Origins of Dirac cone formation in AB3 and A3B (A, B = C, Si, and Ge) binary monolayers

Comparative first principles investigation on the structural, optoelectronic and vibrational properties of strain-engineered graphene-like AlC3, BC3and C3N monolayers

Three cardinal two-dimensional semiconductorsviz., AlC3, BC3and C3N, closely resembling the graphene structure, are intriguing contenders for emerging optoelectronic and thermomechanical applications. Starting from a critical stability analysis, this density functional theory study delves into a quantitative assessment of structural, mechanical, electronic, optical, vibrational and thermodynamical properties of these monolayers as a function of biaxial strain(ε)in a sublinear regime(-2%⩽ε⩽4%)of elastic deformation. The structures with cohesive energies slightly smaller than graphene, manifest exceptional mechanical stiffness, flexibility and breaking stress. The mechanical parameters have been deployed to further cultivate acoustic attributes and thermal conductivity. The hexagonal structures with mixed ionic-covalent molecular bonds have indirect electronic band-gap and work-function acutely sensitive toε. Dispersions of optical dielectric function, energy loss, refractive index, extinction coefficient, reflectivity, absorption coefficient and conductivity are deciphered in the UV-Vis-NIR regime against strain, where particular frequency bands featuring high polarization, dissipation, absorbance or reflectance are identified. Phonon band-structure and density of states testify dynamic stability in the ground state for all systems except the compressed ones. A comprehensive group theoretical analysis is performed to cultivate rotational; infrared and Raman-active modes, and the nature of molecular vibrations is delineated. The red-shifting of phonon bands andE2g/A1gRaman peaks with increasingε, associates estimation of Grüneisen parameter. Finally, strain-induced alterations of thermodynamic quantities such as entropy, enthalpy, free energy, heat capacity and Debye temperature are studied, followed by a molecular dynamics-based stability assessment under canonical ensemble.

Dirac cones in bipartite square-octagon lattice: A theoretical approach

Dirac cones are difficult to achieve in a square lattice with full symmetry. Here, we have theoretically investigated a bipartite tetragonal lattice composed of tetragons and octagons using both Tight-Binding (TB) model and density functional theory (DFT) calculations. The TB model predicts that the system exhibits nodal line semi-metallic properties when the on-site energies of all atoms are identical. When the on-site energies differ, the formation of an elliptical Dirac cone is predicted. Its physical properties (anisotropy, tilting, merging, and emerging) can be regulated by the hopping energies. An exact analytical formula is derived to determine the position of the Dirac point by the TB parameters, and a criterion for the existence of Dirac cones is obtained. The "divide-and-coupling" method is applied to understand the origin of the Dirac cone, which involves dividing the bands into several groups and examining the couplings among inter-groups and intra-groups. Various practical systems computed by DFT methods, e.g., t-BN, t-Si, 4,12,2-graphyne, and t-SiC, are also examined, and they all possess nodal lines or Dirac cones as predicted by the TB model. The results provide theoretical foundation for designing novel Dirac materials with tetragonal symmetry.

Unveiling the effect of 2D silagraphene structural diversity on electronic properties: DFT, DOS, and ELF studies

Recently, fully π-functional two-dimensional (2D) materials have been reported for electronic device applications. Graphene is one of these 2D materials that is attributed to 2D electron confinement effects and exhibits an aromatic character; however, it is characterized by vanishing the bandgap energy. Hence, research was focused on the discovery of graphene-based 2D materials to reduce the bandgap energy. Herein, we investigate the silagraphene structures (Si_xC_y) using DFT calculations to undertake and improve structural, physico-chemical, and electronic properties. Various types of 2D networks have been investigated by considering C-C and C-Si bonds in relative positions. Both conjugation and hyperconjugation phenomenon have been deeply examined and it seemed that they take advantage of each other depending on the C-C and C-Si bond positions. Localized orbital locator (LOL) and electron localization function (ELF) were also performed to examine the electronic densities in the investigated 2D networks and unveil the electronic properties of the studied materials.

The Dirac cone in two-dimensional tetragonal silicon carbides: a ring coupling mechanism

The exploration of novel two-dimensional semimetallic materials is always an attractive topic. We propose a series of two-dimensional silicon carbides with a tetragonal lattice. The band structure of silicon carbides with tetragonal carbon rings and silicon rings exhibits Dirac cones. Interestingly, the Dirac cone of tetragonal SiC originates from a "ring coupling" mechanism. This mechanism refers to the mutual coupling between the four carbon atoms in the tetragonal C ring, and the same coupling in the tetragonal Si ring. Additionally, the "ring coupling" mechanism is applicable to other group IV binary compounds such as monolayer GeC and SnC. This work provides reliable evidence for the argument that two-dimensional tetragonal materials can produce Dirac cones.

Electronic and optical properties of boron-based hybrid monolayers

Anisotropic 2D Dirac cone materials are important for the fabrication of nanodevices having direction-dependent characteristics since the anisotropic Dirac cones lead to different values of Fermi velocities yielding variable carrier concentrations. In this work, the feasibility of the B-based hybrid monolayers BX (X = As, Sb, and Bi), as anisotropic Dirac cone materials is investigated. Calculations based on density functional theory and molecular dynamics method find the stability of these monolayers exhibiting unique electronic properties. For example, the BAs monolayer possesses a robust self-doping feature, whereas the BSb monolayer carries the intrinsic charge carrier concentration of the order of 10¹²cm^-2which is comparable to that of graphene. Moreover, the direction-dependent optical response is predicted in these B-based monolayers; a high IR response in thex-direction is accompanied with that in the visible region along they-direction. The results are, therefore, expected to help in realizing the B-based devices for nanoscale applications.

SiC3 as a Charge-Regulated Material for CO2 Capture

The increasing CO2 emission rate is deteriorating the atmospheric environment, leading to global warming and climate change. The potential of the SiC3 nanosheet as a functioning material for the separation of CO2 from the mixture of CO2, H2, N2 and CH4 by injecting negative charges is studied by DFT calculations in this paper. The results show that in the absence of injecting negative charges, CO2 interacts weakly with the SiC3 nanosheet. While the interaction between CO2 and the SiC3 nanosheet can be strengthened by the injection of negative charges, the absorption mechanism of CO2 changes from physisorption to chemisorption when the injection of negative charges is switched on. H2/N2/CH4 are all physiosorbed on the SiC3 nanosheet with/without the injection of negative charges. The mechanism of CO2 adsorption/desorption on the SiC3 nanosheet could be tuned by switching on/off the injection of negative charges. Our results indicate that the SiC3 nanosheet can be regarded as a charge-regulated material for the separation of CO2 from the CO2/H2/N2/CH4 mixture.

Half-Dirac semimetals and the quantum anomalous Hall effect in Kagome Cd2N3 lattices

Half-Dirac semimetals (HDSs), which possess 100% spin-polarizations for Dirac materials, are highly desirable for exploring various topological phases of matter as low-dimensionality opens unprecedented opportunities for manipulating the quantum state of low-cost electronic nanodevices. The search for high-temperature HDSs is still a current hotspot and yet challenging experimentally. Herein based on first-principles calculations, we propose the realization of Half Dirac semimetals (HDS) in two-dimensional (2D) Kagome transition-metal nitride Cd₂N₃, which is robust against strain engineering. Monte Carlo simulations reveal that Cd₂N₃ possesses a Curie temperature reaching up to T _C = 225 K, which is much higher than that of the reported monolayers CrI₃ (T _C = 45 K) and Cr₂Ge₂Te₆ (T _C = 20 K). The band crossings in Cd₂N₃ are gapped out by the spin-orbit coupling, which brings about the quantum anomalous Hall (QAH) effect with a sizeable band gap of E _g = 4.9 meV, characterized by the nonzero Chern number (C = 1) and chiral edge states. A tight-binding model is further used to clarify the origin of HDSs and nontrivial electronic properties. The results suggest monolayer transition-metal nitrides as a promising platform to explore fascinating physical phenomena associated with novel 2D emergent HDSs and QAH insulators toward realistic spintronics devices, thus stimulating experimental interest.

Mirror symmetry origin of Dirac cone formation in rectangular two-dimensional materials

The Dirac cone (DC) band structure of graphene was thought to be unique to the hexagonal symmetry of its honeycomb lattice. However, two-dimensional (2D) materials possessing rectangular unit cells, e.g. unitary 6,6,12-graphyne and binary t1/t2-SiC, were also found to have DC band features. In this work, a "mirror symmetry parity coupling (MSPC)" mechanism is proposed to elaborate on the DC formation process of 6,6,12-graphyne with the tight-binding method combined with density functional calculations. First, atoms of unit cells are divided into two groups, each of which possesses its own mirror symmetry. Second, wave atom functions within each group are combined into two sets of normalized orthogonal wave functions with an odd and even parity symmetry, respectively, followed by couplings among intragroups and intergroups. The MSPC mechanism, in general, can explain the origins of the DC band structures of a category of 2D materials possessing mirror symmetry and rectangular or hexagonal unit cells. The important role of symmetry analysis, especially mirror symmetry, in understanding DC formation is demonstrated, which may serve as a critical design criterion for novel DC materials.

Recent advances of novel ultrathin two-dimensional silicon carbides from a theoretical perspective

Compared to graphene with semimetallic features, two-dimensional (2D) silicon carbide (Si-C) materials constitute another highly promising family for opto-electronic applications owing to their intrinsic electronic gaps. Recent theoretical studies of 2D Si-C materials thoroughly investigated their structure and properties. Herein, we overview these high-throughput approaches aiming to theoretically design 2D Si-C crystals. Graphene-like siligraphene and non-siligraphene are described in terms of morphology, physicochemical properties and potential applications based on the insights provided by simulations. In addition, the current progress of experimental exploration of 2D Si-C materials and underlying challenges are assessed as well.

Borophosphene: A New Anisotropic Dirac Cone Monolayer with a High Fermi Velocity and a Unique Self-Doping Feature

Two-dimensional (2D) Dirac cone materials exhibit linear energy dispersion at the Fermi level, where the effective masses of carriers are very close to zero and the Fermi velocity is ultrahigh, only 2-3 orders of magnitude lower than the light velocity. Such Dirac cone materials have great promise in high-performance electronic devices. Herein, we have employed the genetic algorithm methods combined with first-principles calculations to propose a new 2D anisotropic Dirac cone material, an orthorhombic boron phosphide (BP) monolayer named borophosphene. Molecular dynamics simulation and phonon dispersion have been used to evaluate the dynamic and thermal stability of borophosphene. Because of the unique arrangements of B-B and P-P dimers, the mechanical and electronic properties are highly anisotropic. Of great interest is the fact that the Dirac cone of the borophosphene is robust, independent of in-plane biaxial and uniaxial strains, and can also be observed in its one-dimensional zigzag nanoribbons and armchair nanotubes. The Fermi velocities are ∼10⁵ m/s, on the same order of magnitude as that of graphene. By using a tight-binding model, the origin of the Dirac cone of borophosphene is analyzed. Moreover, a unique feature of self-doping can be induced by the in-plane biaxial and uniaxial strains of borophosphene and the curvature effect of nanotubes, which is greatly beneficial for realizing high-speed carriers (holes). Our results suggest that the borophosphene holds great promise for high-performance electronic devices, which could promote experimental and theoretical studies for further exploring the potential applications of other 2D Dirac cone sheets.

Stabilization of planar tetra-coordinate silicon in a 2D-layered extended system and design of a high-capacity anode material for Li-ion batteries

Stabilization of planar tetra-coordinate silicon (ptSi) was achieved in compounds and 2D-layered extended systems, in which single molecular ptSi in C₁₂H₈Si captures four additional electrons to maintain a stable planar structure while the extending conjugate interactions are responsible for the stabilization of ptSi in the 2D sheet. Based on the ptSi SiC₁₂ building block, a SiC₈ siligraphene 2D sheet was constructed, and each of its ptSi could accommodate six lithium atoms. The electronic and lithium-storage properties of the ptSi 2D network were explored using first-principles calculations and ab initio molecular dynamics (AIMD) simulations. The newly designed 2D SiC₈ sheet has high thermal and dynamic stability, good electronic conductivity, strong lithium-storage ability, a large theoretical capacity of 1297 mA h g^-1, and facile surface diffusion of Li and Li⁺. The predicted relatively high average cell voltages from 2.24 to 2.47 V are fairly stable as the lithium content varies. These unique properties of the 2D SiC₈ sheet with ptSi make it quite appealing as a novel anode material for high-performance Li-ion batteries (LIBs).

A New Anisotropic Dirac Cone Material: A B2S Honeycomb Monolayer

Different from the isotropic Dirac cones existing in other two-dimensional (2D) materials, anisotropic Dirac cones have the merit of anisotropic carrier mobility for applications in direction-dependent quantum devices. Motivated by the recent experimental finding of an anisotropic Dirac cone in borophene, here we report a new 2D anisotropic Dirac cone material, B₂S monolayer, identified by using a global structure search method and first-principles calculation combined with a tight-binding model. The B₂S monolayer is found to be stable mechanically, thermally, and dynamically and exhibits an anisotropic Dirac cone exactly at the Fermi level, showing a Fermi velocity of 10⁶ m/s in the same order of magnitude as that of graphene. Moreover, B₂S monolayer is the first anisotropy Dirac cone material with a pristine honeycomb structure stabilized by S in free-standing conditions where each atom has four valence electrons on average being isoelectronic to C. This study would expand the Dirac cone material family with new features.