单壁碳纳米管杂化轨道计算

根据轨道杂化理论以及碳纳米管的几何结构,计算了(n,0),(n,n)和(n,m)三种单壁碳纳米管的杂化轨道,给出了杂化轨道s轨道成分和p轨道成分的解析式.对于管径较小的纳米管,锯齿型(n<40),扶手椅型(n<20),手性型(n<30,m相似文献   

The calculation of energy gaps in small single-walled carbon nanotubes within a symmetry-adapted tight-binding model

This paper studies in detail the electronic properties of the semimetallic single-walled carbon nanotubes by applying the symmetry-adapted tight-binding model.It is found that the hybridization of π-σ states caused by the curvature produces an energy gap at the vicinity of the Fermi level.Such effects are obvious for the small zigzag and chiral single-walled carbon nanotubes.The energy gaps decrease as the diameters and the chiral angles of the tubes increase,while the top of the valence band and the bottom of the conduction band of armchair tubes cross at the Fermi level.The numeral results agree well with the experimental results.     

Transport properties of boron nanotubes investigated by ab initio calculation

We investigate atomic and electronic structures of boron nanotubes (BNTs) by using the density functional theory (DFT). The transport properties of BNTs with different diameters and chiralities are studied by the Keldysh nonequilibrium Green function (NEGF) method. It is found that the cohesive energies and conductances of BNTs decrease as their diameters decrease. It is more diffcult to form (N,0) tubes than (M,M) tubes when the diameters of the two kinds of tubes are comparable. However, the (N,0) tubes have a higher conductance than the (M,M) tubes. When the BNTs are connected to gold electrodes, the coupling between the BNTs and the electrodes will affect the transport properties of tubes significantly.     

Smallest nanotube: breaking the symmetry of sp(3) bonds in tubular geometries

We describe how sp(2) carbon, threefold coordinated by other carbons, can be replaced by sp(3) carbon, also threefold carbon coordinated, to produce extremely small-diameter ( approximately 0.4 nm) carbon nanowires with only minimal bond-angle distortion. Under a naming convention analogous to that for ordinary carbon nanotubes, the smallest sp(3) tubes have wrapping indices (3,0) and (2,2). These systems have large band gaps and a stiffness larger even than that of traditional sp(2)-bonded carbon nanotubes. They therefore form the stiffest one-dimensional systems known.     

Manganese silicide single crystal and films deposited on Si(1 1 1): A comparative spectroscopic study

A comparative experimental study is presented of the electronic properties of MnSi films grown on Si(1 1 1) and of MnSi single crystals, using X-ray absorption spectroscopy (XAS), and core level and valence band photoemission spectroscopy (PES). No significant differences in the electronic structure of the two systems can be found.Absorption measurements on the Mn 2p threshold show a mixed valence ground state, where the multiplet structure is washed out by the hybridisation of the Mn 3d states with the Si sp states. These results are also confirmed by photoemission (PE) spectra from the valence band and the Mn 3s, 3p and 2p core levels.Strong attention has been paid to the effect of contamination. The occurrence of multiplet effects in the absorption spectra indicates unambiguously the localisation of the Mn 3d electrons in Mn-O bonds, which strongly influences the electronic properties of these systems.     

掺Pb的Bi-Sr-Ca-Cu-O超导体及非晶体的Raman光谱

本文研究了Bi2Sr2CaCu2O8单晶(Tc=85K)、Bi2-xPbxSr2Ca2Cu3Oy单相(Tc=107K)和Bi(Pb)-Sr-Ca-Cu-O非晶体室温下20-720cm-1的Raman光谱.通过谱图的比较,我们认为456-466、631-650cm-1是高Tc氧化物的特征谱,它们与Cu-O链及Cu-O面上的强电子-声子耦合有关;掺Pb导致Bi(Pb)-O双层之间氧的分布改变,增强了Bi(Pb)-O双层之间的耦合,从而有助于提高超导转变温度Tc.     

First-principles studies on stabilities and electronic properties of AlN nanostructures

Under the generalized gradient approximation (GGA), the stabilities and electronic properties of semiconductor AlN nanostructures have been investigated by using the first-principles projector-augmented wave (PAW) potential within the density function theory (DFT) framework. The single-walled faceted AlNNTs present an obvious structural modification. The larger the diameter, the more stable the nanowire, and the wires without internal surfaces are more stable than the multiwalled tubes or the SWNT. Therefore, the large-size nanowires are easier to be synthesized than the corresponding multiwalled tubes or single-walled nanotube in experiment. The dangling bonds of surface atoms cause the “localized edge-induced states”. These two nanostructures C and F are still wide band gap semiconductors accompanied by a few surface states located in the band gap of bulk AlN and thus extremely suitable for application in flexible pulse wave sensors, nanomechanical resonators and light-emitting diodes.     

Role of inter-tube coupling and quantum interference on electrical transport in carbon nanotube junctions

Due to excellent transport properties, Carbon nanotubes (CNTs) show a lot of promise in sensor and interconnect technology. However, recent studies indicate that the conductance in CNT/CNT junctions are strongly affected by the morphology and orientation between the tubes. For proper utilization of such junctions in the development of CNT based technology, it is essential to study the electronic properties of such junctions. This work presents a theoretical study of the electrical transport properties of metallic Carbon nanotube homo-junctions. The study focuses on discerning the role of inter-tube interactions, quantum interference and scattering on the transport properties on junctions between identical tubes. The electronic structure and transport calculations are conducted with an Extended Hückel Theory-Non Equilibrium Green's Function based model. The calculations indicate conductance to be varying with a changing crossing angle, with maximum conductance corresponding to lattice registry, i.e. parallel configuration between the two tubes. Further calculations for such parallel configurations indicate onset of short and long range oscillations in conductance with respect to changing overlap length. These oscillations are attributed to inter-tube coupling effects owing to changing π orbital overlap, carrier scattering and quantum interference of the incident, transmitted and reflected waves at the inter-tube junction.     

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.     

Far-infrared to visible optical conductivity of single-wall carbon nanotubes

The reflectance of unoriented single-wall carbon nanotube films has been measured over a wide wavelength range (far-IR–UV). The results are consistent with the film being a mixture of conducting (armchair), small bandgap (n≡m, mod 3) and semiconducting nanotubes. The optical conductivity shows peaks corresponding to transitions between density-of-states peaks of these tubes, at energy locations consistent with 1.4 nm diameter tubes. In addition optical absorption spectroscopy of aligned single-wall carbon nanotubes shows that the optical transitions are well-aligned along the tube axis. This behavior is consistent with polarized resonant Raman and electronic structure calculations.     

The geometrical and electronic structures of open-end fully functionalized single-walled carbon nanotubes

We have investigated the geometrical and electronic structures of open-end single-walled carbon nanotubes (SWNTs) having chemically modified tips, using semi-empirical AM1 and density functional theory methods. The hydroxyl (–OH), carboxyl (–COOH) and amide (–CONH₂) functional groups were used to saturate the open-ends of nanotubes. The effects of functional groups were studied by comparison with the pristine tubes, of which the tubular lengths vary from two to ten unit-cells (40 Å). The results show that the C–C bond lengths of all model tubes are only slightly different, and the behavior of converging bond lengths in COOH– and CONH₂-SWNTs is very similar to the pristine tube. Tip functionalization alters the frontier orbitals of the pristine tube, but these effects seem to rapidly decrease as the tubule becomes longer. In general, it can be concluded that the geometrical and electronic structures of pristine tubes after tube-end “full” functionalization will be preserved, hence supporting that more real-world “partially” functionalized SWNTs can be used in the same way as the pristine version in most application areas.     

α-C∶H膜化学结构对光学性能的影响

 选用体积分数为99.999 9%的H₂及反式-2-丁烯（T2B）为工作气体,利用低压等离子体增强化学气相沉积法制备了α-C∶H薄膜。利用傅里叶变换红外光谱仪和X射线光电子能谱对薄膜化学键和电子结构进行分析,并结合高斯分峰拟合分析了薄膜中sp³/sp²杂化键比值和sp³C杂化键分数。结果表明：薄膜中氢含量较高,主要以sp³C—H形式存在;工作气压越高,制备的薄膜中C=C键含量越少,薄膜中sp³/sp²杂化键比值和sp³C杂化键分数增加,薄膜稳定性提高。应用UV-VIS光谱仪,获得了波长在400~1 000 nm范围内薄膜的光吸收特性,结果显示: α-C∶H薄膜透过率可达98%。光学常数公式计算得到工作压强为4~14 Pa时光学带隙在2.66~2.76之间,并均随着工作气压的升高而增大。结果表明,随工作气压的升高,薄膜内sp³键减小,从而促使透过率、光学带隙增大。     

Band structure and optical transitions in semiconducting double-wall carbon nanotubes

The electronic structure of semiconducting double-wall carbon nanotubes (CNTs) is calculated using the linearized augmented cylindrical wave method. The consideration is performed in the framework of the local density functional theory and the muffin-tin (MT) approximation for the one-electron Hamiltonian. The electronic spectrum of a double-wall CNT is determined by the free motion of electrons in the interatomic space of the two cylindrical layers, scattering by the MT spheres, and tunneling through the classically impenetrable region. Calculated results for double-wall CNTs of the (n, 0)@(n′, 0) zigzag type indicate that the shift of the band-gap width depends on whether n and n′ are divided by 3 with a remainder of 1 or 2. It is found that, regardless of the type of the inner tube, the energy gap E _g of the outer tube decreases by 0.15–0.22 eV if the tube belongs to the sequence n = 2 (mod 3). For the outer tubes of the sequence n = 1 (mod 3), the shifts of the band gap ΔE _g are always negative ?0.15 ≤ ΔE _g ≤ ?0.05 eV. In both cases, the shifts ΔE _g weakly oscillate rather than decrease in going to tubes of a larger diameter d. For the inner tubes, the changes in the band gap ΔE _g are more sensitive to the diameter. At 10 ≤ n ≤ 16, the shifts ΔE _g are positive and the maximum value of ΔE _g equals 0.39 and 0.32 for the sequences n = 2 (mod 3) and n = 1 (mod 3), respectively. In going to the inner tubes of a larger diameter, ΔE _g rapidly drops and then oscillates in the range from ?0.05 to 0.06 eV. The calculated results indicate that the shifts of the optical band gaps in core and shell tubes upon the formation of double-wall CNTs are significant, which must hinder the identification of double-wall CNTs by optical methods. On the other hand, the obtained results open up possibilities for a more detailed classification of double-wall nanotubes.     

α-C∶H膜化学结构对光学性能的影响

 选用体积分数为99.999 9%的H₂及反式-2-丁烯（T2B）为工作气体,利用低压等离子体增强化学气相沉积法制备了α-C∶H薄膜。利用傅里叶变换红外光谱仪和X射线光电子能谱对薄膜化学键和电子结构进行分析,并结合高斯分峰拟合分析了薄膜中sp³/sp²杂化键比值和sp³C杂化键分数。结果表明：薄膜中氢含量较高,主要以sp³C—H形式存在;工作气压越高,制备的薄膜中C=C键含量越少,薄膜中sp³/sp²杂化键比值和sp³C杂化键分数增加,薄膜稳定性提高。应用UV-VIS光谱仪,获得了波长在400~1 000 nm范围内薄膜的光吸收特性,结果显示: α-C∶H薄膜透过率可达98%。光学常数公式计算得到工作压强为4~14 Pa时光学带隙在2.66~2.76之间,并均随着工作气压的升高而增大。结果表明,随工作气压的升高,薄膜内sp³键减小,从而促使透过率、光学带隙增大。     

Si tight-binding parameters from genetic algorithm fitting

Quantum mechanical simulations of carrier transport in Si require an accurate model of the complicated Si bandstructure. Tight-binding models are an attractive method of choice since they bear the full electronic structure symmetry within them and can discretize a realistic device on an atomic scale. However, tight-binding models are not simple to parameterize and characterize. This work addresses two issues: (1) the need for an automated fitting procedure that maps tight-binding orbital interaction energies to physical observables such as effective masses and band edges, and (2) the capabilities and accuracy of the nearest and second-nearest neighbor tight-binding sp3s* models with respect to carrier transport in indirect bandgap materials. A genetic algorithm approach is used to fit orbital interaction energies of these tight-binding models in a nine- and 20-dimensional global optimization problem for Si. A second-nearest neighbor sp3s* parameter set that fits all relevant conduction and valence band properties with a high degree of accuracy is presented. No such global fit was found for the nearest neighbor sp3s* model and two sets, one heavily weighed for electron properties and the other for hole properties, are presented. Bandstructure properties relevant for electron and hole transport in Si derived from these three sets are compared with the seminal Vogl et al. [Journal of the Physics and Chemistry of Solids 44, 365 (1983)] parameters.     

A full spd tight-binding treatment for electronic bands of graphitic tubes

A tight-binding (TB) treatment with the inclusion of d orbitals is applied to the electronic structures of graphitic tubes. The results show that the high angular moment bases in TB scheme are necessary to account the severe curvature effect in ultra-thin single wall carbon nanotubes, especially for properly reproducing the band edge overlap behavior in (5, 0) tube, predicted by the existing ab initio calculations. In the large diameter limit, the participation of two symmetry-allowed d bases provides a natural replication to the recent measured electronic dispersions of valence band of graphene when the strong anisotropy due to the two-dimensional planar hexagonal sheet structure is dealt with properly. In addition, the detailed relation between the two sets of quantum numbers of screw symmetry and that of zone folding is formulated in appendix.     

Stone-Wales defects can cause a metal-semiconductor transition in carbon nanotubes depending on their orientation

It has been shown that the two different orientations of Stone-Wales (SW) defects, i.e. longitudinal and circumferential SW defects, on carbon nanotubes (CNTs) result in two different electronic structures. Based on density functional theory we have shown that the longitudinal SW defects do not open a bandgap near the Fermi energy, while a relatively small bandgap emerges in tubes with circumferential defects. We argue that the bandgap opening in the presence of circumferential SW defects is a consequence of long-range symmetry breaking which can spread all the way along the tube. Specifically, the distribution of contracted and stretched bond lengths due to the presence of defects, and hopping energies for low-energy electrons, i.e. the 2p(z) electrons, show two different patterns for the two types of defects. Interplay between the geometric features and the electronic properties of the tubes have also been studied for different defect concentrations. Considering π-orbital charge density, it has also been shown that the deviations of bond lengths from their relaxed length result in different doping for two defect orientations around the defects-electron-rich for a circumferential defect and hole-rich for a longitudinal one. We have also shown that, in the tubes having both types of defects, circumferential defects would dominate and impose their electronic properties.     

Electronic transport through carbon nanotubes: effects of structural deformation and tube chirality

Atomistic simulations using a combination of classical force field and density-functional theory (DFT) show that carbon atoms remain essentially sp(2) coordinated in either bent tubes or tubes pushed by an atomically sharp atomic-force microscope (AFM) tip. Subsequent Green's-function-based transport calculations reveal that for armchair tubes there is no significant drop in conductance, while for zigzag tubes the conductance can drop by several orders of magnitude in AFM-pushed tubes. The effect can be attributed to simple stretching of the tube under tip deformation, which opens up an energy gap at the Fermi surface.     

Doping induced diode behavior with large rectifying ratio in graphyne nanoribbons device

By applying nonequilibrium Green's functions (NEGF) in combination with density functional theory (DFT), the electronic transport properties of α-armchair graphyne nanoribbons (α-AGyNR) device are investigated, in which the left electrode is doped by Boron (B) atoms and the right electrode is doped by Nitrogen (N) atoms. B and N doping atoms that are substituted in SP and SP² sites result in four devices named as A (sp–sp), B (sp²–sp²), C (sp²–sp) and D (sp–sp²). The current–voltage characteristics of these systems reveal that the proposed devices have a striking nonlinear feature that leads to formation of a p–n junction which results rectifying behavior. The results show that the rectification characteristics are strongly dependent on the site of doping atoms. In A and D devices the rectification ratio (RR) can reach to 10³ in the bias region from 0.2 V to 0.6 V while in B and C devices the RR can be enlarged to 10⁴ in the same bias region. Unlike the most of the previous proposed molecular rectifiers that are vertical hetero structures, the proposed rectifier in this work is in-plane or two dimensional structures. The results provide a new insight and a novel effective pathway for developing applicable high-performance Graphyne-based rectifiers.     

Comparison of the X-ray photoelectron and electron-energy-loss spectra of the nitrogen-doped hydrogenated amorphous carbon bond

The composition of nitrogen-doped hydrogenated amorphous carbon (a-C : H : N) films grown in a magnetically confined rf plasma-enhanced chemical vapour deposition system has been determined by X-ray photoelectron spectroscopy (XPS) and compared with that determined using a combination of elastic recoil detection analysis, Rutherford back-scattering and nuclear reaction analysis. The importance of nitrogen doping or 'incorporation' in hydrogenated amorphous carbon (a-C : H) films is discussed in relation to the significant variation in the sp 2 -to-sp 3 ratio that takes place. At 7 at.% N in the a-C : H matrix, a critical change in the microstructure is observed, which governs the resulting mechanical, optical and electronic properties. Finally, the correlation between the sp 2 and sp 3 fractions determined by a non-destructive method of obtaining the bond fractions (XPS) and by electron-energy-loss spectroscopy is discussed, with a view to evaluating accurately the sp 2 fraction in a-C : H : N films.