Heat capacity,electrical and thermal conductivity of silicene

We investigate the electronic heat capacity, electrical and thermal conductivity ofmonolayer planar and buckled silicon sheets (silicene) through tight binding approximationand Kubo-Greenwood formula. Applying and increasing dopant atoms to the system leads toopening a gap in the band structures and density of states that causes to decrease(increase) the heat capacity before (after) the Schottky anomaly. The electrical andelectronic thermal conductivity of doped silicene reduces with increasing impuritystrength.     

石墨烯纳米片电子结构和量子输运特性第一原理研究

用基于密度泛函理论的原子紧束缚方法计算研究单层石墨烯纳米圆片和纳米带的电子结构,并结合第一原理和非平衡函数法计算量子输运特性.通过电子能态和轨道密度分布研究纳米碳原子层的电子成键状态,结合电子透射谱、电导和电子势分布分析电子散射与输运机制.石墨烯纳米带和纳米圆片分别呈现金属和半导体的能带特征,片层边缘上电极化分别沿垂直和切向方向,电子电导出现较大的差异,来源于石墨烯纳米圆片边缘的突出碳原子环对电子的强散射.石墨烯纳米带的电子透射谱表现为近似台阶式变化并在费米能级处存在弹道电导峰,而石墨烯纳米圆片的电子能带和透射谱在费米能级处开口并且因量子限制作用呈现更加离散的多条高态密度窄能带和尖锐谱峰.     

A study of electronic structure and lattice dynamics of CoSb3 skutterudite

The novel filled skutterudite materials have attracted much interest in recent years and experimental studies have revealed that electrical properties (electrical conductivity and Seebeck coefficient) in these materials are dominated by their electronic structure while the effective suppression of thermal conductivity is mainly determined by their lattice dynamics. To clarify the relationship between microstructure and properties in further, we report a systematic study of electronic structures and lattice dynamics of CoSb₃ in this paper using linearized augmented plane waves based on the density functional theory of first principles. By calculating band structure and partial density of states (PDOS), effects of electronic structures of CoSb₃ on electrical properties were investigated. Based on the calculated results of phonon dispersions and phonon density of states of CoSb₃, lattice dynamics of CoSb₃ (heat capacity, Debye temperature, mean free path and lattice thermal conductivity) are discussed in detail. The calculated results are excellently consistent with other work and experimental data.     

Orbital electronic heat capacity of hydrogenated monolayer and bilayer graphene

The tight-binding Harrison model and Green's function approach have been utilized in order to investigate the contribution of hybridized orbitals in the electronic density of states(DOS) and electronic heat capacity(EHC) for four hydrogenated structures, including monolayer chair-like, table-like, bilayer AA- and finally AB-stacked graphene. After hydrogenation, monolayer graphene and bilayer graphene are behave as semiconducting systems owning a wide direct band gap and this means that all orbitals have several states around the Fermi level. The energy gap in DOS and Schottky anomaly in EHC curves of these structures are compared together illustrating the maximum and minimum band gaps are appear for monolayer chair-like and bilayer AA-stacked graphane, respectively. In spite of these, our findings show that the maximum and minimum values of Schottky anomaly appear for hydrogenated bilayer AA-stacked and monolayer table-like configurations, respectively.     

Local electronic and electrical properties of functionalized graphene nano flakes

Based on experimental findings models of amorphous graphene related carbon materials were generated using graphene nano flakes. On the optimized structures detailed local electronic properties were investigated using density functional theory. The electrical conductivities of all these models were also estimated using an in-house program based on tight-binding method. The calculated electrical conductivity values of all the models agreed well with the trend of calculated energy gap and graphitic character.     

Modification of the electronic structure of graphene by intercalation of iron and silicon atoms

The ab initio calculations of the electronic structure of low-dimensional graphene–iron–nickel and graphene–silicon–iron systems were carried out using the density functional theory. For the graphene–Fe–Ni(111) system, band structures for different spin projections and total densities of valence electrons are determined. The energy position of the Dirac cone caused by the p _z states of graphene depends weakly on the number of iron layers intercalated into the interlayer gap between nickel and graphene. For the graphene–Si–Fe(111) system, the most advantageous positions of silicon atoms on iron are determined. The intercalation of silicon under graphene leads to a sharp decrease in the interaction of carbon atoms with the substrate and largely restores the electronic properties of free graphene.

     

Manipulation of inherent characteristics of graphene through N and Mg atom co-doping; a DFT study

First-principles calculations were performed to investigate the structural, electronic, magnetic and optical properties of nitrogen (N) and magnesium (Mg) atom co-doped graphene systems. We observed that, N and Mg atom co-doping in graphene, introduces half-metallic properties in the electronic structure of graphene, introduces ferromagnetism behavior along with new trends in optical properties of graphene. Doping site and concentration of N and Mg atoms in graphene was changed and resulting effects of these changes on aforementioned properties were investigated. Through density of states plots we observed that, Mg atom sp orbitals mainly induced magnetic moments in graphene. It was revealed that, N/Mg atoms substitution in graphene introduces a red shift in absorption spectrum towards visible range and a finite absorption coefficient quantity value in 0 to 3 eV and 7 to 11 eV energy intervals is also produced, that is unavailable for absorption spectrum of intrinsic graphene. Moreover, N/Mg atoms co-doping produces increment in the reflectivity parameter of graphene in low lying energy region, while producing diminishing behavior in the higher energy range. These results offer a possibility to tune electronic, magnetic and optical characteristics of graphene sufficiently for utilization in graphene based spintronic and optoelectronic devices.     

锂改性点缺陷石墨烯储氢性能的第一性原理研究

本研究采用基于密度泛函理论的第一性原理方法计算了两种石墨烯点缺陷处原子的分波态密度(PDOS),能带结构和差分电荷密度等,研究了锂掺杂对两种本征石墨烯缺陷C-Bridge和C7557电子结构的改性,以及对其储氢能力的影响.结果表明Li原子能够稳定的掺杂且不易形成团簇,并且Li原子掺杂石墨烯能够对石墨烯能带中的狄拉克锥和费米面起到调控作用,增强了缺陷石墨烯的电子活性.本征缺陷石墨烯的储氢能力较弱,缺陷石墨烯的储氢能力可以通过Li掺杂来改善.     

Hydrogen storage of beryllium adsorbed on graphene doping with boron: First-principles calculations

Based on density-functional theory, we find that B-doped graphene significantly enhances the Be adsorption energy and prevent Be atoms from clustering. The complex of Be adsorbed on B-doped graphene can serve as a high-capacity hydrogen storage medium: the hydrogen storage capacity (HSC) can reach up to 15.1 wt% with average adsorption energy ?0.298 eV/H₂ for double-sided adsorption. It has exceeded the target specified by US Department of Energy with HSC of 9 wt% and a binding energy of ?0.2 to ?0.6 eV/H₂ at near-ambient conditions. By analyzing the projected electronic density of states of the adsorbed system, we show that the high HSC is due to the change of electron distribution of H₂ molecules and a graphene system decorated with B and Be atoms.     

Optical properties of gapped graphene structure driven by electron electron interaction

We analyze the effects of on-site electronic coulomb repulsion U on the optical absorption and density of states of a graphene like structure with two different sublattice on-site energies in the context of Hubbard model. Mean field approximation has been implemented in order to find excitation spectrum of electronic system. Antiferromagnetic long range ordering has been considered as the ground state of model Hamiltonian. We find that the band gap in both optical conductivity and density of states decreases with strength of coulombic interaction. The absorption spectra of the graphene like structure as a nanoscale system exhibit the prominent peaks, mainly owing to the divergent density of states and excitonic effects.     

Structure of endohedral complexes of carbon nanotubes encapsulated with lithium and sodium

The article provides the results of ab initio calculations employing density functional theory of carbon nanotubes that contain clusters of lithium and sodium atoms. Stable positions of interstitial atoms, the electron density distribution in the system and the density of electronic states are determined. It is shown that the amount of charge transferred from the interstitial atoms in a cluster significantly differs from the corresponding value for a single atom. It is established that the density of electronic states of the system at low concentrations of atoms of the introduced element is determined by the electronic structure of a hollow nanotube, and as the concentration of interstitial atoms increases, this quantity becomes virtually independent on the type of alkali metal (lithium or sodium) and the initial type of the nanotube conductivity.     

Unraveling the effect of uniaxial strain on thermoelectric properties of Mg_2Si: A density functional theory study

In this work, the effect of uniaxial strain on electronic and thermoelectric properties of magnesium silicide using density functional theory(DFT) and Boltzmann transport equations has been studied. We have found that the value of band gap increases with tensile strain and decreases with compressive strain. The variations of electrical conductivity,Seebeck coefficient, electronic thermal conductivity, and power factor with temperatures have been calculated. The Seebeck coefficient and power factor are observed to be modified strongly with strain. The value of power factor is found to be higher in comparison with the unstrained structure at 2% tensile strain. We have also calculated phonon dispersion, phonon density of states, specific heat at constant volume, and lattice thermal conductivity of material under uniaxial strain. The phonon properties and lattice thermal conductivity of Mg_2Si under uniaxial strain have been explored first time in this report.     

氧化石墨烯纳米带能带结构和态密度的第一性原理研究

基于密度泛函理论,采用第一性原理方法,计算了氧化石墨烯纳米带的电荷密度、能带结构和分波态密度。结果表明,石墨烯纳米带被氧化后,转变为间接带隙半导体,带隙值为0.375 e V。电荷差分密度表明,从C原子和H原子到O原子之间有电荷的转移。分波态密度显示,在导带和价带中C-2s、2p,O-2p,H-1s电子态之间存在强烈的杂化效应。在费米能级附近,O-2p态电子局域效应的贡献明显,对于改善氧化石墨烯纳米带的半导体发光效应起到了主要作用。     

Study of the electronic structure and properties of 13C-isotope-based composites

The morphology and electronic structure of ¹³C-isotope-based graphite composites were studied by transmission electron microscopy (TEM), x-ray diffraction, and x-ray fluorescence spectroscopy. High-resolution TEM images showed that composites contain several forms of carbon particles. According to an x-ray diffraction analysis, the size of graphene stacks of graphite particles is 20 and 40 Å. The CK _α x-ray fluorescence spectra of the initial ¹³C isotope powder and composites based on it detected an increase in the density of high-energy occupied states in comparison with the graphite spectrum. Ab initio quantum chemical calculation of the structure of C₁₅₀ graphene showed that the increase in the density of states stems from the electrons of dangling bonds of boundary carbon atoms of particles ～20 Å in size. Electrical properties of ¹³C-isotope-based samples were studied by analyzing the temperature dependence of the conductivity. It was assumed that the change in the logarithmic dependence of the conductivity observed at liquid-helium temperatures to the linear dependence as the temperature increases is caused by carrier transfers between the disordered graphene layers forming a nanocomposite.     

Electronic and magnetic properties of silicon adsorption on graphene

Graphene has proved to be extremely sensitive to its surrounding environment, such as the supporting substrate and guest adatoms. In this work, the structural stabilities, and electronic and magnetic properties of graphene with low-coverage adsorption of Si atoms and dimers are studied using a first-principles method. Our results show that graphene with Si adatoms is metallic and magnetic with a tiny structural change in the graphene, while graphene with Si addimers is semi-metallic and nonmagnetic with a visible deformation of the graphene. The spin-polarized density of states is calculated in order to identify the electronic origin of the magnetic and nonmagnetic states. The present results suggest that the electronic and magnetic behaviors of graphene can be tuned simply via Si adsorptions.     

石墨烯纳米网电导特性的能带机理：第一原理计算

通过第一原理电子结构计算来研究有序多孔纳米网的电导特性变化的能带机理.能带结构分析结果表明:石墨烯纳米网超晶格(3m,3n)(m和n为整数)的电子本征态在布里渊区中心点发生四重简并;碳空位孔洞规则排列形成的石墨烯纳米网具有由简并态分裂形成的宽度可调带隙,无论石墨烯的两个子晶格是否对等.在具有磁性网孔阵列的石墨烯纳米网中,反铁磁耦合使对称子晶格的反演对称性增加了一项量子限制条件,导致能带结构在K点的二重简并态分裂成带隙.通过控制网孔密度能够有效调节石墨烯纳米网的带隙宽度,为实现新一代石墨烯纳米电子器件提供了理论依据.     

Specific heat and electrical resistivity of an icosahedral-structure Zr<Subscript>70</Subscript>Pd<Subscript>30</Subscript> alloy and of its amorphous and crystalline analogs

Binary icosahedral and crystalline phases of the Zr₇₀Pd₃₀ alloy were obtained in crystallization from the amorphous state during heat treatment. The specific heat and electrical resistivity of the icosahedral, amorphous, and crystalline phases were measured and compared. An increase in the electronic density of states on the Fermi surface, lattice softening, and an increase in the electron-phonon coupling constant were observed to occur with decreasing structural order. Despite the high valence electron density in the icosahedral phase, where the electronic densities of states are twice those in the crystal, the electrical resistivity of the icosahedral phase is ～50 times as high. Superconductivity was observed for the first time in the icosahedral phase of a binary system of transition metal atoms, Zr₇₀Pd₃₀.     

B、N掺杂对钠在石墨烯上吸附性能的影响

采用基于密度泛函理论的第一性原理方法,研究了本征石墨烯及缺陷石墨烯对Na原子的吸附行为。主要研究了三种石墨烯：本征石墨烯、B掺杂的石墨烯和N掺杂的石墨烯。结果表明,与本征石墨烯相比,B掺杂的石墨烯和N掺杂的石墨烯在吸附能、电荷密度、态密度和储钠量方面表现出很大的差异。B掺杂的石墨烯对Na原子的吸附能是-1.93 eV,约为本征石墨烯对Na原子吸附能的2.7倍;与本征石墨烯相比,N掺杂的石墨烯对Na原子的吸附能明显增大。态密度计算结果表明,Na原子与B掺杂的石墨烯中的B原子发生轨道杂化,而本征石墨烯和N掺杂的石墨烯中不存在轨道杂化现象。B掺杂的石墨烯对Na原子的吸附量是3个,与本征石墨烯相比显著提高。因此,B掺杂的石墨烯有望成为一种新型的储钠材料。     

B、N掺杂对钠在石墨烯上吸附性能的影响

采用基于密度泛函理论的第一性原理方法,研究了本征石墨烯及缺陷石墨烯对Na原子的吸附行为.主要研究了三种石墨烯:本征石墨烯、B掺杂的石墨烯和N掺杂的石墨烯.结果表明,与本征石墨烯相比,B掺杂的石墨烯和N掺杂的石墨烯在吸附能、电荷密度、态密度和储钠量方面表现出很大的差异.B掺杂的石墨烯对Na原子的吸附能是-1.93 e V,约为本征石墨烯对Na原子吸附能的2.7倍;与本征石墨烯相比,N掺杂的石墨烯对Na原子的吸附能明显增大.态密度计算结果表明,Na原子与B掺杂的石墨烯中的B原子发生轨道杂化,而本征石墨烯和N掺杂的石墨烯中不存在轨道杂化现象.B掺杂的石墨烯对Na原子的吸附量是3个,与本征石墨烯相比显著提高.因此,B掺杂的石墨烯有望成为一种新型的储钠材料.     

氮原子修饰点缺陷石墨烯结构和性质的理论研究

氮原子掺杂石墨烯对基于石墨烯的器件和催化研究具有重要的应用价值.本文采用基于密度泛函理论的计算方法,研究了氮原子修饰的C-Bridge(碳原子吸附在石墨烯碳碳键桥位)、C-Top(碳原子吸附在石墨烯一个碳原子上方)和C7557(碳原子对吸附在石墨烯碳环上方)三种不同点缺陷类型的石墨烯物理性质.讨论不同缺陷石墨烯结构在用氮原子进行修饰前后体系的稳定性、电子结构等;计算得到了缺陷处原子的分波态密度(PDOS)图,分析了原子间的相互作用;模拟出氮原子修饰后缺陷石墨烯恒流模式的STM图像,以便和实验上得出的图像进行对比.计算结果表明,对于所选取的三种不同缺陷,氮原子能够较稳定地吸附在缺陷表面.C-Bridge和C-Top缺陷结构本身具有磁矩,经氮原子修饰后结构磁矩消失.与之相反,C7557缺陷结构本身没有磁矩,经氮原子修饰后缺陷体系带有磁矩.另外,C-Bridge和CTop两种不同缺陷结构石墨烯经过氮原子修饰后,体系几何结构变得完全一样.