Vacancy-defect effect on the electronic and optical properties of Pmm2 BC2N: A first-principles study

Orthorhombic-Pmm2-BC₂N as a superhard photocatalyst simulates great interests in the researches of materials-design and application. To promote the studies of Pmm2 BC₂N as a multifunctional material with both great hardness and good optical properties, we investigated the electronic and optical properties of Pmm2 BC₂N with various vacancy-defects by the systematic first-principles density functional theory (DFT) calculations in this work. The absorption, refractivity, reflectivity, and photoconductivity of considered structures were calculated and explored. The various characteristics of the optical properties were analyzed based on relative computed density of states (DOS).     

二维BC2 N薄片的结构稳定性和电子性质

采用基于密度泛函理论的第一性原理方法,对二维BC₂N薄片的结构稳定性和电子性质进行了系统的研究.计算了BC₂N化合物16种可能的二维单层结构.对它们的能带结构分析发现,对称性最高的构型与石墨烯一样是一种半金属,而其他二维结构则为有不同带隙的半导体,其中最稳定的构型是带隙值为1.63 eV的直接带隙半导体.对最稳定构型的差分电荷密度分析和Bader分析发现:在最稳定的构型中,C–C键、C–N键、C–B键和B–N键主要以共价键的形式呈现,也具有比较明显的离子性.在应力作用下最稳定构型的单层BC₂N的带隙宽度会发生变化,压缩时带隙变宽,而拉伸时带隙变窄,但仍然为直接带隙半导体. 关键词： 2N')" href="#">BC₂N 单层原子薄片 电子结构 从头计算     

拉伸形变下BC3纳米管的能带结构

运用紧束缚能带理论,研究拉伸形变下BC₃纳米管的能带结构. 研究表明:随着拉伸和压缩强度的不断增加,BC₃纳米管的导带能级和价带能级逐渐靠近,最终发生能带交叠. 压缩形变下能带的交叠程度可达05 eV,而拉伸形变下只有02 eV. 对于扶手椅型BC₃纳米管,随着拉伸和压缩的不断增加,BC₃纳米管首先由直接半导体转化为间接半导体,进而发生能带的交叠,表现出金属性. 在无形变时,扶手椅型BC₃纳米 关键词： 3纳米管')" href="#">BC₃纳米管 能隙 拉伸形变 半导体     

First-principles study of ferromagnetism in N doped TiO2 and TiO

N doped TiO is nonmagnetic, in which spin-split impurity states are not induced near the Fermi energy (E_F) by N dopant. N doped TiO₂ along with transition-metal (TM) doped TiO is magnetic, in which spin-split impurity states are induced across E_F. The magnetic moment is determined by the 3d4s electron configurations and the valence states of TM-dopant ions when they substitute Ti. Hence, the origin of ferromagnetism of N doped TiO₂ and TiO is not closely related to the width of the band gaps of host oxides, but would be crucially related to that if the dopant can induce spin-split impurity states near E_F.     

Optical properties of superhard BC5: First-principles calculations

BC₅ is a newly synthesized superhard material. We present a systematic investigation of optical properties of BC₅ in P3m1 and I-4m2 phases at ambient and high pressure in the framework of density functional theory with the generalized gradient approximation (GGA) in this paper. Optical properties such as dielectric function, refractive index, absorption, reflectivity and electron energy-loss spectrum are obtained successfully. The feature in the spectra of the optical parameters is discussed. Through calculation, we find BC₅ is optically anisotropic. Moreover, the dielectric function exhibits a large change at 70 GPa pressure for P3m1 BC₅ phase, but I-4m2 phase not, indicating the stable electronic structure that the I-4m2 phase possesses.     

Mechanical properties of superhard diamondlike BC5

Using first-principles calculations, we systematically studied the mechanical properties and electronic structure of the recently synthesized diamondlike BC₅. Our calculated bulk modulus B, shear modulus G, elastic constant c₄₄, and theoretical hardness H confirm that BC₅ is an ultraincompressible and superhard material. Also, it exhibits mechanical stability and metallic features. Electronic structures show that a strong covalent bond network through sp³ hybridization is the origin of the excellent mechanical properties of BC₅. Our results show that BC₅ has good prospects in electronic application as a superhard material.     

Elastic, electronic, and optical properties of hypothetical SnNNi3 and CuNNi3 in comparison with superconducting ZnNNi3

The elastic, electronic, and optical properties of MNNi₃ (M=Zn, Sn, and Cu) have been calculated using the plane-wave ultrasoft pseudopotential technique, which is based on the first-principle density functional theory (DFT) with generalized gradient approximation (GGA). The optimized lattice parameters, independent elastic constants (C₁₁, C₁₂, and C₄₄), bulk modulus B, compressibility K, shear modulus G, and Poisson's ratio υ, as well as the band structures, total and atom projected densities of states and finally the optical properties of MNNi₃ have been evaluated and discussed. The electronic band structures of the two hypothetical compounds show metallic behavior just like the superconducting ZnNNi₃. Using band structures, the origin of features that appear in different optical properties of all the three compounds has been discussed. The large reflectivity of the predicted compounds in the low energy region might be useful in good candidate materials for coating to avoid solar heating.     

Structure and electronic structure of S-doped graphitic C3N4 investigated by density functional theory

The structures of the heptazine-based graphitic C3N4 and the S-doped graphitic C3N4 are investigated by using the density functional theory with a semi-empirical dispersion correction for the weak long-range interaction between layers.The corrugated structure is found to be energetically favorable for both the pure and the S-doped graphitic C3N4.The S doptant is prone to substitute the N atom bonded with only two nearest C atoms.The band structure calculation reveals that this kind of S doping causes a favorable red shift of the light absorption threshold and can improve the electroconductibility and the photocatalytic activity of the graphitic C3N4.     

C20 fullerene and its boron- and nitrogen-doped counterparts as an efficient catalyst for CO oxidation

DFT calculations are utilised to investigate the CO oxidation on the C₂₀, BC₁₉, and NC₁₉ clusters. For CO oxidation over considered clusters, two continuous steps are proposed that in each step one CO₂ molecule is released from clusters surface. The calculations demonstrate that in the case of the C₂₀ cluster, the first step of CO oxidation takes place through the ER mechanism on two routes with a barrier height of 1.06?eV and 2.57?eV for the rate-limiting step. Also, in the cases of BC₁₉ and NC₁₉ clusters, both reaction paths occur via the ER mechanism. The activation energy of the first reaction step is about 0.53 and 0.46?eV, while it is negligible for the second step that is 0.04 and 0.18?eV for BC₁₉ and NC₁₉ clusters, respectively. Based on the present theoretical results, the catalytic activity of BC₁₉ and NC₁₉ clusters toward the CO oxidation is more than that of the C₂₀ cluster. These results show that the BC₁₉ and NC₁₉ clusters can be recommended as an efficient and metal-free catalyst for CO oxidation at near ambient temperatures.

Research Highlights

• CO oxidation over C₂₀ fullerene has been investigated.

• The effect of N/B-doing on the CO oxidation reaction is also studied.

• The N/B-doping increases the catalytic activity of C₂₀ fullerene.

• Boron/Nitrogen-doped C₂₀ fullerene can be applied as an efficient catalyst for CO oxidation.

     

Effect of contact interface configuration on electronic transport in (C20)2-based molecular junctions

Using first-principles calculations, we study the electronic transport properties in Au(C₂₀)₂Au molecular junctions with different contact interface configurations: point contact and bond contact. We observe that the transmission through the bond contact is considerably higher than that of point contact. Furthermore, the I-V characteristics are rather different. For the bond contact, we get a metallic behavior followed by a varistor-type behavior. While as for the point contact, the current increases very slowly in a nonlinear way and is one order of magnitude smaller than that of bond contact. We attribute these obvious differences to the distinct contact configurations.     

Electronic and optical properties of pure and Mo doped anatase TiO2 using GGA and GGA+U calculations

Comparative GGA and GGA+U calculations for pure and Mo doped anatase TiO₂ are performed based on first principle theory, whose results show that GGA+U calculation provide more reliable results as compared to the experimental findings. The direct band gap nature of the anatase TiO₂ is confirmed, both by using GGA and GGA+U calculations. Mo doping in anatase TiO₂ narrows the band gap of TiO₂ by introducing Mo 4d states below the conduction band minimum. Significant reduction of the band gap of anatase TiO₂ is found with increasing Mo doping concentration due to the introduction of widely distributed Mo 4d states below the conduction band minimum. The increase in the width of the conduction band with increasing doping concentration shows enhancement in the conductivity which may be helpful in increasing electron–hole pairs separation and consequently decreases the carrier recombination. The Mo doped anatase TiO₂ exhibits the n-type characteristic due to the shifting of Fermi level from the top of the valence band to the bottom of the conduction band. Furthermore, a shift in the absorption edge towards visible light region is apparent from the absorption spectrum which will enhance its photocatalytic activity. All the doped models have depicted visible light absorption and the absorption peaks shift towards higher energies in the visible region with increasing doping concentration. Our results describe the way to tailor the band gap of anatase TiO₂ by changing Mo doping concentration. The Mo doped anatase TiO₂ will be a very useful photocatalyst with enhanced visible light photocatalytic activity.     

C42+的几何构型和Jahn Teller效应

用从头计算法QCISD/6-311G得到了C₄²⁺分子的10种不同的几何构型,其中包括C_s,C_∞v,C_2v,D_2h,D_∞h,D_4h,D_2d,C_3v等不同的构型.计算表明C₄²⁺的T_d构型不能稳定存在,详细讨论了T_{d 关键词： 几何构型 4}²⁺')" href="#">C₄²⁺ Jahn Teller效应     

Ab initio electronic structure,bonding and optical properties of two relatively new ceramics Hf2Al3C4 and Hf3Al3C5: A comparative study

The structural, electronic and optical properties of the ternary carbides Hf₂Al₃C₄ and Hf₃Al₃C₅ are studied via first principles orthogonalized linear combination of atomic orbitals (OLCAO) method. Results on crystal structure, interatomic bonding, band structure, total and partial density of states (DOS), localization index (LI), effective charge (Q*), bond order (BO), dielectric function (ε), optical conductivity (σ) and electron energy loss function are presented and discussed in detail. The band structure plots show the conducting nature of Hf₂Al₃C₄ and Hf₃Al₃C₅ carbides. DOS results disclose that the total number of states at Fermi level N(E_F) are 1.89 and 2.38 states/(eV unit cell) for Hf₂Al₃C₄ and Hf₃Al₃C₅ respectively. The Q* calculations show an average charge transfer of 0.723 and 0.711 electrons from Hf and 0.809 and 0.807 electrons from Al to C sites in Hf₂Al₃C₄ and Hf₃Al₃C₅ respectively. The BO results provide the dominating role of Al–C bonds with BO value of 6.62 (BO%?=?59%) and 6.66 (BO%?=?49%) for Hf₂Al₃C₄ and Hf₃Al₃C₅ respectively and are considered responsible for the crystals cohesion. The LI results reflect the presence of highly delocalized states in the vicinity of the Fermi level. The dielectric function plots of the real (?₁(?ω)) and imaginary (?₂(?ω)) parts show the anisotropic behavior of Hf₂Al₃C₄ and Hf₃Al₃C₅. The results on optical conductivity (σ) support the trends observed in dielectric functions. The electron energy loss functions reveal the presence of sharp peaks both in ab-plane and along c-axis around 20?eV in Hf₂Al₃C₄ and Hf₃Al₃C₅ ternary carbides.     

Rotationally resolved electronic spectroscopy of a nonlinear carbon chain radical C6H4

Rotationally resolved electronic spectrum of the origin band in the 2A″-X2A″ transition of a nonlinear carbon chain radical C₆H₄⁺ has been recorded in the 604 nm region using cw cavity ring down spectroscopy. The radical was produced by a discharge through an acetylene-helium mixture in a supersonic planar expansion. The rotational structure has been analysed and precisely determined. A band having a-type prolate rotational structure has also been observed near 581 nm. By considering the results of ab initio calculations this band is assigned to a transition involving the excitation of the ν₁₂ fundamental in the upper 2A″ electronic state of the same C₆H₄⁺ isomer.     

Electron band structure of α-ZnIn2S4 and related polytypes

Pseudopotential calculations have been carried out for the α, β and γ polytypic forms of the layer semiconductor ZnIn₂S₄, respectively, corresponding to space groups C⁵_3v, C¹_3v and D³_3d. The required form factors are consistent with those used in our previous calculations for ZnS and CdIn₂S₄. The band structure of the α phase, the only one up to now for which optical data are available, compares quite satisfyingly with very recent photoemission and reflectivity experimental data. The computed band structures of the β and γ phases are very alike; on the contrary, interesting differences exist between these structures and the α phase which could easily be verified by experimental investigations.     

Size-dependent electronic structures of boron carbonitride (BC2N) nanotubes. A DFT approach

The electronic structures and physical properties of zigzag BC₂N (n,0; n = 4–10) and armchair BC₂N (n,m; n = m = 4–10) nanotubes (type III) are studied by using density functional theory with the generalized gradient approximation. From a comparison of the binding energies, it is inferred that in the large diameter BC₂N nanotubes, the zigzag form is thermally more stable than the armchair form. BC₂N nanotubes (with the exception of (4,0) which is conductor) are gapless semiconductors. Depend on the chirality index, the zigzag forms of BC₂N nanotubes have narrower band gap than the armchair form. Semiconductor character in the studied BC₂N nanotubes is due to contribution of p electrons in the Fermi level. Mulliken population analyses show that significant amounts of electron charge are transferred between atoms; which suggests the existence of polar covalent bonds in the BC₂N nanotubes.     

Modeling electronic,mechanical, optical and thermal properties of graphene-like BC6N materials: Role of prominent BN-bonds

We model monolayer graphene-like materials with BC₆N stoichiometry where the bonding between the B and the N atoms plays an important role for their physical and chemical properties. Two types of BC₆N are found based on the BN bonds: In the presence of BN bonds, an even number of π-bonds emerges indicating an aromatic structure and a large direct bandgap appears, while in the absence of BN bonds, an anti-aromatic structure with an odd-number of π-bonds is found resulting a direct small bandgap. The stress-strain curves shows high elastic moduli and tensile strength of the structures with BN-bonds, compared to structures without BN-bonds. Self-consistent field calculations demonstrate that BC₆N with BN-bonds is energetically more stable than structures without BN-bonds due to a strong binding energy between the B and the N atoms, while their phonon dispersion displays that BC₆N without BN-bonds has more dynamical stability. Furthermore, all the BC₆N structures considered show a large absorption of electromagnetic radiation with polarization parallel to the monolayers in the visible range. Finer detail of the absorption depend on the actual structures of the layers. A higher electronic thermal conductivity and specific heat are seen in BC₆N systems caused by hot carrier–assisted charge transport. This opens up a possible optimization for bolometric applications of graphene based material devices.     

Theoretical investigation of the atomic and electronic structure of Li x BC3 intercalated compounds

Li _x BC₃ intercalated compounds with various configurations are studied for their possible application as electrode materials for lithium current sources. For this purpose, the band structure and the density of states were calculated for each structure, and energy stability and possible deformations due to a change in the unit cell volume during intercalation are investigated.     

The electronic structure and properties of Fe6(N1−xCx)2 carbonitrides by first-principles calculations

Electronic structure and properties of Fe₆(N_1−xC_x)₂ carbonitrides with 0≤x≤1, i.e. the concentrations of N and C elements are respectively in range of 0∼7.69 wt% and 0∼6.67 wt%, have been studied by first-principles calculations based on density functional theory (DFT) implemented in the Cambridge Serial Total Energy Package (CASTEP) code. The calculated results show that the Fe₆(N_1−xC_x)₂ carbonitrides are thermodynamically and mechanically stable. Lattice parameters and stability of the carbonitrides increase when C atoms replace N atoms in Fe₆N₂ unit cell. In Fe₆(N_1−xC_x)₂ unit cell, the hybridization effect between C-2p and Fe-3d states is stronger than that between N-2p and Fe-3d states. Elastic properties and melting points of the carbonitrides change slightly with the substitution of C atoms for N atoms in Fe₆(N_1−xC_x)₂ carbonitrides.     

EHCO studies on Cis polyacetylene

To date, band structure calculations on cis polyacetylene have been used on its translational symmetry, i.e. C₄H₄ is the unit cell. In a band structure calculation based on helical symmetry, the mer C₂H₂ becomes the unit cell. The merits of band structure calculations based on helical symmetry are discussed, and it is demonstrated that calculations on cis polyacetylene in either mode give comparable results.