Pnma-BN: Another Boron Nitride Polymorph with Interesting Physical Properties

Novel III-V Nitride Polymorphs in the P42/mnm and Pbca Phases

In this work, the elastic anisotropy, mechanical stability, and electronic properties for P4₂/mnm XN (XN = BN, AlN, GaN, and InN) and Pbca XN are researched based on density functional theory. Here, the XN in the P4₂/mnm and Pbca phases have a mechanic stability and dynamic stability. Compared with the Pnma phase and Pm-3n phase, the P4₂/mnm and Pbca phases have greater values of bulk modulus and shear modulus. The ratio of the bulk modulus (B), shear modulus (G), and Poisson’s ratio (v) of XN in the P4₂/mnm and Pbca phases are smaller than those for Pnma XN and Pm-3n XN, and larger than those for c-XN, indicating that Pnma XN and Pm-3n XN are more ductile than P4₂/mnm XN and Pbca XN, and that c-XN is more brittle than P4₂/mnm XN and Pbca XN. In addition, in the Pbca phases, XN can be considered a semiconductor material, while in the P4₂/mnm phase, GaN and InN have direct band-gap, and BN and AlN are indirect wide band gap materials. The novel III-V nitride polymorphs in the P4₂/mnm and Pbca phases may have great potential for application in visible light detectors, ultraviolet detectors, infrared detectors, and light-emitting diodes.

Novel porous aluminum nitride monolayer: a first-principles study

Using ab initio calculations within the density functional theory, we explored the possible structures and properties of porous AlN monolayer materials. Two kinds of porous AlN monolayers (H- and T-) are identified. The phonon dispersion spectra together with the ab initio molecular dynamics simulations demonstrate that these structures are stable. We further show that the H- and T-AlN porous monolayers have well-defined porous nanostructures and high specific surface areas of 2863 m² g^-1 and 2615 m² g^-1 respectively, which is comparable to graphene (2630 m² g^-1), and can be maintained stably at high temperatures (>1300 K). Furthermore, both porous monolayers exhibit semiconductor properties, with indirect band gaps of 2.89 eV and 2.86 eV respectively. In addition, the electronic structures of the porous monolayers can be modulated by strain. The band gap of porous T-AlN monolayer experiences an indirect-direct transition when biaxial strain is applied. A moderate  -9% compression can trigger this gap transition. These results indicate that porous AlN monolayers may potentially be used in future optoelectronic and catalyst applications.

Optical, Electronic Properties and Anisotropy in Mechanical Properties of "X" Type Carbon Allotropes

Based on first-principle calculations, the mechanical anisotropy and the electronic and optical properties of seven kinds of carbon materials are investigated in this work. These seven materials have similar structures: they all have X-type structures, with carbon atoms or carbon clusters at the center and stacking towards the space. A calculation of anisotropy shows that the order of elastic anisotropy in terms of the shear modulus, Young's modulus and Poisson's ratio of these seven carbon materials with similar structure is diamond < supercubane < T carbon < Y carbon < TY carbon < cubane-diyne < cubane-yne. As these seven carbon materials exhibit cubic symmetry, Young's modulus has the same anisotropy in some major planes, so the order of elastic anisotropy in the Young's modulus of these seven main planes is (111) plane < (001) plane = (010) plane = (100) plane < (011) plane = (110) plane = (101) plane. It is also due to the fact that their crystal structure has cubic symmetry that the elastic anisotropy in the shear modulus and the Poisson's ratio of these seven carbon materials on the seven major planes are the same. Among the three propagation directions of [100], [110], and [111], the [110] propagation direction's anisotropic ratio of the sound velocity of TY carbon is the largest, while the anisotropic ratio of the sound velocity of cubane-diyne on the [100] propagation direction is the smallest. In addition, not surprisingly, the diamond has the largest Debye temperature, while the TY carbon has the smallest Debye temperature. Finally, TY carbon, T carbon and cubane-diyne are also potential semiconductor materials for photoelectric applications owing to their higher or similar absorption coefficients to GaAs in the visible region.

Physical Properties of XN (X = B, Al, Ga, In) in the Pm-3n phase: First-Principles Calculations

Three direct semiconductor materials and one indirect semiconductor material, Pm-3n XN (X = B, Al, Ga, In), are investigated in our work, employing density functional theory (DFT), where the structural properties, stability, elastic properties, elastic anisotropy properties and electronic properties are included. The shear modulus G and bulk modulus B of Pm-3n BN are 290 GPa and 244 GPa, respectively, which are slightly less than the values of B and G for c-BN and Pnma BN, while they are larger than those of C64 in the I41/amd phase. The shear modulus of Pm-3n BN is the greatest, and the shear modulus of C64 in the I41/amd phase is the smallest. The Debye temperatures of BN, AlN, GaN and InN are 1571, 793, 515 and 242 K, respectively, using the elastic modulus formula. AlN has the largest anisotropy in the Young's modulus, shear modulus, and Poisson's ratio; BN has the smallest elastic anisotropy in G; and InN has the smallest elastic anisotropy in the Poisson's ratio. Pm-3n BN, AlN, GaN and InN have the smallest elastic anisotropy along the (111) direction, and the elastic anisotropy of the E in the (100) (010) (001) planes and in the (011) (101) (110) planes is the same. The shear modulus and Poisson's ratio of BN, AlN, GaN and InN in the Pm-3n phase in the (001), (010), (100), (111), (101), (110), and (011) planes are the same. In addition, AlN, GaN and InN all have direct band-gaps and can be used as a semiconductor within the HSE06 hybrid functional.

The hydrothermal processing of iron oxides from bacterial biofilm waste as new nanomaterials for broad applications

Iron oxides and their hydroxides have been studied and analysed with properties of their mutual transformations under different hydrothermal conditions being indicated. Amorphous bacteria nanowires produced from biofilm waste were investigated under the influence of pH at a fixed duration (20 h) and reaction temperature (200 °C). The morphology, structure, and particle size of the transformation of hematite (α-Fe2O3) was obtained and characterised with SEM, XRD, FTIR, and particle sizer. The optimal conditions for the complete conversion of amorphous iron oxide nanowires to crystalline α-Fe2O3 is under acidic conditions where the pH is 1. The flower-like α-Fe2O3 structures have photocatalytic activity and adsorbent properties for heavy metal ions. This one-pot synthesis approach to produce α-Fe2O3 at a low cost would be greatly applicable to the recycling process of biofilm waste in order to benefit the environment.

Theoretical Investigations of the Hexagonal Germanium Carbonitride

The structural, mechanical, elastic anisotropic, and electronic properties of hexagonal germanium carbonitride (h-GeCN) are systematically investigated using the first-principle calculations method with the ultrasoft pseudopotential scheme in the frame of generalized gradient approximation in the present work. The h-GeCN are mechanically and dynamically stable, as proved by the elastic constants and phonon spectra, respectively. The h-GeCN is brittle because the ratio B/G and Poisson’s ratio v of the h-GeCN are less than 1.75 and 0.26, respectively. For h-GeCN, from brittleness to ductility, the transformation pressures are 5.56 GPa and 5.63 GPa for B/G and Poisson’s ratio v, respectively. The h-GeCN exhibits the greater elastic anisotropy in Young’s modulus and the sound velocities. In addition, the calculated band structure of h-GeCN reveals that there is no band gap for h-GeCN with the HSE06 hybrid functional, so the h-GeCN is metallic.

Structural, Electronic, and Thermodynamic Properties of Tetragonal t-SixGe3-xN₄

The structural, mechanical, anisotropic, electronic, and thermal properties of t-Si₃N₄, t-Si₂GeN₄, t-SiGe₂N₄, and t-Ge₃N₄ in the tetragonal phase are systematically investigated in the present work. The mechanical stability is proved by the elastic constants of t-Si₃N₄, t-Si₂GeN₄, t-SiGe₂N₄, and t-Ge₃N₄. Moreover, they all demonstrate brittleness, because B/G < 1.75, and v < 0.26. The elastic anisotropy of t-Si₃N₄, t-Si₂GeN₄, t-SiGe₂N₄, and t-Ge₃N₄ is characterized by Poisson’s ratio, Young’s modulus, the percentage of elastic anisotropy for bulk modulus A_B, the percentage of elastic anisotropy for shear modulus A_G, and the universal anisotropic index A^U. The electronic structures of t-Si₃N₄, t-Si₂GeN₄, t-SiGe₂N₄, and t-Ge₃N₄ are all wide band gap semiconductor materials, with band gaps of 4.26 eV, 3.94 eV, 3.83 eV, and 3.25 eV, respectively, when using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional. Moreover, t-Ge₃N₄ is a quasi-direct gap semiconductor material. The thermodynamic properties of t-Si₃N₄, t-Si₂GeN₄, t-SiGe₂N₄, and t-Ge₃N₄ are investigated utilizing the quasi-harmonic Debye model. The effects of temperature and pressure on the thermal expansion coefficient, heat capacity, Debye temperature, and Grüneisen parameters are discussed in detail.

Two B-C-O Compounds: Structural, Mechanical Anisotropy and Electronic Properties under Pressure

The structural, stability, mechanical, elastic anisotropy and electronic properties of two ternary light element compounds, B₂CO₂ and B₆C₂O₅, are systematically investigated. The elastic constants and phonon calculations reveal that B₂CO₂ and B₆C₂O₅ are both mechanically and dynamically stable at ambient pressure, and they can stably exist to a pressure of 20 GPa. Additionally, it is found that B₂CO₂ and B₆C₂O₅ are wide-gap semiconductor materials with indirect energy gaps of 5.66 and 5.24 eV, respectively. The hardness calculations using the Lyakhov-Oganov model show that B₂CO₂ is a potential superhard material. Furthermore, the hardness of B₆C₂O₅ is 29.6 GPa, which is relatively softer and more easily machinable compared to the B₂CO₂ (41.7 GPa). The elastic anisotropy results show that B₆C₂O₅ exhibits a greater anisotropy in the shear modulus, while B₂CO₂ exhibits a greater anisotropy in Young's modulus at ambient pressure.

Mechanical, Anisotropic, and Electronic Properties of XN (X = C, Si, Ge): Theoretical Investigations

The structural, mechanical, elastic anisotropic, and electronic properties of Pbca-XN (X = C, Si, Ge) are investigated in this work using the Perdew–Burke–Ernzerhof (PBE) functional, Perdew–Burke–Ernzerhof for solids (PBEsol) functional, and Ceperly and Alder, parameterized by Perdew and Zunger (CA–PZ) functional in the framework of density functional theory. The achieved results for the lattice parameters and band gap of Pbca-CN with the PBE functional in this research are in good accordance with other theoretical results. The band structures of Pbca-XN (X = C, Si, Ge) show that Pbca-SiN and Pbca-GeN are both direct band gap semiconductor materials with a band gap of 3.39 eV and 2.22 eV, respectively. Pbca-XN (X = C, Si, Ge) exhibits varying degrees of mechanical anisotropic properties with respect to the Poisson’s ratio, bulk modulus, shear modulus, Young’s modulus, and universal anisotropic index. The (001) plane and (010) plane of Pbca-CN/SiN/GeN both exhibit greater elastic anisotropy in the bulk modulus and Young’s modulus than the (100) plane.

Theoretical Investigations of Si-Ge Alloys in P4₂/ncm Phase: First-Principles Calculations

The structural, mechanical, anisotropic, electronic and thermal properties of Si, Si_0.667Ge_0.333, Si_0.333Ge_0.667 and Ge in P4₂/ncm phase are investigated in this work. The calculations have been performed with an ultra-soft pseudopotential by using the generalized gradient approximation and local density approximation in the framework of density functional theory. The achieved results for the lattice constants and band gaps of P4₂/ncm-Si and P4₂/ncm-Ge in this research have good accordance with other results. The calculated elastic constants and elastic moduli of the Si, Si_0.667Ge_0.333, Si_0.333Ge_0.667 and Ge in P4₂/ncm phase are better than that of the Si, Si_0.667Ge_0.333, Si_0.333Ge_0.667 and Ge in P4₂/mnm phase. The Si, Si_0.667Ge_0.333, Si_0.333Ge_0.667 and Ge in P4₂/ncm phase exhibit varying degrees of mechanical anisotropic properties in Poisson’s ratio, shear modulus, Young’s modulus, and universal anisotropic index. The band structures of the Si, Si_0.667Ge_0.333, Si_0.333Ge_0.667 and Ge in P4₂/ncm phase show that they are all indirect band gap semiconductors with band gap of 1.46 eV, 1.25 eV, 1.36 eV and 1.00 eV, respectively. In addition, we also found that the minimum thermal conductivity κ_min of the Si, Si_0.667Ge_0.333, Si_0.333Ge_0.667 and Ge in P4₂/ncm phase exhibit different degrees of anisotropic properties in (001), (010), (100) and (01¯0) planes.