A review on role of tetra-rings in graphene systems and their possible applications

Boosting Potassium Adsorption and Diffusion Performance of Carbon Anodes for Potassium-Ion Batteries via Topology and Curvature Engineering: From KT-Graphene to KT-CNTs

We propose a two-dimensional carbon allotrope (named KT-graphene) by incorporating kagome and tetragonal lattices consisting of trigonal, quadrilateral, octagonal, and dodecagonal rings. The introduction of non-hexagonal rings can give rise to the localized electronic states that improve the chemical reactivity toward potassium, making KT-graphene a high-performance anode material for potassium-ion batteries. It shows a high theoretical capacity (892 mAh g-1), a low diffusion barrier (0.33 eV), and a low average open-circuit voltage (0.51 V). The presence of electrolyte solvents is propitious to boost the K-ion adsorption and diffusion capabilities. Moreover, one-dimensional nanotubes (KT-CNTs), rolled up by the KT-graphene sheet, are metallic regardless of the tube diameter. As the curvature increases, KT-CNTs exhibit significantly increased surface activity, which can promote the electron-donating ability of K. Furthermore, the curvature effect greatly enhances the efficiency of K diffusion on the inner surface compared to that on the outer surface.

Graphene oxide-based large-area dynamic covalent interfaces

Dynamic materials, being capable of reversible structural adaptation in response to the variation of external surroundings, have experienced significant advancements in the past several decades. In particular, dynamic covalent materials (DCMs), where the dynamic covalent bonds (DCBs) can reversibly break and reform under defined conditions, present superior dynamic characteristics, such as self-adaptivity, self-healing and shape memory. However, the dynamic characteristics of DCBs are mainly limited within the length scale of covalent bonds, due to the local position exchange or the inter-distance variation between the chemical compositions involved in the reversible covalent reactions. In this minireview, a discussion regarding the realization of long-range migration of chemical compositions along the interfaces of graphene oxide (GO)-based materials via the spatially connected and consecutive occurrence of DCB-based reversible covalent reactions is presented, and the interfaces are termed "large-area dynamic covalent interfaces (LDCIs)". The effective strategies, including water adsorption, interfacial curvature and metal-substrate support, as well as the potential applications of LDCIs in water dissociation and humidity sensing are summarized. Additionally, we also give an outlook on potential strategies to realize LDCIs on other 2D carbon-based materials, including the interfacial morphology and periodic element doping. This minireview provides insights into the realization of LDCIs on a wider range of 2D materials, and offers a theoretical perspective for advancing materials with long-range dynamic characteristics and improved performance, including controlled drug delivery/release and high-efficiency (bio)sensing.

Graphene-like emerging 2D materials: recent progress, challenges and future outlook

Owing to the unique physical and chemical properties of 2D materials and the great success of graphene in various applications, the scientific community has been influenced to explore a new class of graphene-like 2D materials for next-generation technological applications. Consequently, many alternative layered and non-layered 2D materials, including h-BN, TMDs, and MXenes, have been synthesized recently for applications related to the 4th industrial revolution. In this review, recent progress in state-of-the-art research on 2D materials, including their synthesis routes, characterization and application-oriented properties, has been highlighted. The evolving applications of 2D materials in the areas of electronics, optoelectronics, spintronic devices, sensors, high-performance and transparent electrodes, energy conversion and storage, electromagnetic interference shielding, hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and nanocomposites are discussed. In particular, the state-of-the-art applications, challenges, and outlook of every class of 2D material are also presented as concluding remarks to guide this fast-progressing class of 2D materials beyond graphene for scientific research into next-generation materials.

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.

Tunable band gap and enhanced thermoelectric performance of tetragonal Germanene under bias voltage and chemical doping

This paper employs the tight-binding model to investigate the thermal properties of tetragonal Germanene (T-Ge) affected by external fields and doping. T-Ge is a two-dimensional material with unique electronic properties, including zero band gap and two Dirac points. The electronic properties of T-Ge can be influenced by bias voltage, which can open its band gap and convert it to a semiconductor due to its buckling structure. The tunable band gap of biased T-Ge, makes it a a promising option for electronic and optoelectronic devices. The band structure of T-Ge is split by the magnetic field, leading to an increases its band edges due to the Zeeman Effect. The findings demonstrate that the thermoelectric properties of T-Ge are highly sensitive to external parameters and modifications of the band structure. The thermal and electrical conductivity of T-Ge increase with increasing temperature due to the rise in thermal energy of charge carriers. The thermoelectric properties of T-Ge decrease with bias voltage due to band gap opening, increase with the magnetic field due to a modifications of the band structure, and increase with chemical potential due to increasing density of charge carriers. By manipulating the band structure of T-Ge through bias voltage and chemical doping, the electrical conductivity can be optimized to achieve higher figure of merit (ZT) and improved thermoelectric performance. The results demonstrate the potential of T-Ge for use in electronic and magnetic devices, opening up new possibilities for further research and development in this field.

Locally spontaneous dynamic oxygen migration on biphenylene: a DFT study

The dynamic oxygen migration at the interface of carbon allotropes dominated by the periodic hexagonal rings, including graphene and carbon nanotubes, has opened up a new avenue to realize dynamic covalent materials. However, for the carbon materials with hybrid carbon rings, such as biphenylene, whether the dynamic oxygen migration at its interface can still be found remains unknown. Using both density functional theory calculations and machine-learning-based molecular dynamics (MLMD) simulations, we found that the oxygen migration departing away from the four-membered carbon (C₄) ring is hindered, and the oxygen atom prefers to spontaneously migrate toward/around the C₄ ring. This locally spontaneous dynamic oxygen migration on the biphenylene is attributed to a high barrier of about 1.5 eV for the former process and a relatively low barrier of about 0.3 eV for the latter one, originating from the enhanced activity of the C-O bond near/around the C₄ ring due to the hybrid carbon ring structure. Moreover, the locally spontaneous dynamic oxygen migration is further confirmed by MLMD simulations. This work sheds light on the potential of biphenylene as a catalyst for spatially controlled energy conversion and provides the guidance for realizing the dynamic covalent interface at other carbon-based or two-dimensional materials.

Tower carbon: a new large-cell carbon allotrope

The structural development of novel carbon materials has always been a hot spot in theoretical and experimental research, due to carbon possess a wide range of applications in the fields of industry and electronic technology. In this work, ansp²+sp³hybrid carbon allotrope, named tower carbon, is proposed and studied based on density functional theory, including its structure, stability, electronic and mechanical properties. The crystal structure of tower carbon is like a Chinese classical architectural tower, so it is named tower carbon, which belongs to the cubic crystal system, and it is stable in thermodynamics, dynamics, and mechanics. The electronic band structure of tower carbon is calculated by Heyd-Scuseria-Ernzerhof hybrid functional. The results show that tower carbon is metallic material. In addition, the anisotropy factor of tower carbon and the directional dependence of Young's modulus, shear modulus, and Poisson's ratio are estimated. Compared with cF320, the tower carbon has less anisotropy.

Graphdiyne Electrochemistry: Progress and Perspectives

Graphdiyne, a carbon allotrope, was synthesized in 2010 for the first time. It consists of two acetylene bonds between adjacent benzene rings. Graphdiyne and its composites thus exhibit ultrahigh intrinsic electrochemical activities. As "star" electrode materials, they have been utilized for various electrochemical applications. With the aim of giving a full screen of graphdiyne electrochemistry, this review starts from the history of graphdiyne materials, followed by their structural and electrochemical features. Recent progress and achievements in the synthesis of graphdiyne materials and their composites are overviewed. Subsequently, various electrochemical applications of graphdiyne materials and their composites are summarized, covering those in the fields of electrochemical energy conversion, electrochemical energy storage, and electrochemical sensing. The perspectives of graphdiyne electrochemistry are also discussed and outlined.

Emerging properties of carbon based 2D material beyond graphene

Graphene turns out to be the pioneering material for setting up boulevard to a new zoo of recently proposed carbon based novel two dimensional (2D) analogues. It is evident that their electronic, optical and other related properties are utterly different from that of graphene because of the distinct intriguing morphology. For instance, the revolutionary emergence of Dirac cones in graphene is particularly hard to find in most of the other 2D materials. As a consequence the crystal symmetries indeed act as a major role for predicting electronic band structure. Since tight binding calculations have become an indispensable tool in electronic band structure calculation, we indicate the implication of such method in graphene's allotropes beyond hexagonal symmetry. It is to be noted that some of these graphene allotropes successfully overcome the inherent drawback of the zero band gap nature of graphene. As a result, these 2D nanomaterials exhibit great potential in a broad spectrum of applications, viz nanoelectronics, nanooptics, gas sensors, gas storages, catalysis, and other specific applications. The miniaturization of high performance graphene allotrope based gas sensors to microscopic or even nanosized range has also been critically discussed. In addition, various optical properties like the dielectric functions, optical conductivity, electron energy loss spectra reveal that these systems can be used in opto-electronic devices. Nonetheless, the honeycomb lattice of graphene is not superconducting. However, it is proposed that the tetragonal form of graphene can be intruded to form new hybrid 2D materials to achieve novel superconducting device at attainable conditions. These dynamic experimental prospects demand further functionalization of these systems to enhance the efficiency and the field of multifunctionality. This topical review aims to highlight the latest advances in carbon based 2D materials beyond graphene from the basic theoretical as well as future application perspectives.

Electronic and thermal transport in novel carbon-based bilayer with tetragonal rings: a combined study using first-principles and machine learning approach

In this article, the structural, electronic and thermal transport characteristics of bilayer tetragonal graphene (TG) are systematically explored with a combination of first-principles calculations and machine-learning interatomic potential approaches. Optimized ground state geometry of the bilayer TG structure is predicted and examined by employing various stability criteria. Electronic bandstructure analysis confirmed that bilayer TG exhibits a metallic band structure similar to the monolayer T-graphene structure. Thermal transport characteristics of the bilayer TG structure are explored by analysing thermal conductivity, the Seebeck coefficient, and electrical conductivity. The electronic part of the thermal conductivity shows linearly increasing behaviour with temperature, however the lattice part exhibits the opposite character. The lattice thermal conductivity part is investigated in terms of the three phonon scattering rates and weighted phase space. On the other hand, the Seebeck coefficient goes through a transition from negative to positive values with increasing temperature. The Wiedemann-Franz law regarding electrical transport of the bilayer TG is verified and confirms the universal Lorentz number. Specific heat of the bilayer TG structure follows the Debye model at low temperature and constant behaviour at high temperature. Moreover, the Debye temperature of the bilayer TG structure is verified by ab initio calculations as well as fitting the specific heat data using the Debye model.

First-principles study of the optical and thermoelectric properties of tetragonal-silicene

We report the optical and thermoelectric properties of the two-dimensional Dirac material T-silicene (TS) sheet and nanoribbons (NRs) by first-principles calculations. Both the optical and thermoelectric properties of TS can be modified by tailoring the sheet into nanoribbons of different widths and edge geometries. The optical response of the structures is highly anisotropic. A π interband transition occurs in the visible range of incident light with parallel polarization. The optical response for asymmetric arm-chair TS nanoribbons (ATSNRs) is larger than for symmetric ATSNRs. The absorptions of asymmetric ATSNR are redshifted due to a decrease in the bandgap with the width of the NRs. Plasma frequencies of the sheet and the NRs are identified from the imaginary part of the dielectric function and electron energy loss spectra curves. Thermoelectric properties like electrical conductivity, Seebeck coefficient, power factor, and electronic figure of merit are also studied. Compared with graphene, the TS sheet possesses a higher electrical conductivity and a better figure of merit. Among the NRs, asymmetric ATSNRs exhibit a better thermoelectric performance. All these intriguing features of TS may shed light on fabricating smart opto-electronic and thermoelectric devices.

Emergence of magnetic anisotropy by surface adsorption of transition metal dimers on γ-graphyne framework

In this paper a systematic study is carried out to demonstrate the structural stability and magnetic novelty of adsorbing transition metal (TM) dimers (A-B) on graphyne (GY) surface, GY@A-B. Our research points out that the dimers are strongly adsorbed onto GY due to their large natural pores and the electron affinity of the sp-hybridized carbon atoms. Electronic properties of these dimer-graphyne composite systems are of particular importance as they behave as degenerate semiconductors with partial occupation of states atE_F. Furthermore, their remarkable spin polarization (>80%) at Fermi energy (E_F) can be of paramount importance in spintronics applications. Most of the GY@A-B structures exhibit large magnetic anisotropies as well as magnetic moments along the out-of-plane direction with respect to the GY surface. Particularly, GY@Co-Ir, GY@Ir-Ir and GY@Ir-Os structures possess positive magnetic anisotropic energies (MAE) of 121 meV, 81 meV and 137 meV, respectively, which are comparable to other well-known TM dimer doped systems. The emergence of high MAE can be understood using the second-order perturbation theory on the basis of the strong spin-orbit coupling (SOC) between the two TMs and the degeneracy of their d-orbitals nearE_F. A close correspondence between the simulated and the analytical results has been established through our work. Further, a simple estimation shows that, GY@A-B structures have the potential to store data up to 64 PB m^-2. These intriguing electronic characteristics along with magnetism suggest GY@A-B to be a promising material for future magnetic storage devices.

The topology and robustness of two Dirac cones in S-graphene: A tight binding approach

Present work reports an elegant method to address the emergence of two Dirac cones in a non-hexagonal graphene allotrope S-graphene (SG). We have availed nearest neighbour tight binding (NNTB) model to validate the existence of two Dirac cones reported from density functional theory (DFT) computations. Besides, the real space renormalization group (RSRG) scheme clearly reveals the key reason behind the emergence of two Dirac cones associated with the given topology. Furthermore, the robustness of these Dirac cones has been explored in terms of hopping parameters. As an important note, the Fermi velocity of the SG system (vF [Formula: see text] c/80) is almost 3.75 times that of the graphene. It has been observed that the Dirac cones can be easily shifted along the symmetry lines without breaking the degeneracy. We have attained two different conditions based on the sole relations of hopping parameters and on-site energies to break the degeneracy. Further, in order to perceive the topological aspect of the system we have obtained the phase diagram and Chern number of Haldane model. This exact analytical method along with the supported DFT computation will be very effective in studying the intrinsic behaviour of the Dirac materials other than graphene.

Electric field induced band tuning, optical and thermoelectric responses in tetragonal germanene: a theoretical approach

Transverse electric field breaks the sublattice symmetry and generates a band gap in the semi-metallic T-Ge structure.