Adsorption Of Self-assembled Monolayers Research Articles

Eelectrochemical Evaluation of Self-assembled Monolayers of N-Heterocyclic Carbenes on Gold and DFT Studies

Three N-heterocyclic carbenes (NHCs) are synthesized and their self-assembled monolayers (SAMs) are built up on gold substrate. Electrochemical impedance spectroscopy (EIS) tests show that the NHC-based SAMs exhibit high stability in 0.1 M HCl solution, and the loss of NHC molecules on gold surface is less than 3% after immersion of 32 h. The unsaturated level of heterocyclic moiety has a negligible effect on the stability of NHC-SAMs, which is attributed to the special bonding mechanism between NHCs and gold atom. Theoretical calculations reveal that the bonding of NHC molecule is dominantly contributed by the interaction of the d orbital of gold atom with the molecular orbitals mainly distributing on carbene carbon atom of NHC, such as HOMO-1 (σ1) and HOMO-2 (π1), and the σ-type interaction accounts for the main state of bonding in comparison to the π-type one. Those molecular orbitals distributing on heterocyclic moiety nearly take no part in the bonding, and the interaction of isopropyl substituents on N atoms with gold surface is not observed in this study. In addition, it is confirmed that EIS measurement can both sensitively and quantitatively examine the adsorption and desorption of SAMs on gold surface.

Data-Driven Prediction of Protein Adsorption on Self-Assembled Monolayers toward Material Screening and Design.

We attempt to predict the water contact angle (WCA) of self-assembled monolayers (SAMs) and protein adsorption on the SAMs from the chemical structures of molecules constituting the SAMs using machine learning with an artificial neural network (ANN) model. After training the ANN with data of 145 SAMs, the ANN became capable of predicting the WCA and protein adsorption accurately. The analysis of the trained ANN quantitatively revealed the importance of each structural parameter for the WCA and protein adsorption, providing essential and quantitative information for material design. We found that the degree of importance agrees well with our general perception on the physicochemical properties of SAMs. We also present the prediction of the WCA and protein adsorption of hypothetical SAMs and discuss the possibility of our approach for the material screening and design of SAMs with desired functions. On the basis of these results, we also discuss the limitation of this approach and prospects.

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.

Formation and Structure of Highly Ordered Self-Assembled Monolayers by Adsorption of Acetyl-Protected Conjugated Thiols on Au(111) in Tetrabutylammonium Cyanide Solution

The surface structure and binding conditions of self-assembled monolayers (SAMs) on Au(111) derived from 1-acetylthio-4-[(phenyl)ethynyl]benzene (OPE2-SAc) without and with tetrabutylammonium cyanide (TBACN) as a deprotection reagent were examined using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM observation revealed that OPE2-S SAMs on Au(111) formed from direct adsorption of OPE2-SAc in 1 mM methanol solution at room temperature (RT) for 24 h were composed of short-range, ordered phase separated by a disordered phase. In contrast, adsorption of OPE2-SAc SAMs on Au(111) at a higher solution temperature of 50 °C for 24 h led to formation of a fully ordered phase with slightly increased domain size. The structural quality of OPE2-S SAMs on Au(111) was remarkably enhanced when TBACN was used. OPE2-S SAMs at RT had a well-ordered (√3 × √7)R30° structure with a domain size larger than 80 nm. The SAMs deposited at 50 °C contained very uniform and highly ordered domains w...

New insights in self-assembled monolayer of imidazolines on iron oxide investigated by DFT

Adsorption of self-assembled monolayer (SAM) of different imidazolines derivatives was investigated on Fe2O3 (0 0 0 1) surface with DFT calculations to understand their corrosion inhibition mechanism. Quantum chemistry calculations enable us acquiring a better insight on the nature of imidazoline/surface interactions, i.e. partly covalent bonds, and self-organization of imidazoline ring on the surface than the previous force field-based studies. The SAM is composed of a network of H-bonded inhibitors. The influence of the alkyl chain length of imidazoline derivatives, which form a hydrophobic barrier above the surface, on the energetic and structural properties of the SAM was evaluated.

Significant improvement in the electrical characteristics of Schottky barrier diodes on molecularly modified Gallium Nitride surfaces

III-Nitride semiconductors face the issue of localized surface states, which causes fermi level pinning and large leakage current at the metal semiconductor interface, thereby degrading the device performance. In this work, we have demonstrated the use of a Self-Assembled Monolayer (SAM) of organic molecules to improve the electrical characteristics of Schottky barrier diodes (SBDs) on n-type Gallium Nitride (n-GaN) epitaxial films. The electrical characteristics of diodes were improved by adsorption of SAM of hydroxyl-phenyl metallated porphyrin organic molecules (Zn-TPPOH) onto the surface of n-GaN. SAM-semiconductor bonding via native oxide on the n-GaN surface was confirmed using X-ray photoelectron spectroscopy measurements. Surface morphology and surface electronic properties were characterized using atomic force microscopy and Kelvin probe force microscopy. Current-voltage characteristics of different metal (Cu, Ni) SBDs on bare n-GaN were compared with those of Cu/Zn-TPPOH/n-GaN and Ni/Zn-TPPOH/n-GaN SBDs. It was found that due to the molecular monolayer, the surface potential of n-GaN was decreased by ∼350 mV. This caused an increase in the Schottky barrier height of Cu and Ni SBDs from 1.13 eV to 1.38 eV and 1.07 eV to 1.22 eV, respectively. In addition to this, the reverse bias leakage current was reduced by 3–4 orders of magnitude for both Cu and Ni SBDs. Such a significant improvement in the electrical performance of the diodes can be very useful for better device functioning.

Adsorption of self-assembled monolayer on Cu(111): First-principles study

The density function theory is used to explore the structures of alkyl-thiolate (RS, R=CH3, CF3) monolayer on the Cu(111) surface. By performing the total energy calculations for RS at three possible adsorption sites (fcc, hcp, bridge) with five different coverages (1/12, 1/9, 1/6, 1/4, 1/3), we obtained the stable adsorption configurations of the Cu–RS system. Especially, the effect of Van der Waals interaction on the adsorption configurations was studied by the DFT-D2 method. The work functions for Cu–RS (R=CH3, CF3) systems were calculated, we find that the CH3S adsorbed on the Cu(111) surface decreases the metal work function remarkably, and the work functions strongly depend on the coverage. In the case of the Cu–CF3S system, the results are just the opposite. Thus, controlling the kind and coverage of the surface adsorbates would be an effective technique to tune the work function of the metal.

Mesoscopic Simulations of Adsorption and Association of PEO-PPO-PEO Triblock Copolymers on a Hydrophobic Surface: From Mushroom Hemisphere to Rectangle Brush.

The dissipative particle dynamics (DPD) method is used to investigate the adsorption behavior of PEO-PPO-PEO triblock copolymers at the liquid/solid interface. The effect of molecular architecture on the self-assembled monolayer adsorption of PEO-PPO-PEO triblock copolymers on hydrophobic surfaces is elucidated by the adsorption process, film properties, and adsorption morphologies. The adsorption thicknesses on hydrophobic surfaces and the diffusion coefficient as well as the aggregation number of Pluronic copolymers in aqueous solution observed in our simulations agree well with previous experimental and numerical observations. The radial distribution function revealed that the ability of self-assembly on hydrophobic surfaces is P123 > P84 > L64 > P105 > F127, which increased with the EO ratio of the Pluronic copolymers. Moreover, the shape parameter and the degree of anisotropy increase with increasing molecular weight and mole ratio of PO of the Pluronic copolymers. Depending on the conformation of different Pluronic copolymers, the morphology transition of three regimes on hydrophobic surfaces is present: mushroom or hemisphere, progressively semiellipsoid, and rectangle brush regimes induced by decreasing molecular weight and mole ratio of EO of Pluronic copolymers.

Electrical resistivity of nanoporous gold modified with thiol self-assembled monolayers

The electrical resistivity of nanoporous gold (NPG) modified with thiol self-assembled monolayers (SAMs) has been measured at 298K using a four-probe method. We found that the adsorption of thiol SAMs increases the electrical resistivity of NPG by up to 22.2%. Dependence of the electrical resistivity on the atmosphere (air or water) was also observed in SAMs-modified NPG, suggesting that the electronic states of the tail groups affect the electrons of the binding sulfur and adjacent surface gold atoms. The present results suggest that adsorption of thiol molecules can influence the behavior of the conducting electrons in NPG and that modification of NPG with SAMs may be useful for environmental sensing.

Chemisorption Threshold of Thiol-based Monolayer on Copper: Effect of Electric Potential and Elevated Temperature

Coating of self-assembled monolayer (SAM) on metal surfaces for corrosion protection and adhesion promotion is gaining increasing interest. One of the major obstacles for this technology is the time required for SAM deposition. Preparing SAMs electrochemically has recently come to prominence due to the time reduction of the SAM deposition process. Additionally, the externally applied electric potentials are critically important in enhancing the quality and the surface coverage of SAM on metal substrates. In this work, thiol-based SAM adsorption was investigated under various applied electric potential values and its impact on the adsorption rate constant was studied. The results demonstrated that there exists a SAM adsorption potential threshold in which the adsorption rate constant kt could be one order higher than its normal values. At an applied potential near potential of zero charge (ϕpzc), the charge on the Cu electrode surface is close to neutral, making adsorption of SAM energetically favorable. A SAM mass transport model under the influence of an external energy field was proposed in which both calculated and experimental results were compared and discussed. These findings are shown to provide key mechanistic insight and help rationalize the selection of the externally applied electric potential in order to enhance the SAM adsorption process.

Adsorption of Alkylthiol Self-Assembled Monolayers on Gold and the Effect of Substrate Roughness: A Comparative Study Using Scanning Tunneling Microscopy, Cyclic Voltammetry, Second-Harmonic Generation, and Sum-Frequency Generation

Self-assembled monolayers (SAMs) of alkylthiols on gold have called considerable attention due to several applications in the modification and control of surface properties, such as wettability, tribology, and biocompatibility. Therefore, understanding the adsorption and molecular structure of these SAMs is crucial to optimize their quality for a given application. Although many studies in this area have been performed, the effects of substrate morphology on the quality of long chain alkylthiols on gold have not been satisfactorily clarified. This study focuses on the adsorption and conformation of SAMs of 1-hexadecanethiol (C16SH) as a function of adsorption time, substrate roughness, and morphology. The adsorption of C16SH on atomically flat Au surfaces was analyzed via cyclic voltammetry (CV), scanning tunneling microscopy (STM), and second-harmonic generation (SHG), while the molecular conformation and orientation were characterized via sum-frequency generation (SFG) vibrational spectroscopy. We show ...

Electrochemical study of self-assembled monolayer adsorption

The electrochemical behaviour of self-assembled monolayer (SAM) of aliphatic hexadecanethiol was studied by cyclic voltammetry (CV), elimination voltammetry with linear scan (EVLS) and crystal quartz microbalance (QCM). SAMs were electrochemically created on gold-coated QCM crystal through the sulphur in 1-hexadecanethiol molecule head group. The effect of thiol concentration and potential scan rate on the SAM formation was studied. Formation of SAM was confirmed by CV and QCM. EVLS results revealed the kinetically controlled process followed with electrode reaction in adsorbed state characteristic for SAM formation at lower concentration. The electrode reaction of a totally adsorbed electroactive species was indicated by means of a peak-counter peak signal at higher thiol concentration.

Electrochemical fabrication of transparent nickel hydroxide nanostructures with tunable superhydrophobicity/superhydrophilicity for 2D microchannels application

In order to survive in the harsh environments, natural creatures evolve unique wettability properties, for example, lotus leaf exhibits extreme water-repellency and Namib Desert beetle displays high wetting contrast patterns. Inspired by these natural creatures, a multifunctional surface which owns superhydrophobicity, widely tunable wettability (i.e. between superhydrophilic and superhydrophobic) and non-degraded transparency is developed. A facile and cost-effective approach via stepwise electrodeposition of nanosponge-like nickel hydroxide with the aid of self-assembled monolayers (SAMs) of non-fluorinated silane is introduced to well manipulate the pore size and surface energy. The exposed hydroxide ligands of nickel hydroxide films enhance water penetration and SAMs adsorption and thereby achieve tunable wettability from superhydrophilic to superhydrophobic. Meanwhile, the nanoscale roughness and sub-wavelength pore size of the films help maintain their optical transparency. The effectiveness of these achieved functions is demonstrated on the transparent 2D microfluidic channel fabricated by a simple electrochemical attach–detach patterning technique, which shows a fast flow speed of 5 mm s−1 because of the strong capillary force.

Tuning Contact Recombination and Open-Circuit Voltage in Polymer Solar Cells via Self-Assembled Monolayer Adsorption at the Organic–Metal Oxide Interface

We adsorbed fluorinated-alkyl and hydrogenated-alkyl phosphonic acid derivatives onto indium tin oxide (ITO) to form self-assembled monolayers (SAMs). Polymer solar cells having these treated ITOs as anodes display open-circuit voltages (Vocs) that are higher than those with bare ITO as anodes. Although the work function of ITO can be significantly tuned by SAM adsorption, the position of the Fermi level of the anode with respect to the hole transport level in the polymer active layer is essentially the same in all of the devices, suggesting that changes in the work function of the anode are not responsible for the Voc variation. Rather, the barrier for minority carrier transport to ITO is altered through SAM adsorption. The adsorption of fluorinated-alkyl phosphonic acid on ITO, in particular, induces a barrier of 2.4 eV for minority carrier transport, which effectively increases carrier selectivity at the anode and increases the Voc in polymer solar cells comprising such treated ITO as anodes compared to those with untreated anodes.

Colloidal Stability of Self-Assembled Monolayer-Coated Gold Nanoparticles: The Effects of Surface Compositional and Structural Heterogeneity

Surface heterogeneity plays an important role in controlling colloidal phenomena. This study investigated the self-aggregation and bacterial adsorption of self-assembled monolayer coated gold nanoparticles (AuNPs) with different surface compositional and structural heterogeneity. Evaluation was performed on AuNPs coated with (1) one ligand with charged terminals (MUS), (2) two homogeneously distributed ligands with respectively charged and nonpolar terminals (brOT) and (3) two ligands with respectively charged and nonpolar terminals with stripe-like distribution (OT). The brOT particles have less negative electrophoretic mobility (EPM) values, smaller critical coagulation concentration (CCC) and larger adsorption rate on Escherichia coli than that of AuNPs with homogeneously charged groups, in good agreement with DLVO predictions. Although the ligand composition on the surface of AuNPs is the same, OT particles have less negative EPM values and faster rate of bacterial adsorption, but much larger CCC compared to brOT. The deviation of OT particles from brOT and MUS in their self-aggregation behavior reflects the effects of surface heterogeneity on electrical double layer structures at the interface. Results from the present study demonstrated that, besides chemical composition, organization of ligands on particle surface is important in determining their colloidal stability.

Adsorption and stability of self-assembled organophosphonic acid monolayers on plasma modified Zn–Mg–Al alloy surfaces

The adsorption and stability of adhesion promoting organophosphonic acid monolayers on plasma modified Zn–Mg–Al alloy surfaces were investigated by means of microscopic and spectroscopic techniques. A strip hollow cathode (SHC) was chosen for the plasma surface modification. The chemical composition of the plasma treated surfaces and the influence of reducing and oxidizing plasma modifications on the corrosion properties of Zn–Mg–Al alloy surfaces were analyzed by means of X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry, respectively. The adsorption and stability of phosphonic acid monolayers were comparatively studied on plasma modified and non-plasma modified Zn–Mg–Al alloy surfaces. Self-organization of monofunctional monolayers was confirmed by means of polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS) and XPS. Contact angle measurements were performed to prove the stability of the octadecylphosphonic acid (ODPA) monolayer on the native and different plasma treated surfaces in aqueous media. Plasma modification and ODPA adsorption resulted in a synergistic inhibition of the redox current densities of the alloy surface. The strongest inhibition was observed for an Ar/H2 plasma+O2 plasma treatment followed by octadecylphosphonic acid self-assembled monolayer adsorption.

Deposition of Metal-Organic Frameworks by Liquid-Phase Epitaxy: The Influence of Substrate Functional Group Density on Film Orientation

The liquid phase epitaxy (LPE) of the metal-organic framework (MOF) HKUST-1 has been studied for three different COOH-terminated templating organic surfaces prepared by the adsorption of self-assembled monolayers (SAMs) on gold substrates. Three different SAMs were used, mercaptohexadecanoic acid (MHDA), 4’-carboxyterphenyl-4-methanethiol (TPMTA) and 9-carboxy-10-(mercaptomethyl)triptycene (CMMT). The XRD data demonstrate that highly oriented HKUST-1 SURMOFs with an orientation along the (100) direction was obtained on MHDA-SAMs. In the case of the TPMTA-SAM, the quality of the deposited SURMOF films was found to be substantially inferior. Surprisingly, for the CMMT-SAMs, a different growth direction was obtained; XRD data reveal the deposition of highly oriented HKUST-1 SURMOFs grown along the (111) direction.

Self-Assembled Monolayer Induced Au(111) and Ag(111) Reconstructions: Work Functions and Interface Dipole Formation

Adsorption of self-assembled monolayers (SAMs) on metal surfaces leads to interface dipole layers that strongly modify the metal work functions. Recently, alkanethiolate SAMs have been shown to give rise to substantial reconstructions of the Au(111) and Ag(111) surfaces. In this paper, we study by means of first-principles calculations how such reconstructed alkanethiolate SAMs on Au and Ag interfaces modify the interface dipole layer and the work function. The impact of SAM-induced reconstructions is remarkably moderate in the Au case, giving rise to work function changes of ≲0.25 eV as compared to the unreconstructed case. Neither the Au work function is altered much by reconstructions nor the orientation of the molecular dipoles in the SAMs. In contrast, in the Ag case, a SAM-induced reconstruction alters the work function by ≳0.4 eV. The different behavior of Au and Ag substrates is partly explained by the participation of the Au 5d states in the surface electronic structure, moderating the impact of ...

Effect of Coverage and Defects on the Adsorption of Propanethiol on Au(111) Surface: A Theoretical Study

Periodic density functional calculations have been carried out to investigate both the thiol adsorption on Au(111) surface and the reaction mechanism for the formation of the self-assembled monolayers, taking propanethiol as a representative example. The effect of coverage and surface defects (adatoms and vacancies) has been analyzed. It is found that the most stable physisorption (undissociated) site is an adatom site, whereas the chemisorption site for the thiol is a vacancy site or protrusion consisting of a pair of adatoms, followed by one adatom site. The results point out that the thiolate self-assembled monolayer adsorption process occurs preferentially on step edges.

In situ real time monitoring of kinetics of thiol adsorption on gold based on electrochemical steady-state current

The growth kinetics of a self-assembled monolayer (SAM) was measured by decrease in steady-state faradic current related to the formation of an insulating SAM. The steady-state current produced by the oxidation of ferrocene in the presence of a rotating disk electrode decreased by introduction of 11-mercaptoundecanoic acid (MUA) related to formation of an insulating MUA SAM. The time constants derived from fitting the measured current to the rearrange-limited Langmuir model agreed well with previously reported results determined using other techniques, demonstrating that our simple method can reliably characterize SAM adsorption kinetics in situ and real timely.