First-principles Study Research Articles

Concentration effects on the local structures and electronic properties of ErxBaY2-xF8: A first-principles study.

Er3+ doped barium yttrium fluoride (BaY2F8) crystal has gained long-term attention due to its great potential in laser and medical device applications. However, the local structures of Er3+ doped BaY2F8 system (Er:BYF) remain uncertain, and the effect of doping concentration on structures and properties is unknown. Therefore, in this study, the first-principles study of the structural evolution of ErxBaY2-xF8 (x = 0.125, 0.25) crystals was carried out. By means of density functional theory and particle swarm optimization algorithm, the stable structures of Er:BYF crystals with two different concentrations are shown as standard monoclinic structures with P2 symmetry for the first time. The impurity Er3+ ions successfully enter the main lattice, replacing the Y3+ ions, and forming a [ErF8]5- polyhedron with C2 point group symmetry. By calculating the electronic properties, the band gap values of the two structures are significantly reduced compared with that of pure BaY2F8 crystal. However, the conduction band does not break through the Fermi level, and the crystals still maintain the insulation characteristic. According to the calculation of the electron local density function, we conclude that Er-F and Y-F in Er:BYF are connected by ionic bonds. These results fill a theoretical gap in the study of Er:BYF crystals and provide inspiration for structural evolution and material design at different doping concentrations.

First-Principles Study of a Zirconium-Terminated Diamond (100) Surface with Promising Negative Electron Affinity and Surface Stability

First-Principles Study of a Zirconium-Terminated Diamond (100) Surface with Promising Negative Electron Affinity and Surface Stability

Enhanced hydrogen adsorption-desorption reversibility found on NiAl alloy: A first-principles study

Improving the hydrogen storage performance of carbon-based materials via transition metals incorporation has been proved experimentally, where control over the hydrogen adsorption kinetics and hydrogen surface diffusion is the key. In this work, we proved theoretically via density functional theory (DFT) how such incorporation promotes better hydrogen storage performance in Ni-based materials supported on graphene, in which static and dynamic behavior of the system were modeled by plane-wave DFT and ab initio molecular dynamics (AIMD), respectively. In addition, the hydrogen storage materials Ni10Al3 and Ni13 supported on graphene, denoted as Ni10Al3/GP and Ni13/GP, are modeled to realize the experimental observation on Ni3Al alloy under high hydrogen pressure. It was found that the Ni10Al3/GP alloy system exhibited better properties in twofold: (1) increased hydrogen adsorption capacity through providing an additional site for H2 molecular adsorption, particularly on the Al atoms, and (2) enhanced hydrogen desorption at ambient conditions observed from reversible H2 adsorption at all coverages with barrierless migration energy. Therefore, the proof from the DFT point-of-view provides valuable insight into exploring other metal alloy systems that can provide additional sites for molecular hydrogen adsorption and improve hydrogen adsorbate mobility.

First-principles study on edge–edge interactions of bilayer zigzag SiC nanoribbons

We have identified a complex interplay of van der Waals, coulombic, and direct edge-to-edge covalent interactions as key factors in determining the stability of bilayer zigzag SiC nanoribbons (ZSiCNRs). The Si–Si edge of the homo-AA-stacked ZSiCNR displays a unique bond angle, indicative of sp 3-like covalent bonds. In contrast, the C–C edge shows a flat structure, suggesting a formation of the pseudo-chemical bonding between p z orbitals at the edge like bilayer graphene NRs. The homo-AA-stacked ZSiCNR is nonmagnetic and metallic, although the monolayer ZSiCNR exhibits magnetic properties due to spin-polarized p z orbitals localized at the edge Si and C atoms. In contrast, the hetero-AA-stacked ZSiCNR is more stable than the homo one and is indicative of semiconductor properties with a finite band gap, since the topology of the edge states originating from p z orbitals is no longer preserved.

Biaxila strain modulated high anisotropic gas-sensing performance of C5N-based two-dimensional devices: A first-principles study

Two-dimensional carbon nitride materials have been widely used in many applications such as device fabrication, gas adsorption and separation due to their abundant element resources, high physicochemical stability and excellent electronic properties. In this paper, density functional theory and non-equilibrium Green's function method are used to study the electronic structure, transport characteristics and gas sensitivity of C5N-based structures. The results show that the electron transport exhibits obvious anisotropy, in which the electron transport in the armchair direction is more conductive than that in the zigzag direction. It is worth noting that the negative differential resistance effect exists in both directions. Furthermore, the transport and sensing characteristics of inorganic molecules (NO, CO, NO2, SO2, and NH3) adsorbed on the C5N monolayer are studied. The results show that CO, SO2 and NH3 exist on the surface of C5N in the form of physical adsorption, while NO and NO2 adhere to the surface of C5N in the form of chemical adsorption. The designed C5N gas sensor exhibits high sensitivity to NO and NO2 molecules, reaching 81 % sensitivity to NO2 at a 0.1 V bias voltage. Finally, the effect of strain on the adsorption properties of gas sensing devices is studied. Studies have shown that the application of -4 % strain in the armchair direction significantly increases the current of NO and NO2, significantly improving the performance of the gas sensor. Whether strain is applied to the device or gas adsorption, C5N materials always maintain significant anisotropy. This study shows that C5N is a highly anisotropic and sensitive two-dimensional material, which has broad application potential in the field of electronic properties and gas sensing.

First-principles study of binary and ternary phases in Mg-Gd-Ni alloys

First-principles study of binary and ternary phases in Mg-Gd-Ni alloys

First-principles study of charge-transfer-plasmon-enhanced photoemission from a gold-nanoparticle–sodium-atom emitter

First-principles study of charge-transfer-plasmon-enhanced photoemission from a gold-nanoparticle–sodium-atom emitter

First-Principles Study of Adsorption of CH4 on a Fluorinated Model NiF2 Surface

Electrochemical fluorination on nickel anodes, also known as the Simons’ process, is an important fluorination method used on an industrial scale. Despite its success, the mechanism is still under debate. One of the proposed mechanisms involves higher valent nickel species formed on an anode acting as effective fluorinating agents. Here we report the first attempt to study fluorination by means of first principles investigation. We have identified a possible surface model from the simplest binary nickel fluoride (NiF2). A twice oxidized NiF2(F2) (001) surface exhibits higher valent nickel centers and a fluorination source that can be best characterized as an [F2]− like unit, readily available to aid fluorination. We have studied the adsorption of CH4 and the co-adsorption of CH4 and HF on this surface by means of periodic density functional theory. By the adsorption of CH4, we found two main outcomes on the surface. Unreactive physisorption of CH4 and dissociative chemisorption resulting in the formation of CH3F and HF. The co-adsorption with the HF gave rise to four main outcomes, namely the formation of CH3F, CH2F2, CH3 radical, and also physisorbed CH4.

Pressure-induced formation of KrO2 from first-principles study

By first-principles study, we propose a hypothetical crystal (with space group P42/MNM) Krypton dioxide (KrO2) which might be synthesized at a pressure over 95[Formula: see text]GPa. At a pressure around 95[Formula: see text]GPa noticeable charge transfer from Kr atom to O atom occurs and chemical bonds are formed. Mechanism of this phenomena is analyzed using the electron-hopping mechanism. The formed Kr–O bond at high pressure can exist with retreated pressure down to 5[Formula: see text]GPa. The electron hopping mechanism might be applicable to the fabrication of other noble gas compounds.

Pressure dependent physical properties of a potential high-TC superconductor ScYH6: Insights from first-principles study

Cubic ScYH6 is elastically and thermodynamically stable within the pressure range considered. Moreover, the compound is brittle. The elastic anisotropy of ScYH6 is low and the machinability index is moderate which increases gradually with rising pressure. It is a hard material. The electronic band structure shows weakly metallic character with low density of states at the Fermi level. The Debye temperature of the compound is high and increases with increasing pressure. The Grüneisen parameter of ScYH6 is low and the phonon thermal conductivity is high at room temperature. The compound is a very efficient reflector of infrared radiation. ScYH6 is also an efficient absorber of visible and ultraviolet light. We have investigated the pressure induced changes in the predicted superconducting state properties. The superconducting transition temperature is found to increase slowly with increasing pressure.

Two-dimensional SnP3 monolayer for inhibiting the shuttle effect in lithium-sulfur batteries: A first-principles study

The problems of the shuttle effect and sluggish transformation kinetics of lithium polysulfides (LiPSs) severely inhibit the practical application of Li-S batteries. Exploring anchoring materials with catalytic properties is an effective way to overcome these obstacles. In this paper, the electrochemical performance of SnP3 monolayer as an anchoring material is explored by using first-principles calculations. SnP3 monolayer can effectively anchor soluble LiPSs under vacuum and solvent conditions, and provides abundant adsorption sites for anchoring multiple LiPSs. Importantly, the rate-limiting step has changed to the conversion of Li2S8 to Li2S6, suggesting that the monolayer can facilitate the conversion of LiPSs and inhibit their accumulation in the electrolyte. Furthermore, lower decomposition and diffusion barriers of Li2S effectively inhibit the volume expansion of Li-S batteries. Thus, SnP3 monolayer becomes a promising anchoring material with suitable catalytic properties. This work demonstrates the feasibility of SnP3 monolayer as an anchoring material and expands the membership of phosphide family for applications in the field of Li-S batteries.

Influence of Eu and Nd dopants on the photovoltaic properties of monolayer MoS2: A first-principles study

In this paper, the effects of Eu, Nd doping, and Eu–Nd co-doping on the stability, magnetic and optical properties of monolayer MoS2 are investigated by the density functional theory based on the first principle's calculations. It was found that the Eu–Nd co-doped monolayer MoS2 is more stable than other considered doping structures. After the introduction of Eu and Nd atoms, the band gap are decreased to different degrees, indicating that the doping is favorable to the electronic jump, which might enhances the conductivity of the system. All doped systems exhibit magnetic properties. In addition, the absorption spectra of all doped systems show significant red-shift compared with those of pure monolayer MoS2, indicating that the doping improves the absorption capacity in visible light. These results might provide theoretical guidance for the design of novel monolayer MoS2 semiconductor materials.

Topological Phase Transition in Non-centrosymmetric LiCaBi Compound: A First-principles Study

Topological Phase Transition in Non-centrosymmetric LiCaBi Compound: A First-principles Study

First-principles study of structural, elastic, and optoelectronic properties of alkali metal tetrafluoroaurate materials MAuF4 (M = Na, K, and Rb)

First-principles study of structural, elastic, and optoelectronic properties of alkali metal tetrafluoroaurate materials MAuF4 (M = Na, K, and Rb)

Functionalized MBenes as promising anode materials for high-performance alkali-ion batteries: a first-principles study

The discovery of novel electrode materials based on two-dimensional (2D) structures is critical for alkali metal-ion batteries. Herein, we performed first-principles computations to investigate functionalized MXenes, Mo2BT2 (T = O, S), which are also regarded as B-based MXenes, or named as MBenes, as potential anode materials for Li-ion batteries and beyond. The pristine and T-terminated Mo2BT2 (T = O, S) monolayers reveal metallic character with higher electronic conductivity and are thermodynamically stable with an intrinsic dipole moment. Both Mo2BO2 and Mo2BS2 monolayers exhibit high theoretical Li/Na/K storage capacity and low ion diffusion barriers. These findings suggest that functionalized Mo2BT2 (T = O, S) monolayers are promising for designing viable anode materials for high-performance alkali-ion batteries.

Optoelectronic Properties of Nitrogen-Doped Hexagonal Graphene Quantum Dots: A First-Principles Study

Optoelectronic Properties of Nitrogen-Doped Hexagonal Graphene Quantum Dots: A First-Principles Study

First-principles study of the effect of strain on the structural and optoelectronic properties of flexible photovoltaic material Cs2AgInBr6

Abstract The lead-free double-perovskite halide materials are promising materials for photovoltaics. Recently, Cs2AgInBr6 (CAIB) has been synthesized with the estimated direct nature of a band gap value of 1.57 eV. To cover the wide solar spectrum for photo-conversion, the applied strain is one of the promising approaches to achieve it through band gap tuning. The density functional theory (DFT) is used to investigate the effect of compressive strain on the structural, electronic, and optical properties of CAIB. The elastic constants follow the Born-Huang stability criterion and show the mechanical stability of the composition even under compressive strain. The Poisson's ratio in the range of 0.23 to 0.26 and B/G >1.75 indicate the ductile and soft nature of the material. The band gap increases monotonically without changing the direct nature of the band gap by increasing the compressive strain.  However, the larger value of strain reproduces more dispersive conduction band minima and valence band maxima, resulting in lower effective masses and consequently larger carrier mobilities. The variations in the optical properties of CAIB are explored under compressive strain. The structural, electronic, and good photoresponse of the material in the visible and ultraviolet regions indicate the suitability of the material for flexible photovoltaics.

A first-principles study on the hydrodeoxygenation mechanism of the lignin-derived phenolic compounds over MoS2 catalysts

Understanding the hydrodeoxygenation (HDO) mechanism of phenolic compounds is crucial for advancing catalytic upgrading techniques of bio-oil. Herein, a comprehensive theoretical study of the HDO of lignin-derived phenolic compounds (phenol, cresols, and guaiacol) on the MoS2 surface was carried out using periodic density functional theory (DFT) calculations. Adsorption of phenolic compounds on different crystal faces was first compared. The MoS2 (100) crystal face is more favorable for the adsorption of phenolic compounds which show the horizontal adsorption configurations. Three kinds of HDO patterns were explored for the phenolic compounds. Phenol tends to transform into benzene via stepwise deoxygenation and hydrogenation under the induction of active hydrogen. For cresol isomers, their HDO mechanisms differ from each other due to the influence of the methyl group on the dehydroxylation process. Specifically, o-cresol follows a synergistic process of deoxidation and hydrogenation, m-cresol undergoes a two-step reaction involving direct dehydroxylation and hydrogenation, and p-cresol involves the identical HDO pathway as phenol. With regard to guaiacol, the preferred HDO route involves successive dehydroxylation and demethoxylation to benzene with the anisole intermediate. In addition, the adsorption of phenolics over transition metal-doped MoS2 catalysts was further explored to examine the potential of metal doping for the HDO of phenolics. The doping of Ta and W increases the adsorption capacity, which shows the potential to enhance the HDO activity of the catalyst theoretically.

First-principles study of the structures and redox mechanisms of Ni-rich lithium nickel manganese cobalt oxides

To reduce the cobalt (Co) content in lithium-ion batteries, Ni-rich (high-Ni) lithium nickel manganese cobalt oxides (NMC) are pursued as one of the next-generation cathode materials. However, there is still debate on the crystal and electronic structures of the baseline, LiNiO2. Density Functional Theory (DFT) calculations were performed to provide a theoretical understanding of Ni-rich NMC. First, it was found that the commonly used R3¯m structure for LiNiO2 is metallic, contrary to the experimentally reported mix-conducting behavior. Among the four different space groups, R3¯m, C2/m, P21/c, and P2/c, P2/c with charge disproportionation of Ni2+ and Ni4+ is the most energetically stable and semiconducting structure of LiNiO2. Therefore, the atomic structures of representative Ni-rich NMC were built by partially replacing Ni with Co or Mn in the P2/c LiNiO2 to form LixNiyMnzCo1-y-zO2. In the fully lithiated (x = 1.0) high Ni content NMC (y > 0.5), the oxidation state of all Mn ions becomes 4+, while Co ions still maintain 3+, and part of the Ni ions become 2+ to compensate for the charge. Upon delithiation, the local environment shows more variation of the charge states on the transition metal (TM) ions. The average oxidation on each TM follows a sequence of losing electrons that starts from Ni2+ to Ni3+, then oxidizing Ni3+ and Co3+, while Mn4+ remains electrochemically inactive till x = 0. A general relationship for the oxidation state change in each TM as a function of x and y is derived and shows agreement with both modeling and experimental data.

First-principles study of the structural, mechanical, electronic, optical, and elastic properties of non-toxic XGeBr3 (X=K, Rb, and Cs) perovskite for optoelectronic and radiation sensing applications

In this study, a comprehensive analysis of the structural, electronic, optical, and elastic properties of cubic perovskite structure XGeBr3(X=K,Rb,andCs) was carried out using GGA-PBE functional based density functional theory in CASTEP code. The simulated values of the lattice parameter were increased with the changing of X from K to Cs (a = 5.51 Å, 5.54 Å, and 5.59 Å respectively). All the compounds possessed direct band gaps and acquired values for the structures KGeBr3, RbGeBr3 and CsGeBr3 were 0.525 eV, 0.599 eV, and 0.708 eV, respectively. Furthermore, the atomic orbital's contribution to the formation of band structure was clarified thoroughly. The thermodynamic, as well as the mechanical stability, has been accomplished with the assurance of negative formation energy and Born-Huang approximation. In addition, a larger shearing modulus attained by CsGeBr3 manifests its strength. Moreover, all structures bear the proof of having ductility and existing central force inside the structures. Finally, the calculated anisotropic index notifies that all structures exhibit anisotropy with the trend of CsGeBr3 < RbGeBr3 < KGeBr3. This is also authenticated through the 3D anisotropy contour plot generated by utilizing ELATE software, which is for the first time in regard to the comparative scenario among them. With the investigation of the optical characteristics of the compounds XGeBr3, intriguing features such as broad absorption spectrum, high dielectric function and refractive index at zero photon's energy, and high conductivity have been affirmed accompanying the marginal value of loss function as well as reflectivity that elevates the probability of being employed in optoelectronic applications along with different absorber layers of tandem solar cells. In addition, these compounds could be used as radiation detectors since they have broad absorption at extreme ultraviolet rays.