Polymorphism in self-assembled terphenylthiolate monolayers on Au(111)

Collective Mid-Infrared Vibrations in Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) is typically assumed to occur at individual molecules neglecting intermolecular vibrational coupling. Here, we show instead how collective vibrations from infrared (IR) coupled dipoles are seen in SERS from molecular monolayers. Mixing IR-active molecules with IR-inactive spacer molecules controls the intermolecular separation. Intermolecular coupling leads to vibrational frequency upshifts up to 8 cm^-1, tuning with the mixing fraction and IR dipole strength, in excellent agreement with microscopic models and density functional theory. These cooperative frequency shifts can be used as a ruler to measure intermolecular distance and disorder with angstrom resolution. We demonstrate this for photochemical reactions of 4-nitrothiophenol, which depletes the number of neighboring IR-active molecules and breaks the collective vibration, enabling direct tracking of the reaction. Collective molecular vibrations reshape SERS spectra and need to be considered in the analysis of vibrational spectra throughout analytical chemistry and sensing.

Investigation of electron-induced cross-linking of self-assembled monolayers by scanning tunneling microscopy

Ultrathin membranes with subnanometer pores enabling molecular size-selective separation were generated on surfaces via electron-induced cross-linking of self-assembled monolayers (SAMs). The evolution of p-terphenylthiol (TPT) SAMs on Au(111) surfaces into cross-linked monolayers was observed with a scanning tunneling microscope. As the irradiation dose was increased, the cross-linked regions continued to grow and a large number of subnanometer voids appeared. Their equivalent diameter is 0.5 ± 0.2 nm and the areal density is ≈1.7 × 10¹⁷ m^-2. Supported by classical molecular dynamics simulations, we propose that these voids may correspond to free volumes inside a cross-linked monolayer.

Carbon Nanomembranes from Aromatic Carboxylate Precursors

Self-assembled monolayers (SAMs) serve as convenient platform for fabricating carbon nanomembranes (CNMs) of extended lateral dimensions. Highly porous CNMs are emerging as interesting materials for membrane technologies as they exhibit selectivity for water permeation and, owing to their reduced dimensionality, promise increased energy efficiency compared to established systems. In the present study terphenylcarboxylate SAMs, prepared on silver underpotential deposited on Au and irradiated by 100 eV electrons, were successfully converted into free-standing CNMs. Infrared and X-ray photoelectron spectroscopy reveal pronounced chemical changes both of the anchoring carboxylate moiety and the aromatic backbone upon electron irradiation. Permeation studies showed high specificity for water as demonstrated by the separation from tetrahydrofuran. Compared to thiols on gold, the standard CNM precursor system, the carboxylic acid based SAM exhibits equivalent characteristics. This suggests that electron-induced carbonization is insensitive to the particular choice of the anchor moiety and, therefore, the choice of precursor molecules can be extended to the versatile class of aromatic carboxylic acids.

Annealing effect for self-assembled monolayers formed from terphenylethanethiol on Au(111)

Formation of several different structural phases and desorption took place from a standing-up phase at an annealing temperature of 473 K.

Oriented Crystallization on Organic Monolayers to Control Concomitant Polymorphism

Nucleation events and crystal growth can be guided by molecular recognition at interfaces through intermolecular interactions. The short-acting antimicrobial sulfa drug sulfathiazole is known for its concomitant crystallization, which has five known polymorphs, due to conformational flexibility and hydrogen-bond synthon variation. In its development stage of a drug the issue of concomitant crystallization needs to be addressed with respect to patent litigation, including legal actions to protect patents against infringement. A functional self-assembled monolayer (SAM) of organic thiol on a gold surface has been employed as an efficient approach to control concomitant nucleation of such flexible drugs. The crystallization on a SAM surface is mostly kinetically driven and often leads to the nucleation of novel metastable forms. Spectroscopic, thermal analysis and X-ray diffraction studies reveal that a previously unknown, sixth form of the drug nucleates on the designed SAM surface.

Exploring the driving forces behind the structural assembly of biphenylthiolates on Au(111)

In this contribution, we use dispersion-corrected density functional theory to study inter- and intramolecular interactions in a prototypical self-assembled monolayer (SAM) consisting of biphenylthiolates bonded to Au(111) via thiolate groups. The goal is to identify the nature of the interactions that drive the monolayer into a specific conformation. Particular focus is laid on sampling realistic structures rather than high symmetry model configurations. This is achieved by studying conceptually different local minimum structures of the SAM that are obtained via exploring the potential energy surface from systematically varied starting geometries. The six obtained packing motifs differ in the relative arrangement of the two molecules in the unit cell (co-planar versus herringbone) and in the intramolecular configuration (twisted versus planar rings). We find that van der Waals interactions within the organic adsorbate and between the adsorbate and substrate are the main reason that these molecular assemblies can form stable structures at all. The van der Waals interactions are, however, very similar for all observed motifs; by analyzing various types of interactions in the course of three notional SAM-formation steps, we find that the main driving force stabilizing the actual global minimum structure originates from electrostatic interactions between the molecules.

Rapid Water Permeation Through Carbon Nanomembranes with Sub-Nanometer Channels

The provision of clean water is a global challenge, and membrane filtration is a key technology to address it. Conventional filtration membranes are constrained by a trade-off between permeance and selectivity. Recently, some nanostructured membranes demonstrated the ability to overcome this limitation by utilizing well-defined carbon nanoconduits that allow a coordinated passage of water molecules. The fabrication of these materials is still very challenging, but their performance inspires research toward nanofabricated membranes. This study reports on molecularly thin membranes with sub-nanometer channels that combine high water selectivity with an exceptionally high permeance. Carbon nanomembranes (CNMs) of ∼1.2 nm thickness are fabricated from terphenylthiol (TPT) monolayers. Scanning probe microscopy and transport measurements reveal that TPT CNMs consist of a dense network of sub-nanometer channels that efficiently block the passage of most gases and liquids. However, water passes through with an extremely high permeance of ∼1.1 × 10^-4 mol·m^-2·s^-1·Pa^-1, as does helium, but with a ∼ 2500 times lower flux. Assuming all channels in a TPT CNM are active in mass transport, we find a single-channel permeation of ∼66 water molecules·s^-1·Pa^-1. This suggests that water molecules translocate fast and cooperatively through the sub-nanometer channels, similar to carbon nanotubes and membrane proteins (aquaporins). CNMs are thus scalable two-dimensional sieves that can be utilized toward energy-efficient water purification.

Quantified Binding Scale of Competing Ligands at the Surface of Gold Nanoparticles: The Role of Entropy and Intermolecular Forces

A basic understanding of the driving forces for the formation of multiligand coronas or self-assembled monolayers over metal nanoparticles is mandatory to control and predict the properties of ligand-protected nanoparticles. Herein, ¹ H nuclear magnetic resonance experiments and advanced density functional theory (DFT) modeling are combined to highlight the key parameters defining the efficiency of ligand exchange on dispersed gold nanoparticles. The compositions of the surface and of the liquid reaction medium are quantitatively correlated for bifunctional gold nanoparticles protected by a range of competing thiols, including an alkylthiol, arylthiols of varying chain length, thiols functionalized by ethyleneglycol units, and amide groups. These partitions are used to build scales that quantify the ability of a ligand to exchange dodecanethiol. Such scales can be used to target a specific surface composition by choosing the right exchange conditions (ligand ratio, concentrations, and particle size). In the specific case of arylthiols, the exchange ability scale is exploited with the help of DFT modeling to unveil the roles of intermolecular forces and entropic effects in driving ligand exchange. It is finally suggested that similar considerations may apply to other ligands and to direct biligand synthesis.

Thiolate versus Selenolate: Structure, Stability, and Charge Transfer Properties

Selenolate is considered as an alternative to thiolate to serve as a headgroup mediating the formation of self-assembled monolayers (SAMs) on coinage metal substrates. There are, however, ongoing vivid discussions regarding the advantages and disadvantages of these anchor groups, regarding, in particular, the energetics of the headgroup-substrate interface and their efficiency in terms of charge transport/transfer. Here we introduce a well-defined model system of 6-cyanonaphthalene-2-thiolate and -selenolate SAMs on Au(111) to resolve these controversies. The exact structural arrangements in both types of SAMs are somewhat different, suggesting a better SAM-building ability in the case of selenolates. At the same time, both types of SAMs have similar packing densities and molecular orientations. This permitted reliable competitive exchange and ion-beam-induced desorption experiments which provided unequivocal evidence for a stronger bonding of selenolates to the substrate as compared to the thiolates. Regardless of this difference, the dynamic charge transfer properties of the thiolate- and selenolate-based adsorbates were found to be nearly identical, as determined by the core-hole-clock approach, which is explained by a redistribution of electron density along the molecular framework, compensating the difference in the substrate-headgroup bond strength.

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.