Asymmetric XMoGeY2 (X = S, Se, Te; Y = N, P, As) monolayers as potential flexible materials for nano piezoelectric devices and nanomedical sensors.

Highly efficient nano piezoelectric devices and nanomedical sensors are in great demand for high-performance piezoelectric materials. In this work, we propose new asymmetric XMoGeY2 (X = S, Se, Te; Y = N, P, As) monolayers with excellent piezoelectric properties, dynamic stability and flexible elastic properties. The piezoelectric coefficients (d11) of XMoGeY2 monolayers range from 2.92 to 8.19 pm V-1. Among them, TeMoGeAs2 exhibits the highest piezoelectric coefficient (d11 = 8.19 pm V-1), which is 2.2 times higher than that of common 2D piezoelectric materials such as 2H-MoS2 (d11 = 3.73 pm V-1). Furthermore, all XMoGeY2 monolayers demonstrate flexible elastic properties ranging from 96.23 to 253.70 N m-1. Notably, TeMoGeAs2 has a Young's modulus of 96.23 N m-1, which is only one-third of that of graphene (336 N m-1). The significant piezoelectric coefficients of XMoGeY2 monolayers can be attributed to their asymmetric structures and flexible elastic properties. This study provides valuable insights into the potential applications of XMoGeY2 monolayers in nano piezoelectric devices and nanomedical sensors.

High-Performance and Polarization-Sensitive Imaging Photodetector Based on WS2 /Te Tunneling Heterostructure.

Next-generation imaging systems require photodetectors with high sensitivity, polarization sensitivity, miniaturization, and integration. By virtue of their intriguing attributes, emerging 2D materials offer innovative avenues to meet these requirements. However, the current performance of 2D photodetectors is still below the requirements for practical application owing to the severe interfacial recombination, the lack of photoconductive gain, and insufficient photocarrier collection. Here, a tunneling dominant imaging photodetector based on WS2 /Te heterostructure is reported. This device demonstrates competitive performance, including a remarkable responsivity of 402 A W-1 , an outstanding detectivity of 9.28 × 1013 Jones, a fast rise/decay time of 1.7/3.2 ms, and a high photocurrent anisotropic ratio of 2.5. These outstanding performances can be attributed to the type-I band alignment with carrier transmission barriers and photoinduced tunneling mechanism, allowing reduced interfacial trapping effect, effective photoconductive gains, and anisotropic collection of photocarriers. Significantly, the constructed photodetector is successfully integrated into a polarized light imaging system and an ultra-weak light imaging system to illustrate the imaging capability. These results suggest the promising application prospect of the device in future imaging systems.

Pressure-Induced Phase Diagram and Electronic Structure Evolves during the Insulator-Metal Transition of Bulk BiFeO3.

BiFeO3 is the most widely known multiferroic at room temperature, possessing both ferroelectricity and antiferromagnetism. It has high Curie temperature and Néel temperature, i.e., 1103 and 643 K, respectively. Despite these unique properties, the pressure-induced phase diagram of bulk BiFeO3 has remained controversial. Based on the ab initio evolutionary algorithm, we systematically searched for the potential stable structures of bulk BiFeO3 at 0-50 GPa. It is identified that there are five pressure-induced phase transition sequences R3c-G-AFM →(5GPa) C2/m-G-AFM →(15GPa) Pnma-G-AFM →(24GPa) Pnma-FM →(35GPa) Imma-FM →(45GPa) Cmcm-FM, which provided a comprehensive pressure-induced phase diagram. As the pressure increases, we discovered an interesting phenomenon: a pressure-induced magnetic sequence transition, i.e., BiFeO3 transitions from an antiferromagnetic to a ferromagnetic sequence. Concurrently, the electronic structure evolves during the insulator-metal transition, influenced not only by the pressure but also by the phase transition. Our research has elucidated the long-standing question of the phase transition sequence of the BiFeO3 system under pressure and provided theoretical support for the insulator-metal transition.

Layered Li-Co-B as a Low-Potential Anode for Lithium-Ion Batteries.

An anode material is one of the key factors affecting the capacity, cycle, and rate (fast charge) performance of lithium-ion batteries. Using the adaptive genetic algorithm, we found a new ground-state Li2CoB and two metastable states LiCoB and LiCo2B2 in the Li-Co-B system. The Li2CoB phase is a lithium-rich layered structure, and it has an equivalent lithium-ion migration barrier (0.32 eV) in addition to the lower voltage platform (0.05 V) than graphite, which is the most important commercial anode material at present. Moreover, we analyzed the mechanism of delithiation for Li2CoB and found that it maintained metallicity in the process of delithiation, indicating its good conductivity as an electrode material. Therefore, it is an excellent potential anode material for lithium-ion batteries. Our work provides a promising theoretical basis for the experimental synthesis of Li-Co-B and similar new materials.

Prediction of the hardest BiFeO3 from first-principles calculations.

BiFeO3 is the only material with ferroelectric Curie temperature and Néel temperature higher than room temperature, making it one of the most well-studied multiferroic materials. Based on an ab initio evolutionary algorithm, we predicted a new cubic C-type antiferromagnetic structure (Fd3̄m-BiFeO3) at ambient pressure. It was found that Fd3̄m-BiFeO3 is the hardest BiFeO3 (Vickers hardness ∼ 9.12 GPa), about 78% harder than R3c-BiFeO3 (the well-known multiferroic material), which contributes to extending the life of BiFeO3 devices. In addition, Fd3̄m-BiFeO3 has the largest shear modulus (83.74 GPa) and the largest Young's modulus (214.72 GPa). Besides, we found an interesting phenomenon that among the common multiferroic materials (BiFeO3, BaTiO3, PbTiO3, SrRuO3, KNbO3, and BiMnO3), Pnma-BiMnO3 has the largest bulk modulus, and its bulk modulus is about 15% larger than that of Fd3̄m-BiFeO3. However, its Vickers hardness (4.47 GPa) is much smaller than that of Fd3̄m-BiFeO3. This is because the Vickers hardness is proportional to the shear modulus and the shear modulus of Fd3̄m-BiFeO3 is larger than that of Pnma-BiMnO3. This work provides a deeper and more comprehensive understanding of BiFeO3.

Activating HfX2 (X = S, Se and Te) for the hydrogen evolution reaction by introducing defects: a first-principles study.

An ideal catalyst should have a relative hydrogen adsorption Gibbs free energy (ΔGH) close to zero [J. K. Nørskov, et al., J. Electrochem. Soc., 2005, 152, J23]. However, most of the known catalysts cannot reach this standard. Based on first-principles calculations, we studied the hydrogen evolution reaction (HER) catalytic performance of pristine and defect (including vacancy and heteroatom doping) structures in terms of its ΔGH. We found that the ΔGH values of Co-doped HfS2 and P-doped HfSe2 are extremely close to zero, even closer than that of Pt (111), indicating that they are excellent catalysts. Moreover, we found that the source of the HER catalytic performance of Co-doped HfS2 is the reduction of electron accumulation of the active site S atom. Our work provides two potential ideal catalysts and provides guidance for the experimental group to search for suitable catalysts.

Pressure-induced novel ZrN4 semiconductor materials with high dielectric constants: a first-principles study.

In addition to Zr3N4 and ZrN2 compounds, zirconium nitrides with a rich family of phases always exhibit metal phases. By employing an evolutionary algorithm approach and first-principles calculations, we predicted seven novel semiconductor phases for the ZrN4 system at 0-150 GPa. Through calculating phonon dispersions, we identified four dynamically stable semiconductor structures under ambient pressure, namely, α-P1̄, ß-P1̄, γ-P1̄, and ß-P1 (with bandgaps of 1.03 eV, 1.10 eV, 2.33 eV, and 1.49 eV calculated using the HSE06 hybrid density functional, respectively). The calculated work functions and dielectric functions show that the four dynamically stable semiconductor structures are all high dielectric constant (high-k) materials, among which the ß-P1̄ phase has the largest static dielectric constant (3.9 times that of SiO2). Furthermore, we explored band structures using the HSE06 functional and density of states (DOS) and the response of bandgaps to pressure using the PBE functional for the four new semiconductor configurations. The results show that the bandgap responses of the four structures exhibit significant differences when hydrostatic pressure is applied from 0 to 150 GPa.

High-Throughput Screening of Strong Electron-Phonon Couplings in Ternary Metal Diborides.

We perform a high-throughput screening on phonon-mediated superconductivity in a ternary metal diboride structure with alkali, alkaline earth, and transition metals. We find 17 ground states and 78 low-energy metastable phases. From fast calculations of zone-center electron-phonon coupling, 43 compounds are revealed to show electron-phonon coupling strength higher than that of MgB2. An anticorrelation between the energetic stability and electron-phonon coupling strength is identified. We suggest two phases, i.e., Li3ZrB8 and Ca3YB8, to be synthesized, which show reasonable energetic stability and superconducting critical temperature.

Enhanced electronic and optical properties of multi-layer arsenic via strain engineering.

Solar cell is a kind of devices for renewable and environmentally friendly energy conversion. One of the important things for solar cells is conversion efficiency. While much attention has been drawn to improving efficiency, the role of strain engineering in two-dimensional materials is not yet well-understood. Here, we propose aPmc21-As monolayer that can be used as a solar cell absorbing material. The bandgap of single-layerPmc21-As can be tuned from 1.83 to 0 eV by applying tensile strain, while keeping the direct bandgap characteristic. Moreover, it has high light absorption efficiency in the visible and near-infrared regions, which demonstrates a great advantage for improving the conversion efficiency of solar cells. Based on the tunable electronic and optical properties, a novel design strategy for solar cells with a wide absorption range and high absorption efficiency is suggested. Our results not only have direct implication in strain effect on two-dimensional materials, but also give a possible concept for improving the solar cell performance.

Intrinsic type-II van der Waals heterostructures based on graphdiyne and XSSe (X = Mo, W): a first-principles study.

Typical transition-metal dichalcogenides (TMDs) and graphdiyne (GDY) often form type-I heterojunctions, which will limit their applications in optoelectronic devices. Here, type-II heterojunctions based on GDY and TMDs are constructed by introducing Janus structures. An intrinsic type-II heterojunction is presented when the GDY is in contact with a Se-terminated layer, but a type-I heterojunction would appear when it is in contact with the S-terminated surface. Such a difference in band alignment can be attributed to the interaction between the dipole moment formed by the Janus structure and the graphdiyne layer. Furthermore, for heterojunctions in contact with the S-terminated layer, they can be converted into type-II heterojunctions by a small external electric field (for WSSe, only 0.05 V A-1 is required). This approach can suggest a convenient design strategy for the application of graphdiyne in a wider range of applications.

Temperature-driven phase transition of Ti2CN from first-principles calculations.

First-principles evolutionary simulations are used to predict the stable compound of Ti2CN. Body-centered tetragonal I41/amd-Ti2CN is found to be more energetically favorable than the other Ti2CN compounds at 0 K. The phase stability as a function of temperature for all relevant competing Ti2CN phases is investigated by means of first-principles calculations and quasi-harmonic approximation. Our calculations predict that I41/amd-Ti2CN undergoes a phase transition to P42/mmc at 1698 K and then to R3̄m at 1872 K. The different effects from the harmonic, electronic and quasi-harmonic contributions to the Gibbs free energy for I41/amd, P42/mmc and R3̄m phases are compared and analyzed. It is found that both the electronic and quasi-harmonic contributions to the Gibbs free energies significantly affect the phase transition curve of Ti2CN. The calculated temperature-dependent lattice parameter is carefully compared with the previous experimental results. We also provide important thermodynamic quantities as the volumetric expansion coefficient and isothermal bulk modulus and discuss their temperature dependence.

Semiconductors with a chiral crystal structure in group IVB transition metal pernitrides.

Group IVB transition metal (TM) nitrides rarely exhibit the semiconductor phase, except for TM3N4 (TM = Ti, Zr, and Hf) compounds. In this study, using the ab initio calculations based on density functional theory, we report two chiral crystal structures, namely P3121 and P3221, of TMN2, which are dynamically stable at ambient pressure. Unlike conventional metal phases of transition metal dinitrides, the P3121 and P3221 configurations exhibit intriguing semiconductor properties (with bandgaps of 1.076 eV, 1.341 eV, and 1.838 eV for TiN2, ZrN2, and HfN2, respectively). The mechanism of metal-to-semiconductor transition from the I4/mcm to P3121 phase is deeply explored by investigating their crystal structure and electronic structures. When hydrostatic pressure is applied from 0 GPa to 200 GPa, the bandgaps of the P3121 phase of TiN2, ZrN2, and HfN2 exhibit a different response, which is related to the orbital contribution at the conduction band minimum (CBM) and valence band maximum (VBM) and the lattice constants. Furthermore, according to the calculated mechanical properties, P3121 and P3221 phases exhibit higher bulk and shear moduli than the semiconductor phases of c-Zr3N4 and c-Hf3N4 in the corresponding systems.

New multiferroic BiFeO3 with large polarization.

BiFeO3 is one of the most widely studied multiferroic materials, because of its large spontaneous polarization at room temperature, as well as ferroelasticity and antiferromagnetism. Using an ab initio evolutionary algorithm, we found two new dynamically stable BiFeO3 structures (P63 and P6322) at ambient pressure. Their energy is only 0.0662 and 0.0659 eV per atom higher than the famous R3c-BiFeO3, and they have large spontaneous polarization, i.e., 71.82 µC cm-2 and 86.06 µC cm-2, respectively. The spontaneous polarization is caused by the movement of the Bi3+ atom along the [001] direction and mainly comes from the 6s electron of Bi3+. Interestingly, there is no lone pair electron of Bi3+, which is different from R3c-BiFeO3. The new structures have the same magnetic configurations as R3c-BiFeO3 (G-type antiferromagnetism), but they are characterized by one-dimensional channels linked by a group of two via surface-sharing oxygen octahedra. Due to the similarity of the two structures, both of them have indirect bandgap structures, and the bandgaps are 2.62 eV and 2.60 eV, respectively. This work not only broadens the structural diversity of BiFeO3 but also has constructive significance for the study of spontaneous polarization of new structures of multiferroic materials.

New two-dimensional arsenene polymorph predicted by first-principles calculation: robust direct bandgap and enhanced optical adsorption.

In this work, we predict a new polymorph of 2D monolayer arsenic. This structure, namedδ-As, consists of a centrosymmetric monolayer, which is thermodynamically and kinetically stable. Distinctly different from the previously predicted monolayer arsenic with an indirect bandgap, the new allotrope exhibits a direct bandgap characteristic. Moreover, while keeping the direct bandgap unchanged, the bandgap of monolayerδ-As can be adjusted from 1.83 eV to 0 eV by applying zigzag-direction tensile strain, which is pronounced an advantage for solar cell and photodetector applications.

A first principles study of p-type doping in two dimensional GaN.

Similar to most semiconductors, low-dimensional GaN materials also have the problem of asymmetric doping, that is, it is quite difficult to form p-type conductivity compared to n-type conductivity. Here, we have discussed the geometry, structure, and electronic defect properties of a two-dimensional graphene-like gallium nitride (g-GaN) monolayer belonging to the group III-V compounds, doped with different elements (In, Mg, Zn) at the Ga site. Based on first principles calculations, we found that substituting Ga (low concentration impurities) with Mg would be a better choice for fabricating a p-type doping semiconductor under N-rich conditions, which is essential for understanding the properties of impurity defects and intrinsic defects in the g-GaN monolayer (using the "transfer to real state" model). Moreover, the g-GaN monolayer is dynamically stable and can remain stable even in high-temperature conditions. This research provides insight for increasing the hole concentration and preparing potential high-performance optoelectronic devices using low-dimensional GaN materials.

Factors affecting the negative Poisson's ratio of black phosphorus and black arsenic: electronic effects.

Negative Poisson's ratio (NPR) materials (when stretched longitudinally, the thickness of these materials increases along the lateral direction) are widely used in engineering because of their good resistance to shear, denting, and fracture. Observance of a negative Poisson's ratio (NPR) in two-dimensional (2D) single-layer materials presently has two explanations. The first, from mechanical principles, is that it derives from the presence of a special structure (hinge structure), such as in single-layer black phosphorus (BP) or black arsenic (ß-As). The second, from electronic effects, is that it derives from (non-hinge-like) planar honeycomb structures and transition-metal dichalcogenides, MX2. Through first-principle calculations, we show that 2D single-layer materials with a hinge structure also have distinct electronic effects, similar to those observed from 2D planar honeycomb materials. Under strain, electronic effects of Px orbitals lead to the inherent NPR of the 2D single-layer material with a hinge structure. We discuss the influencing factors of the hinge structure on the NPR and demonstrate that the electronic effects inside the hinge structure are the fundamental factor in determining the inherent NPR.

Two-dimensional arsenene polymorph beyond the auxetic foam: high mechanical sensitivity and large, negative NPR.

Single-layer δ-As and γ-P have unique atomic arrangement, which belong to the Pmc21 and Pbcm space groups, respectively. Because of the coupling hinge structure, the physical properties of the two materials have obvious anisotropy. In this paper, we report the mechanical properties of the single-layer δ-As and γ-P. That is, their inherent negative Poisson ratio (NPR) is -0.708 and -0.226, respectively. Surprisingly, the absolute value of the NPR of δ-As is approximately 26.2 times greater than that of single-layer black phosphorus (the NPR of single-layer black phosphorus is -0.027), and remains invariant at a certain strain range. Thus, single-layer δ-As will have huge potential applications in nanosensors and electronic wearable devices due to its invariant and large, negative NPR.

Exfoliated Graphite Nanosheets Coating on Nano-grained SnO2/Li4Ti5O12 as a High-Performance Anode Material for Lithium-Ion Batteries.

The SnO2/Li4Ti5O12/C compound was gained via hydrothermal, sintering, and ball milling methods. Nano-grained SnO2/Li4Ti5O12 are homogeneously wrapped in a sheet-like graphite. Li4Ti5O12 possesses cyclic stability and superior rate capacity. Meanwhile, the SnO2/Li4Ti5O12 hybrid can supply abundant active sites for absorption of Li+, mitigate chemical stress in cycling, and prevent the ultrathin graphite nanosheets from stacking. Besides, the sheet-like graphite could reduce volume variation in cycling and reduce transmission distance for the electron or Li+. Therefore, an outstanding electrochemical property of the SnO2/Li4Ti5O12/C composite can be obtained.

Mo-Doped SnO2 Nanoparticles Embedded in Ultrathin Graphite Nanosheets as a High-Reversible-Capacity, Superior-Rate, and Long-Cycle-Life Anode Material for Lithium-Ion Batteries.

A new ternary Mo-SnO2-graphite composite has been constructed via hydrothermal and ball milling. The Mo/SnO2 hybrids were homogeneously dispersed in graphite nanosheets. In the Mo-SnO2-graphite, Mo can inhibit the Sn nanoparticle aggregation, enhance the reversible conversion reaction in lithiation, and improve the electrochemical performance. Consequently, the Mo-SnO2-graphite composite contributes a high capacity of 1317.4 mAh g-1 at 0.2 A g-1 after 200 cycles, remarkable rate property of 514.0 mAh g-1 at 5 A g-1, and long-term cyclic stabilization of 759.0 mAh g-1 after 950 cycles at 1.0 A g-1. With outstanding electrochemical performance and facial synthesis, the ternary Mo-SnO2-graphite is a hopeful anode material for lithium-ion batteries (LIBs).

Unexpected stable phases of tungsten borides.

Tungsten borides are a unique class of compounds with excellent mechanical properties comparable to those of traditional superhard materials. However, the in-depth understanding of these compounds is hindered by the uncertainty of their phase relations and complex crystal structures. Here, we explored the W-B system systematically by ab initio variable-composition evolutionary simulations at pressures from 0 to 40 GPa. Our calculations successfully found all known stable compounds and discovered two novel stable phases, P4[combining macron]21m-WB and P21/m-W2B3, and three nearly stable phases, R3m-W2B5, Ama2-W6B5, and Pmmn-WB5, at ambient pressure and zero Kelvin. Interestingly, P4[combining macron]21m-WB is much harder than the known α and ß phases, while Pmmn-WB5 exhibits the highest hardness. Furthermore, it is revealed that the much debated WB4 becomes stable as the P63/mmc (2 f.u. per unit cell) phase at pressures above ∼1 GPa, not at ambient pressure as reported previously. Our findings provide important insights for understanding the rich and complex crystal structures of tungsten borides, and indicate WB2, WB4, and WB5 as compounds with the most interesting mechanical properties.