Steric and chain length effects in the (√(3)×√(3))R30° structures of alkanethiol self-assembled monolayers on Au(111)

Effects of chain-chain interaction on the configuration of short-chain alkanethiol self-assembled monolayers on a metal surface

Surface-specific sum frequency generation vibrational spectroscopy is applied to study the molecular configuration of short-chain n-alkanethiol self-assembled monolayers (SAMs with n = 2-6) on the Au surface. For monolayers with n≥ 3, the alkanethiols are upright-oriented, with the CH3 tilt angle varying between ∼33° and ∼46° in clear even-odd dependency. The ethanethiol monolayer (n = 2) is, however, found to exhibit a distinct lying-down configuration with a larger methyl tilt angle (67°-79°) and a smaller CH2 tilt angle (56°-68°). Such a unique configurational transition from n = 2 to n≥ 3 discloses the steric effect owing to chain-chain interaction among neighboring molecules. Through density functional theory calculations, the transition is further confirmed to be energetically favorable for thiols on a defective reconstructed Au(111) surface but not on the pristine one. Our study highlights the roles of the chain-chain interaction and the substrate surface atomic structure when organizing SAMs, offering a strategic pathway for exploiting their applications.

Determination of electric and thermoelectric properties of molecular junctions by AFM in peak force tapping mode

Molecular thin films, such as self-assembled monolayers (SAMs), offer the possibility of translating the optimised thermophysical and electrical properties of high-Seebeck-coefficient single molecules to scalable device architectures. However, for many scanning probe-based approaches attempting to characterise such SAMs, there remains a significant challenge in recovering single-molecule equivalent values from large-area films due to the intrinsic uncertainty of the probe-sample contact area coupled with film damage caused by contact forces. Here we report a new reproducible non-destructive method for probing the electrical and thermoelectric (TE) properties of small assemblies (10-103) of thiol-terminated molecules arranged within a SAM on a gold surface, and demonstrate the successful and reproducible measurements of the equivalent single-molecule electrical conductivity and Seebeck values. We have used a modified thermal-electric force microscopy approach, which integrates the conductive-probe atomic force microscope, a sample positioned on a temperature-controlled heater, and a probe-sample peak-force feedback that interactively limits the normal force across the molecular junctions. The experimental results are interpreted by density functional theory calculations allowing quantification the electrical quantum transport properties of both single molecules and small clusters of molecules. Significantly, this approach effectively eliminates lateral forces between probe and sample, minimising disruption to the SAM while enabling simultaneous mapping of the SAMs nanomechanical properties, as well as electrical and/or TE response, thereby allowing correlation of the film properties.

In Situ Wide-Frequency Surface-Enhanced Infrared Absorption Spectroscopy Enables One to Decipher the Interfacial Structure of a Cu Plating Additive

In situ spectroscopic characterization of the interfacial structure of an organic additive at a Cu electrode is essential for a mechanistic understanding of Cu superfilling at the molecular level. In this work, we demonstrate wide-frequency attenuated total reflection surface-enhanced infrared absorption spectroscopy (wf-ATR-SEIRAS) to elucidate the dissociative adsorption of bis(sodium sulfopropyl)-disulfide (a typical accelerator) on a Cu electrode in conjunction with the electrochemical quartz crystal microbalance measurement and modeling calculations. The wf-ATR-SEIRAS clearly identifies the peaks featuring the sulfonate and methylene groups as well as the C-S_sulfonate and C-S_thiol vibrations of the adsorbate. Analysis of relative peak intensities from 1100 to 650 cm^-1 reveals a more tilted alkyl chain axis for the thiolate on Cu than that on Au, which is supported by comparative density functional theory calculations. This work opens a new avenue for the wf-ATR-SEIRAS to study interfacial structures of electroplating additives related to advanced microelectronics manufacture.

Observing Real-Time Formation of Self-Assembled Monolayers on Polycrystalline Gold Surfaces with Scanning Electrochemical Cell Microscopy

Self-assembled monolayers (SAMs) of alkanethiols on gold have become a central focus of controllable surface chemistry because they can be easily formed from the solution phase and characterized using various techniques. Understanding the formation processes occurring at a nanoscale level is crucial to form defect-free SAMs for tailored applications in bio- and nanotechnology. Although many reports study and characterize SAMs after they are formed on gold surfaces, typical methods have not extensively studied the SAM formation process at the nanoscale. This paper focuses on the formation of defect-free SAMs and elucidates the formation mechanism occurring at the nanoscale level during the formation process. Exploiting the strength of scanning electrochemical cell microscopy, we monitored SAM formation via a soluble redox reporter on a polycrystalline gold foil using voltammetric and amperometric techniques. We formed SAMs by varying the concentration of 3-mercapto-1-propanol [HS(CH₂)₃OH], 6-mercapto-1-hexanol [HS(CH₂)₆OH], and 9-mercapto-1-nonanol [HS(CH₂)₉OH] to determine the effects of the thiol chain length, concentration, and location on the substrate (grain boundaries) on monolayer formation. We observed real-time changes in the quasisteady-state current of our redox reporter during the self-assembly process. Importantly, we formed defect-free SAMs at the nanoscale level using different concentrations of HS(CH₂)₆OH and HS(CH₂)₉OH and found that SAM formation at the nanoscale is concentration-dependent and varies when at a boundary between two crystal grains.

Influence of the terminal group on the thermal decomposition reactions of carboxylic acids on copper: nature of the carbonaceous film

The effect of the terminal groups on the nature of the films formed by the thermal decomposition of carboxylic acids on copper is studied in ultrahigh vacuum using temperature-programmed desorption (TPD), scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). The influence of the presence of vinyl or alkynyl terminal groups and chain length is studied using heptanoic, octanoic, 6-heptenoic, 7-octenoic, 6-heptynoic and 7-octynoic acids. The carboxylic acids form strongly bound carboxylates following adsorption on copper at room temperature, and thermally decompose between ∼500 and 650 K. Previous work has shown that this occurs by the carboxylate plane tilting towards the surface to eliminate carbon dioxide and deposit a hydrocarbon fragment. The fragment can react to evolve hydrogen or form oligomeric species on the surface, where the amount of carbon increases for carboxylic acids that contain terminal functional groups that can anchor to the surface. These results will be used to compare with the carbonaceous films formed by the mechanochemical decomposition of carboxylic acids on copper, which occurs at room temperature. This is expected to lead to less carbon being deposited on the surface than during thermal decomposition.

Adsorption and reaction pathways of 7-octenoic acid on copper

The surface structure and reaction pathways of 7-octenoic acid are studied on a clean copper substrate in ultrahigh vacuum using a combination of reflection-absorption infrared spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption and scanning-tunneling microscopy, supplemented by first-principles density functional theory calculations. 7-Octenoic acid adsorbs molecularly on copper below ∼260 K in a flat-lying configuration at low coverages, becoming more upright as the coverage increases. It deprotonates following adsorption at ∼300 K to form an η2-7-octenoate species. This also lies flat at low coverages, but forms a more vertical self-assembled monolayer as the coverage increases. Heating causes the 7-octenoate species to start to tilt, which produces a small amount of carbon dioxide at ∼550 K and some hydrogen in a peak at ∼615 K ascribed to the reaction of these tilted species. The majority of the decarbonylation occurs at ∼650 K when CO2 and hydrogen evolve simultaneously. Approximately half of the carbon is deposited on the surface as oligomeric species that undergo further dehydrogenation to evolve more hydrogen at ∼740 K. This leaves a carbonaceous layer on the surface, which contains hexagonal motifs connoting the onset of graphitization of the surface.

Computational and experimental approach to understanding the structural interplay of self-assembled end-terminated alkanethiolates on gold surfaces

Molecular organization dictates phases, stability and subsequent electronic structure of self-assembled monolayers. With appropriate density functionals, ab initio molecular dynamics (AIMD) simulations predicted and elucidated experimental orientations.

Computational investigations of electronic structure modifications of ferrocene-terminated self-assembled monolayers: effects of electron donating/withdrawing functional groups attached on the ferrocene moiety

The electrochemical properties of chemically modified electrodes have long been a significant focus of research. Although the electronic states are directly related to the electrochemical properties, there have been only limited systematic efforts to reveal the electronic structures of adsorbed redox molecules with respect to the local environment of the redox center. In this study, density functional theory (DFT) calculations were performed for ferrocene-terminated self-assembled monolayers with different electron-donating abilities, which can be regarded as the simplest class of chemically modified electrodes. We revealed that the local electrostatic potentials, which are changed by the electron donating/withdrawing functional groups at the ferrocene moiety and the dipole field of coadsorbed inert molecules, practically determine the density of states derived from the highest occupied molecular orbital (HOMO) and its vicinities (HOMO-1 and HOMO-2) with respect to the electrode Fermi level. Therefore, to design new, sophisticated electrodes with chemical modification, one should consider not only the electronic properties of the constituent molecules, but also the local electrostatic potentials formed by these molecules and coadsorbed inert molecules.

Alkylthiol self-assembled monolayers on Au(111) with tailored tail groups for attaching gold nanoparticles

Self-assembled monolayers (SAMs) on Au(111) are able to control the functionality of a gold surface. We use scanning tunnelling microscopy (STM) in air and contact angle measurements to compare the morphology and the chemistry of three alkylthiol SAMs differing by their tail groups: 1,9-nonanedithiol (NDT), 1,4-butanedithiol (BDT) and 11-mercaptoundecanol (MUOH). STM reveals very different morphologies: a hexagonal lattice for MUOH and parallel rows for NDT and BDT. In the case of NDT, we find that the thiol tail groups may form disulfide bridges with long immersion times. The availability of the -SH group for chemical reactions is demonstrated by attaching gold nanoparticles (AuNPs). When the thiol tail group is available, AuNPs readily attach as shown with atomic force microscopy (AFM). When disulfide bridges are formed, the gold surface is not able to bind nanoparticles.

Ligand-Exchange Dynamics on Gold Nanocrystals: Direct Monitoring of Nanoscale Polyvinylpyrrolidone-Thiol Domain Surface Morphology

We report direct high-resolution monitoring of an evolving mixed nanodomain surface morphology during thiol adsorption on polyvinylpyrrolidone (PVP)-stabilized single crystal gold nanocrystals. The thiol adsorption and replacement dynamics are much more complex than a simple complete substitution of the initial polymer ligand. We observed that during ligand exchange with linear thiol, the nanocrystal surface evolved from an initial 1 nm uniform PVP coating into a remarkably stable network of globular PVP domains 20-100 nm in size and ∼4 nm in height surrounded by thiol self-assembled monolayers. The final stability of such a mixed thiol-PVP surface morphology can possibly be attributed to the interfacial energy reduction from partially solvophilic surfaces and the entropic gain from mixed ligand surface layers. The ligand-exchange dynamics and the unusual equilibrium morphology revealed here provide important insights into both displacement dynamics of surface-bound molecules and the nanoscale peculiarities of surface functionalization of colloidal metal substrates.

A van der Waals DFT study of chain length dependence of alkanethiol adsorption on Au(111): physisorption vs. chemisorption

Coverage and size dependent chain–chain electronic interactions counteract with the alkyl chain–gold surface interactions and the surface relaxation of the metal in the formation of standing up monolayer structures.

Understanding the Phase Diagram of Self-Assembled Monolayers of Alkanethiolates on Gold

Alkanethiolate monolayers on gold are important both for applications in nanoscience as well as fundamental studies of adsorption and self-assembly at metal surfaces. While considerable experimental effort has been put into understanding the phase diagram of these systems, theoretical work based on density functional theory (DFT) has long been hampered by the inability of conventional exchange-correlation functionals to describe dispersive interactions. In this work, we combine dispersion-corrected DFT calculations using the new vdW-DF-CX functional with the ab initio thermodynamics method to study the stability of dense standing-up and low-coverage lying-down phases on Au(111). We demonstrate that the lying-down phase has a thermodynamic region of stability starting from thiolates with alkyl chains consisting of n ≈ 3 methylene units. This phase emerges as a consequence of a competition between dispersive chain-chain and chain-substrate interactions, where the strength of the latter varies more strongly with n. A phase diagram is derived under ultrahigh-vacuum conditions, detailing the phase transition temperatures of the system as a function of the chain length. The present work illustrates that accurate ab initio modeling of dispersive interactions is both feasible and essential for describing self-assembled monolayers.

A van der Waals density functional investigation of carboranethiol self-assembled monolayers on Au(111)

Isolated and full monolayer adsorption of various carboranethiol (C2B10H12S) isomers on the gold(111) surface has been investigated using both the standard and van der Waals density functional theory calculations. The effect of different molecular dipole moment orientations on the low energy adlayer geometries, the binding characteristics and the electronic properties of the self-assembled monolayers of these isomers has been studied. Specifically, the binding energy and work function changes associated with different molecules show a correlation with their dipole moments. The adsorption is favored for the isomers with dipole moments parallel to the surface. Of the two possible unit cell structures, (5 × 5) was found to be more stable than .

Epitaxial transfer through end-group coordination modulates the odd-even effect in an alkanethiol monolayer assembly

Short spacer length and high end-group coordination lead to the top network acting as a template for the buried sulfur-gold interface of n-alkanethiols (SH-(CH2)n-OH or SH-(CH2)n-CH3) on gold {111}. Annealing and templating both drive toward a higher sampling of the spatially favorable bridge adsorption sites. The hydrogen-bonded network increases in strength by increasing the number of hydrogens participating per oxygen, from 1.75 to 1.98 for n = 14-30. Higher n leads to better packing (five times for hydroxyl-terminated and seven times for methyl-terminated for n = 14-30) and stability of monolayers, while lower n results in better epitaxial transfer (transfer coefficient ratio = 13.5 for {SH-(CH2)14-OH}/{SH-(CH2)30-CH3}) and actuation. Odd values of n for the hydroxyl-terminated n-alkanethiols lead to lattice spacing of an average of 0.04 ± 0.01 Å higher than even values. There is a structural transition in properties around spacer length n = 24-27. Characterization of monolayer assembly through correlation between adatom and network layers provides recursive design principles for actuation and sensing applications.

Self-assembled monolayers of thiolates on metals: a review article on sulfur-metal chemistry and surface structures

A review article on fundamental aspects of thiolate self-assembled monolayers (SAMs) on the (111) and (100) surfaces of the Cu and Ni groups is presented.

From the bottom up: dimensional control and characterization in molecular monolayers

Self-assembled monolayers are a unique class of nanostructured materials, with properties determined by their molecular lattice structures, as well as the interfaces with their substrates and environments. As with other nanostructured materials, defects and dimensionality play important roles in the physical, chemical, and biological properties of the monolayers. In this review, we discuss monolayer structures ranging from surfaces (two-dimensional) down to single molecules (zero-dimensional), with a focus on applications of each type of structure, and on techniques that enable characterization of monolayer physical properties down to the single-molecule scale.

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.

Reverse self-assembly: (111)-oriented gold crystallization at alkylthiol monolayer templates

It has long been known that thiol-terminated molecules self-assemble as commensurate monolayers on Au(111) surfaces. By spreading floating octadecanethiol monolayers on aqueous solutions of chloroauric acid (HAuCl4) and using x rays to reduce the gold ions as well as to probe the structure, we have observed the nucleation of (111)-oriented Au nanoparticles at thiol surfaces. This process may be similar to the formation of biogenic gold by bacteria. The thiol monolayer acts as a "soft template," changing its structure as Au crystals form so that there is a sqrt[3]×sqrt[3] commensurate relationship.