First-principles study on structural stabilities, mechanical properties, and biaxial strain-induced superconductivity in Janus MoWC monolayer.

The unique attributes of hydrophilicity, expansive surface groups, remarkable flexibility, and superior conductivity converge in MXene, a pioneering 2D material. Owing to MXene's exceptional properties, diverse strategies have been explored to enhance its characteristics. Janus MXene and stress-strain response considerations represent the primary avenues of interest today. In this study, we investigated the Janus MXene structure under biaxial stress using first-principles calculations. The most stable configuration of Janus MoWC MXene identified in our analysis exhibits an atomic arrangement known as the hexagonal (2H) phase. Subsequently, we examined the mechanical and electronic properties of 2H-MoWC when subjected to biaxial strain. Our findings indicate that the 2H phase of Janus MoWC MXene demonstrates superior strength compared to the tetragonal (1T) phase. Analysis of the ELF of the 2H-MoWC structure unveiled that the robust C-C bond within the material is the underlying factor enabling the 2H phase to withstand a maximum of 9% tensile strain. Furthermore, we demonstrate that 2H-MoWC is a superconductor with the superconducting temperature (Tc) of 1.6 K, and the superconductivity of 2H phase can be enhanced by biaxial strain with the Tc reaching 7 K. This study offers comprehensive insights into the properties of Janus MoWC monolayer under biaxial stress, positioning it as a promising candidate for 2D straintronic applications.

First-principles study of 2H-Mo2C-based MXenes under biaxial strain as Li-battery anodes.

MXenes, a family of superior 2D materials, have been intensively investigated because they have many promising properties, particularly high-performance energy storage and high flexibility. To approach the expected critical benchmarks of such materials, the strain dependence of the atomic structure is widely considered for tuning the related properties. In this work, by means of density functional theory, we demonstrate the potential application of the strained 2H phase of Mo2C-based MXenes (Mo2C and Mo2CO2) as anode materials for lithium-ion batteries (LIBs). Adsorption and diffusion of Li on the surfaces of both materials and the impact of biaxial strain (εb) in the range of -4% to 4% are insightfully investigated. The lowest adsorption energy of Mo2C is -0.96 eV, and that of Mo2CO2 is -3.13 eV at εb = 0%. The diffusion of Li ions, considering the pathway between the first two most favorable adsorption sites, reveals that the biaxial strain refinement under compressive strain decreases the energy barrier, but the induction of tensile strain increases it in both MXenes. The ranges of the energy barriers of Li-ion adsorption on the surfaces of Mo2C and Mo2CO2 are 31-57 meV and 177-229 meV, respectively. Interestingly, the storage capacity of Li can reach three layers corresponding to a comparably high theoretical capacity of 788.61 mA h g-1 for Mo2C and 681.64 mA h g-1 for Mo2CO2. The atomic configurations are stable, as verified by the negative adsorption energy as well as the slightly distorted structures, by using ab initio molecular dynamics (AIMD) simulations at 400 K. Moreover, average open circuit voltages (OCVs) of 0.35 V and 0.63 V (at εb = 0%) are reported for Mo2C and Mo2CO2, respectively. Furthermore, the tensile strain results in an increase in the OCVs, while compression has the opposite effect. These computational results provide some basic information on the behaviors of Li-ion adsorption and diffusion on Mo2C-based MXenes upon tuning biaxial strain. They also give a guideline on what conditions are appropriate for practically implementing these MXenes as electrode materials in LIBs.

Biaxial stress and functional groups (T = O, F, and Cl) tuning the structural, mechanical, and electronic properties of monolayer molybdenum carbide.

MXenes are a family of novel two-dimensional (2D) materials attracting intensive interest because of the rich chemistry rooted from the highly diversified surface functional groups. This enables the chemical optimization suitable for versatile applications, including energy conversion and storage, sensors, and catalysis. This work reports the ab initio study of the crystal energetics, electronic properties, and mechanical properties, and the impacts of strain on the electronic properties of tetragonal (1T) and hexagonal (2H) phases of Mo2C as well as the surface-terminated Mo2CT2 (T = O, F, and Cl). Our findings indicate that 2H-Mo2C is energetically more stabilized than the 1T counterpart, and the 1T-to-2H transition requires a substantial energy of 210 meV per atom. The presence of surface termination T atoms on Mo2C intrinsically induces variations in the atomic structure. The calculated structures were selected based on the energetic and thermodynamic stabilities (400 K). The O atom prefers to be terminated on 2H-Mo2C, whereas the Cl atom energetically stabilizes on 1T-Mo2C. Meanwhile, with certain configurations, 2H-Mo2CF2 and 1T-Mo2CF2 with slightly different energies could exist simultaneously. The Mo2CO2 possesses the highest mechanical strength and elastic modulus (σmax = 52 GPa at εb = 20% and E = 507 GPa). The nature of the ordered centrosymmetric layer and the strong bonding between 4 d-Mo and 2 p-O of 2H-Mo2CO2 are responsible for its promising mechanical properties. Interestingly, the topological properties of 2H-Mo2CO2 at a wide range of strains (-10% to 12%) are reported. Moreover, 2H-Mo2CF2 is metallic through the range of calculation. Meanwhile, originally semiconducting 1T-Mo2CF2 and 1T-Mo2CCl2 preserve their features under the ranges of the strain of -2% to 10% and -1% to 5%, respectively, beyond which they undergo the semiconductor-to-metal transitions. These findings would guide the potential applications in modern 2D straintronic devices.

Improved Thermoelectric Properties of SrTiO3 via (La, Dy and N) Co-Doping: DFT Approach.

This work considers the enhancement of the thermoelectric figure of merit, ZT, of SrTiO3 (STO) semiconductors by (La, Dy and N) co-doping. We have focused on SrTiO3 because it is a semiconductor with a high Seebeck coefficient compared to that of metals. It is expected that SrTiO3 can provide a high power factor, because the capability of converting heat into electricity is proportional to the Seebeck coefficient squared. This research aims to improve the thermoelectric performance of SrTiO3 by replacing host atoms by La, Dy and N atoms based on a theoretical approach performed with the Vienna Ab Initio Simulation Package (VASP) code. Here, undoped SrTiO3, Sr0.875La0.125TiO3, Sr0.875Dy0.125TiO3, SrTiO2.958N0.042, Sr0.750La0.125Dy0.125TiO3 and Sr0.875La0.125TiO2.958N0.042 are studied to investigate the influence of La, Dy and N doping on the thermoelectric properties of the SrTiO3 semiconductor. The undoped and La-, Dy- and N-doped STO structures are optimized. Next, the density of states (DOS), band structures, Seebeck coefficient, electrical conductivity per relaxation time, thermal conductivity per relaxation time and figure of merit (ZT) of all the doped systems are studied. From first-principles calculations, STO exhibits a high Seebeck coefficient and high figure of merit. However, metal and nonmetal doping, i.e., (La, N) co-doping, can generate a figure of merit higher than that of undoped STO. Interestingly, La, Dy and N doping can significantly shift the Fermi level and change the DOS of SrTiO3 around the Fermi level, leading to very different thermoelectric properties than those of undoped SrTiO3. All doped systems considered here show greater electrical conductivity per relaxation time than undoped STO. In particular, (La, N) co-doped STO exhibits the highest ZT of 0.79 at 300 K, and still a high value of 0.77 at 1000 K, as well as high electrical conductivity per relaxation time. This renders it a viable candidate for high-temperature applications.

The quantum confined Stark effect in N-doped ZnO/ZnO/N-doped ZnO nanostructures for infrared and terahertz applications.

The terahertz (THz) frequency range is very important in various practical applications, such as terahertz imaging, chemical sensing, biological sensing, high-speed telecommunications, security, and medical applications. Based on the density functional theory (DFT), this work presents electronic and optical properties of N-doped ZnO/ZnO/N-doped ZnO quantum well and quantum wire nanostructures. The density of states (DOS), the band structures, effective masses, and the band offsets of ZnO and N-doped ZnO were calculated as the input parameters for the subsequent modeling of the ZnO/N-doped ZnO heterojunctions. The results show that the energy gaps of the component materials are different, and the conduction and valence band offsets at the ZnO/N-doped ZnO heterojunction give type-II alignment. Furthermore, the optical characteristics of N-doped ZnO/ZnO/N-doped ZnO quantum well were studied by calculating the absorption coefficient from transitions between the confined states in the conduction band under the applied electric field (Stark effect). The results indicate that N-doped ZnO/ZnO/N-doped ZnO quantum wells, quantum wires, and quantum cascade structures could offer the absorption spectrum tunable in the THz range by varying the electric field and the quantum system size. Therefore, our work indicates the possibility of using ZnO as a promising candidate for infrared and terahertz applications.

Electronic Properties of h-BCN-Blue Phosphorene van†der†Waals Heterostructures.

Van der Waals heterostructures, a new class of materials made of a vertically selective assembly of various 2D monolayers held together by van der Waals forces, have attracted a great deal of attention due to their promise to deliver novel electronic and optoelectronic properties that are not achievable by using individual 2D crystals. Using density functional theory (DFT), it is revealed that van der Waals heterostructures composed of monolayers of hexagonal boron nitride (h-BN) and the latest P allotrope blue phosphorus (blue phosphorene, BlueP) forms a straddling type I band offset for which the band edges exclusively belong to BlueP. This feature enables h-BN to act as a protective coating material to resolve the air instability of BlueP. Furthermore, substitutional doping of C into h-BN (h-BCN) at a suitable concentration induces h-BCN-BlueP into staggered type II band offset. The type II band alignment triggered by the intensified built-in electric field across the sheets implies improved carrier mobility and the suppressed recombination of photogenerated hole pairs. These major benefits can pave the way for the potential functionality of h-BCN-BlueP to be exploited for efficient photovoltaic devices.

Improved sensing characteristics of methane over ZnO nano sheets upon implanting defects and foreign atoms substitution.

Thanks to the growing interests of metal oxide sensors in environmental and industrial uses, this study presents the sensing mechanism of methane gas (CH4) on recently synthesized two-dimensional form of ZnO, ZnO nano sheets (ZnO-NS). The adsorption energy of CH4 on pristine ZnO-NS, calculated by means of van der Waals corrected first-principles calculations, is found to be insufficient restricting its application as an efficient nano sensor. However, the creation of (O/Zn) vacancies and the substitution of foreign dopants into ZnO-NS considerably intensify the binding energy of CH4. Through a comprehensive energetic analysis, it is observed that among all the substituents, boron (B), sulphur (S) and gallium (Ga) improves the binding of CH4 to 2.75, 6.1 and 7.5 times respectively than its values on pristine ZnO-NS. In addition to the CH4 binding energies falling ideally between physisorption and chemisorption range, a prominent variation in the electronic properties before and after CH4 exposure indicates the promise of substituted Zn-NS as a useful nano sensors.

Revealing an unusual transparent phase of superhard iron tetraboride under high pressure.

First principles-based electronic structure calculations of superhard iron tetraboride (FeB4) under high pressure have been undertaken in this study. Starting with a "conventional" superconducting phase of this material under high pressure leads to an unexpected phase transition toward a semiconducting one. This transition occurred at 53.7 GPa, and this pressure acts as a demarcation between two distinct crystal symmetries, metallic orthorhombic and semiconducting tetragonal phases, with Pnnm and I41/acd space groups, respectively. In this work, the electron-phonon coupling-derived superconducting T(c) has been determined up to 60 GPa and along with optical band gap variation with increasing pressure up to 300 GPa. The dynamic stability has been confirmed by phonon dispersion calculations throughout this study.

Crystal Phase Effects in Si Nanowire Polytypes and Their Homojunctions.

Recent experimental investigations have confirmed the possibility to synthesize and exploit polytypism in group IV nanowires. Driven by this promising evidence, we use first-principles methods based on density functional theory and many-body perturbation theory to investigate the electronic and optical properties of hexagonal-diamond and cubic-diamond Si NWs as well as their homojunctions. We show that hexagonal-diamond NWs are characterized by a more pronounced quantum confinement effect than cubic-diamond NWs. Furthermore, they absorb more light in the visible region with respect to cubic-diamond ones and, for most of the studied diameters, they are direct band gap materials. The study of the homojunctions reveals that the diameter has a crucial effect on the band alignment at the interface. In particular, at small diameters the band-offset is type-I whereas at experimentally relevant sizes the offset turns up to be of type-II. These findings highlight intriguing possibilities to modulate electron and hole separations as well as electronic and optical properties by simply modifying the crystal phase and the size of the junction.

A new, layered monoclinic phase of Co3O4 at high pressure.

We present the crystal structures and electronic properties of a Co3O4 spinel under high pressure. Co3O4 undergoes a first-order transition from a cubic (CB) Fd3̄m to a lower-symmetry monoclinic (MC) P21/c phase at 35 GPa, occurring after the local high-spin to low-spin phase transition. The high-pressure phase exhibits the octahedral coordination of Co(II) and Co(III), whereas the CB phase contains the fourfold coordination of Co(II) and the sixfold coordination of Co(III). The CB-to-MC transition is attributed to the charge-transfer between the di- and trivalent cations via the enhanced 3d-3d interactions.

Reversible hydrogen storage in Y2C MXene under the influence of functional groups (F, Cl, OH).

The hydrogen storage potential of the bare MXenes Y2C and terminated Y2CT2, where T is O, OH, or F, were studied using density functional theory (DFT). Hydrogen adsorption and desorption behaviors are simulated by ab initio molecular dynamic simulations. The interaction of the H2-molecules on the Y2C-family was investigated within the projected density of states. Only bare Y2C and Y2C(OH)2 provided Kubas-type interactions. The calculated maximum capacity of H2 in bare Y2C can reached up to 5.041 wt%, whereas the reversible capacity of H2 was up to 2.036 wt%. In Y2C(OH)2, the maximum capacity of H2 storage is 3.477 wt% and the reversible capacity was 1.769 wt%. This indicated that Y2C and Y2C(OH)2 are practical candidates for hydrogen storage applications.

MoS2/Mayenite Electride Hybrid as a Cathode Host for Suppressing Polysulfide Shuttling and Promoting Kinetics in Lithium-Sulfur Batteries.

The commercial viability of emerging lithium-sulfur batteries (LSBs) remains greatly hindered by short lifespans caused by electrically insulating sulfur, lithium polysulfides (Li2Sn; 1 ≤ n ≤ 8) shuttling, and sluggish sulfur reduction reactions (SRRs). This work proposes the utilization of a hybrid composed of sulfiphilic MoS2 and mayenite electride (C12A7:e-) as a cathode host to address these challenges. Specifically, abundant cement-based C12A7:e- is the most stable inorganic electride, possessing the ultimate electrical conductivity and low work function. Through density functional theory simulations, the key aspects of the MoS2/C12A7:e- hybrid including electronic properties, interfacial binding with Li2Sn, Li+ diffusion, and SRR have been unraveled. Our findings reveal the rational rules for MoS2 as an efficient cathode host by enhancing its mutual electrical conductivity and surface polarity via MoS2/C12A7:e-. The improved electrical conductivity of MoS2 is attributed to the electron donation from C12A7:e- to MoS2, yielding a semiconductor-to-metal transition. The resultant band positions of MoS2/C12A7:e- are well matched with those of conventional current-collecting materials (i.e., Cu and Ni), electrochemically enhancing the electronic transport. The accepted charge also intensifies MoS2 surface polarity for attracting polar Li2Sn by forming stronger bonds with Li2Sn via ionic Li-S bonds than electrolytes with Li2Sn, thereby preventing polysulfide shuttling. Importantly, MoS2/C12A7:e- not only promotes rapid reaction kinetics by reducing ionic diffusion barriers but also lowers the Gibbs free energies of the SRR for effective S8-to-Li2S conversion. Beyond the reported applications of C12A7:e-, this work highlights its functionality as an electrode material to boost the efficiency of LSBs.

Functionalization of hydrogenated silicene with alkali and alkaline earth metals for efficient hydrogen storage.

First principles density functional theory has been employed to investigate the electronic structure along with the stability, bonding mechanism, band gap and charge transfer of metal functionalized hydrogenated silicene (SiH), or silicane, in order to envisage the hydrogen storage capacity. Various metal adatoms including Li, Na, K, Be, Mg and Ca have been doped on the most stable chair like configuration of silicane. The corresponding binding energies and charge transfer mechanism have been discussed from the perspective of H2 storage ability. The Li and Na metal adatoms have been found to be ideally suitable not only for their strong metal to substrate binding and uniform distribution over the substrate but also for their high capacity for storage of hydrogen. The stability of both Li and Na functionalized SiH has also been confirmed by MD simulations. It was found that both Li(+) and Na(+) adsorbed four H2 molecules attaining reasonably high storage capacities of 6.30 wt% and 5.40 wt% respectively with average adsorption energies lying within the range suitable for practical H2 storage applications, in contrast with alkaline earth metals.

Elucidating Synergistic Mechanisms of Adsorption and Electrocatalysis of Polysulfides on Double-Transition Metal MXenes for Na-S Batteries.

Multiple unfavorable features, such as poor electronic conductivity of sulfur cathodes, the dissolution and shuttling of sodium polysulfides (Na2Sn) in electrolytes, and the slower kinetics for the decomposition of solid Na2S, make sodium-sulfur batteries (NaSBs) impractical. To overcome these obstacles, novel double-transition metal (DTM) MXenes, Mo2TiC2T2, (T = O and S) are studied as an anchoring material (AM) to immobilize higher-order polysulfides and to expedite the otherwise slower kinetics of insoluble short-chain polysulfides. Density functional theory (DFT) calculations are carried out to justify and compare the effectiveness of Mo2TiC2S2 and Mo2TiC2O2 as AMs by analyzing their interactions with S8/Na2Sn (n = 1, 2, 4, 6, and 8). Mo2TiC2S2 provides moderate adsorption strength compared to Mo2TiC2O2, therefore, it is expected to effectively inhibit Na2Sn dissolution and shuttling without causing decomposition of Na2Sn. The calculated Gibbs free energies of the rate-determining step for sulfur reduction reactions (SRR) are found to be significantly lower (0.791 eV for S and 0.628 eV for O functionalization) than that in vacuum (1.442 eV), suggesting that the SRR is more thermodynamically favorable on Mo2TiC2T2 during discharge. Additionally, both Mo2TiC2S2 and Mo2TiC2O2 demonstrated effective electrocatalytic activity for the decomposition of Na2S, with a substantial reduction in the energy barrier to 1.59 eV for Mo2TiC2S2 and 1.67 eV for Mo2TiC2O2. While Mo2TiC2O2 had superior binding properties, structural distortion is observed in Na2Sn, which may adversely affect cyclability. On the other hand, because of its moderate binding energy, enhanced electronic conductivity, and significantly faster oxidative decomposition kinetics of polysulfides, Mo2TiC2S2 can be considered as an effective AM for suppressing the shuttle effect and improving the performance of NaSBs.

Conversion of CO2 into Formic Acid on Transition Metal-Porphyrin-like Graphene: First Principles Calculations.

Recently, transition metal (TM)-porphyrin-like graphene has been predicted to be a promising material for CO2 capturing under favorable conditions. Such materials can capture CO2 at 300 K and release it at 450 K. However, the captured CO2 gas is mostly stored in oceans. With the aid of first principles calculations, we herein propose a method in which the captured CO2 is converted into an environmentally friendly product, formic acid. Addition of H2 to CO2 molecules adsorbed on Sc- and Ti-porphyrin-like graphene was found to catalyze this conversion. We also performed nudged elastic band calculations and thermodynamic analysis using the first-order Polanyi-Wigner equation and equilibrium statistical mechanics to investigate the chemical reactions involved in this conversion. In addition, we performed Bader charge analysis to obtain insights into the mechanism of charge transfer and adsorption throughout the conversion. Our study presents a novel method in which the captured CO2 is treated by converting it into an environmentally friendly product. Since this method does not require CO2 storage, it is expected to be an effective strategy to manage the rising CO2 level in the environment.

Defected and Functionalized Germanene-based Nanosensors under Sulfur Comprising Gas Exposure.

Efficient sensing of sulfur containing toxic gases like H2S and SO2 is of the utmost importance due to the adverse effects of these noxious gases. Absence of an efficient 2D-based nanosensor capable of anchoring H2S and SO2 with feasible binding and an apparent variation in electronic properties upon the exposure of gas molecules has motivated us to explore the promise of a germanene nanosheet (Ge-NS) for this purpose. In the present study, we have performed a comprehensive computational investigation by means of DFT-based first-principles calculations to envisage the structural, electronic, and gas sensing properties of pristine, defected, and metal substituted Ge-NSs. Our initial screening has revealed that although interaction of SO2 with pristine Ge-NSs is within the desirable range, H2S binding however falls below the required values to guarantee an effective sensing. To improve the binding characteristics, we have considered the interactions between H2S and SO2 with defected and metal substituted Ge-NS. The systematic removals of Ge atoms from a reasonably large super cell lead to monovacancy, divacancies, and trivacancies in Ge-NS. Similarly, different transition metals like As, Co, Cu, Fe, Ga, Ge, Ni, and Zn have been substituted into the monolayer to realize substituted Ge-NS. Our van der Waals corrected DFT calculations have concluded that the vacancy and substitution defects not only improve the binding characteristics but also enhance the sensing propensity of both H2S and SO2. The total and projected density of states show significant variations in electronic properties of pristine and defected Ge-NSs before and after the exposure to the gases, which are essential in constituting a signal to be detected by the external circuit of the sensor. We strongly believe that our present work would not only advance the knowledge towards the application of Ge-NS-based sensing but also provide motivation for the synthesis of such efficient nanosensor for H2S and SO2 based on Ge monolayer.