Atomic and electronic structures ofα-Al2O3surfaces

The electronic structure of surfaces, which can be determined by various electron/ion spectroscopies including ultraviolet photoemission spectroscopy (UPS), is dependent on the surface atomic geometry. Thus, it is possible to determine the surface atomic structure via structure-dependent theoretical analyses of these experimental spectra. Because of the complexity of such systems, a simple but accurate theoretical scheme is required. We show that the valence electronic states and atomic structures of semiconductor surfaces—both clean and with chemisorbed species as well as relaxed and/or reconstructed—can be determined using a tight-binding model. Such calculations together with UPS data show that the annealed Si(111)7×7 surface contains about 25% vacancies. For GaAs(110), we have found tha the Ga atoms move into, and As atoms move out of the surface with small bond length distortions so that a plane through them makes an angle of about 20° with the surface. We show that H chemisorbed on Si(111) has two distinct structural phases: a monohydride phase at low coverages, and a rather unexpected trihydride phase at high coverages.

A microscopic study investigating the structure of SnSe surfaces

Developing novel catalysts with high efficiency and selectivity is critical for enabling future clean energy conversion technologies. Interfaces in catalyst systems have long been considered the most critical factor in controlling catalytic reaction mechanisms. Interfaces include not only the catalyst surface but also interfaces within catalyst particles and those formed by constructing heterogeneous catalysts. The atomic and electronic structures of catalytic surfaces govern the kinetics of binding and release of reactant molecules from surface atoms. Interfaces within catalysts are introduced to enhance the intrinsic activity and stability of the catalyst by tuning the surface atomic and chemical structures. Examples include interfaces between the core and shell, twin or domain boundaries, or phase boundaries within single catalyst particles. In supported catalyst nanoparticles (NPs), the interface between the metallic NP and support serves as a critical tuning factor for enhancing catalytic activity. Surface electronic structure can be indirectly tuned and catalytically active sites can be increased through the use of supporting oxides. Tuning interfaces in catalyst systems has been identified as an important strategy in the design of novel catalysts. However, the governing principle of how interfaces contribute to catalyst behavior, especially in terms of interactions with intermediates and their stability during electrochemical operation, are largely unknown. This is mainly due to the evolving nature of such interfaces. Small changes in the structural and chemical configuration of these interfaces may result in altering the catalytic performance. These interfacial arrangements evolve continuously during synthesis, processing, use, and even static operation. A technique that can probe the local atomic and electronic interfacial structures with high precision while monitoring the dynamic interfacial behavior in situ is essential for elucidating the role of interfaces and providing deeper insight for fine-tuning and optimizing catalyst properties. Scanning transmission electron microscopy (STEM) has long been a primary characterization technique used for studying nanomaterials because of its exceptional imaging resolution and simultaneous chemical analysis. Over the past decade, advances in STEM, that is, the commercialization of both aberration correctors and monochromators, have significantly improved the spatial and energy resolution. Imaging atomic structures with subangstrom resolution and identifying chemical species with single-atom sensitivity are now routine for STEM. These advancements have greatly benefitted catalytic research. For example, the roles of lattice strain and surface elemental distribution and their effect on catalytic stability and reactivity have been well documented in bimetallic catalysts. In addition, three-dimensional atomic structures revealed by STEM tomography have been integrated in theoretical modeling for predictive catalyst NP design. Recent developments in stable electronic and mechanical devices have opened opportunities to monitor the evolution of catalysts in operando under synthesis and reaction conditions; high-speed direct electron detectors have achieved sub-millisecond time resolutions and allow for rapid structural and chemical changes to be captured. Investigations of catalysts using these latest microscopy techniques have provided new insights into atomic-level catalytic mechanisms. Further integration of new microscopy methods is expected to provide multidimensional descriptions of interfaces under relevant synthesis and reaction conditions. In this Account, we discuss recent insights on understanding catalyst activity, selectivity, and stability using advanced STEM techniques, with an emphasis on how critical interfaces dictate the performance of precious metal-based heterogeneous catalysts. The role of extended interfacial structures, including those between core and shell, between separate phases and twinned grains, between the catalyst surface and gas, and between metal and support are discussed. We also provide an outlook on how emerging electron microscopy techniques, such as vibrational spectroscopy and electron ptychography, will impact future catalysis research.

The investigation and understanding of the atomic structure and electronic properties of semiconductor surfaces and interfaces is a challenging area of current research. Because properties at surfaces are often drastically different from those in the bulk, many fundamental aspects of physics and chemistry are involved. Achieving a better understanding is also important for the related technology. In presenting the material in this chapter, we will continually stress the interdependence of the atomic structure and the electronic structure of surfaces. In almost all cases, the development of surface and interface states is accompanied by a self-consistent atomic rearrangement. Particular examples of this are the many spatial reconstructions that arise on clean semiconductor surfaces. The examples reviewed in this chapter are typical of some of the efforts to provide insight into the atomic and electronic structure of semiconductor surfaces and interfaces. As will be evident, considerable success in determining the atomic and electronic structure of semiconductor surfaces and interfaces has been achieved in the last decade. The application of synchrotron radiation–based techniques has played an important role in this achievement.

There is plenty of room for new structures at the bottom

The atomic and electronic structures of SnO2 surfaces are closely related to their catalysis and gas detection properties. In this study, the atomic structure of SnO2 (100) and (101) surfaces has b...

X-ray absorption spectrum is a powerful tool for atomic structure detection on materials under extreme conditions. Here, we perform first-principles molecular dynamics and X-ray absorption spectrum calculations for warm dense methane under thermodynamical conditions along a Hugoniot curve. From the molecular dynamics trajectories, the detailed atomic structures are examined for each condition. The carbon K-shell X-ray absorption spectrum is calculated, and its change with temperature and pressure is discussed. The methane systems under extreme conditions may contain radicals CHx (x = 1,2,3), molecules CH4, and carbon chains CmHn (m,n >1). These various products show quite different contributions to the total X-ray spectrum due to the different atomic and electronic structures. The change of the total X-ray spectrum along the Hugoniot curve is then attributed to the change of the products induced by the temperature and pressure. Some clear signatures on the X-ray absorption spectrum under different thermodynamical conditions are proposed, which provide useful information for future X-ray experiments.

The surface atomic structure and chemical state of Pt is consequential in a variety of surface-intensive devices. Herein we present the direct interrelationship between the growth scheme of Pt films, the resulting atomic and electronic structure of Pt species, and the consequent activity for methanol electro-oxidation in Pt/TiO(2) nanotube hybrid electrodes. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements were performed to relate the observed electrocatalytic activity to the oxidation state and the atomic structure of the deposited Pt species. The atomic structure as well as the oxidation state of the deposited Pt was found to depend on the pretreatment of the TiO(2) nanotube surfaces with electrodeposited Cu. Pt growth through Cu replacement increases Pt dispersion, and a separation of surface Pt atoms beyond a threshold distance from the TiO(2) substrate renders them metallic, rather than cationic. The increased dispersion and the metallic character of Pt results in strongly enhanced electrocatalytic activity toward methanol oxidation. This study points to a general phenomenon whereby the growth scheme and the substrate-to-surface-Pt distance dictates the chemical state of the surface Pt atoms, and thereby, the performance of Pt-based surface-intensive devices.

The W(100) surface is probably the most extensively studied metallic surface in the last 15 years. Numerous studies have shown that the clean surface is reconstructed at temperatures below about 220 K, and there is now general agreement concerning the structure of the low-temperature phase. On the other hand, there are still unresolved problems and controversies regarding the structure of the surface at higher temperatures.

The atomic and electronic structure of ceria surfaces exhibiting step edges have been studied by means of periodic density functional (LDA+U and GGA+U) calculations. A variety of stoichiometric and nonstoichiometric models of increasing complexity have been designed. The electronic structure has been explored using the topological Bader analysis, the calculated magnetic moments and the ELF (electron localization function) maps. It is concluded that Ce3+ atoms may exist even in stoichiometric extended ceria samples and that the presence of oxygen vacancies in stepped surfaces also induces the presence of Ce3+ atoms although in both cases, the reduced atoms tend to occupy the sites with smallest possible coordination number.

We calculate the atomic and electronic structure of 3C- SiC（001）-（2×1） using density functional calculations within the generalized gradient approximation. The calculated results show that the atomic structure of 3C-SiC（001）-（2×1） surface can be described by dissymmetrical Si dimmer model. The bond length of Si dimmer of 3C-SiC（001）-（2×1） surface is 0.232 nm. The calculated results of electronic structure show that a prominent density of states exists at the Fermi level, so the 3C-SiC（001）-（2×1） surface has the characteristics of metal. There are four surface state bands in the gap, one of which is located near the Fermi level, another at 5 eV above Fermi level, and the others in the valence bands below Fermi level.

The surface atomic and electronic structure of GaAs(110)-p(1×1)–Bi (1 ML) has been examined using a tight-binding total-energy (TBTE) model. Two structural candidates were considered; the epitaxial continued-layer structure (ECLS) and the epitaxial on-top structure (EOTS). The TBTE computations indicate that the ECLS is more stable than the EOTS by ∼0.06 eV/surface atom/unit cell in agreement with a recent low-energy electron diffraction (LEED) intensity analysis which selected the ECLS as the best-fit structure. The predicted TBTE structural parameters for the ECLS are in good agreement with the best-fit LEED results. The energy difference between the ECLS and EOTS is much smaller for the Bi/GaAs interface than for the Sb/GaAs interface (∼0.11 eV/surface atom/unit cell) in agreement with the proposition that the relative stability of the ECLS and EOTS is a function of the overlayer-substrate mismatch; with the EOTS being favored for larger overlayer-substrate mismatch. The primary difference between the bonding of Bi and Sb to the GaAs(110) surface is the occurrence of s–p hybridization in the Bi derived surface states. Both the Bi s and p states hybridize with the substrate atom orbitals and change the character of the adsorbate–substrate bonding. We find that both the ECLS and EOTS have similar surface electronic structures; the energies and dispersions of the predicted surface bound states for both structures are in good agreement with photoemission data.

A review of the applications of the pseudopotential method and total energy techniques to the electronic and structural properties of solids is presented. With this approach, it has recently become possible to determine with accuracy crystal structures, lattice constants, bulk moduli, shear moduli, cohesive energies, phonon spectra, solid-solid phase transformations, and other static and dynamical properties of solids. The only inputs to these calculations, which are performed either with plane wave or LCAO bases, are the atomic numbers and masses of the constituent atoms. Calculations have also been carried out to study the atomic and electronic structure of surfaces, chemisorption systems, and interfaces. Results for several selected systems including the covalent semiconductors and insulators and the transition metals are discussed. The review is not exhaustive but focuses on specific prototype systems to illustrate recent progress.

Atomic structures and migration mechanisms of interphase boundaries have been of scientific interest for many years owing to their significance in the field of phase transformations. Though the interphase boundary structures can be deduced from crystallographic investigations, the detailed atomic structures and migration mechanisms of interphase boundaries during phase transformations are still poorly understood. In this study, a systematic study on atomic structures and migration mechanisms of interphase boundaries in a body-centered cubic (b.c.c.) to face-centered cubic (f.c.c.) massive transformation was carried out using the phase-field crystal model. Simulation results show that the f.c.c./b.c.c. interphase boundaries can be classified into faceted interphase boundaries and side surfaces. The faceted interphase boundaries are semi-coherent with a group of dislocations, leading to a ledge migration mechanism, while the side surfaces are incoherent and thus migrate in a continuous way. After a careful analysis of the simulated migration process of interphase boundaries at atomic scales, a detailed description of the ledge mechanism based on the motion and nucleation of interphase boundary dislocations is presented. The ledge-forming process is accompanied by the nucleation of new heterogeneous dislocations and motions of original dislocations, and thus the barrier of ledge formation comes from the hindrance of these two dislocation behaviors. Once the ledge is formed, the original dislocations continue to advance until the ledge height reaches 1/|Δg|, where Δg represents the difference in reciprocal lattice vectors between two phases. The new heterogeneous dislocation moves along the radial direction of the interphase boundary, resulting in ledge extension. The interface dislocation behaviors greatly affect the migration of the interphase boundary, leading to different migration kinetics of faceted interphase boundaries under the Kurdjumov–Sachs and the Nishiyama–Wasserman orientation relationships. This study revealed the mechanisms and kinetics of complex structure transition during a b.c.c.–f.c.c. massive phase transformation and can shed some light on the process of solid phase transformations.

We present a theory which permits for the first time a detailed analysis of the dependence of the absorption spectrum on atomic structure and cluster size. Thus, we determine the development of the collective excitations in small clusters and show that their broadening depends sensitively on the atomic structure, in particular at the surface. Results for ${\mathrm{Hg}}_{\mathit{n}}^{+}$ clusters show that the plasmon energy is close to its jellium value in the case of spherical-like structures, but is in general between ${\mathrm{\ensuremath{\omega}}}_{\mathit{p}}$/ \ensuremath{\surd}3 and ${\mathrm{\ensuremath{\omega}}}_{\mathit{p}}$/ \ensuremath{\surd}2 for compact clusters. A particular success of our theory is the identification of the excitations contributing to the absorption peaks.