Prediction of a new BeC monolayer with perfectly planar tetracoordinate carbons

Ternary CE2Ba2 (E = As, Sb) Clusters: New Pentaatomic Planar Tetracoordinate Carbon Species with 18 Valence Electrons

18-valence-electron (ve) rule is one important guide for us to design planar tetracoordinate carbon (ptC) species. Using the "polarization of ligands" strategy, the new pentaatomic ptC species CE₂Ba₂ (E = As, Sb) with 18 ve are designed in this work. Computer structural searches and high-level calculations reveal that the ptC CE₂Ba₂ (E = As, Sb) species are global minima (GMs) on the potential energy surfaces, whose C center is coordinated by the interspaced E and Ba atoms. CE₂Ba₂ (E = As, Sb) are also kinetically stable. Chemical bonding analyses reveal that the ptC core is stabilized by two localized C-E σ bonds, one delocalized five-center two-electron (5c-2e) σ bond and one delocalized 5c-2e π bond. One π and three σ bonds collectively conform to the 8-electron counting, which determines the stability of ptC CE₂Ba₂ (E = As, Sb) species. Interestingly, the delocalized 2π and 2σ electrons render the ptC systems π/σ double aromaticity. Additional 10 electrons contribute to peripheral lone pairs of E and E-Ba bonding.

Two-dimensional semiconductor materials with high stability and electron mobility in group-11 chalcogenide compounds: MNX (M = Cu, Ag, Au; N = Cu, Ag, Au; X = S, Se, Te; M ≠ N)

It is still an urgent task to find new two-dimensional (2D) semiconductor materials with a suitable band gap, high stability and high mobility for the applications of next generation electronic devices. Based on first-principles calculations, we report a new class of 2D group-11-chalcogenide trielement monolayers (MNX, where M = Cu, Ag, Au; N = Cu, Ag, Au; X = S, Se, Te; M ≠ N) with a wide band gap, excellent stability (dynamic stability, thermodynamic stability and environmental stability) and high mobility. At the mixed density functional level, the energy band gap extends from 0.61 eV to 2.65 eV, covering the ultraviolet-A and visible light regions, which is critical for a broadband optical response. For δ-MNX monolayers, the carrier mobility is as high as 10⁴ cm² V^-1 s^-1 at room temperature. In particular, the mobility of δ-AgAuS is as high as 6.94 × 10⁴ cm² V^-1 s^-1, which is of great research significance for the application of electronic devices in the future. Based on the above advantages, group-11 chalcogenide MNX monomolecular films have broad prospects in the field of nanoelectronics and optoelectronics in the future.

FeSi2: a two-dimensional ferromagnet containing planar hexacoordinate Fe atoms

As an unconventional bonding pattern different from conventional chemistry, the concept of planar hypercoordinate atoms was first proposed in the molecular system, and it has been recently extended to 2D periodic systems. Using first-principles calculations, herein we predict a stable FeSi₂ monolayer with planar hexacoordinate Fe atoms. Due to its abundant multicenter bonds, the FeSi₂ monolayer shows excellent thermal and kinetic stability, anisotropic mechanical properties and room-temperature ferromagnetism (T _C ∼360 K). Furthermore, we have demonstrated the feasibility of directly growing an FeSi₂ monolayer on a Si (110) substrate while maintaining the novel electronic and magnetic properties of the freestanding monolayer. The FeSi₂ monolayer synthesized in this way would be compatible with the mature silicon semiconductor technology and could be utilized for spintronic devices.

Theoretically evaluating two-dimensional tetragonal Si2Se2 and SiSe2 nanosheets as anode materials for alkali metal-ion batteries

In this work, based on first-principles calculations, we theoretically predict two kinds of two-dimensional tetragonal Si–Se compounds, Si2Se2 and SiSe2, as the anode materials for alkali metal-ion batteries.

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.

Two-dimensional stable Mn based half metal and antiferromagnets promising for spintronics

In this paper, we predict that the tetragonal MnSi and MnC0.5Si0.5 monolayers are mechanically stable metallic ferromagnetic materials. The thermal stability of the MnC0.5Si0.5 monolayer is verified by our ab initio molecular dynamics (AIMD) result at 300 K. Both MnSi and MnC0.5Si0.5 monolayers exhibit room temperature half-metallic properties, which is very promising for spintronic applications. Both monolayers exhibit large perpendicular magnetic anisotropy, which is desirable for maintaining magnetic order and for high density storage spintronics. A bilayer of the MnSi nanosheet has obviously enhanced thermal stability and exhibits antiferromagnetic metal properties. The Néel temperature could be effectively manipulated and improved by surface functionalization. In addition, monolayer and bilayer MnSi nanosheets exhibit nodal lines in the reciprocal space, and the nodal lines are robust against spin orbit coupling.

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.

A novel T-C3N and seawater desalination

A structurally stable stacked multilayer carbonitride is predicted with the aid of ab initio calculations. This carbonitride consists of C3N tetrahedra, and is similar to T-carbon and thus named T-C3N. Its 2-dimensional (2D) monolayer is also carefully investigated in this work. The studies on electronic properties reveal that bulk and 2D T-C3N are insulators with a 5.542 eV indirect band gap and a 5.741 eV direct band gap, respectively. However, the monolayer T-C3N exhibits an excellent uniform porosity. Its 5.50 Å pore size is perfect for water nanofiltration. The adsorption and permeation of water molecules on the monolayer T-C3N are investigated. Its promising potential application in highly efficient nanofiltration membranes for seawater desalination is discussed.

Prediction of staggered stacking 2D BeP semiconductor with unique anisotropic electronic properties

By comprehensive structure design and first-principles calculations, we report a novel two-dimensional (2D) BeP nanomaterial with exotic structural and properties. This BeP 2D material is formed by a couple honeycomb sheets by slab staggered stacking and strong interlayer bondings. It behaves as a natural 2D semiconductor with several notable properties: a modest bandgap (~1.34 eV), high room-temperature electron mobility (~10⁴ cm² V^-1 s^-1) and high visible-light absorption coefficient (~10⁵ cm^-1); Moreover, due to the unique stacking topology, BeP may display distinctive direction-dependent electric transport by the anisotropic polarity of electron and hole mobilities, that is, it exhibits n-type (electron mobility  >  hole mobility) along the armchair direction while acts as p -type (hole mobility  >  electron mobility) in the zigzag direction, thus promising for applications in nanoelectronics. The BeP has good dynamic and thermal stabilities and is also the lowest-energy structure of 2D space indicated by particle swarm search, implying the high feasibility of experimental synthesis.

Mono-silicon isoelectronic replacement in CAl4 : van't hoff/le bel carbon or not?

In cluster studies, the isoelectronic replacement strategy has been successfully used to introduce new elements into a known structure while maintaining the desired topology. The well-known penta-atomic 18 valence electron (ve) species C Al 4 2 - and its Al^- /Si or Al/Si⁺ isoelectronically replaced clusters CAl₃ Si^- , CAl₂ Si₂ , C AlSi 3 - , and C Si 4 2 + , all possess the same anti-van't Hoff/Le Bel skeletons, that is, nontraditional planar tetracoordinate carbon (ptC) structure. In this article, however, we found that such isoelectronic replacement between Si and Al does not work for the 16ve-CAl₄ with the traditional van't Hoff/Le Bel tetrahedral carbon (thC) and its isoelectronic derivatives CAl₃ X (X = Ga/In/Tl). At the level of CCSD(T)/def2-QZVP//B3LYP/def2-QZVP, none of the global minima of the 16ve mono-Si-containing clusters CAl₂ SiX⁺ (X = Al/Ga/In/Tl) maintains thC as the parent CAl₄ does. Instead, X = Al/Ga globally favors an unusual ptC structure that has one long C─X distance yet with significant bond index value, and X = In/Tl prefers the planar tricoordinate carbon. The frustrated formation of thC in these clusters is ascribed to the CSi bonding that prefers a planar fashion. Inclusion of chloride ion would further stabilize the ptC of CAl₂ SiAl⁺ and CAl₂ SiGa⁺ . The unexpectedly disclosed CAl₂ SiAl⁺ and CAl₂ SiGa⁺ represent the first type of 16ve-cationic ptCs with multiple bonds. © 2019 Wiley Periodicals, Inc.

A BPt4S4 cluster: a planar tetracoordinate boron system with three charges all at their global energy minima

The monoanion state of BPt₄S₄⁻ possesses the lowest energy among the three oxidation states with planar tetracoordinate boron.

Predicting two-dimensional semiconducting boron carbides

Carbon and boron can mix to form numerous two-dimensional (2D) compounds with strong covalent bonds, yet very few possess a bandgap for functional applications. Motivated by the structural similarity between graphene and recently synthesized borophene, we propose a new family of semiconducting boron carbide monolayers composed of B₄C₃ pyramids and carbon hexagons, denoted as (B₄C₃)_m(C₆)_n (m, n are integers) by means of the global minimum search method augmented with first-principles calculations. These monolayers are isoelectronic to graphene yet exhibit increased bandgaps with decreasing n/m, due to the enhanced localization of boron multicenter bonding states as a consequence of the electronic transfer from boron to carbon. In particular, the B₄C₃ monolayer is even more stable than the previously synthesized BC₃ monolayer and has a direct bandgap of 2.73 eV, with the promise for applications in optical catalysis and optoelectronics. These results are likely to inform the on-going effort on the design of semiconducting 2D materials based on other light elements.

MnB2 nanosheet and nanotube: stability, electronic structures, novel functionalization and application for Li-ion batteries

In this paper, two kinds of two-dimensional manganese boride monolayers, h-MnB2 and t-MnB2, are predicted to be stable metallic nanosheets, which exhibit favorable mechanical and thermal properties. The Young's moduli of h-MnB2 and t-MnB2 are 77.73 N m-1 and 59.59 N m-1, respectively. Ab initio molecular dynamics results show that h-MnB2 and t-MnB2 can sustain up to 500 K and 1000 K, respectively. The magnetic property of h-MnB2 is frustrated antiferromagnetic with a Néel temperature of about 25 K, and the magnetic property of t-MnB2 is collinear antiferromagnetic with a Néel temperature of about 317 K. In addition, the electronic structure of the h-MnB2 monolayer can be tuned by passivation to exhibit Dirac states. h-MnB2 can also self-assemble to form nanotubes, and is thus very promising for application as the anode for Li-ion batteries because of its high capacity (about 875 mA h g-1), low diffusion barrier (about 0.03 eV) and strong stability.

Dirac cones in a snub trihexagonal tiling lattice with reflective symmetry breaking

The present models of the two-dimensional (2D) Dirac materials always have reflective symmetry, but the essentiality has never been proved. Here we present an exceptional case: a snub trihexagonal tiling (STT) lattice without reflective symmetry. We demonstrate the existence of Dirac cones in this reflection-symmetry-free lattice by using a single-orbital tight-binding (TB) Hamiltonian. The SST lattice is also topologically nontrivial, because the Dirac cone can be gapped by spin-orbit coupling (SOC) effect associated with robust gapless edge states. Using first-principles calculations, we predict a promising candidate 2D material, Be₃C₄ monolayer to realize this toy model. The Fermi velocities in this unique lattice are even higher than that in graphene. The stability and plausibility of the Be₃C₄ monolayer are verified by positive phonon modes, molecular dynamical simulations and mechanical criteria. This work eliminates the need for reflective symmetry in 2D Dirac materials, opening an avenue for designing new 2D Dirac materials.

VC2 and V1/2Mn1/2C2 nanosheets with robust mechanical and thermal properties as promising materials for Li-ion batteries

In this paper, vanadium carbides VC₂ and bi-transition-metal carbides V_1/2Mn_1/2C₂ are predicted to be stable metallic nanosheets showing promising mechanical properties.

CAl3X (X = B/Al/Ga/In/Tl) with 16 valence electrons: can planar tetracoordinate carbon be stable?

As a perpetual chemical curiosity, planar tetracoordinate carbon (ptC) that violates the traditional tetrahedral carbon (thC) has made enormous achievements. In particular, the 18-valence-electron (18ve) counting rule has been found to be very effective in predicting ptC structures, as in CX₄^2- (X = Al/Ga/In/Tl). By contrast, the corresponding neutral CX₄ with 16ve each takes the thC form like methane. Herein, we report a mono-substituted neutral 16ve-CAl₃X (X = Al/Ga/In/Tl). Our theoretical results showed that the competition between thC and ptC can be well tuned upon variation of X, and for X = In and Tl, the ptC structure becomes isoenergetic to and even more stable than thC, respectively. Thus, a low-lying ptC can be achieved in the 16ve-CAl₃X set without acquiring additional electrons. This unintuitive result can be ascribed to the increased energetic preference of the ionic sub-structure [CAl₃^-]X⁺ from X = Al to Tl. We thus predict the first penta-atomic ptC species with 16ve, and the ionic strategy presented in this work is expected to promote novel designs of ptC molecules.

LiCrS2 and LiMnS2 Cathodes with Extraordinary Mixed Electron-Ion Conductivities and Favorable Interfacial Compatibilities with Sulfide Electrolyte

Sulfide-type solid-state electrolytes for all-solid-state lithium ion batteries are capturing more and more attention. However, the electronegativity difference between the oxygen and the sulfur element makes sulfide-type solid-state electrolytes chemically incompatible with the conventional LiCoO₂ cathode. In this work, we proposed a series of chalcopyrite-structured sulfide-type materials and systematically assessed their performances as the cathode materials in all-solid-state lithium ion batteries by first-principle calculations. All the five metallic LiMS₂ (M = Cr, Mn, Fe, Co, and Ni) materials are superionic conductors with extremely small lithium ion migration barriers in the range from 43 to 99 meV, much lower than most oxide- and even sulfide-type cathodes. Voltage and volume calculations indicate that only LiCrS₂ and LiMnS₂ cathodes are structurally stable during cycling with the stable voltage plateaus at ∼3 V, much higher than that of the P3m1-LiTiS₂ cathode. For the first time, we studied the interfacial lithium transport resistance from a new perspective of charge transfer and redistribution at the electrode/solid-state electrolyte interface. LiCrS₂ and LiMnS₂ cathodes exhibit favorable interfacial compatibilities with Li₃PS₄ electrolyte. Our investigations demonstrate that the metallic LiCrS₂ and LiMnS₂ superionic conductors would possess excellent rate capability, high energy density, good structural stability during cycling, and favorable interfacial compatibility with Li₃PS₄ electrolyte in all-solid-state lithium ion batteries.

Optical properties of monolayer BeC under an external electric field: A DFT approach

BeC, a two-dimensional hypercoordinated nanostructure carbon compound, has been the focus of the nanoworld because of its high value of dynamical stability, in-plane stiffness, carrier mobility and the existence of band gap. In this work, we have explored the electronic and the optical properties of this material under the influence of static external perpendicular electric field within the framework of density functional theory. Under the influence of a uniform electric field, the band gap changes within the meV range. The electron energy loss function study reveals that this material has optical band gaps which remain constant irrespective of the applied electric field strength. The optical property also exhibits interesting features when the applied field strength is within 0.4–0.5 V/Å. We have also tried to explain the optical data from the respective band structures and thus paving the way to understand qualitatively the signature of the optical anisotropy from the birefringence study.

Viable aromatic BenHn stars enclosing a planar hypercoordinate boron or late transition metal

Similar to B_n rings, star-like Be_nH_n rings can serve as the n-electron σ-donors for designing species with planar hypercoordinate atom.

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.

A two-dimensional TiB4 monolayer exhibits planar octacoordinate Ti

At present, the concept of planar hypercoordination in chemistry meets the fast development of two-dimensional (2D) nanomaterials, leading to considerable interest in searching for 2D materials with planar hypercoordinate atoms. In this work, by means of the swarm-intelligence structure search method and first-principles calculations, we predict a hitherto unknown 2D TiB₄ monolayer with a planar octacoordinate Ti moiety, in which each Ti atom binds to eight B atoms with equal distances in a perfect plane, and has the highest coordination of Ti known for 2D materials thus far. Systematic ab initio calculations demonstrate the superior thermodynamic and dynamic stabilities of the predicted TiB₄ monolayer, indicating the high feasibility for experimental synthesis. The stabilization of this perfect planar structure originates from the geometric fit between the atomic radius of Ti and the size of the 8-membered B ring, as well as the electron transfer from Ti to B atoms which compensates the electron deficiency of the full sp² hybridized B network. Motivated by the unforeseen geometry of the TiB₄ monolayer, a series of other 2D transition metal borides (TMB₄, TM = V, Cr, Mo, W and Os) with quasi-planar octacoordinate TM atoms are further designed and discussed. The present work provides a useful roadmap for the discovery of 2D hypercoordinate materials.