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The 2D form of tellurium, named tellurene, is one of the latest discoveries in the family of 2D mono-elemental materials.

The phenomenology of resonant scattering has been known since the earliest experiments upon scattering of atomic beams from surfaces and is a means of obtaining experimental information about the fundamentals of weak adsorption systems in the van der Waals regime. We provide an overview of the experimental approach based on new experimental data for the He-Sb2Te3(111) system, followed by a comparative overview and perspective of recent results for topological semimetal and insulator surfaces. Moreover, we shortly discuss the perspectives of calculating helium-surface interaction potentials from ab initio calculations. Our perspective demonstrates that atom-surface scattering provides direct experimental information about the atom-surface interaction in the weak physisorption regime and can also be used to determine the lifetime and mean free path of the trapped atom. We further discuss the effects of elastic and inelastic scattering on the linewidth and lifetime of the trapped He atom with an outlook on future developments and applications.

Poly(triazine imide) or PTI is a promising material for molecular sieving membranes, thanks to its atom-thick ordered lattice with an extremely high density (1.6 × 1014pores per cm2) of triangular-shaped nanopores of ∼0.34 nm diameter. Here, we investigate the application of PTI nanopores in the purification of Kr from Xe to reduce the storage volume of the mixture of85Kr/Xe. Using van der Waals density-functional theory (vdW-DFT) calculations, benchmarked against the random phase approximation (RPA), we calculate the potential energy profiles for Kr and Xe across the nanopores. For each gas, starting from the RPA potential-energy profile, the force-field parameters to be used in the classical restrained molecular dynamics framework are trained to calculate the Helmholtz free energy barrier as a function of temperature, and therefore, the corresponding entropic loss. Overall, due to the much higher activation energy from the adsorbed state in Xe (17.61 and 42.10 kJ per mole for Kr and Xe, respectively), a large Kr/Xe separation selectivity is postulated from the PTI membrane. Furthermore, the combination of the atom-thick PTI lattice and high pore density leads to extremely large yet selective permeances for Kr. For example, a Kr permeance of 1000 gas permeation units (GPU) accompanying a large Kr/Xe selectivity (>10 000) is calculated at 25 °C, which is significantly better than that of the state-of-the-art membranes for Kr/Xe separation, making PTI-based membranes a leading candidate for processing the hazardous waste of85Kr/Xe mixture.

In this work we present NJOY+NCrystal, a tool to generate thermal neutron scattering libraries with support for coherent and incoherent elastic components for crystalline solid materials. This tool, which is a customized version of NJOY, was created by modifying the nuclear data processing program NJOY to call the thermal scattering software library NCrystal, and includes a proposed change in the ENDF-6 format to store both the coherent and incoherent elastic components. Necessary changes to enable this format in NJOY, as well as to sample it in the OpenMC Monte Carlo code, are detailed here. Examples of materials that are coherent–dominant, incoherent–dominant, and mixed elastic scatterers are presented, as well as the creation of novel libraries for MgH2 and MgD2, that are under consideration as advanced neutron reflectors in the HighNESS project at the European Spallation Source. NJOY+NCrystal greatly simplifies the process to generate thermal scattering libraries (TSL) and this is exemplified with 213 new and updated TSL evaluations.

We present recent developments of improved modelling methods for simulating neutron transport in reflector materials of interest for neutron source applications. These include materials to be used as traditional reflectors around the neutron moderator, such as beryllium, and also novel materials, such as nanodiamond particles, to be used as a reflector for very-cold neutrons in the neutron beam extraction area of a neutron scattering instrument. Of particular interest is the inclusion of physical effects that are not modelled in standard thermal scattering libraries used for Monte-Carlo simulations, such as extinction in beryllium reflectors and effects of small-angle neutron scattering from nanodiamond particles.

Liquid phase exfoliation is a commonly used method to produce 2D nanosheets from a range of layered crystals. However, such nanosheets display broad size and thickness distributions and correlations between area and thickness, issues that limit nanosheet application potential. To understand the factors controlling the exfoliation process, we have liquid-exfoliated 11 different layered materials, size-selecting each into fractions before using AFM to measure the nanosheet length, width, and thickness distributions for each fraction. The resultant data show a clear power-law scaling of nanosheet area with thickness for each material. We have developed a simple nonequilibrium thermodynamics-based model predicting that the power-law prefactor is proportional to both the ratios of in-plane-tearing/out-of-plane-peeling energies and in-plane/out-of-plane moduli. By comparing the experimental data with the modulus ratio calculated from first-principles, we find close agreement between experiment and theory. This supports our hypothesis that energy equipartition holds between nanosheet tearing and peeling during sonication-assisted exfoliation., Accepted Manuscript (30 pages) and Supporting Information (68 pages)

Two-dimensional superconductors attract great interest both for their fundamental physics and for their potential applications, especially in the rapidly growing field of quantum computing. Despite intense theoretical and experimental efforts, materials with a reasonably high transition temperature are still rare. Even more rare are those that combine superconductivity with a nontrivial band topology that could potentially give rise to exotic states of matter. Here, we predict a remarkably high superconducting critical temperature of 21 K in the easily exfoliable, topologically nontrivial 2D semimetal W

Closed-shell atoms scattered from a metal surface exchange energy and momentum with surface phonons mostly via the interposed surface valence electrons, i.e., via the creation of virtual electron-hole pairs. The latter can then decay into surface phonons via electron-phonon interaction, as well as into acoustic surface plasmons (ASPs). While the first channel is the basis of the current inelastic atom scattering (IAS) surface-phonon spectroscopy, no attempt to observe ASPs with IAS has been made so far. In this study we provide evidence of ASP in Ni(111) with both Ne atom scattering and He atom scattering. While the former measurements confirm and extend so far unexplained data, the latter illustrate the coupling of ASP with phonons inside the surface-projected phonon continuum, leading to a substantial reduction of the ASP velocity and possibly to avoided crossing with the optical surface phonon branches. The analysis is substantiated by a self-consistent calculation of the surface response function to atom collisions and of the first-principle surface-phonon dynamics of Ni(111). It is shown that in Ni(111) ASP originate from the majority-spin Shockley surface state and are therefore collective oscillation of surface electrons with the same spin, i.e. it represents a new kind of collective quasiparticle: a Spin Acoustic Surface Plasmon (SASP)., The work of I.V.S. is supported by the Ministry of Science and Higher Education of the Russian Federation for funding in framework of State Task (No. 0721-2020-0033). V.M.S. and P.M.E. acknowledge support from the Spanish Ministry of Science and Innovation (Grant No. PID2019-105488GB-I00) and D.F. from Grant No. PID2019-109525RB-I00. E.V.C. acknowledges support from Saint Petersburg State University (project ID No. 51126254). R.M. and D.F. acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M). IMDEA Nanociencia acknowledges support from the ’Severo Ochoa’ Programme for Centres of Excellence in R&D (MINECO, Grant SEV-2016-0686). Part of the calculations was performed at the SKIF-Cyberia supercomputer at the National Research Tomsk State University (Russian Federation) and the Supercomputer Center of D.I.P.C. (Spain).

Metal soaps or metal alkanoates are metal-organic complexes held together with metal cations and the functional groups of hydrocarbon chains. They can be synthesized at a high yield by simply mixing the metal and organic sources, forming crystalline frameworks with diverse topology, and have been studied in the past because of their rich polymorphism-like liquid crystals. Their ability to melt while retaining the crystalline properties upon cooling is unique among nanoporous materials and is especially attractive for membrane fabrication. Herein, metal soaps as a new class of material for molecular separation are reported. Three metal soaps, Ca(SO4C12H25)(2), Zn(COOC6H13)(2), and Cu(COOC9H19)(2), hosting lamellar structure with molecular-sized channels are synthesized. They are processed in thin, intergrown, polycrystalline films on porous substrates by two scalable methods, interfacial crystallization and melting with an extremely small processing time (a minute to an hour). The resulting crystalline films are oriented with the alkyl chains perpendicular to the porous substrate which favors molecular transport. The prepared membranes demonstrate attractive gas separation behavior, e.g., 300-nm-thick Ca(SO4C12H25)(2)membrane prepared in a minute using interfacial crystallization yields H(2)permeance of 6.1 x 10(-7)mol m(-2)s(-1)Pa(-1)with H-2/CO(2)selectivity of 10.5.

C2N is an ordered two-dimensional carbon nitride with a high density (1.7 × 1014 cm-2) of 3.1 Å-sized nanopores, making it promising for high-flux gas sieving for energy-efficient He and H2 purification. Herein, we discuss the accurate calculation of potential energy surfaces for He, H2, N2, and CO2 across C2N nanopores, to characterize the gas-sieving potential of C2N. We compare the potential energy surface derived from density-functional theory calculations using five commonly used van der Waals (vdW) approximations. While all five functionals point that the C2N nanopore yields He/N2 and H2/N2 selectivities over 1000, adsorption energies and energy barriers vary remarkably depending on the approximation chosen. To make progress, we compare the calculations against the results from the adiabatic connection fluctuation dissipation theory, with random-phase approximation, known to be accurate in capturing vdW interactions. The comparison indicates that the interaction energy is less accurate with vdW density functional theory. On the other hand, more empirical corrections work reasonably well, a finding that we also confirm for another carbon nitride lattice, poly(triazine imide). Overall, we recommend these for screening carbon nitride materials for gas separation, but also comparing functionals with higher-order approaches when dealing with different materials.

Due to their remarkable properties, single-layer 2-D materials appear as excellent candidates to extend Moore's scaling law beyond the currently manufactured silicon FinFETs. However, the known 2-D semiconducting components, essentially transition metal dichalcogenides, are still far from delivering the expected performance. Based on a recent theoretical study that predicts the existence of more than 1800 exfoliable 2-D materials, we investigate here the 100 most promising contenders for logic applications. Their current

The theoretical methods currently in use for the calculation of surface phononsurface phonon dispersion curves and how they have evolved from the phenomenological force-constant models to the present day first principles theories are discussed. Aselection of paradigmatic examples for the different classes of crystal surfaces is presented with comparisons to the experimental data obtained from helium atom scattering or electron energy-loss spectroscopy.

Due to their remarkable properties, single-layer 2-D materials appear as excellent candidates to extend Moore’s scaling law beyond the currently manufactured silicon FinFETs. However, the known 2-D semiconducting components, essentially transition metal dichalcogenides, are still far from delivering the expected performance. Based on a recent theoretical study that predicts the existence of more than 1800 exfoliable 2-D materials, we investigate here the 100 most promising contenders for logic applications. Their current versus voltage characteristics are simulated from first-principles, combining density functional theory and advanced quantum transport calculations. Both n- and p-type configurations are considered, with gate lengths ranging from 15 down to 5 nm. From this large collection of electronic materials, we identify 13 compounds with electron and hole currents potentially much higher than those in future Si FinFETs. The resulting database widely expands the design space of 2-D transistors and provides original guidelines to the materials and device engineering community. © 2020 American Chemical Society. ISSN:1936-0851 ISSN:1936-086X

Full-band atomistic quantum transport simulations based on first principles are employed to assess the potential of band-to-band tunneling FETs (TFETs) with a 2-D channel material as future electronic circuit components. We demonstrate that single-layer (SL) transition metal dichalcogenides are not well suited for TFET applications. There might, however, exist a great variety of 2-D semiconductors that have not even been exfoliated yet; this paper pinpoints some of the most promising candidates among them to realize highly efficient TFETs. SL SnTe, As, TiNBr, and Bi are all found to ideally deliver ON-currents larger than 100{\mu}A/{\mu}m at 0.5-V supply voltage and 0.1 nA/{\mu}m OFF-current value. We show that going from single to multiple layers can boost the TFET performance as long as the gain from a narrowing bandgap exceeds the loss from the deteriorating gate control. Finally, a 2-D van der Waals heterojunction TFET is revealed to perform almost as well as the best SL homojunction, paving the way for research in optimal 2-D material combinations.

Tin selenide (SnSe) is one the most promising materials for flexible electronics. However, experiments on the direct determination of its mechanical properties are still missing. By means of depth-sensing nanoindentation experiments, we directly evaluate the Young's modulus of bulk single crystals of tin selenide (25.3 GPa), as well as their hardness (0.82 GPa). Experimental results are compared with predictions by density functional theory, performed using eleven different functionals. The discrepancies between the experimental results and the thoretical predictions can be ascribed to the oxidation of the SnSe surface, detected by X-ray photoelectron spectroscopy.

We search for novel two-dimensional materials that can be easily exfoliated from their parent compounds. Starting from 108423 unique, experimentally known three-dimensional compounds we identify a subset of 5619 that appear layered according to robust geometric and bonding criteria. High-throughput calculations using van-der-Waals density-functional theory, validated against experimental structural data and calculated random-phase-approximation binding energies, allow to identify 1825 compounds that are either easily or potentially exfoliable, including all that are commonly exfoliated experimentally. In particular, the subset of 1036 easily exfoliable cases---layered materials held together mostly by dispersion interactions and with binding energies up to $30-35$ meV$\cdot\text{\AA}^{-2}$---provides a wealth of novel structural prototypes and simple ternary compounds, and a large portfolio to search materials for optimal properties. For the 258 compounds with up to 6 atoms per primitive cell we comprehensively explore vibrational, electronic, magnetic, and topological properties, identifying in particular 56 ferromagnetic and antiferromagnetic systems, including half-metals and half-semiconductors., Comment: 15 pages, 5 figures, supplementary material available at https://www.nature.com/articles/s41565-017-0035-5#Sec15. Data available on the Materials Cloud https://www.materialscloud.org/discover/2dstructures/dashboard/ptable

First principles calculations of phonon dispersion relations have been performed to estimate the phononic contribution to the thermal boundary resistance at the interfaces between GeTe, In $$_3$$ SbTe $$_2$$ and In $$_2$$ GeTe $$_3$$ phase change compounds and metals (Al, Pt and TiN). The diffuse mismatch model has been used. The electron–phonon coupling constants provide an estimate of the electron–phonon contribution to the thermal boundary resistance. The electron–phonon and phononic contributions to the thermal boundary resistance turn out to be comparable and they have to be both included for a reliable estimate of the thermal conductance at the interface with phase change compounds.

Quantum spin Hall insulators make up a class of two-dimensional materials with a finite electronic band gap in the bulk and gapless helical edge states. In the presence of time-reversal symmetry, [Formula: see text] topological order distinguishes the topological phase from the ordinary insulating one. Some of the phenomena that can be hosted in these materials, from one-dimensional low-dissipation electronic transport to spin filtering, could be promising for many technological applications in the fields of electronics, spintronics, and topological quantum computing. Nevertheless, the rarity of two-dimensional materials that can exhibit nontrivial [Formula: see text] topological order at room temperature hinders development. Here, we screen a comprehensive database we recently identified of 1825 monolayers that can be exfoliated from experimentally known compounds to search for novel quantum spin Hall insulators. Using density-functional and many-body perturbation theory simulations, we identify 13 monolayers that are candidates for quantum spin Hall insulators including high-performing materials such as AsCuLi

Two-dimensional materials are emerging as a promising platform for ultrathin channels in field-effect transistors. To this aim, novel high-mobility semiconductors need to be found or engineered. While extrinsic mechanisms can in general be minimized by improving fabrication processes, the suppression of intrinsic scattering (driven e.g. by electron-phonon interactions) requires to modify the electronic or vibrational properties of the material. Since intervalley scattering critically affects mobilities, a powerful approach to enhance transport performance relies on engineering the valley structure. We show here the power of this strategy using uniaxial strain to lift degeneracies and suppress scattering into entire valleys, dramatically improving performance. This is shown in detail for arsenene, where a 2% strain stops scattering into 4 of the 6 valleys, and leads to a 600% increase in mobility. The mechanism is general and can be applied to many other materials, including in particular the isostructural antimonene and blue phosphorene., 14 pages, 14 figures

Quantum spin Hall insulators make up a class of two-dimensional materials with a finite electronic band gap in the bulk and gapless helical edge states. In the presence of time-reversal symmetry, Z(2) topological order distinguishes the topological phase from the ordinary insulating one. Some of the phenomena that can be hosted in these materials, from one-dimensional low-dissipation electronic transport to spin filtering, could be promising for many technological applications in the fields of electronics, spintronics, and topological quantum computing. Nevertheless, the rarity of two-dimensional materials that can exhibit nontrivial Z(2) topological order at room temperature hinders development. Here, we screen a comprehensive database we recently identified of 1825 monolayers that can be exfoliated from experimentally known compounds to search for novel quantum spin Hall insulators. Using density-functional and many-body perturbation theory simulations, we identify 13 monolayers that are candidates for quantum spin Hall insulators including high-performing materials such as AsCuL2 and (platinum) jacutingaite (P2HgS3). We also identify monolayer Pd2HgSe3 (palladium jacutingaite) as a novel Kane-Mele quantum spin Hall insulator and compare it with platinum jacutingaite. A handful of promising materials are mechanically stable and exhibit Z(2) topological order, either unperturbed or driven by small amounts of strain. Such screening highlights a relative abundance of Z(2) topological order of around 1% and provides an optimal set of candidates for experimental efforts.

By means of a combination of surface-science spectroscopies and theory, we investigate the mechanisms ruling the catalytic role of epitaxial graphene (Gr) grown on transition-metal substrates for the production of hydrogen from water. Water decomposition at the Gr/metal interface at room temperature provides a hydrogenated Gr sheet, which is buckled and decoupled from the metal substrate. We evaluate the performance of Gr/metal interface as a hydrogen storage medium, with a storage density in the Gr sheet comparable with state-of-the-art materials (1.42 wt %). Moreover, thermal programmed reaction experiments show that molecular hydrogen can be released upon heating the water-exposed Gr/metal interface above 400 K. The Gr hydro/dehydrogenation process might be exploited for an effective and eco-friendly device to produce (and store) hydrogen from water, i.e., starting from an almost unlimited source.

We present a combined experimental and theoretical study of the surface vibrational modes of the topological insulator Bi2Te3. Using high-resolution helium-3 spin-echo spectroscopy we are able to resolve the acoustic phonon modes of Bi2Te3(111). The low energy region of the lattice vibrations is mainly dominated by the Rayleigh mode which has been claimed to be absent in previous experimental studies. The appearance of the Rayleigh mode is consistent with previous bulk lattice dynamics studies as well as theoretical predictions of the surface phonon modes. Density functional perturbation theory calculations including van der Waals corrections are in excellent agreement with the experimental data. Comparison of the experimental results with theoretically obtained values for films with a thickness of several layers further demonstrate, that for an accurate theoretical description of three-dimensional topological insulators with their layered structure the inclusion of van der Waals corrections is essential. The presence of a prominent surface acoustic wave and the contribution of van der Waals bonding to the lattice dynamics may hold important implications for the thermoelectric properties of thin-film and nanoscale devices.

Density functional perturbation theory is used to study the phonon dispersion relations at the (111) surface of the topological insulator Sb2Te3. Inelastic Helium atom scattering experiments at the (111) surface of the similar Bi2Se3 topological insulator suggested the occurrence of a deep Kohn anomaly related to the presence of the topological surface metallic state. A slab made of three Sb2Te3 quintuple layers is used here to model the (111) surface. This geometry is shown to be sufficient to generate metallic Dirac surface states in good agreement with experimental data from Angle Resolved Photoemission Spectroscopy. The inclusion of spin-orbit interaction in the calculations of surface phonons at wavevectors corresponding to Fermi surface nesting inside the Dirac cone allow us to address the possible occurrence of a Kohn anomaly in Sb2Te3(111) as well. No evidence is found in our calculations for any Kohn anomaly in Sb2Te3 due to electronic transitions across the Dirac cone. (C) 2018 Elsevier B.V. All rights reserved.

An excellent electrostatic control has been early on identified as one of the most critical ingredients to build band- to- band tunneling field-effect transistors (TFETs) with a steep sub-threshold swing (SS) and a high ON-current (I ON ) [1]. These essential features can be obtained by reducing the thickness of ultra-thin-body structures or the diameter of nanowires. Two-dimensional materials, especially their single-layer (SL) configuration, represent a promising alternative to conventional semiconductors due to their intrinsic sub-1nm thickness. Indeed, a TFET implementing an atomically thin MoS 2 channel combined with a Ge layer was recently shown to exhibit a less than 60 mV/dec SS over several orders of magnitude and a decent I ON [2]. In this experiment, however, MoS 2 had to be grouped with Ge to achieve the desired goal, thus raising the question whether 2-D materials alone can provide a suitable platform for high performance TFETs. Various theoretical studies based on empirical tight-binding models and focusing on SL transition metal dichalcogenides (TMDs) [3] and black phosphorus [4] have come to the conclusion that these compounds, in particular WTe 2 , could deliver ON-currents larger than $100\ \mu\mathrm{A}/\mu\mathrm{m}$ at a supply voltage $V_{DD}=0.5\ \mathrm{V}$ and OFF-current $I_{OFF}=1\ \mathrm{nA}/\mu\mathrm{m}$ . Here, by employing an ab-initio quantum transport simulator, we will demonstrate that none of the usual TMDs reaches a $I_{ON} > > 10\ \mu\mathrm{A}/\mu\mathrm{m}$ , contrary to recently discovered 2-D materials [5] that could pave the way for future, highly efficient TFETs.

The structure and the acoustic phonon branches of graphene on Ru(0 0 0 1) have been experimentally investigated with helium atom scattering (HAS) and analyzed by means of density functional theory (DFT) including Grimme dispersion forces. In-plane interactions are unaffected by the interaction with the substrate. The energy of 16 meV for the vertical rigid vibration of graphene against the Ru(0 0 0 1) surface layer indicates an interlayer effective force constant about five times larger than in graphite. The Rayleigh mode observed for graphene/Ru(0 0 0 1) is almost identical to the one measured on clean Ru(0 0 0 1). This is accounted for by the strong bonding to the substrate, which also explains the previously reported high reflectivity to He atoms of this system. Finally, we report the observation of an additional acoustic branch, closely corresponding to the one already observed by HAS in graphite, which cannot be ascribed to any phonon mode and suggests a possible plasmonic origin.

The atomic-scale surface structure of methyl-terminated germanium (111) has been characterized by using a combination of helium atom scattering and density functional theory. High-resolution helium diffraction patterns taken along both the "1I¯21I¯" and the "011I¯" azimuthal directions reveal a hexagonal packing arrangement with a 4.00 ± 0.02 Å lattice constant, indicating a commensurate (1 × 1) methyl termination of the primitive Ge(111) surface. Taking advantage of Bragg and anti-Bragg diffraction conditions, a step height of 3.28 ± 0.02 Å at the surface has been extracted using variable de Broglie wavelength specular scattering; this measurement agrees well with bulk values from CH3-Ge(111) electronic structure calculations reported herein. Density functional theory showed that methyl termination of the Ge(111) surface induces a mild inward relaxation of 1.66% and 0.60% from bulk values for the first and second Ge-Ge bilayer spacings, respectively. The DFT-calculated rotational activation barrier of a single methyl group about the Ge-C axis on a fixed methyl-terminated Ge(111) surface was found to be approximately 55 meV, as compared to 32 meV for a methyl group on the H-Ge(111) surface, sufficient to hinder the free rotation of the methyl groups on the Ge(111) surface at room temperature. However, accurate MD simulations demonstrate that cooperative motion of neighboring methyl groups allows a fraction of the methyl groups to fully rotate on the picosecond time scale. These experimental data in conjunction with theory provide a quantitative evaluation of the atomic-scale surface structure for this largely unexplored, yet technologically interesting, hybrid organic-semiconductor interface.

It is shown that the electron-phonon interaction at a conducting interface between a topological insulator thin film and a semiconductor substrate can be directly probed by means of high-resolution Brillouin light scattering (BLS). The observation of Kohn anomalies in the surface phonon dispersion curves of a 50 nm thick Bi2Te3 film on GaAs, besides demonstrating important electron-phonon coupling effects in the GHz frequency domain, shows that information on deep interface electrons can be obtained by tuning the penetration depth of optically-generated surface phonons so as to selectively probe the interface region, as in a sort of quantum sonar.

Thermal transport is a key feature for the operation of phase change memory devices which rest on a fast and reversible transformation between the crystalline and amorphous phases of chalcogenide alloys upon Joule heating. In this paper we report on the ab initio calculations of bulk thermal conductivity of the prototypical phase change compounds Ge 2 Sb 2 Te 5 and GeTe in their crystalline form. The related Sb 2 Te 3 compound is also investigated for the sake of comparison. Thermal conductivity is obtained from the solution of the Boltzmann transport equation with phonon scattering rates computed within density functional perturbation theory. The calculations show that the large spread in the experimental data on the lattice thermal conductivity of GeTe is due to a variable content of Ge vacancies which at concentrations realized experimentally can halve the bulk thermal conductivity with respect to the ideal crystal. We show that the very low thermal conductivity of hexagonal Ge 2 Sb 2 Te 5 of about 0.45 W m −1 K −1 measured experimentally is also resulting from disorder in the form of a random distribution of Ge/Sb atoms in one sublattice.

Phonon dispersion in periodically rippled graphene/Ru(0001) has been studied by high-resolution electron energy loss spectroscopy and density functional theory. We show that, in addition to the usual phonon modes of the free-standing graphene lattice, the phonon spectrum of graphene/Ru(0001) exhibits additional modes arising from the peculiar nanostructured morphology of this system. In particular, the out-of-plane optical phonon has two components at 83 and 105 meV, related to the spectral contribution arising from carbon atoms in the valleys and in the hills of the ripples, respectively.

Fundamental research and technological applications of topological insulators are hindered by the rarity of materials exhibiting a robust topologically nontrivial phase, especially in two dimensions. Here, by means of extensive first-principles calculations, we propose a novel quantum spin Hall insulator with a sizable band gap of $\ensuremath{\sim}0.5\text{ }\text{ }\mathrm{eV}$ that is a monolayer of jacutingaite, a naturally occurring layered mineral first discovered in 2008 in Brazil and recently synthesized. This system realizes the paradigmatic Kane-Mele model for quantum spin Hall insulators in a potentially exfoliable two-dimensional monolayer, with helical edge states that are robust and that can be manipulated exploiting a unique strong interplay between spin-orbit coupling, crystal-symmetry breaking, and dielectric response.

Author(s): Bosoni, E; Campi, D; Donadio, D; Sosso, GC; Behler, J; Bernasconi, M | Abstract: The thermal conductivity of GeTe crystalline nanowires has been computed by means of non-equilibrium molecular dynamics simulations employing a machine learning interatomic potential. This material is of interest for application in phase change non-volatile memories. The resulting lattice thermal conductivity of an ultrathin nanowire (7.3 nm diameter) of 1.57 W m-1 K-1 is sizably lower than the corresponding bulk value of 3.15 W m-1 K-1 obtained within the same framework. The analysis of the phonon dispersion relations and lifetimes reveals that the lower thermal conductivity in the nanowire is mostly due to a reduction in the phonon group velocities. We further predict the presence of a minimum in the lattice thermal conductivity for thicker nanowires.

Scientific reports 6, 32895 (2016). doi:10.1038/srep32895, Published by Nature Publishing Group, London

The structural defects in two-dimensional transition metal dichalcogenides, including point defects, dislocations and grain boundaries, are scarcely considered regarding their potential to manipulate the electrical and optical properties of this class of materials, notwithstanding the significant advances already made. Indeed, impurities and vacancies may influence the exciton population, create disorder-induced localization, as well as modify the electrical behaviour of the material. Here we report on the experimental evidence, confirmed by ab initio calculations, that sulfur vacancies give rise to a novel near-infrared emission peak around 0.75 eV in exfoliated MoS2 flakes. In addition, we demonstrate an excess of sulfur vacancies at the flake's edges by means of cathodoluminescence mapping, aberration-corrected transmission electron microscopy imaging and electron energy loss analyses. Moreover, we show that ripplocations, extended line defects peculiar to this material, broaden and redshift the MoS2 indirect bandgap emission., Impurities and vacancies are commonly found within the crystalline lattice of transition metal dichalcogenides, however they are usually seen as detrimental for their optical properties. Here, the authors demonstrate that sulfur vacancies in MoS2 can give rise to a near-infrared emission peak.

The elastic constants Cij of bulk Bi2Te3 were calculated by density functional theory by different approximations. Computational results were validated by means of nanoindentation tests, performed along the directions parallel and perpendicular to the cleavage plane of Bi2Te3. The indentation modulus was estimated and compared with the experimental one. The generalized gradient Perdew-Burke-Ernzerhof approximation better reproduces experimental results in the direction perpendicular to the cleavage plane, whereas the occurrence of the nanobuckling in the direction parallel to the cleavage plane causes discrepancies between theoretical and experimental findings., Authors acknowledge MaTeRiA project (Grant No. PONa3_00370) for providing equipment to perform nanoindentation experiments. EVC acknowledge funding from the University of Basque Country UPV/EHU (GIC07-IT-756-13), the Departamento de Educación del Gobierno Vasco and the Spanish Ministerio de Ciencia e Innovación (FIS2010-275 19609-C02-01), the Tomsk State University Academic D.I. Mendeleev Fund Program (Grant No. 8.1.05.2015), the Spanish Ministry of Economy and Competitiveness MINECO (Grant No. FIS2013-48286-C2-1-P), and Saint Petersburg State University (Project No. 11.50.202.2015).

We have studied the electronic excitation spectrum in periodically rippled graphene on Ru(0 0 0 1) and flat, commensurate graphene on Ni(1 1 1) by means of high-resolution electron energy loss spectroscopy and a combination of density functional theory and tight-binding approaches. We show that the periodic moiré superlattice originated by the lattice mismatch in graphene/Ru(0 0 0 1) induces the emergence of an extra mode, which is not present in graphene/Ni(1 1 1). Contrary to the ordinary intra-band plasmon of doped graphene, the extra mode is robust in charge-neutral graphene/metal contacts, having its origin in electron–hole inter-band transitions between van Hove singularities that emerge in the reconstructed band structure, due to the moiré pattern superlattice., The support by the Netherlands National Computing Facilities foundation (NCF) are acknowledged. SY and MIK thank financial support from the European Research Council Advanced Grant program (contract 338957). The research has also received funding from the European Union Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship. RR acknowledges financial support from MINECO (Spain) through grant FIS2014-58445-JIN.

In the realm of layered materials beyond graphene, MoS2 gains a primary role due to its semiconducting nature and n-type transport down to the 2D limit that makes it extremely appealing for electronic and optoelectronic applications. The intrinsic presence of defects causes MoS2 to undergo localization effects. In the present work, solid evidence of Cs impurities in bulky MoS2 crystals in a concentration well beyond the sensitivity threshold of independent compositional spectrometry probes is brought. Unlike conventional intercalation of alkali metals in MoS2, on the basis of the measured crystal structure and ab initio calculations, it is proposed that the incorporation of Cs is stabilized by complex where one Cs atom is associated with a double S vacancy therein resulting in an overall n-type doping of the MoS2. The field effect transistor based on this kind of Cs-doped MoS2 multilayer flakes exhibits a variable range hopping transport and a metal-insulator transition.

The average elastic properties and the interatomic force constants of a quasi-freestanding graphene sample have been evaluated by analyzing the phonon dispersion measured by means of angle-resolved energy loss spectroscopy. We found a Poisson’s ratio of 0.19 and a Young’s modulus of 342 N/m. Such findings are in excellent agreement with calculations performed for a free-standing graphene membrane. Present results indicate that the high crystalline quality of graphene grown on metal substrates leads to macroscopic samples of high tensile strength and bending flexibility to be used for technological applications such as electromechanical devices and carbon-fiber reinforcements.

A combined synthesis, experiment, and theory approach, using elastic and inelastic helium atom scattering along with ab initio density functional perturbation theory, has been used to investigate the vibrational dynamics and band structure of a recently synthesized organic-functionalized semiconductor interface. Specifically, the thermal properties and lattice dynamics of the underlying Ge(111) semiconductor crystal in the presence of a commensurate (1 × 1) methyl adlayer were defined for atomically flat methylated Ge(111) surfaces. The mean-square atomic displacements were evaluated by analysis of the thermal attenuation of the elastic He diffraction intensities using the Debye-Waller model, revealing an interface with hybrid characteristics. The methyl adlayer vibrational modes are coupled with the Ge(111) substrate, resulting in significantly softer in-plane motion relative to rigid motion in the surface normal. Inelastic helium time-of-flight measurements revealed the excitations of the Rayleigh wave across the surface Brillouin zone, and such measurements were in agreement with the dispersion curves that were produced using density functional perturbation theory. The dispersion relations for H-Ge(111) indicated that a deviation in energy and lineshape for the Rayleigh wave was present along the nearest-neighbor direction. The effects of mass loading, as determined by calculations for CD_3-Ge(111), as well as by force constants, were less significant than the hybridization between the Rayleigh wave and methyl adlayer librations. The presence of mutually similar hybridization effects for CH_3-Ge(111) and CH_3-Si(111) surfaces extends the understanding of the relationship between the vibrational dynamics and the band structure of various semiconductor surfaces that have been functionalized with organic overlayers.

Self-assembled Sb2Te3 nanowires with diameters of

A new analysis of inelastic helium atom scattering (HAS) data for the (111) surface of Xe reveals a surprising similarity between the surface phonon dispersion curves of the solid noble gas xenon and of the noble metal copper, the only difference being an energy scale factor. Even more surprising is that the poorly understood X phonon branch observed in Xe(111) actually corresponds to the subsurface S2 phonon branch of Cu(111). A subsurface phonon in a closed-shell insulator surface should not be detectable by HAS, as is instead the case for free electron surfaces, where the electron−phonon interaction can couple the scattering He atoms to phonons several layers beneath the surface (quantum sonar effect). A density functional perturbation theory analysis shows that the large atomic polarizability of surface Xe atoms actually provides the electron−phonon coupling sufficient to explain the comparatively large HAS intensity from the S2 branch of Xe(111). It is concluded that the possibility of directly measuring the surface electron−phonon coupling constants from inelastic HAS intensities, originally demonstrated for conducting surfaces, can be extended to polarizable insulating surfaces, as well as oxides and heavier chalcogenides.

Phonon dispersion relations and electron-phonon coupling of hole-doped trigonal GeTe have been computed by density functional perturbation theory. This compound is a prototypical phase change material of interest for applications in phase change non-volatile memories. The calculations allowed us to estimate the electron-phonon contribution to the thermal boundary resistance at the interface between the crystalline and amorphous phases present in the device. The lattice contribution to the thermal boundary resistance has been computed by non-equilibrium molecular dynamics simulations with an interatomic potential based on a neural network scheme. We find that the electron-phonon term contributes to the thermal boundary resistance to an extent which is strongly dependent on the concentration and mobility of the holes. Further, for measured values of the holes concentration and electrical conductivity, the electron-phonon term is larger than the contribution from the lattice. It is also shown that the presence of Ge vacancies, responsible for the p-type degenerate character of the semiconductor, strongly affects the lattice thermal conductivity of the crystal.

We report on the MOCVD synthesis of Sb2Te3 nanowires that self-assemble in a novel metastable polymorph. The nanowires crystallize in a primitive trigonal lattice (P (3) over bar m1 SG #164) with lattice parameters a = b = 0.422 nm, and c = 1.06 nm. The stability of the polymorph has been studied by first principle calculations: it has been demonstrated that the stabilization is due to the particular side-wall faceting, finding excellent agreement with the experimental observations.