Be,C双受主共掺杂实现P型AlN的第一性原理研究

基于密度泛函理论(Density Functional Theory)的第一性原理平面波超软赝势方法(USPP),首先对Be、C掺杂Al N的晶格结构进行优化,得到其稳定结构.然后对Be、C掺杂Al N的晶格参数、结合能、能带结构、电子态密度和电荷集居数进行了详细地计算和分析.计算结果表明:Be-2C共掺杂Al N的构型具有更稳定的结构,能使受主能级变宽、非局域化特征明显.因此,Be-2C共掺杂Al N有望成为一种更稳定高效的p型掺杂手段.     

Be，C双受主共掺杂实现P型AlN的第一性原理研究

基于密度泛函理论 Density Functional Theory 的第一性原理平面波超软赝势方法USPP,首先对Be、C掺杂AlN的晶格结构进行优化,得到其稳定结构．然后对Be、C掺杂AlN的晶格参数、结合能、能带结构、电子态密度和电荷集居数进行了详细地计算和分析．计算结果表明：Be-2C共掺杂AlN的构型具有更稳定的结构,能使受主能级变宽、非局域化特征明显．因此,Be-2C共掺杂AlN有望成为一种更稳定高效的p型掺杂手段．     

Electronic structures of N- and C-doped NiO from first-principles calculations

The large intrinsic band gap of NiO has hindered severely its potential application under visible-light irradiation. In this Letter, we have performed first-principles calculations on the electronic properties of N- and C-doped NiO to ascertain if its band gap may be narrowed theoretically. It was found that impurity bands driven by N 2p or C 2p states appear in the band gap of NiO and that some of these locate at the conduction band minimum, which leads to a significant band gap narrowing. Our results show that N-doped NiO may serve as a potential photocatalyst relative to C-doped NiO, due to the presence of some recombination centres in C-doped NiO.     

Electronic structure and optical properties of non-metallic modified graphene: a first-principles study

In this paper, the electronic structure and stability of the intrinsic, B-, N-, Si-, S-doped graphene are studied based on first-principles calculations of density functional theory. Firstly, the intrinsic, B-, N-, Si-, S-doped graphene structures are optimized, and then the forming energy, band structure, density of states, differential charge density are analyzed and calculated. The results show that B- and Si-doped systems are p-type doping, while N is n-type doping. By comparing the forming energy, it is found that N atoms are more easily doped in graphene. In addition, for B-, N-, Si-doped systems, it is found that the doping atoms will open the band gap, leading to a great change in the band structure of the doping system. Finally, we systematically study the optical properties of the different configurations. By comparison, it is found that the order of light sensitivity in the visible region is as follows: S-doped> Si-doped> pure > B-doped > N-doped. Our results will provide theoretical guidance for the stability and electronic structure of non-metallic doped graphene.     

Magnetic,electronic, optical,and photocatalytic properties of nonmetal- and halogen-doped anatase TiO2 nanotubes

The structure stability, magnetic, electronic, optical, and photocatalytic properties of nonmetal (B, C, N, P, and S), and halogen (F, Cl, Br, and I)-doped anatase TiO₂ nanotubes (TNTs) have been investigated using spin polarized density functional theory. The N- and F-doped TNTs are the most stable among other doped TNTs. It is found that the magnetic moment of doped TNT is the difference between the number of the valence electrons of the dopant and host anion. All dopants decrease the band gap of TNT. The decrease in the band gap of nonmetal (C, N, P, and S)-doped TNTs, in particular N and P, is larger than that of halogen-doped TNTs due to the created states of the nonmetal dopant in the band gap. There is a good agreement between the calculation results and the experimental observations. Even though C-, N-, and P-doped TNTs have the lowest band gap, they cannot be used as a photocatalysis for water splitting. The B-, S-, and I-doped TiO₂ nanotubes are of great potential as candidates for water splitting in the visible light range.     

应变和C掺杂对单层BN纳米片的电子结构和磁学性质的影响

利用基于密度泛函理论的第一性原理计算方法, 研究了应变和C原子掺杂对单层BN纳米片的电子结构和磁学性质的影响. 计算结果表明未掺杂的单层BN纳米片具有宽的直接带隙, 在压缩和拉伸应变的作用下, 带隙会分别增大和减小, 但应变对带隙的调制整体效果不太明显. 单个C原子掺入BN纳米片的态密度揭示体系呈现出半金属性(Half-metallicity), 磁矩主要源于C 2p态, 而B 2p和N 2p态在极化作用下也能提供部分磁矩. 两个C原子掺入BN纳米片时, 磁性基态会随着C原子的间距发生变化: 当两C原子为最近邻(nn)和次近邻(nnn)时, 反铁磁态为磁性基态; 而当两C原子为次次近邻(nnnn)时, 铁磁态为基态, 并且其态密度也显示出半金属性.     

Half-metallic ferromagnetism and band gap reduction in Cu-doped zinc-blende BeO: First-principle study

In this paper, we report ferromagnetism in copper doped zinc-blende BeO. Our first-principles calculations based on spin density functional theory predicts a total magnetic moment of 1 μB per copper when copper substitutes beryllium in BeO, where 0.58 μB is localized at Cu atom. The results obtained show that the ferromagnetic state is 34 meV lower than the antiferromagnetic state. Calculations indicate an appreciable band gap reduction in BeO. The analysis of the partial density of states reveals that ferromagnetism and reduction of BeO band gap are principally due to the strong p–d coupling of О and Cu.     

Modulation of electronic structure properties of C/B/Al-doped armchair GaN nanoribbons

ABSTRACT

The electronic structures of C/B/Al-doped armchair GaN nanoribbons (aGaNNRs) are systematically studied by using density functional theory. We find that the original aGaNNRs are direct band gap semiconductors and that the gaps monotonically decrease with increasing widths. Interestingly, the B- or Al-doped aGaNNRs are also direct-band gap semiconductors with a slightly larger gap than their undoped aGaNNRs, while the C-doped aGaNNRs display metallic characteristics with an impurity state across the Fermi level in band structures. The semiconducting or metallic behaviours of C/B/Al-doped aGaNNRs can be explained by the orbital coupling between the extrinsic atom and primary Ga, N in their partial density of states. Our results show a useful way to modulate the band gaps of aGaNNRs.

Using the density-functional theory, we performed a theoretical research to study the electronic structures of C/B/Al-doped armchair gallium nitride nanoribbons. The calculated band structures show that the perfect and original aGaNNRs are direct semiconductors regardless of ribbon widths, and gaps monotonically decrease with increasing the widths. The B/Al-doped aGaNNRs are semiconductors with a slightly larger gap, while metallic behavior presents in C-doped aGaNNRs with an impurity band across the EF. The results show a useful way to modulate the band gaps of aGaNNRs.     

Effects of enhanced electronic correlation on magnetic properties of light non-metallic element (B,C, N,and O)-doped CuCl: A first-principles study

Based on density functional calculations within both standard generalized gradient approximation and plus on-site Coulomb interactions approaches, we have investigated the electronic structure and magnetic properties of the first-row element-doped CuCl semiconductors. The electronic correlations in both 2p and 3d orbitals are enhanced by adding the on-site Coulomb repulsion (Hubbard U and Hund exchange J). After a comparative study, we find that, for both standard and beyond approaches, B-doped CuCl is a half-metallic magnet with majority-spin impurity bands touching the Fermi level, C-doped CuCl is a magnetic semiconductor, and N-doped CuCl is a half-metallic magnet with minority-spin impurity bands crossing the Fermi level. Nevertheless, for O-doped CuCl, it transforms from a nonmagnetic semiconductor to a half-metallic magnet with metallic up-spins by considering the correlation effects. The calculation shows that the enhanced electronic correlation not only corrects the error of band-gap, but also influences the magnetic ground state and the distribution of local magnetic moments. The location of impurity bands with different dopants was understood based on the elements' electronegativity and interaction between dopant and host atoms. Strong hybridization between the dopant's 2p states and the filled 3d orbitals of adjacent Cu yields the main contribution to magnetization.     

Structural,electronic and mechanical properties of alkaline earth metal oxides MO (M=Be,Mg, Ca,Sr, Ba)

The structural, electronic and mechanical properties of alkaline earth metal oxides MO (M=Be, Mg, Ca, Sr, Ba) in the cubic (B1, B2 and B3) phases and in the wurtzite (B4) phase are investigated using density functional theory calculations as implemented in VASP code. The lattice constants, cohesive energy, bulk modulus, band structures and the density of states are computed. The calculated lattice parameters are in good agreement with the experimental and the other available theoretical results. Electronic structure reveals that all the five alkaline earth metal oxides exhibit semiconducting behavior at zero pressure. The estimated band gaps for the stable wurtzite phase of BeO is 7.2 eV and for the stable cubic NaCl phases of MgO, CaO, SrO and BaO are 4.436 eV, 4.166 eV, 4.013 eV, and 2.274 eV respectively. A pressure induced structural phase transition occurs from wurtzite (B4) to NaCl (B1) phase in BeO at 112.1 GPa and from NaCl (B1) to CsCl (B2) phase in MgO at 514.9 GPa, in CaO at 61.3 GPa, in SrO at 42 GPa and in BaO at 14.5 GPa. The elastic constants are computed at zero and elevated pressures for the B4 and B1 phases for BeO and for the B1 and B2 phases in the case of the other oxides in order to investigate their mechanical stability, anisotropy and hardness. The sound velocities and the Debye temperatures are calculated for all the oxides using the computed elastic constants.     

First-principles study on ferromagnetism in C-doped AlN

First-principles calculations are performed to study the electronic structures and magnetic properties of C-doped AlN. Both generalized gradient approximation (GGA) and GGA+U calculations show that a substitutional C atom introduces magnetic moment of about 1.0 μB, which comes from the partially occupied 2p orbitals of the C, its first neighboring Al and first neighboring N atoms (GGA) or out-of-plane first and fifth neighboring N atoms (GGA+U), among which the atomic moment of the C is the biggest. The U correction for the anion-2p states obviously changes the magnetic moment distribution of Al and N atoms and transforms the ground state of C-doped AlN to insulating from half-metallic. The C atoms can induce ferromagnetic ground state with long-range couplings between the moments in C-doped AlN. The ferromagnetic coupling can be explained in terms of the two band coupling model.     

ZnO1-xCx稀磁半导体的磁特性的第一性原理和蒙特卡罗研究

利用第一性原理计算得到C掺杂ZnO的电子结构,发现系统具有半金属的电子结构.从态密度的分析可以看到Zn-3d和C-2p电子具有强烈的杂化作用,这是体系具有相对稳定铁磁基态的原因.利用第一性原理得到的磁性耦合强度并结合蒙特卡罗模拟得到了C掺杂浓度为555%,833%,125%的ZnO_1-xC_x分别具有210 K,260 K,690 K的居里温度.同时,详细地分析了C掺杂引起的电子转移和C,Zn,O的s,p和d电子的自旋向上和自旋向下电子数的变化.通过比较研究,发现ZnO_1-xC_x的局域磁矩主要来源于Zn-3d 电子和C-2p 电子之间的相互作用,而局域磁矩耦合倾向于RKKY耦合. 关键词： 1-xC_x')" href="#">ZnO_1-xC_x 磁性 第一性原理 蒙特卡罗模拟     

C掺杂锐钛矿相TiO2吸收光谱的第一性原理研究

运用第一性原理,对C掺杂锐钛矿相TiO₂的电子结构进行了研究,从能带结构理论解释了C掺杂TiO₂吸收光谱的一些实验现象.发现在C掺杂后的锐钛矿相TiO₂的禁带宽度增大,并且在带隙中出现了杂质能级,这些杂质能级主要是由C 2p轨道上的电子构成的,它们之间是独立的,正是这些独立的杂质能级使TiO₂掺杂后可以发生可见光响应.价带上的电子可以吸收一定能量的光子跃迁到杂质能级,而杂质能级上的电子也可以吸收一定能量的光子跃迁到导带,所以从理论上可以计算出掺杂后的TiO₂在可见光范围内存在两个吸收边,与实验中所得到的现象相一致. 关键词： C掺杂 2')" href="#">锐钛矿TiO₂ 能带结构 吸收光谱     

Electronic Correlation Effects on the Optical Properties of SiC,BeO and Boron Nitride Nanotubes and Monolayers

We present numerical calculation of the impact of electron-electron interaction on the behavior of density of states and optical properties of BeO, SiC and Boron-Nitride nanotubes and sheets. Hubbard model hamiltonian is applied to describe the dynamics of electrons on the lattice structure of theses compounds. The excitation spectrum of the system in the presence of local electronic interactions has been found using mean field approach. We find the band gap width in both optical absorption and density of states reduces with local Hubbard electronic interaction parameter. The absorption spectra exhibits the remarkable peaks, mainly owing to the divergence behavior of density of states and excitonic effects. Also we compare optical absorption frequency behavior of BeO, SiC and Boron-Nitride nanotubes with each other. Furthermore we investigate the optical properties of BeO and SiC sheets. A novel feature of optical conductivity of these structures is the decrease of frequency gap in the optical spectrum due to electronic interaction.     

Electronic Correlation Effects on the Optical Properties of SiC,BeO and Boron Nitride Nanotubes and Monolayers

We present numerical calculation of the impact of electron-electron interaction on the behavior of density of states and optical properties of BeO,SiC and Boron-Nitride nanotubes and sheets.Hubbard model hamiltonian is applied to describe the dynamics of electrons on the lattice structure of theses compounds.The excitation spectrum of the system in the presence of local electronic interactions has been found using mean Seld approach.We find the band gap width in both optical absorption and density of states reduces with local Hubbard electronic interaction parameter.The absorption spectra exhibits the remarkable peaks,mainly owing to the divergence behavior of density of states and excitonic effects.Also we compare optical absorption frequency behavior of BeO,SiC and Boron-Nitride nanotubes with each other.Furthermore we investigate the optical properties of BeO and SiC sheets.A novel feature of optical conductivity of these structures is the decrease of frequency gap in the optical spectrum due to electronic interaction.     

Structure and properties of BeO nanotubes

The structure of a new non-carbon (beryllium oxide BeO) nanotube consisting of a rolled-up graphene sheet is proposed, and its physical properties are described. Ab initio calculations of the binding energy, the electronic band structure, the density of states, the dependence of the strain energy of the nanotube on the nanotube diameter D, and the Young’s modulus Y for BeO nanotubes of different diameters are performed in the framework of the density functional theory (DFT). From a comparison of the binding energies calculated for BeO nanotubes and crystalline BeO with a wurtzite structure, it is inferred that BeO nanotubes can be synthesized by a plasma-chemical reaction or through chemical vapor deposition. It is established that BeO nanotubes are polar dielectrics with a band gap of ～5.0 eV and a stiffness comparable to that of the carbon nanotubes (the Young’s modulus of the BeO nanotubes Y _BeO is approximately equal to 0.7Y _C, where Y _C is the Young’s modulus of the carbon nanotubes). It is shown that, for a nanotube diameter D > 1 nm, the (n, n) armchair nanotubes are energetically more favorable than the (n, 0) zigzag nanotubes.     

Dilute magnetic semiconductor and half-metal behaviors in C-codoped BeO nanotubes: A first principles simulations

The geometric, electronic and magnetic properties of C-codoped single walled BeO nanotubes (SWBeONTs) are systematically explored by using ab-initio density functional theory calculations. We performed our calculations for C codoping BeO nanotube in two different chiralities: (8,0) and (5,5). In each case, two different configurations are considered, first the two oxygen atoms replaced by two carbon atoms are on first nearest neighbor sites in the plane of codoping and second they are far from each other. We found when C atoms are at the nearest-neighboring positions; the antiferromagnetism (AFM) phase is stable while increasing the distance between the two C atoms, the ferromagnetism stability increases. In the AFM phase the structures are nonmagnetic semiconductors, but in the FM phase all these systems are half-metallic systems with high magnetic moment and 100% spin polarization which can be used as magnetic nanostructure and possible future applications in permanent magnetism, magnetic recording, and spintronics.     

Tuning the magnetic and electronic properties of strontium titanate by carbon doping

The magnetic and electronic properties of strontium titanate with different carbon dopant configurations are explored using first-principles calculations with a generalized gradient approximation (GGA) and the GGA+U approach. Our results show that the structural stability, electronic properties and magnetic properties of C-doped SrTiO₃ strongly depend on the distance between carbon dopants. In both GGA and GGA+U calculations, the doping structure is mostly stable with a nonmagnetic feature when the carbon dopants are nearest neighbors, which can be ascribed to the formation of a C–C dimer pair accompanied by stronger C–C and weaker C–Ti hybridizations as the C–C distance becomes smaller. As the C–C distance increases, C-doped SrTiO₃ changes from an n-type nonmagnetic metal to ferromagnetic/antiferromagnetic half-metal and to an antiferromagnetic/ferromagnetic semiconductor in GGA calculations, while it changes from a nonmagnetic semiconductor to ferromagnetic half-metal and to an antiferromagnetic semiconductor using the GGA+U method. Our work demonstrates the possibility of tailoring the magnetic and electronic properties of C-doped SrTiO₃, which might provide some guidance to extend the applications of strontium titanate as a magnetic or optoelectronic material.     

Effect of C doping on the structural and optical properties of sol–gel TiO2 thin films

Undoped and C-doped TiO₂ thin films have been prepared by sol–gel process. Their structure and optical properties have been investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV–vis spectroscopy. It has been observed that C dopants retard the transformation from anatase-to-rutile phase. Namely, C doping effect is attributed to the anatase phase stabilization. The optical analyses show that the optical band gap of anatase C-doped TiO₂ decreases with increasing amount of C. Also, it is founded that C dopants have been shown to make TiO₂ have a visible light photoresponse.     

Low-temperature luminescence and thermally stimulated luminescence of BeO: Mg single crystals

Luminescence and thermally stimulated luminescence (TL) of BeO: Mg crystals are studied at T = 6–380 K. The TL glow curves and the spectra of luminescence (1.2–6.5 eV), luminescence excitation, and reflection (3.7–20 eV) are obtained. It is found that the introduction of an isovalent magnesium impurity into BeO leads to the appearance of three new broad luminescence bands at 6.2–6.3, 4.3–4.4, and 1.9–2.6 eV. The first two are attributed to the radiative annihilation of a relaxed near-impurity (Mg) exciton, the excited state of which is formed as a result of energy transfer by free excitons. The impurity VUV and UV bands are compared with those for the intrinsic luminescence of BeO caused by the radiative annihilation of self-trapped excitons (STE) of two kinds: the band at 6.2–6.3 eV of BeO: Mg is compared with the band at 6.7 eV (STE₁) of BeO, and the band at 4.3–4.4 eV is compared with the band at 4.9 eV (STE²) of BeO. In the visible region, the luminescence spectrum is due to a superposition of intracenter transitions in an impurity complex including a magnesium ion. The manifestation of X-ray-induced luminescence bands at T = 6 K in BeO: Mg indicates their excitation during band-to-band transitions and in recombination processes. The energy characteristics of the impurity states in BeO: Mg are determined; the effect of the isovalent impurity on the fluctuation rearrangement of the BeO: Mg structure in the thermal transformation region of STE₁ → STE₂ is revealed.