Abstract: Different allotropes of phosphorene are possible of which black and blue phosphorus are the most stable. While blue phosphorus has isotropic properties, black phosphorus is strongly anisotropic in its electronic and optical properties due to its anisotropic crystal structure. In this work, we systematically investigated the lattice thermal properties of black and blue phosphorene by using first-principles calculations based on the quasiharmonic approximation approach. Similar to the optoelectronic and electronic properties, we predict that black phosphorene has highly anisotropic thermal properties, in contrast to the blue phase. The linear thermal expansion coefficients along the zigzag and armchair direction differ up to 20% in black phosphorene. The armchair direction of black phosphorene is more expandable as compared to the zigzag direction and the biaxial expansion of blue phosphorene under finite temperature. Our comparative analysis reveals that the inclusion of finite-temperature effects makes the blue phase thermodynamically more stable over the black phase above 135 K.

Abstract: Recently penta-graphene was proposed as a stable two-dimensional carbon allotrope consisting of a single layer of interconnected carbon pentagons [Zhang et al., PNAS, 2015, 112, 2372]. Its silicon counterpart, penta-silicene, however, is not stable. In this work, we show that multilayers of penta-silicene form stable materials with semiconducting or metallic properties, depending on the stacking mode. We demonstrate their dynamic stability through their phonon spectrum and using molecular dynamics. A particular type of bilayer penta-silicene is found to have lower energy than all of the known hexagonal silicene bilayers and forms therefore the most stable bilayer silicon material predicted so far. The electronic and mechanical properties of these new silicon allotropes are studied in detail and their behavior under strain is investigated. We demonstrate that strain can be used to tune its band gap.

Abstract: Edge functionalization of graphene nanoribbons with nitrogen atoms for various adatom configurations at armchair and zigzag edges are investigated. We provide comprehensive information on the electronic and magnetic properties and investigate the stability of the various systems. Two types of rippling of the nanoribbons, namely edge and bulk rippling depending on the sign of edge stress induced at the edge, are found. They are found to play the decisive role for the stability of the structures. We also propose a type of edge decoration in which every third nitrogen adatom at the zigzag edges is replaced by an oxygen atom. In this way, the electron count is compatible with a full aromatic structure, leading to additional stability and a disappearance of magnetism that is usually associated with zigzag nanoribbons.

Abstract: MXenes are the newest class of two-dimensional (2D) materials, and they offer great potential in a wide range of applications including electronic devices, sensors, and thermoelectric and energy storage materials. In this work, we combined the outstanding electrical conductivity, that is essential for battery applications, of graphene with MXene monolayers (M2CX2 where M = Sc, Ti, V and X = OH, O) to explore its potential in Li battery applications. Through first principles calculations, we determined the stable stacking configurations of M2CX2/graphene bilayer heterostructures and their Li atom intercalation by calculating the Li binding energy, diffusion barrier and voltage. We found that: (1) for the ground state stacking, the interlayer binding is strong, yet the interlayer friction is small; (2) Li binds more strongly to the O-terminated monolayer, bilayer and heterostructure MXene systems when compared with the OHterminated MXenes due to the H+ induced repulsion to the Li atoms. The binding energy of Li decreases as the Li concentration increases due to enhanced repulsive interaction between the positively charged Li ions; (3) Ti2CO2/graphene and V2CO2/graphene heterostructures exhibit large Li atom binding energies making them the most promising candidates for battery applications. When fully loaded with Li atoms, the binding energy is -1.43 eV per Li atom and -1.78 eV per Li atom for Ti2CO2/graphene and V2CO2/graphene, respectively. These two heterostructures exhibit a nice compromise between storage capacity and kinetics. For example, the diffusion barrier of Li in Ti2CO2/graphene is around 0.3 eV which is comparable to that of graphite. Additionally, the calculated average voltages are 1.49 V and 1.93 V for Ti2CO2/graphene and V2CO2/graphene structures, respectively; (4) a small change in the in-plane lattice parameters (<1%), interatomic bond lengths and interlayer distances (<0.5 angstrom) proves the stability of the heterostructures against Li intercalation, and the impending phase separation into constituent layers and capacity fading during charge-discharge cycles in real battery applications; (5) as compared to bare M2CX2 bilayers, M2CX2/graphene heterostructures have lower molecular mass, offering high storage capacity; (6) the presence of graphene ensures good electrical conductivity that is essential for battery applications. Given these advantages, Ti2CO2/graphene and V2CO2/graphene heterostructures are predicted to be promising for lithium-ion battery applications.

Abstract: The atomically laminated layered ternary transition-metal borides (the MAB phases) have demonstrated outstanding properties and have been applied in various fields. Understanding their thermal and mechanical properties is critical to determining their applicability in various fields such as high-temperature applications. To achieve this, we conducted first-principles calculations based on density-functional theory and the quasiharmonic approximation to determine the thermal expansion coefficients, Gruneisen parameters, bulk moduli, hardness, thermal conductivity, electron-phonon coupling parameters, and the structural and vibrational properties of MoAlB, WAlB, Cr2AlB2, and Ti2InB2. We found varying degrees of anisotropy in the thermal expansion and mechanical properties in spite of similarities in their crystal structures. MoAlB has a mild degree of anisotropy in its thermal expansion coefficient (TEC), while Cr2AlB2 and WAlB display the highest level of TEC anisotropy. We assessed various empirical models to calculate hardness and thermal conductivity, and correlated the calculated values with the material properties such as elastic moduli, Gruneisen parameter, Debye temperature, and type of bonding. Owing to their higher Gruneisen parameters, implying a greater degree of anharmonicity in lattice vibrations and lower phonon group velocities, MoAlB and WAlB have significantly lower lattice thermal conductivity values than those of Cr2AlB2 and Ti2InB2. The hardness and lattice thermal conductivity of MAB phases can be predicted with high accuracy if one utilizes an appropriate model.

Abstract: Advanced materials with various micro-/nanostructures have attracted plenty of attention for decades in energy storage devices such as rechargeable batteries (ion- or sulfur based batteries) and supercapacitors. To improve the electrochemical performance of batteries, it is uttermost important to develop advanced electrode materials. Moreover, the cathode material is also important that it restricts the efficiency and practical application of aluminum-ion batteries. Among the potential cathode materials, sulfur has become an important candidate material for aluminum-ion batteries cause of its considerable specific capacity. Two-dimensional materials are currently potential candidates as electrodes from lab-scale experiments to possible pragmatic theoretical studies. In this review, the fundamental principles, historical progress, latest developments, and major problems in Li-S and Al-S batteries are reviewed. Finally, future directions in terms of the experimental and theoretical applications have prospected.

Abstract: A multiscale modeling and simulation approach, including first-principles calculations, ab initio molecular dynamics simulations, and a tight binding approach, is employed to study band flattening of the electronic band structure of oxidized monolayer graphene. The width offlat bands can be tuned by strain, the external electric field, and the density of functional groups and their distribution. A transition to a conducting state is found for monolayer graphene with impurities when it is subjected to an electric field of similar to 1.0 V/angstrom. Several parallel impurity-induced flat bands appear in the low-energy spectrum of monolayer graphene when the number of epoxy groups is changed. The width of the flat band decreases with an increase in tensile strain but is independent of the electric field strength. Here an alternative and easy route for obtaining band flattening in thermodynamically stable functionalized monolayer graphene is introduced. Our work discloses a new avenue for research on band flattening in monolayer graphene.

Abstract: ZnM2O4 (M = Co, Rh, Ir) spinels are considered as a class of potential p-type transparent conducting oxides (TCOs). We report the formation energy of acceptor-like defects using first principles calculations with an advanced hybrid exchange-correlation functional (HSE06) within density functional theory (DFT). Due to the discrepancies between the theoretically obtained band gaps with this hybrid functional and the – scattered – experimental results, we also perform GW calculations to support the validity of the description of these spinels with the HSE06 functional. The considered defects are the cation vacancy and antisite defects, which are supposed to be the leading source of disorder in the spinel structures. We also discuss the band alignments in these spinels. The calculated formation energies indicate that the antisite defects ZnM (Zn replacing M, M = Co, Rh, Ir) and VZn act as shallow acceptors in ZnCo2O4, ZnRh2O4 and ZnIr2O4, which explains the experimentally observed p-type conductivity in those systems. Moreover, our systematic study indicates that the ZnIr antisite defect has the lowest formation energy in the group and it corroborates the highest p-type conductivity reported for ZnIr2O4 among the group of ZnM2O4 spinels. To gain further insight into factors affecting the p-type conductivity, we have also investigated the formation of localized small polarons by calculating the self-trapping energy of the holes.

Abstract: We have used first-principles calculations based on density functional theory to study point defects in CdO within the local density approximation and beyond (LDA+U). Hydrogen interstitials and oxygen vacancies are found to act as shallow donors and can be interpreted as the cause of conductivity in CdO. Hydrogen can also occupy an oxygen vacancy in its substitutional form and also acts as a shallow donor. Similar to what was found for ZnO and MgO, hydrogen creates a multicenter bond with its six oxygen neighbors in CdO. The charge neutrality level for native defects and hydrogen impurities has been calculated. It is shown that in the case of native defects, it is not uniquely defined. Indeed, this level depends highly on the chemical potentials of the species and one can obtain different values for different end states in the experiment. Therefore, a comparison with experiment can only be made if the chemical potentials of the species in the experiment are well defined. However, for the hydrogen interstitial defect, since this level is independent of the chemical potential of hydrogen, one can obtain a unique value for the charge neutrality level. We find that the Fermi level stabilizes at 0.43 eV above the conduction band minimum in the case of the hydrogen interstitial defect, which is in good agreement with the experimentally reported value of 0.4 eV.

Abstract: The in-plane dc conductivity of twisted bilayer graphene is calculated using an expansion of the real-space Kubo-Bastin conductivity in terms of Chebyshev polynomials. We investigate within a tight-binding approach the transport properties as a function of rotation angle, applied perpendicular electric field, and vacancy disorder. We find that for high-angle twists, the two layers are effectively decoupled, and the minimum conductivity at the Dirac point corresponds to double the value observed in monolayer graphene. This remains valid even in the presence of vacancies, hinting that chiral symmetry is still preserved. On the contrary, for low twist angles, the conductivity at the Dirac point depends on the twist angle and is not protected in the presence of disorder. Furthermore, for low angles and in the presence of an applied electric field, we find that the chiral boundary states emerging between AB and BA regions contribute to the dc conductivity, despite the appearance of localized states in the AA regions. The results agree qualitatively with recent transport experiments in low-angle twisted bilayer graphene.

Abstract: Dynamics of electron wave-packets is studied using the continuum Dirac model in a graphene geometric diode where the propagation of the wave packet is favored in certain direction due to the presence of geometric constraints. Clear rectification is obtained in the THz frequency range with the maximum rectification level of 3.25, which is in good agreement with recent experiments on graphene ballistic diodes. The rectification levels are considerably higher for systems with narrower channels. In this case, the wave packet transmission probabilities and rectification rate also strongly depend on the energy of the incident wave packet, as a result of the quantum nature of energy levels along such channels. These findings can be useful for fundamental understanding of the charge carrier dynamics in graphene geometry diodes.

Abstract: The interaction between the surface of a 3D topological insulator and a skyrmion/anti-skyrmion structure is studied in order to investigate the possibility of electron confinement due to the skyrmion presence. Both hedgehog (Neel) and vortex (Bloch) skyrmions are considered. For the hedgehog skyrmion, the in-plane components cannot be disregarded and their interaction with the surface state of the topological insulator (TI) has to be taken into account. A semi-classical description of the skyrmion chiral angle is obtained using the variational principle. It is shown that both the hedgehog and the vortex skyrmion can induce bound states on the surface of the TI. However, the number and the properties of these states depend strongly on the skyrmion type and the skyrmion topological number N-Sk. The probability densities of the bound electrons are also derived where it is shown that they are localized within the skyrmion region. Published by AIP Publishing.