The Structure of Atoms

ABSTRACTNanotechnology is an area of research that is highly intriguing because of the novel properties often observed for materials whose sizes are reduced to the nanoscale. However, one of the biggest challenges is understanding the underlying principles that dictate the particles resulting properties. The atomic level structure for nanoparticles is suspected to vary from that for the corresponding bulk materials, however, direct observation of this phenomenon has proven difficult. Until recently only indirect information on the atomic level structure of such materials could be obtained with techniques such as XRD, HR-TEM, XPS, etc… However, recent advances in Transmission Electron Microscopy techniques now allow true atomic scale resolution, leading to definitive confirmation of the atomic structure. Namely, Scanning Transmission Electron Microscopy coupled with a High-angle Annular Dark Field detector (STEM-HAADF) has been demonstrated to be capable of achieving a nominal resolution of 0.8 nm (the JEOL JEM-ARM200F instrument). The ability is highly exciting because it will lead to an enhanced understanding of the relationship between atomic structure of nanoparticles and the resulting novel properties. In our own study, we focus on the analysis of the atomic level structure for nanoparticles composed of bismuth, antimony and tellurium for thermoelectric materials. This area has recently received much interest because of the realization that nanotechnology can be employed to greatly enhance the efficiency (dimensionless figure of merit ZT) of this class of materials. One of the most intriguing parameters leading to the enhanced TE activity is the relationship between composition and structure that exists within individual nanoparticles. We report our results on a study of the atomic level structure for both nanowires and nanodiscs composed of bismuth, antimony and tellurium. It was found that the nanoparticles have a complex structure that cannot be elucidated by conventional techniques such as XRD or HR-TEM. In addition, by employing Energy Dispersive Spectroscopy (EDS), a greater understanding of the composition-structure dependence was gained. The results are primarily discussed in terms of the atomic level resolution images obtained with the STEM-HAADF technique.

Clarifying how the atomic structure of interfaces/boundaries in materials affects the magnetic coupling nature across them is of significant academic value and will facilitate the development of state-of-the-art magnetic devices. Here, by combining atomic-resolution transmission electron microscopy, atomistic spin-polarized first-principles calculations, and differential phase contrast imaging, we conduct a systematic investigation of the atomic and electronic structures of individual Fe3O4 twin boundaries (TBs) and determine their concomitant magnetic couplings. We demonstrate that the magnetic coupling across the Fe3O4 TBs can be either antiferromagnetic or ferromagnetic, which directly depends on the TB atomic core structures and resultant electronic structures within a few atomic layers. Revealing the one-to-one correspondence between local atomic structures and magnetic properties of individual grain boundaries will shed light on in-depth understanding of many interesting magnetic behaviors of widely used polycrystalline magnetic materials, which will surely promote the development of advanced magnetic materials and devices.

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.

Thin film reaction based synthesis techniques are promising for large area, uniform two-dimensional transition metal dichalcogenide (TMD) layers such as MoS2. In this work, the impact of the initial molybdenum film composition (metallic versus oxidized) is explored. Alternating steps of Mo sputtering and H2S soaks are used in conjunction with plasma assisted synthesis techniques to synthesize films at low temperatures. Raman, photoluminescence, x-ray photoelectron spectroscopy, and atomic force microscopy are used to physically characterize the films' atomic structure, stoichiometry, and topography, while devices were fabricated to characterize their electronic properties. MoS2 synthesized from metallic Mo films were found to exhibit better atomic and electronic structure than MoS2 synthesized from MoOx films. Additionally, slowing the rate of synthesis by segmenting growth into repeating cycles resulted in much higher film quality. To understand the impact of atomic structure and stoichiometry on device performance, films synthesized at low temperature were exposed to various high temperature annealing conditions to induce changes in film structure and composition. Physical and electrical characterization reveal that stoichiometry has a significantly weaker influence on electronic performance than grain size and atomic structure. These results provide valuable information on the optimization of low temperature thin film reactions for TMD syntheses.

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.

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.

Pair Distribution Function Obtained from Electron Diffraction: An Advanced Real-Space Structural Characterization Tool

Grain boundary structure search by using an evolutionary algorithm with effective mutation methods

By spherical aberration (CS)-corrected high-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS), the atomic and electronic structures at the CrN/Cr interface are studied. A transition layer is formed at the CrN/Cr interface, which is identified as hexagonal Cr2N. The atomic structures at the interfaces are revealed. The elemental concentration distribution across the interface was quantified by EELS. The fine structures of Cr-L2,3 in Cr, CrN, and Cr2N exhibit a subtle difference. The Cr-L2,3 edge in CrN shows a noticeable chemical shift as compared to Cr and Cr2N, accompanied by a slight variation at the corresponding N-K edge.

There is plenty of room for new structures at the bottom

Atomic and electronic structures of the electronically magic $\mathrm{Na}_{59}{}^{+}$ cluster are investigated using density functional theory and an ab initio pseudopotential. Two atomically closed-shell structures are found, but none of these ``double-magic'' structures is the global ground state at $T=0$. Simulated annealing with ab initio molecular dynamics yields a more stable amorphous, but ``spherical'' atomic structure that closely resembles structures obtained from a simple classical model of 59 point charges in a spherical harmonic well. This indicates that the sodium ions arrange into a structure that is dictated by a stiff spherical background of the closed 58-electron configuration. Upon heating, this cluster melts already at about $175\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ displaying a minute latent heat of $2.3\phantom{\rule{0.3em}{0ex}}\mathrm{meV}∕\text{atom}$. The simulated photoelectron spectrum at $260\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ reproduces the $1g$, $2p$, and $1f$ peaks observed previously in the experiment.

The macroscopic properties of many materials are controlled by the structure and chemistry at grain boundaries. A basic understanding of the structure-property relationship requires a technique which probes both composition and chemical bonding on an atomic scale. High-resolution Z-contrast imaging in the scanning transmission electron microscope (STEM) forms an incoherent image in which changes in atomic structure and composition across an interface can be interpreted directly without the need for preconceived atomic structure models (1). Since the Z-contrast image is formed by electrons scattered through high angles, parallel detection electron energy loss spectroscopy (PEELS) can be used simultaneously to provide complementary chemical information on an atomic scale (2). The fine structure in the PEEL spectra can be used to investigate the local electronic structure and the nature of the bonding across the interface (3). In this paper we use the complimentary techniques of high resolution Zcontrast imaging and PEELS to investigate the atomic structure and chemistry of a 25° symmetric tilt boundary in a bicrystal of the electroceramic SrTiO3.

Structural transformations and evolution of the electron band structure in the (Tl, Pb)/Ge(1 1 1) system have been studied using low-energy electron diffraction, scanning tunneling microscopy, angle-resolved photoelectron spectroscopy and density functional theory calculations. The two 2D Tl–Pb compounds on Ge(1 1 1), -(Tl, Pb) and -(Tl, Pb), have been found and their composition, atomic arrangement and electron properties has been characterized. The (Tl, Pb)/Ge(1 1 1) compound is almost identical to the alike (Tl, Pb)/Si(1 1 1) system from the viewpoint of its atomic structure and electronic properties. They contain 1.0 ML of Tl atoms arranged into a honeycomb network of chained trimers and 1/3 ML of Pb atoms occupying the centers of the honeycomb units. The (Tl, Pb)/Ge(1 1 1) compound contains six Tl atoms and seven Pb atoms per unit cell (i.e. ∼0.67 ML Tl and ∼0.78 ML Pb). Its atomic structure can be visualized as consisting of Pb hexagons surrounded by Tl trimers. The (Tl, Pb)/Ge(1 1 1) and (Tl, Pb)/Ge(1 1 1) compounds are metallic and their band structures contain spin-split surface-state bands. By analogy with the (Tl, Pb)/Si(1 1 1), these (Tl, Pb)/Ge(1 1 1) compounds are believed to be promising objects for prospective studies of superconductivity in one-atom-layer systems.

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.

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.