Electronic structure and stability of semiconducting graphene nanoribbons

We present a systematic density functional theory study of the electronic properties, optical spectra, and relative thermodynamic stability of semiconducting graphene nanoribbons. We consider ribbons with different edge nature including bare and hydrogen-terminated ribbons, several crystallographic orientations, and widths up to 3 nm. Our results can be extrapolated to wider ribbons providing a qualitative way of determining the electronic properties of ribbons with widths of practical significance. We predict that in order to produce materials with band gaps similar to Ge or InN, the width of the ribbons must be between 2 and 3 nm. If larger bang gap ribbons are needed (like Si, InP, or GaAs), their width must be reduced to 1-2 nm. According to the extrapolated inverse power law obtained in this work, armchair carbon nanoribbons of widths larger than 8 nm will present a maximum band gap of 0.3 eV, while for ribbons with a width of 80 nm the maximum possible band gap is 0.05 eV. For chiral nanoribbons the band gap oscillations rapidly vanish as a function of the chiral angle indicating that a careful design of their crystallographic nature is an essential ingredient for controlling their electronic properties. Optical excitations show important differences between ribbons with and without hydrogen termination and are found to be sensitive to the carbon nanoribbon width. This should provide a practical way of revealing information on their size and the nature of their edges.     

Structural, electronic and magnetic properties of ultra-narrow NbSe2 nanoribbons

Structural, electronic and magnetic properties of ultra-narrow NbSe2 nanoribbons with different chirality and edge structures are studied by using density functional theory calculations. Armchair-like NbSe2 nanoribbons exhibit nonmagnetic and metal behavior. Zigzag-like NbSe2 nanoribbons always have ferromagnetic ground states with electronic types ranging from semiconductor, metal, even to half-metal. And stability estimation indicates that the zigzag-like NbSe2 nanoribbons with 50% Se passivation at Nb edges are the most energetically stable. All these findings suggest potential applications of NbSe2 nanoribbons in spintronic devices.     

Electronic properties of edge-functionalized zigzag graphene nanoribbons on SiO2 substrate

Based on first-principles calculations, electronic properties of edge-functionalized zigzag graphene nanoribbons (ZGNRs) on SiO(2) substrate are presented. Metallic or semiconducting properties of ZGNRs are revealed due to various interactions between edge-hydrogenated ZGNRs and different SiO(2)(0001) surfaces. Bivalent functional groups decorating ZGNRs serve as the bridge between active edges of ZGNRs and SiO(2). These functional groups stabilize ZGNRs on the substrate, as well as modify the edge states of ZGNRs and further affect their electronic properties. Bandgaps are opened owing to edge state destruction and distorted lattice in ZGNRs.     

On magnetic properties of Fe-Si-Mn alloy sheet

Fe-Si-Mn alloys composed of 3.5-7,5%, 0-3.5%Mn and 90-96.5%Fe were melted in argon and hot-rolled. Measurements of their magnetic properties and electrical resistivity indicate that the addition of manganese to higher silicon-iron alloys improves in permeability and coercive force of the alloys. Maximum permeability as high as 83000 is obtained when the alloy of 6.70%Si, 0.55%Mn and 92.75%Fe is quenched into oil from 400° after annealing at 1200° for 5 hr. The alloy exhibits the coercive force of 0.06 Oe for the maximum induction of 10 kG. The electrical resistivity of the alloy is 86 μΩ-cm, with the Vickers hardness of 355.     

Electronic and structural properties of cholesteryl acetates

Electronic and structural properties of the cholesteryl acetate are investigated using semi-empirical quantum calculation. In the present article we perform an energy minimization through series of different configurations and a particular attention is given to the steroid ring structures. Different orientations and directions for the acetyl and the iso-octyl groups are observed for the suggested configurations. A comparison between calculated and experimental data show that the method not often used for large molecules is reliable and yields to values of bond lengths and bond angles close to those obtained by neutron diffraction at low temperature.     

Electronic states of graphene nanoribbons and analytical solutions

Abstract

Graphene is a one-atom-thick layer of graphite, where low-energy electronic states are described by the massless Dirac fermion. The orientation of the graphene edge determines the energy spectrum of π-electrons. For example, zigzag edges possess localized edge states with energies close to the Fermi level. In this review, we investigate nanoscale effects on the physical properties of graphene nanoribbons and clarify the role of edge boundaries. We also provide analytical solutions for electronic dispersion and the corresponding wavefunction in graphene nanoribbons with their detailed derivation using wave mechanics based on the tight-binding model. The energy band structures of armchair nanoribbons can be obtained by making the transverse wavenumber discrete, in accordance with the edge boundary condition, as in the case of carbon nanotubes. However, zigzag nanoribbons are not analogous to carbon nanotubes, because in zigzag nanoribbons the transverse wavenumber depends not only on the ribbon width but also on the longitudinal wavenumber. The quantization rule of electronic conductance as well as the magnetic instability of edge states due to the electron–electron interaction are briefly discussed.     

Mechanical stability and magnetic properties of austenite

Austenitic alloys have been produced by additional alloying in maraging steel grade 18 Ni at 2400 MPa. The concentration of Mo, Ni and Co was increased individually until the martensite start temperature M _s, was suppressed below ambient value. Charpy impact strength, tensile strength and magnetic properties were determined. The impact strength in the annealed condition ranged between 260 to 294 J. In alloys where martensitic transformation occurred following quenching in liquid nitrogen, the impact strength dropped appreciably and was found to be in the range 120–216 J. The tensile strengths of the austenite and martensite phases ranged between 680 to 890 and 1030 to 1100 MPa, respectively. It was observed that the austenite phase transformed to martensite in the region that under went plastic deformation during Charpy and tensile testing. The degree of transformation incorporated, varied as a function of composition. The magnetic properties of the austenite phases were typical of a very weak magnetic material. The coercive field and saturation magnetization values were in the range 1034–2387 Am^–1 and 1.6–2.9 T, respectively. In contrast to the general observation, the austenite phase containing high Co exhibited ferromagnetic behaviour. The coercive field and saturation magnetization of ferromagnetic austenite was 1034 Am^–1 and 11 T, respectively.     

Graphene nanoribbons in criss-crossed electric and magnetic fields

Graphene nanoribbons (GNRs) in mutually perpendicular electric and magnetic fields are shown to exhibit dramatic changes in their band structure and electron-transport properties. A strong electric field across the ribbon induces multiple chiral Dirac points, closing the semiconducting gap in armchair GNRs. A perpendicular magnetic field induces partially formed Landau levels as well as dispersive surface-bound states. Each of the applied fields on its own preserves the even symmetry E(k)=E(-k) of the sub-band dispersion. When applied together, they reverse the dispersion parity to be odd, which gives E(e,k)=-E(h,-k), and mix the electron and hole sub-bands within the energy range corresponding to the change in potential across the ribbon. This leads to oscillations of the ballistic conductance within this energy range. The broken time-reversal symmetry provides dichroism in the absorption of the circularly polarized light. As a consequence, one can observe electrically enhanced Faraday rotation, since the edges of the ribbon provide formation of the substantial density of states.     

电工钢片磁性的计量问题

综述了电工钢片计量的当前水平和最新进展,重点讨论了在交变场中的标准测试方法:Epstein方圈法和单片(SST)法.所讨论的相关条款是有关磁通密度控制、数据获取和数据计算的最新电子方法,以及磁路设计对这两种方法的准确性和精度特性的影响,也讨论了系统误差来源、再现特性和经济因素等.     

Some structural and magnetic properties of Fe-Al-C and Fe-Mn-Al-C alloys

The remanence and coercivity of Fe-Al-C and Fe-Mn-Al-C alloys have been measured and studied as a function of ageing at 623 K. The resulting formation and growth of carbide precipitates of Fe₃AlC and Mn₃AlC in the same samples have been investigated by transmission and scanning electron micoscope techniques. The addition of Manganese to the Fe-Al-C alloy is shown to be severely deleterious to the magnetic properties of the resulting quaternary alloy; however, simultaneously, the electron diffraction and microscopic studies demonstrate that the structural order of the carbide precipitates is considerably improved. It is concluded that this observed decline in the magnetic properties of this alloy system is related to the formation of Mn₃AlC in preference to Fe₃AlC carbide precipitates.     

Electronic and structural properties of the amorphous/crystalline silicon interface

A review of capacitance and conductance measurements on (n) a-Si:H/(p) c-Si structures is presented. Capacitance measurements performed on cells under AM 1.5 illumination at or close to open-circuit voltage are sensitive to the recombination at interfaces, as evidenced by the comparison with photoluminescence results. Capacitance measurements performed in the dark at zero or reverse bias can reveal the presence of interface defects from trapping and release of carriers, but the sensitivity is limited to a few 10¹² cm^− 2 eV^− 1. This is partly due to the presence of a strong inversion layer at the c-Si surface. Such a layer has been revealed from coplanar conductance measurements, which allow a precise determination of the conduction band offset, found equal to 0.15 (± 0.04) eV. As shown by spectroscopic ellipsometry, a thin undoped silicon layer deposited under conditions that normally produce polymorphous silicon can be epitaxially grown onto c-Si prior to the (n) a-Si:H layer. Electrical measurements indicate that this additional buffer layer is not detrimental and can slightly improve the interface quality.     

Size-dependent structural and magnetic properties of chemically synthesized Co-Ni-Ga nanoparticles

Phase transitions and magnetic properties of shape-memory materials can be tailored by tuning the size of the constituent materials,such as nanoparticles.However,owing to the lack of suitable synthetic methods for size-controlled Heusler nanoparticles,there is no report on the size dependence of their properties and functionalities.In this contribution,we present the first chemical synthesis of size-selected Co-Ni-Ga Heusler nanoparticles.We also report the structure and magnetic properties of the biphasic Co-Ni-Ga nanoparticles with sizes in the range of 30-84 nm,prepared by a SBA-15 nanoporous silicatemplated approach.The particle sizes could be readily tuned by controlling the loading and concentration of the precursors.The fractions and crystallite sizes of each phase of the Co-Ni-Ga nanoparticles are closely related to their particle size.Enhanced magnetization and decreased coercivity are observed with increasing partide size.The Curie temperature (Tc) of the Co-Ni-Ga nanoparticles also depends on their size.The 84 nm-sized particles exhibit the highest Tc (≈ 1,174 K) among all known Heusler compounds.The very high Curie temperatures of the Co-Ni-Ga nanoparticles render them promising candidates for application in high-temperature shape memory alloy-based devices.     

化合物SmCo5.85Si0.90的磁性及电子结构研究

用自旋极化的MS-Xα方法研究了原子簇Sm5Co28Si6的电子态密度、自旋能级劈裂及原子磁矩.结果显示,由于Sm-Co间的轨道杂化效应,使Sm原子的5d0空轨道上占据了少量5d电子.Co(3d)-Sm(5d)电子间的直接交换作用,构成Sm-Co间的主要耦合方式,这是化合物SmCo5.85Si0.90中形成铁磁性长程序的一个重要因素.Si的掺杂使2e晶位的负交换耦合作用明显减弱,Fermi面处电子间键合作用增强,化合物的系统自由能降低,从而有利于形成稳定的铁磁性结构.考虑到深层4f电子的局域性以及轨道杂化效应所产生的少量5d电子,可以得到Sm的原子磁矩为1.24μB,与顺磁盐中Sm3 的磁矩实验值(1.32～1.63μB)基本符合.     

Size-dependent non-linear mechanical properties of graphene nanoribbons

An atomistic, spring-based, non-linear finite element method is implemented in order to predict the non-linear mechanical behavior of graphene nanoribbons. According this method, appropriate non-linear springs are utilized to simulate each interatomic interaction. Their force–displacement curve follows the relation between the first differentiation of the potential energy of the corresponding interaction-bond deformation. The potential which corresponds to the bond angle variation is simulated by a torsional spring, while the bond stretching is simulated by a uniaxial compression/extension spring. The linear approximation, commonly made in the literature for the bond angle bending interaction, is not followed here and thus the overall non-linear response of the specific interaction is accurately introduced into the model. Following the proposed formulation, the tensile uniaxial stress–strain behavior for various graphene nanoribbons, of zigzag as well as armchair orientation, arise. The results demonstrate that the linear and non-linear mechanical properties are strongly dependent on the structure as well as on the size of the graphene strip tested.     

Electronic structure and magnetic properties of a molecular octanuclear chromium-based ring

A comprehensive study of electronic and magnetic properties of Cr8F8Piv16 (HPiv = pivalic acid, trimethyl acetic acid) molecular ring is presented. The total, local and orbital projected density of states are calculated by the first principle density functional theory calculations using the package SIESTA. The original molecule has been approximated by replacing the pivallic groups by H atoms (hydrogen saturation). Electron density, deformation density, electrostatic potential and spin density maps are analyzed and compared with experiment for the first time. Magnetic properties are investigated in detail. Magnetic moments are calculated using two different approaches: the Mulliken one and integration of muffin-tin sphere with a given radius. Different magnetic configurations (ferromagnetic, antiferromagnetic and many more with randomly distributed spins up and down) are considered to extract exchange interaction parameter J and check the stability of its estimate.     

Electronic and magnetic properties of nitrogen-doped finite-size and open-ended zigzag carbon nanotubes

Density functional calculations have been used to investigate the effect of the N doping on the electronic and magnetic properties of finite-size and open-ended zigzag carbon nanotubes (CNT). The carbon atoms at the edge site are more easily replaced by N atoms energetically, compared to other sites. The single N atom substitution can reduce the magnetic moment of edge carbon atoms near the doping site and result in the suppression of spin polarization. Such spin suppression is independent of tube diameter and it can be ascribed into perturbation of the π/π^* state localized at the doped edge. At low N-doping concentrations, the finite-length zigzag CNTs can maintain an antiferromagnetic (AFM) ground state, but the conversion from AFM to a nonmagnetic state can occur at high impurity concentrations. More interestingly, the tunable electronic properties from the half-semiconducting to semiconducting state can be realized in these N-doped finite-size and open-ended CNTs if subjected to a variably external electric field along the tube axis.     

Reactively evaporated BeO films: Preparation and electrical and optical properties

The transmission, conductivity and thermally stimulated exoelectron emission have been measured for reactively evaporated thin BeO films. The composition of the solid system beryllium-oxygen can be varied within wide limits between beryllium oxide and pure beryllium by varying the evaporation rate at a fixed oxygen gas pressure. Increasing the metal content results in a decreasing transmission mainly due to absorption of light by small beryllium precipitates within the BeO film. The increasing conductivity with increasing beryllium content supports this hypothesis.Some results on thermally stimulated exoelectron emission after irradiation of thin BeO films with X-rays and γ-rays are presented. Pure untreated BeO films exhibit only a weak electron emission, but the emission is increased strongly by different heat treatments. These heat treatments indicate that water adsorption on the BeO surface makes an important contribution to the mechanism of thermally stimulated exoelectron emission.The thermally stimulated emission from beryllium oxide may find a useful application as a nearly tissue-equivalent solid state dosimeter for ionizing radiation; some dosimetric aspects of the investigations are discussed.