A two-dimensional CaSi monolayer with quasi-planar pentacoordinate silicon

Large valley splitting induced by spin-orbit coupling effects in monolayer W2NSCl

Two-dimensional (2D) valley materials are promising materials for writing and storing information. The search for 2D materials with large valley splitting is essential for the development of spintronics and valley electronics. In this study, we theoretically design 2D W2NSCl MXenes with large valley splitting based on first-principle calculations. Due to the strong spin-orbit coupling (SOC) and the broken inversion symmetry, the W2NSCl monolayer exhibits valley splitting values of 491 meV and 83 meV at K/K' of the valence and conduction bands, respectively. The valley splitting of W2NSCl is robust to biaxial strain. Because of the broken mirror symmetry of W2NSCl, there is a Rashba effect at Γ with a Rashba parameter of 1.019 V Å. Based on the maximum localization of the Wannier function, we found the non-zero Berry curvature at K/K'. Furthermore, the non-zero Berry curvature at the K/K' valley increases monotonically with an external strain from -4% to 4%. Our finding shows that W2NSCl is a candidate material for valley electronics and spintronics applications.

Anisotropic Mechanical Properties of Orthorhombic SiP2 Monolayer: A First-Principles Study

In recent years, the two-dimensional (2D) orthorhombic SiP2 flake has been peeled off successfully by micromechanical exfoliation and it exhibits an excellent performance in photodetection. In this paper, we investigated the mechanical properties and the origin of its anisotropy in an orthorhombic SiP2 monolayer through first-principles calculations, which can provide a theoretical basis for utilizing and tailoring the physical properties of a 2D orthorhombic SiP2 in the future. We found that the Young's modulus is up to 113.36 N/m along the a direction, while the smallest value is only 17.46 N/m in the b direction. The in-plane anisotropic ratio is calculated as 6.49, while a similar anisotropic ratio (~6.55) can also be observed in Poisson's ratio. Meanwhile, the in-plane anisotropic ratio for the fracture stress of the orthorhombic SiP2 monolayer is up to 9.2. These in-plane anisotropic ratios are much larger than in black phosphorus, ReS2, and biphenylene. To explain the origin of strong in-plane anisotropy, the interatomic force constants were obtained using the finite-displacement method. It was found that the maximum of interatomic force constant along the a direction is 5.79 times of that in the b direction, which should be considered as the main origin of the in-plane anisotropy in the orthorhombic SiP2 monolayer. In addition, we also found some negative Poisson's ratios in certain specific orientations, allowing the orthorhombic SiP2 monolayer to be applied in next-generation nanomechanics and nanoelectronics.

Enhanced Curie temperature and conductivity of van der Waals ferromagnet MgV2S4via electrostatic doping

A van der Waals intrinsic ferromagnet with double magnetic atom layers is of great interest for both revealing fundamental physics and exploring promising applications in low-dimensional spintronics. Here, the magnetic and electronic properties of the van der Waals ferromagnet MgV₂S₄ monolayer are studied under electrostatic doping using first-principles calculations. A MgV₂S₄ monolayer presents the desired physical properties such as that of being a half-semiconductor with a direct bandgap of 1.21 eV and a ferromagnetic ground state, and having a high Curie temperature of 462 K. Unlike the robust ferromagnetic ground state, magnetic anisotropy and Curie temperature are sensitive to electrostatic doping. Meanwhile, the transition from a semiconductor to a half-metal and the significant improvement in conductivity under electrostatic doping make the MgV₂S₄ monolayer a promising candidate for low-dimensional spintronic field-effect transistors.

Two-dimensional Be2Al and Be2Ga monolayer: anti-van't Hoff/Le Bel planar hexacoordinate bonding and superconductivity

Because of the electron deficiency of boron, a triangular network with planar hexacoordination is the most common structural and bonding property for isolated boron clusters and two-dimensional (2D) boron sheets. However, this network is a rule-breaking structure and bonding case for all other main-group elements. Herein, the Be₂M (M = Al and Ga) 2D monolayer with P6/mmm space group was found to be the lowest-energy structure with planar hexacoordinate Be/Al/Ga motifs. More interestingly, Be₂Al and Be₂Ga were observed to be intrinsic phonon-mediated superconductors with a superconducting critical temperature (T_c) of 5.9 and 3.6 K, respectively, where compressive strain could further enhance their T_c. The high thermochemical and kinetic stability of Be₂M make a promising candidate for experimental realization, considering its high cohesive energy, absence of soft phonon modes, and good resistance to high temperature. Moreover, the feasibility of directly growing Be₂M on the electride Ca₂N substrate was further demonstrated, where its intriguing electronic and superconducting properties were well maintained in comparison with the freestanding monolayer. The Be₂M monolayer with rule-breaking planar hexacoordinate motifs firmly pushes the ultimate connection of the "anti-van't Hoff/Le Bel" structure with promising physical properties.

A two-dimensional Be2Au monolayer with planar hexacoordinate s-block metal atoms: a superconducting global minimum Dirac material with two perfect Dirac node-loops

Using a starlike Be6Au7 - cluster as a building block and following the bottom-up strategy, an intriguing two-dimensional (2D) binary s-block metal Be2Au monolayer with a P6/mmm space group was theoretically designed. Both the Be6Au7 - cluster and the 2D monolayer are global minima featuring rule-breaking planar hexacoordinate motifs (anti-van't Hoff/Le Bel arrangement), and their high stabilities are attributed to good electron delocalization and electronic-stabilization-induced steric force. Strikingly, the Be2Au monolayer is a rare Dirac material with two perfect Dirac node-loops in the band structure and is a phonon-mediated superconductor with a critical temperature of 4.0 K. The critical temperature can be enhanced up to 11.0 K by applying compressive strain at only 1.6%. This study not only identifies a new binary s-block metal 2D material, namely Be2Au, which features planar hexacoordination, and a candidate superconducting material for further explorations, but also provides a new strategy to construct 2D materials with novel chemical bonding.

Bare and ligand protected planar hexacoordinate silicon in SiSb3M3 + (M = Ca, Sr, Ba) clusters

The occurrence of planar hexacoordination is very rare in main group elements. We report here a class of clusters containing a planar hexacoordinate silicon (phSi) atom with the formula SiSb3M3 + (M = Ca, Sr, Ba), which have D 3h (1A1') symmetry in their global minimum structure. The unique ability of heavier alkaline-earth atoms to use their vacant d atomic orbitals in bonding effectively stabilizes the peripheral ring and is responsible for covalent interaction with the Si center. Although the interaction between Si and Sb is significantly stronger than the Si-M one, sizable stabilization energies (-27.4 to -35.4 kcal mol-1) also originated from the combined electrostatic and covalent attraction between Si and M centers. The lighter homologues, SiE3M3 + (E = N, P, As; M = Ca, Sr, Ba) clusters, also possess similar D 3h symmetric structures as the global minima. However, the repulsive electrostatic interaction between Si and M dominates over covalent attraction making the Si-M contacts repulsive in nature. Most interestingly, the planarity of the phSi core and the attractive nature of all the six contacts of phSi are maintained in N-heterocyclic carbene (NHC) and benzene (Bz) bound SiSb3M3(NHC)6 + and SiSb3M3(Bz)6 + (M = Ca, Sr, Ba) complexes. Therefore, bare and ligand-protected SiSb3M3 + clusters are suitable candidates for gas-phase detection and large-scale synthesis, respectively.

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.

σ-Aromaticity Planar Pentacoordinate Beryllium Atoms

Six-valence-electron planar pentacoordinate beryllium (ppBe) is explored herein as a global minimum, which is only constructed by s-block metals in BeM₅⁺ (M = Cu, Ag, Au). The bonding in ppBe can be regarded as the excited-stated Be with a 2p_x¹2p_y¹ electronic configuration, forming electron sharing with doublet M₅⁺ motifs followed by two sets of Be(p_∥) → [M₅⁺] σ donations and one Be(s) ← [M₅⁺] σ back-donation. Thus, the σ aromaticity originating from three delocalized σ orbitals gives rise to the whole stability of the high D_5h-symmetry ppBe and strongly enriches s-block planar hypercoordinate bonding.

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.

Half-auxetic effect and ferroelasticity in a two-dimensional monolayer TiSe

Two-dimensional (2D) materials with both auxetic effect and ferroelasticity are rare, however, have great application potential in next generation microelectromechanical and nanoelectronic devices. Here, we report the findings of an extraordinary combination half-auxetic effect and ferroelasticity in a single p2mm-type TiSe monolayer by performing first-principles calculations. The unique half-auxetic effect, namely the material expand laterally under both uniaxial tensile strain, and compressive strain, is reported and explained by considering both the nearest and the next-nearest interactions. The ferroelasticity is stemming from the degeneracy breaking of the3d-orbitals of Ti atoms in a distorted tetrahedron crystal field, or the so-called Jahn-Teller effect. The results provide a guideline for the future design of novel 2D multiple functional materials at the nanoscale.

Effective CO Migration among Multiabsorbed Sites Achieves the Low-Barrier and High-Selective Conversion to C2 Products on the Ni2B5 Monolayer

For the electrochemical reduction of CO₂, CO is a crucial single-carbon product and a major intermediate to multicarbon products. Direct dimerization of CO is the most charming channel to C2 products, although the corresponding kinetic energy barrier causes a huge gap compared with other alternative pathways. The effective CO migration among multiple catalytic sites is predominant but has not been fully explored during the C-C bond formation and further protonation processes. Herein, the entirely planar global-minimum Ni₂B₅ monolayer with multikinds of catalytic sites is selected as an appropriate instance, on which CO can effectively migrate among different types of sites with the highest barrier of 0.64 eV. Most importantly, the computed ultralow barrier of direct *CO dimerization (0.17 eV), the limiting potentials for CH₂CH₂ (-0.13 V), and CH₃CH₂OH (-0.17 V) reach the optimal value until now, which all happen on the p-p type of dual-CO adsorption configurations after CO migration. Moreover, the hydrogen reduction side reaction is uncompetitive with the CO electrochemical reduction on all possible adsorption sites. This study demonstrates the significance of CO migration and opens a new avenue for CO reduction to high-density multicarbon products on the surface of catalysts possessing multikinds of catalytic sites.

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.

Planar pentacoordinate silicon and germanium atoms

The global minimum of XMg₄Y^- (X = Si, Ge; Y = In, Tl) and SiMg₃In₂ contains a planar pentacoordinate atom of group 14 other than carbon. Its design is based on the "localization" approach, replacing one or two peripheral atoms in XMg₅^2- by more electronegative ones. This change diminishes the repulsion and leads to stronger covalent X-Y bonds, stabilizing the planar pentacoordinate atom species.

Stability, electronic, and optical properties of two-dimensional phosphoborane

The structure and properties of two-dimensional phosphoborane sheets were computationally investigated using Density Functional Theory calculations. The calculated phonon spectrum and band structure point to dynamic stability and allowed characterization of the predicted two-dimensional material as a direct-gap semiconductor with a band gap of ~1.5 eV. The calculation of the optical properties showed that the two-dimensional material has a relatively small absorptivity coefficient. The parameters of the mechanical properties characterize the two-dimensional phosphoborane as a relatively soft material, similar to the monolayer of MoS₂ . Assessment of thermal stability by the method of molecular dynamics indicates sufficient stability of the predicted material, which makes it possible to observe it experimentally.

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 sixteen-valence-electron carbon-group 13 family with global penta-atomic planar tetracoordinate carbon: an ionic strategy

The design of planar tetracoordinate carbon (ptC) has always been a challenge due to its unique bonding mode that necessitates the perfect balance between the carbon center and surrounding ligands both electronically and mechanically. A unique type of 18-valence-electron (18ve) template, i.e., CAl42-, has been found to be very effective in designing various novel 18ve-species upon skeletal substitution. In this work, we showed that though ptC is not the global structure for the parent 16ve-CAl4, suitable skeletal substitution can allow for a series of global minimum ptC species. Theoretical calculations at the level of CCSD(T)/def2-QZVP//B3LYP/def2-QZVP for 35 carbon-group 13 systems with 16-ve, i.e., CXaYbZcKd (X, Y, Z, K = Al/Ga/In/Tl; 0 ≤ a, b, c, d ≤ 4, a + b + c + d = 4), showed that 9 systems (CAl3Tl, CGa3Tl, CGa2Tl2, CAl2GaTl, CAl2InTl, CGa2InTl, CAlGa2Tl, CGa2InTl and CAlGaInTl) possess global minimum ptC and 2 systems (CAl3In and CAl2Tl2) have quasi-GM ptC. Except for CAl3Tl and CAl3In, all the ptCs were predicted for the first time. All these stable ptC structures have the same skeleton and can be described as the same ionic sub-structure, i.e., [A-]B+. This study not only enriches 16ve-ptC, but also directly demonstrates that utilizing an ionic strategy, non-ptC CAl4 also can be used as a template to extend the ptC family.

Two-Dimensional Ferroelastic Semiconductors in Nb2SiTe4 and Nb2GeTe4 with Promising Electronic Properties

Two-dimensional crystals with coupling of ferroelasticity and attractive electronic properties offer unprecedented opportunities for achieving long-sought controllable devices. However, to date, the reported proposals are mainly based on hypothetical structures. Here, using first-principles calculations, we identify single-layer Nb₂ATe₄ (A = Si, Ge), which can be exfoliated from its layered bulk, as a promising candidate. Single-layer Nb₂ATe₄ is found to be dynamically, thermally, and chemically stable. It possesses excellent ferroelasticity with high reversible ferroelastic strain and a moderate ferroelastic transition energy barrier, which are beneficial for practical applications. Meanwhile, it harbors outstanding anisotropic electronic properties, including anisotropic carrier mobility and optical properties. More importantly, the anisotropic properties of single-layer Nb₂ATe₄ can be efficiently controlled through ferroelastic switching. These appealing properties combined with the experimental feasibility render single-layer Nb₂ATe₄ an extraordinary platform for realizing controllable devices.

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.

Superoctahedral two-dimensional metallic boron with peculiar magnetic properties

A novel two-dimensional ferromagnetic stable boron material has been predicted and exhaustively studied with DFT methods. Its magnetism can be described by the presence of two unpaired delocalized bonding elements inside each distorted octahedron.

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.

Band offsets in new BN/BX (X = P, As, Sb) lateral heterostructures based on bond-orbital theory

Identifying heterostructures with tunable band alignments remains a difficult challenge. Here, based on bond-orbital theory, we propose a series of new BN/BX (X = P, As, Sb) lateral heterostructures (LHS). Our first principles calculations reveal that the LHS interlines have a substantial impact on the electronic properties. Importantly, we start with the chemical concepts, such as bond length and strength as well as orbital overlap interaction, in an attempt to thoroughly investigate the electronic properties, namely the band offset, the band gap (Eg) and the state of the energy level. We demonstrate that the newly designed BN/BX LHS have profound implications for developing advanced optoelectronics, such as high-performance light-emitting diodes and lasers. Furthermore, the new BN/BX LHS designed from the chemical viewpoint can shed new light on overcoming the enormous hurdle of ineffective and laborious material design.