First-principles Theory Research Articles

Van der Waals density functional study of hydrocarbon adsorption and separation in metal\u2013organic frameworks without open metal sites

Metal–organic frameworks (MOFs) have received significant attention thanks to their promising features in the storage and separation of guest molecules. MOFs without open metal sites are emerging as they are often less susceptible to poisoning compared to those with open metal sites. However, a complete understanding of the binding and gas separation mechanisms in such materials is still missing. In this work, we perform a comparative study of two classes of vanadium-based MOFs without open metal sites: MFM-300-V^text{(III)} and MFM-300-V^text{(IV)}, as well as MIL-47-V^text{(III)} and MIL-47-V^text{(IV)}. We employ first-principles van der Waals density functional theory to find the optimal binding conformations and binding energies of a series of small hydrocarbons within the pores of the aforementioned MOFs. Our study provides insight into the host–guest interactions in such MOFs without open metal sites, especially the role played by the bridging hydroxyl group (mu _2–OH). We conclude that the bridging –OH group acts as a pseudo open metal site in these MOFs.Graphic abstract

Dominant hydrogen complex in natural anatase TiO2

A study combining vibrational spectroscopy and first-principles theory is presented for a hydrogen-related defect in natural anatase TiO2 that is characterized by an O–H vibrational mode at 3373 cm−1 (10 K). Based on complementary Raman scattering, IR absorption, electron paramagnetic resonance, and secondary ion mass spectrometry data supported also by ab initio calculations, it is tentatively proposed that the defect responsible for the 3373 cm−1 line includes two hydroxyl units located next to a substitutional iron atom, FeTiH2. The defect may exist in at least two charge states, whereby the 3373 cm−1 line is associated with the positive charge state of the complex.

An initio study of influence of substitution of Sc with Al on intrinsic mechanical properties of hexagonal high-entropy alloys Hf0.25Ti0.25Zr0.25Sc0.25−xAlx (x ≤ 15%)

The intrinsic mechanical properties of novel hexagonal high entropy alloys (HEAs) Hf0.25Ti0.25Zr0.25Sc0.25−xAlx (x ≤ 15%) have been studied using first-principles theory based on special quasi-random structure, and influence of Al content is stressed. The derived formation enthalpy and elastic constants indicate the thermodynamical and mechanical stability of the studied alloys. With substitution of Al for Sc, bulk modulus, shear modulus and Young's modulus tend to decrease, while HEAs become more ductile. The estimated elastic anisotropies of alloys show an ascending trend with increasing Al content, and the shear and tensile Young's moduli along several typical directions have been obtained for the studied HCP crystal. Furthermore, the studied ideal strength of HCP HEAs suggest that the ideal tensile strength (ITS) of HEAs occurs in [112̅0] direction, and decreases from 5.34 GPa to 3.80 GPa with increasing Al content for x goes from 0.00 to 0.15, whereas corresponding critical tensile strain increases from 0.07 to 0.08. The ideal shear strength (ISS) of HEAs takes place in (101̅0)< 112̅0 > shear system. With increasing Al content for x goes from 0.00 to 0.15, the ISS decrease from 3.71 GPa to 2.46 GPa, the corresponding critical shear strains increase also from 0.10 to 0.11. So HEAs are more ductile as increasing Al concentration at expense of both ITS and ISS. It is worth noting that the initial slopes of tensile and shear stress–strain are in good agreement with the tensile and shear modulus from elastic parameter, also exhibiting descending trend with Al content. The critical resolved shear stresses (CRSS) estimated from the ITS for HEAs is smaller than the corresponding ISS, suggesting that slip is preference to cleavage. The shearability and half-width of the dislocation are also evaluated and discussed. Furthermore, the detailed electronic structure in bond length evolution is further studied, from feature of the Al-M atom pairs due to addition of Al element, the mechanism for enhancement of ductility for HfTiZrSc1−xAlx is uncovered, all of these provide more details for more adequate understanding deeply the intrinsic mechanism of mechanical properties of alloys HfTiZrSc1−xAlx.

Adsorption and sensing performances of ZnO-g-C3N4 monolayer toward SF6 decomposition products

In this study, ZnO-g-C3N4 monolayer was selected as a gas-sensing material for detecting three characteristic SF6 decomposition products (SO2, SOF2 and SO2F2), based on first-principles theory. The result shows that the electrical conductivity of g-C3N4 system has been remarkably improved after ZnO doping with bind energy of −3.54 eV and Qt of 0.205 e. Each gas molecule can be stably captured on the ZnO-g-C3N4 monolayer by chemisorption, and the electrons are enormously redistributed during the process. The adsorption energy of SO2, SOF2 and SO2F2 after doping are −1.14 eV, −1.61 eV and −1.95 eV, respectively. And compared with the original system, the band gap is reduced by 18.7 %, 89.8 % and 71.9 %, respectively. The recovery time at different temperatures shows that a rapid desorption for SO2 can be achieved by increasing the temperature appropriately, while it is hard to desorb for SOF2 and SO2F2. In conclusion, this study provides a reliable theoretical basis for ZnO-g-C3N4 monolayer as gas sensing material for the detection of SF6 characteristic decomposition products.

Computational design of quantum defects in two-dimensional materials.

Missing atoms or atom substitutions (point defects) in crystal lattices in two-dimensional (2D) materials are potential hosts for emerging quantum technologies, such as single-photon emitters and spin quantum bits (qubits). First-principles-guided design of quantum defects in 2D materials is paving the way for rational spin qubit discovery. Here we discuss the frontier of first-principles theory development and the challenges in predicting the critical physical properties of point defects in 2D materials for quantum information technology, in particular for optoelectronic and spin-optotronic properties. Strong many-body interactions at reduced dimensionality require advanced electronic structure methods beyond mean-field theory. The great challenges for developing theoretical methods that are appropriate for strongly correlated defect states, as well as general approaches for predicting spin relaxation and the decoherence time of spin defects, are yet to be addressed.

First-principles nonequilibrium dynamical cluster theory for quantum transport simulations of disordered nanoelectronic devices

As important progress for simulating realistic device materials, we report the first-principles nonequilibrium dynamical cluster theory for simulating the quantum transport properties of nanoelectronics with inevitable disordered defects or dopants. In this method, we formulate the nonequilibrium dynamical cluster theory in Keldysh's Green's function representation, and implement it with the exact muffin-tin orbital based density functional theory. With this method, the important correlation effects of disordered scattering and short-range order effects can be effectively treated for the nonequilibrium electronic structure and quantum transport calculations of devices under finite bias. Moreover, a double-energy-contour technique is devised to considerably improve the numerical convergence in the nonequilibrium electron structure calculation. As the demonstration, the first-principles nonequilibrium dynamical cluster theory is applied to calculate Cu/Co junction with disordered interface and Fe/vacuum/Fe magnetic tunnel junction with surface roughness. We find that a sizable transmission decrease can be induced by including the correlation effects of disorders of few layers in the Cu/Co junction, presenting the important transport channel closing due to disordered quantum interference. For Fe/vacuum/Fe junction, we find that short-range order of surface roughness, with the important clustering and anticlustering tendencies, can dramatically change the transmission properties compared to the case of (or close to) complete randomness. The development of first-principles nonequilibrium dynamical cluster theory provides an important approach for analyzing the process-dependent device performance, extending the capability of first-principles quantum transport simulation.

A pan-Himalayan test of predictions on plant species richness based on primary production and water-energy dynamics

Spatial variation in plant species diversity is well-documented but an overarching first-principles theory for diversity variation is lacking. Chemical energy expressed as Net Primary Production (NPP) is related to a monotonic increase in species richness at a macroscale and supports one of the leading energy-productivity hypotheses, the More individuals Hypothesis. Alternatively, water-energy dynamics (WED) hypothesizes enhanced species richness when water is freely available and energy supply is optimal. This theoretical model emphasises the amount and duration of photosynthesis across the year and therefore we include the length of the growing season and its interaction with precipitation. This seasonal-WED model assumes that biotemperature and available water represent the photosynthetically active period for the plants and hence, is directly related to NPP, especially in temperate and alpine regions. This study aims to evaluate the above-mentioned theoretical models using interpolated elevational species richness of woody and herbaceous flowering plants of the entire Himalayan range based on data compiled from databases. Generalized linear models (GLM) and generalized linear mixed models (GLMM) were used to analyse species richness (elevational gamma diversity) in the six geopolitical sectors of the Himalaya. NPP, annual precipitation, potential evapotranspiration (derived by the Holdridge formula), and length of growing season were treated as the explanatory variables and the models were evaluated using the Akaike Information Criterion (AIC) and explained deviance. Both precipitation plus potential evapotranspiration (PET), and NPP explain plant species richness in the Himalaya. The seasonal-WED model explains the species richness trends of both plant life-forms in all sectors of the Himalayan range better than the NPP-model. Despite the linear precipitation term failing to precisely capture the amount of water available to plants, the seasonal-WED model, which is based on the thermodynamical transition between water phases, is reasonably good and can forecast peaks in species richness under different climate and primary production conditions.

Interfacial Properties and Electronic Structure of Ag(001)/BaTiO3(001): A First Principle Study

The adhesion strength, interface energy, interfacial fracture toughness, and electronic structure of Ag(001)/BaTiO3(001) interface have been calculated by using first-principles theory. Adhesion work is used to analyze interfacial fracture toughness, and electronic structure is used to analyze interfacial charge transfer and bonding. Compared with BaO-terminated, the interfaces composed of TiO2-terminated BaTiO3 and Ag have relatively small interface distance and interface energy, higher adhesion work and interface fracture toughness. Among the six stacking models, M 22 is the most stable interface and has the highest interfacial fracture toughness, the reason of which is that Ag–O bond produced by the hybrid of Ag-5 s orbital and O-2 s orbital. This work provides some theoretical guidance for the experiment and other theoretical research of Ag/BaTiO3 composites, and provides research ideas for the calculation of cermet interface.

Detection of SF6 decomposition components by pristine and Cr-doped GaN based on the first-principles theory

In this work, the first-principles theory is used to simulate the adsorption behavior of four SF6 characteristic decomposed gases on the pristine and Cr-doped GaN (Cr-GaN). The feasibility of the GaN for application as a gas sensor was evaluated in terms of adsorption energy, adsorption distance, transfer charge, density of state, and band structure. The results show that the adsorption energy of pristine GaN upon H2S, SOF2, and SO2F2 are all more than −0.60 eV, and both the gas molecules and pristine GaN have little geometric deformation, which is physical adsorption. The pristine GaN is more sensitive to SO2, in which the adsorption energy is less than −0.80 eV, and the deformation of GaN is obvious, which is chemical adsorption. The adsorption energies of Cr-GaN upon four gases are significantly improved over three times, which indicates the chemisorption nature. Besides, the band gap of the Cr-GaN is largely reduced, making it behave semi-metallic and magnetic properties. All the simulation studies in this paper provide a theoretical basis for the material selection of SF6 decomposition gas detection and scavenge.

Improved electrical properties of potassium-sodium niobate piezoceramics owing to micron-sized non-uniform structure

In the work, compared to previously reported KNN-ZnO piezoceramic by the conventional solid-reaction method (d33~114 pC/N and kp~36%), the improved piezoelectric properties of d33~128 pC/N and kp~41% are obtained in KNN composite piezoceramic with the inclusion of ZnO (abbreviated by KNN-ZN) prepared by femtosecond laser micro-processing and spin coating method. For comparison, the additives of BiFeO3 and Al2O3 are introduced in KNN ceramics adopting the same synthesis method. According to the investigation of microstructures and electrical properties, such enhancement for KNN-ZN ceramic is ascribed to a big dielectric constant which results from a narrow forbidden gap and loosened band structure of ZnO calculated by the first-principles theory based on the density functional theory (DFT). The work provides a new method to prepare composite ceramic and a novel strategy to enhance the piezoelectric properties of KNN piezoceramics through the modulation of the additive’s energy band structure.

Pd-doped SnP3 monolayer: A new 2D buddy for sensing typical dissolved gases in transformer oil

Dissolved gas analysis (DGA) is a workable and simple manner to evaluate the operation status of a transformer. In this paper, we investigated the adsorption performance of Pd-doped SnP3 (Pd-SnP3) monolayer upon three typical dissolved gases, including H2, CH4 and C2H2, using first-principles theory. The electronic behavior, dielectric function and work function were all considered to explore its potential as a chemical gas sensor for sensing application. Results indicated that Pd-doping is kind of n-doping for the pristine SnP3 surface and significantly enhances its adsorption performance upon three gases. Pd-SnP3 monolayer behaves physisorption upon H2 and CH4 with adsorption energy (Ead) of −0.64 and −0.65 eV, respectively, but chemisorption upon C2H2 with Ead of −1.21 eV. For sensing application, we found that Pd-SnP3 monolayer is a promising candidate as a resistance-type, optical or field electron transistor gas sensor to sensitively detect H2 and C2H2, and the electrical response for resistance-type sensor would be 93.87% and 314.29%, respectively. Our calculations aim to propose novel 2D buddy for DGA application, which is meaningful to realize the status-monitoring of the transformers and extend the exploration of SnP3-based material in many fields.

Improvement of the High-Performance Al-Doped LiNi1/3Co1/3Mn1/3O2 Cathode Material for New Electro-Optical Conversion Devices

The ternary cathode material LiNi1/3Co1/3Mn1/3O2 has been extensively focused on as the power sources for new electro-optical conversion devices and lithium-ion batteries. To improve the electrochemical performance, Al doping is one of the effective strategies. Based on the density functional theory of first-principles, the band gap, volume, partial density of states, lithiation formation energy, electron density difference, and electrons’ potential energy of Li1.0-xAlxNi1/3Co1/3Mn1/3O2 were simulated and analyzed with Materials Studio, Nanodcal and Matlab. Results show that Li0.9Al0.1Ni1/3Co1/3Mn1/3O2 has a better conductivity and cycling capability. The potential energy maps of Li1.0-xAlxNi1/3Co1/3Mn1/3O2 simulated in Matlab indicate that the rate capability of LiNi1/3Co1/3Mn1/3O2 is promoted after Al doping. Our theoretical advice could be an important choice for the power application of new optoelectronic devices. In addition, our methods could provide some theoretical guidance for the subsequent electrochemical performance investigations on doping of optoelectronic devices or lithium-ion battery materials.

Gold-black manufacture, microstructure, and optical characterization.

A previous contribution formulates a first-principle dipole antenna theory for predicting the polarization-sensitive directional spectral absorptance of gold-black in the near infrared. The current contribution chronicles a successful effort to validate that theory. After a brief review of gold-black history, we describe in some detail the design and construction of a vapor-deposition cell for laying down gold-black coatings on a mirrorlike gold substrate. The microstructure of 4- and 8-µm-thick coatings is revealed using scanning electron microscopy. An automated bench-level reflectometer has been used to measure the in-plane bidirectional reflectivity of the gold-black coatings in the visible (532 nm) and near-infrared (800 nm) for p and s polarization. Measurements are reported over incident zenith angles ranging between 10 and 50 deg. Results obtained using the apparatus are consistent with the dipole antenna theory in this range of incident zenith angles.

Single transition metal atom modified MoSe2 as a promising electrocatalyst for nitrogen Fixation: A first-principles study

Single atom electrocatalysts show great potential to nitrogen reduction reaction (NRR). Here, we studied transition metals (V, Cr, Nb, Mo, Ta, W) doped on MoSe2 as single atom electrocatalysts by first-principles theory. We investigated different NRR pathways and its corresponding Gibbs free energy. It is found that W/MoSe2 has a better NRR catalytic activity than others due to the larger charge composition with N2 molecule. Furthermore, the limiting potential has a quasi-linear relationship with the N2 adsorption energy, which is also related to the changes of charge composition. The result highlights a new family of efficient TM/MoSe2 catalysts for NRR.

Scaling theory of three-dimensional magnetic reconnection spreading

We develop a first-principles scaling theory of the spreading of three-dimensional (3D) magnetic reconnection of finite extent in the out of plane direction. This theory addresses systems with or without an out of plane (guide) magnetic field, and with or without Hall physics. The theory reproduces known spreading speeds and directions with and without guide fields, unifying previous knowledge in a single theory. New results include the following: (1) reconnection spreads in a particular direction if an x-line is induced at the interface between reconnecting and non-reconnecting regions, which is controlled by the out of plane gradient of the electric field in the outflow direction. (2) The spreading mechanism for anti-parallel collisionless reconnection is convection, as is known, but for guide field reconnection it is magnetic field bending. We confirm the theory using 3D two-fluid and resistive-magnetohydrodynamics simulations. (3) The theory explains why anti-parallel reconnection in resistive-magnetohydrodynamics does not spread. (4) The simulation domain aspect ratio, associated with the free magnetic energy, influences whether reconnection spreads or convects with a fixed x-line length. (5) We perform a simulation initiating anti-parallel collisionless reconnection with a pressure pulse instead of a magnetic perturbation, finding spreading is unchanged rather than spreading at the magnetosonic speed as previously suggested. The results provide a theoretical framework for understanding spreading beyond systems studied here and are important for applications including two-ribbon solar flares and reconnection in Earth's magnetosphere.

Enhanced NOx adsorption and sensing properties of MoTe2 monolayer by Ni-doping: A first-principles study

Based on the first-principles theory, we studied the adsorption behavior of pristine and Ni-doped MoTe2 (Ni-MoTe2) monolayer towards NOx, in order to explore their potentials to be a resistance-type chemical gas sensor. Firstly, we studied the geometric and electronic behaviors of Ni-doping on MoTe2 monolayer, which shows that Ni dopant could be stably adsorbed on the MoTe2 monolayer through the TMo site, causing enhanced electron density and reduced energy gap as well. For adsorption of NOx, Ni-MoTe2 monolayer behaved better performance compared with intrinsic counterpart, due to the pronounced electron hybridization between Ni dopant and gas molecules, leading to larger adsorption energy and charge transfer accordingly. Besides, the adsorption of NOx brought about more remarkable change in the bandgap of Ni-MoTe2 monolayer than that of intrinsic MoTe2 monolayer, exhibiting stronger potential for Ni-MoTe2 monolayer to be explored as NOx sensors or adsorbent. This work would be beneficial to extend the application of TMDs as chemical gas sensors applied in the fields of environmental monitoring and industrial manufacturing.

First-principles theory for Schottky barrier physics

We develop a first-principles theory for Schottky barrier physics. The Poisson equation is solved completely self-consistently with the electrostatic charge density and outside the normal density functional theory (DFT) electronic structure iteration loop, allowing computation of a Schottky barrier entirely from DFT involving thousands of atomic layers in the semiconductor. The induced charge in the bulk consists of conduction and valence band charges from doping and band bending, as well as charge from the evanescent states in the gap of the semiconductor. The Schottky barrier height is determined when the induced charge density and the induced electrostatic potential reach self-consistency.

Nonlinear Tunable Vibrational Response in Hexagonal Boron Nitride.

Nonlinear light–matter interactions in structured materials are the source of exciting properties and enable vanguard applications in photonics. However, the magnitude of nonlinear effects is generally small, thus requiring high optical intensities for their manifestation at the nanoscale. Here, we reveal a large nonlinear response of monolayer hexagonal boron nitride (hBN) in the mid-infrared phonon–polariton region, triggered by the strongly anharmonic potential associated with atomic vibrations in this material. We present robust first-principles theory predicting a threshold light field ∼24 MV/m to produce order-unity effects in Kerr nonlinearities and harmonic generation, which are made possible by a combination of the long lifetimes exhibited by optical phonons and the strongly asymmetric landscape of the configuration energy in hBN. We further foresee polariton blockade at the few-quanta level in nanometer-sized structures. In addition, by mixing static and optical fields, the strong nonlinear response of monolayer hBN gives rise to substantial frequency shifts of optical phonon modes, exceeding their spectral width for in-plane DC fields that are attainable using lateral gating technology. We therefore predict a practical scheme for electrical tunability of the vibrational modes with potential interest in mid-infrared optoelectronics. The strong nonlinear response, low damping, and robustness of hBN polaritons set the stage for the development of applications in light modulation, sensing, and metrology, while triggering the search for an intense vibrational nonlinear response in other ionic materials.

First-principles Insights into Cu-Decorated GaN Monolayers for Sensing CO and HCHO in Dry-Type Transformers.

This work using first-principles theory studies the sensing properties of Cu-decorated GaN (Cu–GaN) monolayers as a promising candidate for the detection of CO and HCHO in dry-type transformers. The Cu dopant prefers to be trapped on the TN site of the GaN surface with an Eb of −1.13 eV. Chemisorption is identified for the two gas adsorption systems, given the large adsorption energy (Ead) of −1.35 and −1.09 eV. Caused by the chemisorption, the electronic property of the Cu–GaN monolayer is significantly deformed, narrowing its band gap of 0.548 eV to 0.00 eV, exhibiting metallic property, in two gas systems. Combined with the desirable recovery property for CO and HCHO desorption from the Cu–GaN surface, it could be proposed that the Cu–GaN monolayer is a promising gas sensor for toxic gas detection in dry-type transformers, so as to evaluate the operation status of the power system and guarantee safe working conditions for the maintenances.

Understanding carbon dioxide capture on metal-organic frameworks from first-principles theory: The case of MIL-53(X), with X = Fe3+, Al3+, and Cu2.

Metal-organic frameworks (MOFs) constitute a class of three-dimensional porous materials that have shown applicability for carbon dioxide capture at low pressures, which is particularly advantageous in dealing with the well-known environmental problem related to the carbon dioxide emissions into the atmosphere. In this work, the effect of changing the metallic center in the inorganic counterpart of MIL-53 (X), where X = Fe3+, Al3+, and Cu2+, has been assessed over the ability of the porous material to adsorb carbon dioxide by means of first-principles theory. In general, the non-spin polarized computational method has led to adsorption energies in fair agreement with the experimental outcomes, where the carbon dioxide stabilizes at the pore center through long-range interactions via oxygen atoms with the axial hydroxyl groups in the inorganic counterpart. However, spin-polarization effects in connection with the Hubbard corrections, on Fe 3d and Cu 3d states, were needed to properly describe the metal orbital occupancy in the open-shell systems (Fe- and Cu-based MOFs). This methodology gave rise to a coherent high-spin configuration, with five unpaired electrons, for Fe atoms leading to a better agreement with the experimental results. Within the GGA+U level of theory, the binding energy for the Cu-based MOF is found to be Eb = -35.85 kJ/mol, which is within the desirable values for gas capture applications. Moreover, it has been verified that the adsorption energetics is dominated by the gas-framework and internal weak interactions.