Optical properties of sub 2 nm long (6,5) single-walled carbon nanotubes: first principles investigation

Optical properties of (6,5) single-walled carbon nanotubes (SWCNTs) of 1 and 2?nm lengths were studied with the help of time-dependent density functional theory and transition density matrix based analysis. Electronic band gap of all SWCNTs is found to be in the range 0.82–1.67?eV. The peak absorptions occur around 600?nm for 1?nm nanotubes and get broader as the length increases to 2?nm. The natural transition orbital analysis was applied to understand the electron delocalisation upon absorption. Finally, the exciton sizes were determined and found to be in the range 6.80–7.25?Å for 1?nm SWCNTs, and 6.82–11.56?Å for 2?nm SWCNTs, which were further used in the electron delocalisation analysis. All the excitons were found to be Frenkel in nature. This study illustrates how the excited state properties in SWCNTs can vary upon length change and improves the understanding of electronic excitations in SWCNTs, which would be beneficial in photovoltaic applications.     

Optical properties of carbon nanotubes and BaTiO3 composite thin films

Multiwalled carbon nanotubes and BaTiO₃ composite films have been prepared by pulsed-laser deposition technique at room temperature and high temperature of 600℃, separately. The structures of the composite films are investigated by using scanning electron microscopy and x-ray diffraction. The optical behaviours of the samples produced at different temperatures are compared with Raman spectroscopy, and UV-visible absorption. And the observation by Z-scan technique reveals that the composite films have a larger optical nonlinearity, and the samples prepared at high temperatures have better transmittance and opposite sign imaginary part of optical third-order nonlinearity.     

Elastic and vibrational properties of Mg2Si1−xSnx alloy from first principles calculations

The structural, elastic and phonon properties of Mg₂Si_1?xSn_x alloy are investigated by performing density functional theory and density functional perturbation theory calculations. The calculated lattice parameter increases with the increase of Sn content obeying Vegard’s Law that is in good agreement with available experimental data. Shear modulus, Young’s modulus and sound velocities are determined from the obtained elastic constants. Phonon dispersion curves show a pronounced softening with increasing of Sn content. The softening mechanism has been discussed based upon the element mass and bond strength. Besides, phonon contribution to the Helmholtz free energy, the entropy and the constant-volume heat capacity are calculated within the harmonic approximation based on the calculated phonon density of states. Results show Mg₂Si_1?xSn_x is thermodynamically more stable with higher Sn content.     

Structure of the (0001) surface of α-Al2O3 from first principles calculations

Fully self-consistent ab initio calculations based on pseudopotentials are used to study the structure and energetics of the basal-plane surface of α-Al₂O₃. The calculated forces on the atoms are used to relax the atomic positions to equilibrium. It is shown that surface relaxations are very large and lead to a reduction of the surface energy by over a factor of two. The results support the validity of earlier work based on pair-interaction models.     

Structure of connections of single-walled carbon nanotubes with the use of the combined 5–7 and 4–8 topological defects

The structures of zigzag-zigzag, armchair-zigzag, zigzag-chiral, armchair-armchair, armchair-chiral, and chiral-chiral pair connections produced by single-walled carbon nanotubes 1.5–5.0 diameter with the use of the combined 5–7 and 4–8 topological defects have been calculated by molecular mechanics methods. It has been established that the use of the combined 5–7 topological defect makes it possible to produce connections between pairs of single-walled carbon nanotubes with any conductivities, chiralities, and diameters, whereas the use of the combined 4–8 topological defect provides a means for forming connections between nanotubes only with the same type of conductivity. The angles between the axes of nanotubes connected by the combined 5–7 and 4–8 topological defects lie in the ranges 145°–180° and 112°–178°, respectively. It has been revealed that there are correlations between structural parameters of the connections and the relative arrangement of the simple topological defects in the combined topological defects.     

First principles calculations of the structural and electronic properties of(CdSe)n clusters

The structural and electronic properties of (CdSe)n(1≤n≤5) clusters are calculated using density functional theory within the pseudopotential and generalized gradient approximations. The calculated binding energies and highest occupied molecular orbital lowest unoccupied molecular orbital gaps are compared with those obtained within local density approximation.     

Investigation on native defects of α-MgAgSb and its effects on thermoelectric properties using first principles calculations

α-MgAgSb is a promising thermoelectric materials having good performance at medium temperature. Native defects in α-MgAgSb are frequently reported experimentally and are tightly involved in the thermoelectric properties of α-MgAgSb. In this paper, all possible native defects in α-MgAgSb are calculated as well as detailed results are given and discussed. The concentrations of several dominant native defects, for example, V_Ag and Ag_Sb, could reach up to 10^?4 cm^?3 at 540 K. Furthermore, the electronic structure and transport properties of α-MgAgSb with dominant native defects are investigated. Results show that the introduction of Ag_Mg and V_Ag contributes to a much lower inertial mass and slight decrease in Seebeck coefficient. The lattice thermal conductivity is greatly reduced with the introduction of native defects. For α-MgAgSb with V_Ag, the peak ZT could reach up to 1.84 at 420 K. Our calculation demonstrates that defect engineering is an effective strategy to enhance thermoelectric performance of the materials.     

Enhancement of the electrical properties of carbon nanotubes with Ar–N2 plasma treatment

Optical emission spectroscopy and Langmuir probe are used to investigate the low pressure inductively coupled Ar–N₂ plasmas as function of rf power, filling pressure and Ar content in N₂ discharge. It is observed that the active species generation, dissociation fraction and electron temperature significantly depends on discharge parameters and may be used to optimize the plasma reactor. Mixture of SWCNTs and MWCNTs are treated for different treatment time (0–120 min) at optimum discharge conditions. Changes induced in the elemental composition, surface morphology, crystallographic structure, and structural disorder in the plasma irradiated CNTs are analyzed by EDX, FTIR, SEM, XRD and Raman spectroscopy, respectively. Ar–N₂ mixture plasma treatment of CNTs lead to a significant increase in the electrical conductivity, modify the microstructure and induce structural disorder and cause a transition of crystalline phase from well crystalline to an amorphous structure.     

Computation of disordered system from the first principles of classical mechanics and ℕℙ hard problem

We study the classical 1D Heisenberg spin glasses in the framework of nearest-neighboring model. Based on the Hamilton equations we obtained the system of recurrence equations which allows to perform node-by-node calculations of a spin-chain. It is shown that calculations from the first principles of classical mechanics lead to ?? hard problem, that however in the limit of the statistical equilibrium can be calculated by ? algorithm. For the partition function of the ensemble a new representation is offered in the form of one-dimensional integral of spin-chains’ energy distribution.     

The structural,elastic, and electronic properties of ZrxNbl-xC alloys from first principle calculations＊

The structural, elastic, electronic, and thermodynamic properties of ZrxNbl xC alloys are investigated using the first principles method based on the density functional theory. The results show that the structural properties of Zr~.Nb1 xC alloys vary continuously with the increase of Zr composition. The alloy possesses both the highest shear modulus （215 GPa） and a higher bulk modulus （294 GPa）, with a Zr composition of 0.21. Meanwhile, the Zr0.2! Nb0.79C alloy shows metallic conductivity based on the analysis of the density of states. In addition, the thermodynamic stability of the designed alloys is estimated using the calculated enthalpy of mixing.     

On the efficiency of bile salt for stable suspension and isolation of single-walled carbon nanotubes—spectroscopic and microscopic investigations

In this contribution we present a systematic study on the dispersion of SWCNTs in a water-based solution of biocompatible detergent: sodium deoxycholate (DOC). By avoiding harsh chemical conditions, which are known to damage nanotubes structure, a stable SWCNTs suspension was created. Long term stirring of the solution led to preparation of a stable transparent solution, containing well-dispersed isolated SWCNTs. The as-prepared dispersion remained stable and clear for two months. Optical absorption spectroscopy was employed to measure SWCNTs suspension stability. Nanotube aggregation was evaluated through the tangential mode (G mode) present in the Raman spectrum. High-resolution transmission electron microscopy was employed to observe the mechanism of debundling process.     

Equilibrium geometries and electronic properties of BenLi （n=2-15） clusters from first principles

This paper studies the equilibrium geometries and electronic properties of Ben and BenLi clusters, up to n=15, by using density-functional theory（DFT） at B3LYP/6-31G（d） level. The lowest-energy structures of Ben and BenLi clusters were determined. The results indicate that a single lithium impurity enhances the stability and chemical reactivity of the beryllium clusters. It finds that the geometries of the host clusters change significantly after the addition of the lithium atom for n ≥8. The lithium impurity prefers to be on the periphery of beryllium clusters, and occupies vertex sites. Both Be4Li, Be9Li, and Be13Li were found to be particularly stable with higher average binding energy, local peaks of second-order energy difference and fragmentation energies. For all the BenLi clusters studied, we found charge transfers from the Li to Be site and co-existence of covalent and metallic bonding characteristics.     

Influence of the annealing atmosphere on the properties of LiCoPO4–graphitic carbon foams composites

The investigation on the properties of LiCoPO₄–graphitic carbon foams (LCP-GCF) composites is reported in this work. The diffraction analysis (XRD) on powders confirmed the presence of LiCoPO₄ as major crystalline phase and Li₄P₂O₇ and Co₂P as secondary phases. The morphological investigation of the composites shows a layer of crystalline spongy-like material on the surface of the GCF for t?=?0 h and of acicular crystallites with different dimensions (5–50 μm) for t?≥?0.1 h. The voltammetric curves (cyclic voltammogramms) show mean values of reduction potential above 5.0 V independently of the annealing time. The LCP-GCF composites deliver a discharge-specific capacity of 76mAh g^?1 (t?=?0 h) and of 102mAh g^?1 (t?=?0.1 h) at a discharge rate of C/10 and room temperature. The electrochemical impedance spectroscopy data reveal a decrease of the electrical resistance and the improvement of the Li-ion conductivity as a function of the annealing time.     

Electronic structure,elastic and thermodynamic properties of α-phase Na3N under pressure from first principles

The structural, electronic, elastic and thermodynamic properties of α-phase Na₃N under pressure are investigated by performing first principles calculations within generalized gradient approximation. The elastic constants, bulk modulus, shear modulus, Young's modulus, and Poisson's ratio dependencies on pressure are also calculated. The thermodynamic properties of the α-phase Na₃N are calculated using the quasi-harmonic Debye model. The dependencies of the heat capacity and the thermal expansion coefficient, as well as the Grüneisen parameter on pressure and temperature are investigated systematically in the ranges of 0–1 GPa and 0–100 K.     

Optical and dielectric properties of the mesogen 4-cyano-4′-n–octyl biphenyl as a function of temperature

The effect of temperature on certain optical and dielectric properties of one member, namely 4-cyano-4′-n–octyl biphenyl (8CB) of the mesogenic series 4-cyano-4′-n–alkyl biphenyl (nCB), is reported. The dependence of the ordinary and extraordinary components of polarisability, α_o and α_e, on temperature is studied using a He–Ne laser beam, and the variation of orientational order parameter with temperature in the entire mesogenic phase determined from the optical studies. The dependence of the effective molecular dipole moment, μ_eff, in the liquid-crystal phase, and the angle of inclination, β, of the dipole moment with the preferred direction have been investigated. An experimental estimate of the correlation factor ‘g’, which is indicative of the nature of the molecular association in the mesophase, has been obtained.     

First principles calculations of the structural,electronic and optical properties of the mixed fluorides SrxCd1−xF2

In the present work, we have investigated the structural, electronic and optical properties of SrF₂ and CdF₂ and their ternary mixed Sr_xCd_1?xF₂ alloys at some selected concentrations x (x?=?0.25, 0.50, 0.75 and 1). The parent compounds SrF₂ and CdF₂ crystallize in Fm-3?m space group, whereas the alloys adopt the cubic structure with Pm-3?m space group for the composition x?=?0.25 and 0.75 and the tetragonal structure with space group P4/mmm for x?=?0.50. The calculations were performed using the full-potential linearized augmented plane wave (FP-LAPW) method. The exchange-correlation potential was handled with Wu and Cohen GGA approximation (WC-GGA). Moreover, the Engel–Vosko's (EV-GGA) formalism and the modified Becke Johnson (mBJ) approximation were also applied to improve the electronic band structure calculations. The computed structural parameters for SrF₂ and CdF₂ such as the equilibrium lattice constants and the bulk moduli are in good agreement with the available experimental and theoretical data. It is found that the lattice parameters increase with increasing composition (x) while the bulk modulus decreases for Sr_xCd_1?xF₂ alloys. The calculated band structures reveal an indirect band gap (W-Γ), (X-Γ) and (M-Γ) for CdF₂, SrF_{2 and} Sr_xCd_1?xF₂ for x?=?0.25, 0.75 and x?=?0.5, respectively. The optical constants, including the dielectric function, refractive index, reflectivity, absorption, extinction coefficient and the energy loss function were calculated using both WC-GGA and mBJ schemes for a radiation up to 40?eV. This is the first quantitative theoretical prediction of the optical properties for these alloys that requires experimental confirmation.     

Effect of sputtered Cu film’s diffusion barrier on the growth and field emission properties of carbon nanotubes by chemical vapor deposition

Growth of carbon nanotube (CNT) films with good field emission properties on glass is very important for low cost field emission display (FED) applications. In addition to Ni, Co and Fe, Cu can be a good catalyst for CNT growth on glass, but due to diffusion into SiO₂ it is difficult to control the CNTs density and uniformity. In this paper, four metal barrier layers (W, Ni, Cr, Ti) were deposited by dc magnetron sputtering on glass to reduce the Cu diffusion. As-grown CNT films showed various morphologies with the use of different barrier metals. CNTs with uniform distribution and better crystallinity can be synthesized only on Ti/Cu and W/Cu. Voltage current measurements indicate that better field emission properties of CNT films can be obtained on titanium and tungsten barriered Cu, while chromium and nickel are not suitable barrier candidates for copper in CNT-FED applications because of the reduced emission performance. PACS 81.05.Uw; 61.46.Fg; 85.45.Db; 66.30.-h