Topologically protected conduction state at carbon foam surfaces: an ab initio study

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.

Regulating the valence level arrangement of high-Al-content AlGaN quantum wells using additional potentials with Mg doping

Quantum states and arrangement of valence levels determine most of the electronic and optical properties of semiconductors. Since the crystal field split-off hole (CH) band is the top valence band in high-Al-content AlGaN, TM-polarized optical anisotropy has become the limiting factor for efficient deep-ultraviolet (DUV) light emission. Additional potentials, including on-site Coulomb interaction and orbital state coupling induced by magnesium (Mg) doping, are proposed in this work to regulate the valence level arrangement of AlN/Al_0.75Ga_0.25N quantum wells (QWs). Diverse responses of valence quantum states |p_i〉 (i = x, y, or z) of AlGaN to additional potentials due to different configurations and interactions of orbitals revealed by first-principles simulations are understood in terms of the linear combination of atomic orbital states. A positive charge and large Mg dopant in QWs introduce an additional Coulomb potential and modulate the orbital coupling distance. For the CH band (p_z orbital), the Mg-induced Coulomb potential compensates the orbital coupling energy. Meanwhile, the heavy/light hole (HH/LH) bands (p_x and p_y orbitals) are elevated by the Mg-induced Coulomb potential. Consequently, HH/LH energy levels are relatively shifted upward and replace the CH level to be the top of the valence band. The inversion of optical anisotropy and enhancement of TE-polarized emission are further confirmed experimentally via spectroscopic ellipsometry.

Structural variety and stability of carbon honeycomb cellular structures

A new synthesized carbon honeycomb allotrope reported previously, built from graphene nanoribbons connected by sp3-bonded carbon junction lines, forms a family of cellular structures with high porosity and sorption capacity. In this work we first propose a complete set of possible honeycomb structures of different wall chiralities both the armchair and zigzag types, including considered earlier only theoretically, for the structural analysis of such structures by means of the high-energy electron diffraction method. The “completeness” of the model set made it possible to obtain nearly perfect coincidence of the experimental and calculated diffraction intensities. The contribution of graphite fragments and random structures, also involved in the analysis, turned out to be zero. Only a limited number of honeycomb structures of different types almost ideally describes the experiment. Thus we conclude that polydomain structures corresponding to a set of basic models formed in this investigation rather than formations dominated by random structures. The samples under study have demonstrated the unique cellular stability since were stored in vacuum ∼4.5 months before the reported measurements. Along with the original results the history of the carbon honeycomb cellular structures is briefly presented.

The spin-polarized edge states of blue phosphorene nanoribbons induced by electric field and electron doping

Edge states of various two-dimensional materials such as graphene are intrinsically spin-polarized. In other materials, electric field and charge doping are required for introducing magnetism to their edges. In this work, by using first-principles calculations, we studied the effects of transverse electric field on the edge states of the armchair blue phosphorene nanoribbon (ABPNR), and found that a transverse electric field drives the edge electronic state occupied and at the same time spin-polarized. We also doped electrons to the ABPNR and found that these additional electrons occupy and spin-polarize the electronic states of both edges of the nanoribbon.

A new two-dimensional semiconducting carbon allotrope with direct band gap: a first-principles prediction

Two-dimensional (2D) carbon materials with an appropriate band gap play important roles in the various electronics fields. Here, based on first-principles calculations, we predict a new 2D carbon allotrope containing 32 atoms, consists of pentagonal, hexagonal, octagonal and decagonal rings. This new allotrope is named as Po-C32, which possesses P4/MMM symmetry with a tetragonal lattice and has a vertical distance of 2.22 Å between the uppermost and undermost atoms. The cohesive energy, phonon band structure,ab initiomolecular dynamics simulations and elastic constants fitting confirm Po-C32 has high stabilities. The fitted in-plane Young's modulus and Poisson's ratio alongaandbdirections areYa=Yb= 244 N m-1andva=vb= 0.14, respectively, exhibiting the same mechanical properties alongaandbdirections. Interestingly, Po-C32 is a semiconductor with a direct band gap of 2.05 eV, comparable to that of phosphorene, exhibiting great potential in nanoelectronics. Moreover, two stable derivative allotropes are also predicted based on Po-C32. Po-C24-3D is an indirect narrow band gap (1.02 eV) semiconductor, while Po-C32-3D possesses a wider indirect band gap of 3.90 eV, which can be also applied in optoelectronic device.

Inorganic gas sensing of green phosphorene nanosheet: insights from density functional theory

Our work highlights the functionality of a novel two-dimensional phosphorene allotrope entitled green phosphorene for inorganic gas detection for the first time. Four inorganic molecules, NH3, SO2, HCN and O3, are considered as adsorbates and the adsorption conformation, adsorption energy, charge transfer, density of states, and electronic band structure are systematically scrutinized based on density functional theory. Our calculations show that the adsorption energy of O3on pristine green phosphorene is the lowest among the four considered gas molecules, suggesting that the substrate is more sensitive to O3. Significant changes in electronic structures confirm the possibility of green phosphorene for O3detection. Biaxial strains and electric fields were applied to investigate the changes in adsorption behavior. The presence of compressive strain could enhance adsorption sensitivity between O3and green phosphorene, while the tensile strain induces the dissociative adsorption that not suitable for reversible sensor. Furthermore, by controlling the orientation of external electric field, it is possible to achieve O3adsorption-desorption cycle, which is of great significance for green phosphorene in the application of reversible gas sensor.

Six novel carbon and silicon allotropes with their potential application in photovoltaic field

By stacking up five novel cagelike structures, three novel three-dimensional (3D)sp3bonding networks, namedhP24,hP30 andhP36, were predicted in this work for the first time. These three newly discovered structures have trigonal unit cell with the space groups ofP-3m1,P-3m1 andP3m1, respectively. Using first-principle calculations, the physical properties, including structural, mechanical, electronic and optical properties of C and Si inhP24,hP30 andhP36 phases were systematically studied. All these newly discovered carbon and silicon allotropes were proven to be thermodynamically and mechanically stable. The wide indirect bandgap value in range of 3.89-4.03 eV suggests that C inhP24,hP30 andhP36 phases have the potential to be applied in high frequency and high power electronic devices. The direct bandgap value in range of 0.60-1.16 eV, the smaller electron and hole effective mass than diamond-Si, and the significantly better photon absorption characteristics than diamond-Si suggest thathP24-Si,hP30-Si andhP36-Si are likely to have better performance in photovoltaic applications than diamond-Si.hP24-Si also has the potential to be applied in infrared detectors.

Absorption of atomic and molecular species in carbon cellular structures (Review article)

The paper presents a brief review of the recent developments in the field of absorption of atomic and molecular species in carbon cellular structures. Such absorbing objects can be distinctly recognized among a large family of carbon porous materials owing to potential and already observed in experiments very high capacity to soak and to keep inside different substances, which at usual conditions outside the porous matrices may often stay only in a gaseous form. High capacity filling is attained owing to single graphene-like walls separating different cells in the whole structures providing their lightweight. This property of cellular structures makes them very promising for numerous technological applications such as hydrogen storage in fuel cells and molecular sieving in membranes made from such structures or for their usage in microelectronics, photovoltaics and production of Li-ion batteries. Independently of the targeted applications gases are good candidates for probing tests of carbon matrices themselves.

Anisotropic thermal conductivity in carbon honeycomb

Carbon honeycomb, a new kind of 3D carbon allotrope experimentally synthesized recently, has received much attention for its fascinating applications in electronic device and energy storage. In the present work, we perform equilibrium molecular dynamics (EMD) to study the thermal transport properties of carbon honeycombs with different chirality. It is found that the thermal conductivity along the honeycomb axis ([Formula: see text]) is three times larger than that normal to the axis ([Formula: see text]), which shows strong anisotropy reflecting their geometric anisotropy. Lattice dynamics calculations reveal that this anisotropy stems from the orientation-dependent phonon group velocities. Moreover, when ambient temperature ([Formula: see text]) increases from 200 K to 800 K, the [Formula: see text] dependence of [Formula: see text] is observed due to the enhanced Umklapp scattering. The detailed phonon spectra analyses indicate phonon group velocities are insensitive to the variation of ambient temperature, and the temperature dependence of the relaxation times of low-frequency phonons (<20 THz) follows [Formula: see text] behavior. Our results have a certain guiding significance to develop carbon honeycomb for effective thermal channeling devices.

Bottom-up Design of Three-Dimensional Carbon-Honeycomb with Superb Specific Strength and High Thermal Conductivity

Low-dimensional carbon allotropes, from fullerenes, carbon nanotubes, to graphene, have been broadly explored due to their outstanding and special properties. However, there exist significant challenges in retaining such properties of basic building blocks when scaling them up to three-dimensional materials and structures for many technological applications. Here we show theoretically the atomistic structure of a stable three-dimensional carbon honeycomb (C-honeycomb) structure with superb mechanical and thermal properties. A combination of sp² bonding in the wall and sp³ bonding in the triple junction of C-honeycomb is the key to retain the stability of C-honeycomb. The specific strength could be the best in structural carbon materials, and this strength remains at a high level but tunable with different cell sizes. C-honeycomb is also found to have a very high thermal conductivity, for example, >100 W/mK along the axis of the hexagonal cell with a density only ∼0.4 g/cm³. Because of the low density and high thermal conductivity, the specific thermal conductivity of C-honeycombs is larger than most engineering materials, including metals and high thermal conductivity semiconductors, as well as lightweight CNT arrays and graphene-based nanocomposites. Such high specific strength, high thermal conductivity, and anomalous Poisson's effect in C-honeycomb render it appealing for the use in various engineering practices.

Are the experimentally observed 3-dimensional carbon honeycombs all-sp2 structures? The dangling p-orbital instability

3-Dimensional all-sp² honeycomb carbon structures are unstable, due to dangling bonds, formed on the junction atom unhybridized p-orbitals.

Spin-dependent Seebeck effect in zigzag black phosphorene nanoribbons

Zigzag black phosphorene nanoribbons are good candidates to realize spin-dependent Seebeck effect due to the particular electronic structures.

Ab initio investigation on the stability of H-6 Carbon

H-6 Carbon is unstable due to the synergetic action of its interchain interactions (U_inter) with its zig-zag chain rotations (U_rot), which eliminate the energy barrier provided by its intrachain interactions (U_intra), transforming it to diamond.

A first-principles study on three-dimensional covalently-bonded hexagonal boron nitride nanoribbons

We studied three-dimensional honeycomb-structure boron nitride (BN) allotrope using first-principles calculations and the tight-binding method. Interconnected by sp(3)-bonding at the vertices, hexagonal BN nanoribbons construct highly-porous, covalently-bonded hexagonal BN nanoribbons (CBBNs). We investigated the structural and mechanical properties of CBBNs with various sizes, compared with those of carbon and other BN allotropes. The mechanical and thermal stabilities are also checked. Our calculations show that, despite the high porosity and low mass density, CBBNs are stable and mechanically hard materials as cubic BN. Moreover, our calculated results suggest that CBBNs can be regarded as a binary alloy of sp(2)- and sp(3)-bonded BNs following the Vegard's rule in average bond lengths and bulk moduli. Calculated band structures show that the band gap of CBBNs has similar variation upon increasing size as BN nanoribbons and is also limited by the second-neighbor interaction between the pz states of sp(2)-bonded atoms in adjacent nanoribbons.

Phase coexistence and metal-insulator transition in few-layer phosphorene: a computational study

Based on ab initio density functional calculations, we propose γ-P and δ-P as two additional stable structural phases of layered phosphorus besides the layered α-P (black) and β-P (blue) phosphorus allotropes. Monolayers of some of these allotropes have a wide band gap, whereas others, including γ-P, show a metal-insulator transition caused by in-layer strain or changing the number of layers. An unforeseen benefit is the possibility to connect different structural phases at no energy cost. This becomes particularly valuable in assembling heterostructures with well-defined metallic and semiconducting regions in one contiguous layer.