Prediction of superconductivity in sandwich XB4 (X = Li, Be, Zn and Ga) films

Borophenes and 2D boron allotropes are metallic and exhibit a BCS superconducting state, unlike graphene. In-plane stretching vibrational modes in bulk MgB2 boron layers induce phonon-mediated superconductivity. However, the effect of stretching vibrational phonon modes on transition temperature (Tc) still requires further investigations. Here, we use first-principles calculations combined with conventional BCS theory to explore the superconducting properties in a series of dynamically stable boron-based sandwich films that have not been realized experimentally. The sandwich films of XB4 (where X = Li, Be, Zn, Ga) are predicted to exhibit good phonon-mediated superconductivity with high Tc values of 25.1 K, 28.7 K, 38.7 K, and 36.2 K, respectively. The origin of the superconducting states is mainly caused by the high metallicity and strong electron-phonon coupling (EPC), which can be attributed to the presence of intercalated atoms within the borophene layers. It is further demonstrated in the XB4 compounds (where X = Li, Be, Zn, Ga) that the pronounced EPC is not solely attributable to the in-plane vibrations of B atoms, but it is also influenced significantly by the out-of-plane vibrations of B atoms. Sandwich (Li,Be,Zn,Ga)B4 films may be a great choice for nanoscale superconductors as the electron-phonon coupling parameter becomes greater than unity, thereby providing a powerful approach for investigating these systems with high critical temperatures.

Machine learning and DFT investigation of CO, CO2 and CH4 adsorption on pristine and defective two-dimensional magnesene

Adsorption study of environmentally toxic small gas molecules on two-dimensional (2D) materials plays a significant role in analyzing the performance of sensors. In this work, density functional theory (DFT) and machine learning (ML) techniques have been employed to systematically study the adsorption properties of CO, CO₂, and CH₄ gas molecules on the pristine and defective planar magnesium monolayer, known as magnesene (2D-Mg). The DFT analysis showed that mechanically robust 2D-Mg retains its metallicity in the presence of both mono and di-vacancy defects. Our observations have shown that 2D-Mg, whether defective or pristine, exhibits distinct adsorption behaviors towards CO, CO₂, and CH₄ gas molecules, including varying chemisorption and physisorption, charge transfer, and distance from the gas molecules. When analyzing the recovery time of gas molecules at room temperature, it is clear that adsorption energy has a direct correlation with the adsorption-desorption cycles, and CH₄ possesses an ultra-low recovery time (15.27 ps) compared to CO₂ (1.04 ns) and CO (0.90 μs) molecules. The analysis showed that defects do not have a significant impact on the work function of 2D-Mg. However, the work function decreased upon adsorption of CH₄, resulting in improved sensitivity due to changes in the electronic properties. Additionally, we explored supervised ML regression models to evaluate their ability to act as a surrogate for the DFT-based adsorption energy calculation. Using both system statistics and smooth overlap of atomic position (SOAP)-based featurization, we observed that adsorption energies can be predicted with a mean absolute error of 0.10 eV.

Superconductivity of monolayer functionalized biphenylene with Dirac cones

Monolayer biphenylene is a new two-dimensional (2D) carbon allotrope, which has been experimentally synthesized and theoretically predicted to show superconductivity. In this work, we investigate functionalized biphenylene with the adsorption of Li. The superconducting critical temperature (T_c) can be pushed from 0.59 K up to 3.91 K after Li adsorption. Our calculations confirm that the adsorption pushes the peak showing a high electronic density of states closer to the Fermi level, which usually leads to a larger T_c. Furthermore, the application of biaxial tensile strain can soften phonons and further enhance the T_c up to 15.86 K in Li-deposited biphenylene. Interestingly, a pair of type-II Dirac cones below the Fermi level has been observed, expanding the range of Dirac materials. It suggests that monolayer biphenylene deposited with Li may be a material with potential applications and improves the understanding of Dirac-type superconductors.

Magnesene: a theoretical prediction of a metallic, fast, high-capacity, and reversible anode material for sodium-ion batteries

Sodium-ion batteries (SIBs) have attracted great attention owing to their low cost and inherent safety. High-performance anode materials for SIBs should possess intrinsically metallic characteristic and be composed of non-toxic, earth abundant, and lightweight elements. We predict a two-dimensional Mg material (named magnesene) to be an excellent anode material, which can meet these design requirements. It is demonstrated to be stable in terms of the cohesive energy, phonon spectrum, ab initio molecular dynamics simulation, and elastic constants. The magnesene monolayer exhibits good SIB performances, including a high storage capacity of 551.3 mA h g^-1, low diffusion energy barrier (0.16-0.19 eV), low open-circuit voltage (0.71-0.82 V), and small volume change (4.7%). Moreover, graphene or h-BN on top of magnesene could serve as a protective cover to preserve the performances of pristine magnesene, such as metallicity, strong Na adsorption capability, and fast ionic mobility. These intriguing theoretical findings make magnesene a promising anode material for SIBs.

Ferromagnetic Dirac half-metallicity in transition metal embedded honeycomb borophene

Exploring two-dimensional (2D) ferromagnetic materials with intrinsic Dirac half-metallicity is crucial for the development of next-generation spintronic devices. Based on first-principles calculations, here we propose a simple valence electron-counting rule to design such materials and endow them with good stability and desirable magnetic properties. Taking honeycomb borophene as a prototype, we demonstrate that embedding open-shell transition metal (like Cr) atoms in the hexagonal ring of boron atoms can provide two valence electrons to fully occupy the in-plane σ and out-of-plane π bands of B atoms. The remaining four valence electrons reside in d orbitals that split under C_6v symmetry, yielding a magnetic moment of ∼2 μ_B per Cr atom. The resulting CrB₂ monolayer exhibits a Dirac half-metal band structure, a high Curie temperature of 175 K, and a large out-of-plane magnetic anisotropy energy of 4 meV per Cr simultaneously. Our work establishes a feasible route for the experimental realization of ferromagnetic Dirac half-metallicity in 2D materials and provides new opportunity to realize high-speed devices with low consumption.

Semiconducting MnB5monolayer as a potential photovoltaic material

Exploring new two-dimensional (2D) materials is of great significance for both basic research and practical applications. Although boron can form various 3D and 2D allotropes due to its ease of forming multi-center bonds, the coexistence of honeycomb and kagome boron structures has never been observed in any 2D material yet. In this article we apply first-principle swarm structural searches to predict the existence of a stable MnB₅structure, consisting of a sandwich of honeycomb and kagome borophenes. More interestingly, a MnB₅nanosheet is a semiconductor with a band gap of 1.07 eV and a high optical absorption in a broad band, which satisfies the requirements of a very good photovoltaic material. Upon moderate strain, MnB₅undergoes a conversion from an indirect to a direct band gap semiconductor. The power conversion efficiency of a heterostructure solar cell made of MnB₅is up to 18%. The MnB₅nanosheet shows a robust dynamical and thermal stability, stemming from the presence of intra- and interlayer multi-center σ and π bonds. These characteristics make MnB₅a promising photovoltaic material.

Exploring structural, electronic, and mechanical properties of 2D hexagonal MBenes

A family of two-dimensional (2D) transition metal borides, referred to as MBenes, is recently emerging as novel materials with great potentials in electronic and energy harvesting applications to the field of materials science and technology. Transition metal borides can be synthesized from chemical exfoliation of ternary-layered transition metal borides, known as MAB phases. Previously it has been predicted that thin pristine 2D Sc-, Ti-, Zr-, Hf-, V-, Nb-, Ta-, Mo-, and W-based transition metal borides with hexagonal phase are more stable than their corresponding orthorhombic phase. Here, using a set of first-principles calculations (at absolute zero temperature), we have examined the geometric, dynamic stability, electronic structures, work function, bond strength, and mechanical properties of the hexagonal monolayer of transition metal borides (M= Sc, Ti, Zr, Hf, V, Nb, Ta, Mo, and W) chemically terminated with F, O, and OH. The results of the formation energies of terminated structures imply that the surface terminations could make a strong bond to the surface transition metals and provide the possibility of the development of transition metal borides with those surface terminations. Except for ScBO, which is an indirect bandgap semiconductor, the other transition metal borides are metallic or semimetal. Particularly, TiBF, ZrBF, and HfBF are metallic systems whose band dispersions close to the Fermi level indicate the coexistence of type-I and type-II nodal lines. Our calculated work functions indicate that 2D transition metal borides with OH (O) functionalization obtain the lowest (highest) work functions. The results of the mechanical properties of the considered structures imply that oxygen functionalized transition metal borides exhibit the stiffest mechanical strength with 248 <E(N m^-1) < 348 while non-terminated transition metal borides are generally the weakest systems with 206 <E(N m^-1) < 283.

Flat Boron: A New Cousin of Graphene

The mechanical exfoliation of graphene from graphite provides the cornerstone for the synthesis of other 2D materials with layered bulk structures, such as hexagonal boron nitride, transition metal dichalcogenides, black phosphorus, and so on. However, the experimental production of 2D flat boron is challenging because bulk boron has very complex spatial structures and a rich variety of chemical properties. Therefore, the realization of 2D flat boron marks a milestone for the synthesis of 2D materials without layered bulk structures. The historical efforts in this field, particularly the most recent experimental progress, such as the growth of 2D flat boron on a metal substrate by chemical vapor deposition and molecular beam epitaxy, or liquid exfoliation from bulk boron, are described.

Theoretical prediction of HfB2 monolayer, a two-dimensional Dirac cone material with remarkable Fermi velocity

A novel 2D boron-based Dirac cone material: a HfB₂ monolayer.

Phonon-mediated superconductivity in Mg intercalated bilayer borophenes

Using first-principles calculations, we investigate the structural, electronic and superconducting properties of Mg intercalated bilayer borophenes B_xMgB_x (x = 2-5). Remarkably, B₂MgB₂ and B₄MgB₄ are predicted to exhibit good phonon-mediated superconductivity with a high transition temperature (T_c) of 23.2 K and 13.3 K, respectively, while B₄MgB₄ is confirmed to be more practical based on the analyses of its stability. The densities of states of in-plane orbitals at the Fermi level are found to be dominant at the superconducting transition temperature in Mg intercalated bilayer borophenes, providing an effective avenue to explore Mg-B systems with high T_cs.

Two-dimensional stable transition metal carbides (MnC and NbC) with prediction and novel functionalizations

MnC and NbC monolayers are predicted to be stable and promising for Li-ion battery, by functionalization, they exhibit half-metallic property and quantum spin Hall effect, respectively.

Boron Triangular Kagome Lattice with Half-Metallic Ferromagnetism

Based on the first-principles evolutionary materials design, we report a stable boron Kagome lattice composed of triangles in triangles on a two-dimensional sheet. The Kagome lattice can be synthesized on a silver substrate, with selecting Mg atoms as guest atoms. While the isolated Kagome lattice is slightly twisted without strain, it turns into an ideal triangular Kagome lattice under tensile strain. In the triangular Kagome lattice, we find the exotic electronic properties, such as topologically non-trivial flat band near the Fermi energy and half-metallic ferromagnetism, and predict the quantum anomalous Hall effect in the presence of spin-orbit coupling.

Predicting a graphene-like WB4 nanosheet with a double Dirac cone, an ultra-high Fermi velocity and significant gap opening by spin–orbit coupling

A stable WB₄ monolayer can deliver a sizable bandgap by spin–orbit coupling whilst maintaining high carrier mobility.

Stable sandwich structures of two-dimensional iron borides FeBxalloy: a first-principles calculation

Multilayer iron borides FeB_x(x= 4, 6, 8, 10) are wide-band-gap semiconductors; the electronic and optical properties of these semiconductors may be modulated by biaxial strains.

Boron based two-dimensional crystals: theoretical design, realization proposal and applications

The successful realization of free-standing graphene and the various applications of its exotic properties have spurred tremendous research interest for two-dimensional (2D) layered materials. Besides graphene, many other 2D materials have been successfully produced by experiment, such as silicene, monolayer MoS2, few-layer black phosphorus and so on. As a neighbor of carbon in the periodic table, element boron is interesting and many researchers have contributed their efforts to realize boron related 2D structures. These structures may be significant both in fundamental science and future technical applications in nanoelectronics and nanodevices. In this review, we summarize the recent developments of 2D boron based materials. The theoretical design, possible experimental realization strategies and their potential technical applications are presented and discussed. Also, the current challenges and prospects of this area are discussed.

Transformative Two-Dimensional Array Configurations by Geometrical Shape-Shifting Protein Microstructures

Two-dimensional (2D) geometrical shape-shifting is prevalent in nature, but remains challenging in man-made "smart" materials, which are typically limited to single-direction responses. Here, we fabricate geometrical shape-shifting bovine serum albumin (BSA) microstructures to achieve circle-to-polygon and polygon-to-circle geometrical transformations. In addition, transformative two-dimensional microstructure arrays are demonstrated by the ensemble of these responsive microstructures to confer structure-to-function properties. The design strategy of our geometrical shape-shifting microstructures focuses on embedding precisely positioned rigid skeletal frames within responsive BSA matrices to direct their anisotropic swelling under pH stimulus. This is achieved using layer-by-layer two photon lithography, which is a direct laser writing technique capable of rendering spatial resolution in the sub-micrometer length scale. By controlling the shape, orientation and number of the embedded skeletal frames, we have demonstrated well-defined arc-to-corner and corner-to-arc transformations, which are essential for dynamic circle-to-polygon and polygon-to-circle shape-shifting, respectively. We further fabricate our shape-shifting microstructures in periodic arrays to experimentally demonstrate the first transformative 2D patterned arrays. Such versatile array configuration transformations give rise to structure-to-physical properties, including array porosity and pore shape, which are crucial for the development of on-demand multifunctional "smart" materials, especially in the field of photonics and microfluidics.