Low-threshold field electron emission from carbon nanoclusters formed upon cold destruction of graphite

The field-emission characteristics of carbon nanoclusters (graphenes, nanotubes, their compositions with microdiamonds) produced by the cold destruction of natural graphite are studied. The structure of a coating of carbon nanoclusters on a tungsten cathode is examined by field emission microscopy, transmission electron microscopy, and electron diffraction. The high-intensity stable field emission of these clusters is shown to be characterized by a low field threshold. The mechanism of the low-threshold emission from carbon nanoclusters is discussed.     

Limitation of the current from nanographite multiemitter field-emission cathodes

The limiting field-emission current of a nanographite film cathode is studied experimentally. The maximal field-emission current density reaches 1 A/cm². Experimental data agree with numerical estimates based on the general theory of field emission with regard to features inherent in nanostructured carbon materials.     

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:     

碳纳米管冷阴极场致发射中栅网特性

利用三维粒子模拟软件对大面积生长无序碳纳米管冷阴级三级结构发射特性进行仿真,对六边形、三角形和正方形3种不同网孔结构栅网下的碳纳米管冷阴极场致发射特性进行研究。通过计算冷阴极表面电场分布及场致发射电子注的通过率,得到不同图形发射效率随网孔尺寸变化的规律。进一步横向对比,得出栅网最佳图形为六边形的结论,其最大电子注通过率为75%。     

Emissivity of a multibeam electron gun with a glassy carbon field-emission cathode

A multibeam triode electron gun with a glassy carbon field-emission cathode that is intended for an O-type microwave amplifier is studied. The electric field strength and the current density at the microtips versus the distance to the center of a cell of the cathode–grid unit are calculated. Calculation data are compared with experimental results. It is shown that about 70% of the cathode current in each cell is accounted for by microtips arranged in a circumferential ring no wider than 20 μm. The field-emission current density inside the ring exceeds 40 A/cm², and the current per microtip equals 43.1 μA.     

单根准直碳纳米纤维的场发射特性

采用等离子体增强热灯丝化学气相沉积方法，以甲烷和氢气为反应气体，在钨丝衬底上制备出准直的碳纳米纤维(CNFs)，其生长密度小于10.6cm-2，长度为6—30μm，直径为60—100nm.并采用自制的双探针扫描电子显微镜系统，对所生长的单根CNF作了场发射特性研究.结果表明，其场发射开启电压约为5V/μm，相应的发射电流达到20μA/cm2，同时，对不同长度的CNFs及单根CNF不同位置的场发射研究表明，场发射电流的大小不仅与材料本身的功函数、外电场场强、材料的微观结构以及宏观的几何结构有关，而且电子在输运过程中所受到的散射也是决定场发射电流大小的关键因素. 关键词： 碳纳米纤维 化学气相沉积 场发射 扫描电子显微镜     

Energy distributions of electrons emitted from tungsten tips covered by diamond-like films

The energy distribution of electrons emitted from the surface of diamond-like pointed cathodes under the action of a high electric field is reported. Diamond-like coatings are applied on thin tungsten tips by ion-beam evaporation in an ultrahigh vacuum. The structure of the carboniferous films covering the tungsten tips is examined by field-emission microscopy. The stability of the field-emission cathode current is considered, and the Fowler-Nordheim I-V characteristics are presented. Based on the results obtained, a model of field-emission cathode covered by a thin diamond-like coating that explains the energy distributions is suggested.     

Doping of graphite by an alkaline-earth metal to reduce the work function

A technique for reducing the work function of a field-emission graphite cathode by doping it by an alkaline-earth metal (barium) is suggested. A model of formation of a barium monolayer on the cathode surface is proposed. Field-emission tests show that the operating voltage of the doped cathode is lower than that of the undoped one with the same emission current.     

Study of the emission properties of a W-Cs3Sb field-emission cathode

A new field-emission cathode based on the W-Cs₂Sb metal-semiconductor system is produced. The emission current of this cathode is one and one-half orders of magnitude higher than that of a tungsten cathode, and the FWHM of the energy spectrum is about 0.3 eV. An energy model of the given emitter is proposed. Using it, one can obtain theoretical values of the main cathode parameters which are in good agreement with the experimental ones.     

Fractal nanostructures arising under the ablation of graphite and silicon by millisecond-laser radiation

A mechanism for the formation of fractal nanostructures with micro- and macroscopic sizes under 10-ms laser-pulse irradiation of carbon, silicon, and their mixtures is investigated. The appearance of linear fragments containing nanoclusters is considered based on the diffusion-limited aggregation model with the dipole-dipole interaction. It is shown that the competition between the thermophoretic and gradient forces acting upon particles and their aggregates in a plume determines the area of congestion and the formation of the macroscopical fractal nanostructures.     

Calculation of a triode field-emission system with a modulator

The electrostatic potential of a triode field-emission system with a modulator that represents a circular diaphragm is calculated. A tip field-emission cathode serves as an emitter. The inner part of the system under study is filled with two insulators. In the calculation of the potential distribution, a real field-emission cathode is changed by a virtual cathode whose surface coincides with an equipotential surface, so that the effect of the cathode on the distribution of the electrostatic potential is simulated as the effect of a finite charged wire that is located at the axis of the system. The separation of variables is used to solve the problem. The potential is represented as an expansion in terms of eigenfunctions, and the expansion coefficients are found from the solution to a system of linear algebraic equations.     

Optimization of the parameters of a carbon nanotube-based field-emission cathode

A procedure for optimizing a field-emission cathode based on carbon nanotubes (CNTs) is developed. An array of identical equidistant vertical CNTs is considered. The optimization procedure takes into account the effect of screening of an electric field by neighboring nanotubes by solving a Laplace equation and the thermal instability of nanotubes, which limits the emission current density of a nanotube, by solving a heat conduction equation. The relation between the emission current and the applied voltage is described by the Fowler-Nordheim relationship containing the CNT tip temperature as a parameter. Upon optimization, the optimum distance between CNTs that ensures the maximum emission current density is calculated. The calculation results demonstrate that this parameter depends substantially on both the applied voltage and the nanotube geometry. These dependences are weakly sensitive to the choice of the transport coefficients (thermal conductivity, electrical conductivity) of nanotubes.     

The use of carbonaceous materials as field-emission cathodes

The advantages and disadvantages of carbon fibers and graphite plates with a developed surface as field-emission cathode materials are discussed. Experimental data for the chemical composition of the materials and the effect of thermal annealing on their structure and emission properties are presented. A correlation between the work function and the amount of cesium implant is studied. The feasibility of preparing planar cold cathodes with a developed surface by means of radiation technologies is considered, and the evolution of the emitting surface during bombardment by low-energy residual gas ions is traced. Cold cathode designs for various applications are recommended.     

On the current state of field-emission electronics

The current state of field-emission electronics is reviewed and the basic types of field-emission cathodes (FECs) are analyzed (the results are presented in the form of diagrams). Special attention is paid to FECs made of carbon materials, which, in our opinion, are the most promising direction in the evolution of field-emission electronics. FEC utilization in modern electronic devices is illustrated by several examples. The main sections of the paper are devoted to analyzing the problems and prospects of FECs and field-emission electronics.     

Electronic structure and field-emission characteristics of open-ended single-walled carbon nanotubes

The field-emission mechanism of open-ended single-walled carbon nanotubes (SWNTs) is studied. Owing to electronic effects that directly alter the bonding mode and remarkably influence the work function, an open-ended SWNT has much better field-emission properties than a closed SWNT; owing to geometrical effects that slightly influence the work function and the amplification factor, an open-ended SWNT with relaxation has higher threshold voltage and higher current density compared to one without relaxation. It is suggested that adjusting the localized electronic states of the emitting regions, by electronic and geometrical means, could improve the field-emission properties of carbon nanotubes.     

Field-emission properties of diamond-like-carbon and nitrogen-doped diamond-like-carbon films prepared by electrochemical deposition

Field-emission studies were carried out on diamond-like-carbon (DLC) and nitrogen-doped diamond-like-carbon (N-DLC) films deposited by an electro-deposition technique. The films were deposited on Si(100) substrates by electrolysis of methanol–urea solution under high voltage, at atmospheric pressure, and low temperature. The microstructure and morphology of the resulting films were analyzed by means of Raman spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. The field-emission measurements were carried out with a parallel plate configuration using the deposited DLC and N-DLC films as the cathode and indium tin oxide coated glass as the anode. The field-emission measurements indicated that the electro-deposited films exhibited good field-emission properties and the nitrogen doping could increase the current density. These behaviors were demonstrated to mainly relate to changes in the microstructure of the samples, in connection with the difference of surface morphology. PACS 79.70.+q; 81.05.Uw; 81.15.Pq     

Clusters of sulfur and selenium in nanoporous carbon

The results of x-ray diffraction, x-ray photoelectron spectroscopy, electrical, and galvanomagnetic studies of nanoporous carbon with sulfur and selenium nanoclusters introduced into pores are analyzed. Conclusions are drawn concerning the short-range order in the introduced clusters and their interaction with carbon.     

Effect of adsorbates on field emission from carbon nanotubes

Recent experiments indicate that water molecules adsorbed on carbon nanotube tips significantly enhance field-emission current. Through first-principles density-functional theory calculations we show that the water-nanotube interaction is weak in zero electric field. However, under emission conditions large electric field present at the tube tip: (a) increases the binding energy appreciably, thereby stabilizing the adsorbate; and (b) lowers the ionization potential (IP), thereby making it easier to extract electrons. Lowering of IP is enhanced further through the formation of a water cluster on the nanotube tip.     

Electric field enhancement in field-emission cathodes based on carbon nanotubes

The problem of determining the field enhancement factor in field-emission cathodes based on carbon nanotubes (CNTs) is considered. The electrostatic problem of finding the field enhancement factor for nanotubes with different shapes of the tip as a function of the angle the nanotube makes with the cathode surface and of the interelectrode spacing is solved. The dependence of the electric field enhancement factor on the spacing between vertically oriented nanotubes constituting an array is derived. Making allowance for this dependence gives an optimal value of the surface density of nanotubes in the array at which the emission current density is maximal. The I—V characteristic of CNT-based cathodes is studied with regard to the statistical straggling of their orientation angles. This I—V characteristic is compared with the characteristic obtained with regard to the statistical straggling of the CNT geometrical parameters.     

Injection of Intense Relativistic Electron Beams into a Toroidal Geometry

Two experimentally verified methods are presented for the injection of high-current relativistic electron beams (30 kA, 400 kV) into strong toroidal magnetic fields (1-8 kGauss). Injection is accomplished by contouring the current flow in the stalk of a field-emission cathode to perturb the flux lines only during beam generation. On conclusion of the emission process the perturbed lines can return to their original positions thereby trapping the beam.