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1.
Mechanism of Carbon Nanotubes Aligning along Applied Electric Field   总被引:1,自引:0,他引:1       下载免费PDF全文
The mechanism of single-walled carbon nanotubes (SWCNTs) aligning in the direction of external electric field is studied by quantum mechanics calculations. The rotational torque on the carbon nanotubes is proportional to the difference between the longitudinal and transverse polarizabilities and varies with the angle of SWCNTs to the external electric field. The longitudinal polarizability increases with second power of length, while the transverse polarizability increases linearly with length. A zigzag SWCNT has larger longitudinal and transverse polarizabilities than an armchair SWCNT with the same diameter and the discrepancy becomes larger for longer tubes.  相似文献   

2.
Engineering Double-Walled Carbon Nanotubes by Ar Plasma   总被引:1,自引:0,他引:1       下载免费PDF全文
Double-walled carbon nanotubes (DWCNTs) have been found to be promising nano-materials for nano-mechanical and nano-electrical devices due to their double-walled structures. Modifying DWCNTs would be one of the key technologies for device construction. We demonstrate engineering the geometry of DWCNTs by etching with Ar plasma. The characterization by atomic force rnicroscopy indicates that single atomic carbon layers could be removed from DWCNTs by Ar plasma. The etching effect is further investigated by electrical measurements on DWCNT field-effect transistors, which allow us to study the interwall screen effect as well.  相似文献   

3.
Using first-principles calculations, we investigate the two-dimensional arsenic nanosheet isolated from bulk gray arsenic. Its dynamical stability is confirmed by phonon calculations and molecular dynamics analyzing. The arsenic sheet is an indirect band gap semiconductor with a band gap of 2.21 e V in the hybrid HSE06 functional calculations. The valence band maximum(VBM) and the conduction band minimum(CBM) are mainly occupied by the 4p orbitals of arsenic atoms,which is consistent with the partial charge densities of VBM and CBM. The charge density of the VBM G point has the character of a π bond, which originates from p orbitals. Furthermore, tensile and compressive strains are applied in the armchair and zigzag directions, related to the tensile deformations of zigzag and armchair nanotubes, respectively. We find that the ultimate strain in zigzag deformation is 0.13, smaller than 0.18 of armchair deformation. The limit compressive stresses of single-layer arsenic along armchair and zigzag directions are-4.83 GPa and-4.76 GPa with corresponding strains of-0.15 and-0.14, respectively.  相似文献   

4.
By capturing the atomic information and reflecting the behaviour governed by a nonlinear potential function, an analytical molecular mechanics approach is applied to establish the constitutive relation for single-walled carbon nanotubes (SWCNTs). The nonlinear tensile deformation curves of zigzag and armchair nanotubes with different radii are predicted, and the elastic properties of these SWCNTs are obtained. A conclusion is made that the nanotube radius has little effect on the mechanical behaviour of SWCNTs subject to simple tension, while the nanotube orientation has larger influence.  相似文献   

5.
This paper reports that the etching morphology of dislocations in 8° off-axis 4H-SiC epilayer is observed by using a scanning electronic microscope.It is found that different types of dislocations correspond with different densities and basal plane dislcation (BPD) array and threading edge dislocation (TED) pileup group lie along some certain crystal directions in the epilayer.It is concluded that the elastic energy of threading screw dislocations (TSDs) is highest and TEDs is lowest among these dislocations,so the density of TSDs is lower than TEDs.The BPDs can convert to TEDs but TSDs can only propagate into the epilyer in spite of the higher elastic energy than TEDs.The reason of the form of BPDs array in epilayer is that the big step along the basal plane caused by face defects blocked the upstream atoms,and TEDs pileup group is that the dislocations slide is blocked by dislocation groups in epilayer.  相似文献   

6.
Effect of the V/III ratio during buffer layer growth on the yellow and blue luminescence in undoped GaN epilayer has been studied by means of photoluminescence spectroscopy and high resolution X-ray diffraction.It is found that the densities of screw and edge threading dislocations increase with the V/III ratio of the buffer layer,and the intensities of the yellow luminescence(YL) and blue luminescence(BL) emissions also increase dramatically.However,the density ratio of the edge threading dislocation to the screw threading dislocation remains invariant,as well as the intensity ratio of YL emission to BL emission.It can be concluded from these phenomena that the edge threading dislocation and screw threading dislocation can enhance the YL and BL emissions,respectively.  相似文献   

7.
杨杰  董全力  江兆潭  张杰 《中国物理 B》2010,19(12):127104-127104
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.  相似文献   

8.
Density functional molecular dynamics are used to study the melting behavior of single-walled SiC nanotubes. The melting of SiC nanotubes starts from the thermally activated Stone—Wales defects. The melting temperature is found to increase with the increasing diameter of nanotubes. The SiC nanotubes have a high melting temperature larger than 4000 K as the diameter larger than 1.0 nm, which indicates that the SiC nanotubes may be the best candidate of nanoscale electronic and optoelectronic devices under high temperatures.  相似文献   

9.
Heat conduction in single-walled carbon nanotubes(SWCNTs) has been investigated by using various methods, while less work has been focused on multi-walled carbon nanotubes(MWCNTs). The thermal conductivities of the double-walled carbon nanotubes(DWCNTs) with two different temperature control methods are studied by using molecular dynamics(MD) simulations. One case is that the heat baths(HBs) are imposed only on the outer wall, while the other is that the HBs are imposed on both the two walls. The results show that the ratio of the thermal conductivity of DWCNTs in the first case to that in the second case is inversely proportional to the ratio of the cross-sectional area of the DWCNT to that of its outer wall. In order to interpret the results and explore the heat conduction mechanisms, the inter-wall thermal transport of DWCNTs is simulated. Analyses of the temperature profiles of a DWCNT and its two walls in the two cases and the interwall thermal resistance show that in the first case heat is almost transported only along the outer wall, while in the second case a DWCNT behaves like parallel heat transport channels in which heat is transported along each wall independently.This gives a good explanation of our results and presents the heat conduction mechanisms of MWCNTs.  相似文献   

10.
The behavior of an extended dislocation near an elliptical blunt crack is investigated. The equilibrium separation between Shockley partials and the critical value of stacking fault energy for the formation of extended dislocations by dissociation reaction as well as the extended dislocation emission criterion are developed. The results show that the equilibrium separation increases as the extended dislocation tends to the blunt crack. If the stacking fault energy is comparable to the critical energy for dissociation in a perfect medium, complete dislocations can dissociate to form extended dislocations near the blunt crack. The critical stress intensity factor (SIF) for extended dislocation emission increases with the stacking fault energy and the curvature radius of the blunt crack tip. Moreover, the critical SIF for extended dislocation emission is far lower than the critical SIF for edge dislocation emission.  相似文献   

11.
朱亚波  鲍振  蔡存金  杨玉杰 《物理学报》2009,58(11):7833-7837
运用分子动力学方法具体模拟研究单个碳纳米管(CNTs)在加热过程中的结构变化.选择多组不同结构的单壁碳纳米管(SWCNTs)和双壁碳纳米管(DWCNTs)作为研究对象,加热温度从室温开始到4000 K,压强保持为1 atm.结果表明单壁碳管中手性型结构热稳定性最好,其次是扶手椅型和锯齿型,当手性角相同时,直径大的热稳定性更高;对于双壁碳管,研究表明当双壁中至少之一为手性结构时其热稳定好,而内外壁均为锯齿结构的稳定性最差,该结果进一步支持了有关单壁碳管的结论;还从理论上探索了描述结构热稳定性的方式,并在键层 关键词: 单壁碳纳米管 双壁碳纳米管 分子动力学方法 热稳定性能  相似文献   

12.
袁剑辉  袁晓博 《物理学报》2008,57(6):3666-3673
用分子动力学方法研究了端口接枝不同数量羟基对扶手椅型和锯齿型单壁碳纳米管弹性模量的影响.结果表明,未接枝的扶手椅型(5, 5),(10,10)管和锯齿型(9, 0),(18, 0)管杨氏模量分别为948,901和804,860GPa.在接枝2—8个羟基情况下,锯齿型单壁碳纳米管拉伸杨氏模量基本不随接枝数量增加发生变化,而扶手椅单壁碳纳米管则不同,接枝状态下的弹性模量比未接枝状态小很多,但接枝一定数量后,其杨氏模量又略增到某一稳定值.分别从接枝后碳纳米管变形电子密度等值线结构、C—C键长和系统结合能变化规律等方面,对单壁碳纳米管弹性模量的接枝效应进行了分析. 关键词: 碳纳米管 羟基 接枝效应 杨氏模量  相似文献   

13.
A low angle twist boundary formed by bonding an ultrathin (001) silicon film onto a (001) silicon wafer is investigated using two-beam transmission electron microscopy to identify positively zigzag lines which separate large interfacial regions formed by square networks of 1/2? 110 ? screw misfit dislocations. An approach to the elastic field of a zigzag line is proposed from the repetitive use of angular dislocations added to a ribbon-like uniform distribution of infinitesimal dislocations parallel to a family of pure screw misfit dislocations. Theoretical and experimental images of successive triple nodes are compared to derive the unique set of Burgers vectors attached to a zigzag line. In principle, this approach can be applied to any elongated hexagonal mesh of a dislocation network.  相似文献   

14.
Adsorption and desorption of an oxygen molecule on carbon nanotubes are investigated using density functional calculations. Several precursor states exist at the edge of armchair nanotubes, whereas an exothermic adsorption takes place at the edge of zigzag nanotubes. We also estimate desorption barriers of a CO molecule from nanotubes as well as fullerenes and amorphous phases. Our calculations suggest that carbon nanotubes can survive selectively during the oxidative etching process with a precise control of annealing temperature, in good agreement with experimental results of purification process of carbon nanotubes.  相似文献   

15.
The classical molecular dynamics method is employed to simulate the interaction of edge dislocations with interstitial Frank loops (2 and 5 nm in diameter) in the Fe-Ni10-Cr20 model alloy at the temperatures T = 300–900 K. The examined Frank loops are typical extended radiation-induced defects in austenitic steels adapted to nuclear reactors, while the chosen triple alloy (Fe-Ni10-Cr20) has the alloying element concentration maximally resembling these steels. The dislocation-defect interaction mechanisms are ascertained and classified, and their comparison with the previously published data concerning screw dislocations is carried out. The detachment stress needed for a dislocation to overcome the defect acting as an obstacle is calculated depending on the material temperature, defect size, and interaction geometry. It is revealed that edge dislocations more efficiently absorb small loops than screw ones. It is demonstrated that, in the case of small loops, the number of reactions accompanied by loop absorption increases with temperature upon interaction with both edge and screw dislocations. It is established that Frank loops are stronger obstacles to the movement of screw dislocations than to the movement of edge ones.  相似文献   

16.
碳纳米管中的群论及一系列新点群   总被引:3,自引:0,他引:3       下载免费PDF全文
杨铮  施毅  刘法  张荣  郑有炓 《物理学报》2004,53(12):4299-4302
在讨论了碳纳米管的几何结构的基础上,对齿型和椅型碳纳米管的对称性进行了分析并将这些对称元进行了抽象和总结.对齿型和椅型碳纳米管的对称元所属的群Dnh点群进行了讨论. 关键词: 点群 碳纳米管 几何结构 对称性  相似文献   

17.
Abstract

The dislocation substructures and mechanical properties of cubic ZrO2 single crystals are analyzed. The temperature dependence of the dislocation velocity is complex, the edge segments moving faster than screw segments at low temperatures, while the screws are faster at high temperatures. Using the loop shrinkage technique, the diffusion coefficient of the slowest diffusing species can be determined. The mechanical properties are analyzed in terms of the Peierls mechanism and of interaction of dislocations and point defects.  相似文献   

18.
We present first principles calculations of the electronic structure of small carbon nanotubes with different chiral angles theta and different diameters (d<1 nm). Results are obtained with a full potential method based on the density functional theory (DFT), with the local density approximation (LDA). We compare the band structure and density of states (DOS) of chiral nanotubes with those of zigzag and armchair tubes with similar diameters. The carbon K-edge energy loss near edge structures (ELNES) have been studied and pi* and sigma* contributions have been evaluated. These contributions give information on the degree of hybridization for the small chiral nanotubes.  相似文献   

19.
The deformation mechanism of zinc oxide (ZnO) nanotube has been first examined by molecular dynamics. The result demonstrated that ZnO nanotubes relax it excess strain via the phase transformation from an armchair structure to a fourfold-coordinated structure, then to a zigzag structure, which is started by a slip deformation. In contrast to carbon, silicon carbide, and boron nitride nanotubes, they relax it local stress via the transformation of the Stone?CWales transformation. After yielding, the 8-4 dislocation loops are found and the numbers of 8-4 dislocation loops grow up, which relax the tensile strain at the necking region and leads the work hardening. Finally, the nanotube is broken down by crack deformation at the interface between different phases.  相似文献   

20.
从能带理论出发,采用电子紧束缚能量色散关系,推导锯齿,扶手椅和手性单壁碳纳米管(SWCNT)的电子能带结构表达式,指出单壁碳纳米管或为金属或为半导体的判据。结果表示:单壁碳纳米管的电子结构与其几何结构密切相关,如扶手椅型单壁碳纳米管是金属性的,而对其它类型的单壁碳纳米管是与碳纳米管的手性指数有关,只有手性指数n和m的差别等于3的倍数时,单壁碳纳米管是金属性的,否则会显出有带隙的半导体特性。这意味着单壁碳纳米管是由特殊的电子传输和光学性质,在纳米电子学领域具有巨大的潜在应用价值。  相似文献   

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