First-principles Calculations Research Articles

2D electrodeposition assembly of Cu/Cu2O nanoarrays for low temperature H2S sensing

Ultra-sensitive hydrogen sulfide (H2S) detection technology at low temperatures provides critical support for the application of one-time alert systems under extreme conditions, ensuring reliable monitoring of H2S in low-temperature environments and enhancing safety and practicability. In this study, we developed a novel low-temperature H2S sensor based on Cu/Cu2O nanoarrays, prepared through a combination of 2D electrodeposition in situ assembly and the hydrothermal method. The heterointerface between Cu nanowires and Cu2O nanocubes presents a high barrier, which can be modulated to zero through the formation of continuous CuxS conductive channels via the vulcanization reaction, thereby achieving excellent sensitivity and selectivity towards H2S at low temperatures. At freezing temperatures and low H2S concentrations (≤20 ppm), the sensors show ideal recovery and reasonable sensitivity (R=150 to 10 ppm H2S at 0 °C). However, the response significantly increases (R=5624 at 10 ppm H2S at 50 °C) and recovery is lost under reversed conditions. First-principles calculations have confirmed that the enhanced H2S adsorption at the heterointerface is the primary reason for the robust response observed at 0 °C. Experimental results demonstrate that the Cu/Cu2O nanoarrays were capable of rapid detection of H2S at low temperatures through modulation of heterointerface barriers.

Migration mechanisms of mono-/di-interstitials in vanadium with alloying Ti/Cr

Self-interstitial atoms (SIAs) are the dominant self-defects at equilibrium in metals, but many questions such as their diffusion characteristics are still open, and studies of the Ti/Cr effect on the kinetics of SIAs are very scarce in vanadium alloy. In this work, we systemically investigated the migration mechanisms of mono/di-interstitials as well as the effect of alloying elements Ti/Cr on the kinetics of SIAs using first-principles calculations. The findings reveal that <111>SIA exhibits three migration mechanisms, including one dimensional (1D) migration, translation-rotation and translation, corresponding energy barriers of 0.008, 0.19 and 0.44 eV. Different migration pathways are activated at high temperatures, which supports the experimental observation of 3D migration of SIAs under high temperatures. We determined the di-SIA (two parallel <111>SIA) migration by both successive and simultaneous jumping paths, the lowest migration barrier is 0.009 eV via successive jumping path passing a translation state. Moreover, based on three migration mechanisms of SIA, SIA migrating to Ti region possesses the lower barrier than pure vanadium, in verse the barrier is relative high and then hinders the SIA diffusion. The alloying Ti has a pinning effect on SIAs. And we explored how SIA to form mixed V-Cr interstitial and its migration of <111>V-Cr. Finally, we estimated the effect of Ti on SIA diffusivity by empirical formulas.

Origins of strength stabilities at elevated temperatures in additively manufactured refractory high entropy alloy

While refractory high entropy alloys (RHEAs) show promising potential for extreme applications, those directly fabricated via additive manufacturing methods have been hindered by their inferior mechanical properties, particularly at high temperatures. In this study, we successfully produced a Hf-Nb-Ti-V RHEA using directed energy deposition (DED) technique, achieving satisfactory tensile properties across a wide temperature range. This was accomplished by inducing severe lattice distortions in the fabricated RHEA, which can be traced back to the local chemical fluctuations present in the newly fabricated RHEA and the significant atomic radius mismatch. Due to strong solute pinning, these factors contribute to the superior yield strength of the DED-fabricated RHEA across a wide temperature range. Furthermore, the elastic constants in the fabricated RHEA show a negligible temperature dependence, revealed by first-principles calculations, ensuring satisfactory strengths even at high temperatures. This alloy design strategy, which involves introducing significant lattice distortion and maintaining the temperature-low sensitivity of the elastic moduli, opens up new possibilities for directly fabricating RHEAs with superior high-temperature properties.

Micro-mechanism study of charge transfer at heterojunction interface based on first-principles theory: MoS2/SnO2 as the prototype

Gas sensors made of semiconductor heterojunctions have excellent gas-sensitive properties. However, sufficient theoretical evidence is still needed to prove that heterojunction improve gas sensitivity of a single-material. A typical case is studied here: MoS2/SnO2 heterojunction absorption of NO2 is investigated by first-principles calculations, to explore the enhancing-mechanism. Interestingly, it is found that electron transfer not only occurs between the material surface of the heterojunction and gas, but also involves the substrate atoms transferring electrons to the gas molecules through the surface atoms. This finding fills the response mechanism of heterojunction gas sensors and provides a theoretical basis for experiments

Strain tunable electronic, optical, and photovoltaic properties of monolayer β2-SrX2Y4 (X = Al, Ga, In, Y = S, Se)

In recent years, the experimentally synthesized two-dimensional material MoSi2N4, known for its excellent mechanical strength and environmental stability, has attracted significant attention as a representative of the MA2Z4 family. However, people's understanding of some MA2Z4 materials is still limited. In this study, we systematically investigated the properties of monolayer β2-SrX2Y4 (X=Al, Ga, In; Y=S, Se) with a seven-layer atomic configuration using first-principles calculations, focusing on their response to external strain engineering. Our results demonstrate that monolayer β2-SrX2Y4 exhibits characteristics of a direct bandgap semiconductor, and the majority of these materials retain these characteristics under applied strain. However, an unexpected transition from a direct bandgap to an indirect bandgap was observed in monolayer β2-SrAl2S4 under strain. Moreover, as strain changes from compressive to tensile, the imaginary part peak of the dielectric function for most β2-SrX2Y4 materials shifted towards lower energies (redshifted). Notably, in their pristine structures, only β2-SrAl2S4 and β2-SrAl2Se4 can facilitate water splitting by crossing the oxidation and reduction potentials of water. By applying strain, we successfully enabled β2-SrGa2S4 and β2-SrIn2S4 to possess the capability to drive water oxidation-reduction reactions. Furthermore, we calculated the second-order elastic constants and analyzed the Grüneisen parameter and thermal expansion coefficient based on the fitted energy density and strain relationships. This study highlights the potential applications of β2-SrX2Y4 in photocatalysis and optoelectronic devices and provides valuable insights for implementing biaxial strain in two-dimensional materials.

Predicting the solid solution structure preference of multi-component alloys

High-entropy alloys (HEAs) provide limitless opportunities to enhance material performance while predicting the thermodynamic properties and the structures rapidly within HEAs remain challenging. Herein, a new method (M2FEn) for rapidly predicting the formation enthalpies is provided. In contrast to Miedema’s semi-empirical methodology, which solely calculates the mixing enthalpy, the M2FEn is capable of calculating the formation enthalpies of various solid solution structures, and by ordering these enthalpies, one can determine the preferred solid-solution structure for the single-phase alloy. The predicted structures using M2FEn are in line with first-principles calculations. The effectiveness of different extrapolation models for formation enthalpy has been assessed. The M2FEn with the Ouyang extrapolation model and the regular solution model have achieved a prediction accuracy of 95.8% and 95.4%, respectively for single-phase HEAs that were experimentally prepared. 28 common HEA elements were selected as the chemical space, and the formation enthalpies of all binary, ternary, quaternary, quinary and senary alloys, a total of 499149 various alloys were calculated by M2FEn. It is found that the BCC structure ratio increases with the increase of the principal element of the alloy. The M2FEn should potentially accelerate the development of novel, high-performance HEAs, enabling more efficient and cost-effective materials design processes.

Improvement of magnetic properties and hardness by alloying Mo to a FeCrCo alloy

In most of the technologies used for improving material properties, the enhancement of some properties usually leads to the degradation of others. In this study, we demonstrated that we were able to simultaneously improve hardness, coercivity, and remanence by alloying an FeCrCo alloy with Mo by engineering its α → α1 + α2 spinodal decomposition. We prepared six Mo-alloyed samples and subjected them to the appropriate aging protocols. As aging progressed, hardness increased from 272 to 447 HV and coercivity increased from 47 to 592 Oe in the alloyed samples. Remanence gradually increased from 52 to 98 Am2/kg, and then decreased to 82 Am2/kg in the final stage of the aging process. By adding Mo to six individual FeCrCo samples aged to different stages, we increased the hardness of these samples by 3%, 8%, 7%, 8%, 5%, and 10%, respectively. At the same time, we increased their coercivity by 74%, 13%, 19%, 4%, 14%, and 9% respectively and their remanence by 82%, 63%, 4%, 4%, 9%, and 13%, respectively. We used data drawn from scanning transmission electron microscopy, first-principles calculations, and magnetic force microscopy in order to elucidate the reasons behind the observed enhancements of magnetic properties in the FeCrCoMo alloy.

Moiré-pattern-tuned interlayer friction of two-dimensional α- and β-tellurene via first-principles calculations and experimental validation

Tellurene, as an emerging class of two-dimensional (2D) materials, exhibit distinctive physicochemical properties derived from their varied structural morphologies, particularly in van der Waals (vdW) heterostructures formed between their allotropes, demonstrating substantial potential for superlubricity applications. This study employs a synergistic approach that combines theoretical calculations with experimental investigations to investigate the tribological performance of two tellurene allotropes (α- and β-Te) and their allotropic homojunctions (α-Te/β-Te). The frictional forces of the tellurene systems under varying twist angles were predicted by applying potential energy surfaces (PESs) to the Prandtl–Tomlinson (PT) model. These findings indicate that a reduction in energy barriers leads to decreased frictional forces, thereby enhancing the system's superlubricity. Notably, the frictional response is influenced not only by the interlayer sliding barriers but also by the shape and periodicity of the potential energy landscapes. Furthermore, leveraging the concept of the lubricating figure of merit, this study provides an in-depth analysis of the intrinsic frictional characteristics within a tellurene system. Experimentally, few-layer α- and β-Te tellurene were successfully synthesized, and their interlayer frictional properties were measured, showing high congruence with theoretical predictions. The outcomes reveal the exceptional interlayer frictional performance of tellurene under controlled twist angles, with the β-Te phase exhibiting superior lubricity over α-Te and the formation of an allotropic homojunction (α-Te/β-Te) further enhancing the interlayer superlubricity. These results not only deepen our understanding of the tribological performance of tellurene but also offer a new perspective on the frictional behavior of 2D materials.

Reaction mechanism of CO2 on the surface δ-Pu(1 0 0):a DFT study

This investigation elucidates the adsorptive and dissociative interactions of CO2 and CO molecules with the δ-Pu (100) surface, utilizing state-of-the-art first-principles calculations. The study unearths that the hollow site epitomizes the most favorable adsorption locus for both CO2 and CO. Within the paramount adsorption configuration, there prevails a propensity for both species to partake in dissociative adsorption, characterized by CO2 cleaving into an O atom and a concomitant CO moiety, whereas CO disintegrates into discrete C and O atoms. The dissociation energy barriers conducive to such configurations are computed to be 3.0568 eV for CO2 and 5.1667 eV for CO. A thorough analysis of electronic charges and density of states intimates a transfer of electrons from the Plutonium surface atoms to the carbonaceous adsorbates during dissociation. Post dissociation of the C-O bond, the 6d orbitals of Plutonium engage in electronic hybridization with the 2p orbitals of Carbon, culminating in the constitution of ionic bonding.

Layer-polarized anomalous Hall effect in the MnBi[formula omitted]Te[formula omitted]/In[formula omitted]Se[formula omitted] (In[formula omitted]Te[formula omitted]) heterostructures

The layer-polarized anomalous Hall effect has emerged as a novel phenomenon in the field of condensed matter physics, holding significant promise for future applications in designing low-dissipation devices. Currently, the layer-polarized anomalous Hall effect has been theoretically predicted or experimentally demonstrated through the application of external electric fields or the utilization of sliding ferroelectricity in diverse systems. Here, through first-principles calculations, we propose a pathway to realize the layer-polarized anomalous Hall effect by constructing A-type antiferromagnetic topological insulator MnBi2Te4 based heterostructures with ferroelectric materials In2Se3/In2Te3. Our results firstly show that the sizeable band splitting (larger than 20 meV) appears in the antiferromagnetic 4 septuple layers MnBi2Te4/In2Se3 system due to broken inversion symmetry. Further calculations approve that the layer-polarized anomalous Hall conductivity with reversal signs can be observed in the antiferromagnetic 4 septuple layers MnBi2Te4/In2Se3 (In2Te3) systems by shifting the Fermi energy level. Additionally, it is also found that ferrimagnetic 4 septuple layers MnBi2Te4/In2Se3 (In2Te3) can be realized by controlling the direction of ferroelectric polarization of ferroelectric materials. Thus, the resulting layer-polarized anomalous Hall effect may be switchable in our suggested systems. This work provides feasible systems for the further experimental realization of the layer-polarized anomalous Hall effect.

Strengthening and deformation mechanisms of laser additively manufactured Ni2CoCrNb0.2V0.2 medium-entropy alloy: Cryogenic to elevated temperatures

To develop a medium-entropy alloy (MEA) with a high yield strength at temperatures ranging from cryogenic (-196 ℃) to elevated (800 ℃) temperatures, a strategy for strengthening nanoparticle precipitation while reducing the MEA stacking fault energy (SFE) is proposed. A novel Ni2CoCrNb0.2V0.2 MEA suitable for additive manufacturing (AM) was designed according to the first-principles calculations and the calculation of phase diagrams (CALPHAD) technique. Bulk MEA samples were printed using laser-directed energy deposition (LDED) with a vibration field. First-principles calculations indicated that V addition could reduce the SFE and stabilize the γ′′ strengthening phase. The aged MEA had excellent yield strengths of ∼1398MPa and ∼751MPa at -196 ℃ and 650 ℃, respectively. The specific strength of the MEA reached 94.16MPag−1 cm−3 at 650 ℃. The deformational mechanism transforms from stacking fault (SF), deformation twin (DT), and Lomer–Cottrell (L-C) lock synergy to slip-dominated plasticity when the tensile temperature increases from -196 ℃ to 800 ℃. This research provides new theoretical guidance for developing AM-ed MEAs with an ultrastrong combination of strength and ductility for extreme temperatures applications.

Four new tetragonal superhard sp3 carbon allotropes with wide direct band gaps

In this study, four new tetragonal sp3 carbon allotropes with high dynamic and mechanical stabilities (I41/acd-C112, I4/mcm-C128, I4/mcm-C112, and I41/amd-C64) were predicted using first-principles calculations. The I41/acd-C112, I4/mcm-C128, I4/mcm-C112, and I41/amd-C64 allotropes exhibited semiconductor behavior, as confirmed by their wide band gaps (calculated to be 3.80, 3.22, 3.56, and 3.51 eV, respectively) and superhardness, as evidenced through hardness and ideal strength calculations.

Thermophysical properties of Yb3+-doped nonstoichiometric gadolinium zirconate ceramics

Low thermal conductivities and high thermal expansion coefficients are desirable for rare-earth zirconate thermal barrier coating materials. Introduction of cation doping and adjustment of the nonstoichiometry are efficient methods for enhancing the thermophysical characteristics of materials. Herein, the thermophysical properties of Yb3+-doped nonstoichiometric gadolinium zirconate ceramic materials were investigated using first-principles calculations and solid-state reaction techniques. As the Yb3+ content increases, additional cations enter the interstitial spaces (for excess Gd3+ gadolinium zirconate) and form vacancy defects (for excess Zr4+ gadolinium zirconate), which serve as phonon scattering centres to decrease the thermal conductivity. When Yb3+ is doped at the same concentration, gadolinium zirconate with excess Zr4+ exhibits a lower thermal conductivity. Specifically, Gd1.875Yb0.25Zr2.125O7 shows the lowest minimum thermal conductivity of 1.133 W/(m⸱K) (according to the Clark model) and 1.241 W/(m⸱K) (according to the Cahill model). In addition, the experimental results also suggest that the optimal content of Yb3+ is 0.25 excess Zr4+ gadolinium zirconate, which has the lowest intrinsic thermal conductivity of 1.037 W/(m⸱K) at 1300 K. Moreover, the thermal expansion coefficient of the Yb3+-doped excess Gd3+ nonstoichiometric material is greater than that of the doped excess Zr4+ material, this is due to the greater electronegativity difference between Yb3+ and Zr4+ than that between Yb3+ and Gd3+ and the lower binding energy of Gd-O than that of Zr-O. The experimental results and the calculated results are in good agreement. The aim of this work is to enhance the thermophysical characteristics of gadolinium zirconate ceramics for use as thermal barrier coating materials.

Linear dependence of potential drop at the passive film/solution interface on film-formation potential and pH: Combining first-principles calculations with experiments

Potential drop at the oxide film/solution interface plays a critical part in numerous electrochemical processes, especially for the passivated metal-electrolyte system, affecting the passive film breakdown and the pitting initiation. There are still some controversies over the dependence of potential drop at the passive film/solution interface (φf/s) on potential and pH of electrolyte. Herein, we develop a model presenting the linear dependence of φf/s on both potential and pH, as represented by φf/s=φf/s(V=0,pH=0)0+αV+βpH. By analyzing the surface charge (i.e., point of zero charge, pHpzc) and performing first-principles calculations, we provide the insights into the effect of potential and pH on the φf/s, and into the linear relation between φf/s with pH beyond the Nernst relation that is attributed to the role of point defects in pHpzc of passive film on iron. In addition, two methods, e.g., a combination of Mott-Schottky measurement with first-principles, contact angle titration, are suggested to determine the values of α, β and φf/s(V=0,pH=0)0, respectively. The study of passivity of iron in borate buffer solutions validates our model.

Research on the electronic properties of TiB2/γ-Fe(1 1 1) and TiB2/Ni(1 1 1) interfaces

In order to explore the interfacial bonding mechanisms of TiB2/γ-Fe and TiB2/Ni in the composites, the adhesion work, electronic properties and fracture toughness of the TiB2(001)/ γ-Fe(111) and TiB2(001)/Ni(111) interfaces were investigated using first-principles calculations. The results reveal that the surface energy of the TiB2 surface at the B-terminated is the smallest and the constructed TiB2(001)/ γ-Fe(111) interface has the largest adhesive energy. The electronic structures of the TiB2(001)/γ-Fe(111) and TiB2(001)/Ni(111) interfaces reveal that bonding at the interfaces is provided by the B-2p orbitals with the Fe-3d and Ni-3d orbitals, respectively, and that the formation of Fe-B and Ni-B covalent/ionic bonds is the main source of bonding and interaction. The bonding and strength of Fe-B in the TiB2(001)/γ-Fe(111) interface is stronger than that in the TiB2(001)/Ni(111) interface, which is due to the higher charge density accumulation of Fe atoms at the TiB2(001)/γ-Fe(111) interface. Using Griffith’s theory, the TiB2(001)/γ-Fe(111) interface is inferred to have the strongest fracture toughness. This study suggests that the chemical bonding stronger Fe-B bonds result in a high bond strength and a more stable interfacial structure at the TiB2(001)/ γ-Fe(111) interface, leading to better resistance to cracking in practice.

First-principles study of Ti adsorption on Al4C3 (0001) surface

Al4C3 phase is hypothesized to undergo a solid–solid phase transition into TiC, thereby facilitating the heterogeneous nucleation of α-Al. However, capturing this transition process experimentally is challenging, which undermines the credibility of this transition theory. In this study, first-principles calculations are employed to investigate the feasibility of Ti atom adsorption on the exposed (0001) surface of the Al4C3 crystal, providing theoretical support for the phase transition hypothesis. The findings indicate that Ti adatoms can effectively adsorb on the C-terminated (0001) surface, whereas adsorption on the Al-terminated surface is thermodynamically unfavorable. Upon structural optimization, Ti adatoms at bridge site B, top site T, and hollow site H1, irrespective of coverage, migrate towards the most stable hollow H2 site. Electronic structure analysis reveals that the bonding between the Ti adatom at the H2 site and the surface layer C atoms is primarily covalent, while the bonding with subsurface layer Al atoms is metallic. Additionally, Ti atom adsorption induces polarization of the bonding electron cloud between Al and C atoms. This study provides theoretical support for the Ti-induced transformation of the Al4C3 to TiC phase via surface adsorption, paving the way for the development of high-performance master alloys based on phase transition theory.

Investigation of phase stability and martensitic transformation of all-d-metal Heusler alloys Fe2VIr and Fe2TaIr in high pressure: A first-principles study

First-principles calculations are used to investigate the phase stability and martensitic transformation of two new all-d-metal Heusler alloys Fe2VIr and Fe2TaIr at 0–50 GPa. The difference in phase stability between Fe2VIr and Fe2TaIr is due to the effect of d-d orbital hybridisation between different atoms and the appearance of triply degenerate in the pressure-induced phonon optical modes. The softening behaviour of the transverse phonon mode along the high symmetry point X is responsible for the martensitic transformation of Fe2TaIr. We also observed the softening of the shear elastic constant C′ = (C11-C12)/2. Strong Fe-Fe magnetic coupling, softening of transverse acoustic phonon modes and anomalous softening of C′ awaken the martensitic transformation. Low-frequency phonon hybridisation of the heavy elements Ta and Ir is the driving force behind the softening. A surprising phenomenon is found during tetragonal deformation of the structure of Fe2TaIr at 45 GPa, whereby the structure break through the energy barrier of the leap and produced a tetragonal martensitic phase with a much lower energy. It may imply that pressure-induced Fe2TaIr produced a premartensitic phase. This work presents a detailed analysis of the anomalous softening behaviour of Fe2TaIr and the mechanism of the pressure-induced martensitic transformation. A new idea is proposed for an in-depth study of the martensitic phase transition precursor reaction.

The structural, electronic, optical and photocatalytic properties of two-dimensional ZrIN monolayer: A first-principles study

This study presents a comprehensive investigation of the structural stability, electronic and optical properties of two-dimensional ZrIN using first-principles calculations. The stability of 2D ZrIN was confirmed through investigation of thermodynamic, mechanical, dynamic and thermal criteria. The 2D ZrIN is predicted to be an indirect bandgap semiconductor with a gap value of 2.28 eV. It demonstrates high transmittance in the violet segment of the visible spectrum. The photocatalytic properties of 2D ZrIN were also investigated under pH values of 0 and 7 by examining the band edge positions. Additionally, the impact of strain on the band structure and photocatalytic activity. It was found that compressive strain limits the photocatalytic working range of ZrIN monolayer, whereas tensile strain expands working range; however, a further increase in tensile strain leads to decrease in the effective catalytic working region. These findings provide valuable insights into the potential applications of 2D ZrIN in both optoelectronics and catalysis.

Bilayer graphene, bilayer GeC and graphene/GeC bilayer heterostructure as anode materials for lithium-ion batteries: first-principles calculations

There is a great effort to develop the high-efficient anode materials for lithium-ion batteries with high stability and mobility. In this paper, we adopt the first-principles calculations to study the electrochemical properties of Li intercalation within bilayer graphene (BLG), bilayer GeC (BLGeC) and graphene/GeC bilayer heterostructure (BLGGeC) as anode materials. Our calculations disclose the following findings: (1) The interlayer is modulated by the stacking patterns and bilayer species. (2) The most energetically favorite intercalation of the Li atom is achieved in BLG with AA stacking because of the lowest adsorption energy. (3) The descending order of energy barriers is BLGGeC > BLGeC > BLG. The low diffusion barriers of BLG (0.025 eV) and BLGeC (0.09 eV) imply their high charging/discharging rates. Finally, the findings underline that the bilayer is a promising anode material and its electrochemical characteristics can be changed by adjusting the stacking configurations and its species.

Impact of titanium modification on the performance improvement and phase change mechanism of ZnSb thin film

Phase change memory has emerged as a promising option for neuro-inspired and data-intensive AI applications, attracting significant attention from the semiconductor field recently. The performance of the device relies on the phase change material, aiming for enhanced thermal stability along with low power consumption. Nanocomposite ZnSb thin films with titanium dopant were prepared and their crystallization properties and mechanisms were investigated. Compared to pure ZnSb material, the addition of titanium can significantly enhance the temperature of crystallization and the 10-year data retention ability, both of which surpassing 16 °C. The obvious enhancement in thermal stability may be ascribed to the ionic bond formation between titanium and antimony elements, as indicated by differential charge calculations. Also, phase change memory cells with a diameter of 190 nm were fabricated using standard CMOS technology. The Ti-doped ZnSb thin film demonstrates rapid crystallization and amorphization within 50 ns while consuming low power at approximately 1.5 × 10−10 J. The decrease in power consumption may stem from the increase in band gap energy and a decrease in electrical conductivity, as confirmed by first-principles calculations. Both experimental and theoretical results prove that titanium doping can remarkably improve the physical properties of ZnSb thin film, facilitating the exploration and design of novel phase change materials with high thermal stability, low power consumption, as well as fast switching speed.