Surface Science Reports最新文献

An atomic projectile colliding with a surface at kinetic energies in the thermal or hyperthermal range interacts with and is reflected by the electronic density well in front of the first layer of target atoms, and it is generally accepted that the repulsive interaction potential is proportional to the density of electrons extending outside the surface. This review develops a complete treatment of the elastic and inelastic scattering of atoms from a conducting surface in which the interaction with the electron density and its vibrations is treated using electron-phonon coupling theory. Starting from the basic principles of formal scattering theory, the elastic and inelastic scattering intensities are developed in a manner that identifies the small overlap region in the surface electron density where the projectile atom is repelled. The effective vibrational displacements of the electron gas, which lead to energy transfer through excitation of phonons, are directly related to the vibrational displacements of the atomic cores in the target crystal via electron-phonon coupling. The effective Debye-Waller factor for atom-surface scattering is developed and related to the mean square displacements of the atomic cores. The complex dependence of the Debye-Waller factor on momentum and energy of the projectile, including the effects of the attractive adsorption well in the interaction potential, are clearly defined. Applying the standard approximations of electron-phonon coupling theory for metals to the distorted wave Born approximation leads to expressions which relate the elastic and inelastic scattering intensities, as well as the Debye-Waller factor, to the well known electron-phonon coupling constant λ. This treatment reproduces the previously obtained result that the intensities for single phonon inelastic peaks in the scattered spectra are proportional to the mode specific mass correction components λ_Q,ν defined by the relationship λ = 〈λ_Q,ν〉. The intensities of elastic diffraction peaks are shown to be a weighted sum over the λ_Q,ν, and the Debye-Waller factor can also be expressed in terms of a similar weighted summation. In the simplest case the Debye-Waller exponent is shown to be proportional to λ and for simple metals, metal overlayers, and other kinds of conducting surfaces values of λ are extracted from available experimental data. This dependence of the elastic and inelastic scattering, and that of the Debye-Waller factor, on the electron-phonon coupling constant λ shows that measurements of elastic and inelastic spectra of atomic scattering are capable of revealing detailed information about the electron-phonon coupling mechanism in the surface electron density.

Water-solid interfaces play important roles across a broad range of scientific and application fields. In the past decades, atomic force microscopy (AFM) has significantly deepened our understanding of water-solid interfaces at molecular scale. In this review, we describe the recent progresses on probing water-solid interfaces by noncontact AFM, highlighting the imaging of interfacial water with ultrahigh spatial resolution. In particular, the recent development of qPlus-based AFM with functionalized tips has made it possible to directly image the H-bonding skeleton of interfacial water under UHV environment. Based on high-order electrostatic forces, such a technique even enables submolecular-level imaging of weakly bonded water structures with negligible disturbance. In addition, the three-dimensional (3D) AFM using low-noise cantilever deflection sensors can achieve atomic resolution imaging at liquid/solid interfaces, which opens up the possibility of probing the hydration layer structures under realistic conditions. We then discuss the application of those AFM techniques to various interfacial water systems, including water clusters, ion hydrates, water chains, water monolayers/multilayers and bulk water/ice on different surfaces under UHV or ambient environments. Some important issues will be addressed, including the H-bonding topology, ice nucleation and growth, ion hydration and transport, dielectric properties of water, etc. In the end, we present an outlook on the directions of future AFM studies of water at interfaces and the challenges faced by this field, as well as the development of new AFM techniques.

Metallic two-dimensional (2D) materials such as transition-metal dichalcogenides (TMDCs) and MXenes exhibit intriguing properties, including superconductivity, magnetism and electrocatalysis. Studies on the correlation between their nanoscale structures and properties can facilitate the development of photodetectors, supercapacitors, nanocatalysts, etc., but this topic has not been reviewed systematically. Here, we provide a comprehensive overview on the key factors that dictate the structures and properties of these 2D metals. We examine their phase transitions induced by structural or electronic modifications based on microscopic imaging, spectral characterization, and electrical measurements. From the perspective of surface and interface engineering, we elucidate the influences of lattice defects, dopants, and intercalated species between adjacent layers. Moreover, heterostructures involving highly conductive 2D component(s) are discussed, which may enable the observation of fascinating phenomena and/or synergistic effects due to the interlayer interactions. Finally, we provide insights into opportunities for new applications, e.g., radio-frequency antennas and electromagnetic interference shields. Feasible routes are also proposed to overcome the current challenges.

Catalytic reactions involve the direct interaction of reactants, intermediates and products with the catalyst surface. We not only need to control the atomic structure and electronic properties of the active site, but also explore the multiple molecular interactions that occur beyond the active site; they play an essential role in altering the binding and reactivity of surface species. In liquid-phase catalysis, solvents provide additional degrees of freedom in the design of the catalytic process for desirable activity and selectivity. The multi-faceted effects of solvents have a profound impact on the catalyst performance by restricting the mass transfer to the site, tuning the chemical potential of the surface species, competing for active sites, stabilizing the initial and transition states, and causing mechanistic changes by participating in the kinetically relevant elementary steps. This review addresses the different aspects of solvent effects, using a few prototype solid-liquid interfaces to illustrate these fundamental features. Recent experimental and computational studies that provide new insight at the molecular level are examined. Solvent structures in the proximity of the catalyst surface are discussed along with their influence in molecular binding and reaction at the solid-liquid interfaces. Furthermore, opportunities to alter such a solid-liquid interaction by tuning the wettability of the catalyst surfaces are explored.

The formation of chiral nanostructures via molecular assembly and reaction on solid surfaces is a ubiquitous surface process due to the symmetry-breaking at 2D surface. Studying chirality during the adsorption, assembly, and reaction of molecules on 2D solid surfaces at molecular level not only sheds deep insights into the enantioselective heterogeneous catalysis, chiral recognition, origin and evolution of chirality, and many important physical chemistry processes but also provides an important strategy to create chiral nanostructures. Here, we give a survey of recent advances in chiral expression and control in molecular assemblies and reactions on surfaces. We firstly give a brief introduction to the general concepts of chiral molecular nanostructures on surfaces. And then we focus on the induction and control of chirality expressed in molecular assemblies. The recent developments in the control strategies such as chiral co-adsorber, chiral auxiliary, chiral solvent, chiral templated surfaces, as well as the underlying mechanism to achieve the chiral induction and amplification, are reviewed. After that, we review the studies of chirality expressed in on-surface synthesis which has been proved to be a promising strategy to fabricate covalently bonded low-dimensional nanostructures and materials. In this respect, we introduce the chiral expression in the intramolecular and intermolecular coupling reactions on surfaces. In addition, we survey the methods to steer the stereoselectivity of on-surface reactions including the design of precursor structure, steric hindrance effect, substrate, kinetic parameters et al. Finally, the future outlook in this field is discussed.

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.

The direct epoxidation of propylene to propylene oxide (PO) using molecular oxygen is an attractive alternative to current production methods using chlorohydrin or hydroperoxide-mediated processes, which are environmentally harmful and expensive. Although direct ethylene epoxidation using Ag-based catalysts has been practiced industrially for decades, due to the presence of allylic hydrogen in propylene the selectivity toward epoxide is generally much lower for propylene than for ethylene. Mechanistic understanding on well-characterized surfaces of model catalysts can potentially provide guidance to effectively alter the electronic properties of the catalyst in order to increase PO selectivity. This review summarizes both experimental and theoretical studies on model catalysts for propylene epoxidation and their contributions to elucidating the reaction mechanism, intermediates, and active sites. We first show examples of experimental studies on Cu, Ag, and Au surfaces, and compare the reaction pathways and intermediates on these surfaces. Novel approaches including plasmon-mediated catalysis and utilization of shape-controlled crystal facets that open new opportunities for improving PO selectivity will also be discussed. We then describe how density functional theory (DFT) calculations have provided important insights into the reaction mechanism and active sites on Cu, Ag, and Au surfaces and clusters. Propylene oxidation pathways on other relevant metal surfaces will also be discussed. The combined experimental and computational studies elucidate the nature of surface oxygen species and the role of the oxametallacycle intermediate. We conclude by highlighting design principles and insights for guiding further development of active and selective propylene epoxidation catalysts.

Layered transition metal dichalcogenides (TMDs) are a diverse group of materials whose properties vary from semiconducting to metallic with a variety of many body phenomena, ranging from charge density wave (CDW), superconductivity, to Mott-insulators. Recent interest in topologically protected states revealed also that some TMDs host bulk Dirac- or Wyle-semimetallic states and their corresponding surface states. In this review, we focus on the synthesis of TMDs by vacuum processes, such as molecular beam epitaxy (MBE). After an introduction of these preparation methods and categorize the basic electronic properties of TMDs, we address the characterization of vacuum synthesized materials in their ultrathin limit-mainly as a single monolayer material. Scanning tunneling microscopy and angle resolved photoemission spectroscopy has revealed detailed information on how monolayers differ in their properties from multi-layer and bulk materials. The status of monolayer properties is given for the TMDs, where data are available. Distinct modifications of monolayer properties compared to their bulk counterparts are highlighted. This includes the well-known transition from indirect to direct band gap in semiconducting group VI-B TMDs as the material-thickness is reduced to a single molecular layer. In addition, we discuss the new or modified CDW states in monolayer VSe₂ and TiTe₂, a Mott-insulating state in monolayer 1T-TaSe₂, and the monolayer specific 2D topological insulator 1T′-WTe₂, which gives rise to a quantum spin Hall insulator. New structural phases, that do not exist in the bulk, may be synthesized in the monolayer by MBE. These phases have special properties, including the Mott insulator 1T-NbSe₂, the 2D topological insulators of 1T′-MoTe₂, and the CDW material 1T-VTe₂. After discussing the pure TMDs, we report the properties of nanostructured or modified TMDs. Edges and mirror twin grain boundaries (MTBs) in 2D materials are 1D structures. In group VI-B semiconductors, these 1D structures may be metallic and their properties obey Tomonaga Luttinger quantum liquid behavior. Formation of Mo-rich MTBs in Mo-dichalcogenides and self-intercalation in between TMD-layers are discussed as potential compositional variants that may occur during MBE synthesis of TMDs or may be induced intentionally during post-growth modifications. In addition to compositional modifications, phase switching and control, in particular between the 1H and 1T (or 1T′) phases, is a recurring theme in TMDs. Methods of phase control by tuning growth conditions or by post-growth modifications, e.g. by electron doping, are discussed. The properties of heterostructures of TMD monolayers are also introduced, with a focus