First-principles Calculations Research Articles

Evidence for Two-Dimensional Weyl Fermions in Air-Stable Monolayer PtTe1.75.

The Weyl semimetals represent a distinct category of topological materials wherein the low-energy excitations appear as the long-sought Weyl Fermions. Exotic transport and optical properties are expected because of the chiral anomaly and linear energy-momentum dispersion. While three-dimensional Weyl semimetals have been successfully realized, the quest for their two-dimensional (2D) counterparts is ongoing. Here, we report the realization of 2D Weyl Fermions in monolayer PtTe1.75, which has strong spin-orbit coupling and lacks inversion symmetry, by combined angle-resolved photoemission spectroscopy, scanning tunneling microscopy, second harmonic generation, X-ray photoelectron spectroscopy measurements, and first-principles calculations. The giant Rashba splitting and band inversion lead to the emergence of three pairs of critical Weyl cones. Moreover, monolayer PtTe1.75 exhibits excellent chemical stability in ambient conditions, which is critical for future device applications. The discovery of 2D Weyl Fermions in monolayer PtTe1.75 opens up new possibilities for designing and fabricating novel spintronic devices.

A first-principles research on the properties of two-dimensional penta-BP2 as an anode material for both Na and K ion batteries

The escalating demand for large-scale energy storage solutions has sparked significant interest in metal-ion batteries, particularly in the realm of high-performance anode materials. This work explores the potential of penta-BP2 as an anode material for sodium and potassium-ion batteries through first-principles calculations. The two-dimensional metallic structure of penta-BP2 exhibits favorable electrical conductivity, making it an ideal candidate for anode materials. Theoretical analysis reveals that penta-BP2 can adsorb two layers of Na and three layers of K, resulting in high storage capacities of 1105 and 1473 mAh/g, along with low open-circuit voltages of 0.40 and 0.30 V, respectively. These characteristics enable the production of high energy density in sodium and potassium-ion batteries. Additionally, the material's small Young's modulus and low diffusion energy barriers further establish penta-BP2 as a flexible anode material capable of rapid charge/discharge processes.

Ultralow Thermal Conductivity in Vacancy-Ordered Halide Perovskite Cs3Bi2Br9 with Strong Anharmonicity and Wave-Like Tunneling of Low-Energy Phonons.

Halide perovskites are of great interest due to their exceptional optical and optoelectronic properties. However, thermal conductivity of many halide perovskites remains unexplored. In this study, an ultralow lattice thermal conductivity κL (0.24W m-1 K-1 at 300 K) is reported and its weak temperature dependence (≈T-0.27) in an all-inorganic vacancy-ordered halide perovskite, Cs3Bi2Br9. The intrinsically ultralow κL can be attributed to the soft low-lying phonon modes with strong anharmonicity, which have been revealed by combining experimental heat capacity and Raman spectroscopy measurements, and first-principles calculations. It is shown that the highly anharmonic phonons originate from the Bi 6s2 lone pair expression with antibonding states of Bi 6s and Br 4p orbitals driven by the dynamic BiBr6 octahedral distortion. Theoretical calculations reveal that these low-energy phonons are mostly contributed by large Br motions induced dynamic distortion of BiBr6 octahedra and large Cs rattling motions, verified by the synchrotron X-ray pair distribution function analysis. In addition, the weak temperature dependence of κL can be traced to the wave-like tunneling of phonons, induced by the low-lying phonon modes. This work reveals the strong anharmonicity and wave-like tunneling of low-energy phonons for designing efficient vacancy-ordered halide perovskites with intrinsically low κL.

Janus nanocellulose membrane by nitrogen plasma: Hydrophilicity to hydrophobicity selective switch

Cellulose nanofibrils are one of the keystone materials for sustainable future, yet their poor water repellency hinders their push into industrial applications. Due to complexity and poor economical outcome and/or processing toxicity of the existing hydrophobization methods, nanocellulose loses against its antagonist plastic in medical and food industries. Herein, we demonstrate for the first time the “one-side selective water-repellency activation” in nanocellulose membranes by the means of mild N2-plasma treatment, exhibiting lowest wettability after 20 s of treatment. Hydrophobicity and accompanying Janus character were justified by the topological, chemical and structural reorganizations in cellulose nanofibrils. The findings suggest that the mechanism behind the hydrophilic/hydrophobic change primarily relies on the interplay between OH removal and appearance of SiCH3, originating from the polysiloxanes-based substrate, as well as complementary CNH2 groups formation. First-principles calculations show that NH2 groups moderately increase hydrophobicity, while various SiCH3 substitutions wholly change the character of the surface to repel water. Using nitrogen is shown to be crucial, as N(H)Si(CH3)3 groups induce greater hydrophobicity than simple OSi(CH3)3. Finally, the obtained materials absorb water on the hydrophilic side, while remaining hydrophobic on the other, exhibit high tensile strength, and protection against UV light, demonstrating applicability over wide range of applications.

Flux Crystal Growth, Structure, and Optical Properties of LiLa3Ti2S3O6: An Oxysulfide Phase Derived from K2NiF4-Type Structure.

Single crystals of a new titanium oxysulfide, LiLa3Ti2S3O6, were grown from a KI molten salt. Single-crystal X-ray diffraction analysis revealed that LiLa3Ti2S3O6 crystallizes in the space group Pnma with lattice parameters of a = 11.7319(4) Å, b = 3.94787(14) Å, and c = 20.6885(6) Å. In this structure, the one-dimensional chains of corner-sharing TiO5S octahedra are further corner-linked via equatorial and apical oxygen atoms to form unique corrugated two-dimensional perovskite-type layers in the ab plane with one octahedral thickness. These layers were intervened along the c-axis by the LaS rock-salt layers corrugated concomitantly with the perovskite-type layers, and LiO2S2 tetrahedral chains were located between these two types of two-dimensional layers. LiLa3Ti2S3O6 can be viewed as a modified K2NiF4-type structure with TiO5S octahedral layers stacked in a zigzag manner along the c axis. The oxysulfide has a direct-type band gap of 1.85 eV, based on UV-vis-NIR diffuse reflectance measurements. First-principles calculations showed that the conduction band minimum mainly consists of Ti 3d orbitals, and the valence band maximum consists of S 3p, O 2p, and Li 2s orbitals. The electronic structures near the Fermi level are similar to those of the structurally related photocatalytic oxysulfides Y2Ti2S2O5 and La5Ti2CuS5O7.

Oxygen-assisted enhancing magnetism of cobalt-doped MoS2 and MoS2/graphene by first-principles calculation

The electronic structure and the magnetic properties are performed on Co doped MoS2 (MoS2-Co) and Co doped MoS2/graphene (MoS2/Gr-Co) systems based on the first principles calculations. When two Mo atoms are substituted by two Co atoms, the magnetic moments of two Co co-doped MoS2 (MoS2-2Co) and MoS2/Gr (MoS2/Gr-2Co) are 5.96 µB and 6.40 µB. The magnetic moment of MoS2-2Co increases by 0.44 µB and the transition from half-metal to metal occurs after introducing the graphene layer. Further introducing an O atom in MoS2-2Co and MoS2/Gr-2Co, the hybridization between Co-3d and O-2p is significantly stronger than that between Co-3d and S-3p.

Crystal and band structures of N-doped Ti3O5 coatings: Experiments and first-principles calculations

N-doped α-phase Ti3O5 has received limited attention in both theoretical and experimental studies compared to N-doped TiO2. This work focuses on the production and analysis of N-doped Ti3O5 coatings, which can be facilely produced using air-based sputtering. First-principles calculations were employed to investigate the crystal and band structures of N-doped Ti3O5. The results revealed that substitutional nitrogen doping within the investigated range preserved the orthorhombic structure of Ti3O5. As the N-doping concentrations increased, Ti3O5 exhibited a transition from metallic to semiconducting characteristics, confirmed by the density of states and refined band calculations. The calculated bandgaps in the semiconductor regime aligned well with the limited experimental results, demonstrating the pronounced narrowing effect of nitrogen doping. Integrating these calculations with experimental findings enhances the understanding of the crystalline and physical properties of N-doped Ti3O5 coatings.

Magnetic structure and lattice properties of R2Cu2In intermetallics (R = Dy, Tm, Lu)

Magnetic and structural properties of tetragonal R2Cu2In intermetallics are presented for R = Dy, Tm, and Lu. The neutron diffraction experiment clarifies the nature of magnetic order in Dy2Cu2In and Tm2Cu2In. While Tm2Cu2In orders ferromagnetically below TC = 31 K with magnetic space group (MSG) P4/mb'm', Dy2Cu2In forms a non-collinear ferromagnetic state below 50 K (MSG P2'/m') with another k = (½, ½, ½) component of magnetic moment (MSG Ibam.1'c) appearing at 20 K. The ground-state magnetic structure is described by the MSG Im'm'a with a magnetic moment of 9.35 μB. This subsequent transition is accompanied by the changes in the T-dependence of anisotropy constants, however, no related anomaly is observed in specific heat dominated by the CF and exchange interaction effects. Magnetic properties are discussed in the context of lattice response to temperature and pressure variations. The experimental results are supported by the analysis of the non-magnetic Lu analogue and by the first-principles calculations (electronic structure and material properties) based on Density Functional Theory (DFT).

Insights into Enhanced Thermoelectric Performance of the N-Type Mg3Sb2-Based Materials by Amphoteric Al Doping

Doping has the potential to alter the levels of anharmonicity in compounds by attenuating bonding strength. In this study, we explore the efficacy of amphoteric Al doping for stimulating anharmonicity in n-type Mg3.25AlxSb1.48Bi0.48Te0.04 to attain enhanced phonon scattering and thermoelectric performance. First-principles calculations and experimental data reveal the occupation of both Sb and Mg2 sites by amphoteric Al atoms in the anionic framework of Mg3.25AlxSb1.48Bi0.48Te0.04. A marginal variation in both carrier concentration and mobility sustains the high power factor without affecting the Seebeck coefficient, implying amphoteric doping induced charge compensation. While phonon velocity, Grüneisen parameter, and crystal orbital Hamilton population (COHP) calculations results indicate that phonon softening and bond weakening are realized via Al doping, leading to an enhanced lattice anharmonicity and a reduced lattice thermal conductivity. A remarkable enhancement ∼16% in the peak figure of merit ZTpeak and the average ZTavg, was attained for the x = 0.015 sample, when compared to the undoped sample. Hence, the amphoteric doping can serve as an effective means to optimize ZT values by decoupling the intertwined thermoelectric transport properties.

Atomic and Electronic Properties of the Metal/Diamond (100) Interfaces by First-principles Calculations

Diamond-based electronic devices have gained much attention owing to their excellent electronic properties, where the metal/diamond contact interface is an important component. Since SBH is one of the key factors influencing the electrical characteristics of the metal/diamond contacts, it is critical to explain the mechanisms affecting SBHs. Herein, systematic first-principles calculations are carried out to investigate the termination impact on the SBHs. The results show a direct correlation between the metal work function and the p-type SBHs at both interfaces. The SBH is reduced by 0.03 eV ∼ 0.54 eV at the H-diamond (100) surface. The Fermi pinning effect is greatly reduced for H-diamond (100), with a higher Fermi pinning factor S=0.42, compared to S=0.29 for non-terminated diamond (100). The computed results are well consistent with previous experimental and theoretical findings. We have achieved a systematic understanding of the mechanisms affecting the interfacial SBH, with effective and controllable regulation of SBH partially realized. These findings provide valuable insights into the strategic choice of surface engineering for contact metal and diamond electronics.

Tunable ferroelectricity of group Ⅳ–Ⅵ nanowires under composition modulation

Low dimensional ferroelectrics are anticipated to play a dominant role in micro/nano-electromechanical field. However, as their intrinsic size decreasing to one-dimension (1D), their ferroelectric polarization will weaken or even disappear, which severely limiting the further development of these promising materials. Therefore, it is of great significance to design one-dimensional ferroelectric materials with high-performance. In this work, first-principles calculations are employed to investigate the ferroelectricity and atomic configuration of MX (MX, M = Ge, Sn; X = S, Se) nanowires with composition tuning. Their ferroelectric characteristics, including spontaneous polarization Ps, transition barrier TL and TQ, Curie temperature Tc, as well as their stabilities, electronic structures, and transformation modes were systematically investigated. The findings indicate that their stability and forming tendency are strongly correlated with the bonding type. Moreover, all nine nanowires retained nanowires exhibit intrinsic 1D ferroelectrics with the maximal Ps of 6.40×10−10C/m for GeS nanowire and the minimum Ps of 3.95×10−10C/m for SnSe nanowire. The obtained calculation results indicate that the ferroelectric properties can be tuned by alteration of composition, which holds significant implications for the design and performance enhancement of 1D ferroelectric materials.

Ultra-high electron mobility in Sn-doped two-dimensional Ga2O3 modified by biaxial strain and electric field

2D Ga2O3 exhibits overwhelming advantages over its bulk counterpart, whereas manipulating the carriers is rare. We report strain-dependent electronic structures and transport properties of Sn-doped 2D Ga2O3 using first-principles calculations with deformation potential theory. The band gaps are tunable from 2.23 eV to 1.20 eV due to the strain-mediated σ* anti-bonding and π bonding state variations. Specifically, ultra-high electron mobility of 22579.32 cm2V−1s−1 is predicated under 8% tensile. Further electric field modulations suggest the retaining of band gap and effective mass. These results highlight its property manipulations and nanoscale electronic applications.

Melt growth of crystalline α-SrSi2 by the vertical Bridgman method and its thermoelectric characteristics

As a candidate thermoelectric material operating in the low-temperature regime (room temperature to 473 K), we have examined α-SrSi2: a narrow bandgap semiconductor material, which is composed of environmentally benign elements. We are able to report the successful production of single-crystalline-like α-SrSi2 with clear semiconducting properties by melting synthesis, a thermodynamically stable and thermally equilibrium process that reduces unexpected process contamination and improves crystallinity.The crystals up to 20 × 20 × 5 mm3 have been obtained for a solid–liquid phase reaction temperature of 1308 K for 10 h with a growth rate averaging at 0.617 K/h in the temperature range from 1403 K to 1373 K. The band gap of this material was measured at 48 meV, which is higher than the values previously reported. The power factor was measured at 2.9 mW/mK2 at 300 K, which is the highest value ever reported for undoped α-SrSi2.Based on the experimental bandgap values, the hybrid functional was used to correct the first-principles calculation method, and the Seebeck coefficient obtained from the first-principles calculation was compared with experimental values. Which values showed good agreement with experimental values.

Unveiling Enhanced Electrochemical Performance of LiMn1-xCoxPO4/C Cathode Materials through Co-Doping Strategy

LiMnPO4(LMP), an olivine-type material, is considered as a cathode for lithium-ion battery material, but faces challenges such as the Jahn-Teller effect, poor conductivity, and low ionic conductivity. To overcome these limitations, we explored Co doping as a potential solution. By utilizing a solvothermal method, we successfully synthesized LiMn1-xCoxPO4/C (x is the mass fraction of Co doping in LiMnPO4 sample) materials with a high (200) exposed surface. Our first-principles calculations indicated that Co doping significantly impacts the material's structure and electrochemical performance. Notably, the LiMn0.95Co0.05PO4/C sample demonstrated the highest performance, delivering capacities of 164.7 mAh g-1 at 0.1C and 97.5 mAh g-1 at 1C, while maintaining a remarkable 95.4% capacity retention after 100 cycles at 0.5C. Furthermore, the preferential growth on the b-c plane results in an enhanced (200) orientation, which exposes more reactive sites and facilitates Li+ migration. This study not only offers a straightforward approach for obtaining high-performance electrode materials but also bridges the gap between theory and experiment, providing a new avenue for understanding cation doping in polyanionic cathode materials.

Revealing the formation mechanisms and interfacial properties of W/CeO2 interfaces from first-principles calculations

Revealing the formation mechanisms and interfacial properties of W/CeO2 interfaces from first-principles calculations

Effect of B and Hf doping on vacancy defects in AlCrMoNbZr high entropy alloy: A first principles study

The AlCrMoNbZr high entropy alloy (HEA) has potential applications as a high-performance cladding coating material. Here, we report the results about the vacancy defects in AlCrMoNbZr HEA using first-principle calculations. We have explored the variations in bulk and vacancy defect properties due to different local chemical environments in AlCrMoNbZr HEA with and without B/Hf doping, using both supercell and special quasi-random structures (SQS) techniques. Vacancy formation energy (VFE) is analyzed in accordance with the statistical distribution across three systems. The VFE exhibits a broad range of values and is characterized by a normal distribution. It is also significantly influenced by the type of doping atoms and the type of vacancy. Specifically, B doping substantially reduces the vacancy formation energy, thereby facilitating the formation of vacancies. Furthermore, the doping of B and Hf exerts a considerable impact on the formation of Zr vacancies (Zr_Vac) and Mo vacancies (Mo_Vac). In addition, Al vacancies (Al_Vac) are most readily formed in the AlCrMoNbZr HEA, while Mo vacancies (Mo_Vac) are most easily formed following the introduction of B and Hf dopants. The VFE in AlCrMoNbZr HEA with and without Hf dopant increases with the increase of the number of Cr and Mo atoms in the local chemical environment, and decreases with the increase of the number of Zr atoms. In conclusion, B doping promotes the formation of vacancy, which is not conducive to the improvement of initial irradiation resistance performance. Fortunately, one can infer from the results of the effect of local chemical environment on VFE that it is rewarding in improving the initial irradiation resistance by increasing the occurrence of Mo-Mo and Zr-Zr arrangement in AlCrMoNbZr HEA.

The A-Ni chemical bond in AIIINiSb (AIII=Sc, Y, Er) half-Heusler materials triggers the formation of anomalous vacancy defects

Defects exert a profound influence on thermoelectric materials by altering electronic band structures and significantly impacting their performance. Despite being a potentially promising thermoelectric material, the mechanism behind defect formation in half-Heusler (HH) compounds ABX remains unclear, impeding the enhancement of their thermoelectric properties. In this study, we investigated the intrinsic defect formation energies for AⅢNiSb (AⅢ = Sc, Y, Er) and other 9 HH compounds, namely AⅣNiSn (AⅣ = Ti, Zr, Hf), AⅤFeSb (AⅤ = V, Nb, Ta), and AⅣCoSb (AⅣ = Ti, Zr, Hf), using first-principles calculations and thermodynamics. The results reveal that AⅢNiSb (AⅢ = Sc, Y, Er) exhibits anomalous B (Ni) vacancy defects, with their formation energies being significantly lower than those of the corresponding B vacancy defects in the other 9 HH compounds. This anomaly can be attributed to the strength of the A-B bonds, where a decrease in bond strength leads to a decrease in the formation energy of B vacancies. This approach of exploring the influence of interatomic bond strength on defect formation is not only insightful for HH compounds but also holds potential applications in defect studies across various materials, offering a broader perspective on the fundamental mechanisms governing defect formation and stability.

Investigation of structural, electrical, dynamical, optical, and thermoelectric properties of Sr-doped Mg2Si systems using first-principles calculations

Investigation of structural, electrical, dynamical, optical, and thermoelectric properties of Sr-doped Mg2Si systems using first-principles calculations

First-principles study of electronic and magnetic properties of Fe atoms on Cu2N/Cu(100)

Abstract First-principles calculations were conducted to investigate the structural, electronic, and magnetic properties of single Fe atoms and Fe dimers on Cu2N/Cu(100). Upon adsorption of an Fe atom onto Cu2N/Cu(100), robust Fe–N bonds form, resulting in the incorporation of both single Fe atoms and Fe dimers within the surface Cu2N layer. The partial occupancy of Fe-3d orbitals lead to large spin moments on the Fe atoms. Interestingly, both single Fe atoms and Fe dimers exhibit in-plane magnetic anisotropy, with the magnetic anisotropy energy (MAE) of an Fe dimer exceeding twice that of a single Fe atom. This magnetic anisotropy can be attributed to the predominant contribution of the component along the x direction of the spin–orbital coupling Hamiltonian. Additionally, the formation of Fe–Cu dimers may further boost the magnetic anisotropy, as the energy levels of the Fe-3d orbitals are remarkably influenced by the presence of Cu atoms. Our study manifests the significance of uncovering the origin of magnetic anisotropy in engineering the magnetic properties of magnetic nanostructures.

Yellow light emitting Cs2Zr0.99Te0.01Cl6 with high efficiency emission via modified co-precipitation method synthesis for the light-emitting device

While lead-free perovskite has attracted widespread interest in the optical field by overcoming their toxicity and stability disadvantages compared to conventional lead-based perovskite, the low photoluminescence quantum yield (PLQY) of lead-free perovskite has greatly limited their commercialization path. In this research, a modified co-precipitation method was used to prepare Cs2ZrCl6, which was achieved by the addition of TeCl4 to shift the emission wavelength from 436 to 556 nm with bright yellow emission, the optimal PLQY of 98.2% was achieved by the addition of Te4+ in the amount of 1 % and the optimization of the reaction concentration. First-principles calculations and spectral analysis of Cs2Zr1-xTexCl6 show that the broadband yellow light emission comes from self-trapped excitons (STEs) in Cs2Zr1-xTexCl6. In addition, the luminous intensity of the prepared Cs2Zr1-xTexCl6 was 57.3, 63.3 and 90.1% of the initial intensity after exposure to air for 30 d, cyclic intermittent heating to 200°C and ultraviolet (UV) irradiation for 7 d, respectively. Finally, Cs2Zr1-xTexCl6 and Cs2ZrCl6:10% Bi were used as phosphors and encapsulated with a UV chip to make three different colors of light-emitting diodes (LED), where the white LED (WLED) had CIE coordinates of (0.3061 0.3244), color temperatures, color rendering indices as well as color tolerances of 6921 K, 84 and 6.52 respectively. This research provides a new approach to improve the performance of lead-free perovskite.