The Materials Project crystal structure database has been searched for materials possessing layered motifs in their crystal structures using a topology-scaling algorithm. The algorithm identifies and measures the sizes of bonded atomic clusters in a structure's unit cell, and determines their scaling with cell size. The search yielded 826 stable layered materials that are considered as candidates for the formation of two-dimensional monolayers via exfoliation. Density-functional theory was used to calculate the exfoliation energy of each material and 680 monolayers emerge with exfoliation energies below those of already-existent two-dimensional materials. The crystal structures of these two-dimensional materials provide templates for future theoretical searches of stable twodimensional materials. The optimized structures and other calculated data for all 826 monolayers are provided at https://materialsweb.org.The combination of modern computational tools and the growing number of available crystal structure databases with high-throughput interfaces have accelerated recent efforts to map the materials genome. One of the most recently discovered branches of the materials genome is the class of two-dimensional (2D) materials, which generally have properties that are markedly different from their three-dimensional counterparts. The canonical example is the graphite/graphene system, but monolayers have been exfoliated from many other layered compounds as well [1][2][3][4]. Stable 2D materials can also be obtained by deposition [5][6][7][8] or chemical exfoliation [9,10]. Because the contribution of interlayer interactions to these materials free energies is typically quite small, the existence of a mechanically exfoliable bulk precursor generally indicates the relative stability of a freestanding single layer, regardless of how it is synthesized.In an effort to discover novel 2D materials, two recent studies searched the inorganic crystal structure database (ICSD) for compounds with large interlayer spacings, which are characteristic of weak interlayer bonding that could be overcome by mechanical exfoliation [11,12]. They used the intuitive criteria of a low packing fraction based on the covalent radii of the atoms and an interlayer gap larger than the sum of the covalent radii of atoms at the layers' surfaces along the c-axis to identify layered compounds in the ICSD. They discovered almost 100 layered phases, nearly half of which had monolayers that had not been the subject of any prior publications.Here, we extend their method to identify a large number of layered compounds that were missed using the packing factor and c-axis interlayer gap criteria. We further add the constraint that a bulk material must be thermodynamically stable to be of interest during our search. Therefore, we use the Materials Project (MP) database [13], an online repository of crystallographic and thermodynamic data for over 65,000 compounds calculated with density-functional theory (DFT). Our algorithm is designed to correctly identify ad...
First-principles calculations are used to compare the binding energies of O, OH, and F on two-dimensional, metal carbide and nitride, or MXene, surfaces in order to predict the dependence of the thermodynamic stability of these compounds on their chemical composition. Solvation effects are implicitly included in the calculations to reproduce experimental conditions as closely as possible. The results indicate that all MXene surfaces are saturated with oxygen when exposed to H 2 O/HF solutions at low hydrogen chemical potential, μ H , and that Sc-based MXenes can also be fluorinated in solutions of higher μ H . After investigating the thermodynamic stability of all 54 MXene compounds M n+1 X n O 2 (M = Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta; X = C, N; n = 1, 2, 3), 38 are predicted to have formation energies below 200 meV/atom. Of these, six are predicted to have formation energies below 100 meV/atom, only one of which has been synthesized. Sc-based MXenes are found to be highly stable when their surfaces are terminated with F, which also results in the easiest exfoliation to produce freestanding single layers.
The cold emission of particles from surfaces under intense electric fields is a process which underpins a variety of applications including atom probe tomography (APT), an analytical microscopy technique with near-atomic spatial resolution. Increasingly relying on fast laser pulsing to trigger the emission, APT experiments often incorporate the detection of molecular ions emitted from the specimen, in particular from covalently or ionically bonded materials. Notably, it has been proposed that neutral molecules can also be emitted during this process. However, this remains a contentious issue. To investigate the validity of this hypothesis, a careful review of the literature is combined with the development of new methods to treat experimental APT data, the modeling of ion trajectories, and the application of density-functional theory simulations to derive molecular ion energetics. It is shown that the direct thermal emission of neutral molecules is extremely unlikely. However, neutrals can still be formed in the course of an APT experiment by dissociation of metastable molecular ions. charged molecules (molecular ions) are regularly detected [30][31][32]. As detailed in thorough review articles by Mathur [33, 34], molecular ions have been the subject of intense studies in the field of mass spectrometry as they can form due to the impact of other charged particles or photons. Molecular ions are metastable and usually dissociate into smaller fragments. Beyond its fundamental interest, the dissociation of molecular ions is also a commonly encountered problem in mass spectrometry of (e.g.) organic compounds, and computational methods have often been used to interpret experimental observations [35]. In addition, molecular ions are known to be very reactive [36] and the dissociative recombination [37] of molecular ions is known to play a role in atmospheric and spatial chemical processes [38].The rate of publication of pulsed-laser APT data has surged in recent years [39,40]. In stark contrast, progress in understanding the fundamental physics of laser-assisted field evaporation, particularly for non-metals, has failed to keep pace. For metals, it is accepted that field evaporation is caused by a sharp increase in the specimen temperature due to the absorption of the light from each laser pulse, which is then quenched as the heat is transported inwards and then along the length of the shank [41] in a transient process that is often referred to as a thermal pulse [42,43]. The least conservative estimates for the case of a metal specimen predict temperature increases of up to a maximum of 600 K in tungsten [41] when the standing electric field is 75% of the intensity required to field evaporate the specimen without laser illumination. However, these are conditions that are not expected to yield good APT performance [44] and are usually avoided.Recent research indicated that the field evaporation mechanisms for semiconductors may differ to that of metals with high and fast phonon excitation that could result in very high ...
Through a systematic search of all layered bulk compounds combined with density functional calculations employing hybrid exchange-correlation functionals, we predict a family of three magnetic two-dimensional (2D) materials with half-metallic band structures. The 2D materials, FeCl, FeBr, and FeI, are all sufficiently stable to be exfoliated from bulk layered compounds. The Fe ions in these materials are in a high-spin octahedral d configuration leading to a large magnetic moment of 4 μ. Calculations of the magnetic anisotropy show an easy-plane for the magnetic moment. A classical XY model with nearest neighbor coupling estimates critical temperatures, T, for the Berezinskii-Kosterlitz-Thouless transition ranging from 122 K for FeI to 210 K for FeBr. The quantum confinement of these 2D materials results in unusually large spin gaps, ranging from 4.0 eV for FeI to 6.4 eV for FeCl, which should defend against spin current leakage even at small device length scales. Their purely spin-polarized currents and dispersive interlayer interactions should make these materials useful for 2D spin valves and other spintronic applications.
The behaviour and the development of the Brent Group in the Northern North Sea is documented by cross-sections and paleogeographic maps. Although this work is based on a previously published Norsk Hydro study, we present recent progress in the understanding of the Brent Group which has been achieved by a better timing of depositional events, by integrating new well information, and by generating new models based on the new information. The early (Aalenian) lateral infill of the Oseberg Formation was deposited as a response to a relative fall of sea-level, probably generated by a tectonic uplift. Fan deltaic sediments built out towards the west and northwest and backfilled the previously emergent areas during the subsequent relative sea-level rise. The progradation of the Brent Delta (Rannoch, Etive and lower Ness Formations) took place in Late Aalenian to Early Bajocian. The thickness of the Etive relative to the Rannoch Formation indicates thickening due to bathymetric deepening in the area south of the maximum regression whereas subsidence controlled the thickening in the area of the maximum extent of the delta. During the retrogradational part of the delta development (the Tarbert and upper Ness Formations, Early Bajocian to Early Bathonian), increasing tectonic activity can be documented, both in terms of enhanced differential subsidence across faults and by early rotational uplift on some fault blocks.
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