Bulk Atomic Structure Research Articles

Identification of Material Dimensionality Based on Force Constant Analysis.

Identification of low-dimensional structural units from the bulk atomic structure is a widely used approach for discovering new low-dimensional materials with new properties and applications. Such analysis is usually based solely on bond-length heuristics, whereas an analysis based on bond strengths would be physically more justified. Here, we study dimensionality classification based on the interatomic force constants of a structure with different approaches for selecting the bonded atoms. The implemented approaches are applied to the existing database of first-principles calculated force constants with a large variety of materials, and the results are analyzed by comparing them to those of several bond-length-based classification methods. Depending on the approach, they can either reproduce results from bond-length-based methods or provide complementary information. As an example of the latter, we managed to identify new non-van der Waals two-dimensional material candidates.

Synchrotron radiation photoemission spectroscopy of the oxygen modified CrCl3 surface.

We investigate the experimentally challenging CrCl3 surface by photon energy dependent photoemission (PE). The core and valence electrons after cleavage of a single crystal, either in a ultra-high vacuum (UHV) or in air, are studied by keeping the samples at 150 °C, aiming at confirming the atomic composition with respect to the expected bulk atomic structure. A common spectroscopic denominator revealed by data is the presence of a stable, but only partially ordered Cl-O-Cr surface. The electronic core levels (Cl 2p, Cr 2p and 3p), the latter ones of cumbersome component determination, allowed us to quantify the electron charge transfer to the Cr atom as a net result of this modification and the increased exchange interaction between metal and ligand atoms. In particular, the analysis of multiplet components by the CMT4XPS code evidenced the charge transfer to be favored, and similarly the reduced crystal field due to the established polarization field. Though it is often claimed that a significant amount of Cl and Cr atomic vacancies has to be included, such a possibility can be excluded on the basis of the sign and the importance of the shift in the binding energy of core level electrons. The present methodological approach can be of great impact to quantify the structure of ordered sub-oxide phases occurring in mono or bi-layer Cr trihalides.

High Resolution Core- and Valence-Level XPS Studies of the Properties (Structural, Chemical and Bonding) of Silicate Minerals and Glasses

Core-level and valence-level X-ray Photoelectron Spectroscopy (XPS), developed in the late 1950’s and 1960’s by Siegbahn and coworkers (Siegbahn et al. 1969; Carlson 1975; Barr 1993; Fadley 2010) has become an invaluable tool over the last 40 years for studying mainly the surface properties and reactivity of a wide range of minerals, predominantly oxides (for reviews, see: Heinrich and Cox 1994; Chambers 2000; Salmeron and Schlogl 2008, and references in Bancroft et al. 2009; Newburg et al. 2011), sulfides (for reviews, see Hochella 1988; Bancroft and Hyland 1990; Nesbitt 2002; Murphy and Strongin 2009) and silicates (for a review see Hochella 1988; references in Biino and Groning 1998; Oelkers 2001; Zakaznova-Herzog et al. 2008). The large majority of these studies have focused on the first few surface monolayers of the minerals because of the surface sensitivity of the technique (~2–20 monolayers for photon energies of ≤ 1486 eV (Hochella 1988; Nesbitt 2002), and in many such cases, XPS has become the technique of choice for surface studies. Silicate XPS studies generally have focused on three surface applications outlined by Hochella (1988): (1) studies of the oxidation state of near surface atoms (e.g., Fe); (2) studies of sorption reactions on mineral surfaces; and (3) studies of the alteration and weathering of mineral surfaces. Fewer reports have focused on the fourth application of Hochella (1988), the study of the bulk atomic structure and chemical state properties of minerals and glasses. This is surprising perhaps, because the large majority (usually >90 %) of XPS line intensities comes from the bulk mineral in XPS studies using the typical laboratory Al K α X-ray sources (1486.6 eV). To emphasize this point, the surface S 2 p peaks from the …

Hard X-ray photoemission with angular resolution and standing-wave excitation

Several aspects of hard X-ray photoemission that make use of angular resolution and/or standing-wave excitation are discussed. These include hard X-ray angle-resolved photoemission (HARPES) from valence levels, which has the capability of determining bulk electronic structure in a momentum-resolved way; hard X-ray photoelectron diffraction (HXPD), which shows promise for studying element-specific bulk atomic structure, including dopant site occupations; and standing wave studies of the composition and chemical states of buried layers and interfaces. Beyond this, standing wave photoemission can be used to derive element-specific densities of states. Some recent examples relevant to all of these aspects are discussed.

Discussion on the surface science of quasicrystals

This paper contains a short review of four aspects of the surface science of quasicrystals, together with a list of challenges for the scientific community in the near future. The first issue concerns the ability of surface science to shed light on bulk atomic structure. The second is the use of surfaces as quasiperiodic templates, particularly for films of periodic metals. Here, enforcing quasiperiodicity in the film may lead to unusual magnetic, tribological or adsorption properties. The third aspect concerns the effects of surface phasons and phonons on dynamical interactions with adsorbates, such as sticking coefficient, as well as on diffusion between the surface and near-surface region. The final area is tribology, where studies of quasicrystals have suggested that both adhesion and phononic friction may be important.

There is plenty of room for new structures at the bottom

The atomic structure of a surface is its most fundamental physical property since it defines the actual system under study, in the first place. Most other physical quantities sensitively depend on the surface structure. For all surfaces currently under investigation, the atomic configuration is by far not as precisely known as the respective bulk atomic structure, resulting, e.g., from X-ray structure determination. Therefore, each and every investigation of any surface property – no matter how sophisticated it may be – also adds to our understanding of its particular atomic structure at the same time. As a consequence, the history of investigations of innumerable surface properties is a history of surface structure determinations, as well. For most surfaces various experimental as well as theoretical studies have been carried out to arrive at convincing surface structure models complying with Charlie Duke’s claim that ‘‘one method is never enough’’. In theory, the optimal structure of clean stoichiometric surfaces is determined by minimizing the total energy of a surface system within the framework of quantum mechanics, which currently is done mainly with density functional theory employing either the local density approximation or some variant of the generalized gradient approximation. Since it is nowadays possible to prepare surfaces by epitaxial growth rather than by cleaving, nonstoichiometric surfaces need to be considered, as well. To do so, one usually refers to ab initio thermodynamics. In such cases a comparison of grand canonical potentials of structures with different stoichiometry or different adatom coverages yielding surface formation energies is a very powerful means to arrive theoretically at optimal surface structures. As suggested by Qian et al. [1] as well as by Northrup and Froyen [2], the formation energy of a binary semiconductor compound AB can be formulated in terms of the gas-phase chemical potential of only one of the two constituents, e.g., lA. The formation energy can then be studied from the A-rich to the B-rich regime just by monitoring the value of lA within its accessible range determined by the heat of formation of the AB bulk compound. There are many examples for which joint experimental and theoretical efforts have converged to generally accepted reconstruction models, to date. To name only a few, the famous Si(111)-(7 · 7), the Si(001)-(2 · 1) or the SiC(001)(3 · 2) surface shall be mentioned, which reconstruct according to the Takayanagy dimer-adatom-stacking-fault (DAS) model [3], the asymmetric dimer model (ADM) [4–8] or the two-adlayer asymmetric-dimer model (TAADM) [9–11], respectively. But there are many other semiconductor surfaces for which either the efforts have not yet lead to generally accepted models or not all possible reconstructions have been worked out, to date. The Letter of Segev and Van de Walle in this issue [12] adds a particularly nice facet to the last class of surface systems. It addresses the reconstruction of polar and nonpolar surfaces of InN and GaN which are wide-band-gap semiconductors whose hexagonal surfaces are currently moving into the focus of interest because of their paramount technological potential for advanced microelectronic and optoelectronic devices. Their specific electronic properties are the very basis of their technological importance. A most quantitative description of their surface atomic and electronic structure is highly desirable, therefore. Now it turns out, that the clean stoichiometric hexagonal surfaces of InN and GaN do not represent

Fabrication of well-aligned Er nanowires on vicinal silicon(001) surfaces

Using vicinal Si(001) surfaces with a4° miscut angle towards the [110] direction as templates, an approach to fabricating well-alignednanowires of Er silicide is demonstrated. At Er coverage of 0.1–0.3 nm and annealing temperature of600–700 °C, ordered nanowires are obtained on the surface several hundreds of nanometres in length.STM images and LEED patterns reveal that the nanowires are all oriented along the direction. The size, shape and density of the nanowires depends uponannealing temperature and duration as well as coverage. The experimentalevidence also suggests a transition of bulk atomic structures betweenAlB2 and ThSi2 for Er silicide nanostructures. The alignment of the Er nanowires will make themeasurements of their physical properties practicable.

Preparation and Structural Study of Metastable Mn Phase Grown on GaAs(OOl) Substrate

Single crystal Mn thin films have been successfully grown on GaAs(001) substrates for the first time by molecular beam epitaxy. The bulk atomic structure of the manganese films is determined to be the face-centered-cubic metastable phase with the lattice parameter of 0.362 nm, by in situ reflection high energy electron diffraction (RHEED) and ex situ x-ray diffraction (XRD).

Atomic scale imaging of surfaces in UHV

Electron Microscopy of surfaces can in principle provide information that is not available by other techniques. Arguably the most significant is information about the surface structure simultaneously with the bulk atomic structure. Whereas bulk atomic scale information is now routinely available using standard high resolution imaging techniques, imaging surface with similar resolution has proved far harder. To date the only extensively used technique is profile imaging, but this has an inherent scientific problem; the imaged area is so thin that there is great uncertainty about how representative the results are of a bulk surface. For instance, one can conclude almost without ambiguity that any results on large unit cell surface structures will be erroneous since the long range elastic strains will be truncated by the specimen thickness.

Structure determination of metastable cobalt films.

The bulk atomic structure of cobalt films deposited on GaAs has been unequivocally determined to be the body-centered-cubic metastable phase. A comparison of the conversion-electron x-ray-absorption fine structure of Co films deposited on GaAs(110) with similar measurements for the stable phases of hcp Co, fcc Cu, and bcc Fe establishes the Co bcc phase with a lattice constant of 2.82\ifmmode\pm\else\textpm\fi{}0.01 A\r{}. Furthermore, this first application of the technique to single-crystal epitaxial films shows it to be a most powerful procedure for establishing their crystal structure.

Low energy ion backscattering angular distributions of 6Li + from Cu(001)

Low energy ion backscattering angular distributions (LEIBAD) from a single crystal may be used to image the atomic structure of the near-surface region. For an incident ion beam oriented randomly with respect to the crystal axes, the ions can penetrate more than 100 Å into the crystal, and the angular distributions of backscattered ions are dominated by the bulk atomic structure. Thus, a bulk stereogram of the crystal may be obtained. For incidence along a low-Miller-index axis of the crystal, shadowing effects limit the penetration depth of the elastically backscattered ions to a region near the surface of the crystal. However, neutralized Li atoms, which are not filtered out of the backscattered yield by the retarding field analyzer used in this study, provide a background characteristic of the bulk of the sample that adds to the signal from the surface plane. These effects are demonstrated by comparing experimentally determined backscattered-ion angular distributions with Monte Carlo simulations for 20 keV 6Li + scattering from a Cu(001) surface. Modifications necessary to improve the surface sensitivity of the technique are discussed.