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.

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 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.

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.

Defect-Induced Ultrafast Nonadiabatic Electron-Hole Recombination Process in PtSe2 Monolayer.

Defects are inevitable in two-dimensional materials due to the growth condition, which results in many unexpected changes in materials' properties. Here, we have mainly discussed the nonradiative recombination dynamics of PtSe2 monolayer without/with native point defects. Based on first-principles calculations, a shallow p-type defect state is introduced by a Se antisite, and three n-type defect states with a double-degenerate shallow defect state and a deep defect state are introduced by a Se vacancy. Significantly, these defect states couple strongly to the pristine valence band maximum and lead to the enhancement of the in-plane vibrational Eg mode. Both factors appreciably increase the nonadiabatic coupling, accelerating the electron-hole recombination process. An explanation of PtSe2-based photodetectors with the slow response, compared to conventional devices, is provided by studying this nonradiative transitions process.

Accurately Controlling the Occupation of Eu2+ in Cs(K, Na)3(Li3SiO4)4 to Achieve Narrow-Band Green Emission for Wide Color Gamut Displays.

The development of narrow-band phosphors for wide color gamut displays in multimodal phosphors through selective site occupancy engineering is an important challenge. In this work, by replacing Na ions with K ions in the cyan-green double-band emitting phosphor CsK0.9Na2(Li3SiO4)4: 10%Eu2+, the occupation of Eu2+ in Cs(K, Na)3(Li3SiO4)4 was accurately controlled from occupying three sites of Cs, K1, and Na to occupying only one site of K2/Na. The obtained phosphor CsK1.9Na(Li3SiO4)4: 10%Eu2+ exhibits a single narrow-band green emission at 531 nm (the full width at half-maximum of 46 nm) with excellent thermal stability of luminescence from 80 to 523 K (96.3% @423 K of the intensity of integrated emission at room temperature and 94.9% @300 K of the intensity of integrated emission at 80 K). The white light-emitting diode (wLED) that was fabricated by combining a blue LED chip with this narrow-band green phosphor and red phosphor K2SiF6: Mn4+ presents a satisfactory wide color gamut of 128.1% of the National Television Standards Committee, which demonstrates the important application value of the phosphor in the wide color gamut displays field. This work provides an effective design strategy for exploring narrow-band phosphors through selective site occupancy engineering, which will facilitate the exploration of relevant narrow-band emitters in the future.

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.

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.

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.

Zero Poisson's ratio in single-layer arsenic.

Zero (or near-zero) Poisson's ratio (ZPR) materials have important applications in the field of precision instruments because one of their faces is stable and will not be affected by strain. However, ZPR materials are extremely rare. Here, we report a novel ZPR material, two-dimensional P2/m arsenene, by first principles calculations. Its Poisson's ratio is -0.00021 (strain along zigzag direction), which is smaller than all the known near-zero Poisson's ratio crystal materials, and even 10 times smaller than Me-graphene (0.002). This feature makes it have huge potential applications in the field of precision instruments such as aviation, medicine, and flexible electronic devices. Besides, the band-gap range of P2/m arsenene is 1.420-2.154 eV (the corresponding wavelength is 873-575 nm) under strain from -5% to 5% along the zigzag direction, which is suitable for infrared and visible optoelectronic devices.

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.

The design of dual-switch fluorescence intensity ratio thermometry with high sensitivity and thermochromism based on a combination strategy of intervalence charge transfer and up-conversion fluorescence thermal enhancement.

Currently, the temperature sensing performances of inorganic photoluminescence materials based on fluorescence intensity ratio technology have become a research hotspot in the optical thermometry field due to their non-contact sensing, fast response and high stability. However, several problems have obstructed the development of optical temperature sensing materials, including low sensitivity and narrow temperature measurement ranges. In view of the above dilemma, a new optical thermometer La2Mo3O12:Yb3+,Pr3+ designed based on the combination strategy of intervalence charge transfer and up-conversion fluorescence thermal enhancement was developed. Under excitation at 450 nm, the thermometer can work in a range from 298 to 648 K and the relative sensitivity reaches as high as 2.000% K-1 at 648 K. Under excitation at 980 nm, the thermometer can sense temperature with a wide range from 298 to 748 K and the relative sensitivity reaches as high as 4.325% K-1 at 598 K. A dual-switch optical temperature sensing material with high-sensitivity and a wide temperature measurement range has been successfully developed. Our research design strategies will give inspiration to the research on multi-switch temperature sensing materials with high sensitivity and a wide temperature measurement range.

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.

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.

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.

Anti-twinning in nanoscale tungsten.

Nanomaterials often surprise us with unexpected phenomena. Here, we report a discovery of the anti-twinning deformation, previously thought impossible, in nanoscale body-centered cubic (BCC) tungsten crystals. By conducting in situ transmission electron microscopy nanomechanical testing, we observed the nucleation and growth of anti-twins in tungsten nanowires with diameters less than about 20 nm. During anti-twinning, a shear displacement of 1/3〈111〉 occurs on every successive {112} plane, in contrast to an opposite shear displacement of 1 / 6 〈 1 ¯ 1 ¯ 1 ¯ 〉 by ordinary twinning. This asymmetry in the atomic-scale shear pathway leads to a much higher resistance to anti-twinning than ordinary twinning. However, anti-twinning can become active in nanosized BCC crystals under ultrahigh stresses, due to the limited number of plastic shear carriers in small crystal volumes. Our finding of the anti-twinning phenomenon has implications for harnessing unconventional deformation mechanisms to achieve high mechanical preformation by nanomaterials.

Tuning element distribution, structure and properties by composition in high-entropy alloys.

High-entropy alloys are a class of materials that contain five or more elements in near-equiatomic proportions1,2. Their unconventional compositions and chemical structures hold promise for achieving unprecedented combinations of mechanical properties3-8. Rational design of such alloys hinges on an understanding of the composition-structure-property relationships in a near-infinite compositional space9,10. Here we use atomic-resolution chemical mapping to reveal the element distribution of the widely studied face-centred cubic CrMnFeCoNi Cantor alloy2 and of a new face-centred cubic alloy, CrFeCoNiPd. In the Cantor alloy, the distribution of the five constituent elements is relatively random and uniform. By contrast, in the CrFeCoNiPd alloy, in which the palladium atoms have a markedly different atomic size and electronegativity from the other elements, the homogeneity decreases considerably; all five elements tend to show greater aggregation, with a wavelength of incipient concentration waves11,12 as small as 1 to 3 nanometres. The resulting nanoscale alternating tensile and compressive strain fields lead to considerable resistance to dislocation glide. In situ transmission electron microscopy during straining experiments reveals massive dislocation cross-slip from the early stage of plastic deformation, resulting in strong dislocation interactions between multiple slip systems. These deformation mechanisms in the CrFeCoNiPd alloy, which differ markedly from those in the Cantor alloy and other face-centred cubic high-entropy alloys, are promoted by pronounced fluctuations in composition and an increase in stacking-fault energy, leading to higher yield strength without compromising strain hardening and tensile ductility. Mapping atomic-scale element distributions opens opportunities for understanding chemical structures and thus providing a basis for tuning composition and atomic configurations to obtain outstanding mechanical properties.

Brittle Fracture of 2D MoSe2.

An in situ quantitative tensile testing platform is developed to enable the uniform in-plane loading of a freestanding membrane of 2D materials inside a scanning electron microscope. The in situ tensile testing reveals the brittle fracture of large-area MoSe2 crystals and measures their fracture strength for the first time.