Assembled on the TiO2 Substrate.

The self-assembled monolayer (SAM) on inorganic metal oxides is highly applicable in making different kinds of surface phenomena such as superhydrophobicity, functional group-modified surfaces, corrosion resistance, and so on. The formation of stearic acid SAMs on the TiO2 substrate depends on a few factors, and the cleanability of the substrate surface can be considered as the critical criterion for the formation of the SAM layer. The solvent, concentration of the adsorbate, immersion time, and temperature can be identified as other factors that are crucial for growing a uniform and highly dense monolayer. SAM layers always build up spontaneously on a suitable substrate, but the growth rate and arrangement can be changed by varying the external factors. These factors highly affect the chemisorption of stearic acid molecules onto the TiO2 substrate and building a well-ordered pattern on the surface without defects. This study mainly focuses on identifying the critical conditions of the external factors in obtaining a high-performance superhydrophobic surface. The crystal structure and surface morphologies of the substrate materials are characterized by powder X-ray diffraction and scanning electron microscopy, and the surface wettability is characterized by contact angle measurements. High superhydrophobicity is observed at the optimum conditions of the factors. Ethanol is used as the solvent; the temperature is about 40 °C; and 600 ppm of stearic acid is the critical concentration in obtaining a superhydrophobic surface with 100 min of immersion time, while the contact angle is 151.38°. Simultaneously, if the concentration is 1000 ppm and the immersion time is 120 min, the surface shows high superhydrophobicity with a contact angle of 162.06°. These critical conditions are found to be adequate for building well-ordered stearic acid SAMs on the TiO2 substrate.

Area-selective atomic layer deposition of Ru thin films using phosphonic acid self-assembled monolayers for metal/dielectric selectivity

• For potential AS-ALD applications, metal/dielectric selectivity was investigated. • Selective chemisorption of phosphonic acid SAMs was confirmed on TiN and W surfaces. • Blocking capability of SAMs with different ligands was evaluated against Ru ALD. • Ru AS-ALD was successfully demonstrated on metal/dielectric-patterned substrates. We comparatively investigate inhibitory efficacy of four different phosphonic acid (PA) SAMs, octadecylphosphonic acid (ODPA), octylphosphonic acid (OPA), ethylphosphonic acid (EPA), and phenylphosphonic acid (PPA) on TiN and W versus SiO 2 for technologically important metal/dielectric selectivity. Herein, blocking ability of various PA SAM-treated surfaces was evaluated by using Ru atomic layer deposition (ALD) as a representative model process toward metal/dielectric selectivity. Although chemo-selective adsorption features of PA SAMs were obtained on TiN and W relative to SiO 2 , their blocking quality was shown to improve with increasing alkyl chain lengths via strong van der Waals interactions between adjacent SAMs. With the best efficacy of ODPA SAMs, we successfully demonstrated area selective ALD (AS-ALD) of Ru thin films over 15 nm on patterned TiN/SiO 2 surfaces.

Selective Copper Electroplating on Patterned Self-Assembled Monolayers for Photovoltaic Applications

The photovoltaic industry is expected to consume 30–50% of the global silver supply by 2030, indicating the need for using other materials for solar cell metallization processes. We report herein a low-cost and scalable mask of phosphonic acid (PA) self-assembled monolayers (SAMs) on indium tin oxide (ITO) for nickel and copper electroplating on solar cells. Octadecylphosphonic acid and 12-azidododecylphosphonic acid were deposited using different procedures and characterized by cyclic voltammetry and etching resistance tests to assess the barrier properties of the SAMs. It was found that the PA chemical functionalization and the deposition method strongly affect the masking properties of SAMs. The underlying PA binding modes were investigated using attenuated total reflectance Fourier transform infrared spectroscopy, which revealed that the heterocondensation reaction between 12-azidododecylphosphonic acid and ITO was promoted during heating. Patterning of SAMs was performed using UV-C radiations to achieve selective electroplating of nickel and copper on ITO.

Investigating the substitution of intermolecular hydrogen bonds on the surface of self-assembled monolayer by scanning electrochemical microscopy

In this work, the substitution of intermolecular hydrogen bonds on the surface of 3-mercaptopropionic acid self-assembled monolayer (MPA SAM) was investigated using scanning electrochemical microscopy (SECM). • The substitution of intermolecular hydrogen bonds on the surface of MPA SAM was investigated by scanning electrochemical microscopy. • The formation of charge-assisted hydrogen bonds was more likely to induce intermolecular hydrogen bonds substitution. • All phenomena were properly characterized by the changes in current of substrate and tip electrodes. • The substitution of intermolecular hydrogen bonds is conductive to understanding the molecular selective recognition process on biofilm surface. Investigating the substitution of intermolecular hydrogen bonds on the simulated biofilm surface at molecular level is beneficial to understand the mechanism of molecular selective recognition occurring on biofilm surface. In this paper, the substitution of intermolecular hydrogen bonds on the surface of simulated biofilm, self-assembled monolayer of 3-mercaptopropionic acid (MPA SAM), is studied using scanning electrochemical microscopy (SECM). When acetic acid exists on MPA SAM surface, the intermolecular hydrogen bonds between acetic acid and MPA SAM are able to replace those between the redox mediator ferrocenemethanol (FcCH 2 OH) and MPA SAM, because the stronger charge-assisted hydrogen bonds (CAHB) formed between acetic acid and MPA SAM have the ability of substitution. The effect of the substitution of intermolecular hydrogen bonds is more significant with increasing the concentration of acetic acid within certain limits. This work may provide ideas for deeper understanding of molecular selective recognition on biofilm surface.

Parameters controlling octadecyl phosphonic acid self-assembled monolayers on titanium dioxide for anti-fouling biomedical applications

Octadecylphosphonic acid (ODPA) self-assembled monolayers (SAMs) have demonstrated potential for deterring bacterial attachment to titanium, however the coating process is time consuming and uses toxic solvents. In this study, ODPA SAM quantity, quality, and structure were evaluated on titanium quartz crystal microbalance (QCM) sensors by varying solvent type (anisole or cyclopentyl methyl ether (CPME), environmentally friendly non-polar solvents); ODPA concentration (0.5 mM or 1 mM); and temperature (21 °C or 60 °C). Surfaces were characterised using QCM frequency and dissipation measurements and Sauerbrey mass calculations; X-ray photoelectron spectroscopy; water contact angle measurements; and temperature-programmed desorption mass spectrometry (TPD-MS). Anti-fouling ability was established against Staphylococcus aureus. Incubation in 0.5 mM ODPA in CPME at 21 °C rapidly formed uniform rigid ODPA SAMs as demonstrated by high Sauerbrey mass (≈285-290 ng/cm2), viscoelastic modelling, high atomic percentage surface phosphorus (1.1%) and high water contact angles (117.6 ± 2.5°), consistent across the entire sample surface. High temperatures or the use of anisole resulted in suspected multilayer formation, which reduced bacterial attachment. TPD-MS confirmed covalent bonding of ODPA SAMs on TiO2 at ≈ 110–120 °C and thermal stability below 300 °C. This study demonstrates the key parameters that control ODPA SAM formation on titanium and their future potential for biomedical applications.

Probing Ca2+-induced electron transfer on the surface of self-assembled monolayer using SECM

The electron transfer process of protein on the surface of biomembrane is closely related to various physiological phenomena occurring on the biomembrane. Calcium cations (Ca2+), as a ubiquitous messenger, induced electron transfer of negatively charged protein on the surface of negatively charged biomembrane is a problem worth exploring. In this work, the 3-Mercaptopropionic acid self-assembled monolayer (MPA SAM) with terminal carboxyl groups was constructed on gold surface as a simplified model for simulated surface of negatively charged biomembrane. The electron transfer of negatively charged redox ion induced by Ca2+ on the surface of MPA SAM was investigated using scanning electrochemical microscopy. It was found that Ca2+ can induce electron transfer and enhance the electron transfer of negatively charged redox ions. The result is attributed to the reduction of the energy barrier of the electron transfer of negatively charged redox ion on the surface of MPA SAM because Ca2+ neutralizes the electrostatic repulsion interaction between negatively charged redox ion and negatively charged carboxylate surface of MPA SAM. The conditions of lower pH also benefit the above-mentioned electron transfer, which further confirms the mechanism of Ca2+-induced electron transfer. This work may provide a reference for studying the electron transfer process on the surface of simulated biomembrane.

Probing the interactions between collector molecules and hydrophobic graphite surfaces using chemical force microscopy

Collectors are widely used in flotation to improve the interfacial hydrophobicity of valuable minerals. Clarifying the mechanism of action is therefore a prerequisite for developing new collectors for flotation intensification. In this study, chemical force microscopy was applied to directly characterize the interaction forces between a mineral surface (hydrophobic graphite) and two collector molecules (undecane and polar undecanoic acid). An undecane/undecanoic acid self-assembled monolayer was formed on the Au-coated tip of an atomic force microscopy (AFM) cantilever through chemical deposition. Jump-in phenomena were observed between the collectors and the hydrophobic surface, indicating the existence of an attractive force, which conformed to the EDLVO theories. Various experiments indicated that the interactions between undecane and the hydrophobic surface were stronger than those involving undecanoic acid, indicating that hydrophobic forces dominated the interactions. Using molecular dynamics simulations, the adsorption energies of undecane and undecanoic acid on the graphite surface were determined to be − 17.1 ± 0.3 and − 14.1 ± 0.5 kcal/mol, respectively. The consistency between the force and the adsorption energy results demonstrated that AFM is a credible technique to investigate the interaction mechanism between a mineral and a collector at a molecular level, and so will provide critical guidance for future collector screening and design.