Improved description of the structure of molecular and layered crystals: ab initio DFT calculations with van der Waals corrections

Electronic and Optical Properties of Twin T-Graphene Co-Doped with Boron and Phosphorus

Twin T-graphene (TTG) is a new two-dimensional carbon allotrope of graphene. Heteroatom co-doping is an effective method for the modulation of the physical and chemical properties of two-dimensional materials. This study explored the structural stability, electronic structures, and optical properties of boron and phosphorus co-doped TTG using first-principles calculations. TTG was doped with B and P atoms (BP) at different positions considering 13 different configurations. Pristine TTG has a band gap of 1.89 eV, and all BP co-doped TTG (TTG/BP) systems remain semiconducting with band gaps that gradually decrease with increasing doping concentration. For a given doping concentration, the TTG/BP-ortho systems had a narrower band gap than the corresponding TTG/BP-para systems. The TTG and TTG/BP systems exhibited significant optical anisotropy. In the infrared region, BP co-doping increased the absorption coefficient, and the reflectance and refractive index increased with increasing doping concentration, except for the vertical component of the TTG/BP-ortho system. In the visible region, the absorption coefficient, reflectance, and refractive index decreased with increasing doping concentration for the vertical component, and the peaks were red-shifted from the near-ultraviolet region to the visible region. In the near-ultraviolet region, the reflectance also decreased with increasing doping concentration. The BP co-doping concentration can regulate the electronic structures and optical properties of the TTG, showing that the BP co-doped TTG has potential for application in nanoelectronics and optoelectronics.

High-Accuracy Semiempirical Quantum Models Based on a Minimal Training Set

A great need exists for computationally efficient quantum simulation approaches that can achieve an accuracy similar to high-level theories at a fraction of the computational cost. In this regard, we have leveraged a machine-learned interaction potential based on Chebyshev polynomials to improve density functional tight binding (DFTB) models for organic materials. The benefit of our approach is two-fold: (1) many-body interactions can be corrected for in a systematic and rapidly tunable process, and (2) high-level quantum accuracy for a broad range of compounds can be achieved with ∼0.3% of data required for one advanced deep learning potential. Our model exhibits both transferability and extensibility through comparison to quantum chemical results for organic clusters, solid carbon phases, and molecular crystal phase stability rankings. Our efforts thus allow for high-throughput physical and chemical predictions with up to coupled-cluster accuracy for systems that are computationally intractable with standard approaches.

Quantifying the Contribution of the Dispersion Interaction and Hydrogen Bonding to the Anisotropic Elastic Properties of Chitin and Chitosan

The elastic tensors of chitin and chitosan allomorphs were calculated using density functional theory (DFT) with and without the dispersion correction and compared with experimental values. The longitudinal Young's moduli were 114.9 or 126.9 GPa for α-chitin depending on the hydrogen bond pattern: 129.0 GPa for β-chitin and 191.5 GPa for chitosan. Furthermore, the moduli were found to vary between 17.0 and 52.8 GPa in the transverse directions and between 2.2 and 15.2 GPa in shear. Switching off the dispersion correction led to a decrease in modulus by up to 63%, depending on the direction. The transverse Young's moduli of α-chitin strongly depended on the hydroxylmethyl group conformation coupled with the dispersion correction, suggesting a synergy between hydrogen bonding and dispersion interactions. The calculated longitudinal Young's moduli were, in general, higher than experimental values obtained in static conditions, and the Poisson's ratios were lower than experimental values obtained in static conditions.

Assessing the Accuracy of Machine Learning Thermodynamic Perturbation Theory: Density Functional Theory and Beyond

Machine learning thermodynamic perturbation theory (MLPT) is a promising approach to compute finite temperature properties when the goal is to compare several different levels of ab initio theory and/or to apply highly expensive computational methods. Indeed, starting from a production molecular dynamics trajectory, this method can estimate properties at one or more target levels of theory from only a small number of additional fixed-geometry calculations, which are used to train a machine learning model. However, as MLPT is based on thermodynamic perturbation theory (TPT), inaccuracies might arise when the starting point trajectory samples a configurational space which has a small overlap with that of the target approximations of interest. By considering case studies of molecules adsorbed in zeolites and several different density functional theory approximations, in this work we assess the accuracy of MLPT for ensemble total energies and enthalpies of adsorption. It is shown that problematic cases can be detected even without knowing reference results and that even in these situations it is possible to recover target level results within chemical accuracy by applying a machine-learning-based Monte Carlo (MLMC) resampling. Finally, on the basis of the ideas developed in this work, we assess and confirm the accuracy of recently published MLPT-based enthalpies of adsorption at the random phase approximation level, whose high computational cost would completely hinder a direct molecular dynamics simulation.

Exceptional Thermoelectric Properties of Bilayer GeSe: First Principles Calculation

The geometry structures, vibrational, electronic, and thermoelectric properties of bilayer GeSe, bilayer SnSe, and van der Waals (vdW) heterostructure GeSe/SnSe are investigated by combining the first-principles calculations and semiclassical Boltzmann transport theory. The dynamical stability of the considered structures are discussed with phonon dispersion. The phonon spectra indicate that the bilayer SnSe is a dynamically unstable structure, while the bilayer GeSe and vdW heterostructure GeSe/SnSe are stable. Then, the electronic structures for the bilayer GeSe and vdW heterostructure GeSe/SnSe are calculated with HSE06 functional. The results of electronic structures show that the bilayer GeSe and vdW heterostructure GeSe/SnSe are indirect band gap semiconductors with band gaps of 1.23 eV and 1.07 eV, respectively. The thermoelectric properties, including electrical conductivity, thermal conductivity, Seebeck coefficient, power factor, and figure of merit (ZT) are calculated with semiclassical Boltzmann transport equations (BTE). The results show that the n-type bilayer GeSe is a promising thermoelectric material.

Ab initio molecular dynamics investigation of the co-adsorption of iodine species with CO and H2O in silver-exchanged chabazite

In the field of nuclear energy, there is particular interest for the trapping of harmful iodine species (I2 and CH3I) that could be released during a nuclear accident, due to their dangereous impact on the human metabolic processes and on nature.

The Role of Cation Acidity on the Competition between Hydrogen Evolution and CO2 Reduction on Gold Electrodes

CO₂ electroreduction (CO₂RR) is a sustainable alternative for producing fuels and chemicals. Metal cations in the electrolyte have a strong impact on the reaction, but mainly alkali species have been studied in detail. In this work, we elucidate how multivalent cations (Li⁺, Cs⁺, Be²⁺, Mg²⁺, Ca²⁺, Ba²⁺, Al³⁺, Nd³⁺, and Ce³⁺) affect CO₂RR and the competing hydrogen evolution by studying these reactions on polycrystalline gold at pH = 3. We observe that cations have no effect on proton reduction at low overpotentials, but at alkaline surface pH acidic cations undergo hydrolysis, generating a second proton reduction regime. The activity and onset for the water reduction reaction correlate with cation acidity, with weakly hydrated trivalent species leading to the highest activity. Acidic cations only favor CO₂RR at low overpotentials and in acidic media. At high overpotentials, the activity for CO increases in the order Ca²⁺ < Li⁺ < Ba²⁺ < Cs⁺. To favor this reaction there must be an interplay between cation stabilization of the *CO₂^– intermediate, cation accumulation at the outer Helmholtz plane (OHP), and activity for water reduction. Ab initio molecular dynamics simulations with explicit electric field show that nonacidic cations show lower repulsion at the interface, accumulating more at the OHP, thus triggering local promoting effects. Water dissociation kinetics is increasingly promoted by strongly acidic cations (Nd³⁺, Al³⁺), in agreement with experimental evidence. Cs⁺, Ba²⁺, and Nd³⁺ coordinate to adsorbed CO₂ steadily; thus they enable *CO₂^– stabilization and barrierless protonation to COOH and further reduction products.

Ferromagnetic barrier induced large enhancement of tunneling magnetoresistance in van der Waals perpendicular magnetic tunnel junctions

van der Waals (vdW) intrinsic magnets are promising for miniaturization of devices beyond Moore's law for future energy efficient nanoelectronic devices and have been successfully used for constructing high performance vdW magnetic tunnel junctions (vdW MTJs). Here, using first principles calculations, we investigate the magnetic anisotropy, spin-dependent transport and tunneling magnetoresistance (TMR) effect of vdW MTJs formed by sandwiching a ferromagnetic (FM) monolayer CrI₃ or non-magnetic monolayer ScI₃ barrier between two vdW FM Fe₃GeTe₂ electrodes, respectively. It is found that two vdW MTJs possess strong perpendicular magnetic anisotropy. Moreover, due to no barrier for majority-spin transmission within half-metallic CrI₃ barrier and the difference between majority- and minority-spin conduction channels of the Fe₃GeTe₂ electrode, a high TMR ratio of about 3100% is achieved in vdW MTJs based on the Fe₃GeTe₂/CrI₃/Fe₃GeTe₂ vdW heterostructure. In contrast, a smaller TMR ratio of about 1200% is produced in vdW MTJs based on the Fe₃GeTe₂/ScI₃/Fe₃GeTe₂ vdW heterostructure due to the strong suppression of ScI₃ for majority-spin transmission in the case of the parallel state of magnetization of two FM electrodes. Our results provide a promising route for the design of vdW perpendicular MTJs with a high TMR ratio.

Combining computational and experimental studies for a better understanding of cellulose and its analogs

Over the last decade, the structural refinement of cellulose allomorphs and their analogs has been advanced using high-resolution fiber diffraction. This also includes structures of crystals complexed with small molecules, which can inherently involve disorders. Computational methods, including density functional theory, in combination with molecular modeling are leading to improved structural analyses. Spectroscopic techniques such as vibrational spectroscopy give quantitative and robust data directly related to structural insights on cellulose. These data will benefit from improved molecular modeling's capacity for interpretation and will also serve as a gauge to measure the capacity of molecular modeling as an aid in structural determinations. Improvement in the capacity to directly simulate experimental data such as that from scattering, diffraction, and spectra will be the key for further integration of modeling and experimental approaches.

Controllable fabrication and photocatalytic performance of nanoscale single-layer MoSe2 islands with substantial edges on an Ag(111) substrate

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are emerging as new electrocatalysts and photocatalysts. The edge sites of 2D TMDs show high catalytic activity and are thus favored at the catalyst surface over TMD inert basal planes. However, 2D TMDs that predominantly expose edges are thermodynamically unfavorable, limiting the number of edge sites at the surface. Herein, we demonstrate a controllable synthesis strategy of single-layer 2D MoSe₂ islands with a lateral size of approximately 5-12 nm on an Ag(111) substrate by pre-deposition of excess Se atoms. The surplus Se atoms react with the Ag(111) substrate and form silver selenide compounds to separate MoSe₂ islands and further prevent MoSe₂ islands from growing up. The nanoscale MoSe₂ islands greatly increase the ratio of exposed edge sites relative to the basal plane sites, which leads to excellent photocatalytic activity for the degradation of a methylene blue (MB) organic pollutant. This work paves the way to limit the size of 2D TMDs at the nanoscale and enables new opportunities for enhancing the catalytic activity of 2D TMD materials.

Highly adjustable piezoelectric properties in two-dimensional LiAlTe2by strain and stacking

Two-dimensional (2D) piezoelectric materials have attracted wide attention because they are of great significance to the composition of piezoelectric nanogenerators. In this work, we have systematically studied the piezoelectric properties of 2D LiAlTe₂by using the first-principles calculation and found the 2D LiAlTe₂monolayer exhibits both large in-plane piezoelectric coefficientd₁₁(3.73 pm V^-1) and out-of-plane piezoelectric coefficientd₃₁(0.97 pm V^-1). Moreover, the piezoelectric coefficients of 2D LiAlTe₂are highly tunable by strain and stacking. When different uniaxial strains are applied,d₁₁changes dramatically, butd₃₁changes little. When 2% stretching is applied to 2D LiAlTe₂monolayer along thex-axis,d₁₁reaches 7.80 pm V^-1, which is twice as large as the previously reported 2D piezoelectric material MoS₂. Both AA stacking and AB stacking can enhance the piezoelectric properties of 2D LiAlTe₂, but they have different effects on in-plane and out-of-plane piezoelectric coefficients. AA stacking can greatly increased₃₁but has little impact ond₁₁. In the case of four-layer AA stacking, thed₃₁reaches 3.32 pm V^-1. AB stacking can both increased₁₁andd₃₁, butd₁₁grows faster thand₃₁as the number of layers increases. In the case of four-layer AB stacking,d₁₁reaches 18.05 pm V^-1. The excellent and highly tunable piezoelectric performance provides 2D LiAlTe₂greater potential for the application of piezoelectric nano-generators and other micro-nano piezoelectric devices.

Transferred metal gate to 2D semiconductors for sub-1 V operation and near ideal subthreshold slope

Ultrathin two-dimensional (2D) semiconductors are regarded as a potential channel material for low-power transistors with small subthreshold swing and low leakage current. However, their dangling bond–free surface makes it extremely difficult to deposit gate dielectrics with high-quality interface in metal-oxide-semiconductor (MOS) field-effect transistors (FETs). Here, we demonstrate a low-temperature process to transfer metal gate to 2D MoS₂ for high-quality interface. By excluding extrinsic doping to MoS₂ and increasing contact distance, the high–barrier height Pt-MoS₂ Schottky junction replaces the commonly used MOS capacitor and eliminates the use of gate dielectrics. The MoS₂ transferred metal gate (TMG) FETs exhibit sub-1 V operation voltage and a subthreshold slope close to thermal limit (60 mV/dec), owing to intrinsically high junction capacitance and the high-quality interface. The TMG and back gate enable logic functions in a single transistor with small footprint.

Theoretical Design of Novel Boron-Based Nanowires via Inverse Sandwich Clusters

Borophene has important application value, boron nanomaterials doped with transition metal have wondrous structures and chemical bonding. However, little attention was paid to the boron nanowires (NWs). Inspired by the novel metal boron clusters Ln₂B_n⁻ (Ln = La, Pr, Tb, n = 7–9) adopting inverse sandwich configuration, we examined Sc₂B₈ and Y₂B₈ clusters in such novel structure and found that they are the global minima and show good stability. Thus, based on the novel structural moiety and first-principles calculations, we connected the inverse sandwich clusters into one-dimensional (1D) nanowires by sharing B−B bridges between adjacent clusters, and the 1D-Sc₄B₂₄ and 1D-Y₂B₁₂ were reached after structural relaxation. The two nanowires were identified to be stable in thermodynamical, dynamical and thermal aspects. Both nanowires are nonmagnetic, the 1D-Sc₄B₂₄ NW is a direct-bandgap semiconductor, while the 1D-Y₂B₁₂ NW shows metallic feature. Our theoretical results revealed that the inverse sandwich structure is the most energy-favored configuration for transition metal borides Sc₂B₈ and Y₂B₈, and the inverse sandwich motif can be extended to 1D nanowires, providing useful guidance for designing novel boron-based nanowires with diverse electronic properties.

Benchmark test of a dispersion corrected revised Tao-Mo semilocal functional for thermochemistry, kinetics, and noncovalent interactions of molecules and solids

In the density functional theory, dispersion corrected semilocal approximations are often used to benchmark weekly interacting finite and extended systems. Here, the focus is on providing a broad overview of the performance of D3 dispersion corrected revised Tao-Mo (revTM) semilocal functionals [A. Patra et al., J. Chem. Phys. 153, 084 117 (2020)] for thermochemistry and kinetics of molecules, molecular crystals, ice polymorphs, metal-organic systems, atom/molecular adsorption on solids, water interacting with nano-materials, binding energies of layered materials, and properties of weekly and strongly bonded solids. We show that the most suitable "optimized power" function for the revTM functional needs a modification to make it suitable for properties related to the diverse nature of finite and extended systems. The present work is an extension of the previously proposed revTM+D3 method with the motivation to design and benchmark the dispersion corrected cost-effective method based on this semilocal approximation. We show that the revised revTM+D3 functional provides various general purpose molecular and solid properties with the closest to experimental findings than its predecessor. The present assessment and benchmarking can be practically useful for performing cost-effective method based simulations of various molecular and solid-state properties.

Adsorption of water in Na-LTA zeolites: an ab initio molecular dynamics investigation

The very wide range of applications of LTA zeolites, including the storage of tritiated water, implies that a detailed and accurate atomic-scale description of the adsorption processes taking place in their structure is crucial. To unravel with an unprecedented accuracy the mechanisms behind the water filling in NaA, we have conducted a systematic ab initio molecular dynamics investigation. Two LTA structural models, the conventional Z4A and the reduced one ZK4, have been used for static and dynamic ab initio calculations, respectively. After assessing this reduced model with comparative static DFT calculations, we start the filling of the α and β cages by water, molecule by molecule. This allowed us to thoroughly study the interaction of water molecules with the zeolite structure and between water molecules, progressively forming H-bond chains and ring patterns as the cage is being filled. The adsorption energies could then be calculated with an unprecedented accuracy, which showed that the interaction of the molecules with the zeolite weakens as their number increases. By these methods, we have been able to highlight the primary role of Na⁺ cations in the interaction of water with zeolite, and inversely, the role of water in the displacement of cations when it is sufficiently solvated, allowing the passage between the α and β cages. This phenomenon is possible thanks to the inhomogeneous distribution of water molecules on the cationic sites, as shown by our AIMD simulations, which allows the formation of water clusters. These results are important because they help in understanding how the coverage of cationic sites by water will affect the adsorption of other molecules inside the Na-LTA zeolite.

CrSbS3 monolayer: a potential phase transition ferromagnetic semiconductor

Two dimensional intrinsic ferromagnetic semiconductors with controllable magnetic phase transition are highly desirable for spintronics. Nevertheless, reports on their successful experimental realization are still rare. Herein, based on first principles calculations, we propose to achieve such a functional material, namely CrSbS₃ monolayer by exfoliating from its bulk crystal. Intrinsic CrSbS₃ monolayer is a ferromagnetic half semiconductor with a moderate bandgap of 1.90 eV. It features an intriguing magnetic phase transition from ferromagnetic to antiferromagnetic when applying a small compressive strain (∼2%), making it ideal for fabricating strain-controlled magnetic switches or memories. In addition, the predicted strong anisotropic absorption of visible light and small effective masses make the CrSbS₃ monolayer promising for optoelectronic applications.

Understanding the Fundamentals of Microporosity Upgrading in Zeolites: Increasing Diffusion and Catalytic Performances

Hierarchical zeolites are regarded as promising catalysts due to their well-developed porosity, increased accessible surface area, and minimal diffusion constraints. Thus far, the focus has been on the creation of mesopores in zeolites, however, little is known about a microporosity upgrading and its effect on the diffusion and catalytic performance. Here the authors show that the "birth" of mesopore formation in faujasite (FAU) type zeolite starts by removing framework T atoms from the sodalite (SOD) cages followed by propagation throughout the crystals. This is evidenced by following the diffusion of xenon (Xe) in the mesoporous FAU zeolite prepared by unbiased leaching with NH4 F in comparison to the pristine FAU zeolite. A new diffusion pathway for the Xe in the mesoporous zeolite is proposed. Xenon first penetrates through the opened SOD cages and then diffuses to supercages of the mesoporous zeolite. Density functional theory (DFT) calculations indicate that Xe diffusion between SOD cage and supercage occurs only in hierarchical FAU structure with defect-contained six-member-ring separating these two types of cages. The catalytic performance of the mesoporous FAU zeolite further indicates that the upgraded microporosity facilitates the intracrystalline molecular traffic and increases the catalytic performance.

Study of the molybdenum dichalcogenide crystals: recent developments and novelty of the P-MoS2 structure

Nontoxicity and economic production have turned some of the molybdenum disulfide (MoS₂) polytypes into very interesting thermoelectric materials. Therefore, these materials with privileged applications have urged theoretical and experimental investigation for understanding and development of new crystals for particular applications. We present the results of computational-theoretical studies on the structural, vibrational thermoelectric, and thermodynamic properties of five crystal structures, known and newly developed, of MoS₂ based on first-principles density functional theory (DFT). While all crystals of MoS₂ were explored by undertaking several methods, the DFT method corrected for dispersion interaction (DFT-D2) confirmed the production of the cell parameters closer to the experimental. The variation of the bandgap and density of states (DOS) in all structures represents crystals comprising both semiconductors (2H- and 3R-MoS₂ crystals) and metals (1T-, P-MoS₂, and FCC-MoS₂). According to spectroscopic studies, two typical [Formula: see text] and [Formula: see text] Raman peaks are indicators of in-plane and out-of-plane vibrational modes of S atoms. From the two newly reported crystals (P-MoS₂ and FCC-MoS₂), P-MoS₂ exhibits exclusive thermoelectric properties (within 300-1000 K) such as high electrical conductivity, Seebeck coefficient, and low thermal conductivity. The thickness dependence of thermoelectric properties in 1T-, 2H-, and 3R-MoS₂ crystals is substantiated. A low thermal conductivity at room temperature along with an extremely high power factor at 1000 K exhibited by P-MoS₂ suggests P-MoS₂ crystal as a potential thermoelectric material. Finally, the present computations can introduce P-MoS₂ crystal as a new thermoelectric material with unique and extraordinary properties.

Ab initiostudies for characterization and identification of nanocrystalline copper pyrophosphate confined in mesoporous silica

Here we employ a novel method for preparing the homogeneous copper pyrophosphate nanocrystals inside silica mesopores. In order to characterize and identify synthesized nanocrystals we performed theab initiostudies of theαphase of Cu₂P₂O₇. The electronic and crystal structure were optimized within the density functional theory with the strong electron interactions in the3dstates on copper atoms and van der Waals corrections included in calculations. The relaxed lattice parameters and atomic positions agree very well with the results of the diffraction measurements for nanocrystalline copper pyrophosphates embedded inside SBA-15 silica pores. The obtained Mott insulating state with the energy gap of 3.17 eV exhibits the antiferromagnetic order with magnetic moments on copper atoms (0.8μB) that is compatible with the experimental studies. The phonon dispersion relations were obtained to study the dynamical properties of the Cu₂P₂O₇lattice and the element-specific atomic vibrations were analyzed using the partial phonon density of states. The calculated Raman spectrum revealed the consistency of typical bands of Cu₂P₂O₇with the experimental data. The investigation that combines a new synthesis of nanomaterials with the first-principles calculations is important for better characterization and understanding of the physical properties relevant for nanotechnological applications.

Understanding Plant Biomass via Computational Modeling

Plant biomass, especially wood, has been used for structural materials since ancient times. It is also showing great potential for new structural materials and it is the major feedstock for the emerging biorefineries for building a sustainable society. The plant cell wall is a hierarchical matrix of mainly cellulose, hemicellulose, and lignin. Herein, the structure, properties, and reactions of cellulose, lignin, and wood cell walls, studied using density functional theory (DFT) and molecular dynamics (MD), which are the widely used computational modeling approaches, are reviewed. Computational modeling, which has played a crucial role in understanding the structure and properties of plant biomass and its nanomaterials, may serve a leading role on developing new hierarchical materials from biomass in the future.

Conversation from antiferromagnetic MnBr2 to ferromagnetic Mn3Br8 monolayer with large MAE

A pressing need in low energy spintronics is two-dimensional (2D) ferromagnets with Curie temperature above the liquid-nitrogen temperature (77 K), and sizeable magnetic anisotropy. We studied Mn3Br8 monolayer which is obtained via inducing Mn vacancy at 1/4 population in MnBr2 monolayer. Such defective configuration is designed to change the coordination structure of the Mn-d5 and achieve ferromagnetism with sizeable magnetic anisotropy energy (MAE). Our calculations show that Mn3Br8 monolayer is a ferromagnetic (FM) half-metal with Curie temperature of 130 K, large MAE of - 2.33 meV per formula unit, and atomic magnetic moment of 13/3μB for the Mn atom. Additionally, Mn3Br8 monolayer maintains to be FM under small biaxial strain, whose Curie temperature under 5% compressive strain is 160 K. Additionally, both biaxial strain and carrier doping make the MAE increases, which mainly contributed by the magneto-crystalline anisotropy energy (MCE). Our designed defective structure of MnBr2 monolayer provides a simple but effective way to achieve ferromagnetism with large MAE in 2D materials.

First-principles calculations of hybrid inorganic-organic interfaces: from state-of-the-art to best practice

The computational characterization of inorganic-organic hybrid interfaces is arguably one of the technically most challenging applications of density functional theory. Due to the fundamentally different electronic properties of the inorganic and the organic components of a hybrid interface, the proper choice of the electronic structure method, of the algorithms to solve these methods, and of the parameters that enter these algorithms is highly non-trivial. In fact, computational choices that work well for one of the components often perform poorly for the other. As a consequence, default settings for one materials class are typically inadequate for the hybrid system, which makes calculations employing such settings inefficient and sometimes even prone to erroneous results. To address this issue, we discuss how to choose appropriate atomistic representations for the system under investigation, we highlight the role of the exchange-correlation functional and the van der Waals correction employed in the calculation and we provide tips and tricks how to efficiently converge the self-consistent field cycle and to obtain accurate geometries. We particularly focus on potentially unexpected pitfalls and the errors they incur. As a summary, we provide a list of best practice rules for interface simulations that should especially serve as a useful starting point for less experienced users and newcomers to the field.

Kinetic and mechanistic analysis of NH3 decomposition on Ru(0001), Ru(111) and Ir(111) surfaces

We investigated the catalytic NH₃ decomposition on Ru and Ir metal surfaces using density functional theory. The reaction mechanisms were unraveled on both metals, considering that, on the nano-scale, Ru particles may also present an fcc structure, hence, leading to three energy profiles. We implemented thermodynamic and kinetic parameters obtained from DFT into microkinetic simulations. Batch reactor simulations suggest that hydrogen generation starts at 400 K, 425 K and 600 K on Ru(111), Ru(0001) and Ir(111) surfaces, respectively, in excellent agreement with experiments. During the reaction, the main surface species on Ru are NH, N and H, whereas on Ir(111), it is mainly NH. The rate-determining step for all surfaces is the formation of molecular nitrogen. We also performed temperature-programmed reaction simulations and inspected the desorption spectra of N₂ and H₂ as a function of temperature, which highlighted the importance of N coverage on the desorption rate.

Magnetoelectric Response of Antiferromagnetic CrI3 Bilayers

We predict that layer antiferromagnetic bilayers formed from van der Waals (vdW) materials with weak interlayer versus intralayer exchange coupling have strong magnetoelectric response that can be detected in dual-gated devices where internal displacement fields and carrier densities can be varied independently. We illustrate this strong temperature-dependent magnetoelectric response in bilayer CrI₃ at charge neutrality by calculating the gate voltage-dependent total magnetization through Monte Carlo simulations and mean-field solutions of the anisotropic Heisenberg model informed from density functional theory and experimental data and present a simple model for electrical control of magnetism by electrostatic doping.

Two-dimensional carbon nitride C6N nanosheet with egg-comb-like structure and electronic properties of a semimetal

In this study, the structural, electronic and optical properties of theoretically predicted C₆N monolayer structure are investigated by means of Density Functional Theory-based First-Principles Calculations. Phonon band dispersion calculations and molecular dynamics simulations reveal the dynamical and thermal stability of the C₆N single-layer structure. We found out that the C₆N monolayer has large negative in-plane Poisson's ratios along bothXandYdirection and the both values are almost four times that of the famous-pentagraphene. The electronic structure shows that C₆N monolayer is a semi-metal and has a Dirac-point in the BZ. The optical analysis using the random phase approximation method constructed over HSE06 illustrates that the first peak of absorption coefficient of the C₆N monolayer along all polarizations is located in theIRrange of spectrum, while the second absorption peak occurs in the visible range, which suggests its potential applications in optical and electronic devices. Interestingly, optically anisotropic character of this system is highly desirable for the design of polarization-sensitive photodetectors. Thermoelectric properties such as Seebeck coefficient, electrical conductivity, electronic thermal conductivity and power factor are investigated as a function of carrier doping at temperatures 300, 400, and 500 K. In general, we predict that the C₆N monolayer could be a new platform for study of novel physical properties in two-dimensional semi-metal materials, which may provide new opportunities to realize high-speed low-dissipation devices.

Anharmonic Correction to Adsorption Free Energy from DFT-Based MD Using Thermodynamic Integration

Adsorption processes are often governed by weak interactions for which the estimation of entropy contributions by means of the harmonic approximation is prone to be inaccurate. Thermodynamic integration (TI) from the harmonic to the fully interacting system (λ-path integration) can be used to compute anharmonic corrections. Here, we combine TI with (curvilinear) internal coordinates in periodic systems to make the formalism available in computational studies. Our implementation of ab initio molecular dynamics in VASP is independent of the reaction path and can be thus applied to study adsorption processes relative to the gas phase and does hence provide a useful tool for computational catalysis. We discuss the application of the approach on three model systems for which exact semianalytical solutions exist and illustrate and quantify the importance of anharmonic vibrations, hindered rotations, and hindered translations (dissociation). Eventually, we apply the method to study the adsorption of small adsorbates in a zeolite (H-SSZ-13).

Graphene adlayer growth between nonepitaxial graphene and the Ni(111) substrate: a theoretical study

Understanding the fundamentals of chemical vapor deposition bilayer graphene growth is crucial for its synthesis. By employing density functional theory calculations and classical molecular dynamics simulations, we have investigated the evolution of carbon structures and the kinetics of the adlayer graphene nucleation between the graphene top layer (GTL) and the Ni(111) substrate. Compared to the epitaxial GTL, the weaker interaction between the nonepitaxial GTL and the Ni(111) substrate makes the nucleation of the adlayer more favorable. Furthermore, the defects involving in the adlayer graphene are easier to be healed by adopting the nonepitaxial GTL. Our results agree well with the experimental observation and demonstrate that the adlayer graphene with a high quality can be grown underneath the nonepitaxial GTL on Ni-like substrates.

First-principles explorations on P8 and N2 assembled nanowire and nanosheet

‘Bottom-up’ method is a powerful approach to design nanomaterials with desired properties. The bottle neck of being oxidized of phosphorous structures may be conquered by cluster assembling method. Here, we used P8 and N2 as assembling units to construct one-dimensional (1D) nanowire (NW) and two-dimensional (2D) nanosheet (NS), the stability, electronic and magnetic properties of these assembled nanomaterials are investigated using density functional theory (DFT) calculations. The assembled 1D-P8N2 NW and 2D-P8N4 NS are identified to possess good stability, as demonstrated by their high cohesive energies, positive phonon dispersions, and structural integrity through molecular dynamics simulations at 300 and 500 K. Moreover, they also exhibit good anti-oxidization property. The 2D-P8N4 NS is a direct bandgap semiconductor with the HSE06 gap of 2.61 eV, and shows appropriate band-edge aliments and moderate carrier mobility for photocatalyzing water splitting. The 1D-P8N2 NW is an indirect bandgap semiconductor, and Mn doping could convert it into a dilute magnetic semiconductor (DMS) with one Dirac cone in the spin-up channel, while the vdW-type sheet composed of Mn1@1D-P8N2 NWs is a ferromagnetic metal. Our theoretical study is helpful to design stable phosphorus-based nanomaterials with diverse properties and potential applications.

Machine Learning in Computational Surface Science and Catalysis: Case Studies on Water and Metal-Oxide Interfaces

The goal of many computational physicists and chemists is the ability to bridge the gap between atomistic length scales of about a few multiples of an Ångström (Å), i. e., 10⁻¹⁰ m, and meso- or macroscopic length scales by virtue of simulations. The same applies to timescales. Machine learning techniques appear to bring this goal into reach. This work applies the recently published on-the-fly machine-learned force field techniques using a variant of the Gaussian approximation potentials combined with Bayesian regression and molecular dynamics as efficiently implemented in the Vienna ab initio simulation package, VASP. The generation of these force fields follows active-learning schemes. We apply these force fields to simple oxides such as MgO and more complex reducible oxides such as iron oxide, examine their generalizability, and further increase complexity by studying water adsorption on these metal oxide surfaces. We successfully examined surface properties of pristine and reconstructed MgO and Fe₃O₄ surfaces. However, the accurate description of water–oxide interfaces by machine-learned force fields, especially for iron oxides, remains a field offering plenty of research opportunities.

Tuning adlayer-substrate interactions of graphene/h-BN heterostructures on Cu(111)-Ni and Ni(111)-Cu surface alloys

The evolution of the interface and interaction of h-BN and graphene/h-BN (Gr/h-BN) on Cu(111)-Ni and Ni(111)-Cu surface alloys versus the Ni/Cu atomic percentage on the alloy surface were comparatively studied by the DFT-D2 method, including the critical long-range van der Waals forces. Our results showed that the interaction strength and interface distance of Gr/h-BN/metal can be distinctly tuned by regulating the chemical composition of the surface alloy at the interface. The initially weak interaction of h-BN/Cu(111)-Ni increased linearly with increasing Ni atomic percentage, and the interface distances decreased from ∼3.10 to ∼2.10 Å. For the h-BN/Ni(111)-Cu interface, the strong interaction of the NtopBfcc/hcp stacking decreased sharply with increasing Cu atomic percentage from 0% to 50%, and the interface distances increased from ∼2.15 to ∼3.00 Å; meanwhile, the weak interaction of the BtopNfcc/hcp stacking decreased slightly with increasing Cu atomic percentage. The absorption of graphene on h-BN/Cu(111)-Ni with BtopNhollow/NtopBfcc and BtopNhollow/BtopNfcc stacking was more energetically favorable than that with NtopBhollow/NtopBfcc and NtopBhollow/BtopNfcc at Ni atomic percentages under 75%, while the interaction energy of graphene on h-BN/Cu(111)-Ni increased sharply at Ni atomic percentages higher than 75% for the BtopNhollow/NtopBfcc and NtopBhollow/NtopBfcc stacking. In contrast, the interaction between graphene and the h-BN/Ni(111)-Cu surface increased sharply at Cu atomic percentages lower than 25% and decreased sharply at Cu atomic percentages higher than 75%. The interaction energies were higher when the percentage of Cu atom was between 25% and 75%. The analysis of charge transfer and density of states provided further details on the changing character and evolution trends of the interactions among graphene, h-BN, and Cu-Ni surface alloy versus the Ni/Cu atomic percentage.

Entropy driven preference for alkene adsorption at the pore mouth as the origin of pore-mouth catalysis for alkane hydroisomerization in 1D zeolites

This work presents a theoretical basis for a pore-mouth catalysis model developed to explain the positional selectivity of skeletal isomerization on bifunctional metal acid-zeolite catalysts.

Layer-dependent photocatalysts of GaN/SiC-based multilayer van der Waals heterojunctions for hydrogen evolution

The interlayer interaction has a great influence on the formation of type-II heterojunctions, which can efficiently decompose water.

Adsorption mechanism of NH3, NO, and O2 molecules over the FexOy/AC catalyst surface: a DFT-D3 study

The surface model of the Fe_xO_y/AC catalyst was constructed and the adsorption mechanism of gas molecules on its surface was revealed.

Thickness-induced band-gap engineering in lead-free double perovskite Cs2AgBiBr6 for highly efficient photocatalysis

In recent years, two-dimensional (2D) lead-free double perovskites have been attracting much attention because of their unique performance in photovoltaic solar cells and photocatalysis. Nonetheless, how thickness affects the photoelectric properties of lead-free double perovskite remains unclear. In this work, by means of density functional theory (DFT) with a spin orbit coupling (SOC) effect, we have investigated the electronic and optical properties systemically, including band structures, carrier mobility, optical absorption spectra, exciton-binding energies, band edges alignment and molecule adsorption performance of Cs2AgBiBr6 with different thicknesses. The calculated results revealed the thickness-induced band gap and optical performance for Cs2AgBiBr6. It shows a low band gap and outstanding optical absorption of visible and ultraviolet light. When the thickness is reduced to a monolayer, Cs2AgBiBr6 moves from an indirect band gap to a direct band gap. Moreover, the carrier mobility of Cs2AgBiBr6 is excellent and the exciton-binding energy increases with the decreased thickness. Importantly, an analysis of molecule adsorption and band edge alignment indicates that Cs2AgBiBr6 is prone to H2O adsorption and H2 desorption theoretically, which is conducive to the photocatalytic water splitting for hydrogen generation and other photovatalytic reactions. Our work suggests that Cs2AgBiBr6 is a potential candidate as a solar cell or a photocatalyst, and we provide theoretical explorations into reducing the layers of lead-free double perovskite materials to 2D atomic thickness for a better photocatalytic application, which can serve as guidelines for the design of excellent photocatalysts.

C9N4 and C2N6S3 monolayers as promising anchoring materials for lithium-sulfur batteries: weakening the shuttle effect via optimizing lithium bonds

The notorious polysulfide shuttle effect is a crucial factor responsible for the degradation of Li-S batteries. A good way to suppress the shuttle effect is to effectively anchor dissoluble lithium polysulfides (LPSs, Li2Sn) on appropriate substrates. Previous studies have revealed that Li of Li2Sn is prone to interact with the N of N-containing materials to form Li-N bonds. In this work, by means of density functional theory (DFT) computations, we explored the possibility to form Li bonds on ten different N-containing monolayers, including BN, C2N, C2N6S3, C9N4, a covalent triazine framework (CTF), g-C3N4, p-C3N4, C3N5, S-N2S, and T-N2S, by examining the adsorption behavior of Li2Sn (n = 1, 2, 3, 4, 6, 8) on these two-dimensional (2D) anchoring materials (AMs), and investigated the performance of the formed Li bonds (if any) in inhibiting the shuttle effect. By comparing and analyzing the nitrogen content, the N-containing pore size, charge transfer, and Li bonds, we found that the N content and N-containing pore size correlate with the number of Li bonds, and the formed Li-N bonds between LPSs and AMs correspond well with the adsorption energies of the LPSs. The C9N4 and C2N6S3 monolayers were identified as promising AMs in Li-S batteries. From the view of Li bonds, this work provides guidelines for designing 2D N-containing materials as anchoring materials to reduce the shuttle effect in Li-S batteries, and thus improving the performance of Li-S batteries.

Development of a Cyclic Periodic Wave Function Approach for the Study of Infinitely Periodic Solid-State Systems

The ab initio cyclic periodic wave function (CPWF) approach is developed for the treatment of infinitely periodic systems. Using the full infinite Hamiltonian operator, as well as symmetrically identical basis set wave functions that preserve the translational symmetry of the electron density of the system, this approach can be applied at the Hartree-Fock level, or correlation can be directly included by the usual modes. In this approach, all many-body interactions are included, and no edge effects occur. Initial test calculations of the CPWF method at the ab initio Hartree-Fock level are performed on the chains of hydrogen fluoride molecules.