First-principles Study Research Articles

First-Principles Study of Doped CdX(X = Te, Se) Compounds: Enhancing Thermoelectric Properties.

Isovalent doping offers a method to enhance the thermoelectric properties of semiconductors, yet its influence on the phonon structure and propagation is often overlooked. Here, we take CdX (X=Te, Se) compounds as an example to study the role of isovalent doping in thermoelectrics by first-principles calculations in combination with the Boltzmann transport theory. The electronic and phononic properties of Cd8Se8, Cd8Se7Te, Cd8Te8, and Cd8Te7Se are compared. The results suggest that isovalent doping with CdX significantly improves the thermoelectric performance. Due to the similar properties of Se and Te atoms, the electronic properties remain unaffected. Moreover, doping enhances anharmonic phonon scattering, leading to a reduction in lattice thermal conductivity. Our results show that optimized p-type(n-type) ZT values can reach 3.13 (1.33) and 2.51 (1.21) for Cd8Te7Se and Cd8Se7Te at 900 K, respectively. This research illuminates the potential benefits of strategically employing isovalent doping to enhance the thermoelectric properties of CdX compounds.

The behaviors of He atoms at Ti3SiC2(001)/V(110) interface: A first-principles study

Ti3SiC2 nano-particles and He atom play an important role for the degradation of properties in vanadium alloys under irradiation environment. Herein, the behaviors of He atoms at C-Ti2-H Ti3SiC2(001)/V(110) interface model by first-principles calculations, which is the most thermodynamically stable, is studied to elucidate the effect of Ti3SiC2 nano-particles on the He bubble formation in the vanadium alloys. He atom tends to dissolve at the Ti3SiC2/V interface and at the near Si layer of Ti3SiC2 side at the interface. The vacancy type defects formation energies for V1, C1, C2 and Si, which both lower than V monovacancy formation energy in bulk vanadium, are more likely to form at the interface. Moreover, The existence of monovacancy at the interface makes it easier for neighboring He atoms to dissolve, which has a spatial effect on He atom. The result of the trapping energy and the interface distortion of He atom trapped by Si and V1 vacancies, more He atoms can be trapped as the number of vacancies increases. The diffusion of He atom located at Ti3SiC2 side towards the interface hampered by the small spacing Ti and C atom layers (dlsTi2−C2= 1.18 Å) anddlsTi1−C1 = 1.31 Å) He atom tends to aggerate at the interface due to the higher energy barrier. The Ti3SiC2-praticle/V-matrix interface can act as a sink for He atom. These findings provide a new insight into the understanding about the influence of Ti3SiC2 nano-particles on the mechanisms of He bubble formation and the guidance for further the preparation of vanadium alloys.

First-Principles Study on Valley Polarization of Ferromagnetic PrBr2 Monolayer under Strain

First-Principles Study on Valley Polarization of Ferromagnetic PrBr₂ Monolayer under Strain

Topological and superconducting properties of monolayered CoN and CoP: A first-principles comparative study

Topological and superconducting properties of monolayered CoN and CoP: A first-principles comparative study

Insights into the electronic, optical, and catalytic properties of finite biphenylene nanoribbons: First-principles study

The electronic, optical, and catalytic properties of finite two-dimensional biphenylene nanoribbons are investigated using density functional theory calculations. The nanoribbons are energetically and dynamically stable as confirmed by the negative formation energy ∼ -7 eV and the real vibrational frequencies, respectively. Increasing the length of nanoribbons in the zigzag or armchair directions reduces the energy gap, with modest oscillations in the former case. In contrast to zigzag graphene nanoribbons, biphenylene ones have nonmagnetic spin ordering due to the passivation of the unpaired electrons at the zigzag edges by the 4C- rings connecting the benzene rings. The UV–vis spectra exhibit redshifts, with size increases in both the armchair and zigzag directions, which validate the electronic computations' results. Simulating the catalytic performance of oxygen evolution reaction shows remarkable performance with a low overpotential of 0.12 V. Thus, biphenylene nanoribbons with desirable electronic and catalytic properties are attractive anode materials for water-splitting cells.

First-principles study of the elastic tensor of GaP at high electronic temperature

First-principles study of the elastic tensor of GaP at high electronic temperature

First-principles study of the titanium-doping effects on the properties of O3-type NaNi0.25Fe0.25Mn0.5O2 cathode material for sodium-ion batteries

We carried out first-principles calculations on titanium-doped O3-type NaNi0.25Fe0.25Mn0.5O2 oxide material for titanium substitution at 0.02, 0.04, 0.06, 0.08, and 0.1. The results obtained show that materials doped with titanium have higher structural stability than undoped material. Also after doping, the conductivity of the materials increases greatly but Na(Ni0.25Fe0.25Mn0.5)0.98Ti0.02O2 is slightly more conductive (ΔEg = 0.017 eV). Na(Ni0.25Fe0.25Mn0.5)1-xTixO2 (NNFM-Tix) (x = 0.02, 0.04, 0.06, 0.08, and 0.1) indicates a larger sodium layer, which allows easy migration of sodium ions during charge/discharge processes than NNFM. Sodium ions diffuse faster in NNFM-Ti0.06 and NNFM-Ti0.08 compared to other doped-NNFMs. In addition, the Pugh index value of NNFM (1.8222) is lower than that of NNFM-Ti0.02, NNFM-Ti0.04, NNFM-Ti0.06, NNFM-Ti0.08, and NNFM-Ti0.1 (2.1376), demonstrating enhancement of elastic ductility after doping. Titanium-doped materials exhibited improved stability under temperature effects between −20 and 60 °C, which represent the operating temperature range of sodium-ion batteries, although this effect did not vary much with the doping rate. The improved thermodynamic stability indicates that the doped materials can better resist degradation, have high efficiency, and a long cycle life. These Na(Ni0.25Fe0.25Mn0.5)1-xTixO2 materials, particularly those with the titanium rate at 0.06 and 0.08 are therefore powerful materials for the accomplishment of efficient sodium-ion batteries cathodes.

First-principles study of layer-dependent band alignment and work function in MoS[formula omitted] nanoflakes

MoS2 is renowned for its intriguing electronic and optical properties, which exhibit variations dependent on the number of layers and the atomic arrangement within its layers. In this work, we employ first-principles theory calculations to systematically investigate how interlayer spacing and layer number impact the electronic properties of MoS2 nanoflakes. The primary focus of this work is to comprehend the layer-dependent behaviors of the band gap, work function, and band alignment of MoS2. Our findings reveal that the MoS2 band gap experiences a reduction as the number of layers increases, following a power-law relationship, while the work function rises with an increasing layer count. Moreover, both band gap and work function increase with the expansion of the interlayer spacing in MoS2 layers. This study suggests that the layer-dependent characteristics of band edges and work functions in few-layer MoS2 offer valuable opportunities for the manipulation of Schottky barriers, ultimately enhancing carrier injection efficiency.

Enhancing mechanical properties of MgB2 superconductors through Nb substitution: A first-principles study

MgB2 has a wide range of applications as a crucial superconducting material in fields such as electronics and communications, but its weak mechanical properties limit its use in extreme environments such as high pressure. Therefore, this article explores the enhancement mechanism of the mechanical properties for MgB2 by substituting Mg sites with Nb element based on first principles. After substitution, NbB2 has better thermodynamic stability, and the introduction of Nb-d state enhances light absorption. In terms of mechanics, due to the higher electron accumulation density in the Nb–B bond and a shorter bond length (2.451 Å), the atomic interactions become tighter and stronger. As a result, both elastic constants and modulus are enhanced, exhibiting high stiffness, high compression resistance, etc. The spatial distribution of 3D elastic anisotropy is closer to a spherical shape and the universal anisotropy index (AU) tends to approach 0, which indicates the mechanical properties in all directions are more consistent, making it easier to predict and control. Meanwhile, the elastic modulus rises smoothly with increasing hydrostatic pressure and is not affected by lattice compression at low hydrostatic pressure. The change rate of B–B bond length is 6.8 % at 100 GPa, which becomes the main reason for the enhancement of mechanical properties. The article provides a theoretical foundation for the preparation of superconductors with excellent mechanical properties under extreme conditions.

Effect of metal (Cr, Sr, Ag, Cu) doping on the performance of lead-free RbSnI3 based perovskite solar cells: A theoretical approach

Perovskite solar cells based on lead have witnessed unprecedented growth over the past decade, achieving an impressive power conversion efficiency (PCE) of 26.1%. However, lead toxicity remains a concern for commercialization. In order to resolve the matter, scientists have been investigating alternative materials; in this context, rubidium-based lead-free perovskites like RbSnI3 may be a promising alternative because it has a high optical conductivity and absorption coefficient. Density Functional Theory (DFT)-based first-principles studies are used in this work to examine the effect of metal doping (specifically Cr, Sr, Ag, and Cu) on the optoelectronic and structural characteristics of orthorhombic RbSnI3 perovskite. In addition, we conducted a comprehensive study to investigate the impact of metal doping on the formation energy, structural stability, and HOMO–LUMO energy levels of RbSnI3 perovskite. Introducing transition metal cations (Cr2+, Ag+, and Cu+) at the Rb site results in a flat band in the conduction band region, transforming the RbSnI3’s indirect band gap into a direct one and significantly affecting the optoelectronic properties. The DFT results are then integrated into the Solar Cell Capacitance Simulator (SCAPS-1D) to estimate the effectiveness of the modeled device. The Cu-doped RbSnI3 device exhibits the highest PCE of 20.2%. Furthermore, Ag and Cu doping in RbSnI3 increases bond length, which reduces exciton binding energy and helps with charge carrier generation.

Speed of sound in dense strong-interaction matter

We study the speed of sound in strong-interaction matter at zero temperature and in density regimes which are expected to be governed by the presence of a color-superconducting gap. At (very) high densities, our analysis indicates that the speed of sound approaches its asymptotic value associated with the non-interacting quark gas from below, in agreement with first-principles studies which do not take the presence of a color-superconducting gap into account. Towards lower densities, however, the presence of a gap induces an increase of the speed of sound above its asymptotic value. Importantly, even if gap-induced corrections to the pressure may appear small, we find that derivatives of the gap with respect to the chemical potential can still be sizeable and lead to a qualitative change of the density dependence of the speed of sound. Taking into account constraints on the density dependence of the speed of sound at low densities, our general considerations suggest the existence of a maximum in the speed of sound. Interestingly, we also observe that specific properties of the gap can be related to characteristic properties of the speed of sound which are indirectly constrained by observations.

The role of graphene in SnS2@Graphene for rechargeable lithium batteries: A view from the electronic structure

Based on the first-principles study, the adsorption and electron transfer properties of Li atom at different sites of SnS2 monolayer, SnS2@Graphene 2D-nanocomposite are analyzed. The differential charge density and density of states (DOS) analysis show that the graphene substrate as an electron donor can change the 2D-nanocomposite from a semiconductor to a metal, and reduce the adsorption energy of Li atom by decreasing the charge transferring from Li atom to SnS2. This indicates that graphene substrate is beneficial for improving the performance of SnS2@Graphene. Meanwhile, the Li atoms tend not to cluster on the SnS2@Graphene 2D-nanocomposite, which is useful to prolong the lifespan of the SnS2@Graphene. The functionality of graphene in SnS2@Graphene 2D-nanocomposite is proved by other electron donor substrates, such as a two-H-atom model and a Sn (111) substrate model. All the results indicate that the graphene, as an electron donor in SnS2@Graphene 2D-nanocomposite, plays a key role in improving the performance of SnS2 in rechargeable lithium batteries.

Structural, Opto-Electronic and Magneto-Elastic Properties of Tetra One Type Layered EuCuFS/Se Magnetic Semiconductor for Diverse Applications: A First-Principles Study

Structural, Opto-Electronic and Magneto-Elastic Properties of Tetra One Type Layered EuCuFS/Se Magnetic Semiconductor for Diverse Applications: A First-Principles Study

First-principle study of CrO2-BNNT-CrO2 based MTJ device using EHTB model and its application in a MRAM circuit

In this work we investigated the performance of Single-Walled Boron Nitride Nanotube (SWBNNT) sandwiched between CrO2 based Magnetic Tunnel Junction (MTJ) structures with atomistic simulations. MTJ based device-level simulation was performed using the Extended Huckel based Tight Binding (EHTB) model. The SWBNNT as a dielectric layer and CrO2 as Ferromagnetic (FM) layers based Magnetic Tunnel Junction (MTJ) heterostructure was computed. The proposed MTJ device operates at the nano-Ampere (nA) range depicting its potential application in a low-powered nanoelectronics device. It also shows a promising Tunnel Magnetoresistance (TMR) effect. The practical circuit-level implementation of the proposed MTJ device in Magnetic Random-Access Memory (MRAM) was validated using LTspice. Hence, the calculated static power consumption and writing energy per bit was minimal for S1 and S2 based MRAM circuit. The perfect switching operation was depicted by the proposed MTJ device based MRAM.

Piezoelectric characteristics of doped β-Ga2O3 monolayer: a first-principles study

Two-dimensional (2D) piezoelectric materials have been widely concerned because of their important applications in nano-piezoelectric generators. Finding two-dimensional materials with large piezoelectric effects is still a great challenge. In this work, the inversion center of the β-Ga2O3 monolayer was broken by substitutional doping. Not only the in-plane piezoelectric effect but also the uncommon out-of-plane piezoelectric effect is induced in the doped β-Ga2O3 monolayer. In addition, we analyzed the cause of the piezoelectric effects from their electronic properties. The values of out-of-plane (in-plane) piezoelectric coefficient for Cu-doped and Al-doped β-Ga2O3 reach −4.04 (3.95) pm/V and −2.91 (0.37) pm/V, respectively. The results are comparable with those of the commonly used bulk piezoelectric materials such as α-quartz (d11 = 2.3 pm V−1), AlN (d33 = 5.1 pm V−1), and GaN (d33 = 3.1 pm V−1), even though they are both two-dimensional structures. Our study shows a great potential application of doped β-Ga2O3 monolayer in micro and nano-electromechanical devices such as smart wearables, sensors, energy converters, and micro energy collectors.

Structural, mechanical and electronic properties and phase transitions in superhard BC1-xNx: A first-principles study

First-principles investigations of the stability, mechanical properties, electronic and phonon structures, as well as molecular dynamics simulations of temperature- and pressure-induced phase transitions in boron carbonitrides, BC1-xNx, for x = 1.0 (C0), 0.75 (C1), 0.5 (C2), 0.25 (C3) and 0.0 (C4), were carried out. The plausible mechanisms of the phase transitions in BNs were proposed and the transformation paths were established for the h → w, r → c, c → r and w → h’ transitions, where h’ was the new hP4-187 phase (space group P-6m2, #187). The non-layered C0–C3 compounds transformed into the layered ones at 2000–3000 K. The new BC1-xNx structures are brittle materials and exhibit the best combination of hardness (23.1–60.8 GPa) and fracture toughness (4.03–5.23 MPa m1/2). The C1–C4 structures are metallic, except for three compounds: C1-oP8-25 and C4-tP8-136 are semi-metals, and C4-mP8-10 is a semiconductor with a very narrow bandgap of 0.2 eV.

First-principles study of behaviors of point defects in Mo-14 Re alloy

Molybdenum alloys with rhenium additions of 14 % (Mo-14 Re) are regarded as structural materials for advanced nuclear reactors. The performance of Mo-14 Re alloy under irradiation is crucial for its application. Irradiation performance is related to the behaviors of point defects produced by irradiation. The behaviors of point defects in Mo-14 Re alloy were investigated using first-principles methods in this work. Compared with the results in pure Mo alloy, the formation energy and the migration energy of vacancy in Mo-14 Re were both reduced, and the migration energy of interstitial dumbbell increased while the formation energy decreased in Mo-14 Re. In Mo-14 Re, vacancy preferred to form and diffuse faster in Re-enriched environment. Knowing the number of Re atoms in the 1NN (first nearest neighbor) shell of the vacancy and saddle point, the vacancy formation energy and migration energy can be estimated. Since the interstitial atom has a certain orientation, the distribution of the atoms around it is anisotropic. To estimate the interstitial formation energy, it was necessary to count the number of Re atoms at the tensile and compressive sites at the 1NN shell around the interstitial. The interstitial motion was three-dimensional (3D) in Mo-14 Re. At 1200 K, the diffusion coefficient of Re atom was a little higher than that of Mo atom, which could lead to the segregation of Re atoms at defect sinks and the formation of Re-enriched phase. It is possible to investigate the longer-term evolution of microstructure using cluster dynamics (CD) or kinetic monte carlo (KMC) techniques based on these results.

Engineering Mn3Ga/GaAs interfaces: a first-principles study on energetic stability and magnetic anisotropy

Mn3Ga-based ferrimagnets have emerged as a promising platform for energy-efficient spintronics. However, the challenge of identifying an appropriate substrate with minimal lattice misfit for the growth of high-quality Mn3Ga films while achieving the desired interfacial magnetic properties persists. Here we investigate the interfacial energetic stability and magnetic anisotropy at the Mn3Ga/GaAs interface through first-principles electronic structure calculations. Four types of interface models based on different configurations, including MnMn/As, MnMn/Ga, MnGa/As, and MnGa/Ga, are systematically examined. Our findings reveal that the MnGa/Ga interface exhibits the largest perpendicular magnetic anisotropy (PMA) but is energetically the least favorable. Conversely, the MnMn/As interface is energetically the most favorable but demonstrates a lower interfacial PMA. Our calculations align well with the experimentally observed formation of the secondary phase Mn2As. This study deepens our understanding of the interfacial properties in Mn3Ga/GaAs and offers some insights for their optimization through strategic interfacial engineering.

Selective adsorption and sensing mechanism of ZnFe2O4 (111) surface towards toxic gases:A first-principles study

ZnFe2O4 possesses an excellent gas-sensing performance, but its sensing mechanism towards different toxic gas molecules requires further exploration. In this study, the competitive adsorption and sensing properties of several toxic gases (NO2, NO, SO2, CO, H2S, and NH3) on the ZnFe2O4 (111) surface were investigated using density functional theory (DFT) calculations. The adsorption energy, charge transfer (QT), occupation function, adsorption free energy, charge density difference (CDD), and density of states (DOS) were compared. The results reveal that the ZnFe2O4 (111) surface exhibits obvious adsorption for NH3, H2S, NO2, and H2O, besides the selectivity of NH3 molecule is highest. Strong chemical interactions exist between these harmful gas molecules and the ZnFe2O4 (111) surface. This study offers valuable theoretical insights into the selective adsorption and sensing mechanism, contributing to the development of high-performance gas sensors to detect toxic gases.

Bandgap engineering and enhanced optical properties of Hf3X2O2 (X = N, P, As) novel 2D MXene structures using first-principles study

Two-dimensional (2D) MXenes, having comparable transport properties like graphene and a wide spectrum application, are often limited to being used in optoelectronics due to metallic bandgap. Here, by employing density functional theory we report the bandgap engineering and tuning optoelectronic properties through modulating the anions of novel 2D spinel Hf3X2O2 (X = N, P and As) MXenes structures and show that the material class can be among the few semiconducting MXenes. Phonon spectra and cohesive energies confirm that these structures are dynamically stable and chemically exothermic. Modulating anions X = N, P, and As in Hf3X2O2, the electronic bandgaps are found ∼0.46 eV for N, metallic for P, and ∼48 meV for As atoms, suggesting the semiconducting, metallic, and semi-metallic MXenes. The biaxial strains are incorporated to tune the features: In the Hf3N2O2 structure, the bandgap is increased with both compressive and tensile strains, while for the Hf3As2O2 structure, the gap decreased at the GGA-PBE level. For Hf3P2O2 structures, the bandgaps are all metallic irrespective of pristine or biaxial strain. Spin–orbit coupling SOC+GGA reveals that Hf3N2O2 is highly spin responsive while Hf3As2O2 shows semi-metal-to-metallic bandgap transition for pristine as well as biaxial strained conditions. From optical properties analysis, optical absorptions are found located in the visible spectral regions that are also highly receptive to biaxial strains. These properties we have unleashed for the novel Hf3X2O2 (X = N, P, As) semiconducting MXene, thus, show the potentiality of the utilization of the material class in nanoelectronics and optoelectronics applications.