Be 2 C Monolayer with Quasi‐Planar Hexacoordinate Carbons: A Global Minimum Structure

Investigation of N 3 C 5 and B 3 C 5 bilayers as anode materials for Li-ion batteries by first-principles calculations

The best choice today for a realistic method of increasing the energy density of a metal-ion battery is to find novel, effective electrode materials. In this paper, we present a theoretical investigation of the properties of hitherto unreported two-dimensional B3 C5 and N3 C5 bilayer systems as potential anode materials for lithium-ion batteries. The simulation results show that N3 C5 bilayer is not suitable for anode material due to its thermal instability. On the other hand B3 C5 is stable, has good electrical conductivity, and is intrinsically metallic before and after lithium intercalation. The low diffusion barrier (0.27 eV) of Li atoms shows a good charge and discharge rate for B3 C5 bilayer. Moreover, the high theoretical specific capacity (579.57 mAh/g) connected with moderate volume expansion effect during charge/discharge processes indicates that B3 C5 is a promising anode material for Li-ion batteries.

From Porphyrin-Like Rings to High-Density Single-Atom Catalytic Sites: Unveiling the Superiority of p-C2N for Bifunctional Oxygen Electrocatalysis

With effective utilization of the catalytic site, single-atom catalysts (SACs) supported by nitrogen atoms surrounding built-in pores of two-dimensional (2D) materials, such as porphyrin/phthalocyanine-based covalent organic frameworks, have been highly promising electrocatalysts in the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) processes for the air electrode of the metal-air battery. However, the number of stable single-atom anchoring sites, i.e., accessible single-atom metal sites, has been concerning as a result of the appearance of heterogeneous or large and even supersized pores in substrate materials. 2D porous graphitic carbon nitride (PGCN) with a stronger stability and smaller component is regarded as a more potential alternative owing to similar controllability and designability. In this work, inspired by the robust coordinated TM-N4 environment of porphyrin/phthalocyanine molecules, novel p-C2N with a high density of porphyrin-like organic units is rationally designed. In well-designed p-C2N, a higher homogeneity and uniformity of coordination sites can enhance the electrocatalytic activity in the whole catalytic material and better prevent SACs from sintering and agglomerating into thermodynamically stable nanoclusters. Utilizing density functional theory (DFT), the stability of the p-C2N monolayer, TM@p-C2N, and OER/ORR catalytic activities of TM@p-C2N (TM including Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) are systematically evaluated. Among them, Ir@p-C2N (0.31 V of the OER and 0.36 V of the ORR), Co@p-C2N (0.47 and 0.22 V), and Rh@p-C2N (0.55 and 0.27 V) are screened as promising SACs for the bifunctional ORR and OER. The proposal of p-C2N guides a new direction for the development of TM-N-C-based SAC bifunctional electrocatalysts.

The Tunable Electronic and Optical Properties of Two-Dimensional Bismuth Oxyhalides

Two-dimensional (2D) bismuth oxyhalides (BiOX) have attracted much attention as potential optoelectronic materials. To explore their application diversity, we herewith systematically investigate the tunable properties of 2D BiOX using first-principles calculations. Their electronic and optical properties can be modulated by changing the number of monolayers, applying strain, and/or varying the halogen composition. The band gap shrinks monotonically and approaches the bulk value, the optical absorption coefficient increases, and the absorption spectrum redshifts as the layer number of 2D BiOX increases. The carrier transport property can be improved by applying tensile strain, and the ability of photocatalytic hydrogen evolution can be obtained by applying compressive strain. General strain engineering will be effective in linearly tuning the band gap of BiOX in a wide strain range. Strain, together with halogen composition variation, can tune the optical absorption spectrum to be on demand in the range from visible to ultraviolet. This suggests that 2D BiOX materials can potentially serve as tunable novel photodetectors, can be used to improve clean energy techniques, and have potential in the field of flexible optoelectronics.

Two-dimensional β-noble-transition-metal chalcogenide: novel highly stable semiconductors with manifold outstanding optoelectronic properties and strong in-plane anisotropy

In this work, five two-dimensional (2D) noble-transition-metal chalcogenide (NTMC) semiconductors, namely β-NX (N = Au, Ag; X = S, Se, Te), were designed and predicted by first-principles simulations. Structurally, the monolayer β-NX materials have good energetic, mechanical, dynamical, and thermal stability. They contain two inequivalent noble-transition-metal atoms in the unit cell, and the N-X bond comprises a partial ionic bond and a partial covalent bond. Regarding the electronic properties, the β-NX materials are indirect-band-gap semiconductors with appropriate band-gap values. They have tiny electron effective masses. The hole effective masses exhibit significant differences in different directions, indicating strongly anisotropic hole mobility. In addition, the coexistence of linear and square-planar channels means that the diffusion and transport of carriers should be anisotropic. In terms of optical properties, the β-NX materials show high absorption coefficients. The absorption and reflection characteristics reveal strong anisotropy in different directions. Therefore, the β-NX materials are indirect-band-gap semiconductors with good stability, high absorption coefficients, and strong mechanical, electronic, transport, and optical anisotropy. In the future, they could have great potential as 2D semiconductors in nano-electronics and nano-optoelectronics.

Searching for Systems with Planar Hexacoordinate Carbons

Here, we present evidence that the D2h M2C50/2+ (M = Li-K, Be-Ca, Al-In, and Zn) species comprises planar hexacoordinate carbon (phC) structures that exhibit four covalent and two electrostatic interactions. These findings have been made possible using evolutionary methods for exploring the potential energy surface (AUTOMATON program) and the Interacting Quantum Atoms (IQA) methodology, which support the observed bonding interactions. It is worth noting, however, that these structures are not the global minimum. Nonetheless, incorporating two cyclopentadienyl anion ligands (Cp) into the CaC52+ system has enhanced the relative stability of the phC isomer. Moreover, cycloparaphenylene ([8]CPP) provides system protection and kinetic stability. These results indicate that using appropriate ligands presents a promising approach for expanding the chemistry of phC species.

Computational studies on functionalized Janus MXenes MM'CT2, (M, M' = Zr, Ti, Hf, M ≠ M'; T = -O, -F, -OH): photoelectronic properties and potential photocatalytic activities

Motivated by the successful synthesis of Janus monolayers of transition metal dichalcogenides (i.e., MoSSe), we computationally investigated the structural, electronic, optical, and transport properties of functionalized Janus MXenes, namely MM'CT2 (M, M' = Zr, Ti, Hf, M ≠ M', T = -O, -F, -OH). The results of the calculations demonstrate that five stable O-terminated Janus MXenes (ZrTiCO2-I, ZrHfCO2-I, ZrHfCO2-III, HfTiCO2-I, and HfTiCO2-III), exhibit modest bandgaps of 1.37-1.94 eV, visible-light absorption (except for ZrHfCO2-I), high carrier mobility, and promising oxidization capability of photoinduced holes. Additionally, their indirect-gap, spatially separated electron-hole pairs, and the dramatic difference between the mobilities of electrons and holes could significantly limit the recombination of photoinduced electron-hole pairs. Our results indicate that the functionalized Janus MXene monolayers are ideal and promising materials for application in visible light-driven photocatalysis.

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.

Ultrahigh Lithium Storage Capacity of Al2C Monolayer in a Restricted Multilayered Growth Mechanism

Designing anode materials with high lithium specific capacity is crucial to the development of high energy density lithium (ion) batteries. Herein, a distinctive lithium growth mechanism, namely, the restricted multilayered growth for lithium, and a strategy for lithium storage are proposed to achieve a balance between ultrahigh specific capacity and the need to avert uncontrolled dendritic growth of lithium. In particular, based on first-principles computation, we show that the Al₂C monolayer with a planar tetracoordinate carbon structure can be an ideal platform for realizing the restricted multilayered growth mechanism as a two-dimensional (2D) anode material. Furthermore, the Al₂C monolayer exhibits the ultrahigh specific capacity of lithium of 4059 mAh/g, yet with a low diffusion barrier of 0.039-0.17 eV and low open circuit voltage in the range of 0.002-0.34 V. These novel properties render the Al₂C monolayer a promising anode material for future lithium (ion) batteries. Our study also offers a design of promising 2D anode materials with a high specific capacity, fast lithium-ion diffusion, and safe lithium storage.

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.

Design of Novel Transition-Metal-Doped C6N2 with High-Efficiency Polysulfide Anchoring and Catalytic Performances toward Application in Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries are highly expected because of their high theoretical specific capacity and energy density. However, its application still faces challenges, including the shuttle effect affecting the sulfur reduction reaction, the high decomposition energy barrier of Li₂S during charging, the volume change of sulfur, and the poor conductivity during charging and discharging. Here, combined with density functional theory and particle swarm optimization algorithm for the nitrogen carbide monolayer structural search (C_mN_8-m, m = 1-8), the surprising discovery is that a single metal-atom-doped C₆N₂ monolayer could effectively accelerate the conversion of lithium polysulfide and anchor lithium polysulfide during discharging and decrease the decomposition energy barrier of Li₂S during charging. This "anchoring and catalyzing" mechanism effectively reduces the shuttle effect and greatly improves the reaction kinetics. Among a series of metal atoms, Cr is the best doping element, and it exhibits suitable adsorption energy for polysulfides and the lowest decomposition energy barrier for Li₂S. This work opens up a new way for the development of transition-metal-doped carbon-nitrogen materials with an excellent catalytic activity for lithium polysulfide as cathode materials for Li-S batteries.

Particle Swarm Predictions of a SrB8 Monolayer with 12-Fold Metal Coordination

Materials containing planar hypercoordinate motifs greatly enriched the fundamental understanding of chemical bonding. Herein, by means of first-principles calculations combined with global minimum search, we discovered the two-dimensional (2D) SrB₈ monolayer, which has the highest planar coordination number (12) reported so far in extended periodic materials. In the SrB₈ monolayer, bridged B₈ units are forming the boron monolayer consisting of B₁₂ rings, and the Sr atoms are embedded at the center of these B₁₂ rings, leading to the Sr@B₁₂ motifs. The SrB₈ monolayer has good thermodynamic, kinetic, and thermal stabilities, which is attributed to the geometry fit between the size of the Sr atom and cavity of the B₁₂ rings, as well as the electron transfer from Sr atoms to electron-deficient boron network. Placing the SrB₈ monolayer on the Ag(001) surface shows good commensurability of the lattices and small vertical structure undulations, suggesting the feasibility of its experimental realization by epitaxial growth. Potential applications of the SrB₈ monolayer on metal ions storage (for Li, Na, and K) are explored.

Simultaneously Enhancing Catalytic Performance and Increasing Density of Bifunctional CuN3 Active Sites in Dopant-Free 2D C3N3Cu for Oxygen Reduction/Evolution Reactions

Atomically dispersed M-N-C has been considered an effective catalyst for various electrochemical reactions such as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), which faces the challenge of increasing metal load while simultaneously maintaining catalytic performance. Herein, we put forward a strategy for boosting catalytic performances of a single Cu atom coordinated with three N atoms (CuN₃) for both ORR and OER by increasing the density of connected CuN₃ moieties. Our calculations first show that a single CuN₃ moiety exhibiting no catalytic performance for ORR and OER can be activated by increasing the density of metal centers, which weakens the binding affinity to *OH due to the lowered d-band center of the metal atoms. These findings stimulate the further theoretical design of a two-dimensional compound of C₃N₃Cu with a high concentration of homogeneously distributed CuN₃ moieties serving as bifunctional active sites, which demonstrates efficient catalytic performance for both ORR and OER as reflected by the overpotentials of 0.71 and 0.43 V, respectively. This work opens a new avenue for designing effective single-atom catalysts with potential applications as energy storage and conversion devices possessing high density of metal centers independent of the doping strategy and defect engineering, which deserves experimental investigation in the future.

2D Nb3SBr7 and Ta3SBr7: Experimentally Achievable Janus Photocatalysts with Robust Coexistence of Strong Optical Absorption, Intrinsic Charge Separation, and Ultrahigh Solar-to-Hydrogen Efficiency

Recently, two-dimensional (2D) Janus semiconductors have attracted great attention in photocatalytic applications owing to their extraordinary properties, especially the intrinsic polarization-induced spontaneous carrier separation, strong optical absorption, and ultrahigh solar-to-hydrogen (STH) efficiency. However, experimental achievable candidates for 2D intrinsic Janus semiconductors are rarely reported. Herein, based on density functional theory (DFT) calculations, we uncovered two new 2D photocatalysts, namely, Janus Nb₃SBr₇ and Ta₃SBr₇ bilayers. We revealed that both structures are highly feasible to be obtained from their bulk counterparts. Excitingly, intrinsic charge separations emerge in both structures, which are beneficial to the repression of recombinations of their photoexcited carriers. Optical absorptions of both structures can be activated in the visible and even infrared regions. Most interestingly, Nb₃SBr₇ and Ta₃SBr₇ bilayers can exhibit ultrahigh STH efficiencies of 35% and 31%, respectively, which are larger than those of most 2D Janus structures. In addition, we further found that these distinguished photocatalytic properties are rather robust and are independent of their stacking modes. Experimental feasibilities and robust coexistences of intrinsic charge separations, ultrahigh STH efficiencies, and strong absorptions endow Nb₃SBr₇ and Ta₃SBr₇ bilayers as hopeful photocatalysts for water splitting.

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.

A topological superconductor, named Be₂Au monolayer, containing planar hexacoordinate s-block metal (Be and Au) atoms was theoretically designed by rationally assembling related clusters.

Metallic C5N monolayer as an efficient catalyst for accelerating redox kinetics of sulfur in lithium-sulfur batteries

Lithium-sulfur battery is one of the most promising applicants for the next generation of energy storage devices whose commercial applications are impeded by the key issue of the shuttle effect. To overcome this obstacle, various two-dimensional (2D) carbon-based metal-free compounds have been proposed to serve as anchoring materials for immobilizing soluble lithium polysulfides (LiPs), which however suffer from low electronic conductivity implying unsatisfactory performance for catalyzing sulfur redox. Therefore, we have predicted metallic C₅N monolayers, possessing hexagonal (H) and orthorhombic (O) phases, exhibiting excellent performance for suppressing the shuttle effect. First-principles simulations demonstrate that O-C₅N could serve as a bifunctional anchoring material due to its strong adsorption capability to LiPs and excellent catalytic performance for sulfur redox with active sites from both basal plane and zigzag edges. Furthermore, the rate of Li₂S oxidation over O-C₅N is fast due to the low energy barrier of 0.93 eV for Li₂S decomposition. While for H-C₅N, only N atoms located at the armchair edges can efficiently trap LiPs and boost the formation and dissociation of Li₂S during discharge and charge processes, respectively. The current work opens an avenue of designing 2D metallic carbon-based anchoring materials for lithium-sulfur batteries, which deserves further experimental research efforts.

Planar Hypercoordinate Carbons in Alkali Metal Decorated CE3 2- and CE2 2- Dianions

After exploring the potential energy surfaces of M_m CE₂ ^p (E=S-Te, M=Li-Cs, m=2, 3 and p=m-2) and M_n CE₃ ^q (E=S-Te, M=Li-Cs, n=1, 2, q=n-2) combinations, we introduce 38 new global minima containing a planar hypercoordinate carbon atom (24 with a planar tetracoordinate carbon and 14 with a planar pentacoordinate carbon). These exotic clusters result from the decoration of V-shaped CE₂ ^2- and Y-shaped CE₃ ^2- dianions, respectively, with alkali counterions. All these 38 systems fulfill the geometrical and electronic criteria to be considered as true planar hypercoordinate carbon systems. Chemical bonding analyses indicate that carbon is covalently bonded to chalcogens and ionically connected to alkali metals.

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.

BAl4Mg−/0/+: Global Minima with a Planar Tetracoordinate or Hypercoordinate Boron Atom

We have explored the chemical space of BAl4Mg−/0/+ for the first time and theoretically characterized several isomers with interesting bonding patterns. We have used chemical intuition and a cluster building method based on the tabu-search algorithm implemented in the Python program for aggregation and reaction (PyAR) to obtain the maximum number of possible stationary points. The global minimum geometries for the anion (1a) and cation (1c) contain a planar tetracoordinate boron (ptB) atom, whereas the global minimum geometry for the neutral (1n) exhibits a planar pentacoordinate boron (ppB) atom. The low-lying isomers of the anion (2a) and cation (3c) also contain a ppB atom. The low-lying isomer of the neutral (2n) exhibits a ptB atom. Ab initio molecular dynamics simulations carried out at 298 K for 2000 fs suggest that all isomers are kinetically stable, except the cation 3c. Simulations carried out at low temperatures (100 and 200 K) for 2000 fs predict that even 3c is kinetically stable, which contains a ppB atom. Various bonding analyses (NBO, AdNDP, AIM, etc.) are carried out for these six different geometries of BAl4Mg−/0/+ to understand the bonding patterns. Based on these results, we conclude that ptB/ppB scenarios are prevalent in these systems. Compared to the carbon counter-part, CAl4Mg−, here the anion (BAl4Mg−) obeys the 18 valence electron rule, as B has one electron fewer than C. However, the neutral and cation species break the rule with 17 and 16 valence electrons, respectively. The electron affinity (EA) of BAl4Mg is slightly higher (2.15 eV) than the electron affinity of CAl4Mg (2.05 eV). Based on the EA value, it is believed that these molecules can be identified in the gas phase. All the ptB/ppB isomers exhibit π/σ double aromaticity. Energy decomposition analysis predicts that the interaction between BAl4−/0/+ and Mg is ionic in all these six systems.

Stereomutation in Tetracoordinate Centers via Stabilization of Planar Tetracoordinated Systems

The quest for stabilizing planar forms of tetracoordinate carbon started five decades ago and intends to achieve interconversion between [R]- and [S]-stereoisomers without breaking covalent bonds. Several strategies are successful in making the planar tetracoordinate form a minimum on its potential energy surface. However, the first examples of systems where stereomutation is possible were reported only recently. In this study, the possibility of neutral and dications of simple hydrocarbons (cyclopentane, cyclopentene, spiropentane, and spiropentadiene) and their counterparts with the central carbon atom replaced by elements from groups 13, 14, and 15 are explored using ab initio MP2 calculations. The energy difference between the tetrahedral and planar forms decreases from row II to row III or IV substituents. Additionally, aromaticity involving the delocalization of the lone pair on the central atom appears to help in further stabilizing the planar form compared to the tetrahedral form, especially for the row II substituents. We identified 11 systems where the tetrahedral state is a minimum on the potential energy surface, and the planar form is a transition state corresponding to stereomutation. Interestingly, the planar structures of three systems were found to be minimum, and the corresponding tetrahedral states were transition states. The energy profiles corresponding to such transitions involving both planar and tetrahedral states without the breaking of covalent bonds were examined. The systems showcased in this study and research in this direction are expected to realize molecules that experimentally exhibit stereomutation.

Discovery of intrinsic two-dimensional antiferromagnets from transition-metal borides

Intrinsic two-dimensional (2D) magnets are promising materials for developing advanced spintronic devices. A few have already been synthesized from the exfoliation of van der Waals magnetic materials. In this work, by using ab initio calculations and Monte Carlo simulation, a series of 2D MBs (M = Cr, Mn or Fe; B = boron) are predicted possessing robust magnetism, sizeable magnetic anisotropy energy, and excellent structural stability. These 2D MBs can be respectively synthesized from non-van der Waals compounds with low separation energies such as Cr2AlB2, Mn2AlB2, and Fe2AlB2. 2D CrB is a ferromagnetic (FM) metal with a weak in-plane magnetic anisotropy energy of 23.6 μeV per atom. Metallic 2D MnB and FeB are Ising antiferromagnets with an out-of-plane magnetic easy axis and robust magnetic anisotropy energies up to 222.7 and 482.2 μeV per atom, respectively. By using Monte Carlo simulation, the critical temperatures of 2D CrB, MnB, and FeB were calculated to be 440 K, 300 K, and 320 K, respectively. Our study found that the super-exchange interaction plays the dominant role in determining the long-range magnetic ordering of 2D MBs. Moreover, most functionalized 2D MBTs (T = O, OH or F) are predicted to have AFM ground states. Alternating transition metals or functional groups can significantly modulate the magnetic ground state and critical temperature of 2D MBTs. This study suggests that the 2D MBs and MBTs are promising metallic 2D magnets for spintronic applications.

Planar Hexacoordinate Carbons: Half Covalent, Half Ionic

Herein, the first global minima containing a planar hexacoordinate carbon (phC) atom are reported. The fifteen structures belong to the CE₃ M₃ ⁺ (E=S-Te and M=Li-Cs) series and satisfy both geometric and electronic criteria to be considered as a true phC. The design strategy consisted of replacing oxygen in the D_3h  CO₃ Li₃ ⁺ structure with heavy and less electronegative chalcogens, inducing a negative charge on the C atom and an attractive electrostatic interaction between C and the alkali-metal cations. The chemical bonding analyses indicate that carbon is covalently bonded to three chalcogens and ionically connected to the three alkali metals.

CAl4Mg0/−: Global Minima with a Planar Tetracoordinate Carbon Atom

Isomers of CAl4Mg and CAl4Mg− have been theoretically characterized for the first time. The most stable isomer for both the neutral and anion contain a planar tetracoordinate carbon (ptC) atom. Unlike the isovalent CAl4Be case, which contains a planar pentacoordinate carbon atom as the global minimum geometry, replacing beryllium with magnesium makes the ptC isomer the global minimum due to increased ionic radii of magnesium. However, it is relatively easier to conduct experimental studies for CAl4Mg0/− as beryllium is toxic. While the neutral molecule containing the ptC atom follows the 18 valence electron rule, the anion breaks the rule with 19 valence electrons. The electron affinity of CAl4Mg is in the range of 1.96–2.05 eV. Both the global minima exhibit π/σ double aromaticity. Ab initio molecular dynamics simulations were carried out for both the global minima at 298 K for 10 ps to confirm their kinetic stability.

A first-principles comparative study of lithium, sodium, potassium and calcium storage in two-dimensional Mg2C

Two-dimensional (2D) materials have recently emerged as potential candidates for high-capacity lithium-ion batteries anode materials because of their compelling physicochemical and structural properties. In the present study, we use first-principles calculations to investigate the performance of 2D Mg₂C as anode materials for Li, Na, K and Ca-ions batteries. The calculated average open-circuit voltage are 0.37, 0.50, 0.03 and 0.06 eV vs Li, Na, K, Ca. No significant structural deformations are observed on the 2D Mg₂C upon the adsorption of Li, Na, K or Ca and the metallic characteristic of the 2D Mg₂C is retained. The metallic behaviour of both pristine and adsorbed Mg₂C ensures the desirable electric conductivity, implying the advantages of 2D Mg₂C for batteries. The Na and K atoms show an extremely high diffusivity on the 2D Mg₂C with a low energy barrier of 0.08 and 0.04 eV respectively, which is about an order of magnitude smaller than that of Li atom. For the Na and K atoms, the theoretical storage capacity can reach up to 1770 mAh g^-1, nearly two times that of the Li atom of 885 mAh g^-1. Our study suggests that the 2D Mg₂C is a promising anode material which offers a fast ion diffusion and high storage capacity.

Computational investigation of two-dimensional borides with planar octacoordinated main group elements

In this communication, by means of first principles calculations, we searched stable two-dimensional borides with octacoordinated main group elements, and we found that only AlB4 monolayer is stable in the planar octacoordinate motif through our stability screenings, which can be used for electrocatalyzing the hydrogen evolution reaction.

Beryllene: A Promising Anode Material for Na- and K-Ion Batteries with Ultrafast Charge/Discharge and High Specific Capacity

We predict two-dimensional Be materials, α- and β-beryllene. In α-beryllene each Be atom binds to six other Be atoms in a planar scheme, whereas β-beryllene consists of two stacked α-beryllene monolayers. Both α- and β-beryllene are found to be highly stable, as demonstrated by high cohesive energies close to that of bulk Be, an absence of imaginary phonon modes, and high melting points. Both materials are metallic, indicating potential applications in Na-ion and K-ion batteries, which are explored in detail. The diffusion barriers of Na (K) on α- and β-beryllene are found to be only 9 (3) and 4 (5) meV, respectively. In particular, the diffusion barrier of K on α-beryllene exhibits the lowest ever recorded value in two-dimensional materials, suggesting the possibility of ultrafast charge/discharge. As the theoretical specific capacities of Na/K on α- and β-beryllene are found to be 1487/1322 and 743/743 mA h g^-1, respectively, the storage capacity is ultrahigh.

Pressure-Induced Modulation of Electronic and Optical Properties of Surface O-Functionalized Ti2C MXene

Functionalized MXenes have gained increasing interest in the fields of thermoelectric materials, hydrogen storage, and so forth. In this work, pressure-induced band modulation and optical properties of the Ti2CO2 monolayer are investigated by using density functional theory with the hybrid (HSE06) functional. The calculation reveals that Ti2CO2 MXenes under pressure are stable because of the positive E coh. Ti2CO2 undergoes a semiconductor-to-metal phase transition at about 7 GPa. The metallization of Ti2CO2 mainly results from the Ti-d state. Research indicates that there exist strong interactions between Ti-d and C-p, and Ti-d and O-p states, which are further confirmed by the charge analysis. In addition, the absorption is enhanced in the visible region with increasing pressure. We also observed some new absorption peaks in the visible region.

A new approach to separate hydrogen from carbon dioxide using graphdiyne-like membrane

In order to separate a mixture of hydrogen (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {H}_{2}$$\end{document}H2) and carbon dioxide (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {CO}_{2}$$\end{document}CO2) gases, we have proposed a new approach employing the graphdiyne-like membrane (GDY-H) using density functional theory (DFT) calculations and molecular dynamics (MD) simulations. GDY-H is constructed by removing one-third diacetylenic (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{-}\text {C}{\equiv}\text {C}{-}\text {C}{\equiv}\text {C}{-}}$$\end{document}-C≡C-C≡C-) bonds linkages and replacing with hydrogen atoms in graphdiyne structure. Our DFT calculations exhibit poor selectivity and good permeances for \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {H}_{2}$$\end{document}H2/\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {CO}_{2}$$\end{document}CO2 gases passing through this membrane. To improve the performance of the GDY-H membrane for \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {H}_{2}$$\end{document}H2/\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {CO}_{2}$$\end{document}CO2 separation, we have placed two layers of GDY-H adjacent to each other which the distance between them is 2 nm. Then, we have inserted 1,3,5-triaminobenzene between two layers. In this approach, the selectivity of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {H}_{2}$$\end{document}H2/\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {CO}_{2}$$\end{document}CO2 is increased from 5.65 to completely purified \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {H}_{2}$$\end{document}H2 gas at 300 K. Furthermore, GDY-H membrane represents excellent permeance, about \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$10^8$$\end{document}108 gas permeation unit (GPU), for \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {H}_{2}$$\end{document}H2 molecule at temperatures above 20 K. The \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {H}_{2}$$\end{document}H2 permeance is much higher than the value of the usual industrial limits. Moreover, our proposed approach shows a good balance between the selectivity and permeance parameters for the gas separation which is an essential factor for \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {H}_{2}$$\end{document}H2 purification and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\text {CO}_{2}$$\end{document}CO2 capture processes in the industry.

Highly Stable Two-Dimensional Iron Monocarbide with Planar Hypercoordinate Moiety and Superior Li-Ion Storage Performance

Stable planar hypercoordinate motifs have been recently demonstrated in two-dimensional (2D) confinement systems, while perfectly planar hypercoordinate motifs in 2D carbon-transition metal systems are rarely reported. Here, by using comprehensive ab initio computations, we discover two new iron monocarbide (FeC) binary sheets stabilized at 2D confined space, labeled as tetragonal-FeC (t-FeC) and orthorhombic-FeC (o-FeC), which are energetically more favorable compared with the previously reported square and honeycomb lattices. The proposed t-FeC is the global minimum configuration in the 2D space, and each carbon atom is four-coordinated with four ambient iron atoms, considered as the quasi-planar tetragonal lattice. Strikingly, the o-FeC monolayer is an orthorhombic phase with a perfectly planar pentacoordinate carbon moiety and a planar seven-coordinate iron moiety. These monolayers are the first example of a simultaneously pentacoordinate carbon and planar seven-coordinate Fe-containing material. State-of-the-art theoretical calculations confirm that all these monolayers have significantly dynamic, mechanical, and thermal stabilities. Among these two monolayers, the t-FeC monolayer shows a higher theoretical capacity (395 mAh g^-1) and can stably adsorb Li up to t-FeCLi₄ (1579 mAh g^-1). The low migration energy barrier is predicted as small as 0.26 eV for Li, which results in the fast diffusion of Li atoms on this monolayer, making it a promising candidate for lithium-ion battery material.

Avoided spin coupling: an unexpected σ-σ diradical in global planar pentacoordinate carbon

We present a global planar pentacoordinate carbon (ppC) featuring a hitherto unreported σ-σ diradical characteristic. Using the multi-reference approach combined with the CCSD(T)/aug-cc-pVTZ method, the ppC C₃Li₃^- was found to be an intriguing triplet ground state, in which the unpaired density is mostly located at three Li ligands. Chemical bonding analysis reveals that the 2p_zπ electrons of C₃Li₃^- are fully located at the C₃ ring formed by C-C multiple bonds, in contrast to the perfect 2p_zπ-delocalization found in the well-known ppCs.

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 systematic investigation of the catalytic performances of monolayer carbon nitride nanosheets C1-xNx

Graphitic carbon nitrides (CNs) are potential candidate materials for the electro-catalytic industry due to their unique physical and chemical properties. However, to date, a full understanding of the electro-catalytic properties of CNs is still lacking. Herein, by using density functional theory calculations, we systematically investigate the catalytic performances in the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), N2 reduction reaction (NRR), and CO2 reduction reaction (CO2RR) of monolayer graphitic carbon nitrides (C1-xNx), C3N (x = 1/4), C2N (x = 1/3), and g-C3N4 (x = 4/7). We also evaluated the NRR activity of B doped C1-xNx, and the CO2RR activity of Cu and Pd modified C1-xNx. The cohesive energy and ab initio molecular dynamics (AIMD) results show that C3N, C2N, and g-C3N4 are stable at room temperature. The C3N-C1 site is predicted to deliver the best HER catalytic performance with a reaction Gibbs free energy (ΔGH*) of -0.03 eV (close to the ideal value (0 eV)). Among the studied C1-xNx materials, the C3N-C2 site is predicted to possess a favorable ηOER of 0.82 V for OER. Pure C3N, C2N, and g-C3N4 are not suitable for NRR and CO2RR. Due to the strong hybridization between the N 2p orbital and the B 2p orbital, the NRR performances of B doped BN-C2N, BN-C3N, and BN-g-C3N4 are greatly enhanced, with corresponding overpotential ηNRR of 0.57 V, 0.70 V, and 0.72 V, respectively. The transition metals Cu and Pd can enhance the CO2RR activity of C3N, C2N, and g-C3N4. The limiting potentials UL of pure C3N, C2N, and g-C3N4 are 0.96 V, 0.86 V, and 2.37 V, respectively, while these values are 0.63 V, 0.68 V, and 0.77 V with Cu or Pd modification. This work provides deep understanding of the catalytic properties of monolayer C1-xNx and guidance for synthesizing higher activity catalysts in the future.

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.

Two-Dimensional BAs/InTe: A Promising Tandem Solar Cell with High Power Conversion Efficiency

Tandem solar cells (SCs) connecting two subcells with different absorption bands have the potential to reach the commercialized photovoltaic standard. However, the performance improvement of tandem architectures is still a challenge, primarily owing to the mismatch of band gaps in two subcells. Here, we demonstrate a two-dimensional (2D) BAs/InTe-based tandem SC, which could achieve solar-to-electric conversion efficiency higher than 30%. First, the narrow band gap of hexagonal single-layer BX (X = P and As) and wide band gap of single-layer YZ (Y = Ga and In, Z = S, Se, and Te) are found to have high thermodynamic stability based on density functional theory calculations. Next, considering narrow and wide band gaps at the HSE06 functional, single-layer BX/YZ-based tandem SCs are built to effectively capture a broad-band solar spectrum by combining such two subcells. Since the band gap of single-layer BAs matches well with that of the InTe monolayer, the power conversion efficiency of BAs/InTe-based tandem SC can reach as high as 30.2%. Moreover, it is important to note that the used materials, including few-layer GaZ and InSe, have been experimentally prepared, which strongly supports the high feasibility of the designed 2D tandem SCs in this work. Our constructed 2D-material-based devices can be competitive in realizing commercialized high-performance tandem SCs.

High-efficiency helium separation through an inorganic graphenylene membrane: a theoretical study

Appropriate interactions between an IGP membrane and He molecules result in efficient helium separation.

First-principles study of structural, elastic and electronic properties of naphyne and naphdiyne

The structural, elastic and electronic properties of 2D naphyne and naphdiyne sheets, which consist of naphthyl rings and acetylenic linkages, are investigated using first-principles calculations. Both naphyne and naphdiyne belong to the orthorhombic lattice family and exhibit the Cmmm plane group. The structural stability of naphyne and naphdiyne are comparable to those of experimentally synthesized graphdiyne and graphtetrayne, respectively. The increase of acetylenic linkages provides naphdiyne with a larger pore size, a lower planar packing density and a lower in-plane stiffness than naphyne. Naphyne is found to be an indirect semiconductor with a band gap of 0.273 eV, while naphdiyne has no band gap and has a Dirac point. The band gaps of naphyne and naphdiyne are found to be modified by applied strain in the elastic range. These facts make naphyne and naphdiyne potential candidates for a wide variety of membrane separations and for fabrication of soft and strain-tunable nanoelectronic devices.

Naphyne and naphdiyne exhibit comparable stability to synthesized graphdiyne and graphtetrayne, and they show potential applications on membrane separations and fabrication of strain-tunable nanoelectronic devices.

Two-dimensional CaFCl: ultra-wide bandgap, strong interlayer quantum confinement, and n-type doping

Two-dimensional (2D) ultra-wide bandgap (UWBG) semiconductors have attracted tremendous attention because of their unique electronic properties and promising applications. Using first-principles calculations, monolayer (bilayer) CaFCl has a cleavage energy of 0.93 J m-2 (0.72 J m-2), suggesting that the exfoliation of monolayer and few-layer materials from the bulk phase could be feasible. The CaFCl monolayer is an UWBG semiconductor with a direct bandgap of 6.62 eV. In addition to the dynamic and thermodynamic stability, it can remain thermally stable at 2200 K, suitable for operation in high-temperature environments. The bandgap of monolayer CaFCl can be tuned by external strain and layer thickness. The decrease of the layer thickness leads to not only a bandgap increase but also an indirect-to-direct bandgap transition, suggesting a strong interlayer quantum confinement effect. Under biaxial strain, the direct bandgap can also be turned into an indirect one. The adsorption of a tetrathiafulvalene (TTF) molecule introduces deep donor states in the gap of CaFCl. Under an external electric field with direction from CaFCl to TTF, the TTF-derived donor states move closer to the conduction band edge of CaFCl and then the adsorption complex becomes effectively n-doped. Furthermore, monolayer CaFCl exhibits pronounced optical absorption in the ultraviolet range of the solar spectrum. These results render CaFCl an attractive 2D material for applications in flexible nanoelectronic and optoelectronic devices.

2D BeP2 monolayer: investigation of electronic and optical properties by driven modulated strain

Recently, the two-dimensional (2D) material beryllium diphosphide (BeP₂) has attracted significant attention for potential device applications due to its Dirac semimetal state, dynamic and thermal stability, and high carrier mobility. In this work, we investigated its electronic and optical properties under biaxial Lagrangian strain using density functional theory (DFT). Electronic band gaps and effective charge carrier mass were highly sensitive to the Lagrangian strain of BeP₂ monolayer. The bandgaps of BeP₂ varied from 0 eV to 0.30 eV for 2% to 8% strain, where the strain range is based on the final stable condition of the system. The absorption spectra for the dielectric properties show the highest absorption peaks in the infrared (IR) region. These abundant strain-dependent studies of the BeP₂ monolayer provide guidelines for its application in infrared sensors and devices.

BeP₂ monolayer is a promising material for the novel IR optical device.

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.

Isoelectronic analogues of graphene: the BCN monolayers with visible-light absorption and high carrier mobility

By employing particle-swarm optimization (PSO) and first-principles computations, we theoretically predicted five stable phases of graphene-like borocarbonitrides (g-BCN) with the stoichiometric ratio of 1:1:1 and uniformly distributed B, C, N atoms, which are the isoelectronic analogues of graphene. These g-BCN monolayers are effectively stabilized by their relatively high proportion of robust C-C or B-N bonds and strong partial ionic-covalent B-C and C-N bonds within them, leading to pronounced thermal and kinetic stability. The visible-light absorption and high carrier mobility of the investigated g-BCN monolayers indicate their possible applications in high-efficiency photochemical processes and electronic devices. Our computations could provide some guidance for designing the graphene-like materials with earth-abundant elements, as well as some clues for the experimental synthesis and practical applications of ternary BCN nanosheets.

Tetragonal and trigonal Mo2B2 monolayers: two new low-dimensional materials for Li-ion and Na-ion batteries

In this study, we report two new Mo₂B₂ monolayers and investigate their stabilities, electronic structures, lattice dynamics, and properties as anode materials for energy storage by using the crystal structure prediction technique and first-principles method.

Copper(i) sulfide: a two-dimensional semiconductor with superior oxidation resistance and high carrier mobility

Two-dimensional (2D) semiconductors with suitable direct band gaps, high carrier mobility, and excellent open-air stability are especially desirable for material applications. Herein, we show theoretical evidence of a new phase of a copper(i) sulfide (Cu₂S) monolayer, denoted δ-Cu₂S, with both novel electronic properties and superior oxidation resistance. We find that both monolayer and bilayer δ-Cu₂S have much lower formation energy than the known β-Cu₂S phase. Given that β-Cu₂S sheets have been recently synthesized in the laboratory (Adv. Mater.2016, 28, 8271), the higher stability of δ-Cu₂S than that of β-Cu₂S sheets suggests a high possibility of experimental realization of δ-Cu₂S. Stability analysis indicates that δ-Cu₂S is dynamically and thermally stable. Notably, δ-Cu₂S exhibits superior oxidation resistance, due to the high activation energy of 1.98 eV for the chemisorption of O₂ on δ-Cu₂S. On its electronic properties, δ-Cu₂S is a semiconductor with a modest direct band gap (1.26 eV) and an ultrahigh electron mobility of up to 6880 cm² V^-1 s^-1, about 27 times that (246 cm² V^-1 s^-1) of the β-Cu₂S bilayer. The marked difference between the electron and hole mobilities of δ-Cu₂S suggests easy separation of electrons and holes for solar energy conversion. Combination of these novel properties makes δ-Cu₂S a promising 2D material for future applications in electronics and optoelectronics with high thermal and chemical stability.