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Designing single-molecule magnets (SMMs) for potential applications in quantum computing and high-density data storage requires tuning their magnetic properties, especially the strength of the magnetic interaction. These properties can be characterized by first-principles calculations based on density functional theory (DFT). In this work, we study the experimentally synthesized Co(II) dimer (Co

The electronic and optical properties of two-dimensional materials can be strongly influenced by defects, some of which can find significant implementations, such as controllable doping, prolonged valley lifetime, and single-photon emissions. In this work, we demonstrate that defects created by remote N

The performance of x-ray free electron lasers and ultrafast electron diffraction experiments is largely dependent on the brightness of electron sources from photoinjectors. The maximum brightness from photoinjectors at a particular accelerating gradient is limited by the mean transverse energy (MTE) of electrons emitted from photocathodes. For high quantum efficiency (QE) cathodes like alkali-antimonide thin films, which are essential to mitigate the effects of non-linear photoemission on MTE, the smallest possible MTE and, hence, the highest possible brightness are limited by the nanoscale surface roughness and chemical inhomogeneity. In this work, we show that high QE Cs3Sb films grown on lattice-matched strontium titanate (STO) substrates have a factor of 4 smoother, chemically uniform surfaces compared to those traditionally grown on disordered Si surfaces. We perform simulations to calculate roughness induced MTE based on measured topographical and surface-potential variations on the Cs3Sb films grown on STO and show that these variations are small enough to have no consequential impact on the MTE and, hence, the brightness.

In this contribution, we use heavy ion irradiation and photoluminescence (PL) spectroscopy to demonstrate that defects can be used to tailor the optical properties of two-dimensional molybdenum disulfide (MoS2). Sonicated MoS2 flakes were deposited onto Si/SiO2 substrate and subjected to 3 MeV Au2+ ion irradiation at room temperature to fluences ranging from 1 × 1012 to 1 × 1016 cm−2. We demonstrate that irradiation-induced defects can control optical excitations in the inner core shell of MoS2 by binding A1s- and B1s-excitons, and correlate the exciton peaks to the specific defects introduced with irradiation. The systematic increase of ion fluence produced different defect densities in MoS2, which were estimated using B/A exciton ratios and progressively increased with ion fluence. We show that up to the fluences of 1 × 1014 cm−2, the MoS2 lattice remains crystalline and defect densities can be controlled, whereas at higher fluences (≥1 × 1015 cm−2), the large number of introduced defects distorts the excitonic structure of the material. In addition to controlling excitons, defects were used to split bound and free trions, and we demonstrate that at higher fluences (1 × 1015 cm−2), both free and bound trions can be observed in the same PL spectrum. Most importantly, the lifetimes of these states exceed trion and exciton lifetimes in pristine MoS2, and PL spectra of irradiated MoS2 remains unchanged weeks after irradiation experiments. Thus, this work demonstrated the feasibility of engineering novel optical behaviors in low-dimensional materials using heavy ion irradiation. The insights gained from this study will aid in understanding the many-body interactions in low-dimensional materials and may ultimately be used to develop novel materials for optoelectronic applications.

The commonly employed supercell approach for defects in crystalline materials may introduce spurious interactions between the defect and its periodic images. A rich literature is available on how the interaction energies can be estimated, reduced, or corrected. A simple and seemingly straightforward approach is to extrapolate from a series of finite supercell sizes to the infinite-size limit, assuming a smooth polynomial dependence of the energy on inverse supercell size. In this work, we demonstrate by means of explict density-functional theory supercell calculations and simplified models that wave-function overlap and electrostatic interactions lead to more complex dependencies on supercell size than commonly assumed. We show that this complexity cannot be captured by the simple extrapolation approaches and that suitable correction schemes should be employed. Published version Open access publication funded by the Max Planck Society.

High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB

The discovery of substrate materials has been dominated by trial and error, opening the opportunity for a systematic search. We generate bonding networks for materials from the Materials Project and systematically break up to three bonds in the networks for three-dimensional crystals. Successful cleavage reduces the bonding network to two periodic dimensions. We identify 4693 symmetrically unique cleavage surfaces across 2133 bulk crystals, 4626 of which have a maximum Miller index of one. We characterize the likelihood of cleavage by creating monolayers of these surfaces and calculating their thermodynamic stability using density functional theory to discover 3991 potential substrates. Following, we identify distinct trends in the work of cleavage and relate them to bonding in the three-dimensional precursor. We illustrate the potential impact of the substrate database by identifying several improved epitaxial substrates for the transparent conductor BaSnO3. The open-source databases of predicted and commercial substrates are available at MaterialsWeb.org.

With ${T}_{c}\ensuremath{\sim}9.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}, {\mathrm{Be}}_{22}\mathrm{Re}$ exhibits one of the highest critical temperatures among Be-rich compounds. We have carried out a series of high-pressure electrical resistivity measurements on this compound to 30 GPa. The data show that the critical temperature ${T}_{c}$ is suppressed gradually at a rate of $d{T}_{c}/dP=\ensuremath{-}0.05\phantom{\rule{0.16em}{0ex}}\mathrm{K}/\mathrm{GPa}$. Using density functional theory (DFT) calculations of the electronic and phonon density of states (DOS) and the measured critical temperature, we estimate that the rapid increase in lattice stiffening in ${\mathrm{Be}}_{22}\mathrm{Re}$ overwhelms a moderate increase in the electron-ion interaction with pressure, resulting in the decrease in ${T}_{c}$. High-pressure x-ray diffraction measurements show that the ambient pressure crystal structure of ${\mathrm{Be}}_{22}\mathrm{Re}$ persists to at least 154 GPa.

Elemental gallium can exist in several phases under ambient pressure. The stable $\ensuremath{\alpha}$ phase has a superconducting transition temperature, ${T}_{c}$, of 0.9 K. By contrast, the ${T}_{c}$ of the metastable $\ensuremath{\beta}$ phase is around 6 K. To understand the significant improvement in ${T}_{c}$ in the $\ensuremath{\beta}$ phase, we first calculate the electronic structure, phonon dispersion, and the electron-phonon coupling of gallium in the $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ phases. Next, we solve the Eliashberg equations to obtain the superconducting gaps and the transition temperatures. Using these results, we relate the increased ${T}_{c}$ in the $\ensuremath{\beta}$ phase to structural differences between the phases that affect the electronic and phonon properties. The structure motif of the $\ensuremath{\alpha}$ phase is ${\mathrm{Ga}}_{2}$ dimers, which form strong covalent bonds leading to bonding and antibonding states that reduce the density of states at the Fermi level. The $\ensuremath{\beta}$-Ga structure consists of arrays of Ga chains that favor strong coupling between the lattice vibrations and the electronic states near the Fermi level. The increased density of states and strong coupling to the phonons for the $\ensuremath{\beta}$-Ga chains compared to the $\ensuremath{\alpha}\phantom{\rule{4pt}{0ex}}{\mathrm{Ga}}_{2}$ dimers enhance superconductivity in the $\ensuremath{\beta}$-Ga phase.

We present a machine learning approach to calculate unrelaxed and relaxed formation energies of compounds relative to the ground state crystal structure of the pure components in the context of structure predictions in binary systems. Typical methods for structure predictions such as genetic algorithms often rely on density-functional theory codes to perform such calculations at a relatively high computational cost. In this work, we explore two commonly used kernel-based learning algorithms, kernel ridge regression and support vector regression. The efficiency of machine learning approaches relies on suitable data representations that encode the relevant physical information about the crystal structures. We select partial radial distribution functions to represent this structural information. We apply the machine learning approaches to the binary Li-Ge system and show that these methods provide small root-mean square prediction errors of about 20 meV/atom across the composition and structure space. Furthermore, we demonstrate that the model can be trained to predict the formation energies of the relaxed structures with the same accuracy when given unrelaxed structures as input. The high accuracy for the prediction of the relaxed energies of unrelaxed structures suggests that the machine-learning method can eliminate unlikely candidate structures from a genetic algorithm search, thus reducing the computational cost required for the explorations of energy landscapes and improving the performance of genetic algorithms for structure predictions.

We show that Pourbaix diagrams generated by combining first principles and tabulated experimental data can determine the electrochemical conditions needed to synthesize metastable phases in solutio...

The phenomenology of solid-state transformations in nanoparticles is important for applications utilizing their reactivity and for investigations into how nearby interfaces interact with the defect...

Due to its toxicity, there is a compelling need to replace lead in the organic-inorganic hybrid perovskite CH3NH3PbI3, a high-performance photovoltaic material. We use density functional theory (DFT) calculations with the generalized gradient functional PBEsol, the meta-GGA functional SCAN, and the hybrid functional HSE06, including spin-orbit coupling, to screen substituent elements based on their fundamental bandgap. Of the 27 candidate elements explored, we identify six (Mg, Si, Ge, Cd, Hg, and Zn) to be viable candidates for replacing Pb in the hybrid perovskites. We further characterize the optical absorption spectrum, with and without spin-orbit coupling, for these candidate replacements.

Scalable substitutional doping of two-dimensional (2D) transition metal dichalcogenides (TMDCs) is a prerequisite to developing next-generation logic and memory devices based on 2D materials. To date, doping efforts are still nascent. Here, we report scalable growth and vanadium (V) doping of 2D WSe2 at front-end-of-line (FEOL) and back-end-of-line (BEOL) compatible temperatures of 800 {\deg}C and 400 {\deg}C, respectively. A combination of experimental and theoretical studies confirm that vanadium atoms substitutionally replace tungsten in WSe2, which results in p-type doping via the introduction of discrete defect levels that lie close to the valence band maxima. The p-type nature of the V dopants is further verified by constructed field-effect transistors, where hole conduction becomes dominant with increasing vanadium concentration. Hence, our study presents a method to precisely control the density of intentionally introduced impurities, which is indispensable in the production of electronic-grade wafer-scale extrinsic 2D semiconductors.

Synthetic two-dimensional polymorphs of boron, or borophene, have attracted attention because of their anisotropic metallicity, correlated-electron phenomena, and diverse superlattice structures. Although borophene heterostructures have been realized, ordered chemical modification of borophene has not yet been reported. Here, we synthesize "borophane" polymorphs by hydrogenating borophene with atomic hydrogen in ultrahigh vacuum. Through atomic-scale imaging, spectroscopy, and first-principles calculations, the most prevalent borophane polymorph is shown to possess a combination of two-center-two-electron boron-hydrogen and three-center-two-electron boron-hydrogen-boron bonds. Borophane polymorphs are metallic with modified local work functions and can be reversibly returned to pristine borophene through thermal desorption of hydrogen. Hydrogenation also provides chemical passivation because borophane reduces oxidation rates by more than two orders of magnitude after ambient exposure.

Substitutional doping of two-dimensional semiconducting transition metal dichalcogenides such as $\mathrm{Mo}{\mathrm{S}}_{2}$ offers a stable and promising route for tailoring their electrical, optical, and magnetic properties. We perform density functional theory calculations for two promising transition metal dopants, Re and Nb, and their defect complexes with intrinsic S vacancies in $\mathrm{Mo}{\mathrm{S}}_{2}$. We compute the formation energy of each dopant and complex in different charge states utilizing a charge correction scheme that enables us to accurately predict the charge transition levels and complex binding energies, as well as characterize their electronic properties. We predict remarkably different behavior between Re and Nb dopants and their defect complexes: Re dopants can form complexes with S vacancies which quench the $n$-type doping of ${\mathrm{Re}}_{\mathrm{Mo}}$, while Nb dopants are unlikely to form such complexes and their $p$-type doping properties appear to be less sensitive to the presence of S vacancies.

We investigate the nature of strain in $\mathrm{Mo}{\mathrm{S}}_{2}$ and correlate it to defect types and densities, while systematically assessing the tolerance of this low dimensional material to He and Au ion irradiations. Through a series of theoretical predictions and experimental observations, we establish the onset of the crystalline-to-amorphous transition in $\mathrm{Mo}{\mathrm{S}}_{2}$ and identify sulfur vacancies as the most favorable defects introduced during irradiation. We note the presence of both tensile and compressive strains, which depend on the types of defects introduced into the lattice and vary with increasing fluence. The results show that defects can be used to tune strain in two-dimensional materials and provide an exciting pathway for using external stimuli to control properties of low dimensional materials.

The transition metal oxides MnO, FeO, NiO, and CoO are essential materials systems for catalysis applications and energy technologies. These materials exhibit a magnetic phase transition from paramagnetism to antiferromagnetism upon cooling. In this work, we show that an accurate treatment of the surfaces requires a description of the disordered local magnetic moments of the paramagnetic phase. We determine how the magnetic phase transition and the presence of solvent affect the surface energies using density-functional theory and a solvation model. To accurately account for the correlations in the $d$-electron system, and to match the observed magnetic order and band gaps of the room-temperature phases, we include the Hubbard-$U$ correction. In the case of MnO, FeO, and CoO, which are paramagnetic semiconductors or insulators at room temperature, we demonstrate the importance of the local magnetic moments and model the materials using special quasirandom structures. To determine the equilibrium shape of MnO, FeO, NiO, and CoO nanocrystals, we calculate the surface energies of their low-energy (100), (110), and (111) facets and perform the Wulff construction. For the (111) facet, we consider various reconstructions that remove the polar nature of the unreconstructed surface. The processing conditions of these oxide nanoparticles, in most cases, involve a solvent. To analyze the influence of the solvent environment on the surface energies of different facets and thereby the crystal shape, we calculate the surface energies of these oxides in water using the continuum solvation model VASPsol. We find that the surface energies decrease due to the dielectric screening. However, as the ratio of surface energies remains sufficiently similar, we predict that the equilibrium crystal shape is only weakly affected by the presence of the solvent.

Two-dimensional (2D) semiconducting transition metal dichalcogenides such as MoS$_2$ have attracted extensive research interests for potential applications in optoelectronics, spintronics, photovoltaics, and catalysis. To harness the potential of these materials for electronic devices requires a better understanding of how defects control the carrier concentration, character, and mobility. Utilizing a correction scheme developed by Freysoldt and Neugebauer to ensure the appropriate electrostatic boundary conditions for charged defects in 2D materials, we perform density functional theory calculations to compute formation energies and charge transition levels associated with sulfur vacancies in monolayer and layered bulk MoS$_2$. We investigate the convergence of these defect properties with respect to vacuum spacing, in-plane supercell dimensions, and different levels of theory. We also analyze the electronic structures of the defects in different charge states to gain insights into the effect of defects on bonding and magnetism. We predict that both vacancy structures undergo a Jahn-Teller distortion, which helps stabilize the sulfur vacancy in the $-1$ charged state., Comment: 10 pages, 6 figures. Submitted to Physical Review Materials journal

The defect relaxation volumes obtained from density-functional theory (DFT) calculations of charged vacancies and interstitials are much larger than their neutral counterparts, seemingly unphysically large. We focus on UO2 as our primary material of interest, but also consider Si and GaAs to reveal the generality of our results. In this work, we investigate the possible reasons for this and revisit the methods that address the calculation of charged defects in periodic DFT. We probe the dependence of the proposed energy corrections to charged defect formation energies on relaxation volumes and find that corrections such as potential alignment remain ambiguous with regards to its contribution to the charged defect relaxation volume. We also investigate the volume for the net neutral defect reactions comprising individual charged defects, and find that the aggregate formation volumes have reasonable magnitudes. This work highlights the issue that, as is well-known for defect formation energies, the defect formation volumes depend on the choice of reservoir. We show that considering the change in volume of the electron reservoir in the formation reaction of the charged defects, analogous to how volumes of atoms are accounted for in defect formation volumes, can renormalize the formation volumes of charged defects such that they are comparable to neutral defects. This approach enables the description of the elastic properties of isolated charged defects within an overall neutral material.

Solid state chemists have yet to find a targeted approach based on simple rules to predict new materials with desired physical properties. Recent advances in computational high-throughput methods have led to the creation of large databases with predicted new compounds. While many of these compounds are unstable, some may be stabilized inside heterostructures. BiSe is an example for such a compound where the rock-salt structure is unstable in bulk but can be found in misfit layer compounds and ferecrystals. In some of these heterostructures, BiSe also exhibits antiphase boundaries (APBs), periodic Bi–Bi pairings that interrupt the alternating pattern of the rock-salt structure. Understanding the behavior of BiSe may aid in the discovery of new heterostructure components where no stable bulk analog exists. We used density functional theory (DFT) and crystal orbital Hamilton populations (COHPs) to explain the different stabilities of rock-salt structured BiSe. COHPs show that rock-salt structured BiSe has oc...

A15 Nb$_3$Si is, until now, the only high temperature superconductor produced at high pressure (~110 GPa) that has been successfully brought back to room pressure conditions in a metastable condition. Based on the current great interest in trying to create metastable-at-room-pressure high temperature superconductors produced at high pressure, we have restudied explosively compressed A15 Nb$_3$Si and its production from tetragonal Nb$_3$Si. First, diamond anvil cell pressure measurements up to 88 GPa were performed on explosively compressed A15 Nb$_3$Si material to trace Tc as a function of pressure. Tc is suppressed to ~ 5.2 K at 88 GPa. Then, using these Tc (P) data for A15 Nb$_3$Si, pressures up to 92 GPa were applied at room temperature (which increased to 120 GPa at 5 K) on tetragonal Nb$_3$Si. Measurements of the resistivity gave no indication of any A15 structure production, i.e., no indications of the superconductivity characteristic of A15 Nb$_3$Si. This is in contrast to the explosive compression (up to P~110 GPa) of tetragonal Nb$_3$Si, which produced 50-70% A15 material, Tc = 18 K at ambient pressure, in a 1981 Los Alamos National Laboratory experiment. Our theoretical calculations show that A15 Nb$_3$Si has an enthalpy vs the tetragonal structure that is 0.07 eV/atom smaller at 100 GPa, implying that the accompanying high temperature (1000 deg C) caused by explosive compression is necessary to successfully drive the reaction kinetics of the tetragonal -> A15 Nb$_3$Si structural transformation. Annealing experiments on the A15 explosively compressed material reaching time scales of 39 years are consistent with this viewpoint., 6 figures

Controlling the phononic properties of materials provides opportunities for better thermal insulation, reduction of sound noise, and conversion of wasted heat into electricity. Phononic crystals are periodically structured media composed of two or more dissimilar materials offering a unique pathway to control the transmission of phonons, responsible for sound and heat transport. In particular, phononic crystals with cubic network structure possessing complete phononic bandgaps are highly desirable for energy applications but have not been thoroughly investigated, hampering progress in this field. Here we computationally obtained phononic band structures of 16 cubic network structures that could be made by fabrication techniques including block copolymer self-assembly and identified six structures that exhibit complete phononic bandgaps. The champion phononic bandgap structure is the so-called I-WP structure with a bandgap width of 0.41. On the basis of simulation results, design rules to tailor network st...

The discovery of emergent phenomena in two-dimensional (2D) materials has sparked substantial research efforts in the materials community. A significant experimental challenge for this field is exerting atomistic control over the structure and composition of the constituent 2D layers and understanding how the interactions between layers drives both structure and properties. Segregation of Pb to the surface of three bilayer thick PbSe-SnSe alloy layers was discovered within [(PbxSn1-xSe)1+δ]n(TiSe2)1 heterostructures using electron microscopy. We demonstrate that this segregation is thermodynamically favored to occur when PbxSn1-xSe layers are interdigitated with TiSe2 monolayers. Density-functional theory (DFT) calculations indicate that the observed segregation depends on what is adjacent to the PbxSn1-xSe layers. The interplay between interface and volume free energies controls both the structure and composition of the constituent layers, which can be tuned using layer thickness.

The friction of adjacent Tin+1Cn (n = 1, 2, and 3) MXene layers is investigated using density functional theory (DFT) calculations and classical molecular dynamics simulations with ReaxFF potentials. The calculations reveal the sliding pathways in all three MXene systems with low energy barriers. The friction coefficients for interlayer sliding are evaluated using static calculations. Both DFT and ReaxFF methods predict friction coefficients between 0.24 and 0.27 for normal loads less than 1.2 GPa. The effect of titanium (Ti) vacancies in sublayers and terminal oxygen (O) vacancies at surfaces on the interlayer friction is further investigated using the ReaxFF potential. These defects are found to increase the friction coefficients by increasing surface roughness and creating additional attractive forces between adjacent layers. However, these defective MXenes still maintain friction coefficients below 0.31. We also consider functionalized Ti3C2 MXene terminated with −OH and −OCH3 and find that compared t...

Predicting the critical temperature ${T}_{c}$ of new superconductors is a notoriously difficult task, even for electron-phonon paired superconductors, for which the theory is relatively well understood. Early attempts to obtain a simple ${T}_{c}$ formula consistent with strong-coupling theory, by McMillan and by Allen and Dynes, led to closed-form approximate relations between ${T}_{c}$ and various measures of the phonon spectrum and the electron-phonon interaction appearing in Eliashberg theory. Here we propose that these approaches can be improved with the use of machine-learning algorithms. As an initial test, we train a model for identifying low-dimensional descriptors using the ${T}_{c}l10$ K dataset by Allen and Dynes, and show that a simple analytical expression thus obtained improves upon the Allen-Dynes fit. Furthermore, the prediction for the recently discovered high-${T}_{c}$ material ${\mathrm{H}}_{3}\mathrm{S}$ at high pressure is quite reasonable. Interestingly, ${T}_{c}$'s for more recently discovered superconducting systems with a more two-dimensional electron-phonon coupling, which do not follow Allen and Dynes's expression, also do not follow our analytic expression. Thus, this machine-learning approach appears to be a powerful method for highlighting the need for a new descriptor beyond those used by Allen and Dynes to describe their set of isotropic electron-phonon coupled superconductors. We argue that this machine-learning method, and its implied need for a descriptor characterizing Fermi-surface properties, represents a promising approach to superconductor materials discovery which may eventually replace the serendipitous discovery paradigm begun by Kamerlingh Onnes.

(BiSe)1+δ(NbSe2)n heterostructures with n = 1–4 were synthesized using modulated elemental reactants. The BiSe bilayer structure changed from a rectangular basal plane with n = 1 to a square basal plane for n = 2–4. The BiSe in-plane structure was also influenced by small changes in the structure of the precursor, without significantly changing the out-of-plane diffraction pattern or value of the misfit parameter, δ. Density functional theory calculations on isolated BiSe bilayers showed that its lattice is very flexible, which may explain its readiness to adjust shape and size depending on the environment. Correlated with the changes in the BiSe basal plane structure, analysis of scanning transmission electron microscope images revealed that the occurrence of antiphase boundaries, found throughout the n = 1 compound, is dramatically reduced for the n = 2–4 compounds. X-ray photoelectron spectroscopy measurements showed that the Bi 5d3/2, 5d5/2 doublet peaks narrowed toward higher binding energies as n in...

A Materials Project based open-source Python tool, MPInterfaces, has been developed to automate the high-throughput computational screening and study of interfacial systems. The framework encompasses creation and manipulation of interface structures for solid/solid hetero-structures, solid/implicit solvents systems, nanoparticle/ligands systems; and the creation of simple system-agnostic workflows for in depth computational analysis using density-functional theory or empirical energy models. The package leverages existing open-source high-throughput tools and extends their capabilities towards the understanding of interfacial systems. We describe the various algorithms and methods implemented in the package. Using several test cases, we demonstrate how the package enables high-throughput computational screening of advanced materials, directly contributing to the Materials Genome Initiative (MGI), which aims to accelerate the discovery, development, and deployment of new materials.

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 H2O/HF solutions at low hydrogen chemical potential, μH, and that Sc-based MXenes can also be fluorinated in solutions of higher μH. After investigating the thermodynamic stability of all 54 MXene compounds Mn+1XnO2 (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 w...

A new empirical variable charge potential has been developed for AlN within the third-generation charge optimized many-body (COMB3) potential framework. The potential is able to reproduce the fundamental physical properties of AlN, including cohesive energy, elastic constants, defect formation energies, surface energies and phonon properties of AlN obtained from experiments and first-principles calculations. The thermodynamic properties of the Al(1 1 1)-AlN ( 1 0 1 ¯ 0 ) and Al 2 O 3 (0 0 0 1)-AlN ( 1 0 1 ¯ 0 ) interfaces and the tensile response of AlN nanowires and nanotubes are investigated in classical molecular dynamical (MD) simulations using this COMB3 potential. The results demonstrate that the potential is well suited to model heterogeneous materials in the Al–O–N system. Most importantly, the fully transferrable potential parameters can be seamlessly coupled with existing COMB3 parameters of other elements to enable MD simulations for an even wider range of heterogeneous materials systems.

The functionality of an electrocatalyst depends sensitively on the surface chemistry. In the case of transition-metal compounds, both the transition metal cation and the anion must be controlled to maximize the electrocatalytic activity. This realization has driven many efforts devoted to engineering the cation chemistry, producing many state-of-the-art electrocatalysts. Motivated by the critical role the cation plays in electrocatalysis, we seek to understand whether a similar effect can be achieved with the anion. Herein, we present a study on the effect of the anion substitution on the hydrogen evolution reaction (HER) electrocatalysis on cobalt oxysulfide nanoparticles. To control the sulfur substitution, we use ammonium sulfide to introduce sulfur to the cobalt oxide nanoparticles at low temperature without inducing secondary phase formation. We find that a lightly doped oxysulfide catalyst, which has the composition CoOxS0.18, exhibits a metastable, distorted S-substituted CoO phase and is 2–3 times more active for the HER than either end-member of the substitution series. Our first-principles calculations attribute the HER enhancement to the stronger surface H adsorption which is maximally favorable at a relatively low doping level. Our work provides a protocol for synthesizing metastable mixed-anion materials and reveals the critical role of the anion on the surface physiochemical properties and the HER electrocatalysis.