There is plenty of room for new structures at the bottom

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.

Molecular cluster calculations within the Local Density Approximation have been performed to analyze the electronic structure of stoichiometric and different non-stoichiometric (111) surfaces of CaF2. The effect of the surrounding crystal ions, i.e. the long range electrostatic potential, have been included by a Fourier summation. Calculations for clusters representing the bulk and stoichiometric surfaces give similar results while calculations for non-stoichiometric surfaces show the existence of occupied surface states in the upper half of the bandgap. Existence of these types of occupied surface states are supported by experimental EELS studies on CaF2 as well as by observation of laser induced emission of ions and electrons from surfaces of BaF2.KeywordsLocal Density ApproximationIonic CrystalMolecular ClusterSurface ClusterSwedish Natural Science Research CouncilThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Ordered intermetallic PtCo alloys are promising candidates for next-generation low-Pt catalysts in proton exchange membrane fuel cells (PEMFCs) due to their high activity and stability originating from ligand and strain effects. However, the influence of the ordered phase on the surface structure, especially after Pt-rich shell formation, remains poorly understood. In this study, we systematically investigated the structural and electrochemical properties of PtCo catalysts with varying degrees of ordering, prepared by controlling the annealing temperature and time. We combined X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and pair distribution function (PDF) analysis to elucidate the correlation between the ordering degree, the atomic structure, and the electrochemical performance. For the first time, our detailed X-ray total scattering measurements revealed the true structural characteristics of PtCo alloys, indicating that the phase types and their relative contents vary significantly with the ordering degree. The ordered PtCo catalysts develop a Pt-rich surface layer with anisotropic strain, featuring contracted Pt-Pt distances along the surface and elongated distances across the surface, which likely contributes to its enhanced ORR activity and stability compared to their disordered counterparts. The electrochemical studies and PDF analysis suggested that the ordering transition occurs concurrently with particle growth, leading to an abrupt increase in the ORR activity at 350 °C before forming a long-range ordered phase, suggesting that local structural changes at the particle surface play a crucial role in enhancing the ORR activity. Further, operando XAS studies confirm lesser Pt-oxidation and Pt-OH formation for ordered structures than disordered ones. We believe that our findings provide new insights into the relationship between the ordering degree, particle growth, and catalytic properties of PtCo catalysts, offering guidance for the design of high-performance electrocatalysts with optimized surface structures.

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.

We perform first-principles density-functional calculations to investigate the electronic and atomic structure and formation energies of native defects and selected impurities ~O, Si, and Mg! in InN. For p-type material, the nitrogen vacancy has the lowest formation energy. In n-type material all defect formation energies are high. We discuss the effect of the band-gap underestimate in density functional theory ~DFT!, and compare the defect electronic structure obtained using DFT ~in the local-density approximation, LDA! with a recently developed self-interaction and relaxation-corrected ~SIRC! pseudopotential treatment. The SIRC calculations affect the positions of some of the defect states in the band gap, but the general conclusions obtained from the standard DFT-LDA calculations remain valid. Indium nitride is the least studied of the group-III-nitride materials, which are currently under intense investigation. Bulk InN is difficult to prepare due to its low thermal stability; 1 reliable experimental information about the properties of InN is therefore scarce. Indium-containing nitride alloys are an important constituent in devices: for example, the active layer in short-wavelength light-emitting diodes and laser diodes usually consists of In xGa12xN. Not intentionally doped InN has often been found to have very high electron densities—an observation similar to GaN before better doping control of that material was achieved. The unintentional n-type conductivity of InN has been attributed to the nitrogen vacancy (VN) or to the nitrogen antisite. 2 In order to control the material properties and ultimately the device characteristics, an understanding of native defects and impurities in the III nitrides and their alloys is essential. The calculations reported here show that neither vacancies nor antisites can explain the observed n-type conductivity of InN. We have therefore examined oxygen and silicon impurities, finding that they act as donors and that they can easily be incorporated during growth. First-principles calculations based on density functional theory ~DFT! within the local density approximation ~LDA! have produced important information about defects and impurities in semiconductors in general, and the nitrides in particular. 3,4 It is well known, however, that DFT-LDA produces band gaps significantly smaller than experiment. 5 Defects can introduce levels in the band gap; when these levels are occupied with electrons, they contribute to the total energy of the system. If the energetic position of the defect levels is incorrect due to the band-gap error, the resulting total energy may also be affected. The band-gap error results largely from a discontinuity in the exchange-correlation potential upon addition of an extra electron. 6 This discontinuity is inherent to the Kohn-Sham treatment of DFT; indeed, we have found that use of the generalized gradient approximation ~GGA! offers no improvement over LDA with respect to the band-gap problem. 7

Atomistic structures and stabilities of MgAl2O4 surfaces under processing conditions: A first-principles thermodynamics study

Within the framework of the GGA+U implementation of density functional theory, we investigate atomistic and electronic structures of Au adsorbed on the stoichiometric and the defective CeO(2){111} surfaces, in the latter of which either O or Ce vacancies are presented. We show that on the stoichiometric surface, the most stable adsorption site of Au is not on the top of the outermost O atoms, as previously suggested, but on a bridgelike site in which the Au directly binds to two O atoms. We suggest that on both sites, the original empty Ce 4f states near the Fermi level facilitate the oxidation of the Au; the preference of the Au for being on the bridgelike site is due to the larger O 2p-d(Au) mixing, accompanied by more significant electron redistributions. On the reduced surface with O vacancies, the most stable adsorption site of Au is near the vacancy position. Unlike that on the stoichiometric surface, strong ionic bonding character exists between Au and Ce, as the former becomes Au(delta-) due to the occupation of the 6s(Au) orbitals. Upon substitution for one of the Ce atoms in the lattice, the Au possesses a much stronger positive charge than that in other cases. We find that although Au is strongly bonded when it is at the Ce vacancy site, the overall binding (i.e., with the Ce vacancy formation energy being taken into account) is weaker than that for Au adsorbed at the stoichiometric surface.

FIRST-PRINCIPLES CALCULATIONS OF THE ELECTRONIC STRUCTURE OF GRAIN BOUNDARIES

We present the first-principles total-energy electronic-structure calculations that provide a firm theoretical framework to consider atomic and electronic structures of alumina surfaces. Exploring detailed atomic structures and electron states of stable and metastable surfaces of three important planes, C plane [the (0001) surface], R plane [the $(1\overline{1}02)$ surface], and A plane [the $(11\overline{2}0)$ surface], of $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}$, we find that the stoichiometric surface of the C plane has the lowest surface energy, followed by the stoichiometric surfaces of the R plane and then the A plane, irrespective of the chemical potentials of constituting elements. Detailed atomic structures for stable and metastable surfaces of each plane have been obtained, which is imperative for atom-scale clarification of reactions on the $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}$ surface. The electron states of each surface have been calculated in detail. The obtained surface energy bands offer a possibility to identify atomic structures of $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}$ surface by spectroscopic measurements. It is found that the ionic interaction between Al and O and the covalent interaction among Al atoms or O atoms are both important to determine the surface atomic structures.

Atomic structures of Si(111) surface during silicon epitaxial growth

The surface-stress tensor is an important quantity entering the elastic theory of interaction between atomic steps on solid surfaces. For the clean Si(001) surface the difference between its diagonal elements, the surface stress anisotropy Δσ, comes out by about a factor of two smaller in experiment than compared to the value obtained from previous first-principles calculations. This discrepancy was usually assigned to the presence of surface defects; as other plausible causes one could quote unrealistic atomic surface structures used for the calculations or even a failure of the local-density approximation (LDA) on which the calculations were based. This constituted a serious deficiency in the understanding of the Si(001) surface. We demonstrate that, when first-principles LDA calculations are carried out for the perfect Si(001) surface with the correct character of reconstruction, the calculated and measured Δσ agree within the experimental and numerical uncertainties. We conclude that standard LDA calculations can properly describe elastic properties of the Si(001) surface and that the effect of surface defects on the measured surface stress anisotropy is, at least for the measurements reported in the literature, much smaller than it has been previously anticipated.

The reconstructions on semipolar GaN(112̄2) and InN(112̄2) surfaces are systematically investigated on the basis of first-principles total-energy calculations. The surface formation energy clarifies that there are several reconstructions depending on the growth conditions: The surface with a Ga adatom is stable under N-rich conditions, whereas metallic reconstructions are favorable under Ga-rich (In-rich) conditions. The accumulation of electrons on the InN(112̄2) surface is also predicted from the analysis of its band structures. Furthermore, we provide immediate access to the atomic structures of semipolar surfaces under realistic growth conditions using surface phase diagrams.

We report a systematic and comprehensive investigation of reconstructions of semipolar GaN(1011) and InN(1011) surfaces by first-principles total-energy calculations. The surface formation energy indicates that there are several reconstructions depending on the growth conditions: The 1×2 surface consisting of a single Ga–Ga dimer and Ga vacancies is stable under N-rich conditions, whereas metallic reconstructions are favorable under Ga-rich (In-rich) conditions. We also characterize atomic structures of semipolar surfaces under realistic growth conditions using surface phase diagrams. For InN, the analysis of density of states predicts charge accumulations on semipolar surfaces.

The project has been productive: 47 refereed publications in about 5 years. While confined to the area of surfaces and thin films, the project has covered a wide range of physical properties and different materials: rare earths, bulk and surface alloys, metal surfaces, magnetism, and (especially) atomic and electronic structure of ultrathin films. Notable achievements include quantitative studies of atomic structure of clean rare-earth surfaces: Tb(0001), Tb(11{ovr 2}0), Gd(0001), and Gd(11{ovr 2}0). Surface alloys studied included Cu{l_brace}001{r_brace}c(2 {times} 2)-Au and Cu{l_brace}001{r_brace}c(2 {times} 2)-Pd. The most important achievement of the project lies in the application of quantitative low-energy electron diffraction to ultrathin films, particularly magnetic metals on nonmagnetic substrates (e.g., Fe on Ag{l_brace}001{r_brace}, etc.) (No data given.)

Charge-dependent atomic-scale structures of high-index and ( 110) gold electrode surfaces as revealed by scanning tunneling microscopy