Preparation Of Self-assembled Monolayers Research Articles

Functionalized N-Heterocyclic Carbene Monolayers on Gold for Surface-Initiated Polymerizations

Although N-heterocyclic carbenes (NHCs) are superior to thiol adsorbates in that they form remarkably stable bonds with gold, the generation of NHC-based self-assembled monolayers (SAMs) typically requires a strong base and an inert atmosphere, which limits the utility of such films in many applications. Herein, we report the development and use of bench-stable NHC adsorbates, benzimidazolium methanesulfonates, for the direct formation of NHC films on gold surfaces under an ambient atmosphere at room temperature without the need for extraordinary precautions. The generated NHC SAMs were fully characterized using ellipsometry, X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and contact angle measurements, and they were compared to analogous SAMs generated from an NHC bicarbonate adsorbate. Based on these findings, a unique radical initiator α,ω-bidentate azo-terminated NHC adsorbate, NHC15AZO[OMs], was designed and synthesized for the preparation of SAMs on gold surfaces with both NHC headgroups bound to the surface. The adsorbate molecules in NHC15AZO SAMs can exist in a hairpin or a linear conformation depending on the concentration of the adsorbate solution used to prepare the SAM. These conformations were studied by a combination of ellipsometry, XPS, PM-IRRAS, and scanning electron microscopy using gold nanoparticles (AuNPs) as a tag material. Moreover, the potential utility of these unique radical-initiating NHC films as surface-initiated polymerization platforms was demonstrated by controlling the thickness of polystyrene brush films grown from azo-terminated NHC monolayer surfaces simply by adjusting the reaction time of the photoinitiated radical polymer growth process.

Efficient Routes for the Preparation of Urazole Radical Self-Assembled Monolayers on Gold Surfaces

The functionalization of substrates with radical species has been shown to be a promising strategy to confer novel physical and chemical properties to a surface. Urazole radicals are persistent nitrogen-centered radicals that are insensitive to oxygen, making them desirable targets for the functionalization of surfaces. Here, we succeed in the preparation of self-assembled monolayers (SAMs) of a 1-arylurazole radical on Au surfaces using two different approaches. In the first approach, a SAM of a radical precursor on the gold surface is prepared followed by its chemical oxidation to generate the active urazole radical. In the second route, the radical is generated in solution and subsequently grafted on the Au surface. In both cases, the SAMs exhibit active radical behavior, but the SAM prepared by the first approach demonstrates greater surface coverage of the electroactive urazole radical species.

Achieving High-Performance Molecular Rectification through Fast Screening Alkanethiol Carboxylate-Metal Complexes Electroactive Units

Achieving High-Performance Molecular Rectification through Fast Screening Alkanethiol Carboxylate-Metal Complexes Electroactive Units

Electrochemical deposition of N-heterocyclic carbene monolayers on metal surfaces

N-heterocyclic carbenes (NHCs) have been widely utilized for the formation of self-assembled monolayers (SAMs) on various surfaces. The main methodologies for preparation of NHCs-based SAMs either requires inert atmosphere and strong base for deprotonation of imidazolium precursors or the use of specifically-synthesized precursors such as NHC(H)[HCO3] salts or NHC–CO2 adducts. Herein, we demonstrate an electrochemical approach for surface-anchoring of NHCs which overcomes the need for dry environment, addition of exogenous strong base or restricting synthetic steps. In the electrochemical deposition, water reduction reaction is used to generate high concentration of hydroxide ions in proximity to a metal electrode. Imidazolium cations were deprotonated by hydroxide ions, leading to carbenes formation that self-assembled on the electrode’s surface. SAMs of NO2-functionalized NHCs and dimethyl-benzimidazole were electrochemically deposited on Au films. SAMs of NHCs were also electrochemically deposited on Pt, Pd and Ag films, demonstrating the wide metal scope of this deposition technique.

A theoretical study of 4-Mercaptobenzoic acid assembled on Ag for surface-enhanced raman scattering applications

Thiols are widely used in the preparation of self-assembled monolayers (SAMs) on metallic surfaces such as Ag, Cu and Au because of their strong affinity for such surfaces. These SAMs have various applications in nanotechnology and supramolecular chemistry. However, the chemistry of the metal surface and the thiol end is still vaguely understood, yet it fundamentally governs the properties of the SAMs. This paper reports density functional theory (DFT) and corroborating experimental findings focusing on vibrational Raman spectroscopy of Ag-Mercaptobenzoic acid (AgMBA) SAMs. The 4-MBA thiol was self-assembled onto Ag (111) and (100) fcc surfaces via both the COO- and S- ends. The experimental AgMBA vibrational frequencies showed the interaction of the 4-MBA with the Ag surface from both the S- and COO- atoms and the observation was supported by the our own n-layered integrated molecular orbital and molecular mechanics (ONIOM) calculated surface-enhanced Raman spectroscopy spectra and DFT thermodynamic calculations. The Ag(100)MBA self-assembled via the COO- end had the highest enthalpy of adsorption of -310.34 kJ/mol and it was the most stable plane contradicting the sulphur end consensus on previous papers. The adsorption of the 4-MBA moiety on the Ag surface was interplayed by the corrugation potential of the exposed surface and the thiol’s aromatic nature which resulted in the adsorption of 4-MBA predominately via the carbonyl end at slightly acidic pH conditions and a deprotonated thiol end.

Electrodepositing DNA Self-Assembled Monolayers on Au: Detailing the Influence of Electrical Potential Perturbation and Surface Crystallography.

The preparation of DNA self-assembled monolayers (SAMs) on single-crystal gold bead electrodes using an applied potential is evaluated with in situ electrochemical fluorescence microscopy. Applying a constant deposition potential or a square-wave potential perturbation during the formation of DNA SAMs is compared for two different modification methods: DNA SAM formation on a clean gold surface followed by alkythiol backfilling (as is typically done in the literature) or via thiol-exchange on an alkylthiol-modified gold surface. DNA SAMs prepared from a chloride-containing deposition buffer were not significantly different when using either square-wave potential perturbation or at a constant applied potential even when considering different surface crystallographies. Greater variations were observed when applying more positive potentials for both DNA thiol-exchange and DNA adsorption on clean Au. Our results suggest that using either a constant potential or a square-wave potential perturbation for 5 min both create defects by weakening the gold-thiol interaction. When the deposition is performed with the adsorption of chloride ions from the electrolyte, the electrodeposition results in a similar increase in DNA coverage when compared to depositions performed at open circuit potentials.

Gold cleaning methods for preparation of cell culture surfaces for self-assembled monolayers of zwitterionic oligopeptides

Self-assembled monolayers (SAMs) have been used to elucidate interactions between cells and material surface chemistry. Gold surfaces modified with oligopeptide SAMs exhibit several unique characteristics, such as cell-repulsive surfaces, micropatterns of cell adhesion and non-adhesion regions for control over cell microenvironments, and dynamic release of cells upon external stimuli under culture conditions. However, basic procedures for the preparation of oligopeptide SAMs, including appropriate cleaning methods of the gold surface before modification, have not been fully established. Because gold surfaces are readily contaminated with organic compounds in the air, cleaning methods may be critical for SAM formation. In this study, we examined the effects of four gold cleaning methods: dilute aqua regia, an ozone water, atmospheric plasma, and UV irradiation. Among the methods, UV irradiation most significantly improved the formation of oligopeptide SAMs in terms of repulsion of cells on the surfaces. We fabricated an apparatus with a UV light source, a rotation table, and HEPA filter, to treat a number of gold substrates simultaneously. Furthermore, UV-cleaned gold substrates were capable of detaching cell sheets without serious cell injury. This may potentially provide a stable and robust approach to oligopeptide SAM-based experiments for biomedical studies.

Grand Canonical Quantum Mechanical Study of the Effect of the Electrode Potential on N-Heterocyclic Carbene Adsorption on Au Surfaces

Increasing interest has been focused on using N-heterocyclic carbenes (NHCs) as surface ligands to replace thiols in the preparation of self-assembled monolayers (SAMs) on gold due to their larger adsorption energies. However, one of the drawbacks of these NHC-based SAMs is that they are unstable under electrochemically reducing conditions. In this study, grand canonic quantum mechanics (GC-QM) were used to study the effect of the electrode potential (U) on the adsorption of NHC on Au(111). The NHC adsorption energies were significantly weaker (∼0.92 eV) under constant U conditions compared to those under constant charge conditions, demonstrating the importance of using GC-QM for studying electrochemical systems. Consistent with experiments, the results from our calculations indicated that the adsorption energy decreased as U became more negative but increased as U became more positive. These results were rationalized using the frontier orbital theory. Importantly, based on the same analysis, when NHCs or...

Photo-Electrochemical Characteristics of Two-Dimensional Porphyrin Molecular Layers on Semiconductor Surfaces

Metallo-porphyrins have planar structure and effectively absorb visible-light normal to the ring plane. Thus, they can be applied to artificial photosynthetic systems and photovoltaic cells when the porphyrin ring was adsorbed parallel to the solid surface [1]. Si is one of the most basic and important semiconductor materials and used to the crystal Si-based solar cell. Si mainly absorbs infrared-light and cannot absorb visible-light. On the other hand, TiO2 is a widely used semiconductor material and strongly absorbs ultraviolet-light. Because most of the sunlight is in a visible-light region, combination of Si or TiO2 with metallo-porphyrin should be required in order to construct the solar-active devices. However, the interaction between the Si and TiO2 substrate surfaces and metallo-porphyrin is very weak. Thus, we have to construct the self-assembled monolayers (SAMs), with a binding group such a carboxylate group to metallo-porphyrin, on the Si and TiO2 surface as a first and then the metallo-porphyrin layers were constructed on them. In this study, we constructed 5,10,15,20- tetraphenyl-porphyrin zinc (ZnTPP) in a planar array on the Si(111) and TiO2(110) single-crystal substrates through the carboxy-terminated SAM and investigated photo-electrochemical characteristics for the visible-light active photodevices. Ester-terminated SAMs of methyl acrylate (MA) and ethyl undecylenate (EU) SAMs were self-assembled on the hydrogen terminated Si(111) substrate by heating. After the SAM preparation on the Si surface, the terminated ester groups of the SAMs were hydrolyzed by the immersion into a hot HCl solution [2,3]. Carboxylate-terminated TiO2 surface was prepared just by dipping the pre-cleaned TiO2(110) substrate into an ethanol solution containing saturated 1,4-benzenedicarboxylic acid (BDC) for overnight. These carboxylate-terminated Si(111) and TiO2(110) substrates were dipped into the ethanol solution containing ZnTPP. In each step, structure, orientation, and surface coverage of the molecular layers were estimated by attenuated total reflection Fourier transformed infrared spectroscopy (ATR-FTIRS) and x-ray photoelectron spectroscopy (XPS), in order to determine the optimum condition. Finally, the photo-electrochemical characteristics of the constructed porphyrin molecular layers on Si(111) and TiO2(110) were investigated in the electrolyte solution containing electron donor and/or acceptor with and without Xe lamp irradiation. From the results of ATR-FTIRS and XPS [4], it was found that the longer the dipping time of each SAM-modified Si(111) and TiO2(110) substrates, the more the absorbed amount of ZnTPP. On the both surfaces, potential-dependent anodic photocurrent was observed by visible-light irradiation. Excited energy dependence is now under investigation.

Formation and Stability of Phenylphosphonic Acid Monolayers on ZnO: Comparison of In Situ and Ex Situ SAM Preparation.

Self-assembled monolayers (SAMs) enable an electronic interface tailoring of conductive metal oxides and offer an alternative to common transparent electrodes in optoelectronic devices. Here, the influence of surface orientation and pretreatment on the formation and stability of SAMs has been studied for the case of phenylphosphonic acid (PPA) on ZnO single crystals. Using thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS), near-edge X-ray adsorption fine structure spectroscopy (NEXAFS) and density-functional theory (DFT) calculations, the thermal stability and orientational ordering of PPA-SAMs on the polar and mixed-terminated ZnO surfaces were analyzed. On all surfaces, PPA-SAMs remain stable up to 550 K, while at higher temperatures a C-P bond cleavage and dissociative desorption takes place yielding two distinct desorption peaks. Based on DFT calculations, these desorption channels are attributed to protonated and deprotonated chemisorbed PPA molecules, which can be related to tri- and bidentate species, hence allowing to determine their relative abundance from the intensity ratio. Beside immersion, an alternative monolayer preparation based on vacuum deposition in combination with controlled desorption of excess multilayers is demonstrated. This enables a SAM preparation on bare ZnO surfaces without any precoating due to exposure to ambient air, which is further compared with SAM formation on intentionally hydroxylated substrates. Corresponding TDS data indicate that initial hydroxylation favors the formation of tridentate and deprotonated bidentate, while the OMBD preparation on bare surfaces yields a larger fraction of protonated bidentate species. The orientation of PPA molecules adopted in the SAMs was determined from the dichroism of K-edge NEXAFS measurements and reveals an almost upright orientation for the deprotonated species, while a slight tilting is obtained for monolayer films with a large fraction of protonated bidentate molecules.

Self-assembled alkanethiol monolayers on gold surfaces: resolving the complex structure at the interface by STM.

The surface properties of metals and metal oxides can be modified by adding a single layer of organic molecules. A most popular route for depositing such a molecular layer is via the formation of self-assembled monolayers (SAMs). The molecules that form SAMs have a functionality which binds to the surface and the adsorption is self-regulated to terminate at exactly one single molecular layer. The very first example, which has become the most widely studied system, of SAMs on metal surfaces consists of chemisorbed alkylthiolate on gold. Despite the simplicity in the preparation of alkanethiol SAMs and the seemingly straightforward structure of such SAMs, the detailed bonding between the sulfur head group and gold is still subject to debate. Experimental and theoretical effort in the last six years has led to a much improved understanding of this classical system of SAMs. In this review, we will highlight the most recent progress in the study of the interfacial structure of alkanethiol SAMs on gold. We focus on the important phenomenon of phase transition that occurs from n-propanethiol to n-butanethiol, and propose a unified structural model to explain how the (3 × 4) phase for short chain alkanethiol monolayers (methyl-, ethyl- and propylthiolate monolayers) changes into the (3 × 2√3)-rect./c(4 × 2) phase for long chain molecular monolayers.

The Preparation and Corrosion Performance of Self-Assembled Monolayers of Stearic Acid and MgO Layer on Pure Magnesium

Self-assembled monolayers (SAMs) of stearic acid (SA) film were developed on pure magnesium with heat-pretreatment in order to improve both the corrosion resistance and the bioactivity of pure magnesium. A new method using a high concentration of SA solution (10 ¹1 mol/L) is proposed to reduce the process time of the conventional SAMs methods. The effect of ultrasonic cleaning treatment on SAMs was investigated. Contact angle measurements were employed to evaluate the hydrophobic properties of the SAMs. The corrosion resistance and the bioactivity of the specimens were evaluated under biological conditions (simulated body fluid, 37°C) using potentiodynamic polarization method and long term immersion test. Surface morphology, composition and structure analysis of samples were characterized by the scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR) after immersion test. The results showed the corrosion resistance of pure Mg was improved by stearic acid-SAMs and the MgO layer made by heat-pretreatment, the SAMs made in high concentration (10 ¹1 mol/L) of stearic acid solution just for 1.5h showed better properties than conventional SAMs. Furthermore, ultrasonic cleaning treatment led to the formation of more dense and orderly SAMs and improved the anticorrosion property of SAMs prepared in high concentration of SA solution. Moreover, SAMs-Mg had better bioactivity than untreated magnesium. [doi:10.2320/matertrans.M2014117]

A new striped-phase of decanethiol self-assembled monolayers on Au(1 1 1) formed at a high solution temperature

Self-assembled monolayers (SAMs) of decanethiol (C10) on Au(111) prepared at room temperature (RT), 60°C, and 70°C have been investigated using high resolution scanning tunneling microscopy (STM). For SAMs prepared at RT, the c(4×2) superlattice of the basic (3×3)R30° structure was hardly observed in small-scale STM images. Increasing the solution temperature to 60°C results in formation of SAMs having large ordered domains of the c(4×2) superlattice. In contrast to the previous studies, our results indicate the presence of four different topographical heights for the C10 molecules per c(4×2) unit cell. Preparation of C10 SAMs from 70°C of solution temperature leads to the formation of new striped phase with (43×23)R30° unit cell. The area of a single molecule in the (43×23)R30° striped structure amounts to 57.73Å2, which is about 167% larger than that (21.6Å2) for a molecule in the c(4×2) structure.

Carboranedithiols: Building Blocks for Self-Assembled Monolayers on Copper Surfaces

Two different positional isomers of 1,2-dicarba-closo-dodecaboranedithiols, 1,2-(HS)(2)-1,2-C(2)B(10)H(10) (1) and 9,12-(HS)(2)-1,2-C(2)B(10)H(10) (2), have been investigated as cluster building blocks for self-assembled monolayers (SAMs) on copper surfaces. These two isomers represent a convenient system in which the attachment of SH groups at different positions on the skeleton affects their acidic character and thus also determines their reactivity with a copper surface. Isomer 1 exhibited etching of polycrystalline Cu films, and a detailed investigation of the experimental conditions showed that both the acidic character of SH groups and the presence of oxygen at the copper surface play crucial roles in how the surface reaction proceeds: whether toward a self-assembled monolayer or toward copper film etching. We found that each positional isomer requires completely different conditions for the preparation of a SAM on copper surfaces. Optimized conditions for the former isomer required the exposure of a freshly prepared Cu surface to vapor of 1 in vacuum, which avoided the presence of oxygen and moisture. Adsorption from a dichloromethane solution afforded a sparsely covered Cu(0) surface; isomer 1 effectively removes the surface copper(I) oxide, forming a soluble product, but apparently binds only weakly to the clean Cu(0) surface. In contrast, adsorption of the latter, less volatile isomer proceeded better from a dichloromethane solution than from the vapor phase. Isomer 2 was even able to densely cover the copper surface cleaned up by the dichloromethane solution of 1. Both isomers exhibited high capacity to remove oxygen atoms from the surface copper(I) oxide that forms immediately after the exposure of freshly prepared copper films to ambient atmosphere. Isomer 2 showed suppression of Cu film oxidation. A number of methods including X-ray photoelectron spectroscopy (XPS), X-ray Rutherford back scattering (RBS), proton-induced X-ray emission (PIXE) analysis, atomic force microscopy (AFM), cyclic voltammetry, and contact angle measurements were used to investigate the experimental conditions for the preparation of SAMs of both positional isomers on copper surfaces and to shed light on the interaction between these molecules and a polycrystalline copper surface.

Studies on the Effect of Solvents on Self-Assembled Monolayers Formed from Organophosphonic Acids on Indium Tin Oxide

The preparation of self-assembled monolayers (SAMs) of organophosphonic acids on indium tin oxide (ITO) surfaces from different solvents (triethylamine, ethyl ether, tetrahydofuran (THF), pyridine, acetone, methanol, acetonitrile, dimethyl sulfoxide (DMSO), or water) has been performed with some significant differences observed. Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and contact angle measurement demonstrated that the quality of SAMs depends critically on the choice of solvents. Higher density, more stable monolayers were formed from solvents with low dielectric constants and weak interactions with the ITO. It was concluded low dielectric solvents that were inert to the ITO gave monolayers that were more stable with a higher density of surface bound molecules because higher dielectric constant solvents and solvents that coordinate with the surface disrupted SAM formation.

Protein resistant oligo(ethylene glycol) terminated self-assembled monolayers of thiols on gold by vapor deposition in vacuum

Protein resistant oligo(ethylene glycol) (OEG) terminated self-assembled monolayers (SAMs) of thiols on gold are commonly used for suppression of nonspecific protein adsorption in biology and biotechnology. The standard preparation for these SAMs is the solution method (SM) that involves immersion of the gold surface in an OEG solution. Here the authors present the preparation of 11-(mercaptoundecyl)-triethylene glycol [HS(CH(2))(11)(OCH(2)CH(2))(3)OH] SAMs on gold surface by vapor deposition (VD) in vacuum. They compare the properties of SAMs prepared by VD and SM using x-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection absorption spectroscopy, and surface plasmon resonance measurements. VD and SM SAMs exhibit similar packing density and show a similar resistance to the nonspecific adsorption of various proteins (bovine serum albumin, trypsin, and myoglobin) under physiological conditions. A very high sensitivity of the OEG SAMs to x-ray radiation is found, which allows tuning their protein resistance. These results show a new path to in situ engineering, analysis, and patterning of protein resistant OEG SAMs by high vacuum and ultrahigh vacuum techniques.

Surface engineering for high performance organic electronic devices: the chemical approach

Self-assembled monolayers (SAMs) are rapidly becoming an essential part of organic electronics such as light emitting diodes (LEDs), organic field-effect transistors (OFETs), and complementary circuits, where they are employed to control the morphology and energetics of the interfaces. This review will focus on interface engineering and its influence on such devices. We will describe the growth of SAMs of organic molecules on various surfaces of interest for this purpose (mainly metals and oxides). For the growth of SAMs on metal electrodes, the most common approach makes use of thiols to modify the metal surface, but the preparation of SAMs on dielectrics requires other reactive groups such as carboxylic or phosphonic acids. We review also the control of the interfacial properties by appropriate molecular design of the SAMs and their effect on device performance. SAMs can modify the morphology of the organic semiconductor, the dielectric properties of the insulator, the electronic states at the dielectric interface and the level alignment at the electrode interfaces. All these factors can influence current transport mainly by modifying the charge mobility, the contact resistance and the trap density at the interfaces.

Potential-assisted deposition of mixed alkanethiol self-assembled monolayers

Preparation of self-assembled monolayers (SAMs) usually involves passive incubation. The recently developed potential-assisted deposition is indeed more selective as well as 100-fold faster than passive adsorption, thereby enhancing the reproducibility of the monolayer deposition. This article aims to identify the electrodeposition conditions necessary to prepare mixed alkanethiol SAMs on gold surface. Parameters such as concentrations in solution, electrode polarization and deposition time were examined for two chain lengths, C 3 (mercaptopropionic acid, MPA) and C 18 (octadecanethiol, ODT). The kinetics and composition of the SAMs were systematically characterized by reductive stripping analysis and by surface plasmon enhanced ellipsometry. Control of the surface concentration of the stable binary SAM was achieved by carefully adjusting the MPA fraction at between 70% and 95% in the deposition solution. Three theoretical models were developed. For MPA or ODT SAMs, whatever the adsorbate and the time period, the second order model gives the best fit. For the binary monolayer (MPA–ODT), a co-adsorption model was developed to describe the kinetics before 10 s.

Self-assembled Monolayers Preparation and Characterization of BiFeO<SUB>3</SUB> Thin Films

Self-assembled Monolayers Preparation and Characterization of BiFeO<SUB>3</SUB> Thin Films

Copper−Metal Deposition on Self Assembled Monolayer for Making Top Contacts in Molecular Electronic Devices

Molecular electronics is an attractive option for low-cost devices because it involves highly uniform self-assembly of molecules with a variety of possible functional groups. However, the potential of molecular electronics can only be turned into practical applications if reliable contacts can be established without damaging the organic layer or contaminating its interfaces. Here, a method is described to prepare tightly packed carboxyl-terminated alkyl self-assembled monolayers (SAMs) that are covalently attached to silicon surfaces and to deposit thin metallic copper top contact electrodes without damage to this layer. This method is based on a two-step procedure for SAM preparation and the implementation of atomic layer deposition (ALD) using copper di-sec-butylacetamidinate [Cu(sBu-amd)](2). In situ and ex situ infrared spectroscopy (IRS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electrical measurements are used to characterize the chemical modification of the Si/SAM interface, the perturbation of the SAM layer itself, and the metal homogeneity and interaction with the SAM headgroups. This work shows that (i) carboxyl-terminated alkyl monolayers can be prepared with the same high density and quality as those achieved for less versatile methyl-terminated alkyl monolayers, as evidenced by electrical properties that are not dominated by interface defects; (ii) Cu is deposited with ALD, forming a bidentate complexation between the Cu and the COOH groups during the first half cycle of the ALD reaction; and (iii) the Si/SAM interface remains chemically intact after metal deposition. The nondamaging thin Cu film deposited by ALD protects the SAM layer, making it possible to deposit a thicker metal top contact leading ultimately to a controlled preparation of molecular electronic devices.