New metallic allotropes of planar and tubular carbon

Proposing TODD-graphene as a novel porous 2D carbon allotrope designed for superior lithium-ion battery efficiency

The category of 2D carbon allotropes has gained considerable interest due to its outstanding optoelectronic and mechanical characteristics, which are crucial for various device applications, including energy storage. This study uses density functional theory calculations, ab initio molecular dynamics (AIMD), and classical reactive molecular dynamics (MD) simulations to introduce TODD-Graphene, an innovative 2D planar carbon allotrope with a distinctive porous arrangement comprising 3-8-10-12 carbon rings. TODD-G exhibits intrinsic metallic properties with a low formation energy and stability in thermal and mechanical behavior. Calculations indicate a substantial theoretical capacity for adsorbing Li atoms, revealing a low average diffusion barrier of 0.83 eV. The metallic framework boasts excellent conductivity and positioning TODD-G as an active layer for superior lithium-ion battery efficiency. Charge carrier mobility calculations for electrons and holes in TODD-G surpass those of graphene. Classical reactive MD simulation results affirm its structural integrity, maintaining stability without bond reconstructions at 2200 K.

Prediction of highly stable 2D carbon allotropes based on azulenoid kekulene

Despite enormous interest in two-dimensional (2D) carbon allotropes, discovering stable 2D carbon structures with practically useful electronic properties presents a significant challenge. Computational modeling in this work shows that fusing azulene-derived macrocycles - azulenoid kekulenes (AK) - into graphene leads to the most stable 2D carbon allotropes reported to date, excluding graphene. Density functional theory predicts that placing the AK units in appropriate relative positions in the graphene lattice opens the 0.54 eV electronic bandgap and leads to the appearance of the remarkable 0.80 eV secondary gap between conduction bands - a feature that is rare in 2D carbon allotropes but is known to enhance light absorption and emission in 3D semiconductors. Among porous AK structures, one material stands out as a stable narrow-multigap (0.36 and 0.56 eV) semiconductor with light charge carriers (me = 0.17 m0,  mh = 0.19 m0), whereas its boron nitride analog is a wide-multigap (1.51 and 0.82 eV) semiconductor with light carriers (me = 0.39 m0,  mh = 0.32 m0). The multigap engineering strategy proposed here can be applied to other carbon nanostructures creating novel 2D materials for electronic and optoelectronic applications.

Bespoke Tailoring of Graphenoid Sheets: A Rippled Molecular Carbon Comprising Cyclically Fused Nonbenzenoid Rings

The incorporation of nonbenzenoid rings into the hexagonal networks of graphenoid nanostructures is of immense importance for electronic, magnetic, and mechanical properties, but the underlying mechanisms of nonbenzenoid ring fusion are rather unexplored. Here, we report the synthesis and characterization of a rippled C84 molecular carbon, which contains 10 nonbenzenoid rings (five-, seven-, and eight-membered rings) that are contiguously fused to give a cyclic geometry. The fused nonbenzenoid rings impart high solubility, configurational stability, multiple reversible redox behaviors, unique aromaticity, and a narrow band gap to the system. Moreover, this carbon nanostructure allows for further functionalization via electrophilic substitution and metalation reactions, enabling access to finely tuned derivatives. Interestingly, both the bowl-shaped and planar conformations of the core in molecular carbon are observed in the solid state. Additionally, this molecular carbon displays ambipolar transport characteristics.

Prospects of 2D graphdiynes and their applications in desalination and wastewater remediation

Water is an indispensable part of human life that affects health and food intake. Water pollution caused by rapid industrialization, agriculture, and other human activities affects humanity. Therefore, researchers are prudent and cautious regarding the use of novel materials and technologies for wastewater remediation. Graphdiyne (GDY), an emerging 2D nanomaterial, shows promise in this direction. Graphdiyne has a highly symmetrical π-conjugated structure consisting of uniformly distributed pores; hence, it is favorable for applications such as oil-water separation and organic-pollutant removal. The acetylenic linkage in GDY can strongly interact with metal ions, rendering GDY applicable to heavy-metal adsorption. In addition, GDY membranes that exhibit 100% salt rejection at certain pressures are potential candidates for wastewater treatment and water reuse via desalination. This review provides deep insights into the structure, properties, and synthesis methods of GDY, owing to which it is a unique, promising material. In the latter half of the article, various applications of GDY in desalination and wastewater treatment have been detailed. Finally, the prospects of these materials have been discussed succinctly.

Mechanical properties of hydrogenated ψ-graphene

CONTEXT

Hydrogenation is an effective way to open a band gap of the metallic ψ-graphene, expanding its application in electronics. Evaluating the mechanical properties of hydrogenated ψ-graphene, especially the effect of hydrogen coverage, is also crucial to the application of ψ-graphene. Here, we demonstrate the mechanical properties of ψ-graphene depend closely on the hydrogen coverage and arrangement. Upon hydrogenation, Young's modulus and intrinsic strength of ψ-graphene decrease due to breaking of sp² carbon networks. Both the ψ-graphene and hydrogenated ψ-graphene exhibit mechanical anisotropy. During changing the hydrogen coverage, the variation of mechanical strength of the hydrogenated ψ-graphene relies on the tensile direction. In addition, the arrangement of hydrogen also contributes to the mechanical strength and fracture behavior of hydrogenated ψ-graphene. Our results not only present a comprehensive understanding of the mechanical properties of hydrogenated ψ-graphene, but also provide a reference to tailor the mechanical properties of other graphene allotropes, which are of potential interest in materials science.

METHODS

Vienna ab initio simulation package based on the plane-wave pseudopotential technique was employed for the calculations. The exchange-correlation interaction was described by the Perdew-Burke-Ernzerhof functional within the general gradient approximation and the ion-electron interaction was treated with the projected augmented wave pseudopotential.

On-Surface Synthesis of a Carbon Nanoribbon Composed of 4-5-6-8-Membered Rings

From the structure point of view, there are a number of ways of tiling a carbon sheet with different polygons, resulting in prospects of tailoring electronic structures of low-dimensional carbon nanomaterials. However, up to now, the experimental fabrication of such structures embedded with periodic nonhexagon carbon polygons, especially ones with more than three kinds, is still very challenging, leaving their potential properties unexplored. Here we report the bottom-up synthesis of a nanoribbon composed of 4-5-6-8-membered rings via lateral fusion of polyfluorene chains on Au(111). Scanning probe microscopy unequivocally determines both the geometric structure and the electronic properties of such a nanoribbon, revealing its semiconducting property with a bandgap of ∼1.4 eV on Au(111). We expect that this work could be helpful for designing and synthesizing complicated carbon nanoribbons.

Raman Study of the Diamond to Graphite Transition Induced by the Single Femtosecond Laser Pulse on the (111) Face

The use of the ultrafast pulse is the current trend in laser processing many materials, including diamonds. Recently, the orientation of the irradiated crystal face was shown to play a crucial role in the diamond to graphite transition process. Here, we develop this approach and explore the nanostructure of the sp² phase, and the structural perfection of the graphite produced. The single pulse of the third harmonic of a Ti:sapphire laser (100 fs, 266 nm) was used to study the process of producing highly oriented graphite (HOG) layers on the (111) surface of a diamond monocrystal. The laser fluence dependence on ablated crater depth was analyzed, and three different regimes of laser-induced diamond graphitization are discussed, namely: nonablative graphitization, customary ablative graphitization, and bulk graphitization. The structure of the graphitized material was investigated by confocal Raman spectroscopy. A clear correlation was found between laser ablation regimes and sp² phase structure. The main types of structural defects that disrupt the HOG formation both at low and high laser fluencies were determined by Raman spectroscopy. The patterns revealed give optimal laser fluence for the production of perfect graphite spots on the diamond surface.

Evaluating the performance of ReaxFF potentials for sp2 carbon systems (graphene, carbon nanotubes, fullerenes) and a new ReaxFF potential

We study the performance of eleven reactive force fields (ReaxFF), which can be used to study sp² carbon systems. Among them a new hybrid ReaxFF is proposed combining two others and introducing two different types of C atoms. The advantages of that potential are discussed. We analyze the behavior of ReaxFFs with respect to 1) the structural and mechanical properties of graphene, its response to strain and phonon dispersion relation; 2) the energetics of (n, 0) and (n, n) carbon nanotubes (CNTs), their mechanical properties and response to strain up to fracture; 3) the energetics of the icosahedral C₆₀ fullerene and the 40 C₄₀ fullerene isomers. Seven of them provide not very realistic predictions for graphene, which made us focusing on the remaining, which provide reasonable results for 1) the structure, energy and phonon band structure of graphene, 2) the energetics of CNTs versus their diameter and 3) the energy of C₆₀ and the trend of the energy of the C₄₀ fullerene isomers versus their pentagon adjacencies, in accordance with density functional theory (DFT) calculations and/or experimental data. Moreover, the predicted fracture strain, ultimate tensile strength and strain values of CNTs are inside the range of experimental values, although overestimated with respect to DFT. However, they underestimate the Young’s modulus, overestimate the Poisson’s ratio of both graphene and CNTs and they display anomalous behavior of the stress - strain and Poisson’s ratio - strain curves, whose origin needs further investigation.

Shape Entropy of a Reconfigurable Ising Surface

Disclinations in a 2D sheet create regions of Gaussian curvature whose inversion produces a reconfigurable surface with many distinct metastable shapes, as shown by molecular dynamics of a disclinated graphene monolayer. This material has a near-Gaussian "density of shapes" and an effectively antiferromagnetic interaction between adjacent cones. A∼10  nm patch has hundreds of distinct metastable shapes with tunable stability and topography on the size scale of biomolecules. As every conical disclination provides an Ising-like degree of freedom, we call this technique "Isigami."

Machine learning the metastable phase diagram of covalently bonded carbon

Conventional phase diagram generation involves experimentation to provide an initial estimate of the set of thermodynamically accessible phases and their boundaries, followed by use of phenomenological models to interpolate between the available experimental data points and extrapolate to experimentally inaccessible regions. Such an approach, combined with high throughput first-principles calculations and data-mining techniques, has led to exhaustive thermodynamic databases (e.g. compatible with the CALPHAD method), albeit focused on the reduced set of phases observed at distinct thermodynamic equilibria. In contrast, materials during their synthesis, operation, or processing, may not reach their thermodynamic equilibrium state but, instead, remain trapped in a local (metastable) free energy minimum, which may exhibit desirable properties. Here, we introduce an automated workflow that integrates first-principles physics and atomistic simulations with machine learning (ML), and high-performance computing to allow rapid exploration of the metastable phases to construct "metastable" phase diagrams for materials far-from-equilibrium. Using carbon as a prototypical system, we demonstrate automated metastable phase diagram construction to map hundreds of metastable states ranging from near equilibrium to far-from-equilibrium (400 meV/atom). We incorporate the free energy calculations into a neural-network-based learning of the equations of state that allows for efficient construction of metastable phase diagrams. We use the metastable phase diagram and identify domains of relative stability and synthesizability of metastable materials. High temperature high pressure experiments using a diamond anvil cell on graphite sample coupled with high-resolution transmission electron microscopy (HRTEM) confirm our metastable phase predictions. In particular, we identify the previously ambiguous structure of n-diamond as a cubic-analog of diaphite-like lonsdaelite phase.

Hexatetra-Carbon: A Novel Two-Dimensional Semiconductor Allotrope of Carbon

Employing first-principles calculations based on density functional theory (DFT), we designed a novel two-dimensional (2D) elemental monolayer allotrope of carbon called hexatetra-carbon. In the hexatetra-carbon structure, each carbon atom bonds with its four neighboring atoms in a 2D double layer crystal structure, which is formed by a network of carbon hexagonal prisms. Based on our calculations, it is found that hexatetra-carbon exhibits a good structural stability as confirmed by its rather high calculated cohesive energy −6.86 eV/atom, and the absence of imaginary phonon modes in its phonon dispersion spectra. Moreover, compared with its hexagonal counterpart, i.e., graphene, which is a gapless material, our designed hexatetra-carbon is a semiconductor with an indirect band gap of 2.20 eV. Furthermore, with a deeper look at the hexatetra-carbon, one finds that this novel monolayer may be obtained from bilayer graphene under external mechanical strain conditions. As a semiconductor with a moderate band gap in the visible light range, once synthesized, hexatetra-carbon would show promising applications in new opto-electronics technologies.

The electronic properties of non-conventional -graphyne nanoribbons

We study the electronic and transport properties of non-conventional graphyne nanoribbons with spin-polarized states, highlighting spin-filtering and spin-switching mechanisms.

High-Throughput Screening of Two-Dimensional Planar sp2 Carbon Space Associated with a Labeled Quotient Graph

The configurational space of two-dimensional planar sp² carbon has been systematically scanned by a random strategy combined with group and graph theory, and 1114 new carbon allotropes have been identified. These allotropes are energetically more favorable than most of the previously predicted 120 carbon allotropes. By fitting the HSE06 band structures of six old structures, we optimize the parameters for a general and transferable tight-binding model for high-throughput band structure calculations. We identified that there are 190 Dirac semimetals, 241 semiconductors, and 683 normal metals among the new allotropes. Interestingly, several stable low-energy carbon systems with exotic electronic properties are proposed, such as type III, type I/II mixed, and type I/III mixed semimetals, which are very rare in planar carbon systems. In particular, one nodal-line semimetal has been discovered among these thousands of allotropes, which is the first nodal-line semimetal in sp² carbon systems. Our discoveries greatly enrich our knowledge of the structures and electronic properties of the two-dimensional carbon family.

Electronic properties of 2D and 1D carbon allotropes based on a triphenylene structural unit

Several bottom-up chemical routes have been developed in the last few years to find ways to grow new forms of nanocarbon by devising a strategical selection of molecular precursors. Here, theoretical calculations are performed on 2D nanocarbon allotropes obtained from the fusion of triphenylene-like units through tetragonal rings. This 2D triphenylene structure has a metallic character in a closed shell configuration, but it also features a spin-polarized semiconducting state. The behavior of the electronic properties of the system is investigated when the structure is cast into nanoribbon forms. It is found that to be metallic in the nonpolarized case, the ribbons must be sufficiently wide while narrow 1D systems are semiconducting. A lower threshold width is also needed for the emergence of a spin-polarized semiconducting configuration in these nanoribbons. These behaviors are robust as they do not depend on edge geometry and chirality, thus offering opportunities for their possible applications in nanoscale devices.

Biphenylene monolayer as a two-dimensional nonbenzenoid carbon allotrope: a first-principles study

In a very recent accomplishment, the two-dimensional form of biphenylene network (BPN) has been fabricated. Motivated by this exciting experimental result on 2D layered BPN structure, herein we perform detailed density-functional theory-based first-principles calculations, in order to gain insight into the structural, mechanical, electronic and optical properties of this promising nanomaterial. Our theoretical results reveal the BPN structure is constructed from three rings of tetragon, hexagon and octagon, meanwhile the electron localization function shows very strong bonds between the C atoms in the structure. The dynamical stability of BPN is verified via the phonon band dispersion calculations. The mechanical properties reveal the brittle behavior of BPN monolayer. The Young's modulus has been computed as 0.1 TPa, which is smaller than the corresponding value of graphene, while the Poisson's ratio determined to be 0.26 is larger than that of graphene. The band structure is evaluated to show the electronic features of the material; determining the BPN monolayer as metallic with a band gap of zero. The optical properties (real and imaginary parts of the dielectric function, and the absorption spectrum) uncover BPN as an insulator along thezzdirection, while owning metallic properties inxxandyydirections. We anticipate that our discoveries will pave the way to the successful implementation of this 2D allotrope of carbon in advanced nanoelectronics.

Effect of Graphene on Ice Polymorph

Recently, ice with stacking disorder structure, consisting of random sequences of cubic ice (Ic) and hexagonal ice (Ih) layers, was reported to be more stable than pure Ih/Ic. Due to a much lower free energy barrier of heterogeneous nucleation, in practice, the freezing process of water is controlled by heterogeneous nucleation triggered by an external medium. Therefore, we carry out molecular dynamic simulations to explore how ice polymorphism depends on the lattice structure of the crystalline substrates on which the ice is grown, focusing on the primary source of atmospheric aerosols, carbon materials. It turns out that, during the nucleation stage, the polymorph of ice nuclei is strongly affected by graphene substrates. For ice nucleation on graphene, we find Ih is the dominant polymorph. This can be attributed to structural similarities between graphene and basal face of Ih. Our results also suggest that the substrate only affects the polymorph of ice close to the graphene surface, with the preference for Ih diminishing as the ice layer grows.

On-surface synthesis of π-conjugated ladder-type polymers comprising nonbenzenoid moieties

On-surface synthesis provides a powerful approach toward the atomically precise fabrication of π-conjugated ladder polymers (CLPs). We report herein the surface-assisted synthesis of nonbenzenoid CLPs from cyclopenta-annulated anthracene monomers on Au(111) under ultrahigh vacuum conditions. Successive thermal annealing steps reveal the dehalogenative homocoupling to yield an intermediate 1D polymer and the subsequent cyclodehydrogenation to form the fully conjugated ladder polymer. Notably, neighbouring monomers may fuse in two different ways, resulting in six- and five-membered rings, respectively. The structure and electronic properties of the reaction products have been investigated via low-temperature scanning tunneling microscopy and spectroscopy, complemented by density-functional theory calculations. Our results provide perspectives for the on-surface synthesis of nonbenzenoid CLPs with the potential to be used for organic electronic devices.

On-surface synthesis provides a powerful approach toward the fabrication of π-conjugated ladder polymers (CLPs). The synthesis of nonbenzenoid CLPs is achieved following two activation steps, including the formation of an intermediate 1D polymer.

Vitreous Carbon, Geometry and Topology: A Hollistic Approach

Glass-like carbon (GLC) is a complex structure with astonishing properties: isotropic sp2 structure, low density and chemical robustness. Despite the expanded efforts to understand the structure, it remains little known. We review the different models and a physical route (pulsed laser deposition) based on a well controlled annealing of the native 2D/3D amorphous films. The many models all have compromises: neither all bad nor entirely satisfactory. Properties are understood in a single framework given by topological and geometrical properties. To do this, we present the basic tools of topology and geometry at a ground level for 2D surface, graphene being the best candidate to do this. With this in mind, special attention is paid to the hyperbolic geometry giving birth to triply periodic minimal surfaces. Such surfaces are the basic tools to understand the GLC network architecture. Using two theorems (the classification and the uniformisation), most of the GLC properties can be tackled at least at a heuristic level. All the properties presented can be extended to 2D materials. It is hoped that some researchers may find it useful for their experiments.

Atomically Thin Pythagorean Tilings in Two Dimensions

We perform a theoretical study of an atomically thin, two-dimensional layer obtained by positioning atoms at the vertices of the classical Pythagorean tiling. This leads to an unusual geometrical pattern that is only stable for the three halogens Cl, Br, and I. In this Pythagorean structure, halogen atoms are arranged in strongly bound diatomic units that bind together by weaker electrostatic bonds. The energy of these phases is competitive with those of the low-temperature phase of the halogens and the two-dimensional layer obtained by exfoliating it. The Pythagorean layers are semiconducting, with an unusual band structure composed of very mobile holes and extremely heavy electrons. They are also soft, exhibiting small values of the elastic constants and a very low energy flexural mode. Analysis of the allowed Raman transitions reveals breathing-like modes that might be used to fingerprint, experimentally, the Pythagorean structure. Finally, we present a series of substrates that, due to lattice matching and compatible symmetry, can be used to stabilize these peculiar two-dimensional layers.

Electronic structure evolution of the transition metals substituted tetragonal graphene: a first-principles investigations

Motivated by the possibilities of tuning the Fermi level of the metallic band structure of the planar tetragonal graphene (T-graphene), by using the transition metals (TMs) substitution (3d, 4dand 5dseries), the electronic structure investigation has been carried out at low concentration level (≈2.7%) throughab initiodensity functional theory method. We have investigated the influence of the valence electrons of the TM on the evolution of the electronic structure and magnetization and the induced magnetic moments at the carbon atoms in the T-graphene network. The investigations also explored the possibilities of inducing long-range magnetic ordering. In the case of multi TMs substitutions we found the dominance signature of the antiferromagnetic correlations for most of the TM substituted cases. The critical analysis of the magnetization densities indicated the important role of the hybridization between the carbonπandσorbitals with the TM-dstates. We explored that the observed non-monotonic nature of the magnetization and evolution of electronic structure was due to the competing energy scales of electronic correlation, hybridization and crystal field splitting. This study opens up the route for further investigations towards the possibilities of using T-graphene as a potential polymorph of graphene for device applications.

Novel 2D allotropic forms and nanoflakes of silicon, phosphorus, and germanium: a computational study

The structural variability offered by 2D materials is an essential feature in materials design. Despite its significance, obtaining assemblies with suitable stability remains a challenge. In this work, we theoretically explore novel silicon, phosphorus, and germanium, analogues of haeckelites at hybrid DFT level. Both 2D systems and nanoflakes (NF) have been studied. All materials have been found dynamically stable; Si-, P-, and Ge- analogues of haeckelites were found to be more stable in comparison to the corresponding honeycomb structure than haeckelites in comparison with graphene. All 2D materials showed metallic behavior; however, the difference between inorganic haeckelites and the corresponding honeycomb allotropes is less than that between haeckelites and graphene. Si-, P-, and Ge-, allotropes have much higher electron affinities (EAs) compared to carbon allotropes, while haeckelites have higher EAs than honeycomb structures. Furthermore, Si-, P-, and Ge-structures also exhibit low hopping activation energies for lithium atoms. It makes these materials potentially promising as a component in Li-ion batteries.

Doxorubicin Encapsulation in Carbon Nanotubes Having Haeckelite or Stone-Wales Defects as Drug Carriers: A Molecular Dynamics Approach

Doxorubicin (DOX), a recognized anticancer drug, forms stable associations with carbon nanotubes (CNTs). CNTs when properly functionalized have the ability to anchor directly in cancerous tumors where the release of the drug occurs thanks to the tumor slightly acidic pH. Herein, we study the armchair and zigzag CNTs with Stone-Wales (SW) defects to rank their ability to encapsulate DOX by determining the DOX-CNT binding free energies using the MM/PBSA and MM/GBSA methods implemented in AMBER16. We investigate also the chiral CNTs with haeckelite defects. Each haeckelite defect consists of a pair of square and octagonal rings. The armchair and zigzag CNT with SW defects and chiral nanotubes with haeckelite defects predict DOX-CNT interactions that depend on the length of the nanotube, the number of present defects and nitrogen doping. Chiral nanotubes having two haeckelite defects reveal a clear dependence on the nitrogen content with DOX-CNT interaction forces decreasing in the order 0N > 4N > 8N. These results contribute to a further understanding of drug-nanotube interactions and to the design of new drug delivery systems based on CNTs.

High temperature stability, metallic character and bonding of the Si2BN planar structure

The family of monolayered Si₂BN structures constitute a new class of 2D materials exhibiting metallic character with remarkable stability. Topologically, these structures are very similar to graphene, forming a slightly distorted honeycomb lattice generated by a union of two basic motifs with AA and AB stacking. In the present work we study in detail the structural and electronic properties of these structures in order to understand the factors which are responsible for their structural differences as well as those which are responsible for their metallic behavior and bonding. Their high temperature stability is demonstrated by the calculations of finite temperature phonon modes which show no negative contributions up to and beyond 1000 K. Presence of the negative thermal expansion coefficient, a common feature of one-atom thick 2D structures, is also seen. Comparison of the two motifs reveal the main structural differences to be the differences in their bond angles, which are affected by the third nearest neighbor interactions ofcis-transtype. On the other hand, the electronic properties of these two structures are very similar, including the charge transfers occurring between orbitals and between atoms. Their metallicity is mainly due to thep_zorbitals of Si with a minor contribution from thep_zorbitals of B, while the contribution from thep_zorbitals of N atoms is negligible. There is almost no contributions from the Np_zelectrons to the energy states near the Fermi level, and they form a band well below it. I.e., thep_zelectrons of N are localized mostly at the N atoms and therefore cannot be considered as mobile electrons of thep_zcloud. Moreover, we show that due to the relative positions in the energy axis of the atomic energies of thep_zorbitals of B, N and Si atoms, the density of states (DOS) of Si₂BN can be considered qualitatively as a combination of the DOS of planar hexagonal BN (h-BN) and hypothetically planar silicene (ph-Si). As a result, the Si₂BN behaves electronically at the Fermi level as slightly perturbed ph-Si, having very similar electronic properties as silicene, but with the advantage of having kinetic stability in planar form. As for the bonding, the Si-Si bonds are covalent, while theπback donation mechanism occurs for the B-N bonding, in accordance with the B-N bonding in h-BN.

The philosophy of carbon: meso-entropy materials

The meso-entropy concept is proposed to re-understand the material world and to develop new carbon-rich materials.

Structural parameters and electronic properties of 2D carbon allotrope: Graphene with a kagome lattice structure

In this paper, the electronic properties of a carbon allotrope, graphene with a kagome lattice structure, are investigated. Spin-polarized density functional theory (DFT) calculations with Grimme dispersion corrections were done. Bond lengths, electronic band structure, and projected density of states were calculated. Electronic band structure calculations show kagome flat-band formation with higher d-orbital contributed bonding behavior than the pristine graphene structure. The structural parameters and electronic band results of this 2D carbon allotrope show wider possible usage in many applications from desalination membranes to possible high-temperature superconductors.

Tripentaphenes: two-dimensional acepentalene-based nanocarbon allotropes

Tripentaphenes are 2D nanocarbon lattices conceptually obtained from the assembly of acepentalene units. In this work, density functional theory is used to investigate their structural, electronic, and vibrational properties. Their bonding configuration is rationalized with a resonance mechanism, which is unique to each of the 2D assemblies. Their formation energies are found to lie within the range of other previously synthesized carbon nanostructures and phonon calculations indicate their dynamical stability. In addition, all studied tripentaphenes are metallic and display different features (e.g., Dirac cone) depending on the details of the atomic structure. The resonance structure also plays an important role in determining the electronic properties as it leads to delocalized electronic states, further highlighting the potential of the structures in nanoelectronics.

Me-graphene: a graphene allotrope with near zero Poisson's ratio, sizeable band gap, and high carrier mobility

The exploration of new two-dimensional (2D) allotropes of carbon has attracted great research attention after graphene, but experiment-feasible graphene allotropes with novel properties are still rare. Here, we predict a new allotrope of graphene, named Me-graphene, composed of both sp2- and sp3-hybridized carbon by topological assembly of C-(C3H2)4 molecules. With a transitional ratio of sp2- and sp3-hybridized carbon atoms (12 : 1) between those of graphene (1 : 0) and penta-graphene (2 : 1), Me-graphene has transition properties between those of graphene and penta-graphene, such as energy, band gap, and Poisson's ratio. Unusually, Me-graphene exhibits a near zero Poisson's ratio of from -0.002 to 0.009 in the xy-plane (or called "anepirretic"), different from that of graphene (0.169) and penta-graphene (-0.068). More importantly, the near zero Poisson's ratio behavior remains in a large strain range, being less than ±0.02 for strain from -15% to +3%. Me-graphene possesses an indirect band gap of 2.04 eV, as a transition of graphene (semimetal) and penta-graphene (wide band gap), and turns into a direct-bandgap semiconductor with an enlarged band gap of 2.62 eV under compressive strain. It possesses high hole mobility of 1.60 × 105 cm2 V-1 s-1 at 300 K. Me-Graphene has potential applications in electronic, photoelectric and high-speed mechatronic devices. The transitional properties related to the ratio of sp2- and sp3-hybridized carbon atoms are inspiring for searching for new graphene allotropes with combinational properties.

Semiconducting two-dimensional group VA-VA haeckelite compounds with superior carrier mobility

A series of two-dimensional (2D) single-layer binary group VA-VA crystals, where VA represents P, As, Sb and Bi, are explored by the first-principles calculations. Unlike the orthorhombic α-phase and hexagonal β-phase, these crystals have a tetragonal haeckelite lattice and are named as T-VA-VAs. These ultrathin 2D materials have high thermal stability and are semiconductors with moderate band gaps ranging from 0.80 to 2.68 eV (HSE06). The band gaps show a prevalent linear correlation with average ionization energies (AIEs) of different composites, and thus can be effectively designed. Furthermore, these materials exhibit superior carrier mobility, e.g. 2.96 × 10³ cm^-2 V^-1 s^-1 of T-SbBi, and considerable visible light absorption index. These novel 2D binary materials are expected to be fabricated and used as nanoelectronics and for solar energy harvesting.

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

Inspired by the success of graphene, various two-dimensional (2D) non-hexagonal graphene allotropes having sp²-bonded tetragonal rings in free-standing (hypothetical) form and on different substrates have been proposed recently. These systems have also been fabricated after modifying the topology of graphene by chemical processes. In this review, we would like to indicate the role of tetra-rings and the local symmetry breaking on the structural, electronic and optical properties of the graphene system. First-principles computations have demonstrated that the tetragonal graphene (TG) allotrope exhibits appreciable thermodynamic stability. The band structure of the TG nanoribbons (TGNRs) strongly depends on the size and edge geometry. This fact has been supported by the transport properties of TGNRs. The optical properties and Raman modes of this graphene allotrope have been well explored for characterisation purposes. Recently, a tight-binding model was used to unravel the metal-to-semiconductor transition under the influence of external magnetic fluxes. Even the introduction of transition metal atoms into this non-hexagonal network can control the magnetic response of the TG sheet. Furthermore, the collective effect of B-N doping and confinement effect on the structural and electronic properties of TG systems has been investigated. We also suggest future directions to be explored to make the synthesis of T graphene and its various derivatives/allotropes viable for the verification of theoretical predictions. It is observed that these doped systems act as a potential candidate for carbon monoxide gas sensing and current rectification devices. Therefore, all these experimental, numerical and analytical studies related to non-hexagonal TG systems are extremely important from a basic science point of view as well as for applications in sensing, optoelectronic and photonic devices.

Nano-makisu: highly anisotropic two-dimensional carbon allotropes made by weaving together nanotubes

Graphene and carbon nanotubes (CNT) are the representatives of two-dimensional (2D) and one-dimensional (1D) forms of carbon, both exhibiting unique geometric structures and peculiar physical and chemical properties. Herein, we propose a family or series of 2D carbon-based highly anisotropic Dirac materials by weaving together an array of CNTs by direct C-C bonds or by graphene ribbons. By employing first-principles calculations, we demonstrate that these nano-makisus are thermally and dynamically stable and possess unique electronic properties. These 2D carbon allotropes are all metals and some nano-makisus show largely anisotropic Dirac cones, causing very different transport properties for the Dirac fermions along different directions. The Fermi velocities in the k_x direction could be ∼170 times higher than those in the k_y direction, which is the strongest anisotropy among 2D carbon allotropes to the best of our knowledge. This intriguing feature of the electronic structure has only been observed in heavy element materials with strong spin-orbit coupling. These results indicate that carbon based materials may have much broader applications in future nanoelectronics.

Twin T-graphene: a new semiconducting 2D carbon allotrope

Two dimensional carbon allotropes with multiple atomic layers have attracted significant interest recently.

Superhard three-dimensional carbon with one-dimensional conducting channels

C₁₀: A superhard sp² + sp³ hybridized carbon allotrope with a three-dimensional network and one-dimensional conducting channels as predicted by first principles calculations.

First-principles calculation of the electronic and optical properties of a new two-dimensional carbon allotrope: tetra-penta-octagonal graphene

A novel sp² hybridized planar 2D carbon allotrope consisting of tetra, penta and octagonal (TPO) rings is proposed in this work. Its thermodynamic stability is confirmed by molecular dynamics in the canonical ensemble at 600 K and the analysis shows that it can also remain stable at 1000 K. The mechanical stability of this material has been estimated by the Born-Huang criterion. Its in-plane stiffness constants are found to be 85% of that of graphene ensuring its high strength quality. The investigation of the electronic properties reveals that the material is metallic in nature with a Dirac cone at 3.7 eV above its Fermi level at an asymmetric position in the conduction band. The study of its optical property for parallel and perpendicular polarization yields the absence of any plasma frequency. Besides, its absorption is mostly spread within 10-20 eV. Further electrical transport study shows negative differential resistance (NDR) above 3.5 V for one nano device. Nano ribbons made out of a TPO-graphene sheet exhibit metallic character. When the porous sheet of TPO-graphene is exposed to Li and S atoms, it is found that the Li atoms pass through the pores unlike the S atoms owing to the less barrier energy compared to S atoms. Substitutional doping with boron and nitrogen at different sites of TPO-graphene showed splitting of the Dirac feature. Also suitable B and N doping brings about semiconducting properties with tunability in band gap with a maximum band gap of 1.09 eV for an isoelectronic structure. All these theoretical predictions might trigger further new avenues involving this novel TPO graphene.

Tuning two-dimensional phase formation through epitaxial strain and growth conditions: silica and silicate on NixPd1-x(111) alloy substrates

Two-dimensional (2D) materials can have multiple phases close in energy but with distinct properties, with the phases that form during growth dependent on experimental conditions and the growth substrate. Here, the competition between 2D van der Waals (VDW) silica and 2D Ni silicate phases on Ni_xPd_1-x(111) alloy substrates was systematically investigated experimentally as a function of Si surface coverage, annealing time and temperature, O₂ partial pressure, and substrate composition and the results were compared with thermodynamic predictions based on density functional theory (DFT) calculations and thermochemical data for O₂. Experimentally, 2D Ni silicate was exclusively observed at higher O₂ pressures (∼10^-6 Torr), higher annealing temperatures (1000 K), and more prolonged annealing (10 min) if the substrate contained any Ni and for initial Si coverages up to 2 monolayers. In contrast, decreasing the O₂ pressure to ∼10^-8 Torr and restricting the annealing temperature and time enabled 2D VDW silica formation. Amorphous 2D VDW silica was observed even when the substrate composition was tuned to lattice match crystalline 2D VDW silica. The trend of decreased O₂ pressure favoring 2D VDW silica was consistent with the theoretical predictions; however, theory also suggested that sufficient Si coverage could avoid Ni silicate formation. The effect of epitaxial strain on 2D Ni silicate was investigated by modifying the solid solution alloy substrate composition. It was found that 2D Ni silicate will stretch to match the substrate lattice constant up to 1.12% tensile strain. When the lattice mismatch was over 1.40%, incommensurate crystalline domains were observed, indicating relaxation of the overlayer to its favored lattice constant. The limited epitaxial strain that could be applied was attributed to a combination of the 2D silicate stiffness, the insensitivity of its bonding to the substrate to its alignment with the substrate, and its lack of accessible structural rearrangements that can reduce the strain energy. The results demonstrate how the resulting 2D material can be manipulated through the growth conditions and how a solid solution alloy substrate can be used to maximize the epitaxial strain imparted to the 2D system.

Carbon Nanotubes Having Haeckelite Defects as Potential Drug Carriers. Molecular Dynamics Simulation

Carbon nanotubes (CNTs) are valuable drug carriers since when properly functionalized they transport drugs and anchor directly to cancerous tumors whose more acidic pH causes the drug release. Herein, we study the so-called zigzag and armchair CNTs with haeckelite defects to rank their ability to adsorb doxorubicin (DOX) by determining the DOX-CNT binding free energies using the MM/PBSA and MM/GBSA methods implemented in AMBER. Our results reveal stronger DOX-CNT interactions for encapsulation of the drug inside the nanotube compared to its adsorption onto the defective nanotube external surface. Armchair CNTs with one and two defects exhibit better results compared with those with four and fifteen defects. Each haeckelite defect consists of a pair of square and octagonal rings. DOX-CNT binding free energies are predicted to be dependent on: (i) nanotube chirality and diameter, (ii) the number of defects, (iii) nitrogen doping and (iv) the position of the encapsulated DOX inside the nanotube. Armchair (10,10) nanotubes with two haeckelite defects, doped with nitrogen, exhibit the best drug-nanotube binding free energies compared with zigzag and fully hydrogenated nanotubes and, also previously reported ones with bumpy defects. These results contribute to further understanding drug-nanotube interactions and their potential application to the design of new drug delivery systems.

Universal Spin Diffusion Length in Polycrystalline Graphene

Graphene grown by chemical vapor deposition (CVD) is the most promising material for industrial-scale applications based on graphene monolayers. It also holds promise for spintronics; despite being polycrystalline, spin transport in CVD graphene has been measured over lengths up to 30 μm, which is on par with the best measurements made in single-crystal graphene. These results suggest that grain boundaries (GBs) in CVD graphene, while impeding charge transport, may have little effect on spin transport. However, to date very little is known about the true impact of disordered networks of GBs on spin relaxation. Here, by using first-principles simulations, we derive an effective tight-binding model of graphene GBs in the presence of spin-orbit coupling (SOC), which we then use to evaluate spin transport in realistic morphologies of polycrystalline graphene. The spin diffusion length is found to be independent of the grain size, and it is determined only by the strength of the substrate-induced SOC. This result is consistent with the D'yakonov-Perel' mechanism of spin relaxation in the diffusive regime, but we find that it also holds in the presence of quantum interference. These results clarify the role played by GBs and demonstrate that the average grain size does not dictate the upper limit for spin transport in CVD-grown graphene, a result of fundamental importance for optimizing large-scale graphene-based spintronic devices.

Carbon nanotubes: Evaluation of toxicity at biointerfaces

Carbon nanotubes (CNTs) are a class of carbon allotropes with interesting properties that make them productive materials for usage in various disciplines of nanotechnology such as in electronics equipments, optics and therapeutics. They exhibit distinguished properties viz., strength, and high electrical and heat conductivity. Their uniqueness can be attributed due to the bonding pattern present between the atoms which are very strong and also exhibit high extreme aspect ratios. CNTs are classified as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) on the basis of number of sidewalls present and the way they are arranged spatially. Application of CNTs to improve the performance of many products, especially in healthcare, has led to an occupational and public exposure to these nanomaterials. Hence, it becomes a major concern to analyze the issues pertaining to the toxicity of CNTs and find the best suitable ways to counter those challenges. This review summarizes the toxicity issues of CNTs in vitro and in vivo in different organ systems (bio interphases) of the body that result in cellular toxicity.

Interfacial properties of 3D metallic carbon nanostructures (T6 and T14)-reinforced polymer nanocomposites: A molecular dynamics study

Herein, the interfacial properties of new three-dimensional (3D) configurations of metallic carbon, namely T6 and T14, incorporated to different polymer matrices (T6 and T14@polymers) are studied using molecular dynamics (MD) simulations. The effects of two types of shape models for T6 and T14, i.e. beam- and plate-like models, various square cross-sectional areas for the reinforcements, pull-out velocity and polymer structure on the interaction energy and pull-out force of final system are investigated. The results reveal that the interfacial resistance of the system is improved by imposing a high pull-out velocity to the nanofillers. For each pull-out velocity, the effect of beam-like T6 and T14@polycarbonate (beam-like T6 and T14@PC) on increasing average pull-out force is more remarkable than that of similar models surrounded by polypropylene (PP). The beam- and plate-like structures@polymers possess the lowest and highest interfacial resistance, respectively. As the aspect ratio (length-to-width ratio) of nanofillers changes from the lowest value to the highest one, the average pull-out force decreases. The average pull-out force of plate-like T6@polymers is higher than their plate-like T14 counterparts. Besides, higher absolute values of interaction energy in plate-like T6 and T14@polymers in comparison with others imply that the load-carrying capacity from the surrounding matrix to the plate-like nanofillers is significantly increased.

Nanomechanics of graphene

The super-high strength of single-layer graphene has attracted great interest. In practice, defects resulting from thermodynamics or introduced by fabrication, naturally or artificially, play a pivotal role in the mechanical behaviors of graphene. More importantly, high strength is just one aspect of the magnificent mechanical properties of graphene: its atomic-thin geometry not only leads to ultra-low bending rigidity, but also brings in many other unique properties of graphene in terms of mechanics in contrast to other carbon allotropes, including fullerenes and carbon nanotubes. The out-of-plane deformation is of a 'soft' nature, which gives rise to rich morphology and is crucial for morphology control. In this review article, we aim to summarize current theoretical advances in describing the mechanics of defects in graphene and the theory to capture the out-of-plane deformation. The structure-mechanical property relationship in graphene, in terms of its elasticity, strength, bending and wrinkling, with or without the influence of imperfections, is presented.

Thermal Conductivity of Two Types of 2D Carbon Allotropes: a Molecular Dynamics Study

The thermal properties of the two novel 2D carbon allotropes with five-five-eight-membered rings are explored using molecular dynamics simulations. Our results reveal that the thermal conductivity increases monotonically with increasing size. The thermal conductivities of infinite sizes are obtained by linear relationships of the inverse length and inverse thermal conductivity. The converged thermal conductivity obtained by extrapolation in the reverse non-equilibrium molecular dynamics method is found to be in reasonable agreement with that in the equilibrium molecular dynamics method. The much lower thermal conductivity, compared with graphene, is attributed to the lower phonon group velocity and phonon mean free path. Temperature and strain effects on thermal conductivity are also explored. The thermal conductivity decreases with increasing temperature and it can also be tuned through strain engineering in a large range. The effect of strain on TC is well explained by spectra analysis of phonon vibration. This study provides physical insight into thermal properties of the two carbon allotropes under different conditions and offers design guidelines for applications of novel two-dimensional carbon allotropes related devices.

On-surface synthesis of nitrogen-doped nanographenes with 5–7 membered rings

Nitrogen-doped nanographenes containing five- and seven-membered rings obtained by cyclodehydrogenation on Au(111).