Cluster modeling of three types of double-walled armchair silicon carbide nanotubes

A systematic study of three types of armchair double-walled SiC nanotubes (DWNT) (n,n)@(m,m) (3 ≤ n ≤ 6; 7 ≤ m ≤ 12) using the finite cluster approximation is presented. The geometries of the tubes have been spin optimized using the hybrid functional B3LYP (Becke’s three-parameter exchange functional and the Lee-Yang-Parr correlation functional) and the all electron 3-21G* basis set. The study indicates that the stabilities of the double-walled SiC nanotubes are of the same order as those of single-walled SiC nanotubes suggesting the possibility of experimental synthesis of both single-walled and double-walled SiC nanotubes. The binding energy per atom or the cohesive energy of the double-walled nanotubes depends not only on the number of atoms but also on the coupling of the constituent singlewalled nanotubes and their types. A study of the binding energies, Mulliken charges, density of states and HOMO-LUMO gaps has been performed for all nanotubes from (n,n)@(n + 3,n + 3) to (n,n)@(n + 6,n + 6) (n = 3 ? 6). Type 2 DWNTs do not preserve the coaxial geometry when the difference in the chirality of the outer and inner tubes is 5 or less. For type 3, this occurs when the chirality difference is 4 or less. The gaps of types 2 and 3 DWNTs are lower than those of the corresponding single-walled nanotubes and are significantly less than those of type 1 DWNTs.     

Existence and stability of co-axial and meshed double-walled armchair silicon nanotubes

A systematic study of armchair double-walled Si nanotubes (DWNT) (n, n)@(m, m) (3≤n≤6; 7≤m≤12) using the finite cluster approximation is presented. The geometries of the tubes have been spin optimized with an all electron 3–21 G^⁎ basis set and the B3LYP functional. Analysis of the electronic structure properties of these tubes has also been performed with a larger basis set. The study indicates that the stabilities of the Si nanotubes are of the same order as those of single-walled Si nanotubes. It should be possible to experimentally synthesis both single-walled and double-walled Si nanotubes. The binding energy per atom or the cohesive energy of the nanotubes depends not only on the number of atoms but also on the coupling of the constituent single-walled nanotubes. Nanotubes with small interlayer separations, called meshed tubes, do not hold the coaxial cylindrical structure after optimization. The SiNTs (n, n)@(n+3, n+3) are found to have large formation energies and binding energies per atom. For example, (3, 3)@(6, 6), (4, 4)@(7, 7), (5, 5)@(8, 8), and (6, 6)@(9, 9) all have large binding energies per atom, around 3.7 eV/atom. All Si nanotubes are found to be semiconductors. However, the band gap, in general, is observed to decrease from single walled nanotubes to double walled nanotubes.     

Insulating state of Na clusters and their metallic transition in low-silica X zeolite

Optical and magnetic properties are investigated for low-silica X (LSX) zeolite loaded with Na metals at various loading densities. In LSX, β-cages with the inside diameter of ∼7 Å are arrayed in a diamond structure and the supercages with that of ∼13 Å are formed among them. When the average number of guest Na atoms per β cage, n, is smaller than ∼2, optical reflectance shows a peak at ∼2.5 eV. This corresponds to the 1s-1p transition of the cluster formed in β cage. At n>2, this peak disappears and several strong peaks are seen at 1-3 eV. The oscillator strength increases with n. They can be attributed to surface-plasmon-like excitations of the Na clusters formed in supercages. At n≤10, clear absorption tails with energy gaps are observed at near IR region indicating insulating states. However, when n is increased up to ∼12, a clear Drude reflection suddenly appears in the IR region, indicating that an insulator-to-metal transition abruptly occurs. This dramatic change of the electronic state may be caused by the successive connection between adjacent clusters in supercages due to some special arrangement of Na⁺ ions as well as the delocalized wave function of electrons at high Na-loading density.     

Interwall conductance in double-walled armchair carbon nanotubes

The dependence of the interwall conductance on distance between walls and relative positions of walls are calculated at the low voltage by Bardeen method for (n,n)@(2n,2n) double-walled carbon nanotubes (DWCNTs) with n=5,6,…,10. The calculations show that interwall conductance does not depend on temperature (for T?500 K) and current-voltage characteristic is linear. The conductance decreases by 6 orders of magnitude when the interwall distance is doubled. Thus, depending on the interwall distance, DWCNTs can be used as temperature stable nanoresistors or nanocapacitors.     

Theoretical study of the adsorption of CO2 on tungsten carbide nanotubes

The adsorption of CO₂ on the single-walled tungsten carbide nanotubes has been investigated employing density functional theory method. The center of a hexagon of tungsten and carbon atoms in sites on tungsten carbide nanotube surfaces is the most stable adsorption site for CO₂ molecule, with a binding energy of −1.68 eV (−38.72 kcal/mol) and a WO binding distance of 1.95 Å. Furthermore, the adsorption of CO₂ on the single-walled carbon nanotubes has been investigated. Our first-principles calculations predict that the CO₂ adsorptive capacity of tungsten carbide nanotubes is about quadruple that of carbon nanotubes. This might have potential for greenhouse gas detection and bioremediation.     

Effect of doping on electronic properties of double-walled carbon and boron nitride hetero-nanotubes

The effect of boron nitride (BN) doping on electronic properties of armchair double-walled carbon and hetero-nanotubes is studied using ab initio molecular dynamics method. The armchair double-walled hetero-nanotubes are predicted to be semiconductor and their electronic structures depend strongly on the electronic properties of the single-walled carbon nanotube. It is found that electronic structures of BN-doped double-walled hetero-nanotubes are intermediate between those of double-walled boron nitride nanotubes and double-walled carbon and boron nitride hetero-nanotubes. Increasing the amount of doping leads to a stronger intertube interaction and also increases the energy gap.     

Third-order nonlinear optical response in double-walled carbon nanotubes

Resonant behavior and magnitudes of third-order nonlinear optical susceptibilities in double-walled carbon nanotubes (DWNTs) have been investigated by means of femtosecond pump-probe spectroscopy with different pump-photon energies. With the selective excitation of the E₂₂ exciton transition of the inner tubes labeled by the chiral vector indices (7,5) and (7,6), the imaginary part of nonlinear susceptibility Imχ⁽³⁾ has shown the resonant enhancement compared with the case of the nonresonant excitation of the specific tube. The nonlinear response signal at the E₂₂ transition energy of the (8,7) tube has been also enhanced for the excitation of the G-band phonon sideband of its E₂₂ transition. This result is consistent with the phonon-mediated nonlinear optical process observed for the E₂₂ transitions in single-walled carbon nanotubes (SWNTs). It has been also found that the values of the figure of merit Im χ⁽³⁾/α (α: absorption coefficient) of the inner tubes in DWNTs are smaller than those of the corresponding SWNTs, which is interpreted in terms of decay time shortening due to the energy relaxation between the inner and outer tubes.     

Formation of a cubic Sr2MnWO6 phase at elevated temperature; a neutron powder diffraction study

In a temperature dependent neutron powder diffraction (NPD) study we observed the high temperature cubic phase at 973 K in the polycrystalline double perovskite Sr₂MnWO₆. Rietveld analysis of the NPD data shows that the room temperature tetragonal phase exists up to 573 K (space group P4₂/n, a=8.0119 (4) Å, c=8.0141(8) Å). At 773 K, the primitive tetragonal symmetry change to body-centred tetragonal (space group I4/m, a=5.6935(5) Å, c=8.077(1) Å) and finally at 973 K it becomes face-centred cubic (space group Fm-3m, a=8.0864(8) Å). The changes in the structural symmetry are connected to the small distortion of the B-site octahedra, which are insensitive to the Differential Thermal Analysis (DTA) signal.     

Electronic properties of the metal/organic interlayer/inorganic semiconductor sandwich device

In this study, we prepared a Metal(Al)/Organic Interlayer(Congo Red=CR)/Inorganic Semiconductor (p-Si) (MIS) Schottky device formed by coating of an organic film on p-Si semiconductor wafer. The Al/CR/p-Si MIS device had a good rectifying behavior. By using the forward bias I-V characteristics, the values of ideality factor (n) and barrier height (Φ_b) for the Al/CR/p-Si MIS device were obtained as 1.68 and 0.77 eV, respectively. It was seen that the Φ_b value of 0.77 eV calculated for the Al/CR/p-Si MIS device was significantly higher than value of 0.50 eV of the conventional Al/p-Si Schottky diodes. Modification of the interfacial potential barrier of the Al/p-Si diode was achieved by using a thin interlayer of the CR organic material. This was attributed to the fact that the CR organic interlayer increased the effective barrier height by influencing the space charge region of Si. The interface-state density of the MIS diode was found to vary from 1.24×10¹³ to 2.44×10¹² eV⁻¹ cm⁻².     

Excitons in optical spectra of boron-nitride (BN) nanotubes

Exciton effects are studied in single-wall boron-nitride nanotubes. The Coulomb interaction dependence of the band gap, the optical gap, and the binding energy of excitons are discussed. The optical gap of the (5,0) nanotube is about 6 eV at the on-site interaction U=2t with the hopping integral t=1.1 eV. The binding energy of the exciton is 0.50 eV for these parameters. This energy agrees well with that of other theoretical investigations. We find that the energy gap and the binding energy are almost independent of the geometries of nanotubes. This novel property is in contrast with that of the carbon nanotubes, which show metallic and semiconducting properties depending on the chiralities.     

Perpendicular anisotropy and magneto-optical Kerr effect of (Ni/Pd) multilayers

Nickel film, with total thickness t_Ni in the range 1000-2000 Å, is known to exhibit perpendicular magnetic anisotropy (PMA), if the film has been deposited at room temperature. This phenomenon is due to the magneto-elastic (ME) effect. The same is also true for the (Ni/Pd)_n multilayers, where n is the period (n≥3). In this paper, we have made two kinds of multilayers: one, which does not have a Pd cap layer, belongs to the A-group, and the other, which has, belongs to the B-group. The polar Kerr rotation θ_k, the polar Kerr ellipticity ε_k, and the figure of merit (θ_k)²R, where R is the reflectance, were measured for the two wavelengths, i.e. λ=633 and 442 nm, respectively. The effective PMA energy K_⊥ was measured from the vibrating sample magnetometer. It was found that the most favorable multilayer for the magneto-optical (MO) application exists among the A-group samples: i.e. the t_Ni=1300 Å, t_Pd=50 Å (seed layer), and n=1 sample. We obtained θ_k=−9.76 min, ε_k=−9.13 min, (θ_k)²R=1.51 (rad)² at λ=442 nm, and K_⊥=3.21×10⁶ erg/cc for this optimal multilayer. Finally, the effects of the Pd seed layer on PMA and MO are also studied.     

On the characterization of the elastic properties of asymmetric single-walled carbon nanotubes

In order to characterize asymmetric single-walled carbon nanotubes, an algorithm has been developed based on numerical simulation to relate the physical geometry to the elastic properties of asymmetric single-walled carbon nanotubes (SWCNTs). A large number of finite element results for the stiffness of asymmetric SWCNTs has been used to develop a best surface fitting function to define the relationship between the geometry of SWCNTs and their stiffness. However, since the stiffness of asymmetric nanotubes depends upon the configuration parameters, n and m, it was impossible to define any diameter dependency. Based on the maximum reaction force concept and in order to account for the hidden mechanical behavior of asymmetric SWCNTs, the chiral factor (CF) has been employed in this study. The proposed CF converts any asymmetric geometry (n and m) into a value between 0 and 1. A group of the SWCNTs with the same applied boundary condition (n+m=30) and different range of the CF was also used for studying of the shear contribution. The chiral factor dependency, which is developed in this study, is applicable for characterising and selecting asymmetric SWCNTs in the design of advanced nanomaterials. Furthermore, the equation which is calculated in this study can be useful for finding the best criteria for selecting asymmetric SWCNTs.     

Derivation of a universal estimate for the stiffness of carbon nanotubes

An algorithm has been developed based on numerical simulation to relate physical geometry to the Young’s modulus of symmetric and asymmetric single-walled carbon nanotubes (SWCNTs). A large number of finite element results for the stiffness of SWCNTs has been categorized into three main classes (i.e., armchair, zigzag and chiral) and the best curve fitting function has been obtained to describe the relation between the geometry of SWCNTs and their stiffness. For two standard configurations of carbon nanotubes (i.e., armchair and zigzag), four equations referring to geometry parameters (n, m) and diameter (d) have been introduced. To find the size dependence of asymmetric nanotubes, three-dimensional surfaces of stiffness (E(n, m)) have been used. However, since the stiffness of asymmetric nanotubes depends upon n and m, it was impossible to define any diameter dependency. To account for the hidden mechanical behavior of asymmetric SWCNTs, a new physical factor (CF) was introduced as the major novelty in this work. The proposed CF converts any asymmetric geometry (n, m) into a value between 0 and 1. The CF for a chiral nanotube can imply the percentage of similarity in its mechanical properties to the two standard symmetric configurations. Based on the CF concept, a new equation is derived to predict the Young’s modulus of asymmetric carbon nanotubes based on the symmetric prediction of standard models. The new physical factor (CF) which is developed in this study can be useful for the characterization of SWCNTs and the selection of all configurations.     

From anatase (1 0 1) surface to TiO2 nanotubes: Rolling procedure and first principles LCAO calculations

A simple procedure of 1D nanotubes (NT) construction based on the supercell of 2D (1 0 1) slab rolling and subsequent cylindrical coordinate system introduction is suggested. This procedure is applicable for any of five 2D lattices as well as both single- and multi-wall nanotubes provided that the chirality and translation vectors orthogonality condition is satisfied. The procedure suggested is applied to the centered rectangular 2D lattice, formed by (1 0 1) sheet of the bulk anatase. It is shown that (n, 0) and (0, m) nanotubes can be constructed by rolling an anatase (1 0 1) sheet along the and [0 1 0] directions respectively. The orthogonal to chirality vector translation vector does not exist for n ≠ m ≠ 0 general case. The first principles LCAO calculations of (n, 0) and (0, m) NT’s are made with hybrid HF/KS (PBE0) exchange-correlation functional for systems containing up to 180 atoms (n = 6, 9, 12, 15; m = 3−6). It is demonstrated that the energy band gap increases (from 4.7 to 5.4 eV) when the NT radius changes from 3.61 to 9.92 Å. The strain energy is larger for (n, 0) than for (0, m) nanotubes of a similar radius. The changes of the unit cell periodicity and radius of titania nanotubes after the structure optimization are negligible, however the atomic relaxations are noticeable.     

Impedance response of polycrystalline tungsten oxide

Polycrystalline tungsten oxide (WO₃) pellets were prepared by conventional ceramic processing technology. The ac small-signal electrical data acquired in the frequency (f) range 100 Hz≤f≤1 MHz at temperature (T) ranging the 31-100 °C revealed distinct semicircular relaxation in the impedance plane. This relaxation indicates device behavior originating from the grain boundaries. The lumped grain impedance associated with the device action remained too small to detect when the large resistance scale is realized. The semicircular relaxation is thermally activated indicating 0.58 eV as the activation energy for the relaxation time.     

超长(n,n)型碳纳米管的密度泛函理论研究

用密度泛函B3LYP/3-21G(d)方法,并利用周期边界条件,研究了n=2—20不同管径的超长(n, n)型单壁碳纳米管的结构、能量、能带结构和能隙.结果表明,管径和能量(或生成焓)都随n有很好的变化规律,并可拟合成很好的解析函数.当n为2和3时,碳纳米管的能隙分别为1.836eV和0.228eV,呈半导体特征,且具有间接带隙;当n=4—20时,能隙介于0.027 eV和0.079 eV之间,呈较强的金属性,且具有直接带 关键词： 扶手椅型碳纳米管 周期边界条件(PBC) 超长模型 能带     

Structural, dielectric, optical and ferroelectric property of urea succinic acid crystals grown in aqueous solution containing maleic acid

Urea-succinic acid crystals have been grown at room temperature from aqueous solution in the presence of maleic acid by a slow evaporation technique. The structural parameters were determined using powder X-ray diffraction (XRD) and found to have monoclinic symmetry (space group P2₁/m) with a=9.902, b=17.510, c=5.555 Å and α=γ=90°, β=96.46°. The transparency and optical analysis were carried out using UV-vis analysis. The optical band gap is found to be 4.71 eV. The presence of various functional groups was confirmed by FTIR analysis. The samples have shown piezoelectric behavior with a fairly good piezoelectric charge coefficient (d₃₃) of 5 pC/N, when it is poled at 7 kV/cm. The hysteresis loop was plotted and the remnant polarization and coercive field were found to be 2.8 μC/cm² and 4 kV/cm, respectively. The dielectric analysis was carried out as a function of temperature at various frequencies and the results were also discussed.     

Carbon- and silicon-capped silicon carbide nanotubes: An ab initio study

A systematic study of fullerene hemisphere capped finite SiC nanotubes is presented. The tubes are spin optimized using the hybrid functional B3LYP (Becke?s three-parameter exchange and the Lee-Yang-Parr correlation functionals) and an all electron 3-21G^? basis. Capping of a SiC nanotube changes cohesive energy, HOMO-LUMO gap and other electronic and geometric properties of a SiC nanotube. Also, the carbon-capped SiC nanotubes are energetically preferable compared to silicon-capped tubes. For example, the binding energy per atom for hydrogen-terminated “infinite” SiC nanotube (5,5) having five unit cells is 4.993 eV, the corresponding numbers being 5.989 eV and 4.812 eV for C-capped and Si-capped nanotubes, respectively.     

Electrical properties of SrBi2(Nb0.5Ta0.5)2O9 ceramics

Aurivillius SrBi₂(Nb_0.5Ta_0.5)₂O₉ (SBNT 50/50) ceramics were prepared using the conventional solid-state reaction method. Scanning electron microscopy was applied to investigate the grain structure. The XRD studies revealed an orthorhombic structure in the SBNT 50/50 with lattice parameters a=5.522 Å, b=5.511 Å and c=25.114 Å. The dielectric properties were determined by impedance spectroscopy measurements. A strong low frequency dielectric dispersion was found to exist in this material. Its occurrence was ascribed to the presence of ionized space charge carriers such as oxygen vacancies. The dielectric relaxation was defined on the basis of an equivalent circuit. The temperature dependence of various electrical properties was determined and discussed. The thermal activation energy for the grain electric conductivity was lower in the high temperature region (T>303.6 °C, E_a−ht=0.47 eV) and higher in the low temperature region (T<303.6 °C, E_a−lt=1.18 eV).