Two-dimensional honeycomb borophene oxide: strong anisotropy and nodal loop transformation

Advances In Borophene: Synthesis, Tunable Properties, and Energy Storage Applications

Monolayer boron nanosheet, commonly known as borophene, has garnered significant attention in recent years due to its unique structural, electronic, mechanical, and thermal properties. This review paper provides a comprehensive overview of the advancements in the synthetic strategies, tunable properties, and prospective applications of borophene, specifically focusing on its potential in energy storage devices. The review begins by discussing the various synthesis techniques for borophene, including molecular beam epitaxy (MBE), chemical vapor deposition (CVD), and chemical methods, such as ultrasonic exfoliation and thermal decomposition of boron-containing precursors. The tunable properties of borophene, including its electronic, mechanical, and thermal characteristics, are extensively reviewed, with discussions on its bandgap engineering, plasmonic behavior, and thermal conductivity. Moreover, the potential applications of borophene in energy storage devices, particularly as anode materials in metal-ion batteries and supercapacitors, along with its prospects in other energy storage systems, such as sodium-oxygen batteries, are succinctly, discussed. Hence, this review provides valuable insights into the synthesis, properties, and applications of borophene, offering much-desired guidance for further research and development in this promising area of nanomaterials science.

Wet-Chemical Synthesis of Elemental 2D Materials

Wet-chemical synthesis refers to the bottom-up chemical synthesis in solution, which is among the most popular synthetic approaches towards functional two-dimensional (2D) materials. It offers several advantages, including cost-effectiveness, high yields,, precious control over the production process. As an emerging family of 2D materials, elemental 2D materials (Xenes) have shown great potential in various applications such as electronics, catalysts, biochemistry,, sensing technologies due to their exceptional/exotic properties such as large surface area, tunable band gap,, high carrier mobility. In this review, we provide a comprehensive overview of the current state-of-the-art in wet-chemical synthesis of Xenes including tellurene, bismuthene, antimonene, phosphorene,, arsenene. The current solvent compositions, process parameters utilized in wet-chemical synthesis, their effects on the thickness, stability of the resulting Xenes are also presented. Key factors considered involves ligands, precursors, surfactants, reaction time, temperature. Finally, we highlight recent advances, existing challenges in the current application of wet-chemical synthesis for Xenes production, provide perspectives on future improvement.

Superhard and Superconducting Bilayer Borophene

Two-dimensional superconductors, especially the covalent metals such as borophene, have received significant attention due to their new fundamental physics, as well as potential applications. Furthermore, the bilayer borophene has recently ignited interest due to its high stability and versatile properties. Here, the mechanical and superconducting properties of bilayer-δ6 borophene are explored by means of first-principles computations and anisotropic Migdal-Eliashberg analytics. We find that the coexistence of strong covalent bonds and delocalized metallic bonds endows this structure with remarkable mechanical properties (maximum 2D-Young's modulus of ~570 N/m) and superconductivity with a critical temperature of ~20 K. Moreover, the superconducting critical temperature of this structure can be further boosted to ~46 K by applied strain, which is the highest value known among all borophenes or two-dimensional elemental materials.

Theoretical prediction on net boroxene as a promising Li/Na-ion batteries anode

Novel two-dimensional (2D) electrode materials have become a new frontier for mining electrode materials for Li-ion batteries (LIBs) and Na-ion batteries (NIBs). Herein, based on first-principles calculations, we present a systematic study on the Li and Na storage behaviors in Calypso-predicted completely flat 2D boron oxide (l-B₂O) with large mesh pores. We start our calculations from geometrical optimization, followed by a performance evaluation of Li/Na adsorption and migration processes. Finally, the specific capacity and average open-circuit voltage are evaluated. Our study reveals that l-B₂O has good electrical conductivity before and after Li/Na adsorption and the Li/Na diffusion barrier height and average open-circuit voltage are both low, which is beneficial to the rate performance and full-cell operation voltage, respectively. Furthermore, it suffers a small lattice change (<1.7%), ensuring good cycling performance. In particular, we find that the Li and Na theoretical specific capacities of l-B₂O can reach up to 1068.5 mA h g^-1 and 712.3 mA h g^-1, respectively, which are almost 2-3 times higher than graphite (372 mA h g^-1). All the above outcomes indicate that 2D l-B₂O is a promising anode material for LIBs and NIBs.

A two-dimensional borophene monolayer with ideal Dirac nodal-line fermions

As a relatively new member of two-dimensional materials, borophene has gained huge interest over the past years, especially in the field of discovering new topological materials, such as Dirac nodal line semimetals. Here, based on first-principles calculations, for the first time, we find a completely flat borophene monolayer (named χ_2/9) with ideal Dirac nodal line states around the Fermi level. A tight-binding model using the Slater-Koster approach is proposed to demonstrate that the unique electronic feature of χ_2/9 that mainly originated from the first-nearest neighbor interactions of the p_z orbitals of boron. According to our symmetry analysis, the Dirac nodal line in χ_2/9 is guaranteed by the out-of-plane mirror or C₂ rotational symmetry and the negligible p_z orbital coupling. The chemical bonding analysis reveals the rare electronic properties of this material, which can be attributed to the multicentered π bonds.

Two -dimensional semimetal AlSb monolayer with multiple nodal-loops and extraordinary transport properties under uniaxial strain

Two-dimensional (2D) nodal-loop semimetal (NLSM) materials have attracted much attention for their high-speed and low-consumption transporting properties as well as their fantastic symmetry protection mechanisms. In this paper, using systematic first-principles calculations, we present an excellent NLSM candidate, a 2D AlSb monolayer, in which the conduction and valence bands cross with each other forming fascinating multiple nodal-loop (NL) states. The NLSM properties of the AlSb monolayer are protected by its glide mirror symmetry, which was confirmed using a symmetry-constrained six-band tight-binding model. The transport properties of the AlSb monolayer under in-plane uniaxial strains are also studied, based on a non-equilibrium Green's function method. It is found that both compressive and tensile strains from -10% to 10% improve the transporting properties of AlSb, and it is interesting to see that flexure configurations are energetically favored when compressive uniaxial strains are applied. Our studies not only provide a novel 2D NLSM candidate with a new symmetry protection mechanism, but also raise the novel possibility for the detection of out-of-plane flexure in 2D semimetal materials.

Prediction of novel two-dimensional Dirac nodal line semimetals in Al2B2 and AlB4 monolayers

Topological semimetal phases in two-dimensional (2D) materials have gained widespread interest due to their potential applications in novel nanoscale devices. Despite the growing number of studies on 2D topological nodal lines (NLs), candidates with significant topological features that combine nontrivial topological semimetal phase with superconductivity are still rare. Herein, we predict Al₂B₂ and AlB₄ monolayers as new 2D nonmagnetic Dirac nodal line semimetals with several novel features. Our extensive electronic structure calculations combined with analytical studies reveal that, in addition to multiple Dirac points, these 2D configurations host various highly dispersed NLs around the Fermi level, all of which are semimetal states protected by time-reversal and in-plane mirror symmetries. The most intriguing NL in Al₂B₂ encloses the K point and crosses the Fermi level, showing a considerable dispersion and thus providing a fresh playground to explore exotic properties in dispersive Dirac nodal lines. More strikingly, for the AlB₄ monolayer, we provide the first evidence for a set of 2D nonmagnetic open type-II NLs coexisting with superconductivity at a rather high transition temperature. The coexistence of superconductivity and nontrivial band topology in AlB₄ not only makes it a promising material to exhibit novel topological superconducting phases, but also a rather large energy dispersion of type-II nodal lines in this configuration may offer a platform for the realization of novel topological features in the 2D limit.

Monoelemental two-dimensional boron nanomaterials beyond theoretical simulations: From experimental preparation, functionalized modification to practical applications

During the past decade, there is an explosive growth of theoretical and computational studies on 2D boron-based nanomaterials. In terms of extensive predictions from theoretical simulations, borophene, boron nanosheets and 2D boron derivatives show excellent structural, electronic, photonic and nonlinear optical characteristics, and potential applications in a wide range of fields. In recent years, previous studies have reported the successful experimental preparations, superior properties, multi-functionalized modifications of various 2D boron and its derivatives, which show many practical applications in significant fields. To further promote the ever-increasing experimental studies, this present review systematically summarizes recent progress on experimental preparation methods, functionalized modification strategies and practical applications of 2D boron-based nanomaterials and multifunctional derivatives. Firstly, this review summarizes the experimental preparation methods, including molecular beam epitaxy, chemical vapor deposition, liquid-phase exfoliation, chemical reaction, and other auxiliary methods. Then, various strategies for functionalized modification are introduced overall, focusing on borophene derivatives, boron-based nanosheets, atom-introduced, chemically-functionalized borophene and boron nanosheets, borophene or boron nanosheet-based heterostructures, and other functionalized 2D boron nanomaterials. Subsequently, various potential applications are discussed in detail, involving energy storage, catalysis conversion, photonics, optoelectronics, sensors, bio-imaging, biomedicine therapy, and adsorption. We comment the state-of-the-art related studies concisely, and also discuss the current status, probable challenges and perspectives rationally. This review is timely, comprehensive, in-depth and highly attractive for scientists from multiple disciplines and scientific fields, and can facilitate further development of advanced functional low-dimensional nanomaterials and multi-functionalized systems toward high-performance practical applications in significant fields.

Electronic, Magnetic, and Optical Performances of Non-Metals Doped Silicon Carbide

The configurations of nine different non-metals doped silicon carbide (NM-SiC) were structured by using the density functional theory (DFT). The magnetic, electronic, and optical properties of each NM-SiC are investigated at the most stable structure with the maximum binding energy. Although the O-, Si-, and S-SiC systems are still non-magnetic semiconductors, the N- and P-SiC systems have the properties of the magnetic semiconductors. The H-, F-, and Cl-SiC systems exhibit the half-metal behaviors, while the B-SiC system converts to magnetic metal. The redistribution of charges occurs between non-metals atoms and adjacent C atoms. For the same doping position, the more charges are transferred, the greater the binding energy of the NM-SiC system. The work function of the NM-SiC systems is also adjusted by the doping of NM atoms, and achieves the minimum 3.70 eV in the P-SiC, just 77.1% of the original SiC. The absorption spectrum of the NM-SiC systems occurs red-shift in the ultraviolet light region, accompanying the decrease of absorption coefficient. These adjustable magnetic, electronic, and optical performances of NM-SiC expand the application fields of two-dimensional (2D) SiC, especially in designing field emission and spintronics devices.

Robust non-integer conductance in disordered 2D Dirac semimetals

We study the conductanceGof 2D Dirac semimetal nanowires at the presence of disorder. For an even nanowire lengthLdetermined by the number of unit cells, we find non-integer values forGthat are independent ofLand persist with weak disorder, indicated by the vanishing fluctuations ofG. The effect is created by a combination of the scattering effects at the contacts (interface) between the leads and the nanowire, an energy gap present in the nanowire for evenLand the topological properties of the 2D Dirac semimetals. Unlike conventional materials the reducedGdue to the scattering at the interface, is stabilized at non-integer values inside the nanowire, leading to a topological phase for weak disorder. For strong disorder the system leaves the topological phase and the fluctuations ofGare increased as the system undergoes a transition/crossover toward the Anderson localized (insulating) phase, via a non-standard disordered phase. We study the scaling and the statistics ofGat these phases. In addition we have found that the effect of robust non-integerGdisappears for oddL, which results in integerG, determined by the number of open channels in the nanowire, due to resonant scattering.

Liquid crystalline 2D borophene oxide for inorganic optical devices

Borophene has been recently proposed as a next-generation two-dimensional material with promising electronic and optical properties. However, its instability has thus far limited its large-scale applications. Here, we investigate a liquid-state borophene analogue with an ordered layer structure derived from two-dimensional borophene oxide. The material structure, phase transition features and basic properties are revealed by using X-ray analysis, optical and electron microscopy, and thermal characterization. The obtained liquid crystal exhibits high thermal stability at temperatures up to 350 °C and an optical switching behaviour driven by a low voltage of 1 V.

Borophene as an emerging 2D flatland for biomedical applications: current challenges and future prospects

Recently, two-dimensional (2D)-borophene has emerged as a remarkable translational nanomaterial substituting its predecessors in the field of biomedical sensors, diagnostic tools, high-performance healthcare devices, super-capacitors, and energy storage devices. Borophene justifies its demand due to high-performance and controlled optical, electrical, mechanical, thermal, and magnetic properties as compared with other 2D-nanomaterials. However, continuous efforts are being made to translate theoretical and experimental knowledge into pragmatic platforms. To cover the associated knowledge gap, this review explores the computational and experimental chemistry needed to optimize borophene with desired properties. High electrical conductivity due to destabilization of the highest occupied molecular orbital (HOMO), nano-engineering at the monolayer level, chemistry-oriented biocompatibility, and photo-induced features project borophene for biosensing, bioimaging, cancer treatment, and theragnostic applications. Besides, the polymorphs of borophene have been useful to develop specific bonding for DNA sequencing and high-performance medical equipment. In this review, an overall critical and careful discussion of systematic advancements in borophene-based futuristic biomedical applications including artificial intelligence (AI), Internet-of-Things (IoT), and Internet-of-Medical Things (IoMT) assisted smart devices in healthcare to develop high-performance biomedical systems along with challenges and prospects is extensively addressed. Consequently, this review will serve as a key supportive platform as it explores borophene for next-generation biomedical applications. Finally, we have proposed the potential use of borophene in healthcare management strategies.

Anisotropic nodal loop in NiB2 monolayer with nonsymmorphic configuration

Two-dimensional (2D) materials featuring a nodal-loop (NL) state have been drawing considerable attention in condensed matter physics and materials science. Owing to their structural polymorphism, recent high-profile metal-boride films have great advantages and the potential to realize a NL. Herein, a 2D NiB₂ monolayer with an anisotropic NL nature is proposed and investigated using first-principles calculations. We show that the NiB₂ monolayer has excellent thermal dynamics stability, suggesting the possibility of its synthesis in experiments. Remarkably, the NL with a considerable Fermi velocity is demonstrated to be protected by nonsymmorphic glide mirror symmetry, instead of the widely known horizontal mirror symmetry. Accompanied by the proper preservation of the NL, strain engineering can not only regulate the anisotropy of the NL but also give rise to a self-doping phenomenon characterized by effective modulation of the carrier type and concentration. Moreover, this NL state is robust against the correlation effect. These findings pave the way for exploring nonsymmorphic-symmetry-enabled NL nature in 2D metal-borides.

Borophene as a rising star in materials chemistry: synthesis, properties and applications in analytical science and energy devices

Borophene is a two-dimensional material that has shown outstanding applications in energy storage devices and analytical chemistry.

Tuning magnetism at the two-dimensional limit: a theoretical perspective

The discovery of two-dimensional (2D) magnetic materials provides an ideal testbed for manipulating the magnetic properties at the atomically thin and 2D limit. This review gives recent progress in the emergent 2D magnets and heterostructures, focusing on the theory side. We summarize different theoretical models, ranging from the atomic to micrometer-scale, used to describe magnetic orders. Then, the current strategies for tuning magnetism in 2D materials are further discussed, such as electric field, magnetic field, strain, optics, chemical functionalization, and spin-orbit engineering. Finally, we conclude with the future challenges and opportunities for 2D magnetism.

Band Bending Mechanism in CdO/Arsenene Heterostructure: A Potential Direct Z-scheme Photocatalyst

For the few years, two-dimensional (2D) materials have aroused general focus. In order to expand the properties and application range of 2D materials, two different layered materials are usually combined into heterostructure through van der Waals (vdW) interaction. In this research, based on first-principles simulation, we propose CdO/Arsenene (CdO/As) vdW heterostructure as a semiconductor possessing a direct bandgap by 2.179 eV. Besides, the CdO/As vdW heterostructure presents type-II band alignment, which can be used as a remarkable photocatalyst. Importantly, the CdO/As heterostructure demonstrates a direct Z-type principle photocatalyst by exploring the band bending mechanism in the heterostructure. Furthermore, we calculated the light absorption characteristics of CdO/As vdW heterostructure by optical absorption spectrum and conversion efficiency of a novel solar-to-hydrogen efficiency (η STH) about 11.67%, which is much higher than that of other 2D photocatalysts. Our work can provide a theoretical guidance for the designing of Z-scheme photocatalyst.

Reversible Hydrogen Storage in Metal-Decorated Honeycomb Borophene Oxide

Two-dimensional (2D) boron-based materials are receiving much attention as H2 storage media due to the low atomic mass of boron and the stability of decorating alkali metals on the surface, which enhance interactions with H2. This work investigates the suitability of Li, Na, and K decorations on 2D honeycomb borophene oxide (B2O) for H2 storage, using dispersion corrected density functional theory (DFT-D2). A high theoretical gravimetric density of 8.3 wt % H2 is achieved for the Li-decorated B2O structure. At saturation, each Li binds to two H2 with an average binding energy of -0.24 eV/H2. Born-Oppenheimer molecular dynamics simulations at temperatures of 100, 300, and 500 K demonstrate the stability of the Li-decorated structure and the H2 desorption behavior at different temperatures. Our findings indicate that Li-decorated 2D B2O is a promising material for reversible H2 storage and recommend experimental investigation of 2D B2O as a potential H2 storage medium.

The Dirac half-semimetal and quantum anomalous Hall effect in two-dimensional Janus Mn2X3Y3 (X, Y = F, Cl, Br, I)

The quantum anomalous Hall (QAH) effect has been experimentally observed in magnetically-doped topological insulators. However, the QAH effect is only seen at extremely low temperatures due to the weak magnetic coupling, small band gap and low carrier mobility. Here, based on first-principles density functional theory, we predict that the Janus Mn₂X₃Y₃ (X, Y) = F, Cl, Br, I are high Curie temperature ferromagnets. Furthermore, we find that they are Dirac half-metals characterized by a Dirac cone in one spin channel with carrier mobilities comparable to freestanding germanene and a large band gap in the other spin channel except for Mn₂F₃I₃. Simultaneously, when the spin-orbital coupling interaction is considered, the Janus Mn₂F₃Cl₃, Mn₂Cl₃Br₃, and Mn₂Br₃I₃ exhibit a nontrivial band gap, indicating that they host a QAH phase. More interestingly, both the Chern number sign and the chiral edge current are tuned by changing the direction of magnetization. Moreover, we find that topological properties are related to the lattice constant and magnetocrystalline anisotropy. Our findings would suggest the possibility of not only realizing the QAH effect but also designing the flow direction of the edge current.

An ideal two-dimensional nodal-ring semimetal in tetragonal borophene oxide

Free-standing stable two-dimensional (2D) boron monolayers, i.e., borophenes, usually settle into triangular lattices with different ratios of monoatomic vacancies. However, a stable polymorph can be drastically distinct from a free-standing one upon charge doping or on a substrate, as evidenced by the free-standing unstable hexagonal borophene that was prepared on the Al(111) substrate [Sci. Bull., 2018, 63, 282]. Moreover, 2D borophenes prefer to be oxidized to form more stable borophene oxides under ambient conditions. In this work, with the help of first-principles calculations, we propose a stable borophene oxide (t-B₂O) through oxidizing the free-standing unstable T-borophene. More interestingly, t-B₂O is a topological nodal-ring semimetal protected by in-plane mirror symmetry and characterized by a topological index. The energy fluctuation of the nodal ring is small and no extraneous bands are entangled with the nodal ring around the Fermi level. Two tight-binding models are developed to elucidate the orbital interactions and the formation of the nodal ring. Our work not only discovers a new ideal 2D topological nodal-ring semimetal, but the method used here also provides a fresh view in the search for 2D materials.

Density-independent plasmons for terahertz-stable topological metamaterials

To efficiently integrate cutting-edge terahertz technology into compact devices, the highly confined terahertz plasmons are attracting intensive attention. Compared to plasmons at visible frequencies in metals, terahertz plasmons, typically in lightly doped semiconductors or graphene, are sensitive to carrier density (n) and thus have an easy tunability, which leads to unstable or imprecise terahertz spectra. By deriving a simplified but universal form of plasmon frequencies, here, we reveal a unified mechanism for generating unusual n-independent plasmons (DIPs) in all topological states with different dimensions. Remarkably, we predict that terahertz DIPs can be excited in a two-dimensional nodal line and one-dimensional nodal point systems, confirmed by the first-principle calculations on almost all existing topological semimetals with diverse lattice symmetries. Besides n-independence, the feature of Fermi velocity and degeneracy factor dependencies in DIPs can be applied to design topological superlattice and multiwalled carbon nanotube metamaterials for broadband terahertz spectroscopy and quantized terahertz plasmons, respectively. Surprisingly, high spatial confinement and quality factor, also insensitive to n, can be simultaneously achieved in these terahertz DIPs. Our findings pave the way for developing topological plasmonic devices for stable terahertz applications.

Oxygen-substituted borophene as a potential anode material for Li/Na-ion batteries: a first principles study

Two-dimensional graphene-like hexagonal borophene sheets (HBSs) have a thermodynamically unsteady configuration since boron has one electron less than the carbon in graphene. To overcome this problem, we proposed a novel 2D graphene-like HBS oxide (h-B3O) in theory, which is designed by substituting partial boron atoms in a HBS with oxygen atoms. Molecular dynamics simulations indicate that h-B3O has good thermal stability. Besides, we also explored the potential of h-B3O monolayers as anodes for Li-ion batteries (LIBs) and Na-ion batteries (NIBs) by using first-principles calculations. The results indicated that the h-B3O monolayer has high adsorption energies (-2.33/-1.70 eV for Li/Na), low diffusion barriers (0.67/0.42 eV for Li/Na) and suitable average open-circuit voltages (0.36/0.32 V for LIBs/NIBs). Particularly, h-B3O has a large theoretical specific capacity of 1161 mA h g-1 for LIBs. Thus, benefiting from these characteristics, the h-B3O monolayer is considered as a promising candidate for an anode material for LIBs/NIBs.

Two-dimensional Weyl points and nodal lines in pentagonal materials and their optical response

Two-dimensional pentagonal structures based on the Cairo tiling are the basis of a family of layered materials with appealing physical properties. In this work we present a theoretical study of the symmetry-based electronic and optical properties of these pentagonal materials. We provide a complete classification of the space groups that support pentagonal structures for binary and ternary systems. By means of first-principles calculations, the electronic band structures and the local spin textures in momentum space are analyzed for four examples of these materials, namely, PdSeTe, PdSeS, InP₅ and GeBi₂, all of which are dynamically stable. Our results show that pentagonal structures can be realized in chiral and achiral lattices with Weyl nodes pinned at high-symmetry points and nodal lines along the Brillouin zone boundary; these degeneracies are protected by the combined action of crystalline and time-reversal symmetries. Additionally, we computed the linear and nonlinear optical features of the proposed pentagonal materials and discuss some particular features such as the shift current, which shows an enhancement due to the presence of nodal lines and points, and their possible applications.

Various half-metallic nodal loops in organic Cr2N6C3 monolayers

Topological nodal-line semimetals, as a type of exotic quantum electronic state, have drawn considerable research interest recently. In this work, we propose a new two-dimensional covalent-organic Cr₂N₆C₃ monolayer (ML) material, which has a combined honeycomb and effective Kagome lattice and has various half-metallic nodal loops (HMNLs). First-principles calculations show that the Cr₂N₆C₃ ML is dynamically and thermally stable and has an out-of-plane ferromagnetic order. Remarkably, various nodal loops, including types I-III, are found coexisting in the material, all of which are rare half-metallic states. The obtained HMNLs, simultaneously possessing the merits of spintronics and semimetals, are robust against spin-orbit coupling and biaxial strain. A topological phase transition, characterized by loop-winding indexes, can be induced in the ML by applying uniaxial strain. Tight-binding model calculations show that the obtained HMNLs originate primarily from the band inversion between Cr d_x²-y²/xy and N p_z orbitals, accommodated on the honeycomb and Kagome sublattices, respectively. The various predicted HMNLs and topological behaviors mean that the Cr₂N₆C₃ MLs have promisingly versatile applications in future low-power-consuming spintronics and electronics.

A novel spin-valley-coupled nodal-ring semimetal in single-layer Ta2C3

Nodal-ring semimetals with band crossing are the new type of quantum materials that have attracted considerable interest from scholars for research. In general, the spin-orbit coupling (SOC) effect opens a band gap at the Dirac point. Therefore, finding 2D nodal-ring semimetals with resistance to SOC has more challenges. Based on first-principles calculations, we propose here that the two-dimensional (2D) Ta2C3 monolayer is a novel nodal-ring semimetal. In particular, Ta2C3 forms six closed rings in the Brillouin zone (BZ) with SOC, which originates from the dxy,x2-y2 orbitals of Ta and the pz orbitals of C. The nodal-ring bands at the K point in Ta2C3 exhibits characteristics of valley splitting and spin polarization due to the breaking of inversion symmetry and SOC. The masximal spin-splitting at the K point is as large as 268.87 meV and 61.90 meV for the conduction band minimum (CBM) and valence band maximum (VBM), respectively. The massless Dirac fermions in the non-equivalent valley have the opposite Berry curvature and spin moment. Therefore, 2D Ta2C3 is novel spin-valley-coupled nodal-ring semimetal. In addition, we found interesting negative differential resistance effects when calculating its transport properties. Our results not only provide an ideal platform for studying the combination of new physical properties, spintronics and valleytronics, but also open the way for designing low-power and fast-transport electronic devices.

Realization of Opened and Closed Nodal Lines and Four- and Three-fold Degenerate Nodal Points in XPt (X = Sc, Y, La) Intermetallic Compound: A Computational Modeling Study

Realizing rich topological elements in topological materials has attracted increasing attention in the fields of chemistry, physics, and materials science. Topological semimetals/metals are classified into three main types: nodal-point, nodal-line, and nodal-surface types with zero-, one-, and two-dimensional topological elements, respectively. This study reports that XPt (X = Sc, Y, La) intermetallic compounds are topological metals with opened and closed nodal lines, and triply degenerate nodal points (TNPs) when the spin-orbit coupling (SOC) is ignored. Based on the calculated phonon dispersions, one can find that ScPt and YPt are dynamically stable whereas LaPt is not. When SOC is added, the one-dimensional nodal line and zero-dimensional TNPs disappear. Interestingly, a new zero-dimensional topological element, that is, Dirac points with 4-fold degenerate isolated band crossings with linear band dispersion appear. The proposed materials can be considered a good platform to realize zero- and one-dimensional topological elements in a single compound and to study the relationship between zero- and one-dimensional topological elements.

Unique topological nodal line states and associated exceptional thermoelectric power factor platform in Nb3GeTe6 monolayer and bulk

To date, ideal topological nodal line semimetal (TNLS) candidates in high dynamically stable and high thermally stable two-dimensional (2D) materials are still extremely scarce. Herein, by performing first-principles calculations, on the one hand, we found that three-dimensional Nb₃GeTe₆ bulk possesses a single closed TNL in the k_x = 0 plane and a fourfold TNL in the S-R direction without considering spin-orbit coupling (SOC). Under the SOC effect, a new topological signature, i.e., hourglass-like Dirac nodal line, occurs in Nb₃GeTe₆ bulk. On the other hand, we found that the 2D Nb₃GeTe₆ monolayer features a doubly degenerate TNL along surface X-S paths. Importantly, this monolayer enjoys the following advantages: (i) it has high thermal stability at room temperature and above; (ii) its TNL is nearly flat in energy and is very close to the Fermi level (E_F), which provides a fantastic maximum value platform of the thermoelectric power factor around the E_F; and (iii) no extraneous bands are close to the TNL, near the Fermi level. Moreover, we explore the entanglement between the topological states and thermolectric properties for the 2D Nb₃GeTe₆ monolayer. Our work not only reports the discovery of a novel TNL material, but also builds the link between the TNL and thermoelectric properties.

Strain tuning of closed topological nodal lines and opposite pockets in quasi-two-dimensional α-phase FeSi2

Following topological nodal point semimetals, topological nodal line semimetals with one dimensional (1D) topological elements have recently aroused great interest worldwide in the fields of quantum chemistry and condensed matter physics. In this study, by means of first-principles, we predict that quasi-two-dimensional (2D) α-FeSi2 with a P4/mmm space group is a topological nodal line semimetal with two nodal lines close to the Fermi level, in the kz = 0 and kz = π planes. Usually, topological nodal line semimetals can be classified into type I, type II, and hybrid-type categories, each type with different physical properties. Importantly, for the first time, we find that type I, type II, and hybrid-type nodal lines can be realized in a realistic material, i.e., quasi-2D α-FeSi2, by strain switching. The realization of tunable nodal line types occurs because quasi-2D α-FeSi2 has special opposite-pocket-behaving bands around the Fermi level. The results presented herein reflect that α-FeSi2 is a valuable candidate for spintronics application by utilization of type I, type II, and hybrid-type topological nodal line semimetals in a single material tuned by mechanical strain.

Pnma metal hydride system LiBH: a superior topological semimetal with the coexistence of twofold and quadruple degenerate topological nodal lines

To date, a handful of topological semimetals (TMs) with multiple types of topological nodal line (TNL) states have been theoretically predicted in novel materials. However, their TNLs are often affected by many factors, such as spin-orbit coupling (SOC) effect, strain, and the extraneous bands near the band crossing points, and therefore, the TNL states cannot be easily verified by experiments. Here, by using first-principles calculations, we report that the Pnma LiBH is a potential TM with twofold and quadruple degenerate topological nodal lines. These TNLs situate very close to the Fermi level, and do not coexist with other extraneous bands. More importantly, the TNLs of this material are very robust to the effect of SOC, uniform strain, and biaxial strain. The nontrivial band structure in LiBH produces drum-head-like surface states in the (001) surface projection. Our result reveals that LiBH material is an excellent candidate to study the multiple kinds of TNLs.

First-principles study of χ3-borophene for charge-modulated switchable CO2 capture

A first-principles calculation was performed to investigate the switchable CO2 capture on χ3-borophene by injecting/removing the extra electrons. The results show that the CO2 adsorption energy on the neutral χ3-borophene is 0.150 eV. After extra 2.5 e are injected, the adsorption energy is raised up to 0.802 eV, showing a significant enhancement with the change from the physical adsorption to chemical adsorption. Furthermore, both the CO2 capture and release processes are exothermic reactions involving injecting/removing extra electrons. χ3-borophene possesses a metallic electronic structure, which is conducive to the injection of extra electrons. The minimum charge density for CO2 capture on the negatively charged χ3-borophene is 1.6 × 1014 e cm-2. The CO2 capture capacity of χ3-borophene is 4.09 × 1014 cm-2. Finally, we study the selectivity of negatively charged χ3-borophene. The results show that the negatively-charged χ3-borophene possesses a high selectivity for CO2 from its mixtures with CO, CH4, NH3, N2, H2S, and H2. χ3-borophene is a new promising charge-modulated switchable CO2 capture material with good stability, high CO2 capture capacity, high selectivity, and excellent electrical conductivity.

Rich novel zero-dimensional (0D), 1D, and 2D topological elements predicted in the P63/m type ternary boride HfIr3B4

Topological semimetals, including topological nodal point semimetals (TNPSs), topological nodal line state semimetals (TNLSs), and topological nodal surface semimetals (TNSSs), featuring zero-dimensional (0D), one-dimensional (1D), and two-dimensional (2D) topological elements (TEs), respectively, have attracted widespread attention in recent years. In this work, based on first-principles calculations, we propose for the first time that three different (0D, 1D, and 2D) TEs are simultaneously present in a synthetic compound, HfIr₃B₄, with a P6₃/m type structure. In detail, HfIr₃B₄ hosts a Dirac point (DP) state at the K point, a TNL state in the k_z = 0 plane, and a 2D TNS state in the k_z = π plane, respectively. All sorts of topological elements, 0D, 1D, and 2D TEs, coexisting in the P6₃/m type HfIr₃B₄, provide an ideal platform to study the rich fermionic states and their related physical properties in this type of compound. In addition, because the 0D, 1D, and 2D TEs of HfIr₃B₄ are equally distributed in different energy ranges relative to the Fermi level, an approach is proposed to utilize individual TEs to build on-demand devices.

2D honeycomb borophene oxide: a promising anode material offering super high capacity for Li/Na-ion batteries

Rational design of novel two-dimensional (2D) electrode materials with high capacity is crucial for the further development of Li-ion and Na-ion batteries. Herein, based on first-principles calculations, we systemically investigate Li and Na storage behaviors in the recently discovered 2D topological nodal-loop metal-the honeycomb borophene oxide (h-B₂O). We show that h-B₂O is an almost ideal anode material. It has good conductivity before and after Li/Na adsorption, fast ion diffusion with diffusion barrier less than 0.5 eV, low open-circuit voltage (<1 V), and small lattice change (<6.2%) during intercalation. Most remarkably, its theoretical storage capacity is extremely high, reaching up to 2137 mAh · g^-1 for Li and 1425 mAh · g^-1 for Na. Its Li storage capacity is more than six times higher than graphite (~372 mAh · g^-1), and is almost the highest among all 2D materials discovered to date. Our results strongly suggest that 2D h-B₂O is an exceedingly promising anode material for both Li- and Na-ion batteries with super high capacity.

Fully spin-polarized quadratic non-Dirac bands realized quantum anomalous Hall effect

The quantum anomalous Hall effect is an intriguing quantum state that exhibits chiral edge states in the absence of a magnetic field. While the search for quantum anomalous Hall insulators is still active, researchers mainly search for the systems containing a magnetic atom. Here, based on first-principles density functional theory, we predict a new family of Chern insulators with fully spin-polarized quadratic px,y non-Dirac bands in the alkaline earth metal BaX (X = Si, Ge, and Sn) system. We show that BaX monolayer has a half-metallic ferromagnetic ground state. The ferromagnetism mainly originates from the p orbitals of Si, Ge and Sn atoms. The 2D BaSn monolayer exhibits a large magnetocrystalline anisotropic energy of 12.20 meV per cell and a nontrivial band gap of 159.10 meV. Interestingly, both the chiral edge current direction and the sign of Chern number can be tuned by doping. Furthermore, the 4% compressive strain in the 2D BaX systems can drive a structural phase transition but the nontrivial topological properties remain reserved. Our findings not only extend the novel topological physics but also provide fascinating opportunities for the realization of the quantum anomalous Hall effect experimentally.

Rotational design of BP/XY2 (X = Mo, W; Y = S, Se) composites for overall photocatalytic water-splitting

Photocatalytic water-splitting for hydrogen generation is a promising way to solve the energy crisis, yet the design of efficient photocatalysts is still a challenge. By utilization of first principles calculations, we predict the photocatalytic properties of monolayer boron phosphide (BP) based BP/XY₂ (X  =  Mo, W; Y  =  S, Se) composites of different rotated configurations. Our results suggest that the BP/XY₂ composites can be stably formed, and the narrowed bandgaps ensure these composites are suitable for absorbing visible light. The bandgaps and band edge positions are slightly affected by the rotation angles. The BP/MoS₂, BP/MoSe₂, and BP/WSe₂ are type II heterostructures. Furthermore, the transferred charge from BP to XY₂ layers leads to the formation of electric fields, which efficiently separate the photoinduced carriers. The band alignments of BP/MoS₂, BP/MoS₂, BP/MoSe₂, and BP/WSe₂ satisfy the requirements of overall water-splitting within the pH scope of 3.6-7.9, 6.8-7.9, 4.0-8.0, and 8.7-8.8. This work will provide valuable insight for designing efficient water-splitting photocatalysts.

Freestanding Borophene and Its Hybrids

Borophene, an elemental metallic Dirac material is predicted to have unprecedented mechanical and electronic character. Need of substrate and ultrahigh vacuum conditions for deposition of borophene restricts its large-scale applications and significantly hampers the advancement of research on borophene. Herein, a facile and large-scale synthesis of freestanding atomic sheets of borophene through a novel liquid-phase exfoliation and the reduction of borophene oxide is demonstrated. Electron microscopy confirms the presence of β₁₂ , X₃ , and their intermediate phases of borophene; X-ray photoelectron spectroscopy, and scanning tunneling microscopy, corroborated with density functional theory band structure calculations, validate the phase purity and the metallic nature. Borophene with excellent anchoring capabilities is used for sensing of light, gas, molecules, and strain. Hybrids of borophene as well as that of reduced borophene oxide with other 2D materials are synthesized, and the predicted superior performance in energy storage is explored. The specific capacity of borophene oxide is observed to be ≈4941 mAh g^-1 , which significantly exceeds that of existing 2D materials and their hybrids. These freestanding borophene materials and their hybrids will create a huge breakthrough in the field of 2D materials and could help to develop future generations of devices and emerging applications.

From Two- to Three-Dimensional van der Waals Layered Structures of Boron Crystals: An Ab Initio Study

A remarkable recent advancement has been the successful synthesis of two-dimensional boron monolayers on metal substrates. However, although up to 16 possible bulk allotropes of boron have been reported, none of them possess van der Waals (vdW) layered structures. In this work, starting from the experimentally synthesized monolayer boron sheet (β12 borophene), we explored the possibility for forming vdW layered bulk boron. We found that two β12 borophene sheets cannot form a stable vdW bilayer structure, as covalent-like B-B bonds are formed between them because of the peculiar bonding. Interestingly, when the covalently bonded bilayer borophene sheets are stacked on top of each other, three-dimensional (3D) layered structures are constructed via vdW interlayer interactions, rather than covalent. The 3D vdW layered structures were found to be dynamically stable. The interlayer binding energy is about 20 meV/Å2, which is close to the weakly bound graphene layers in graphite (∼16 meV/Å2). Furthermore, the density functional theory predicted electronic band structure testifies that these vdW bulk boron crystals can behave as good conductors. The insights obtained from this work suggest an opportunity to discover new vdW layered structures of bulk boron, which is expected to be crucial to numerous applications ranging from microelectronic devices to energy storage devices.

Inorganic Boron-Based Nanostructures: Synthesis, Optoelectronic Properties, and Prospective Applications

Inorganic boron-based nanostructures have great potential for field emission (FE), flexible displays, superconductors, and energy storage because of their high melting point, low density, extreme hardness, and good chemical stability. Until now, most researchers have been focused on one-dimensional (1D) boron-based nanostructures (rare-earth boride (REB₆) nanowires, boron nanowires, and nanotubes). Currently, two-dimensional (2D) borophene attracts most of the attention, due to its unique physical and chemical properties, which make it quite different from its corresponding bulk counterpart. Here, we offer a comprehensive review on the synthesis methods and optoelectronics properties of inorganic boron-based nanostructures, which are mainly concentrated on 1D rare-earth boride nanowires, boron monoelement nanowires, and nanotubes, as well as 2D borophene and borophane. This review paper is organized as follows. In Section I, the synthesis methods of inorganic boron-based nanostructures are systematically introduced. In Section II, we classify their optical and electrical transport properties (field emission, optical absorption, and photoconductive properties). In the last section, we evaluate the optoelectronic behaviors of the known inorganic boron-based nanostructures and propose their future applications.