An appropriate electrode material is crucial for two-dimensional (2D) semiconductors, where a vanishing Schottky barrier is ideal but is a great challenge. Blue phosphorene (BlueP) is a promising 2D semiconductor for electronic and optoelectronic applications. Here, we report that Zr-, Hf-, and Nb-based 2D transition metal carbides (MXenes) are ideal electrode materials for BlueP based on extensive investigations of the electronic properties and interfacial Schottky barrier characteristics of BlueP/MXene heterojunctions by first-principles calculations. Our results show that the strong interaction between BlueP and bare MXenes destroys the semiconducting character of BlueP, and thus bare MXenes are not ideal contact electrodes. With the surface functionalization of MXene, the intrinsic electronic feature of BlueP is well preserved in the BlueP/surface-engineered MXene heterojunctions. Furthermore, the interfacial Schottky barriers of the heterojunctions are affected by the terminal surface groups on MXenes, and vanishing Schottky barriers are achieved in some MXenes with the formula Zrn+1CnF2, Hfn+1CnF2, Zrn+1Cn(OH)2, Hfn+1Cn(OH)2, and Nbn+1Cn(OH)2. Finally, we demonstrate that the work functions of MXenes and the interface dipole induced by charge rearrangement are two underlying factors to determine the magnitude of Schottky barriers. This work provides fundamentals for selecting ideal electrode material for BlueP and is also beneficial for optimizing electrodes for other 2D semiconductors.

Atomically thin two-dimensional (2D) materials have received considerable research interest due to their extraordinary properties and promising applications. Here we predict the monolayered indium triphosphide (InP3) as a new semiconducting 2D material with a range of favorable functional properties by means of ab initio calculations. The 2D InP3 crystal shows high stability and promise of experimental synthesis. It possesses an indirect band gap of 1.14 eV and a high electron mobility of 1919 cm2 V–1 s–1, which can be strongly manipulated with applied strain. Remarkably, the InP3 monolayer suggests tunable magnetism and half-metallicity under hole doping or defect engineering, which is attributed to the novel Mexican-hat-like bands and van Hove singularities in its electronic structure. A semiconductor–metal transition is also revealed by doping 2D InP3 with electrons. Furthermore, monolayered InP3 exhibits extraordinary optical absorption with significant excitonic effects in the entire range of the visible light spectrum. All these desired properties render 2D InP3 a promising candidate for future applications in a wide variety of technologies, in particular for electronic, spintronic, and photovoltaic devices.

•Stacking fault energy for pure Nb along habit slip systems are identified.•Occupation preferences of C, N, O, H and Si in niobium are figured out.•C, N, O and H deteriorate the hardness and ductility of Nb, while Si enhances them.•Micro alloying of Si in Nb phase in Nb-Si based alloy is mechanically beneficial.Nonmetallic elements C, H, N, O and Si which normally are introduced into the metallic matrix unintentionally, usually have significant effects on the mechanical properties of alloys, which, however, is unclear for niobium. Hardness and ductility are two important properties for alloys, which can be characterized by the unstable stacking fault energy γus and the ratio between surface energy γs and γus, respectively. In this study, by using first-principles calculation, we find that C, N, O and H decrease the hardness and ductility of niobium. On the other hand, minor alloying of Si in niobium can improve the hardness and show excellent ductilization capability.

Development of novel van der Waals (vdW) heterostructures from various two-dimensional (2D) materials shows unprecedented possibilities by combining the advantageous properties of their building layers. In particular, transforming the vdW heterostructures from type-I to type-II is of great interest and importance to achieve efficient charge separation in photocatalytic, photovoltaic, and optoelectronic devices. In this work, by means of ab initio calculations, we have systematically investigated the electronic structures, optical properties, and mechanical properties of MXene/Blue Phosphorene (BlueP) vdW heterostructures under various deformations. We highlight that, under strain, the type-I heterostructures can be transformed to type-II with their conduction band minimum (CBM) and valence band maximum (VBM) separated in different layers. Interestingly, the locations of the CBM or VBM in MXene/BlueP vdW heterostructures can also be reversed by compressive or tensile strain between the building layers, which indicates that either layer can be utilized as an electron donor or acceptor by varying its deformation conditions. Meanwhile, this compressive (tensile) strain can also induce a red (blue) shift in the optical absorption spectra of MXene/BlueP vdW heterostructures. Finally, our results on the mechanical flexibility and deformation mechanism of MXene/BlueP vdW heterostructures suggest their great long-term stability as well as promising applications in flexible devices. We believe that our findings will open a new way for the modulation and development of vdW heterostructures in flexible optical/electronic devices.

Exploring new two-dimensional (2D) crystals attracts great interest in the materials community due to their potential intriguing properties. Here, we report a new family of 2D transition metal borides (labeled as MBenes) that can be produced by selectively etching the A layer from a family of layered transition metal borides (MAB phases). The emerged MBenes are demonstrated to possess great stability with isotropic and ultrahigh Young's modulus. Meanwhile, our results show that 2D Mo2B2 and Fe2B2 MBenes are metallic with excellent electronic conductivity, which are highly desirable for applications in Li-ion batteries (LIB) and electrocatalysis. Furthermore, 2D Mo2B2 and Fe2B2 are confirmed to have an omnidirectional small diffusion energy barrier and high storage capacity for Li atoms, which highlight MBenes as appealing electrode materials for LIBs. Moreover, 2D Fe2B2 MBene also exhibits superior catalytic activity for the hydrogen evolution reaction (HER) with hydrogen adsorption Gibbs free energy close to the optimal value (0 eV), indicating its promising application as an electrocatalyst for hydrogen evolution. Considering the large number of possible MAB phases, more MBenes with attractive applications are anticipated theoretically and/or experimentally in the near future.

•Prediction of few-layer arsenic trichalcogenides with broad and tunable band-gaps.•Strain-induced indirect to direct band-gap transition in these layered compounds.•Low exfoliation energy renders them attractive for artificial hetrostructures.•Promising candidates for applications in photocatalysis and optoelectronics.Since the discovery of graphene, two-dimensional (2D) layered nanomaterials have been receiving continuous attention owing to their extraordinary properties and promising applications in nanoelectronics. However, many 2D nanomaterials are gapless or possess a small band-gap (≤2 eV), which greatly restricts their applications. Here, by means of ab initio calculations and molecular dynamics simulcations, we report a class of emerging 2D semiconductors, mono- and few-layer arsenic trichalcogenides (As2S3 and As2Se3), with a broad band-gap range from 2.06 eV to 3.18 eV, which can be manipulated by the number of layers or external strains. Interestingly, under moderate tensile strain, the nanolayers undergo a transition from indirect to direct band-gap semiconductors. More importantly, these 2D semiconductors exhibit suitable band-edge alignment and desirable optical absorption, suggesting their potential applications for photocatalysis and optoelectronics. Thanks to the small exfoliation energies, these 2D layered materials could be fabricated from experiments feasibly and serve as promising candidates in constructing van der Waals heterostructures for future nanoelectronics.Download high-res image (562KB)Download full-size image

Oxygen is widely used to tune the performance of chalcogenide phase-change materials in the usage of phase-change random access memory (PCRAM), which is considered as the most promising next-generation non-volatile memory. However, the microscopic role of oxygen in the write–erase process, i.e., the reversible phase transition between crystalline and amorphous state of phase-change materials, remains unclear. Using oxygen doped GeTe as an example, this study unravels the role of oxygen at the atomic scale by means of ab initio total energy calculations and ab initio molecular dynamics simulations. Our main finding is that after the amorphization and the subsequent re-crystallization process simulated by ab initio molecular dynamics, oxygen will drag one Ge atom out of its lattice site and both atoms will stay in the interstitial region near the Te vacancy that was originally occupied by oxygen, forming a “dumbbell-like” defect (O–VTe–Ge), which is in sharp contrast to the results of ab initio total energy calculations at 0 K showing that the oxygen prefers to substitute Te in crystalline GeTe. This specific defect configuration leads to a slower crystallization speed and hence the improved data retention of oxygen doped GeTe. Moreover, we find that the local oxygen configuration will increase the effective mass of the carrier and thus increase the resistivity of GeTe. Our results unravel the microscopic mechanism of the oxygen-doping optimization of GeTe phase-change material, and the present reported mechanism can be applied to other oxygen doped ternary chalcogenide phase-change materials.

•Y, Ti Cr and Si all tend to segregate at the grain boundary of Nb.•Atomic size and stacking density of matrix-layers determine the occupation site.•Alloying effects on the cohesion of Nb grain boundary are identified.•Strengthening energy is decomposed into mechanical and chemical contributions.•Variation of host bond strength after alloying determines the segregation effect.The intergranular fracture of materials is sensitive to the grain-boundary segregation of solutes, which is not yet clear for niobium alloys, a vital promising high-temperature material in the aerospace industry. In this study, first-principles calculations are performed to study the segregation effects of four mostly-used solutes (Y, Ti, Cr and Si) on the cohesion of two distinct grain boundaries (GBs) of Nb. Based on Rice-Wang Model, it is found that Y embrittles the GB; while Ti and Si strengthen it; the effect of Cr on the GB cohesion is structure-dependent. The strengthening/embrittling of the GBs induced by the segregation are attributed to the local structural distortion and charge redistributions around GBs. The underlying physical origin of different alloying effects on the intergranular fracture are identified to be the variation of bonding characters between the host NbNb atoms across the GB regions.Download high-res image (192KB)Download full-size image

By using an ab initio evolutionary algorithm structure search, low enthalpy criterion as well as stability analysis, we have found that cubic Fmm Ca2Si can be achieved under a negative external pressure, and the cubic phase is dynamically and mechanically stable at ambient conditions and high pressure. From first-principle hybrid functional calculations, we have unraveled the direct bandgap nature and bandgap variation of cubic Fmm Ca2Si with respective to pressure. Moreover, by combining with Boltzmann transport theory and the phonon Boltzmann transport equation, we have predicted that the figure of merit ZT for the cubic Fmm Ca2Si reaches the maximum value of 0.52 by p-type doping. Our results provide an interesting insight and feasible guidelines for the potential applications of cubic Fmm Ca2Si and related alkaline-earth metals silicides as the thermoelectric materials for heat-electricity energy convertors.

By using an ab initio evolutionary algorithm structure search, low enthalpy criterion as well as stability analysis, we have found that cubic Fmm Ca2Si can be achieved under a negative external pressure, and the cubic phase is dynamically and mechanically stable at ambient conditions and high pressure. From first-principle hybrid functional calculations, we have unraveled the direct bandgap nature and bandgap variation of cubic Fmm Ca2Si with respective to pressure. Moreover, by combining with Boltzmann transport theory and the phonon Boltzmann transport equation, we have predicted that the figure of merit ZT for the cubic Fmm Ca2Si reaches the maximum value of 0.52 by p-type doping. Our results provide an interesting insight and feasible guidelines for the potential applications of cubic Fmm Ca2Si and related alkaline-earth metals silicides as the thermoelectric materials for heat-electricity energy convertors.

MXenes are a large family of two-dimensional (2D) early transition metal carbides that have shown great potential for a host of applications ranging from electrodes in supercapacitors and batteries to sensors to reinforcements in polymers. Here, on the basis of first-principles calculations, we predict that Mo2MC2O2 (M = Ti, Zr, or Hf), belonging to a recently discovered new class of MXenes with double transition metal elements in an ordered structure, are robust quantum spin Hall (QSH) insulators. A tight-binding (TB) model based on the dz2-, dxy-, and dx2–y2-orbital basis in a triangular lattice is also constructed to describe the QSH states in Mo2MC2O2. It shows that the atomic spin–orbit coupling (SOC) strength of M totally contributes to the topological gap at the Γ point, a useful feature advantageous over the usual cases where the topological gap is much smaller than the atomic SOC strength based on the classic Kane–Mele (KM) or Bernevig–Hughes–Zhang (BHZ) model. Consequently, Mo2MC2O2 show sizable gaps from 0.1 to 0.2 eV with different M atoms, sufficiently large for realizing room-temperature QSH effects. Another advantage of Mo2MC2O2 MXenes lies in their oxygen-covered surfaces which make them antioxidative and stable upon exposure to air.Keywords: first-principles calculations; large gap; MXene; topological insulator; triangular lattice;

Identifying suitable photocatalysts for photocatalytic water splitting to produce hydrogen fuel via sunlight is an arduous task by the traditional trial-and-error method. Thanks to the progress of density functional theory, one can nowadays accelerate the process of finding candidate photocatalysts. In this work, by ab initio calculations, we investigated 48 two-dimensional (2D) transition metal carbides also referred to as MXenes to understand their photocatalytic properties. Our results highlight 2D Zr2CO2 and Hf2CO2 as the candidate single photocatalysts for possible high efficiency photocatalytic water splitting. A significant property of 2D Zr2CO2 and Hf2CO2 is that they exhibit unexpectedly high and directionally anisotropic carrier mobility, which may effectively facilitate the migration and separation of photogenerated electron–hole pairs. Meanwhile, these two MXenes also exhibit very good optical absorption performance in the wavelength ranging approximately from 300 to 500 nm. The stability of 2D Zr2CO2 and Hf2CO2 in liquid water is expected to be good based on ab initio molecular dynamics simulations. Finally, the adsorption and decomposition of water molecules on the 2D Zr2CO2 surface and the subsequent formation process of hydrogen were studied, which contributes to the unravelling of the micro-mechanism of photocatalytic hydrogen production on MXenes. Our findings will open a new way to facilitate the discovery and application of MXenes for photocatalytic water splitting.

In this work, we introduce a series of two dimensional (2D) group IV chalcogenides (AX)2 with the building block X–A–A–X (A = Si, Ge, Sn, and Pb, and X = Se and Te) on the basis of ab initio calculations. The analysis of energy evaluation, lattice vibration as well as the chemical bonding demonstrate the good stability of these 2D materials. Furthermore, the pictures for the chemical bonding and electronic features of the 2D (AX)2 are drawn. Their narrow gapped semiconducting nature is unraveled. Especially, strong interactions between the electrons and phonons as well as the topological insulating nature in (SiTe)2 are observed. The present results indicate that such remarkable artificial 2D (AX)2 are building blocks for van der Waals heterostructure engineering, which shows potential applications in nanoscaled electronics and optoelectronics.

Sb2Te3 exhibits outstanding performance among the candidate materials for phase-change memory; nevertheless, its low electrical resistivity and thermal stability hinder its practical application. Hence, numerous studies have been carried out to search suitable dopants to improve the performance; however, the explored dopants always cause phase separation and thus drastically reduce the reliability of phase-change memory. In this work, on the basis of ab initio calculations, we have identified yttrium (Y) as an optimal dopant for Sb2Te3, which overcomes the phase separation problem and significantly increases the resistivity of crystalline state by at least double that of Sb2Te3. The good phase stability of crystalline Y-doped Sb2Te3 (YST) is attributed to the same crystal structure between Y2Te3 and Sb2Te3 as well as their tiny lattice mismatch of only ∼1.1%. The significant increase in resistivity of c-YST is understood by our findings that Y can dramatically increase the carrier’s effective mass by regulating the band structure and can also reduce the intrinsic carrier density by suppressing the formation of SbTe antisite defects. Y doping can also improve the thermal stability of amorphous YST based on our ab initio molecular dynamics simulations, which is attributed to the stronger interactions between Y and Te than that of Sb and Te.Keywords: electrical resistivity; phase-change material; Sb2Te3; thermal stability; Y doping

•The mechanism of the cleavage fracture of α-Nb5Si3 is figured out.•The occupation preference of titanium in α-Nb5Si3 is identified.•When the concentration of Ti exceeds 12.5 at.%, cleavage resistance of α-Nb5Si3 decreases.•Origin of the concentration dependent effect of Ti on the toughness of Nb5Si3 is explained.NbSi-based alloy, which primarily consists of niobium solid solution (NbSS) and α-Nb5Si3, is a promising ultra-high temperature material. The overall fracture toughness of this alloy is limited by the brittle α-Nb5Si3 phase. Alloying Ti, which can improve the toughness of NbSS, is typically introduced into NbSi-based alloy. The overall toughening effect of the alloying elements on multi-composites is dependent on the alloying effect on each component. However, the influence of Ti on the brittleness of α-Nb5Si3 is unclear, which makes the beneficial effect of Ti on the mechanical properties of NbSi-based alloy ambiguous. By calculating the ideal cleavage energy and cleavage strength using a first-principles method, we find that a low amount of Ti will improve the toughness of Nb5Si3, while Ti-rich Nb5Si3 becomes more brittle, which is in good agreement with experimental results. The concentration-sensitive effect of alloying Ti is induced by the variation of its substitution site and can be well explained by the electronic structure analysis. Our findings demonstrate that the alloying amount of Ti should be carefully selected in the NbSi-based alloy.Figure optionsDownload full-size imageDownload as PowerPoint slide

It is commonly believed that early transition metal monoxides (TM–MOs) crystallize in simple rock-salt structures (symmetry FM3̅M) for their ground states. Here, by combining structure-searching algorithm and first-principles calculations, we identified structures that are more stable than the ideal rock-salt for the early TM–MOs (TM = Ti, Hf, V, Ta). For TiO, HfO, and TaO, ground state symmetries of P6̅2M), I41/AMD and P1̅ are obtained, respectively, which have distinct structural and electronic properties compared to the rock-salt structure. However, it is rather complex for the case of VO due to the existence of magnetic ordering. For VO, magnetic ordering behavior exists in the rock-salt and the predicted P1̅ structure according to the hybrid functional calculations. After relaxation, the magnetic ordering causes local distortion in the original rock-salt structure, leading to a R3̅M symmetry, which becomes more stable than the predicted P1̅ structure. Furthermore, the ionic TM–O bonding of the predicted phases is rather weaker than that of their rock-salt counterparts. While the enhanced metal–metal bonding characterized by the distances between the nearest-neighboring metallic atoms is found to be responsible for the stabilization of the ground state structures discovered here. Our findings deepen the understanding of the ground state of early TM–MOs, which is vital for the unraveling of the complete physical picture for transition metal monoxides.

Ta2O5 is extensively studied as a data-storage material for resistance random access memory (RRAM). The resistive switching (RS) in Ta2O5-based RRAM is generally believed to be due to the diffusion of oxygen vacancy (Vo) inside the oxide, while the role of metal interstitials is paid less attention. Here, on the basis of first-principles calculations, we show that the role of interstitial Ta (Tai) is competitive under the oxygen-poor condition and should also contribute to RS in Ta2O5. This is obvious by our calculated comparable energy barriers for the diffusion of Vo and Tai, which are 3.5 and 3.7 eV, respectively. Furthermore, the presence of electric field in working devices will enhance the migration of Tai due to its higher charge states compared to Vo. Meanwhile, Tai will introduce more defect states closer to the conduction bands and, thus, is more effective on tuning the electronic structure of Ta2O5. The present work unravels the contribution of Ta cations in RS of tantalum oxide-based RRAM, presenting a synergistic RS mechanism of oxygen vacancies and metal interstitials, which should be helpful for optimizing and designing novel RRAM devices.

Resistance random access memory (RRAM) is known to be a promising candidate for next generation non-volatile memory devices, in which the diffusion of oxygen vacancies plays a key role in resistance switching. Based on first principles calculations and transition state theory, using SrZrO3 (SZO) as an example, we found that the diffusion energy of an oxygen vacancy strongly depends on its charge states and V2+O contributes mostly to the resistance switching due to its lowest activation energy. To adjust the performance of SZO RRAM, the effects of dopants (Y, V, Nb and Ta) were revealed according to their modifications on the diffusion of V2+O. We found that doping of Y or V has the most significant effect on the performance of RRAM devices. Furthermore, for dopants with various numbers of valence electrons and atomic radius, general design principles were proposed based on their different effects on the RRAM characteristics. Our results will guide the experimentations and pave a new way for the optimization of RRAM devices.

Graphene-like two-dimensional materials have garnered tremendous interest as emerging device materials for nanoelectronics due to their remarkable properties. However, their applications in spintronics have been limited by the lack of intrinsic magnetism. Here, using hybrid density functional theory, we predict ferromagnetic behavior in a graphene-like two-dimensional Cr2C crystal that belongs to the MXenes family. The ferromagnetism, arising from the itinerant Cr d electrons, introduces intrinsic half-metallicity in Cr2C MXene, with the half-metallic gap as large as 2.85 eV. We also demonstrate a ferromagnetic–antiferromagnetic transition accompanied by a metal to insulator transition in Cr2C, caused by surface functionalization with F, OH, H, or Cl groups. Moreover, the energy gap of the antiferromagnetic insulating state is controllable by changing the type of functional groups. We further point out that the localization of Cr d electrons induced by the surface functionalization is responsible for the ferromagnetic–antiferromagnetic and metal to insulator transitions. Our results highlight a new promising material with tunable magnetic and electronic properties toward nanoscale spintronics and electronics applications.Keywords: ferromagnetism; functionalization; half-metallicity; MXene; spintronics

Two-dimensional (2D) transition metal carbides/nitrides Mn+1Xn labeled as MXenes are attracting increasing interest due to promising applications as Li-ion battery anodes and hybrid electro-chemical capacitors. To realize MXenes devices in future flexible practical applications, it is necessary to have a full understanding of the mechanical properties of MXenes under deformation. In this study, we extensively investigated the stress–strain curves and the deformation mechanisms in response to tensile stress by first principles calculations using 2D Tin+1Cn (n = 1, 2 and/or 3) as examples. Our results show that 2D Ti2C can sustain large strains of 9.5%, 18% and 17% under tensions of biaxial and uniaxial along x and y, respectively, which respectively increase to 20%, 28% and 26.5% for 2D Ti2CO2 due to surface functionalizing oxygen, which is much better than graphene (15% biaxial). The failure of 2D Tin+1Cn MXene is due to the significant collapse of the surface atomic layer; however, surface functionalization could slow down this collapse, resulting in the improvement of mechanical flexibility. We have also discussed the critical strains and Young's modulus of 2D Tin+1Cn and Tin+1CnO2. Our results provide an insight into the microscopic deformation mechanism of MXenes and hence benefit their applications in flexible electronic devices.

Two-dimensional transition metal carbides/nitrides Mn+1Xns labeled as MXenes derived from layered transition metal carbides/nitrides referred to as MAX phases attract increasing interest due to their promising applications as Li-ion battery anodes, hybrid electro-chemical capacitors and electronic devices. To predict the possibility of forming various MXenes, it is necessary to have a full understanding of the chemical bonding and mechanical properties of MAX phases. In this work, we investigated the chemical bonding changes of MAX phases in response to tensile and shear stresses by ab initio calculations using M2AlC (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W) as examples. Our results show that the M2C layer is likely to separate from the Al layer during the tensile deformation, where the failure of M2AlC is characterized by an abrupt stretch of the M–Al bonds. While under shear deformation, the M2C and Al layers slip significantly relative to each other on the (0001) basal planes. It is found that the ideal strengths of M2AlC are determined by the weak coupling of the M2C and Al layers, closely related to the valence-electron concentration. Our results unravel the possibility as well as the microscopic mechanism of the fabrication of MXenes through mechanical exfoliation from MAX phases.

We report that complete spin polarization and controllable spin polarization of carriers can be simultaneously realized in the Heusler alloy Mn2CoAl simply by applying external pressures based on first-principles studies. At ambient conditions, Mn2CoAl is a ferromagnetic spin-gapless semiconductor (SGS) with complete spin polarization. Under hydrostatic pressures up to 40 GPa, Mn2CoAl undergoes a series of electronic transitions from SGS with spin-up as a conducting channel to a ferromagnetic semiconductor and then to SGS with spin-down as a conducting channel and finally to a half metal, during which the magnetic moment remains as 2 μB. Such rich electronic transitions are attributed to different responses of the spin-up and spin-down electrons under pressure. This work highlights a desirable way to control the carrier's spin polarization and provides a new insight into the electron behavior in Mn2CoAl related Heusler alloys under pressure.

•The high pressure stability of Ni2Zn11 γ brasses was investigated.•Ni2Zn11 remains mechanically and dynamically stable under high pressure.•The high pressure stabilization mechanism was unraveled by electronic structure.•The pseudogap at the EF accounts for the high pressure stabilization of Ni2Zn11.The γ brasses, a family of structurally complex intermetallic compounds containing 52 atoms in the cubic unit cell, attract increasing interest due to their highly ordered but unusual symmetry structure. To investigate their stability and performance under high pressure, we have extensively studied the elastic, thermodynamic and electronic properties as well as their dependences on pressures up to 71 GPa by first-principles calculations using one γ brasses phase (N2Zn11) as an example. We found that Ni2Zn11 remains energetically, mechanically and dynamically stable under the present studied pressure range. Further analysis on the electronic structure of Ni2Zn11 unravels that under various pressures, the pseudogap at the Fermi level which reduces the electronic energy of the system accounts for the stability of γ brasses, i.e., the well-known stabilization mechanism still holds at high pressure. Besides, various thermodynamic quantities of Ni2Zn11 under high pressure were systematically calculated and analyzed. Our present results extend the knowledge of the stabilization mechanism and performance of γ brasses to a high pressure condition.

•Site preferences of alloying elements in α-Nb5Si3 depends on their atomic radii.•Temperature effects on solution of various elements in Nb5Si3 systems are deduced.•All the alloyed Nb5Si3 phases are mechanically stable.•Mechanical properties of the alloyed Nb5Si3 systems are not improved.•Ionic bonding is beneficial to the stability of alloyed α-Nb5Si3 systems.The tendency to dissolve in the matrix for alloying elements such as transition metals and some main group elements in α-Nb5Si3 phase as well as their effects on the structure stability and mechanical properties are important for the performance of niobium-silicide based alloys, which, however, are not clear yet. In this work, we unravel the above problems based on ab-initio calculations. Our results show that the alloyed Nb5Si3 systems become less stable as the alloying elements change from group IVB to VIB in the periodic table. The occupation preferences of the alloying elements depend on their relative atomic radii respect to either Nb or Si. Furthermore, the dissolution of the alloying elements is easier at high temperatures by the Debye model analysis, from which the homogenization-treatment temperatures of alloyed Nb5Si3 phases are also deduced. All alloyed Nb5Si3 phases are mechanically stable, even though their mechanical properties like ductility are not improved. Finally, the electron localized function, Bader charge and densities of states are used to understand the structure stability of alloyed Nb5Si3, and we find that ionic bonding has quite significant effects on the stability of these intermetallic compounds.

•Some general rules of material selection for resistive random access memory (RRAM) are proposed.•Mechanisms of resistance switch in RRAM are reviewed.•Computational material science will play an important role in the design of RRAM.Resistive random access memory (RRAM) is a very promising next generation non-volatile RAM, with quite significant advantages over the widely used silicon-based Flash memories. For RRAM, material with switchable resistance, working as the storage medium, is the most important part for the performance of the memory. In this review, as a start, some general hints for the materials selection are proposed. Then most recent studies on this emerging memory from the perspective of materials science are summarized: various materials with resistance switch (RS) behavior and the underlying mechanisms are introduced; as a complementary to the previous review articles, here the increasingly important role of computational materials science in the research of RRAM is presented and highlighted. By incorporating the framework of high-throughput calculation and multi-scale simulations, design process of new RRAM could be accelerated and more cost-effective.Figure optionsDownload full-size imageDownload as PowerPoint slide

•Hybrid density functional study of huge gap SrZrO3 for efficient photocatalyst.•Electronic structures and optical absorptions of mono-doped and co-doped SrZrO3.•Electronic band edge positions with respect to water redox potential levels.•Cationic–anionic co-doped SrZrO3 can be used for hydrogen production.Using SrZrO3 (SZO, the intrinsic band gap being 5.6 eV) as an example, we have investigated the design principles for huge-gap semiconductors with band gap larger than 5 eV for the application of efficient visible-light driven photocatalysts for splitting water into hydrogen. Based on the hybrid density function calculations, the electronic structures of mono-doped and co-doped SZO are investigated to obtain design principles for improving their photocatalytic activity in hydrogen generation. The cationic–anionic co-doping in SZO could reduce the band gap significantly and its electronic band position is excellent for the visible-light photocatalysis. This work reports a new type of candidate material for visible-light driven photocatalysis, i.e., huge-gap semiconductors with band gap larger than 5 eV. Furthermore, based on the present results we have proposed the design principles for band gap engineering that provides general guideline for other huge-gap semiconductors.

•We have investigated trigonal Ge2Sb2Te5 using DFT-D2 method.•DFT-D2 improves the structure but not the electronic structure.•The band gap of ∼0.5 eV has been obtained from DFT-D2+HSE06 hybrid functions.•The elastic constants were further compared with the experimental Young’s modulus.The Te–Te weak van der Waals-type bonding plays an important role in Ge2Sb2Te5, a widely investigated phase-change material and a potential topological insulator. In this work, we have studied the electronic and mechanical properties of stable Ge2Sb2Te5 using ab initio calculations with the van der Waals corrections. The results show that the van der Waals corrections combined with hybrid functions improve the descriptions of the electronic structure of stable Ge2Sb2Te5. The band gap of ∼0.5 eV in very good agreement with the experimental value for stable Ge2Sb2Te5 has been successfully reproduced. Furthermore, we have predicted the elastic constants and mechanical properties of stable trigonal Ge2Sb2Te5.

Hydrogen fuel produced from water splitting using solar energy and a catalyst is a clean and renewable future energy source. Great efforts in searching for photocatalysts that are highly efficient, inexpensive, and capable of harvesting sunlight have been made for the last decade, which, however, have not yet been achieved in a single material system so far. Here, we predict that MoS2/AlN(GaN) van der Waals (vdW) heterostructures are sufficiently efficient photocatalysts for water splitting under visible-light irradiation based on ab initio calculations. Contrary to other investigated photocatalysts, MoS2/AlN(GaN) vdW heterostructures can separately produce hydrogen and oxygen at the opposite surfaces, where the photoexcited electrons transfer from AlN(GaN) to MoS2 during the photocatalysis process. Meanwhile, these vdW heterostructures exhibit significantly improved photocatalytic properties under visible-light irradiation by the calculated optical absorption spectra. Our findings pave a new way to facilitate the design of photocatalysts for water splitting.

Resistance random access memory (RRAM) is known to be a promising candidate for next generation non-volatile memory devices, in which the diffusion of oxygen vacancies plays a key role in resistance switching. Based on first principles calculations and transition state theory, using SrZrO3 (SZO) as an example, we found that the diffusion energy of an oxygen vacancy strongly depends on its charge states and V2+O contributes mostly to the resistance switching due to its lowest activation energy. To adjust the performance of SZO RRAM, the effects of dopants (Y, V, Nb and Ta) were revealed according to their modifications on the diffusion of V2+O. We found that doping of Y or V has the most significant effect on the performance of RRAM devices. Furthermore, for dopants with various numbers of valence electrons and atomic radius, general design principles were proposed based on their different effects on the RRAM characteristics. Our results will guide the experimentations and pave a new way for the optimization of RRAM devices.

Oxygen is widely used to tune the performance of chalcogenide phase-change materials in the usage of phase-change random access memory (PCRAM), which is considered as the most promising next-generation non-volatile memory. However, the microscopic role of oxygen in the write–erase process, i.e., the reversible phase transition between crystalline and amorphous state of phase-change materials, remains unclear. Using oxygen doped GeTe as an example, this study unravels the role of oxygen at the atomic scale by means of ab initio total energy calculations and ab initio molecular dynamics simulations. Our main finding is that after the amorphization and the subsequent re-crystallization process simulated by ab initio molecular dynamics, oxygen will drag one Ge atom out of its lattice site and both atoms will stay in the interstitial region near the Te vacancy that was originally occupied by oxygen, forming a “dumbbell-like” defect (O–VTe–Ge), which is in sharp contrast to the results of ab initio total energy calculations at 0 K showing that the oxygen prefers to substitute Te in crystalline GeTe. This specific defect configuration leads to a slower crystallization speed and hence the improved data retention of oxygen doped GeTe. Moreover, we find that the local oxygen configuration will increase the effective mass of the carrier and thus increase the resistivity of GeTe. Our results unravel the microscopic mechanism of the oxygen-doping optimization of GeTe phase-change material, and the present reported mechanism can be applied to other oxygen doped ternary chalcogenide phase-change materials.

Identifying suitable photocatalysts for photocatalytic water splitting to produce hydrogen fuel via sunlight is an arduous task by the traditional trial-and-error method. Thanks to the progress of density functional theory, one can nowadays accelerate the process of finding candidate photocatalysts. In this work, by ab initio calculations, we investigated 48 two-dimensional (2D) transition metal carbides also referred to as MXenes to understand their photocatalytic properties. Our results highlight 2D Zr2CO2 and Hf2CO2 as the candidate single photocatalysts for possible high efficiency photocatalytic water splitting. A significant property of 2D Zr2CO2 and Hf2CO2 is that they exhibit unexpectedly high and directionally anisotropic carrier mobility, which may effectively facilitate the migration and separation of photogenerated electron–hole pairs. Meanwhile, these two MXenes also exhibit very good optical absorption performance in the wavelength ranging approximately from 300 to 500 nm. The stability of 2D Zr2CO2 and Hf2CO2 in liquid water is expected to be good based on ab initio molecular dynamics simulations. Finally, the adsorption and decomposition of water molecules on the 2D Zr2CO2 surface and the subsequent formation process of hydrogen were studied, which contributes to the unravelling of the micro-mechanism of photocatalytic hydrogen production on MXenes. Our findings will open a new way to facilitate the discovery and application of MXenes for photocatalytic water splitting.

Two-dimensional (2D) transition metal carbides/nitrides Mn+1Xn labeled as MXenes are attracting increasing interest due to promising applications as Li-ion battery anodes and hybrid electro-chemical capacitors. To realize MXenes devices in future flexible practical applications, it is necessary to have a full understanding of the mechanical properties of MXenes under deformation. In this study, we extensively investigated the stress–strain curves and the deformation mechanisms in response to tensile stress by first principles calculations using 2D Tin+1Cn (n = 1, 2 and/or 3) as examples. Our results show that 2D Ti2C can sustain large strains of 9.5%, 18% and 17% under tensions of biaxial and uniaxial along x and y, respectively, which respectively increase to 20%, 28% and 26.5% for 2D Ti2CO2 due to surface functionalizing oxygen, which is much better than graphene (15% biaxial). The failure of 2D Tin+1Cn MXene is due to the significant collapse of the surface atomic layer; however, surface functionalization could slow down this collapse, resulting in the improvement of mechanical flexibility. We have also discussed the critical strains and Young's modulus of 2D Tin+1Cn and Tin+1CnO2. Our results provide an insight into the microscopic deformation mechanism of MXenes and hence benefit their applications in flexible electronic devices.

Development of novel van der Waals (vdW) heterostructures from various two-dimensional (2D) materials shows unprecedented possibilities by combining the advantageous properties of their building layers. In particular, transforming the vdW heterostructures from type-I to type-II is of great interest and importance to achieve efficient charge separation in photocatalytic, photovoltaic, and optoelectronic devices. In this work, by means of ab initio calculations, we have systematically investigated the electronic structures, optical properties, and mechanical properties of MXene/Blue Phosphorene (BlueP) vdW heterostructures under various deformations. We highlight that, under strain, the type-I heterostructures can be transformed to type-II with their conduction band minimum (CBM) and valence band maximum (VBM) separated in different layers. Interestingly, the locations of the CBM or VBM in MXene/BlueP vdW heterostructures can also be reversed by compressive or tensile strain between the building layers, which indicates that either layer can be utilized as an electron donor or acceptor by varying its deformation conditions. Meanwhile, this compressive (tensile) strain can also induce a red (blue) shift in the optical absorption spectra of MXene/BlueP vdW heterostructures. Finally, our results on the mechanical flexibility and deformation mechanism of MXene/BlueP vdW heterostructures suggest their great long-term stability as well as promising applications in flexible devices. We believe that our findings will open a new way for the modulation and development of vdW heterostructures in flexible optical/electronic devices.