大电流碳纳米管场发射阴极研究

报道了在较大发射面积上获得较大场发射电流的碳纳米管场发射阴极。为了加强场发射电流,在丝网印刷浆料中增加一种金属纳米颗粒,金属颗粒增强了碳纳米管发射体和衬底的接触,提高碳纳米管和衬底的粘附作用。利用改进后的丝网印刷方法制备了大电流碳纳米管场发射阴极,测得最大发射电流为68.0 mA,阴极有效发射面积约1.1 mm2,发射电流密度约6.2 A/cm2;并成功将改进方法制备的大电流场发射碳纳米管阴极应用于场发射真空器件原型。实验证明这种具有较大发射电流和较大发射电流密度的场发射能够满足部分大功率电子器件的需求。收稿日期:; 修订日期:     

A cytosine-assisted carbon nanotubes junction: DFT studies

Since nucleobase-functionalized carbon nanotubes (CNTs) are important in the biological applications; the junction of a pair of CNTs through a bridging cytosine linkage is investigated based on density functional theory (DFT) calculations. In the exact model of study, the CNTs are bound to N₁ and C₅ atomic sites of cytosine to make possible the CNT–cytosine–CNT model. To systematically investigate the purpose, the models of original CNT, original cytosine, and primary models of cytosine–CNT in which one CNT is only bound to N₁ or C₅ atomic site of cytosine are also considered. The results of dipole moments and binding energies indicated that the CNT–cytosine–CNT model is the most stable one among all three possible models cytosine-functionalized CNT. The values of energy gaps indicated that the conducting properties of primary cytosine–CNT models are not changed referring to the original CNT but better conductivity could be observed for the CNT–cytosine–CNT model. The values of evaluated quadrupole coupling constants indicated that the electronic densities of nitrogen and oxygen atoms of cytosine detect notable affects during the functionalization processes by the zigzag CNTs and the oxygen atom of CNT–cytosine–CNT model could be proposed as the most proper interacting site of cytosine among other functionalized zigzag models and also the original cytosine. However, the changes of quadrupole coupling constants for the atoms of cytosine are almost negligible during the functionalization processes by the armchair CNTs.     

The feasibility of producing MWCNT paper and strong MWCNT film from VACNT array

This study sought to produce carbon nanotube (CNT) pulp out of extremely long, vertically aligned CNT arrays as raw materials. After high-speed shearing and mixing nitric acid and sulfuric acid, which served as the treatment, the researchers produced the desired pulp, which was further transformed into CNT paper by a common filtration process. The paper’s tensile strength, Young’s modulus and electrical conductivity were 7.5 MPa, 785 MPa and 1.0×10⁴ S/m, respectively, when the temperature of the acid treatment was at 110°C. Apart from this, the researchers also improved the mechanical property of CNT paper by polymers. The CNT paper was soaked in polyethylene oxide, polyvinyl pyrrolidone, and polyvinyl alcohol (PVA) solution, eventually making the CNT/PVA film show its mechanical properties, which increased, while its electrical conductivity decreased. To diffuse the polymer into the CNT paper thoroughly, the researchers used vacuum filtration to fabricate a CNT/PVA film by penetrating PVA into the CNT paper. After a ten-hour filtration, the tensile strength and Young’s modulus of CNT/PVA film were 96.1 MPa and 6.23 GPa, respectively, which show an increase by factors of 12 and 7, respectively, although the material’s electrical conductivity was lowered to 0.16×10⁴ S/m.     

Computer simulations in the study of gold nanowires: the effect of impurities

Suspended gold nanowires have recently been made in an ultra-high vacuum ambient and were imaged by electron microscopy. Two puzzles were presented: one atom thick wires are produced and some of the atomic distances between these atoms before their breaking were too large. Simulations using realistic molecular dynamics method were able to unveil some processes to explain the mechanisms of formation, evolution, and breaking of these atomically thin Au nanowires under stress. The calculations showed how defects induce the formation of constrictions that eventually will form the one-atom chains. Atomically thin chains, five atoms long were obtained, before breaking. The results were in excellent agreement with experimental results except for the large Au-Au distances. In fact no theoretical calculation of pure gold nanowires have been able to produce such large distances. Light impurities that cannot be imaged in these experiments may be responsible for these large Au-Au distances. Using ab initio total energy calculations based on the density functional theory, we have studied the effect of H, C, O, N, B, S, CH, CH₂, and H₂ impurities on the nanowire’s electronic and structural properties, in particular how they affect the rupture of the nanowire. We find that the impurities tend to locally increase the nanowire’s strength, in such a way that its rupture always occurs at an Au-Au bond and never at an Au-X bond (X being an impurity). In particular, oxygen seems to form very stable bonds that may be used to pull longer Au chains. Regarding the observed large Au-Au bond lengths, it was found, based on quasi-static calculations, that the best candidate to explain the large distances is H. However, some particular experimental conditions may lead to different results. PACS 68.65.-k; 68.37.Lp; 71.15.Pd     

Electron field emission properties of highly dense carbon nanotube arrays

In this paper, we have studied field emission properties of highly dense arrays of multi-walled carbon nanotubes (CNTs) used as cathodes in diode-type field emission devices with a phosphor screen. For the high-density CNT emitters it is demonstrated that the emission sites are located on the CNT-cathode edges, which is direct experimental evidence of the ‘edge effect’. The results of computer simulations (using ‘ANSYS Electromagnetic’ software) are presented to confirm the experimental data and to analyze the effect of patterning on the electric field distribution for high-density CNT arrays. It is shown that selective-area removal of nanotubes in the arrays leads to the formation of additional edges characterized by the high field enhancement factor and enhanced emission from the CNT cathodes. In addition, scanning probe microscopy techniques are employed to examine surface properties of the high-density CNT arrays. For CNT arrays of ‘short’ nanotubes, the work function distribution over the sample surface is detected using a scanning Kelvin microscopy method.     

Stable C-atom displacements on HOPG surface under plasma low-energy argon-ion bombardment

Surface defects are generated by an Ar plasma on crystalline graphite. In situ scanning tunneling microscopy reveals localized defects surrounded by a R30° superstructure (Friedels charge oscillations) for short treatment times and long-range disordered lattices for longer treatment times. In situ X-ray photoelectron spectroscopy C1s core-level spectra exhibit a broadening attributed to a distribution of C atoms in inequivalent sites. As implantation of heteroatoms and formation of vacancies can be ruled out, defects are attributed to stable displaced surface carbon atoms. PACS 81.65.Cf; 68.35.Dv; 68.37.Ef; 79.60.Ht     

A new method for the analysis of transmission property in carbon nanotubes using Green’s function

A new method for the analysis of electron transmission property in single-walled carbon nanotubes (SWCNTs) using Green’s function is presented in this paper for the first time. Using the proposed method, a new relation for the transmission function through a deformed SWCNT is obtained, which depends on the energy variations and the coupling matrices related to the mechanical deformations applied to the structure of CNT. The obtained new relation is explained by the presented results in the literature.     

Inhomogeneous strains in semiconducting nanostructures

We have developed a numerical technique for calculating inhomogeneous strains in stressed semiconducting nanostructures, such as quantum wires and dots manufactured by nanolithography from stressed InGaAs/GaAs quantum wells. The technique is based on solving a linear problem of elasticity theory by the Green’s function method and presumes a lack of defects and dislocations in nanostructure heterojunctions. Spatial distributions of strain tensor components and shifts of electron and hole potentials in a nanostructure due to the strain have been calculated. Zh. éksp. Teor. Fiz. 115, 1906–1914 (May 1999)     

Effect of metallic surface microroughness on the probability and spectrum of plasmon excitation by a fast charged particle moving parallel to the surface

The Green’s function of the electric field of plasmons is determined in a semi-infinite medium with an abrupt plasma boundary where nonequilibrium conduction electrons either undergo elastic reflection from the boundary or “stick” to it and give rise to a stationary surface charge. The angular reflection of elastically scattered electrons can be either specular or diffuse. The Green’s function is used to find the singleevent spectrum of energy loss by a fast electron moving parallel to the boundary. The effect of electronboundary scattering parameters on the structure of bulk and surface plasmon resonances is analyzed. The probability of transition radiation of bulk plasmon by an electron moving in vacuum is examined. A new type of surface resonance is found under conditions of perfectly elastic scattering of conduction electrons from the plasma boundary, similar in structure to a tangential surface plasmon.     

Orientation imaging microscopy in two-dimensional crystals via undersampled microscopy

A novel microscopy analysis technique is presented, with applications in imaging two-dimensional grains and grain boundaries. The method allows the identification of grain shapes and orientations from large area micrographs, via the moiré pattern obtained in a raster image. The observed moiré pattern originates from the aliasing between a micrographs regular sampling raster and the inherent periodicity of the elements forming the grain under study. The technique presented is very general, allowing grain analysis via many types of microscopy. We demonstrate it in this paper by using Tapping Mode Atomic Force Microscopy and Scanning Electron Microscopy on diblock copolymer thin films. PACS 68.37.Ps; 68.37.Hk; 68.37.Ef; 68.55.Jk; 61.72.Ff; 61.72.Mm     

Carbon nanotubes as nanoparticles collector

This communication reports on a new method for the collection of nanoparticles using carbon nanotubes (CNT) as collecting surfaces, by which the problem of agglomeration of nanoparticles can be circumvented. CNT (10–50 nm in diameter, 1–10 μm in length) were grown by thermal CVD at 923 K in a 7 v/v% C₂H₂ in N₂ mixture on electroless nickel-plated copper transmission electron microscopy (TEM) grids and Monel coupons. These samples were then placed downstream of an arc plasma reactor to collect individual copper nanoparticles (5–30 nm in diameter). It was observed that the Cu nanoparticles preferentially adhere onto CNT and that the macro-particles (diameter >1 μm), a usual co-product obtained with metal nanoparticles in the arc plasma synthesis, are not collected. Cu–Ni nanoparticles, a catalyst for CNT growth, were deposited on CNT to grow multibranched CNT. CNT-embedded thin films were produced by re-melting the deposited nanoparticles.     

Study of J-T effect on the self-energy of electrons in manganite systems

We present here a theoretical study of the effect of Jahn-Teller(J-T) distortion on the self-energy of electrons in the CMR manganites. The model consists of the itinerant e _g electrons distorted by J-T effect and the localized t _2g core electrons carrying strong ferromagnetism due to Hund’s rule. The phonon interacts with the e _g electrons as well as the J-T distorted e _g band. The electron Green’s functions are calculated by Zubarev’s technique. The electron self-energy which carries all the information of the model is calculated from the Green’s function. The effect of J-T distortion, magnetism on the frequency and temperature dependent dynamic self-energy is presented in this paper. The results are discussed.     

Novel unconventional inorganic nanowires: fabrication, structural analysis and electrical property evaluation

The authors’ endeavors in recent years to synthesize novel inorganic semiconducting nanowires, to analyze their atomic structures using state-of-the-art electron microscopy facilities and techniques, and to evaluate electrical properties are highlighted. Along with a general survey of nanostructures prepared and thoroughly characterized in the laboratory, particular emphasis is placed on analysis of non-standard nanowires in the Si-ZnS and B-C-N inorganic systems. Si-ZnS biaxial nanowire heterojunctions are found to be n-type semiconductors, due primarily to a lower resistance Si-path. Fractal-like B-C-N nanofibers display high-resistivity I-V curves arising from enrichment of the nanostructure periphery by the insulating BN-rich phase, as revealed by spatially-resolved chemical mapping during energy-filtering electron microscopy. PACS 61.46.w; 68.37.Lp; 68.55.Jk; 64.70.Nd; 72.15.Eb; 78.20.-e     

STM/STS investigation of carbon nanotube junctions

In this paper we present scanning tunneling microscopy/spectroscopy investigations of multiwall carbon nanotube junctions. We concentrated on bent and narrowing junctions, which my be formed by introducing pentagon–heptagon defects into a hexagonal network of a carbon nanotube. It was expected that the defects introduced to the nanotube could cause changes in the local density of states. The scanning tunneling spectroscopy results were used to search for and identify these defects. We also discuss a hypothesis for a combination of a telescope junction and a pentagon–heptagon induced junction. PACS 68.37.Ef; 73.22.–f; 61.46.Fg; 71.20.Tx     

Effective defect detection in thin film transistor liquid crystal display images using adaptive multi-level defect detection and probability density function

A new automated inspection algorithm is proposed for detecting critical defects based on adaptive multi-level defect detection and probability density function in thin film transistor liquid crystal display (TFT-LCD) images containing a background region’s non-uniform and random noises. To improve the detecting capability for a critical-defect-detecting algorithm, the background region’s non-uniformity is eliminated using statistical values such as the mean and standard deviation of a test image. For the defect detection, the candidate defects are collected on each detection level and used to find a probability density function based on Parzen-window technique. Through simulation it was verified that the proposed method has superior capability for detecting critical defects which results in smaller brightness difference between a defect and its neighbors.     

Vacancy cluster-limited electronic transport in metallic carbon nanotube

We investigate the electronic properties of metallic (7,7) carbon nanotubes (CNT) in the presence of a variety of tetra- and hexa-vacancy defects, by using the first principles density functional theory (DFT) combined with the non-equilibrium Green’s function technique. From the view point of energetic stability large vacancies tend to split into pentagon and heptagon (5-7) defects. However, this does not preclude the presence of “holes” in the carbon nanotube by the nanoelectronic lithography technique. We show that the states linked to large vacancies hybridize with the extended states of the nanotubes to modify their band structure. As a consequence, the hole-like defects in the CNT lead to more prominent electronic transport compared to the situation in the defective CNT consisting of pentagon-heptagon pair defects. Our study suggests the possibility to improve the electronic properties of a defective carbon nanotube via morphological modifications induced by irradiation techniques.     

STEM (scanning transmission electron microscopy) analysis of femtosecond laser pulse induced damage to bulk silicon

This work reports on the structural changes that take place in wafer grade silicon when it is micro-machined with ultra-short laser pulses of 150 fs duration. A Chirped Pulse Amplification (CPA) Ti:Sapphire laser was used, with an operating wavelength centered on 775 nm and a maximum repetition rate of 1 KHz. The laser induced damage was characterized over the fluence range 0.43–14 Jcm^-2, and for each fluence a progressively increasing number of pulses was used. The analytical tools used to characterize the samples were all based upon electron microscopy. A 30 KeV scanning transmission electron microscope (STEM) imaging technique was developed to observe defects in the crystal lattice and the thermal-mechanical damage in the area surrounding the laser machined region. Mechanical cross sectioning (in conjunction with Scanning Electron Microscope (SEM) surface imaging) was also used to reveal the internal structure, composition, and dimensions of the laser machined structures. Based on this analysis, it will be shown that laser machining of silicon with femtosecond pulses can produce features with minimal thermal damage, although lattice damage created by mechanical stresses and the deposition of ablated material both limit the extent to which this can be achieved, particularly with high aspect ratios. A key feature of the work presented here is the high-resolution STEM images of the laser-machined structures. PACS  42.65.Re; 42.62.Cf; 61.80.Ba; 61.82.Fk; 68.37.Hk; 68.37.Lp     

First-time observation of Mastro Giorgio masterpieces by means of non-destructive techniques

For the first time some excellent pieces belonging to the majolica production of the great master Giorgio Andreoli from Gubbio (Central Italy) have been characterized from a chemical and structural point of view with the aim to identify the composition of both pigments and lustres. A series of particle-induced X-ray emission (PIXE), Rutherford backscattering spectrometry (RBS) and Raman analyses have been performed on some plates coming from Museo del Palazzo dei Consoli (Gubbio) and several French museums (Louvre, Musée National de la Céramique, Musée National de la Renaissance) lustred by Giorgio Andreoli and decorated by famous majolica painters such as Francesco Xanto Avelli. The three techniques are complementary and useful in the investigation of art objects since they are non-destructive. Furthermore, the low detection limits allow the identification of all elements and compounds present, and RBS allows concentration profiling, too. It is worth noticing that the examined objects are characterized by the presence of both gold and ruby-red lustres, a peculiarity of Mastro Giorgio’s technique. The measurements by PIXE and RBS have been carried out on the AGLAE accelerator at C2RMF, Louvre Palace. PACS 78.67.Bf; 81.05.Je; 68.37.Lp; 68.37.Hk; 68.90.+g     

Effect of electron inertia on large amplitude solitary waves in presence of kinematic viscosity in dusty plasma

Nonlinear dust acoustic solitary waves in a dusty plasma are studied for nonzero kinematic viscosity. Sagdeev’s potential can be obtain upto any order in ϕ. The existence of soliton solution is determined by pseudopotential approach. It is seen that the electron inertia has a significant effect on the existence of solitary waves in presence of kinematic viscosity.