Electronic states and atomic structure at the Pd2Si–Si interface

Grain boundaries (GBs) modulate the macroscopic properties in polycrystalline materials because they have different atomic and electronic structures from the bulk. Despite the progress on the understanding of GB atomic structures, knowledge of the localized electronic band structures is still lacking. Here, we experimentally characterized the atomic structures and the band gaps of four typical GBs in α-Al2O3 by scanning transmission electron microscopy and valence electron energy-loss spectroscopy (EELS). It was found that the band gaps of the GBs are narrowed by 0.5-2.1 eV compared with that of 8.8 eV in the bulk. By combing core-loss EELS with first-principles calculations, we elucidated that the band gap reductions directly correlate with the decrease of the coordination numbers of Al and O ions at the GBs. These results provide in-depth understanding between the local atomic and electronic band structures for GBs and demonstrate a novel electronic-structure analysis for crystalline defects.

CONSPECTUS: The ultimate goal of solid state chemistry is to gain a clear correlation between atomic, defect, and electronic structure and intrinsic properties of solid state materials. Solid materials can generally be classified as amorphous, quasicrystalline, and crystalline based on their atomic arrangement, in which crystalline materials can be further divided into single crystals, microcrystals, and nanocrystals. Conventional solid state chemistry mainly focuses on studying single crystals and microcrystals, while recently nanocrystals have become a hot research topic in the field of solid state chemistry. As more and more nanocrystalline materials have been artificially fabricated, the solid state chemistry for studying those nanosolids has become a new subdiscipline: solid state nanochemistry. However, solid state nanochemistry, usually called "nanochemistry" for short, primarily studies the microstructures and macroscopic properties of a nanomaterial's aggregation states. Due to abundant microstructures in the aggregation states, it is only possible to build a simple but imprecise correlation between the microscopic morphology and the macroscopic properties of the nanostructures. Notably, atomically thin two-dimensional inorganic materials provide an ideal platform to establish clear structure-property relationships in the field of solid state nanochemistry, thanks to their homogeneous dispersion without the assistance of a capping ligand. In addition, their atomic structures including coordination number, bond length, and disorder degree of the examined atoms can be clearly disclosed by X-ray absorption fine structure spectroscopy. Also, their more exposed interior atoms would inevitably induce the formation of various defects, which would have a non-negligible effect on their physicochemical properties. Based on the obtained atomic and defect structural characteristics, density-functional calculations are performed to study their electronic structures. Then, after the properties of the individual ultrathin two-dimensional materials or their assembled highly oriented thin film-based nanodevices are measured, the explicit relationship between atomic, defect, and electronic structure and intrinsic properties could be established. In this Account, we focus on our recent advances in the field of solid state nanochemistry, including atomic structure characterization of ultrathin two-dimensional inorganic materials by X-ray absorption fine structure spectroscopy, characterization of their different types of structural defects by positron annihilation spectra and electron spin resonance, and investigation of their electronic structure by density-functional calculations. In addition, we summarize the close correlation between atomic, defect, and electronic structure variations and the optoelectronic, electrical, magnetic, and thermal properties of ultrathin two-dimensional materials. Finally, we also propose the major challenges and opportunities that face solid state nanochemistry. We believe that all the past achievements in ultrathin two-dimensional materials could bring new opportunities for solid state nanochemistry.

Obtaining electronic and atomic structure of material simultaneously is very important for developing the nano-technology. In this paper, we demonstrate that atomic and electronic structure of an interface can be extracted with combination of Gerchberg-Saxton indirect microscopy and electron spectroscopy imaging (ESI) technique. Basically, Gerchberg-Saxton algorithm includes two projections. Projection in the real space is a maximum entropy (ME) de-convolution process and in reciprocal space is an amplitude substitution process. It has been shown that Gerchberg-Saxton algorithm can extend the structural resolution to near 0.1nm. An application case of Gerchberg-Saxton algorithm to solve the atomic structure for 3C-polytypic SiC boundary is shown.ESI spectrum processed by FFT interpolation, maximum entropy de-convolution and wavelet transformation allow us to extract 2-dimensional map of the sp2/sp3 with a sub-nanometer resolution. Grain boundary and interface at SiC are good candidates for this study, since the bond distance of Si-C is slightly less than 0.1nm which is not routinely resolvable using a FEG TEM and Si-L (99eV) and C-K-edges (283 eV) locate in a reasonable energy range. The resultant electronic structure can be compared with that calculated using WIEN97. An example of quantitative analysis on 2-dimensional sp3/sp2 map deduced from the C K-edge of ESI spectra acquired from 6H-SiC is given.

Atomistic structures and electronic states on the initial stage of Fe/H:Si(111) interface formation

Monodisperse faceted nanocrystals, with controllable shapes and sizes, have been becoming increasingly important for applications in catalysis, gas sensing, and energy conversion. Such highly shape sensitive and selective physical and chemical properties inherently stem from the atomic and electronic structures on the faceted surfaces. For elemental nanocrystals, the atomic structure on the surfaces is determined by the geometric shape itself. However, for compound materials such as alloys and complex oxides, the compositional segregation and different terminating lattice planes on the surfaces have to be taken into account. In order to understand the unique property and growth mechanism of these nanocrystals, atomic details on the faceted surfaces need to be studied on the atomic level. Strontium titanate (SrTiO 3 ), strontium zirconate (SrZrO 3 ) and their solid solutions (SrTi 1−x Zr x O 3 ) are important members in the class of perovskite structures with a general formula ABO 3 (Figure 1a). These materials are of great technological and fundamental importance not only because of their interesting properties, but also because of their ability to combine and to adjust these properties by chemical substitution with a wide variety of cations. However, despite the success of the synthesis of the {1 0 0}‐faceted BaTiO 3 , SrTiO 3 , and Ba 1−x Sr x TiO 3 nanocubes, whether the {1 0 0} facets of the nanocubes are terminated with AO (SrO) or BO 2 (TiO 2 ) is a question which still remains open for speculation and investigation. A comprehensive understanding of the growth mechanisms of these faceted nanocubes has not been achieved. Direct experimental evidence for the atomic structure on these nanocube surfaces has become one of the key steps in exploring the growth mechanisms. In this work, we report on detailed studies of monodisperse {1 0 0}‐faceted nanocubes of SrTi 1−x Zr x O 3 (x = 0.25 to 0.5) which were synthesized using the oil‐water two‐phase solvothermal method. The surface atomic structure of the monodisperse faceted nanocrystals is determined by means of aberration‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM). On the basis of the structural features on the faceted surfaces, a deeper insight into the growth mechanisms could be obtained.

In this article, the theoretical photoionization spectroscopy of the aziridine (C2H5N) molecule is investigated. To start with, we have optimized the geometry of this molecule at the neutral electronic ground state at the density functional theory/augmented correlation-consistent polarized valence triple zeta level of theory using the G09 program. The electronic structure calculations were restricted to the first six low-lying electronic states in order to account for the experimental photoelectron spectrum of the C2H5N molecule. The first six low-lying electronic states (X̃2A', Ã2A', B̃2A″, C̃2A″, D̃2A', and Ẽ2A') of the potential energy surfaces (PESs) are calculated by both equation of motion-ionization potential-coupled cluster singles and doubles and multi-configuration quasi-degenerate perturbation theory abinitio quantum chemistry methods along the dimensionless normal displacement coordinates in which multiple conical intersections were established among the considered electronic states. A (6 × 6) model vibronic Hamiltonian is constructed on a diabatic electronic basis, using the symmetry selection rules and Taylor series expansion. The Cs symmetry point group of the aziridine molecule leads to electronic states symmetry of either A' or A″, and these states are close in energy, due to which the same symmetry electronic states avoid each other. To get a smooth diabatic PES, a fourfold diabatization scheme is used, which is implemented in the General Atomic and Molecular Electronic Structure Systems suite of programs. All the parameters used in the diabatic vibronic coupling model Hamiltonian are calculated in terms of the normal modes of vibrational coordinates. Finally, the vibronic model Hamiltonian constructed for the coupled six electronic states is used to solve both time-independent and time-dependent Schrödinger equations using the multi-configuration time-dependent Hartree program module to obtain the dynamical observables. The theoretical vibronic band structure is found to be in good accord with the available experimental results.

There is plenty of room for new structures at the bottom

Calculations of the atomic and electronic structure of Ca-Mg glasses and of the crystalline intermetallic compound ${\mathrm{CaMg}}_{2}$ are presented. For the glasses, the calculations are based on realistic models of the atomic structure constructed by a molecular-dynamics computer simulation linked to a steepest-descent potential-energy mapping. Interatomic forces are described in terms of pseudopotential-derived effective pair potentials. Our results for the atomic structure are in very good agreement with x-ray-diffraction data. They show that the topological short-range order in the glass is similar to the atomic arrangement in the crystalline Laves phase and best described as a highly defective tetrahedral close packing. The analogy between the glassy and the crystalline phases extends to the electronic structure. In both the crystalline and the amorphous alloys we find an essentially free-electron-like density of states (DOS), but close to the Fermi level the DOS is enhanced due to the onset of Ca 3d states. We find no evidence for the existence of a structure-induced minimum in the DOS near ${E}_{F}$. Our results are in good agreement with photoemission measurements and electronic specific-heat data.

Selective synthesis of Ni12P5 and Ni2P nanoparticles: Electronic structures, magnetic and optical properties

The atomic structures and electronic states of CaCO${}_{3}$ surfaces in calcite and aragonite polymorph are theoretically investigated by performing electronic structure calculations within the density-functional theory. The calculated surface energies demonstrate that in both calcite and aragonite structures the surfaces, containing the same number of Ca atoms as CaCO${}_{3}$ groups around the top layer, are stabilized irrespective of the chemical potentials of constituting elements. The analysis of the electronic structures clarifies that the transfer of electrons between 4$s$ orbitals of Ca atoms into 2$p$ orbitals of O atoms, which are located near the surface, is crucial for the stabilization. Furthermore, we find the surface states caused by 2$p$ orbitals of the O atoms in triangular CO${}_{3}^{2\ensuremath{-}}$ located at the top or second layer appear above the valence band maximum of bulk CaCO${}_{3}$, leading to the reduction of C-O bonds of carbonate groups near the surfaces. The emergence of these surface states implies a possibility to identify the atomic structures by spectroscopic measurements.

Ab-initio calculations of the atomic and electronic structure of metallic glasses

Unveiling the mechanism of charge compensation in Li2RuxMn1−xO3 by tracking atomic structural evolution

First-principles study of structure, electronic properties and stability of tungsten adsorption on TiC(111) surface with disordered vacancies

Effects of stress on the formation of titanium and nickel silicides on silicon-on-insulator (SOI) wafers have been investigated by sheet resistance measurements, glancing-angle x-ray diffraction analysis and transmission electron microscopy. Tensile stress was induced by the difference of thermal expansion coefficients of Si and SiO/sub 2/. The thinner the top silicon layer, the higher tensile stress was induced. The formation of both Ti and Ni silicides was found to be enhanced by the tensile stress. The results indicated that the tensile stress promotes the atomic diffusion at the interface to facilitate the formation of metal silicides.

In the current decade, perovskite solar cell research has emerged as a remarkably active, promising, and rapidly developing field. Alongside breakthroughs in synthesis and device engineering, halide perovskite photovoltaic materials have been the subject of predictive and explanatory computational work. In this Minireview, we focus on a subset of this computation: density functional theory (DFT)-based work highlighting the ways in which the electronic structure and band gap of this class of materials can be tuned via changes in atomic structure. We distill this body of computational literature into a set of underlying design principles for the band gap engineering of these materials, and rationalize these principles from the viewpoint of band-edge orbital character. We hope that this perspective provides guidance and insight toward the rational design and continued improvement of perovskite photovoltaics.