Molecular switching on surfaces

Molecular switching has established itself as a key functionality of building blocks developed for addressable materials and surfaces over the last two decades. Many challenges in their use and characterisation have been presented by the wide variation in interfaces studied, these ranging from truly single-molecule devices to two-dimensional self-assembled monolayers and thin films that bridge the gap between surface and macroscopically bulk materials (polymers, MOFs, COFs), and further still to other interfaces (solid–liquid, liquid–air, etc.). The low number density of molecules on monolayer-coated interfaces as well as in thin films, for example, presents substantial challenges in the characterisation of the composition of modified interfaces. The switching of molecular structure with external stimuli such as light and electrode potential adds a further layer of complexity in the characterisation of function. Such characterisation “in action” is necessary to correlate macroscopic phenomena with changes in molecular structure. In this review, key classes of molecular switches that have been applied frequently to interfaces will be discussed in the context of the techniques and approaches used for their operando characterisation. In particular, we will address issues surrounding the non-innocence of otherwise information-rich techniques and show how model – non-switching – compounds are often helpful in confirming and understanding the limitations and quirks of specific techniques.     

Interface response theory of discrete composite systems

A general theory of “interface responses” in discrete composite d-dimensional systems for operators with two-body interactions is presented. It is shown that the “interface responses” of all the internal and external interfaces of any composite system are the linear superposition of the responses to a coupling operator of all individual interfaces and of the responses to a cleavage operator of the corresponding ideal free surfaces of the same but non-interacting subsystems. The response function and its elements between two space points of the system are given by a new simple general equation as a function of these “interface responses” and of the bulk response functions of each subsystem contained in the complete real system. The present paper establishes this new general two-body theory of interface responses for surfaces, interfaces, adsorbates, membranes, superlattices, defects of any kind and dimension, ... and for the first time, to the knowledge of the author, for any d-dimensional composite system. The presentation of the theory is followed here by a few general applications.     

Catalyst design using an inverse strategy: From mechanistic studies on inverted model catalysts to applications of oxide-coated metal nanoparticles

Metal-oxide interfaces are of great importance in catalytic applications since each material can provide a distinct functionality that is necessary for efficient catalysis in complex reaction pathways. Moreover, the synergy between two materials can yield properties that exceed the superposition of single sites. While interfaces between metals and metal oxides can play a key role in the reactivity of traditional supported catalysts, significant attention has recently been focused on using “inverted” oxide/metal catalysts to prepare catalytic interfaces with unique properties. In the inverted systems, metal surfaces or nanoparticles are covered by oxide layers ranging from submonolayer patches to continuous films with thickness at the nanometer scale. Inverse catalysts provide an alternative approach for catalyst design that emphasizes control over interfacial sites, including inverted model catalysts that provide an important tool for elucidation of mechanisms of interfacial catalytic reactions and oxide-coated metal nanoparticles that can yield improved stability, activity and selectivity for practical catalysts.This review begins by providing a summary of recent progress in the use of inverted model catalysts in surface science studies, where oxides are usually deposited onto the surface of metal single crystals under ultra-high vacuum conditions. Surface-level studies of inverse systems have yielded key insights into interfacial catalysis and facilitated active site identification for important reactions such as CO oxidation, the water-gas shift reaction, and CO₂ reduction using well-defined model systems, informing strategies for designing improved technical catalysts. We then expand the scope of inverted catalysts, using the “inverse” strategy for preparation of higher-surface area practical catalysts, chiefly through the deposition of metal oxide films or particles onto metal nanoparticles. The synthesis techniques include encapsulation of metal nanoparticles within porous oxide shells to generate core-shell type catalysts using wet chemical techniques, the application of oxide overcoat layers through atomic layer deposition or similar techniques, and spontaneous formation of metal oxide coatings from more conventional catalyst geometries under reaction or pretreatment conditions. Oxide-coated metal nanoparticles have been applied for improvement of catalyst stability, control over transport or binding to active sites, direct modification of the active site structure, and formation of bifunctional sites. Following a survey of recent studies in each of these areas, future directions of inverted catalytic systems are discussed.     

Effect of elastic characteristics of the surface layer on the deformation properties of materials with an interface-controllable structure

Computer simulation is used for analyzing the possibility of changing the ultimate strain in samples of “interface” materials whose mechanical behavior is determined by strain localization at the interfaces of structural elements (blocks, grains, etc.) by controlled modification of surface layers. It is shown that a considerable improvement of the deformability of samples subjected to cyclic loading can be attained by reducing the Young modulus and the elastic limit of interfaces in the surface layer. This effect can be explained by the large volume of the material involved in irreversible strain accumulation, which suppresses strain localization in the vicinity of stress macroconcentrators and delays the crack formation.     

Magnetic surfaces

The magnetic ground state, magnetic anisotropies and spin excitations of surfaces, interfaces and ultra-thin films of ferromagnetic 3d-metals are discussed. Enhanced magnetic ground state moments and altered hyperfine fields as predicted by ab initio band calculations have not been conclusively verified by experiments up to now. Future calculations should take into account dipolar fields and the role of interface roughness. Very large magnetic anisotropies are observed at magnetic surfaces and interfaces. In Ni/Cu multilayered films, the superposition of surface and stress-induced anisotropies was used to switch the easy axis of magnetization from the film plane to a perpendicular orientation by a proper choice of the Ni layer thickness. This could be an attractive possibility to develop new magnetic materials for technical applications. The temperature dependence of the spontaneous magnetization at surfaces and in ultra-thin films deviates from the behaviour of bulk material. Size effects as well as surface effects of spin wave excitations are discussed, comparing theoretical and experimental results. The need for more complete theories including surface exchange, surface anisotropy and realistic surface structures is emphasized.     

Solid-state NMR concepts for the investigation of supported transition metal catalysts and nanoparticles

In recent years, solid-state NMR spectroscopy has evolved into an important characterization tool for the study of solid catalysts and chemical processes on their surface. This interest is mainly triggered by the need of environmentally benign organic transformations (“green chemistry”), which has resulted in a large number of new catalytically active hybrid materials, which are organized on the meso- and nanoscale. Typical examples of these catalysts are supported homogeneous transition metal catalysts or transition metal nanoparticles (MNPs). Solid-state NMR spectroscopy is able to characterize both the structures of these materials and the chemical processes on the catalytic surface. This article presents recent trends both on the characterization of immobilized homogeneous transition metal catalysts and on the characterization of surface species on transition metal surfaces.     

Surface chemistry of hot electron and metal-oxide interfaces

Fundamental mechanisms for energy conversion and dissipation on surfaces and at interfaces have been significant issues in the community of surface science. Electronic excitation in exothermic chemical reactions or photon absorption involves the generation of energetic or hot electrons that are not in thermal equilibrium via non-adiabatic electronic excitation. A number of experimental and theoretical studies have demonstrated the influence of excited hot electrons on atomic and molecular processes, and it is a key moderator in the surface energy conversion process. The charge transfer through the metal-oxide interfaces has a significant impact on catalytic performance in mixed metal-oxide catalysts. In order to understand the influence of hot electrons and metal-oxide interfaces on the surface reactions, various detection schemes of exoelectron detection, including metal-insulator-metal and metal-semiconductor Schottky diodes, have been developed. Catalysts coupled with surface plasmons exhibit peculiar catalytic performance related to hot electron flow. In this review, we outline recent research efforts to relate hot electron flow with surface reactions occurring at metal-oxide interfaces. We report recent studies on the observation of hot electrons and the correlation between hot electrons and catalytic activity and selectivity on metallic surfaces. We show recent results from studies of surface reactions on nanocatalysts coupled with surface plasmons, where hot electron transport is the key process in energy dissipation and conversion processes.     

Chemical reactions on palladium surfaces studied with Pd-MOS structures

It is pointed out that metal-oxide-semiconductor structures can be used to stfidy catalytic reactions on metal surfaces (like Pd and Pt surfaces). The flatband voltage shift induced by hydrogen at the metal-oxide interface is a measure of the amount of hydrogen in the metal, which in turn reflects the chemical reactions on the surface. Experimental results on hydrogen in argon and in air detected with Pd-MOS structures are compared with absolute reaction rate theory. The agreement between theory and experiments is surprisingly good.     

Effects of madelung potentials at surfaces of ZnO on photoemission spectra and work functions

Madelung potentials have been calculated for sites in and near polar and nonpolar, primitive and reconstructed surfaces of wurtzite ZnO. Surface versus bulk potential differences result in electron binding energy differences which should be observable in photoemission spectra even on nonpolar surfaces. In polar planes, the average site charges in the outermost ion layer are “naturally” reduced relative to charges in the crystal interior, and observable effects should be larger. Results for polar surfaces of singly bonded atoms are indicative of the Madelung potential effects expected for dense layers of chemisorbed species. These potentials are especially sensitive to details of charge transfer. If a crystal has uniform site charges except for the above-mentioned charge reduction on polar surfaces, there exist surface dipoles which would cause considerably greater variation in work function from one crystal face to another than that observed experimentally; this discrepancy is resolved if, in the outermost cation-anion layer, charges are further reduced to three-quarters to one-half the “natural” values.     

表面科学研究回顾与21世纪发展展望

表面与界面是材料物理、化学性质发生空间突变的二维区域,材料的许多重要物理、化学过程首先发生在表面,同时材料的很多破坏和失效也首先起源于表面和界面。因此,表面是材料与外部环境直接发生联系的窗口,从研究材料表面界面的各种物理化学过程入手最终可以达到改善材料性能的目的。过去１０年中,材料表面科学在促进材料科学基础研究、推动新材料新技术发展中发挥了关键作用。世纪之交乃至下一世纪,表面科学将面临新的挑战和机     

Nanoscale depth-resolved cathodoluminescence spectroscopy of ZnO surfaces and metal interfaces

The electronic properties of ZnO surfaces and interfaces has until recently been relatively unexplored. We have used a complement of ultrahigh vacuum scanning electron microscope (SEM)-based, depth-resolved cathodoluminescence spectroscopy (DRCLS), temperature-dependent charge transport, trap spectroscopy, and surface science techniques to probe the electronic and chemical properties of clean surfaces and interfaces on a nanometer scale. DRCLS reveals remarkable nanoscale correlations of native point defect distributions with surface and sub-surface defects calibrated with capacitance trap spectroscopies, atomic force microscopy, and Kelvin probe force microscopy. The measurement of these near-surface states associated with native point defects in the ZnO bulk and those induced by interface chemical bonding is a powerful extension of cathodoluminescence spectroscopy that provides a guide to understanding and controlling ZnO electronic contacts.     

Elementary processes at semiconductor/electrolyte interfaces: perspectives and limits of electron spectroscopy

Semiconductor device properties based on electrolyte contacts or modified by electrochemical reactions are dominated by the electronic structure of the interface. Electron spectroscopy as e.g. photoemission is the most appropriate surface science techniques to investigate elementary processes at semiconductor/electrolyte interfaces. For such investigations a specific experimental set-up (SoLiAS) has been built-up which allows performing model experiments as well as surface analysis after emersion under different experimental conditions. The experimental approach is presented by a number of experiments performed during the last years with GaAs as substrate material. Model experiments by adsorption and coadsorption of electrolyte species give information on fundamental aspects of semiconductor/electrolyte interactions. Emersion experiments give information on a final composition and the related electronic structure of electrodes after electrochemical reactions. The use of frozen electrolytes will help to bridge the gap between these two approaches. With the combination of the experimental procedures one may expect a detailed analysis of electrolyte (modified) interfaces covering chemical composition, electronic structure of surfaces/interfaces as well as surface/interface potentials.     

Photoemission spectroscopy investigation of the electronic structure of carbidic and graphitic carbon on Ni (111)

Photoemission measurements have been carried out on different forms of carbon deposits of catalytic importance on the (111) face of nickel, and give a picture of the electronic structure of “carbidic” and “graphitic” carbon.The photoemission spectra of graphitic overlayers strongly resemble spectra taken on graphite bulk samples but show an (almost) rigid shift of the levels with respect to the Fermi level.The results we obtain for carbidic carbon can be understood on the basis of recent surface electronic structure calculations. In particular a p-like state has been observed very near the Fermi level, which is probably responsible for the high chemical reactivity of carbidic carbon.     

Work function changes during low pressure oxidation of aluminum at room temperature

The oxidation kinetics of clean bulk aluminum surfaces exposed to low oxygen pressures (10^?8 to 2 × 10^?6 torr) at room temperature were followed by measurement of work function changes. The interaction of oxygen with aluminum resulted in a decrease in the work function, indicating that the chemisorbed surface oxygen was unstable and incorporation into the subsurface region took place even at very small surface coverages. A “stable layer” of constant work function was finally formed on the aluminum surface. The limiting work functio nchange (work function change to form the “stable layer”) became less negative with increase in oxygen pressure. It was not possible to explain these results on the basis of previously published oxygen uptake models. A new model based on a pressure dependent limiting amount of outer surface and incorporated oxygen is proposed to explain the pressure dependence of the limiting work function change.     

Peculiarities of Dielectric Barrier Discharges

The dielectric barrier discharge (DBD) is a highly transient, non‐thermal discharge form, which exists in a broad pressure range. It occurs in arrangements, where a dielectric layer covers at least one electrode. The dielectric quenches the current and distributes the discharge over the whole surface. Depending on the geometrical conditions three basic types of DBD arrangements are distinguished. In arrangements with a gas gap a filamentary or a homogeneous‐diffuse discharge mode appears. The (stable) filamentary mode consists of a multitude of microdischarges, which in some extent can be rather easily tailored for e. g. plasma‐chemical applications. In arrangements with a long electrode (or several in parallel) on a dielectric surface and a plane counter‐electrode on the reverse side of the dielectric, pure surface discharges can be observed. They are characterised by low ignition voltages. The extension of the discharge on the surface depends on the voltage amplitude. If pairs of long electrodes are within the bulk of a dielectric, discharge phenomena appear on the surface of the dielectric. As these devices can be produced with small and precise electrode gaps, high mean field strengths in the discharge region can be realised. The properties of the discharges in these arrangements as well as their dynamics are described in detail and compared with one another. The advantages of each type are highlighted. Some aspects, which may be of interest for plasma‐chemical reactions on surfaces and in the gas space are discussed.     

The interaction of water with solid surfaces: fundamental aspects revisited

Water is perhaps the most important and most pervasive chemical on our planet. The influence of water permeates virtually all areas of biochemical, chemical and physical importance, and is especially evident in phenomena occurring at the interfaces of solid surfaces. Since 1987, when Thiel and Madey (TM) published their review titled ‘The interaction of water with solid surfaces: fundamental aspects’ in Surface Science Reports, there has been considerable progress made in further understanding the fundamental interactions of water with solid surfaces. In the decade and a half, the increased capability of surface scientists to probe at the molecular-level has resulted in more detailed information of the properties of water on progressively more complicated materials and under more stringent conditions. This progress in understanding the properties of water on solid surfaces is evident both in areas for which surface science methodology has traditionally been strong (catalysis and electronic materials) and also in new areas not traditionally studied by surface scientists such as electrochemistry, photoconversion, mineralogy, adhesion, sensors, atmospheric chemistry and tribology. Researchers in all these fields grapple with very basic questions regarding the interactions of water with solid surfaces such as how is water adsorbed, what are the chemical and electrostatic forces that constitute the adsorbed layer, how is water thermally or non-thermally activated and how do coadsorbates influence these properties of water. The attention paid to these and other fundamental questions in the past decade and a half has been immense. In this review, experimental studies published since the TM review are assimilated with those covered by TM to provide a current picture of the fundamental interactions of water with solid surfaces.     

Structure and fluctuations of the premelted liquid film of ice at the triple point

ABSTRACT

In this paper, we study the structure of the ice/vapour interface in the neighbourhood of the triple point for the TIP4P/2005 model. We probe the fluctuations of the ice/film and film/vapour surfaces that separate the liquid film from the coexisting bulk phases at basal, primary prismatic and secondary prismatic planes. The results are interpreted using a coupled sine Gordon plus Interface Hamiltonian model. At large length scales, the two bounding surfaces are correlated and behave as a single complex ice/vapour interface. For small length, on the contrary, the ice/film and film/vapour surfaces behave very much like independent ice/water and water/vapour interfaces. The study suggests that the basal facet of the TIP4P/2005 model is smooth, the prismatic facet is close to a roughening transition, and the secondary prismatic facet is rough. For the faceted basal face, our fluctuation analysis allows us to estimate the step free energy in good agreement with experiment. Our results allow for a quantitative characterisation of the extent to which the adsorbed quasi-liquid layer behaves as water and explains experimental observations which reveal similar activation energies for crystals grown in bulk vapour or bulk water.     

Static polarizability effects on counterion distributions near charged dielectric surfaces: A coarse-grained Molecular Dynamics study employing the Drude model

Coarse-grained implicit solvent Molecular Dynamics (MD) simulations have been used to investigate the structure of the vicinal layer of polarizable counterions close to a charged interface. The classical Drude oscillator model was implemented to describe the static excess polarizability of the ions. The electrostatic layer correction with image charges (ELCIC) method was used to include the effects of the dielectric discontinuity between the aqueous solution and the bounding interfaces for the calculation of the electrostatic interactions. Cases with one or two charged bounding interfaces were investigated. The counterion density profile in the vicinity of the interfaces with different surface charge values was found to depend on the ionic polarizability. Ionic polarization effects are found to be relevant for ions with high excess polarizability near surfaces with high surface charge.     

The Atomic Structure of Oxide/Oxide Interface

The physical and chemical properties of thin or ultrathin oxide film deposited on another oxide bulk or thin film usually differ strongly from the bulk. The properties of the heterostructures ultimately rely on the structure and the chemistry of the oxide/oxide interface. Data in the literature indicated that atomically abrupt interfaces between oxides show abnormal electronic and magnetic properties. This article reviews the interfacial structures of oxide/oxide interfaces in an atomic scale. The origins of the unique physical and chemical properties at the oxide/oxide interfaces are also discussed.