Long-chain Alkanethiols Research Articles

Metastable Metal-Monolayer-Semiconductor Junctions: Diverse Chain-Length-Dependent and Ultraslow Electrical Progression

Molecularly tunable metal-semiconductor (MS) junctions have been fabricated by modifying mercury drop electrodes with n-alkanethiols, 1-CH3(CH2)n–1SH (n = 10, 11, 12, 14, 16, and 18) prior to the formation of intimate contact with hydrogen-terminated silicon (H-Si≡) and characterized by both solid-state electrical and electrochemical measurements. We have demonstrated that the current-voltage properties of these molecular junctions change with time and that diverse time-dependent progression “patterns” were observed between the systems of long-chain alkanethiols (C14SH, C16SH, and C18SH) and short-chain alkanethiols (C10SH, C11SH, and C12SH). It is remarkable that for mercury contact electrodes modified with long-chain alkanethiolate self-assembled monolayers (SAMs), the junctions became more rectifying and stabilized over a prolonged period (∼2 h). For short-chain counterparts, surprisingly, they initially became more rectifying, then changed to ohmic over a similar period. It was proposed that at the MS interface, short-chain alkanethiolate SAMs first reorganize to be more ordered before eventually collapse. Long-chain alkanethiolate SAMs, on the other hand, achieve a more uniform, oriented, and stable packing as time goes by. These novel findings shed light on “long-term” intermolecular interactions that drive molecular systems to undergo ultraslow reorganizations, which can be readily modulated by simply varying the precursor structures (e.g., chain length of n-alkanethiols).

Electroreductive Desorption of Alkanethiols on Gold and UPD Copper/Gold Surfaces Using In Situ Second Harmonic Generation

The reductive desorption of three alkanethiols: 1-decanethiol (DT), 1-dodecanethiol (DDT), and 1-octadecanethiol (ODT) was studied. Real-time second harmonic generation (SHG) was used to monitor changes at the surface as the self-assembled monolayer was electrodesorbed from a gold and UPD Cu/Au electrode through cyclic voltammetry (CV). Before and after reductive desorption, samples are examined ex situ with sum frequency generation (SFG) spectroscopy, X-ray photoelectron spectroscopy (XPS), and contact angle (CA). These techniques confirmed the electrodesorption of short-chain alkanethiols, while long-chain alkanethiols were proven to remain near the surface of the electrode. However, a difference was observed for a DDT monolayer on the UPD Cu/Au electrode. While DDT was easily desorbed from a gold substrate, DDT on the UPD Cu/Au electrode behaved similarly to a long-chain alkanethiol on gold. Significantly, SHG was able to determine the relative alkanethiol coverage even while the desorption-features in the CV were obscured by the hydrogen evolution current.

Structures of self-assembled n-alkanethiols on gold by reflection high-energy electron diffraction.

The structures of long-chain alkanethiols (C18H37SH) chemisorbed on an Au(111) single crystal were investigated using reflection high-energy electron diffraction (RHEED). The primary structure observed as a major species in the as-deposited films contains gold adatoms below the sulfur headgroups. Between the small ordered domains with the alkyl chains tilting toward six directions are azimuthally disorderly packed regions, with a similar average tilt of 30.2°. In contrast, a significant reduction in the coverage of gold adatoms is found in the thermally-induced phase. This superlattice is shown to contain a mixture of two sulfur arrangements, both of which exhibit a small S-S distance, and the pairing of the aliphatic chains. A microscopic picture is then given for the structural transition. These findings demonstrate how the RHEED technique may be used to resolve structures of nanometer-thick thin films with multiple orders at the interfaces.

Characterization of self-assembled monolayers for Cu[sbnd]Cu bonding technology

CuCu bonding process is intensively investigated nowadays because of the possibility of obtaining fine-pitch interconnects in 3D stacks for efficient electronic and sensing applications. These interconnects possess high shear strength, as well as excellent electrical and thermal conductivity compared to commonly used solder joints. The bottleneck of CuCu bonding technology is the rapid oxidation of Cu and adsorption of a contamination layer upon exposure to air. This effect can be strongly reduced by using self-assembled monolayers (SAMs) as temporary protective coatings on Cu substrates.The results of X-ray photoelectron spectroscopy (XPS) yielded a drastic decrease in oxidation rate of both sputtered and electroplated Cu surfaces during storage of passivated samples at −40 °C for 7 days. The results of infrared spectroscopic ellipsometry (IRSE) showed that after 10 days of air exposure at room temperature the long-chain 1-hexadecanethiol (C16) SAMs appear to still remain on the Cu surface and to have much slower oxidation rate in comparison to the short-chain 1-hexanethiol (C6) SAMs. Therefore, we can make a conclusion that the application of long-chain alkanethiols or storage of passivated samples at low temperature can provide a long-term oxidation protection for Cu surfaces stored in air.

Superhydrophobic three-dimensional porous ethyl cellulose absorbent with micro/nano-scale hierarchical structures for highly efficient removal of oily contaminants from water

The development of efficient absorbent materials is of global importance for oil spillage cleanup and environmental protection. In this work, a novel superhydrophobic micro/nano-scale hierarchical structured ethyl cellulose sponge was successfully fabricated via an eco-friendly salt-templating method followed by immobilizing silver nanoparticles on the surface and subsequent modification with long-chain alkanethiols. The as-prepared sponge with unique micro-nano structure and porous interconnected network exhibited low density (<17 kg m−3), high porosity (>98%) and robust superhydrophobicity (θwater = 161.3°, θoil = 0°, sliding angle = 3.6°). The sponge could collect a wide range of organic solvents and oils with absorption capacity of 36–48 times of its own weight. Furthermore, the absorption capacity decreased slightly to 89.8% of its initial value after 50 cycles, demonstrating excellent recyclability of the sponge. It was believed that the superhydrophobic/superoleophilic sponge would be a promising absorbent material for the selective oil removal and recovery in environmental remediation.

Solving the Long-Standing Controversy of Long-Chain Alkanethiols Surface Structure on Au(111)

The determination of the amount of gold adatoms and vacancies present at the thiol–gold interface of the (√3 × √3)-R30° lattice on Au(111), usually observed for long-chain alkanethiols, is crucial ...

Influence of amines and alkanethiols on the spectral and luminescent properties of InP@ZnS colloidal quantum dots

It has been shown that the precursors (long-chain amines and alkanethiols) used in liquid-phase colloidal synthesis of InP@ZnS quantum dots exert a significant influence on their spectral and luminescent properties. It has been found that dodecylamine and 1-dodecanethiol facilitate obtaining the particles with a narrow luminescence band and a low quantum yield, whereas oleylamine, 1-octanethiol, and 1,6-hexanedithiol stimulate the formation of the quantum dots with broad and intense luminescence. Conditions have been found under which the narrowing of the emission band to 46.3 nm becomes possible.

A Highly Sensitive Scanning Electrochemical Microscope Based Method for Studying Enzymatic Proton-Coupled Electron Transfer

Proton-coupled electron transfer (PCET) reactions, such as those used in the electron transport chain in mitochondria for adenosine triphosphate production, photosynthesis or nucleic acid repair, are often important to proper cellular functioning and survival. Despite this, many of the mechanisms and energetics that govern PCET in proteins are not fully understood and call for further examination. One of the limitations to understanding enzymatic PCET reactions includes the limited availability of sensitive bioelectroanalytical techniques for studying this class of biological redox chemistry. In this presentation, we report a new, highly sensitive approach to probe PCET in biochemical systems through the use of a scanning electrochemical microscope (SECM) based chronoamperometric method. The proof-of-concept and feasibility measurements were carried out on surface-immobilized cytochrome c oxidase (CcO) and coenzyme Q10 / cytochrome c reductase (bc 1). The proteins were embedded into unilamellar liposomes and then immobilized onto the surface of a gold-coated silicon wafer electrode through integration of the membrane with various gold-bonded tethers or supported lipid membranes, such as long-chain alkanethiol or thiolated-PEG-cholesterol, while in HEPES-Na buffer (at various pH). Upon contact with the surface of the lipid membrane, the platinum ultramicroelectrode SECM tip (~600 nm) was held at a potential (–0.8 V vs. Ag/AgCl) so that diffusion-controlled hydrogen evolution can take place. This reaction current is proportional to the near-tip proton concentration. Upon application of a potential via the gold-coated silicon wafer, or injection of the protein’s native electron donor, such as reduced cytochrome c, a change in the localized proton concentration was measured (shown in Figure 1). The density of immobilized protein on the surface of the electrode was analyzed by AFM in order to normalize current responses. The technique proved to be successful in detecting sub-picoampere current changes in response to proton-pumping of the transmembrane proteins towards the platinum ultramicroelectrode, which could be used for further investigations in the kinetics of other proton-coupled electron transfer proteins. Figure 1

Influence of Order within Nonpolar Monolayers on Hydrophobic Interactions.

We report an experimental investigation of the influence of molecular-level order (crystallinity) within nonpolar monolayers on hydrophobic interactions. The measurements were performed using gold film-supported monolayers formed from alkanethiols (CH3(CH2)nSH, with n ranging from 3 to 17), which we confirmed by using polarization-modulation infrared reflection-adsorption spectroscopy to exhibit chain-length-dependent order (methylene peak moves from 2926 to 2919 cm-1, corresponding to a transition from liquid- to quasi-crystalline-like order) in the absence of substantial changes in chain density (constant methyl peak intensity). By using monolayer-covered surfaces immersed in either aqueous triethanolamine (TEA, 10 mM, pH 7.0) or pure methanol, we quantified hydrophobic and van der Waals contributions to adhesive interactions between identical pairs of surfaces (measured using an atomic force microscope) as a function of the length and order of the aliphatic chains within the monolayers. In particular, we measured pull-off forces arising from hydrophobic adhesion to increase in a nonlinear manner with chain length (abrupt increase between n = 5 and 6 from 2.1 ± 0.3 to 14.1 ± 0.7 nN) and to correlate closely with a transition from a liquid-like to crystalline-like monolayer phase. In contrast, adhesion in methanol increased gradually with chain length from 0.3 ± 0.1 to 3.2 ± 0.3 nN for n = 3 to 7 and then did not change further with an increase in chain length. These results lead to the hypothesis that order within nonpolar monolayers influences hydrophobic interactions. Additional support for this hypothesis was obtained from measurements reported in this paper using long-chain alkanethiols (ordered) and alkenethiols (disordered). The results are placed into the context of recent spectroscopic studies of hydrogen bonding of water at nonpolar monolayers. Overall, our study provides new insight into factors that influence hydrophobic interactions at nonpolar monolayers.

Inhibiting Metal Oxide Atomic Layer Deposition: Beyond Zinc Oxide.

Atomic layer deposition (ALD) of several metal oxides is selectivity inhibited on alkanethiol self-assembled monolayers (SAMs) on Au, and the eventual nucleation mechanism is investigated. The inhibition ability of the SAM is significantly improved by the in situ H2-plasma pretreatment of the Au substrate prior to the gas-phase deposition of a long-chain alkanethiol, 1-dodecanethiol (DDT). This more rigorous surface preparation inhibits even aggressive oxide ALD precursors, including trimethylaluminum and water, for at least 20 cycles. We study the effect that the ALD precursor purge times, growth temperature, alkanethiol chain length, alkanethiol deposition time, and plasma treatment time have on Al2O3 ALD inhibition. This is the first example of Al2O3 ALD inhibition from a vapor-deposited SAM. The inhibitions of Al2O3, ZnO, and MnO ALD processes are compared, revealing the versatility of this selective surface treatment. Atomic force microscopy and grazing-incidence X-ray fluorescence further reveal insight into the mechanism by which the well-defined surface chemistry of ALD may eventually be circumvented to allow metal oxide nucleation and growth on SAM-modified surfaces.

Assembly of Mid-Nanometer-Sized Gold Particles Capped with Mixed Alkanethiolate SAMs into High-Coverage Colloidal Films.

We investigated the influence of the mixed n-alkanethiolate self-assembled monolayer (SAM) formed on gold nanoparticles (AuNPs: 50.0 ± 3.2 nm in diameter) on their assembly into colloidal films. Dodecanethiol and octadecanethiol were selected as the short- and long-chain alkanethiols, respectively. The mixed SAMs were formed by immersing AuNPs in a mixed alkanethiol solution at different molar ratios. Au colloidal films were fabricated on indium tin oxide substrates by our previously reported hybrid method. The composition of the two alkanethiolates in the SAM was deduced from the intensity ratio of two Raman bands at 1080 and 1105 cm(-1). The surface coverage of the colloidal films increased by forming equimolar or dodecanethiolate-dominant mixed SAMs on AuNPs instead of a pure dodecanethiolate or octadecanethiolate SAM. The highest coverage exceeded 80%. This improvement is attributed to the high dispersion stability of AuNPs covered with equimolar or dodecanethiolate-dominant mixed SAMs.

Probing molecular interactions with methylene blue derivatized self-assembled monolayers

The emergence of stratified and personalised medicine and the associated need for highly multiplexed detection strategies are driving the development of innovative sensor technology. Electronic immunosensor arrays capable of label-free and highly parallel monitoring of ligand binding have emerged as a particularly promising technology capable of meeting these new diagnostic challenges. In this study, we present an approach for interrogating molecular interactions electronically using redox active molecular monolayers. Specifically, we have synthesised self-assembled molecular monolayers assembled from long-chain alkanethiols (LCAT) incorporating oligoethyleneglycol (OEG) linkers that can be derivatized with a range of functional groups, including the redox active molecule methylene blue. Critically, we show that the electron transport properties of this redox-active monolayer are highly sensitive to the electrochemical environment, including the local concentration of protons and the electrostatic potential at the plane of electron transfer. Using a combination of cyclic voltammetry and QCM-D to study in detail the behaviour of the monolayer during functionalisation and analyte binding, we demonstrate that these redox properties can be exploited for the electrochemical sensing of molecular interactions (biotin–avidin in our case) on SAMs. Given the versatility of LCAT-OEG monolayers, in terms of linker lengths, choice of functional group, and ability to create mixed component layers and the straight-forward assembly of mixed SAMs of high quality, our electrochemical sensing approach forms an excellent and generic label-free platform for probing a wide range of molecular interactions.

Real-time observation of atomic layer deposition inhibition: metal oxide growth on self-assembled alkanethiols.

Through in situ quartz crystal microbalance (QCM) monitoring, we resolve the growth of a self-assembled monolayer (SAM) and subsequent metal oxide deposition with high resolution. We introduce the fitting of mass deposited during each atomic layer deposition (ALD) cycle to an analytical island-growth model that enables quantification of growth inhibition, nucleation density, and the uninhibited ALD growth rate. A long-chain alkanethiol was self-assembled as a monolayer on gold-coated quartz crystals in order to investigate its effectiveness as a barrier to ALD. Compared to solution-loading, vapor-loading is observed to produce a SAM with equal or greater inhibition ability in minutes vs days. The metal oxide growth temperature and the choice of precursor also significantly affect the nucleation density, which ranges from 0.001 to 1 sites/nm(2). Finally, we observe a minimum 100 cycle inhibition of an oxide ALD process, ZnO, under moderately optimized conditions.

Enhancement of self-assembly of large (>10 nm) gold nanoparticles on an ITO substrate

Densely arraying gold nanoparticles (AuNPs) immobilized on substrates is still a challenge. For the arraying, surface modification of AuNPs with alkanethiols is a key technology. However, if the particle size is larger than 20 nm, short-chain alkanethiols have very weak interparticle interaction such that self-assembly is not realized, whereas long-chain alkanethiols have very strong interaction such that self-aggregation is induced in solution. One solution is their combination. Eventually, a mixture gave a dense array of 55 nm AuNPs on an ITO substrate with a coverage of approximately 80%. Our approach provides high-performance plasmonic optics with a resonance wavelength of more than 700 nm.

Improvement of Thermal Stability of Gold Nanoparticles by Synergistic Interligand Interactions

Abstract Three types of novel gold nanoparticles containing a different condensed ring in their ligands were prepared. Their thermal stability was found to be much higher than that of typical gold nanoparticles surrounded by long-chain alkanethiols because of strong interligand interaction formed by the condensed rings (π–π stacking). A clear correlation was found between the thermal stability and the strength of π–π stacking.

Selective Penetration of Metal Atoms — New Evidence and Application for the Simple Ideal Penetration Model of the Long-Chain Close-Packed Alkanethiol Self-Assembled Monolayers on Au(111)

Abstract In the study of the vapor deposition of metal atoms onto long-chain alkanethiol self-assembled monolayers (CnSH-SAMs, n>10) on Au(111) substrate, metal atoms with different diameters can penetrate selectively into the CnSH-SAMs or not, which looks like the “size exclusion effect”. In this paper, we proposed a new mechanism for the metal atoms penetration by establishing a simple ideal penetration model of the long-chain CnSH-SAMs with defect-free structure on Au(111). The intrinsic and regular–distributed channels with ∼ 3 A diameters (which are decided by the framework size of the three-aggregate of the CnSH molecules) were deduced. This new penetration mechanism can explain reasonably the selective penetration of metal atoms into the long-chain CnSH-SAMs on Au(111): When the driving force is applied, the metal atoms with diameter smaller than 3 A will overcome the van der Waals interaction among the neighboring hydrocarbon chains and penetrate into the CnSH-SAMs through the ∼ 3 A channels reaching to the surface of Au(111), while the metal atoms with large diameter (> 3 A) can not penetrate.

Initial Reaction Probability and Dynamics of Ozone Collisions with a Vinyl-Terminated Self-Assembled Monolayer

The gas–surface reaction dynamics of ozone with a model unsaturated organic surface have been explored through a series of molecular beam scattering experiments. Well-characterized organic surfaces were reproducibly created by adsorption of C═C-terminated long-chain alkanethiols onto gold, while the incident molecular beams were created by supersonic expansion of ozone seeded in an inert carrier gas to afford control over collision energy. Time-of-flight distributions for the scattered molecules showed near complete thermal accommodation of ozone for incident energies as high as 70 kJ/mol. Reflection–absorption infrared spectroscopy, performed in situ with ozone exposure, revealed that oxidation of the double bond depends significantly on the translational energy of O3. For energies near room temperature, 5 kJ/mol, the initial reaction probability (γ0) for the formation of the primary ozonide was determined to be γ0 = 1.1 × 10–5. As translational energy increased to 20 kJ/mol, the reaction probability dec...

Improving the Surface Biocompatibility with the Use of Mixed Zwitterionic Self-Assembled Monolayers Prepared by a Proper Solvent

In this study, the mixed self-assembled monolayers (SAMs) containing the mixture of long-chain alkanethiol, SH(CH(2))(11)NH(2) and SH(CH(2))(10)SO(3)H, was prepared as a model surface to examine the interaction between the biological environment and artificial surface. The 10% (v/v) NH(4)OH ethanolic solution and DMSO were chosen as the solvents for the preparation of these mixed SAMs and the "solvent effect" was discussed. X-ray photoelectron spectroscopy (XPS) has indicated that -SO(3)H/-NH(2) mixed SAMs formed from 10% (v/v) NH(4)OH ethanolic solution were surface "-SO(3)H poor", while a nearly equivalent amount of surface -SO(3)H functionality was presented on the mixed SAMs formed from DMSO. This has resulted from the different solvation capability between solvent molecules and the alkanethiol. Such solvent effects were also reflected in various surface properties such as surface wettability and surface zeta potential. The mixed SAMs formed from DMSO were more surface hydrophilic and less negatively surface charged than from 10% (v/v) NH(4)OH ethanolic solution. In addition, these mixed SAMs formed from DMSO exhibited the least amount of protein adsorbed as well as a better platelet compatibility than its counterpart from 10% (v/v) NH(4)OH ethanolic solution. These findings indicated that choosing a proper solvent for mixed zwitterionic SAM can greatly affect its surface properties and biocompatibility, such as to form a surface with near neutrality for reducing protein adsorption and subsequent platelet adhesion and activation.

Surface characterization and platelet compatibility evaluation of binary mixed self-assembled monolayers containing novel sulfonic acid terminated alkanethiol

The interactions between the biological environment and artificial surface are greatly influenced by the surface characteristics of substrate. Self-assembled monolayer prepared by long-chain alkanethiol on gold has been considered as a good model surface to study the effects of surface characteristics upon biological responses. In this study, two different series of mixed SAMs prepared by lab-synthesized sulfonic acid terminated alkanethiol with hydrophobic –CH 3 or hydrophilic –OH terminated one were characterized. It was noted that the surface hydrophilicity of –SO 3H/–CH 3 mixed SAMs was increased with the solution mole fraction of –SO 3H terminated thiol. However, the surfaces were all hydrophilic on the –SO 3H/–OH mixed SAMs. All of these mixed SAMs were “–SO 3H poor” and negatively charged on surface. In vitro platelet adhesion study indicated that –SO 3H/–OH mixed SAMs exhibited a better platelet compatibility than –SO 3H/–CH 3 mixed ones, suggesting a surface with fair hydrophilicity and least negative surface charges might be of potential as a candidate for developing a platelet/blood compatible artificial surface.

Adsorption of Long-Chain Alkanethiols on Au(111): A Look from the Substrate by High Resolution X-ray Photoelectron Spectroscopy

The structure of the interface between a self-assembled monolayer (SAM) of alkanethiolates (AT) and the underlying Au(111) substrate is a longstanding puzzle in surface science. To cast further light on this problem, we took SAMs of hexanethiolate and dodecanethiolate on Au(111) as test systems and studied them by a combination of synchrotron-based high resolution X-ray photoelectron spectroscopy (HRXPS) and scanning tunneling microscopy (STM). The emphasis of the HRXPS characterization was put on the Au 4f emission of the substrate, which could be decomposed into the components related to the bulk and surface. The behavior of the surface component upon formation of hexanethiolate and dodecanethiolate SAMs was monitored in detail. We observed both a shift of this component and its branching into two independent emissions, which can be associated with two groups of Au atoms differently affected by the adsorption. This behavior, along with the relative intensity of both surface emissions, was correlated wit...