Performing First-principles Density Functional Theory Research Articles

Symmetry lowering through surface engineering and improved thermoelectric properties in Janus MXenes.

Despite ample evidence of their influence on the transport properties of two-dimensional solids, the interrelations of reduced symmetry, electronic and thermal transport have rarely been discussed in the context of thermoelectric materials. With the motivation to design new thermoelectric materials with improved properties, we have addressed these by performing first-principles density functional theory based calculations in conjunction with semi-classical Boltzmann transport theory on a number of compounds in the MXene family. The symmetry lowering in parent M2CO2 (M = Ti, Zr, Hf, Mo) MXenes is achieved by replacing the transition metal M on one surface, resulting in Janus compounds MM'CO2 (M = Ti, Zr, Hf and M' = Mo, Zr, Hf; M ≠ M'). Our calculations show that the thermoelectric figure-of-merit can be improved significantly by such surface engineering. We discuss in detail, both qualitatively and quantitatively, the origin behind high thermoelectric parameters for these compounds. Our in-depth analysis shows that the modifications in the electronic band structures and degree of anharmonicity driven by the dispersions in the bond strengths due to the lowering of symmetry, an artefact of surface engineering, are the factors behind the trends in the thermoelectric parameters of the MXenes considered. The results also substantiate that the compositional flexibility offered by the MXene family of compounds can generate a complex interplay of symmetry, electronic structure, bond strength and anharmonicity which can be exploited to engineer thermoelectric materials with improved properties.

Reversible hydrogen storage tendency of light-metal (Li/Na/K) decorated carbon nitride (C9N4) monolayer

The emergence of hydrogen (H2) as a future carrier of energy and the issue related to its storage leads to the exploration of two-dimensional materials, which have the potential to be explored as H2 storage materials. In this regard, potential of alkali metals (Li/Na/K) decorated two-dimensional carbon-nitride (C9N4) monolayer is studied for H2 storage by performing first-principles density functional theory (DFT) computations. Metal dopants, Li, Na, and K show strong binding interactions with the C9N4 due to the transfer of charges from the formers to the later. In addition to strong bindings with C9N4 high diffusion barriers further nullify the cluster formation among metal dopants. The effect of temperature on the stability of alkali metal decorated C9N4 is studied in terms of ab-initio molecular dynamics (AIMD) simulations. The effect of metal decoration on electronic as well as magnetic properties of C9N4 are studied in terms of partial density of states (PDOS) plots. We find that each Li of Li-doped C9N4 (Li@C9N4) can stably binds six H2 molecules without disintegration and results in the average adsorption energy of −0.20 eV, leading to a notably high H2 storage capability of 11.9 wt%. Similarly, the storage capacities and average H2 adsorption energies in the case of Na@C9N4 and K@C9N4 complexes fulfil the US Department of Energy (DOE) criteria. The electronic, AIMD, and thermodynamics analysis in this study provide an insight into that alkali metal decorated C9N4 as reversible H2 storage material.

Quantum capacitance of iron metal doped boron carbide monolayer-based for supercapacitors electrodes: A DFT study

In the last decades, many research studies have been carried out into carbon-based nanostructures as electrode materials for supercapacitors. The current research study introduced a new carbon-based electrode by using a two-dimensional (2D) Fe-doped boron carbide monolayer (FBC3ML). The capacitance, electronic, energy and structural properties of the FBC3ML were investigated by performing first-principles density functional theory (DFT) calculations. The results demonstrated that the formation energies of FBC3ML are low, which indicate its easy experimental synthesis. Interestingly, due to the shift in the Fermi level to the conduction/valence bands, Fe-doping changed the FBC3ML from a semi-conductor into a metal. By calculating the quantum capacitance of the FBC3ML, it was found that its capacitance reached 150.09 μF/cm2, which substantially outperformed other 2D carbon-based electrodes. The FBC3ML with tunable electronic properties and high specific surface area can be regarded as a promising candidate for energy storage devices and supercapacitors.

A computational study on the potential application of carbon nitride nanosheets in Na-ion batteries.

In the current study, the C3N monolayer, as a two-dimensional material, was considered as a suitable anode material in sodium-ion (Na-ion) batteries (NIBs) by performing first-principles density functional theory (DFT) computations. It was shown that the metallic feature of the Na-ion is retained after its adsorption onto various sites on the surface of the C3N monolayer. The C3N monolayer has negative adsorption energy values for the Na-ion. Afterwards, the C3N monolayer was shown to have a greater maximum theoretical capacity of 543.89 mAh/g for the Na-ion in comparison with other materials based on our computations. Due to the low activation energy barrier of approximately 0.112eV, migration of the Na-ion, which includes diffusion on the surface of the C3N monolayer, is rapid, demonstrating its great capability for fast charge-discharge processes. Ultimately, it was shown that, for the Na, the C3N monolayer possesses a low average open-circuit voltage, i.e., 0.83V. These results show that it is possible to utilize the C3N monolayer in NIBs as an anode material.

Tunable vacuum ultraviolet cross-luminescence from KMgF under high pressure as potential fast-response scintillator.

We report on the potential of the potassium magnesium fluoride (KMgF) crystal as a fast-response scintillator with tunable cross-luminescence (CL) emission wavelength through high-pressure applications. By performing first-principles density functional theory calculations using the Perdew-Burke-Ernzerhof (PBE) hybrid functional including exact exchange (PBE0) and Green's function and screened Coulomb interaction approximation as implemented in the Vienna Ab initio Simulation Package using plane-wave basis sets within the projector-augmented wave method, we identify the specific valence-to-core band transition that results in the experimentally observed CL emission at 148 nm (8.38 eV) and 170 nm (7.29 eV) wavelengths with intrinsically fast decay times of 290 ps and 210 ps, respectively. Uniform volume compression through hydrostatic high-pressure applications could decrease the energy gap between the valence and core bands, potentially shifting the CL emission wavelength to the ultraviolet (UV) region from 200 nm (6.2 eV) to 300 nm (4.1 eV). The ability to tune and shift the CL emission to UV wavelengths allows for the detection of the CL emission using UV-sensitive photodetectors in ambient atmosphere instead of highly specialized vacuum UV detectors operating in vacuum while maintaining the intrinsically fast CL decay times, thereby opening up new possibilities for KMgF as a fast-response scintillator.

Indirect-to-direct bandgap transition in bilayer InSe: roles of twistronics

We report a general approach to trigger indirect-to-direct bandgap transition in twisted bilayer two-dimensional (2D) systems. By performing first-principles density functional theory calculation, this indirect-to-direct bandgap transition is demonstrated to be realized in twisted bilayer InSe (tbIS). Through analyzing the spatial distribution of wave functions with respect to variations of twist angles, it is found that the shift of location of valence band maximum arises from the formation of Moiré flat bands associated with the evolution of electronic localization. Meanwhile, the relaxation effect introduced by out-plane displacement in relaxed tbIS Moiré superlattice is inevitable when considering the twist-angle dependent electronic properties. Our prediction in tbIS systems could provide an alternative strategy to realize indirect-to-direct bandgap transition through twistronics for designing nano-electronic devices.

Electronic properties of dilute-As InGaNAs alloys: A first-principles study

The dilute-As InyGa1 − yN1 − xAsx alloys are explored by performing first-principles density functional theory (DFT) calculations, where the In-content is varied from 0% up to 18.75% and the As-content is varied from 0% up to 3.13%. The obtained band structures indicate a direct bandgap semiconductor, whose electronic properties are modified significantly by the addition of As-atoms into the InGaN-based system. The lattice parameters (a and c) are analyzed, and the bowing parameters of the dilute-As InGaNAs are analyzed and discussed. The electronic properties indicate the dilute-As InyGa1 − yN1 − xAsx alloys as a potential new material alternative for achieving longer wavelength emission, while utilizing a low In-content (<20%).

Effect of Nonmagnetic Ion Deficiency on Magnetic Structure: Density Functional Study of Sr2MnO2Cu2-xTe2, Sr2MO2Cu2Te2 (M = Co, Mn), and the Oxide-Hydrides Sr2VO3H, Sr3V2O5H2, and SrVO2H.

Two seemingly puzzling observations on two magnetic systems were analyzed. For the oxide-hydrides Sr2VO3H, Sr3V2O5H2, and SrVO2H, made up of VO4H2 octahedra, the spin orientations of the V3+ (d2, S = 1) ions were reported to be different, namely, perpendicular to the H-V-H bond in Sr2VO3H but parallel to the H-V-H bond in Sr3V2O5H2 and SrVO2H, despite that the d-state split patterns of the VO4H2 octahedra are similar in the three oxide-hydrides. Another puzzling observation is the contrasting magnetic structures of Sr2CoO2Cu2Te2 and Sr2MnO2Cu1.58Te2, consisting of the layers made up of corner-sharing MO4Te2 (M = Co, Mn) octahedra. The Co2+ spins in each CoO2Te2 layer are antiferromagnetically coupled with spins perpendicular to the Te-Co-Te bond, whereas the Mn3+/Mn2+ ions of each MnO2Te2 layer are ferromagnetically coupled with spins parallel to the Te-Mn-Te bonds. We investigated the cause for these observations by performing first-principles density functional theory (DFT) calculations for stoichiometric phases Sr2VO3H, Sr3V2O5H2, SrVO2H, Sr2CoO2Cu2Te2, and Sr2MnO2Cu2Te2, as well as nonstoichiometric phase Sr2MnO2Cu1.5Te2. Our study reveals that the V3+ ions in all three oxide-hydrides should have the spin orientation parallel to the H-V-H bond. The unusual magnetic structure of the MnO2Te2 layers of Sr2MnO2Cu1.52Te2 arises from the preference of a Mn3+ spin to be parallel to the Te-Mn-Te bond, the ferromagnetic spin exchange between adjacent Mn3+ and Mn2+ ions, and the nearly equal numbers of Mn3+ and Mn2+ ions in each MnO2Te2 layer. We show that the spin orientation of the magnetic ions in an antiferromagnetically coupled perovskite layer, expected in the absence of nonmagnetic ion vacancies, cannot be altered by the magnetic ions of higher oxidation that result from trace vacancies at the nonmagnetic ion sites.

Controlling the Magnetic Properties of LaMnO3 /SrTiO3 Heterostructures by Stoichiometry and Electronic Reconstruction: Atomic-Scale Evidence.

Interface-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking are of importance for fundamental physics and device applications. How interfaces affect the interplay between charge, spin, orbital, and lattice degrees of freedom is the key to boosting device performance. In LaMnO3 /SrTiO3 (LMO/STO) polar-nonpolar heterostructures, electronic reconstruction leads to an antiferromagnetic to ferromagnetic transition, making them viable for spin filter applications. The interfacial electronic structure plays a critical role in the understanding of the microscopic origins of the observed magnetic phase transition, from antiferromagnetic at 5 unit cells (ucs) of LMO or below to ferromagnetic at 6 ucs or above, yet such a study is missing. Here, an atomic scale understanding of LMO/STO ambipolar ferromagnetism is offered by quantifying the interface charge distribution and performing first-principles density functional theory (DFT) calculations across this abrupt magnetic transition. It is found that the electronic reconstruction is confined within the first 3 ucs of LMO from the interface, and more importantly, it is robust against oxygen nonstoichiometry. When restoring stoichiometry, an enhanced ferromagnetic insulating state in LMO films with a thickness as thin as 2 nm (5 uc) is achieved, making LMO readily applicable as barriers in spin filters.

A first-principles study of the structural, electronic and elastic properties of solid nitromethane under pressure

The structural, electronic and elastic properties of solid nitromethane are investigated under pressure by performing first-principles density functional theory (DFT) calculations within the generalized gradient approximation (GGA) and the local density approximation (LDA). The obtained ground state structure properties are found to be consistent with existing experimental and theoretical results. The pressure-induced variations of structure parameters (a, b, c and V) indicate that the solid nitromethane has an anisotropic compressibility, and the compression along the c direction is more difficult than along a and b directions. From the vibration curves of intermolecular bond length and bond angle, we find that the C-N bond is the most sensitive among these bonds under pressure, suggesting that the C-N bonds may be broken first under external loading. The influence of pressure on the electronic properties of solid NM has been studied, indicating that solid NM is an insulating compound with a large indirect band gap and tends to be a semiconductor with increasing pressure. Finally, we predict the elastic constants and their pressure dependence for the solid NM with the bulk modulus, Young’s modulus, shear modulus and the Poisson’s ratio derived.

Magnetism in graphene nanoribbons on Ni(111): First-principles density functional study

We study magnetism of zigzag graphene nanoribbons (ZGNRs) whose ribbon widths are 1.8--2.2 nm by performing first-principles density functional theory calculations. In contrast with freestanding ZGNRs, ZGNRs directly adsorbed on Ni(111) do not show flat-band magnetism due to strong orbital hybridization between edge-localized $\text{C}\text{ }p$ orbitals and $\text{Ni}\text{ }d$ orbitals. The flat-band magnetism of the ZGNR is recovered by introduction of a graphene sheet between the ZGNR and Ni(111) as a buffer layer which weakened the orbital hybridization. In this case, a parallel configuration of spin moments at the two edges has lower energy than the antiparallel spin configuration whereas the magnetic ground state of the freestanding ZGNR has an antiparallel spin configuration. We explore the effects of orbital hybridization and charge transfer on the magnetic stability of ZGNRs on graphene/Ni(111).

Ultrafast dissociation of a core-ionized water molecule in liquid phase: Density functional theory based simulations

We investigate the dynamics of a core-ionized molecule in liquid water by performing first-principles density functional theory (DFT)-based simulations. We observe that molecular dissociation is reached within the lifetime of the induced vacancy. Our findings show that at least one of the intramolecular hydrogens migrates from the core- ionized molecule to one molecule of the first solvation shell.

Characterization of the Magnetic and Structural Properties of Copper Carbodiimide, CuNCN, by Neutron Diffraction and First-Principles Evaluations of Its Spin Exchange Interactions

The crystal structure of copper carbodiimide, CuNCN, was determined from neutron diffraction data at room temperature and at 4 K, and the electrical resistivity, specific heat, and magnetic susceptibility measurements were carried out. The spin exchange interactions of CuNCN were evaluated by performing first-principles density functional theory electronic structure calculations. CuNCN is a semiconductor containing Jahn−Teller distorted CuN6 octahedra around the divalent copper ions, and the material shows a very small and almost temperature-independent magnetic susceptibility. Our electronic structure calculations evidence that the spin exchange interactions of CuNCN are dominated by two antiferromagnetic spin exchange paths leading to a triangular lattice antiferromagnet within the ab plane. Because the coupling between the layers (along the c axis) is small, CuNCN may be regarded a two-dimensional S = 1/2 frustrated triangular Heisenberg quantum antiferromagnet.

Size dependence of structural, electronic, elastic, and optical properties of selenium nanowires: A first-principles study

We have studied the structural, elastic, and optical properties of selenium nanowires, as well as bulk selenium, by performing first-principles density functional theory calculations. The nanowires are structurally similar to bulk trigonal Se, in that they consist of hexagonal arrays of helices, though there is a slight structural rearrangement in response to the finite size of the nanowires. These small structural changes result in Young's modulus decreasing slightly for progressively thinner nanowires. However, there is a significant effect on electronic structure and optical properties. The thinner the nanowire, the greater the band gap, and the greater the anisotropy in optical conductivity. The latter is due to the effects of finite size being much more marked for the case where the electric field is polarized perpendicular to the helical axis, than in the case where the polarization is parallel to c. For the case of bulk Se, we obtain good agreement with experimental data on the structure, elastic constants, and dielectric function.

Cooperative effect of electron correlation and spin-orbit coupling on the electronic and magnetic properties ofBa2NaOsO6

The electronic and magnetic properties of the cubic double perovskite Ba2NaOsO6 were examined by performing first-principles density functional theory calculations and analyzing spin-orbit coupled states of an Os7+ (d1) ion at an octahedral crystal field. The insulating behavior of Ba2NaOsO6 was shown to originate from a cooperative effect of electron correlation and spin-orbit coupling. This cooperative effect is responsible not only for the absence of orbital ordering in Ba2 NaOsO6 but also for a small magnetic moment and a weak magnetic anisotropy in Ba2NaOsO6.