碳纳米管阀门的分子动力学研究

以Lennard-Jones位能式与Brenner-Tersoff位能式为基础,经由分子动力学模拟,探讨流体分子与碳管间质、能传递的关系.首先在一(5,5)armchair碳管侧面,分别移除不同数目的碳原子,形成阀口(A_av=17.3~116.9Å2),进行模拟.结果显示,常用的自扩散行为在该环境下不足以完全说明物性,即在相同系统温度下,阀口的大小也会改变氢原子逸出速度V_b(Breakthrough velocity).为此,必须考虑麦克斯韦-波尔兹曼能量分布方程(Maxwell-Boltzmann energy distribution)修正,此外,原子释放率与阀口尺寸有明显的相依性.同时研究中亦发现,阀门不同几何尺寸引起位能障(Potential energy barrier)、功函数(Work function)以及能隙(Energy gap)的改变,进而影响粒子通过时流率、流速等动力行为.可利用该特性,作为控制原子、分子流动的纳米阀门、粒子分离或化学反应器等基础设计依据.     

氢、氮、铍、锂原子在缺陷富勒烯CX（x=58-62）中的吸附与渗透

碳富勒烯包合物由于潜在的应用前景受到广泛关注。我们采用第一性原理方法计算了氢、氮、锂、铍原子在不同碳富勒烯中的吸附和穿越。根据原子在不同碳富勒烯笼中的势能曲线,我们给出了原子穿越碳富勒烯笼的势垒和势阱,归纳出原子穿越碳笼机理分为插入机理、渗透机理以及插入机理和渗透机理的混合机理。     

氢、氮、铍、锂原子在缺陷富勒烯C_x(X=58～62)中的吸附与渗透

碳富勒烯包合物由于潜在的应用前景受到广泛关注.我们采用第一性原理方法计算了氢、氮、锂、铍原子在不同碳富勒烯中的吸附和穿越.根据原子在不同碳富勒烯笼中的势能曲线,我们给出了原子穿越碳富勒烯笼的势垒和势阱,归纳出原子穿越碳笼机理分为插入机理、渗透机理以及插入机理和渗透机理的混合机理.     

电场穿透对高温超导体场致发射电流的影响

高温超导体在场致发射时伴随正常态部分的空穴的产生 ,会出现非平衡现象。这会引起超导体电子平衡态的化学势偏移 ,增加外电场在高温超导体发射体内的穿透深度 ,使导带底弯曲影响超导体场致发射的电流。     

Field penetration and band bending near semiconductor surfaces in high electric fields

The existing theory of band bending in the surface space charge region of semiconductors is adapted to problems in field emission, field ionization and field evaporation. The surface field in the space charge layers of semiconductors appropriate for these phenomena ranges from ~10^?2 V/Å to a few 10^?1 V/Å, similar to those encountered in many interface phenomena involving semiconductors. We found that the surface potential resulting from band bending may amount to a few eV. The field penetrates ~10 Å into the semiconductor surface for intrinsic cases, and ~200Å for an n-type semiconductor in a positive field, or for a p-type semiconductor in a negative field. Both the surface potential and the field penetration will affect significantly the electronic properties of the near surface layers. In particular, the photon adsorption edge will be shifted toward the red by the field penetration effect.     

镜像势对碳纳米管阵列场发射特性的影响

采用多尺度量子化学方法模拟了碳纳米管阵列的场发射特性.碳纳米管镜像势的作用可以等效地用原子尺寸的理想金属球的镜像势来代替.模拟计算结果表明,考虑了镜像势作用后的碳纳米管阵列发射电流密度比没有考虑镜像势的结果增大了约6倍. 关键词： 镜像势 碳纳米管 场致发射     

Features of surface and volume magnetostatic waves in a tangentially magnetized slab

A special direction of the wave vector was found for volume magnetostatic waves (MSWs) that coincides with the cutoff angle for surface MSWs; when crossing this direction, the field configuration for MSWs with small wavenumbers changes sharply. It is shown that this direction corresponds to the maximum asymmetry of magnetostatic potential distribution over the film depth and the largest wave penetration depth into the film bulk from the surface. This effect occurs also for volume MSWs propagating in a wave channel.     

Specific features of the thermodynamic equilibrium state of a thin water film in an electric field of surface active centers of a mica crystal

The structural features of a thin polar liquid film in an electric field of a charged substrate have been theoretically investigated using methods of statistical physics. It has been established that electrical interactions in this system lead to a change in the structure of the polar liquid and to the formation of clusters in it. A self-consistent nonlinear equation has been derived for describing the distribution of the potential and strength of the electric field inside the film under investigation. The depth of penetration of this field into the liquid medium has been studied as a function of the electrical activity of the substrate, temperature, degree of polarity of the liquid, and concentration of interface heterocharges. It has been revealed that the extent of structured regions in the liquid increases with an increase in the potential gradient of the internal field and with a decrease in the temperature.     

Effect of the percentage of normal phase on the penetration of a magnetic field into textured superconducting metallic oxides of yttrium

The penetration of a magnetic field into textured high-temperature superconductors (HTSCs) based on yttrium is studied experimentally. Samples contain impregnations of normal phase (Y-211) in sizes of 1 to 30 micron. A pronounced effect of the percentage of the normal phase on the value of the penetration field is revealed: penetration occurs at lower values of the magnetic field upon a reduction in the percentage of Y₂BaCuO₅. Vortices are shown to penetrate the superconductor in the form of bundles.     

Magnetic flux penetration into high-temperature superconductors in the vortex liquid state under the blow-up conditions

The problem of magnetic field penetration into a type-II high-temperature superconductor that is in the weakly pinned vortex-liquid phase is considered. A magnetic field on the superconductor boundary rises with time in the blow-up regime. A model hydrodynamic equation describing the magnetic induction distribution in the vortex-liquid phase for thermomagnetic motion of the flux is derived. Analytical expressions for the depth and rate of magnetic field penetration into the superconductor are found. It is demonstrated that these quantities depend on parameters of the problem: index of power n in the boundary regime characterizing the penetration rate of vortices into the superconducting half-space and a parameter describing the effect of random pinning forces and thermal fluctuations on the magnetic flux distribution.     

等离子体融断开关磁场Hall渗透机制的模拟研究

 利用自行研制的2-1/2维全电磁柱坐标粒子模拟程序对等离子体融断开关磁场渗透机制进行了模拟研究。模拟结果表明在磁场Hall渗透机制特征长度远远小于等离子体离子的无碰撞趋肤深度的条件下,等离子体内部磁场渗透过程主要由电子流体运动的Hall项来控制。对于等离子体空间分布存在较大的密度梯度的物理问题,必须考虑二维空间特性对磁场渗透速度的影响。在磁场已渗透经过的等离子体区域中,等离子体呈现非电中性,离子受静电场的作用会加速运动到达阴极,最终形成真空鞘层。     

The probability of potential barrier penetration in the generalized wkB approximation

The approximation method, discussed in [1], of calculating the coefficients of penetration through one-dimensional potential barriers is unsuitable at small values of the energy. In the present work this insufficiency is removed by applying some additional conditions to the basic potential W(S). A new solution is given for the problem of particle penetration through a symmetric barrier.     

Electromagnetic response in kinetic energy driven cuprate superconductors: Linear response approach

Within the framework of the kinetic energy driven superconductivity, the electromagnetic response in cuprate superconductors is studied in the linear response approach. The kernel of the response function is evaluated and employed to calculate the local magnetic field profile, the magnetic field penetration depth, and the superfluid density, based on the specular reflection model for a purely transverse vector potential. It is shown that the low temperature magnetic field profile follows an exponential decay at the surface, while the magnetic field penetration depth depends linearly on temperature, except for the strong deviation from the linear characteristics at extremely low temperatures. The superfluid density is found to decrease linearly with decreasing doping concentration in the underdoped regime. The problem of gauge invariance is addressed and an approximation for the dressed current vertex, which does not violate local charge conservation is proposed and discussed.     

Field emission of carbon nanotubes and electronic structure of carbon nanopeapods

We calculate the electron emission from nanostructures under an applied electric field using the ab initio pseudopotential method. The transition rates of the electrons are calculated by integrating the time-dependent Schrödinger equation for the states initially inside the emitter. A localized basis set is used for obtaining the eigenstates before emission and the potential that drives the field emission. The calculated electron-tunneling graph is quite linear in the short-time region, giving the transition rate within the simulation time. We have applied this new method to the field emission of carbon nanotubes as a test. Then we calculate the electronic structure of the fullerenes encapsulated inside the carbon nanotubes, the so-called carbon “nanopeapods”. The fullerenes may or may not contain metallic atoms such as gadolinium or potassium. There is an interesting effect of strain when the diameter of the carbon nanotube is smaller than that of the inserted fullerene plus twice the van der Waals distance.     

Preparation and Biomedical Applications of Multicolor Carbon Dots: Recent Advances and Future Challenges

Multicolor carbon dots (CDs) as an emerging subclass of carbonaceous nanomaterials have inspired intensive attention due to the fascinating fluorogenic properties of quantum dots, exhibiting great potential applications in the biomedical field. In some cases, reported CDs with blue or green fluorescence are not desirable for further biological applications owing to the conflicting background autofluorescence, low penetration, and relatively large damage to biological tissue. However, multicolor CDs that can work in the longer wavelength region are being developed to address this issue by overcoming the autofluorescence from the cellular components (usually in the blue and green range). In this review, the development of multicolor CDs is described comprehensively, including for red-emissive or NIR-emissive CDs. Additionally, the preparation methods of multicolor CDs are summarized. Moreover, the factors affecting the luminescence of multicolor CDs are discussed in detail, and the biomedical applications are emphasized from multiple perspectives.     

Effect of vacancy defect on electrical properties of chiral single-walled carbon nanotube under external electrical field

Ab initio calculations demonstrated that the energy gap modulation of a chiral carbon nanotube with mono-vacancy defect can be achieved by applying a transverse electric field. The bandstructure of this defective carbon nanotube varying due to the external electric field is distinctly different from those of the perfect nanotube and defective zigzag nanotube. This variation in bandstructure strongly depends on not only the chirality of the nanotube and also the applied direction of the transverse electric field. A mechanism is proposed to explain the response of the local energy gap between the valence band maximum state and the local gap state under external electric field. Several potential applications of these phenomena are discussed.     

Nonequilibrium charge states of light ions penetrating through carbon films

The formation of helium, carbon, and nitrogen ion charge equilibrium upon their penetration through thin carbon films is examined. The ions under consideration have different initial charges and energies ranging from 0.3 to 3 MeV/nucleon. A semiempirical formula based on measured data is proposed for calculating the dependence of the mean nonequilibrium ion charge on the target thickness and estimating the carbon target thickness corresponding to the achievement of an equilibrium charge state.     

Mechanism of field electron emission from carbon nanotubes

Field electron emission (FE) is a quantum tunneling process in which electrons are injected from materials (usually metals) into a vacuum under the influence of an applied electric field. In order to obtain usable electron current, the conventional way is to increase the local field at the surface of an emitter. For a plane metal emitter with a typical work function of 5 eV, an applied field of over 1 000 V/μm is needed to obtain a significant current. The high working field (and/or the voltage between the electrodes) has been the bottleneck for many applications of the FE technique. Since the 1960s, enormous effort has been devoted to reduce the working macroscopic field (voltage). A widely adopted idea is to sharpen the emitters to get a large surface field enhancement. The materials of emitters should have good electronic conductivity, high melting points, good chemical inertness, and high mechanical stiffness. Carbon nanotubes (CNTs) are built with such needed properties. As a quasi-one-dimensional material, the CNT is expected to have a large surface field enhancement factor. The experiments have proved the excellent FE performance of CNTs. The turn-on field (the macroscopic field for obtaining a density of 10 μA/cm²) of CNT based emitters can be as low as 1 V/μm. However, this turn-on field is too good to be explained by conventional theory. There are other observations, such as the non-linear Fowler-Nordheim plot and multi-peaks field emission energy distribution spectra, indicating that the field enhancement is not the only story in the FE of CNTs. Since the discovery of CNTs, people have employed more serious quantum mechanical methods, including the electronic band theory, tight-binding theory, scattering theory and density function theory, to investigate FE of CNTs. A few theoretical models have been developed at the same time. The multi-walled carbon nanotubes (MWCNTs) should be assembled with a sharp metal needle of nano-scale radius, for which the FE mechanism is more or less clear. Although MWCNTs are more common in present FE applications, the single-walled carbon nanotubes (SWCNTs) are more interesting in the theoretical point of view since the SWCNTs have unique atomic structures and electronic properties. It would be very interesting if people can predict the behavior of the well-defined SWCNTs quantitatively (for MWCNTs, this is currently impossible). The FE as a tunneling process is sensitive to the apex-vacuum potential barrier of CNTs. On the other hand, the barrier could be significantly altered by the redistribution of excessive charges in the micrometer long SWCNTs, which have only one layer of carbon atoms. Therefore, the conventional theories based upon the hypothesis of fixed potential (work function) would not be valid in this quasi-one-dimensional system. In this review, we shall focus on the mechanism that would be responsible for the superior field emission characteristics of CNTs. We shall introduce a multi-scale simulation algorithm that deals with the entire carbon nanotube as well as the substrate as a whole. The simulation for (5, 5) capped SWCNTs with lengths in the order of micrometers is given as an example. The results show that the field dependence of the apex-vacuum electron potential barrier of a long carbon nanotube is a more pronounced effect, besides the local field enhancement phenomenon.     

Large-eddy simulation of equilibrium plasma-assisted combustion in supersonic flow

A large-eddy simulation (LES) model with a new localized dynamic subgrid closure for the magnetohydrodynamics (MHD) equations is used to investigate plasma-assisted combustion in supersonic flow. A 16-species and 74-reactions kinetics model is used to simulate hydrogen-air combustion and high-temperature air dissociation. The numerical model is validated with experimental data for non-reacting and reacting supersonic flow over a rearward-facing step. The creation of a plasma source near the step corner is shown to have a strong localized effect with the high temperature region resulting in an increase of the radical species concentration in the mixing region. This has the potential for enhancing combustion. In addition, downstream fuel–air mixing is improved, primarily by the creation of a strong baroclinic torque effect in the near field of the plasma source. Furthermore, by adding an uniform external magnetic field, the Lorentz force effect helps to further enhance mixing by lifting the shear layer and increasing fuel penetration by approximately 20%.