掺杂三角形硼氮片的锯齿型石墨烯纳米带的磁电子学性质

利用基于密度泛函理论的第一性原理方法,研究了三角形BN片掺杂的锯齿型石墨烯纳米带(ZGNR)的磁电子学特性.研究表明:当处于无磁态时,不同位置掺杂的ZGNR都为金属;当处于铁磁态时,随着杂质位置由纳米带的一边移向另一边时,依次可以实现自旋金属-自旋半金属-自旋半导体的变化过程,且只要不在纳米带的边缘掺杂,掺杂的ZGNR就为自旋半金属;当处于反铁磁态时,在中间区域掺杂的ZGNR都为自旋金属,而在两边缘掺杂的ZGNR没有反铁磁态.掺杂ZGNR的结构稳定,在中间区域掺杂时反铁磁态是基态,而在边缘掺杂时铁磁态为基态.研究结果对于发展基于石墨烯的纳米电子器件具有重要意义.     

Negative differential resistance and rectification effects in zigzag graphene nanoribbon heterojunctions: Induced by edge oxidation and symmetry concept

By applying non-equilibrium Green's functions (NEGF) in combination with tight-binding (TB) model, we investigate and compare the electronic transport properties of H-terminated zigzag graphene nanoribbon (H/ZGNR) and O-terminated ZGNR/H-terminated ZGNR (O/ZGNR–H/ZGNR) heterostructure under finite bias. Moreover, the effect of width and symmetry on the electronic transport properties of both models is also considered. The results reveal that asymmetric H/ZGNRs have linear I–V characteristics in whole bias range, but symmetric H-ZGNRs show negative differential resistance (NDR) behavior which is inversely proportional to the width of the H/ZGNR. It is also shown that the I–V characteristic of O/ZGNR–H/ZGNR heterostructure shows a rectification effect, whether the geometrical structure is symmetric or asymmetric. The fewer the number of zigzag chains, the bigger the rectification ratio. It should be mentioned that, the rectification ratios of symmetric heterostructures are much bigger than asymmetric one. Transmission spectrum, density of states (DOS), molecular projected self-consistent Hamiltonian (MPSH) and molecular eigenstates are analyzed subsequently to understand the electronic transport properties of these ZGNR devices. Our findings could be used in developing nanoscale rectifiers and NDR devices.     

Doping-induced giant rectification and negative differential conductance (NDC) behaviors in zigzag graphene nano-ribbon junction

By p-type and n-type doping on the electrode edges of V-notched zigzag graphene nano-ribbons (ZGNRs), four V-notched ZGNR-based PN-junctions are designed theoretically. The electronic transport properties of the doped and un-doped V-notched ZGNRs are studied applying non-equilibrium Green's function method combined with the density functional theory. The numerical results show that, the doped systems are less conductive than the un-doped system, because after doping the transition states become localized. To our surprise, the ZGNR-based PN-junctions do not show obvious rectification by purely doping the boron atoms and nitrogen atoms on the edges of two ZGNR electrodes respectively. However, after hydrogenated the doped boron atoms and nitrogen atoms, the ZGNR systems present giant rectifications with the maximum rectification ratios up to ${10}^6～{10}^7$, which attributed to the vanishing of overlap between left-electrode sub-band and right-electrode sub-band in the negative bias regime after the doped boron and nitrogen atoms being hydrogenated. Due to the same reason, the hydrogenated doping systems also show large negative differential conductance behaviors.     

Effect of N doping and Stone-Wales defects on the electronic properties of graphene nanoribbons

The effects of nitrogen substitutional doping in the Stone-Wales (SW) defect on the electronic transport properties of zigzag-edged graphene nanoribbon (ZGNR) are studied by using density functional theory combined with nonequilibrium Green’s function. The transformation energies of all doped nanostructures are evaluated in terms of total energies and, furthermore, it is found that the impurity placed on the center of the ribbon is the most energetically favorable site. Nitrogen substitution gives rise to a complete electron backscattering region in doped configurations, and the location of which is dependent on the doping sites. The electronic and transport properties of doped ZGNRs are discussed. Our results suggest that modification of the electronic properties of ZGNR with topological defects by substitutional doping might not be significant for some doping sites.     

Electronic properties of Li-doped zigzag graphene nanoribbons

Zigzag graphene nanoribbons (ZGNRs) are known to exhibit metallic behavior. Depending on structural properties such as edge status, doping and width of nanoribbons, the electronic properties of these structures may vary. In this study, changes in electronic properties of crystal by doping Lithium (Li) atom to ZGNR structure are analyzed. In spin polarized calculations are made using Density Functional Theory (DFT) with generalized gradient approximation (GGA) as exchange correlation. As a result of calculations, it has been determined that Li atom affects electronic properties of ZGNR structure significantly. It is observed that ZGNR structure exhibiting metallic behavior in pure state shows half-metal and semiconductor behavior with Li atom.     

Optical and electrical properties of some sulphates doped with cobalt

Doubly ionized cobalt ion which has a ⁴F ground state exhibits several optical bands in orthorhombic sulphates. In view of the low symmetry, many degenerate states split and at low temperature (77°K) well resolved bands have been observed which enable the detailed calculation of the crystal field parameters in orthorhombic symmetry.Electrical conductivity measurements in pure potassium and ammonium sulphates show only the extrinsic unassociated region while in cobalt doped crystals, extrinsic as well as association regions are observed. Three distinct regions with slopes 0·86, 1·2 and 0·5 eV are obtained in cobalt doped K₂SO₄.X-irradiated pure crystals give two prominent bands at 2200 and 3300 Å which are attributed to SO₃^? and SO₂^?. Divalent cobalt doped crystals give additional bands at 2100 and 3100 Å. These bands are attributed to Co⁺ in different surroundings. Three ESR lines with ‘g’ values 2·042, 2·02 and 2·004 are obtained in support of the assignments.     

Structural, optical and magnetic properties of Cr doped ZnO microrods prepared by spray pyrolysis method

A series of Cr-doped ZnO micro-rod arrays were fabricated by a spray pyrolysis method. X-ray diffraction patterns of the samples showed that the undoped and Cr-doped ZnO microrods exhibit hexagonal crystal structure. Surface morphology analysis of the samples has revealed that pure ZnO sample has a hexagonal microrod morphology. From X-ray photoelectron spectroscopy studies, the Cr 2p3/2 binding energy is found to be 577.3 eV indicating that the electron binding energy of the Cr in ZnO is almost the same as the binding energy of Cr³⁺ states in Cr₂O₃. The optical band gap E_g decreases slightly from 3.26 to 3.15 eV with the increase of actual Cr molar fraction from x = 0.00 to 0.046 in ZnO. Photoluminescence studies at 10 K show that the incorporation of chromium leads to a relative increase of deep level band intensity. It was also observed that Cr doped samples clearly showed ferromagnetic behavior; however, 2.5 at.% Cr doped ZnO showed remnant magnetization higher than that of 1.1 at.% and 4.6 at.% Cr doped samples, while 4.6 at.% Cr doped ZnO samples had a coercive field higher than the other dopings.     

First-principles study on the magnetism and electronic structure in 3d transition metal (X=Sc,V, Cr,Mn, Fe,Ni, Cu) doped CoO

We have studied the electronic structure and magnetism of the single transitional metal element X=Sc, V, Cr, Mn, Fe, Ni, Cu-doped CoO systems by first-principles calculations. At X=Sc, Cr, Cu, the binding energy of the doped systems is lower than pure CoO, suggesting that these systems are energetically stable. In the Sc, V, Cr, Mn, Fe, Ni, Cu-doped 2×2×2 CoO supercells, the total magnetic moments are 3.03, 5.64, 6.80, 7.70, 6.93, 2.30 and 1.96 μ_B, respectively. At X=Cr and Fe, the doped CoO systems are half-metallic with a high spin polarization. The large magnetic moment and high spin polarization in the Cr and Fe-doped CoO are important for the design of the spintronic devices.     

Surface characterisation of ethylene-propylene-diene rubber upon exposure to aqueous acidic solution

Two types of pure ethylene propylene diene rubbers were exposed to two different acids for varying period of time. Surface characterisation was carried out using X-ray photoelectron spectroscopy (XPS). Two EPDM rubbers selected for this study were comparable in co-monomer compositions but significantly different with respect to molar mass and the presence of long chain branching. Both rubbers contained 5-ethylidene-2-norbornene (ENB) as diene. Solution cast films of pure EPDM samples were exposed in two different acidic solutions, viz. chromosulphuric (Cr (VI)/H₂SO₄) and sulphuric acid (H₂SO₄) (20%, v/v) at ambient temperature from 1 to 12 weeks. XPS analysis indicated that several oxygenated species were formed on the surface of both rubbers after exposure. It was postulated from the XPS analyses that both aqueous acidic solutions attacked the olefinic double bonds (CC) of ENB. Furthermore, 20% Cr (VI)/H₂SO₄ also attacked the allylic carbon-hydrogen (CH) bonds of ENB resulting in more oxygenated species on the surface compared to 20% H₂SO₄ under identical conditions. Cr (VI) in the 20% Cr (VI)/H₂SO₄ was found to play an important role in alteration of surface chemistry. Studies using a model system consisting of EPDM mixed with Cr (VI) and Cr (III) salts revealed that the change of oxidation state from Cr (VI) to Cr (III) as a consequence of direct involvement of Cr (VI) in the chemical alteration of EPDM surfaces. Interestingly, the presence of long chain branching and molar mass did not significantly influence the chemical processes owing to the acid treatment.     

Novel conductance step in carbon nanotube with wing-like zigzag graphene nanoribbons

Connecting one armchair carbon nanotube(CNT) to several zigzag graphene nanoribbons(ZGNRs) we find that the topologically-protected edge states of ZGNRs and the massless Dirac particle inherited from CNT still hold from the analysis of the band structure and the edge state. Furthermore, the lowest conductance step at the valley bottom increases proportionally with increasing the number of ZGNR wings. A novel conductance step of a peak occurs in the valley, which is two steps higher than the lowest step at the valley bottom. In addition, with increasing the number of ZGNR wings the width of the novel conductance step becomes narrow.     

Sulfur dioxide adsorbed on graphene and heteroatom-doped graphene: a first-principles study

The adsorption of sulfur dioxide (SO₂) on intrinsic graphene and heteroatom-doped (B, N, Al, Si, Cr, Mn, Ag, Au, and Pt) graphene samples was theoretically studied using first-principles approach based on density functional theory to exploit their potential applications as SO₂ gas sensors. The structural and electronic properties of the graphene-molecule adsorption adducts are strongly dependent on the dopants. SO₂ molecule is adsorbed weakly on intrinsic graphene, and B-, N-doped graphene; in general, strong chemisorption is observed on Al-, Si-, Cr-, Mn-, Ag-, Au-, and Pt-doped graphene. The adsorption mechanisms are discussed from charge transfers and density of states. This work reveals that the sensitivity of graphene-based chemical gas sensors for SO₂ can be drastically improved by introducing appropriate dopant, and Cr, as well as Mn, may be the best choices among all the dopants.     

Structural and electrical studies on Zn2+ doped LiCoPO4

Zinc doped LiCoPO₄ was prepared by two step solid state reaction method. XRD studies ensure the formation of phase pure LiCoPO₄ with olivine phase orthorhombic structure with p_nma phase group. 2θ value of XRD peaks of LiCoPO₄ is found to be shifted upon Zn²⁺ doping. The conductivity of pure LiCoPO₄ is noticed to be increased by one order upon doping of Zn²⁺. The scaling behavior of Z″ indicates that the ion dynamical process are temperature independent for both pure and Zn²⁺ doped LiCoPO₄. The temperature dependence of conductivity for both pure and Zn²⁺ doped sample obeys Arrhenius law of conduction.     

Ab initio study of gold-doped zigzag graphene nanoribbons

The electronic transport properties of zigzag graphene nanoribbons (ZGNRs) through covalent functionalization of gold (Au) atoms is investigated by using non-equilibrium Green’s function combined with density functional theory. It is revealed that the electronic properties of Au-doped ZGNRs vary significantly due to spin and its non-inclusion. We find that the DOS profiles of Au-adsorbed ZGNR due to spin reveal very less number of states available for conduction, whereas non-inclusion of spin results in higher DOS across the Fermi level. Edge Au-doped ribbons exhibit stable structure and are energetically more favorable than the center Au-doped ZGNRs. Though the chemical interaction at the ZGNR–Au interface modifies the Fermi level, Au-adsorbed ZGNR reveals semimetallic properties. A prominent qualitative change of the I–V curve from linear to nonlinear is observed as the Au atom shifts from center toward the edges of the ribbon. Number of peaks present near the Fermi level ensures conductance channels available for charge transport in case of Au-center-substituted ZGNR. We predict semimetallic nature of the Au-adsorbed ZGNR with a high DOS peak distributed over a narrow energy region at the Fermi level and fewer conductance channels. Our calculations for the magnetic properties predict that Au functionalization leads to semiconducting nature with different band gaps for spin up and spin down. The outcomes are compared with the experimental and theoretical results available for other materials.     

Effects of contact oxidization on the transport properties of Au/ZGNR junctions

The transport properties of the junction assembled by zigzag graphene nanoribbons (ZGNRs) and Au electrode (Au/ZGNR) are investigated using first‐principles calculations. It is found that the Au/ZGNR junction behaves as a typical diode with Schottky barrier at the contact. Our results indicate that although the oxidization at the contact slightly influences the Schottky barrier, the I –V characteristic is effectively modulated. Such effect derives from the impact of the oxidization on the coupling between the ZGNRs and Au electrode.

     

Investigation of the electronic structure of La1−x(M2+)xCrO3, Cr2O3 and La2O3 by X-ray photoelectron spectroscopy

The electronic structures of lanthanum chromites, pure and doped with magnesium and strontium, have been studied in comparison with Cr₂O₃ and La₂O₃ through the use of X-ray photo-electron spectroscopy. The main peaks and satellites of inner and outer electrons are properly assigned. The band structure of LaCrO₃ is determined by using the XPS data and a calculation based on point charge model. The partially filled Cr(3d) band is localized. The conduction is mainly extrinsic. The measured Fermi-level is close to the valence band indicating a low activation energy in agreement with the results of conductivity measurements.     

Two and four photon absorption and nonlinear refraction in undoped,chromium doped and copper doped ZnS quantum dots

The ZnS quantum dots (QDs) with Cr and Cu doping were synthesized by chemical co-precipitation method. The nanostructures of the prepared undoped and doped ZnS QDs were characterized by UV–vis spectroscopy, Transmission electron microscopy (TEM) and X-ray diffraction (XRD). The sizes of QDs were found to be within 3–5 nm range. The nonlinear parameters viz. Two photon absorption coefficient (β₂), nonlinear refractive index (n₂), third order nonlinear susceptibility (χ³) at wavelength 532 nm and Four photon absorption coefficient (β₄) at wavelength 1064 nm have been calculated by Z-scan technique using nanosecond Nd:YAG laser in undoped, Cr doped and Cu doped ZnS QDs. Higher values of nonlinear parameters for doped ZnS infer that they are potential material for the development of photonics devices and sensor protection applications.     

Ultrasonic attenuation in pure and Gd-Doped superconductors: LaSn3 and LaAl2

We have investigated the ultrasonic attenuation (UA) in the pure and magnetically doped superconductors LaSn₃ and LaAl₂, the latter being an ideal Abrikosov-Gorkov system. The UA of pure LaSn₃ and LaAl₂ agrees well with BCS prediction. The experiments in doped LaSn₃ proved difficult due to high background attenuation. The results in La_1?nGd_nAl₂ (_n = 0.377 at .%), though, show a significant discrepancy with the theories which is not understood.     

6,7Li NMR study of ion mobility on the molecular scale in lithated imidazole complexes

Two model compounds, lithium imidazolium (LiIm) and lithium 2-undecylimidazolium (und-LiIm), were synthesized. These materials are chosen as models of potential lithium ion conductors for use as electrolytes in lithium batteries. Solid-state NMR was used to provide information on the microscopic interactions including ionic mobility and ring reorientations which govern the efficiency of conductivity. Lithium imidazolium was mixed with lithium methylsulfonate, generating a doped complex in which a doubly lithiated imidazole ring was inferred based on the ⁷Li NMR chemical shifts. Our research includes ^6,7Li variable temperature MAS NMR experiments at intermediate spinning speeds, relaxation studies to determine spin-lattice relaxation times (T₁) of lithium ion hopping, and 2D exchange spectroscopy to determine possible chemical exchange processes. The possibility of 2-site ring reorientation for the doubly lithiated imidazole ring was supported by exchange spectroscopy. Comparisons of spin-lattice relaxation times and corresponding activation energies of the lithium imidazolium and the doped complex point to a higher degree of mobility in the latter.Lithium 2-undecylimidazolium was prepared and exhibited a lower melting point than the parent lithium imidazolium, as expected. This small molecule was chosen as representative of a side-chain functionalized polyethylene-based material. ⁷Li MAS spectra show mainly the presence of the doubly lithiated imidazole ring in pure und-LiIm, and in the LiCH₃SO₃–und-LiIm mixture. The data clearly indicate local mobility of the lithium ions in the materials.     

An inelastic electron tunneling study of the insulator formed on aluminum in a low pressure discharge containing SO2

Vacuum deposited films of aluminum were exposed to a 400 V AC discharge plasma initially composed of SO₂ and of either oxygen or water gas. ¹⁸O substituted water and SO₂ were also used. The surfaces thus formed were studied by tunneling spectroscopy. From the observed peak positions and isotopic shifts it appears that, when SO₂ is initially about half of the discharge gas, the principal species present in the insulating layer is AlS_xO_y, where x and y are of the order of 3 and 7, respectively. A thin layer of AlO_z probably underlies the SO containing film. Initial concentrations of SO₂ in excess of about 60% gave barriers having low reproducibility and showing large charge-trapping effects. The tunneling spectrum of the sulphate ion, doped from water solution, is also reported.     

堆叠石墨片对锯齿型石墨纳米带电子输运的影响

采用格林函数方法研究了堆叠石墨片对锯齿型石墨纳米带电子输运性质的影响,计算了两种不同堆叠方式下锯齿型石墨纳米带的电导.研究发现,由于堆叠石墨片与石墨纳米带的耦合作用,锯齿型石墨纳米带的电导谱出现了电导谷.在远离费米能处,两种堆叠方式下的电导谷位置相近甚至重合;而在费米能附近,两种堆叠方式下的电导谷存在差异.此外,讨论了堆叠石墨片的几何尺寸对锯齿型石墨纳米带电子输运的影响.结果显示,随石墨片几何尺寸的增大,锯齿型石墨纳米带在两种堆叠方式下远离费米能处的电导谷逐渐向费米能方向移动,同时其费米能附近的电导谷在两种堆叠方式下的差异随石墨片尺寸的增大变得更为明显.研究结果表明,堆叠石墨片能够有效地调制锯齿型石墨纳米带的电子输运性质.