A first-principles prediction of two-dimensional superconductivity in pristine B₂C single layers

Theoretical study on the superconductivity of graphene-like TMB6 (TM = Cr, Fe and Co) monolayer and its potential anchoring and catalytic properties for lithium-sulfur batteries

In recent years, two-dimensional materials have aroused enormous interest owing to their superior electrochemical performance, abundant exposed active sites, high specific surfaces and so on. Unlike many stable allotropes, honeycomb hexagonal borophene is kinetically unstable. In this study, we introduce transition metal atoms (Cr, Fe and Co) to stabilize honeycomb hexagonal borophene, forming stable graphene-like TMB6 (TM = Cr, Fe and Co) monolayers. Moreover, we explored the possibility of superconductivity and the anchoring materials of lithium-sulfur batteries using the first-principles density functional theory (DFT) calculation. Our results show that CoB6 exhibited the best superconductivity with a superconducting transition temperature of 33.3 K. Furthermore, CoB6 and FeB6 are promising anchoring materials because of the suppression of lithium polysulfides shuttling in lithium-sulfur batteries because they can accelerate sulfur reduction reaction kinetics.

High-Throughput DFT-Based Discovery of Next Generation Two-Dimensional (2D) Superconductors

High-throughput density functional theory (DFT) calculations allow for a systematic search for conventional superconductors. With the recent interest in two-dimensional (2D) superconductors, we used a high-throughput workflow to screen over 1000 2D materials in the JARVIS-DFT database and performed electron-phonon coupling calculations, using the McMillan-Allen-Dynes formula to calculate the superconducting transition temperature (T_c) for 165 of them. Of these 165 materials, we identify 34 dynamically stable structures with transition temperatures above 5 K, including materials such as W₂N₃, NbO₂, ZrBrO, TiClO, NaSn₂S₄, Mg₂B₄C₂, and the previously unreported Mg₂B₄N₂ (T_c = 21.8 K). Finally, we performed experiments to determine the T_c of selected layered superconductors (2H-NbSe₂, 2H-NbS₂, ZrSiS, FeSe) and discuss the measured results within the context of our DFT results. We aim that the outcome of this workflow can guide future computational and experimental studies of new and emerging 2D superconductors by providing a roadmap of high-throughput DFT data.

Two-dimensional B[Formula: see text]C as a potential anode material for Mg-ion batteries with extremely high theoretical capacity

The development of new high-capacity anode materials using ions other than lithium as a charge carrier is one of the essential strategies in searching for next-generation high-performance rechargeable batteries. Herein, using first-principles computations, we explore a B[Formula: see text]C monolayer as a potential anode material for Mg-ion batteries. The high stability of the free-standing B[Formula: see text]C monolayer has been demonstrated via calculating the adsorption energy, phonon dispersion, and ab-initio molecular dynamics simulations. The metallic character of the B[Formula: see text]C monolayer, desirable from the point of view of energy storage, ensures good electronic conductivity during the battery charge/discharge process. The calculated migration energy barrier, open-circuit voltage, and theoretical specific capacity of the B[Formula: see text]C monolayer are much better than those of some other two-dimensional materials. These findings provide the B[Formula: see text]C monolayer as a potential candidate for Mg-ion battery anode material with a high theoretical specific capacity of 3187.55 mAh/g.

Phonon-mediated superconductivity in two-dimensional hydrogenated phosphorus carbide: HPC3

In recent years, three-dimensional (3D) high-temperature superconductors at ultrahigh pressure have been reported, typical examples are the polyhydrides H₃S, LaH₁₀, YH₉, etc. To find high-temperature two-dimensional (2D) superconductors at atmospheric pressure is another research hotspot. Here, we investigated the possible superconductivity in a hydrogenated monolayer phosphorus carbide based on first-principles calculations. The results reveal that monolayer PC₃ transforms from a semiconductor to a metal after hydrogenation. Interestingly, the C-π-bonding band contributes most to the states at the Fermi level. Based on the electron-phonon coupling mechanism, it is found that the electron-phonon coupling constant of HPC₃ is 0.95, which mainly originates from the coupling of C-π electrons with the in-plane vibration modes of C and H. The calculated critical temperature T_c is 31.0 K, which is higher than those in most 2D superconductors. By further applying a biaxial tensile strain of 3%, the T_c can be boosted to 57.3 K, exceeding the McMillan limit. Thus, hydrogenation and strain are effective ways for increasing the superconducting T_c of 2D materials.

Surface passivation induced a significant enhancement of superconductivity in layered two-dimensional MSi2N4 (M = Ta and Nb) materials

Two-dimensional (2D) transition metal di-nitrides (TMN₂) have been arousing great interest for their unique mechanic, electronic, optoelectronic, and magnetic properties. The recent successful growth of monolayer MSi₂N₄ (M = Mo and W) further motivates us to explore new physics and unusual properties behind this family. By using first-principles calculations and Bardeen-Cooper-Schrieffer theory, we predicted the existence of the superconductivity in single-layer (SL) 1T- and 1H-TaN₂ with superconducting transition temperatures (T_c) of ∼0.86 and 1.3 K. Specifically, the T_c could be greatly enhanced to ∼24.6 K by passivating the TaN₂ monolayer with Si-N bilayers. Furthermore, the superconductivity could be increased to ∼30.4 K via substituting lighter Nb for Ta. This enhancement of superconductivity mainly stems from the softer vibration modes consisting of in-plane Ta/Nb vibrations mixed with Si-xy vibrations. The superconductivity can be further tuned by applying external strains and carrier doping. This enhancement strategy of surface passivation and light atom substitution would suggest a new platform for 2D superconductors and provide an instructive pathway for next-generation nanoelectronics.

Bonding, structure, and mechanical stability of 2D materials: the predictive power of the periodic table

This tutorial review describes the ongoing effort to convert main-group elements of the periodic table and their combinations into stable 2D materials, which is sometimes called modern 'alchemy'. Theory is successfully approaching this goal, whereas experimental verification is lagging far behind in the synergistic interplay between theory and experiment. The data collected here gives a clear picture of the bonding, structure, and mechanical performance of the main-group elements and their binary compounds. This ranges from group II elements, with two valence electrons, to group VI elements with six valence electrons, which form not only 1D structures but also, owing to their variable oxidation states, low-symmetry 2D networks. Outside of these main groups reviewed here, predominantly ionic bonding may be observed, for example in group II-VII compounds. Besides high-symmetry graphene with its shortest and strongest bonds and outstanding mechanical properties, low-symmetry 2D structures such as various borophene and tellurene phases with intriguing properties are receiving increasing attention. The comprehensive discussion of data also includes bonding and structure of few-layer assemblies, because the electronic properties, e.g., the band gap, of these heterostructures vary with interlayer layer separation and interaction energy. The available data allows the identification of general relationships between bonding, structure, and mechanical stability. This enables the extraction of periodic trends and fundamental rules governing the 2D world, which help to clear up deviating results and to estimate unknown properties. For example, the observed change of the bond length by a factor of two alters the cohesive energy by a factor of four and the extremely sensitive Young's modulus and ultimate strength by more than a factor of 60. Since the stiffness and strength decrease with increasing atom size on going down the columns of the periodic table, it is important to look for suitable allotropes of elements and binaries in the upper rows of the periodic table when mechanical stability and robustness are issues. On the other hand, the heavy compounds are of particular interest because of their low-symmetry structures with exotic electronic properties.

A novel two-dimensional beryllium diphosphide (BeP2) with superconductivity: the first-principles exploration

In this study, three stable two-dimensional beryllium diphosphide (2D-BeP₂) structures with the wrinkle and planar monolayers, namely MoS₂-like 6[combining macron]m-BeP₂ phase (1H-BeP₂), pentagonal 4[combining macron]2m-BeP₂ (Penta-BeP₂) and planar mm2-BeP₂ (Planar-BeP₂), have been successfully predicted through the first-principles calculation combined with a global structure search method. The structural stabilities, mechanical properties, electron properties and superconductivities are also systematically investigated. Results indicated that the 2D MoS₂-like 1H-BeP₂ showed higher stability than the Penta- and Planar-BeP₂ structures. The 1H-BeP₂ structure possessed an intrinsic metallic characteristics with the bands crossing the Fermi level. Notably, the Penta-BeP₂ is a typical semiconductor, and the planar-BeP₂ is semi-metal with Dirac corn. Based on the calculation results of the electron properties, phonon properties and electron-phonon coupling (EPC), the layer 1H-BeP₂ sheet is a phonon-mediated superconductor with a critical temperature (T_c) of about 1.32 K.

First-Principles Observation of Bonded 2D B4C3 Bilayers

Two-dimensional (2D) B-C compounds possess rich allotropic structures with many applications. Obtaining new 2D B₄C₃ structures is highly desirable due to the novel applications of three-dimensional (3D) B₄C₃ in protections. In this work, we proposed a new family of 2D B₄C₃ from the first-principles calculations. Distinct from previous observations, this family of 2D B₄C₃ consists of bonded 2D B₄C₃ bilayers. Six different types of bilayers with distinct bonded structures are found. The phonon spectrum calculations and ab initio molecular dynamics simulations at room temperature demonstrate their dynamic and thermal stabilities. Low formation energies suggest the high possibility of realizing such structures in experiments. Rich electronic structures are found, and the predicted Young's moduli are even higher than those of the previous ones. It is revealed that the unique electronic and mechanical properties are rooted in the bonding structures, indicating the prompting applications of this family of 2D B₄C₃ materials in photovoltaics, nanoelectronics, and nanomechanics.

Theoretical Prediction of Two-Dimensional Materials, Behavior, and Properties

Predictive modeling of two-dimensional (2D) materials is at the crossroad of two current rapidly growing interests: 2D materials per se, massively sought after and explored in experimental laboratories, and materials theoretical-computational models in general, flourishing on a fertile mix of condensed-matter physics and chemistry with advancing computational technology. Here the general methods and specific techniques of modeling are briefly overviewed, along with a somewhat philosophical assessment of what "prediction" is, followed by selected practical examples for 2D materials, from structures and properties, to device functionalities and synthetic routes for their making. We conclude with a brief sketch-outlook of future developments.

Prediction of superconductivity in bilayer borophenes

Borophenes and related two-dimensional materials have exhibited many exotic properties, especially for superconductivity, although the superconductivity of single-layer borophene is suppressed by the strains or doping from its substrates. Intriguingly, bilayer (BL) borophenes can be stabilized by appropriate pillar density and hexagonal holes density, rather than being supported by Ag(111) or Cu(111) substrates. Thus, we studied the two most stable structures, namely BL-B8 and BL-B30, stabilized by the above-mentioned two methods. Within density functional theory and Bardeen–Cooper–Schrieffer theory framework, their stability, electron structures, and phonon properties, as well as possible superconductivity are systematically scrutinized. The metallic BL-B8 and BL-B30 exhibit intrinsic superconducting features with superconductivity transition temperatures (T_c) of 11.9 and 4.9 K, respectively. The low frequency (below 400 cm⁻¹) consisting of out-of-plane vibrations of boron atoms plays crucial rule in their superconductivity. In particular, a Kohn anomaly appears at the Γ point in BL-B8, leading to substantial electron–phonon coupling. Here, our findings will provide instructive clues for experimentally determining the superconductivity of borophene and will broaden the two-dimensional superconductor family.

Bilayer borophene B8 and B30 are BCS-superconductors with T_c of 11.9 and 4.9 K, respectively.

Monolayer NbF4: a 4d1-analogue of cuprates

The electronic structure and possible electronic orders in monolayer NbF₄ are investigated by density functional theory and functional renormalization group. Because of the niobium-centered octahedra, the energy band near the Fermi level is found to derive from the 4d_xy orbital, well separated from the other bands. Local Coulomb interaction drives the undoped system into an antiferromagnetic insulator. Upon suitable electron/hole doping, the system is found to develop [Formula: see text] -wave superconductivity with sizable transition temperature. Therefore, the monolayer NbF₄ may be an exciting 4d¹ analogue of cuprates, providing a new two-dimensional platform for high-T_c superconductivity.

Planar Hypercoordinate Motifs in Two-Dimensional Materials

ConspectusAs one of the most important and versatile elements, carbon renders itself as one of the most fundamental and cutting-edge topics in chemistry, physics, and materials science. Many carbon-based chemical rules were established accordingly. While the tetrahedral predilection of tetracoordinate carbon has been a cornerstone of organic chemistry since 1874, almost a century later tetracoordinate carbon was found to be able to adopt planar structures known as planar tetracoordinate carbon (ptC), which are stabilized electronically by good π-acceptor (delocalization of a lone electron pair of ptC) or σ-donor (promoting electron transfer to electron-deficient bonding) substituents or mechanically by appropriate steric enforcement. The experimental and theoretical achievements for the rule-breaking ptC species totally refreshed our understanding of chemical bonding and triggered exploration of peculiar molecules featuring planar pentacoordinate carbon (ppC) and planar hexacoordinate carbon (phC) as well as other outlandish species such as planar hypercoordinate silicon.While the planar hypercoordinate carbon chemistry has been gradually established for molecules in the past five decades, there is growing interest in pursuing their extension systems, especially in two-dimensional (2D) space as a result of the recent extensive studies of graphene and its analogues. Though the natural 2D layered crystals do not contain any planar hypercoordinate carbon or silicon, several 2D nanosheets featuring planar or quasi-planar hypercoordinate ones have been theoretically suggested. Encouragingly, these unique planar configurations possess decent stabilities, and some of them are even the global minimum structure, exhibiting great potential for experimental realization. As the nature of a material is mainly determined by its structural characteristics (e.g., dimensionality, crystallography, and bonding), the combination of planar hypercoordinate chemistry and 2D nanoscience not only endows these rule-breaking systems with the merits of 2D materials but also may offer various promising properties and applications. For example, an unusual negative Poisson's ratio can be found in ppC-containing Be₅C₂ and planar pentacoordinate silicon (ppSi)-containing CaSi monolayers, of which the former has an anisotropic Dirac cone and the latter is a semiconductor with a desirable band gap for the semiconductor industry. Specially, shortly after the theoretical prediction, a planar hexacoordinate silicon (phSi)-containing Cu₂Si monolayer was experimentally synthesized and characterized with the 2D Dirac nodal line fermion, which offers a platform to achieve high-speed, low-dissipation nanodevices.In this Account, we review the recent progress, mostly by density functional theory (DFT) computations, in designing 2D materials with planar hypercoordinate motifs. We describe the key achievements in this field, paying special attention to the "bottom-up" and "isoelectronic substitution" design strategies. In addition, the fundamental stabilization mechanisms of planar hypercoordinate motifs in an infinite layer are discussed. We hope that this Account will inspire more experimental and theoretical efforts to explore nanomaterials with such unconventional chemical bonding.

Theoretical prediction of superconductivity in monolayer CoO2

Motivated by the synthesis of the layered structure CoO2via Li atom deintercalation from LixCoO2, herein, we investigated the electronic structure, lattice dynamics, electron-phonon interaction, and superconductivity of monolayer CoO2 using first-principles calculations. This 2D material was predicted to have a ferromagnetic ground state with a metallic band structure and the total magnetization of 0.83μB. Remarkably, the non-spin polarized calculations show that the monolayer CoO2 possesses phonon-mediated superconductivity at 25-28 K owing to its intermediate to strong electron-phonon coupling (EPC). The rather strong EPC in this compound is mainly driven by the acoustic phonons, making this compound one of the highest-temperature superconductors among the existing 2D materials. Moreover, the CoO2 sheets could be synthesized via exfoliation from bulk CoO2 owing to the relatively small interlayer binding energy while maintaining its stability under normal experimental conditions. Compared to its bulk and bilayer counterparts, monolayer CoO2 was found to have highest EPC.

Intrinsic phonon-mediated superconductivity in graphene-like BSi lattice

The research of new superconductors is an ongoing field for the fundamental significances and potential applications, and two-dimensional (2D) nanomaterials open a new alluring branch for exploration. Here we predict by first-principles calculations that 2D pristine graphene-like BSi monolayer is a phonon-mediated superconductor above the boiling point of liquid helium. The intrinsic covalent-metallic ground state, large density of states at Fermi energy, proper electronic organization as well as strong coupling of out-of-plane phonons and electrons endow an intermediate electron-phonon coupling of ~1.12, rendering this honeycomb sheet as a conventional superconductor with a relatively high T _c ~ 11 K. As the global minimum structure in the 2D space previously predicted, this superconducting BSi monolayer may be feasible experimentally. Our finding provides a new field of superconducting nanomaterials for study.

Novel structures of two-dimensional tungsten boride and their superconductivity

We predict four new tungsten boride monolayers and demonstrate that two of them are phonon-mediated superconductors with superconducting transition temperatures of 7.8 and 1.5 K.

Electronic and Optical Properties of 2D Materials Constructed from Light Atoms

Boron, carbon, nitrogen, and oxygen atoms can form various building blocks for further construction of structurally well-defined 2D materials (2DMs). Both in theory and experiment, it has been documented that the electronic structures and optical properties of 2DMs are well tunable through a rational design of the material structure. Here, the recent progress on 2DMs that are composed of B, C, N, and O elements is introduced, including borophene, graphene, h-BN, g-C₃ N₄ , organic 2D polymers (2DPs), etc. Attention is put on the band structure/bandgap engineering for these materials through a variety of methodologies, such as chemical modifications, layer number and atomic structure control, change of conjugation degree, etc. The optical properties, such as photoluminescence, thermoluminescence, single photon emission, as well as the associated applications in bioimaging and sensing, are discussed in detail and highlighted.

Germanene on single-layer ZnSe substrate: novel electronic and optical properties

In this work, the structural, electronic and optical properties of germanene and ZnSe substrate nanocomposites have been investigated using first-principles calculations. We found that the large direct-gap ZnSe semiconductors and zero-gap germanene form a typical orbital hybridization heterostructure with a strong binding energy, which shows a moderate direct band gap of 0.503 eV in the most stable pattern. Furthermore, the heterostructure undergoes semiconductor-to-metal band gap transition when subjected to external out-of-plane electric field. We also found that applying external strain and compressing the interlayer distance are two simple ways of tuning the electronic structure. An unexpected indirect-direct band gap transition is also observed in the AAII pattern via adjusting the interlayer distance. Quite interestingly, the calculated results exhibit that the germanene/ZnSe heterobilayer structure has perfect optical absorption in the solar spectrum as well as the infrared and UV light zones, which is superior to that of the individual ZnSe substrate and germanene. The staggered interfacial gap and tunability of the energy band structure via interlayer distance and external electric field and strain thus make the germanene/ZnSe heterostructure a promising candidate for field effect transistors (FETs) and nanoelectronic applications.

Unique Zigzag-Shaped Buckling Zn2C Monolayer with Strain-Tunable Band Gap and Negative Poisson Ratio

Designing new materials with reduced dimensionality and distinguished properties has continuously attracted intense interest for materials innovation. Here we report a novel two-dimensional (2D) Zn₂C monolayer nanomaterial with exceptional structure and properties by means of first-principles calculations. This new Zn₂C monolayer is composed of quasi-tetrahedral tetracoordinate carbon and quasi-linear bicoordinate zinc, featuring a peculiar zigzag-shaped buckling configuration. The unique coordinate topology endows this natural 2D semiconducting monolayer with strongly strain tunable band gap and unusual negative Poisson ratios. The monolayer has good dynamic and thermal stabilities and is also the lowest-energy structure of 2D space indicated by the particle-swarm optimization (PSO) method, implying its synthetic feasibility. With these intriguing properties the material may find applications in nanoelectronics and micromechanics.

Superconductivity in two-dimensional phosphorus carbide (β0-PC)

The out-of-plane P_z vibrational modes in two-dimensional phosphorus carbide lead to intrinsic superconductivity with a Kohn anomaly.

Metal-semiconductor transition of two-dimensional Mg2C monolayer induced by biaxial tensile strain

Designing new two-dimensional (2D) materials with novel band topologies has continuously attracted intense interest in fundamental science and potential applications. Here, we report a unique 2D Mg₂C monolayer featuring quasi-planar hexa-coordinate magnesium and hexa-coordinate carbon, which can be tuned from a metal to a semiconductor. The system has been studied using density functional theory, including electronic structure calculations and molecular dynamics simulations. In the freestanding state, the Mg₂C monolayer behaves as a weak metal; however, by increasing the biaxial tensile strains, it can gradually be modulated to a gapless semimetal and then to a semiconductor. The Mg₂C monolayer exhibits excellent dynamic and thermal stabilities and is also the global minimum of the 2D Mg₂C system, implying the feasibility of its experimental synthesis. With unique band structures, the material may find applications in optoelectronics and electromechanics.

Monolayer BC2: an ultrahigh capacity anode material for Li ion batteries

Uniformly doped monolayered BC₂sheets show the highest ever reported specific capacity of 1667 mA h g⁻¹for B doped graphene sheets.

Electronic and excitonic properties of two-dimensional and bulk InN crystals

Motivated by potential extensive applications in nanoelectronics devices, we calculate structural and optoelectronic properties of two-dimensional InN as well as its three-dimensional counterparts by using density functional theory.

Can Two-Dimensional Boron Superconduct?

Two-dimensional boron is expected to exhibit various structural polymorphs, all being metallic. Additionally, its small atomic mass suggests strong electron-phonon coupling, which in turn can enable superconducting behavior. Here we perform first-principles analysis of electronic structure, phonon spectra, and electron-phonon coupling of selected 2D boron polymorphs and show that the most stable structures predicted to feasibly form on a metal substrate should also exhibit intrinsic phonon-mediated superconductivity, with estimated critical temperature in the range of Tc ≈ 10-20 K.

Post-anti-van't Hoff-Le Bel motif in atomically thin germanium–copper alloy film

We predict a novel planar hypercoordinate Cu₂Ge material.