Interfacial Electrostatics of Self-Assembled Monolayers of Alkane Thiolates on Au(111): Work Function Modification and Molecular Level Alignments

Stimuli-Responsive Oligourea Molecular Films

We have designed and synthesized a helical cysteamine-terminated oligourea foldamer composed of ten urea residues featuring side carboxyl and amine groups. The carboxyl group is located in proximity to the C-terminus of the oligourea and hence at the negative pole of the helix dipole. The amine group is located close to the N-terminus and hence at the positive pole of the helix dipole. Beyond the already remarkable dipole moment inherent in oligourea 2.5 helices, the incorporation of additional charges originating from the carboxylic and amine groups is supposed to impact the overall charge distribution along the molecule. These molecules were self-assembled into monolayers on a gold substrate, allowing us to investigate the influence of an electric field on these polar helices. By applying surface-enhanced infrared reflection-absorption spectroscopy, we proved that molecules within the monolayers tend to reorient themselves more vertically when a negative bias is applied to the surface. It was also found that surface-confined oligourea molecules affected by the external electric field tend to rearrange the electron density at urea groups, leading to the stabilization of the resonance structure with charge transfer character. The presence of the external electric field also affected the nanomechanical properties of the oligourea films, suggesting that molecules also tend to reorient in the ambient environment without an electrolyte solution. Under the same conditions, the helical oligourea displayed a robust piezoresponse, particularly noteworthy given the slender thickness of the monolayer, which measured approximately 1.2 nm. This observation demonstrates that thin molecular films composed of oligoureas may exhibit stimulus-responsive properties. This, in turn, may be used in nanotechnology systems as actuators or functional films, enabling precise control of their thickness in the range of even fractions of nanometers.

Capturing the Coverage Dependence of Aromatics' Adsorption through Mean-Field Models

To capture the dominant interactions (surface-mediated and through-space) in catalytic hydrodeoxygenation systems, coverage-dependent mean-field models of aromatic adsorption are developed on Pt(111) and Ru(0001). We derive three key insights from this work: (1) we can universally apply mean-field models to capture the coverage-dependent behavior of oxygenated aromatics on transition-metal surfaces, (2) we can deconvolute surface-mediated and through-space interactions from the mean-field model, and (3) we can develop relatively accurate models that predict the adsorption energy of aromatics on transition-metal surfaces for the full coverage range using the work function at the lowest modeled coverage. Our approach enables the rapid prediction of the coverage-dependent behavior of oxygenated aromatics on transition-metal surfaces, reducing the computational cost associated with these studies by an order of magnitude.

A Paradigm Shift from 2D to 3D: Surface Supramolecular Assemblies and Their Electronic Properties Explored by Scanning Tunneling Microscopy and Spectroscopy

Exploring supramolecular architectures at surfaces plays an increasingly important role in contemporary science, especially for molecular electronics. A paradigm of research interest in this context is shifting from 2D to 3D that is expanding from monolayer, bilayers, to multilayers. Taking advantage of its high-resolution insight into monolayers and a few layers, scanning tunneling microscopy/spectroscopy (STM/STS) turns out a powerful tool for analyzing such thin films on a solid surface. This review summarizes the representative efforts of STM/STS studies of layered supramolecular assemblies and their unique electronic properties, especially at the liquid-solid interface. The superiority of the 3D molecular networks at surfaces is elucidated and an outlook on the challenges that still lie ahead is provided. This review not only highlights the profound progress in 3D supramolecular assemblies but also provides researchers with unusual concepts to design surface supramolecular structures with increasing complexity and desired functionality.

The application of QM/MM simulations in heterogeneous catalysis

The QM/MM simulation method is provenly efficient for the simulation of biological systems, where an interplay of extensive environment and delicate local interactions drives a process of interest through a funnel on a complex energy landscape. Recent advances in quantum chemistry and force-field methods present opportunities for the adoption of QM/MM to simulate heterogeneous catalytic processes, and their related systems, where similar intricacies exist on the energy landscape. Herein, the fundamental theoretical considerations for performing QM/MM simulations, and the practical considerations for setting up QM/MM simulations of catalytic systems, are introduced; then, areas of heterogeneous catalysis are explored where QM/MM methods have been most fruitfully applied. The discussion includes simulations performed for adsorption processes in solvent at metallic interfaces, reaction mechanisms within zeolitic systems, nanoparticles, and defect chemistry within ionic solids. We conclude with a perspective on the current state of the field and areas where future opportunities for development and application exist.

Rationalizing energy level alignment by characterizing Lewis acid/base and ionic interactions at printable semiconductor/ionic liquid interfaces

Charge transfer and energy conversion processes at semiconductor/electrolyte interfaces are controlled by local electric field distributions, which can be especially challenging to measure. Herein we leverage the low vapor pressure and vacuum compatibility of ionic liquid electrolytes to undertake a layer-by-layer, ultra-high vacuum deposition of a prototypical ionic liquid EMIM⁺ (1-ethyl-3-methylimidazolium) and TFSI^- (bis(trifluoromethylsulfonyl)-imide) on the surfaces of different electronic materials. We consider a case-by-case study between a standard metal (Au) and four printed electronic materials, where interfaces are characterized by a combination of X-ray and ultraviolet photoemission spectroscopies (XPS/UPS). For template-stripped gold surfaces, we observe through XPS a preferential orientation of the TFSI anion at the gold surface, enabling large electric fields (∼10⁸ eV m^-1) within the first two monolayers detected by a large surface vacuum level shift (0.7 eV) in UPS. Conversely, we observe a much more random orientation on four printable semiconductor surfaces: methyl ammonium lead triiodide (MAPbI₃), regioregular poly(3-hexylthiophene-2,5-diyl (P3HT)), sol-gel nickel oxide (NiO_x), and PbI_x-capped PbS quantum dots. For the semiconductors considered, the ionization energy (IE) of the ionic liquid at 3 ML coverage is highly substrate dependent, indicating that underlying chemical reactions are dominating interface level alignment (electronic equilibration) prior to reaching bulk electronic structure. This indicates there is no universal rule for energy level alignment, but that relative strengths of Lewis acid/base sites and ion-molecular interactions should be considered. Specifically, for P3HT, interactions are found to be relatively weak and occurring through the π-bonding structure in the thiophene ring. Alternatively, for NiO_x, PbS/PbI_x quantum dots, and MAPbI₃, our XPS data suggest a combination of ionic bonding and Lewis acid/base reactions between the semiconductor and IL, with MAPbI₃ being the most reactive surface. Collectively, our results point towards new directions in interface engineering, where strategically chosen ionic liquid-based anions and cations can be used to preferentially passivate and/or titrate surface defects of heterogeneous surfaces while simultaneously providing highly localized electric fields. These opportunities are expected to be translatable to opto-electronic and electrochemical devices, including energy conversion and storage and biosensing applications.

Interplay of Fermi Level Pinning, Marcus Inverted Transport, and Orbital Gating in Molecular Tunneling Junctions

This Letter examines the interplay of important tunneling mechanisms-Fermi level pinning, Marcus inverted transport, and orbital gating-in a molecular rectifier. The temperature dependence of the rectifying molecular junction containing 2,2'-bipyridyl terminated n-alkanethiolate was investigated. A bell-shaped trend of activation energy as a function of applied bias evidenced the dominant occurrence of unusual Marcus inverted transport, while retention of rectification at low temperatures implied that the rectification obeyed the resonant tunneling regime. The results allowed reconciling two separately developed transport models, Marcus-Landauer energetics and Fermi level pinning-based rectification. Our work shows that the internal orbital gating can be substituted with the pinning effect, which pushes the transport mechanism into the Marcus inverted regime.

Structural Rearrangement of Au-Pd Nanoparticles under Reaction Conditions: An ab Initio Molecular Dynamics Study

The structure, composition, and atomic distribution of nanoalloys under operating conditions are of significant importance for their catalytic activity. In the present work, we use ab initio molecular dynamics simulations to understand the structural behavior of Au-Pd nanoalloys supported on rutile TiO₂ under different conditions. We find that the Au-Pd structure is strongly dependent on the redox properties of the support, originating from strong metal-support interactions. Under reducing conditions, Pd atoms are inclined to move toward the metal/oxide interface, as indicated by a significant increase of Pd-Ti bonds. This could be attributed to the charge localization at the interface that leads to Coulomb attractions to positively charged Pd atoms. In contrast, under oxidizing conditions, Pd atoms would rather stay inside or on the exterior of the nanoparticle. Moreover, Pd atoms on the alloy surface can be stabilized by hydrogen adsorption, forming Pd-H bonds, which are stronger than Au-H bonds. Our work offers critical insights into the structure and redox properties of Au-Pd nanoalloy catalysts under working conditions.

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 .

The role of reducible oxide-metal cluster charge transfer in catalytic processes: new insights on the catalytic mechanism of CO oxidation on Au/TiO2 from ab initio molecular dynamics

To probe metal particle/reducible oxide interactions density functional theory based ab initio molecular dynamics studies were performed on a prototypical metal cluster (Au20) supported on reducible oxides (rutile TiO2(110)) to implicitly account for finite temperature effects and the role of excess surface charge in the metal oxide. It is found that the charge state of the Au particle is negative in a reducing chemical environment whereas in the presence of oxidizing species coadsorbed to the oxide surface the cluster obtained a net positive charge. In the context of the well-known CO oxidation reaction, charge transfer facilitates the plasticization of Au20, which allows for a strong adsorbate induced surface reconstruction upon addition of CO leading to the formation of mobile Au-CO species on the surface. The charging/discharging of the cluster during the catalytic cycle of CO oxidation enhances and controls the amount of O2 adsorbed at oxide/cluster interface and strongly influences the energetics of all redox steps in catalytic conversions. A detailed comparison of the current findings with previous studies is presented, and generalities about the role of surface-adsorbate charge transfer for metal cluster/reducible oxide interactions are discussed.

Exploring the tilt-angle dependence of electron tunneling across molecular junctions of self-assembled alkanethiols

Electronic transport mechanisms in molecular junctions are investigated by a combination of first-principles calculations and current-voltage measurements of several well-characterized structures. We study self-assembled layers of alkanethiols grown on Au(111) and form tunnel junctions by contacting the molecular layers with the tip of a conductive force microscope. Measurements done under low-load conditions permit us to obtain reliable tilt-angle and molecular length dependencies of the low-bias conductance through the alkanethiol layers. The observed dependence on tilt-angle is stronger for the longer molecular chains. Our calculations confirm the observed trends and explain them as a result of two mechanisms, namely, a previously proposed intermolecular tunneling enhancement as well as a hitherto overlooked tilt-dependent molecular gate effect.

Investigations of an O-H...S hydrogen bond via Car-Parrinello and path integral molecular dynamics

The presence of intramolecular hydrogen bonds influences the binding energy, tautomeric equilibrium, and spectroscopic properties of various classes of organic molecules. This article discusses the O-H...S bridge, one of the less commonly investigated types of intramolecular interactions. 3-mercapto-1,3-diphenylprop-2-en-1-one was considered as the model structure. This compound exhibits photochromic properties. Car-Parrinello molecular dynamics (CPMD) was applied to investigate the spectroscopic and molecular properties of this compound in the gas phase and in the solid state. The second part of the study is devoted to the effects of the quantization of nuclear motions, with special attention to the O-H...S moiety. Path integral molecular dynamics (PIMD) of the molecular crystal of 3-mercapto-1,3-diphenylprop-2-en-1-one was carried out for this purpose. The employment of this fully quantum mechanical technique enables one to study, in a time-averaged sense, the zero-point motion important for flat potential energy surfaces. Finally, the potentials of mean force (Pmfs) were calculated from the CPMD and PIMD data obtained for the solid-state calculations. The effect of including quantum nuclear motion was investigated. In the studied compound, quantum effects shortened the H-bridge and provided a better description of the free energy minimum. The computational results place this uncommon intramolecular H-bonding among the class of strong hydrogen bonds with large red shifts of O-H stretching modes, which correspond well with previously presented experimental data in the literature concerning this structure.

Determination of the Potential of Zero Charge of Au(111) Modified with Thiol Monolayers

A new method is proposed for the determination of the potential of zero charge of gold electrodes modified with thiol monolayers. It makes use of the immersion technique, in combination with a vapor deposition protocol to build the thiol monolayers. As compared to previous methods, the present approach provides more accurate results, particularly in the case of long-chain alkanethiol monolayers, and it is applicable to any monolayer irrespective of its degree of hydrophilicity. Results are presented for a series of 12 alkanethiol monolayers and for 11-mercaptoundecanol and 11-mercaptoundecanoic acid monolayers. Good agreement is found between the variation of the potential of zero charge along the alkanethiol series with the corresponding change of the surface work function. The potential of zero charge of the 11-mercaptoundecanoic acid monolayer is shown to depend on the extent of dissociation of the acid, thus opening the possibility of applying this type of measurements to the study of surface ionization processes.

Interface and Molecular Electronic Structure vs Tunneling Characteristics of CH3- and CF3-Terminated Thiol Monolayers on Au(111)

By means of density functional theory calculations, we investigate work functions, energy level alignments, charge transfers, and tunneling characteristics of CH3- and CF3-terminated alkane- and diphenylthiol monolayers on Au(111). While the alignments of the energy levels and the charge transfers at the metal-molecule interface are found to be determined by the value of the clean Au surface work function relative to the HOMO ionization potential (IP) at the thiolate end of the monolayer, the change of work function for the modified Au(111) surface is dominated by the properties of the thiolate monolayer, including the character, saturated or conjugated, of the molecule and the chemical nature and orientation of the terminal group. The tunneling currents through the adsorbed molecular monolayers are calculated using the Tersoff-Hamann approach. The computed difference between the I-V characteristics for the CH3- and CF3-terminated alkanethiol monolayers agree well with available experimental data. The energy barrier at the metal-molecule interface, the molecular electronic structure, and the IP of the terminal group are the key parameters which determine the tunneling properties.