Room-Temperature Half-Metallicity in La(Mn,Zn)AsO Alloy via Element Substitutions

Exploring half-metallic materials with high Curie temperature, wide half-metallic gap, and large magnetic anisotropy energy is one of the effective solutions to develop high-performance spintronic devices. Using first-principles calculations, we design a practicable half-metal based on a layered La­(Mn_0.5Zn_0.5)­AsO alloy via element substitutions. At its ground state, the pristine La­(Mn_0.5Zn_0.5)­AsO alloy is an antiferromagnetic semiconductor. Either hole doping via (Ca²⁺/Sr²⁺,La³⁺) substitutions or electron doping via (H^–/F^–,O^2–) substitutions in the [LaO]⁺ layer induce half-metallicity in the La­(Mn_0.5Zn_0.5)­AsO alloy. The half-metallic gap is as large as 0.74 eV. Monte Carlo simulations based on the Ising model predict a Curie temperature of 475 K for 25% Ca doping and 600 K for 50% H doping, respectively. Moreover, the quasi two-dimensional structure endows the doped La­(Mn,Zn)­AsO alloy a sizable magnetic anisotropy energy with the magnitude of at least one order larger than those of Fe, Co, and Ni bulks

Droplet Oscillation as an Arbitrary Waveform Generator

Oscillating droplets and bubbles have been developed into a novel experimental platform for a wide range of analytical and biological applications, such as digital microfluidics, thin film, biophysical simulation, and interfacial rheology. A central effort of developing any droplet-based experimental platform is to increase the effectiveness and accuracy of droplet oscillations. Here, we developed a novel system of droplet-based arbitrary waveform generator (AWG) for feedback-controlling single-droplet oscillations. This AWG was developed through closed-loop axisymmetric drop shape analysis and based on the hardware of constrained drop surfactometry. We have demonstrated the capacity of this AWG in oscillating the volume and surface area of a millimeter-sized droplet to follow four representative waveforms, sine, triangle, square, and sawtooth. The capacity of oscillating the surface area of a droplet across the frequency spectrum makes the AWG an ideal tool for studying interfacial rheology. The AWG was used to determine the surface dilational modulus of a commonly studied nonionic surfactant, dodecyldimethylphosphine oxide. The droplet-based AWG developed in this study is expected to achieve accuracy, versatility, and applicability in a wide range of research areas, such as thin film and interfacial rheology

Electronic Stability of Phosphine-Protected Au₂₀ Nanocluster: Superatomic Bonding

A recent experiment reported that a newly crystallized phosphine-protected Au₂₀ nanocluster [Au₂₀(PPhy₂)₁₀Cl₄]­Cl₂ [PPhpy₂ = bis­(2-pyridyl)­phenylphosphine] owns a very stable Au₂₀ core, but the number of valence electrons of the Au₂₀ core is 14e, which is not predicted by the superatom model. So we apply the density functional theory to further study this cluster from its molecular orbital and chemical bonding. The results suggest that the Au₂₀⁽⁺⁶⁾ core is an analogue of the F₂ molecule based on the super valence bond model, and the 20-center–14-electron Au₂₀⁽⁺⁶⁾ core can be taken as a superatomic molecule bonded by two 11-center–7-electron superatoms, where the two 11c superatoms share two Au atoms and two electrons to meet an 8-electron closed shell for each. The electronic shell closure enhances the stability of the Au₂₀ core, besides the PN bridges. Exceptionally, the theoretical HOMO–LUMO gap (1.03 eV) disagrees with the experimental value (2.24 eV), and some possible reasons for this big difference are analyzed in this paper

Two-Dimensional Stoichiometric Boron Oxides as a Versatile Platform for Electronic Structure Engineering

Oxides of two-dimensional (2D) atomic crystals have been widely studied due to their unique properties. In most 2D oxides, oxygen acts as a functional group, which makes it difficult to control the degree of oxidation. Because borophene is an electron-deficient system, it is expected that oxygen will be intrinsically incorporated into the basal plane of borophene, forming stoichiometric 2D boron oxide (BO) structures. By using first-principles global optimization, we systematically explore structures and properties of 2D BO systems with well-defined degrees of oxidation. Stable B–O–B and OB₃ tetrahedron structure motifs are identified in these structures. Interesting properties, such as strong linear dichroism, Dirac node-line (DNL) semimetallicity, and negative differential resistance, have been predicted for these systems. Our results demonstrate that 2D BO represents a versatile platform for electronic structure engineering via tuning the stoichiometric degree of oxidation, which leads to various technological applications

Three-Dimensional Covalently Linked Allotropic Structures of Phosphorus

The discovery of new phosphorus allotropes has attracted continuous attention over recent decades, partly due to the importance of phosphorus in life and their fantastic structural diversity. Generally, phosphorus allotropes consist of covalently linked substructures, stacked together with van der Waals interactions, and a few phosphorus allotropes possess three-dimensional covalently linked structures only at high pressure. On the basis of first-principles calculations, five new phosphorus allotropes with three-dimensional covalently linked structures are predicted by assembling phosphorus units at ambient pressure, which are energetically more favorable than white phosphorus. Particularly, three of them share the same structures as those of previously reported three-dimensional nitrogen allotropes. These new allotropes are semiconductors with band gaps ranging from 0.52 to 2.39 eV, and the Young’s modulus varies from 39 to 72 GPa. The structural stability of the new phosphorus allotropes are confirmed with a phonon spectrum and Born–Oppenheimer molecular dynamic simulation at temperatures up to 700 K. Our findings enrich the phosphorus allotrope family with three-dimensional covalently linked structures at ambient pressure and versatile electronic properties

Tetrahedral Au₁₇⁺: A Superatomic Molecule with a Au₁₃ FCC Core

A unique tetrahedral structure of Au₁₇⁺ (<i>T</i>_d) is found by using first-principles global optimization, which lies 0.40 eV lower in energy than the previously known structure and has a fairly large HOMO–LUMO gap (1.46 eV) at the TPSS/def2-TZVP level. For neutral Au₁₇, this tetrahedral structure is distorted to <i>D</i>_2d symmetry but is also 0.18 eV lower in energy than the previous flat cage structure. Au₁₇⁺ (<i>T</i>_d) has a FCC Au₁₃ octahedral core, and the other four gold atoms are above its four triangular faces. Magic electronic stability of the cluster is explained by the super valence bond model, of which it can be seen as a superatomic molecule in the electronic structure. Moreover, the cluster can also be viewed as a network of eight 2e-superatoms. This Au₁₇⁺ cluster mimics the behavior of the Au₂₀ pyramid, known as a unique one among the family of gold clusters since its discovery in 2003, in electronic structures

Droplet Oscillation as an Arbitrary Waveform Generator

Oscillating droplets and bubbles have been developed into a novel experimental platform for a wide range of analytical and biological applications, such as digital microfluidics, thin film, biophysical simulation, and interfacial rheology. A central effort of developing any droplet-based experimental platform is to increase the effectiveness and accuracy of droplet oscillations. Here, we developed a novel system of droplet-based arbitrary waveform generator (AWG) for feedback-controlling single-droplet oscillations. This AWG was developed through closed-loop axisymmetric drop shape analysis and based on the hardware of constrained drop surfactometry. We have demonstrated the capacity of this AWG in oscillating the volume and surface area of a millimeter-sized droplet to follow four representative waveforms, sine, triangle, square, and sawtooth. The capacity of oscillating the surface area of a droplet across the frequency spectrum makes the AWG an ideal tool for studying interfacial rheology. The AWG was used to determine the surface dilational modulus of a commonly studied nonionic surfactant, dodecyldimethylphosphine oxide. The droplet-based AWG developed in this study is expected to achieve accuracy, versatility, and applicability in a wide range of research areas, such as thin film and interfacial rheology

Dominant Kinetic Pathways of Graphene Growth in Chemical Vapor Deposition: The Role of Hydrogen

The most popular way to produce graphene nowadays is chemical vapor deposition, where, surprisingly, H₂ gas is routinely supplied even though it is a byproduct itself. In this study, by identifying dominant growing pathways via multiscale simulations, we unambiguously reveal the central role hydrogen played in graphene growth. Hydrogen can saturate the edges of a growing graphene island to some extent, depending on the H₂ pressure. Although graphene etching by hydrogen has been observed in experiment, hydrogen saturation actually stabilizes graphene edges by reducing the detachment rates of carbon-containing species. Such a new picture well explains some puzzling experimental observations and is also instrumental in growth protocol optimization for two-dimensional atomic crystal van der Waals epitaxy

Two-Dimensional Phosphorus Porous Polymorphs with Tunable Band Gaps

Exploring stable two-dimensional (2D) crystalline structures of phosphorus with tunable properties is of considerable importance partly due to the novel anisotropic behavior in phosphorene and potential applications in high-performance devices. Here, 21 new 2D phosphorus allotropes with porous structure are reported based on topological modeling method and first-principles calculations. We establish that stable 2D phosphorus crystals can be obtained by topologically assembling selected phosphorus monomer, dimer, trimer, tetramer, and hexamer. Nine of reported structures are predicted to be more stable than white phosphorus. Their dynamic and thermal stabilities are confirmed by the calculated vibration spectra and Born–Oppenheimer molecular dynamic simulation at temperatures up to 1500 K. These phosphorus porous polymorphs have isotropic mechanic properties that are significantly softer than phosphorene. The electronic band structures calculated with the HSE06 method indicate that new structures are semiconductors with band gaps ranging widely from 0.15 to 3.42 eV, which are tuned by the basic units assembled in the network. Of particular importance is that the position of both conduction and valence band edges of some allotropes matches well with the chemical reaction potential of H₂/H⁺ and O₂/H₂O, which can be used as element photocatalysts for visible-light-driven water splitting

Obtaining Two-Dimensional Electron Gas in Free Space without Resorting to Electron Doping: An Electride Based Design

Nearly free electron (NFE) states are widely existed on atomically smooth surfaces in two-dimensional materials. Since they are mainly distributed in free space, these states can in principle provide ideal electron transport channels without nuclear scattering. Unfortunately, NFE states are typically unoccupied, and electron doping is required to shift them toward the Fermi level and, thus, to be involved in electron transport. Instead of occupying these NFE states, it is more desirable to have intrinsic nucleus-free two-dimensional electron gas in free space (2DEG-FS) at the Fermi level without relying on doping. Inspired by a recently identified electride material, we suggest that Ca₂N monolayer should possess such a 2DEG-FS state, which is then confirmed by our first-principles calculations. Phonon dispersion in Ca₂N monolayer shows no imagery frequency indicating that the monolayer structure is stable. A mechanical analysis demonstrates that Ca₂N bulk exfoliation is feasible to produce a freestanding monolayer. However, in real applications, the strong chemical activity of 2DEG-FS may become a practical issue. It is found that some ambient molecules can dissociatively adsorb on the Ca₂N monolayer, accompanying with a significant charge transfer from the 2DEG-FS state to adsorbates. To protect the 2DEG-FS state from molecule adsorption, we predict that graphane can be used as an effective encapsulating material. A well-encapsulated intrinsic 2DEG-FS state is expected to play an important role in low-dimensional electronics by realizing nuclear scattering free transport