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.     

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.     

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.     

XPS study on the evaporation of gold submonolayers on carbon surfaces

We investigated the early nucleation stages of evaporated gold submonolayers on different carbon surfaces (pristine HOPG, argon-ion irradiated HOPG and amorphous carbon). Gold core-level and valence band spectra were measured by monochromatised X-ray photoelectron spectroscopy (MXPS). The Au 4f spectra for the lowest coverages (0.1 Å equivalent thickness) on irradiated HOPG and amorphous carbon surprisingly exhibited two well-separated doublets. We attribute this phenomenon to a bimodal particle size distribution caused by gold atom pinning at carbon defect sites. Deposition at elevated temperatures (on irradiated HOPG) opens a possibility to grow particles preferentially on defect sites. The influence of carbon surface defects on the cluster morphology was checked by SEM imaging. These results are interesting for future applications as they help to improve control over metal nanodots growth.     

Effect of the glassy carbon structure on the aspect ratio of micropoints of matrix field-emission cathodes prepared by thermochemical etching

The application of thermochemical etching technology makes it possible to reveal and investigate the structure of SU-2000 glassy carbon using electron microscopy. The glassy carbon structure at the microscopic and nanoscopic levels is inhomogeneous and consists of pockets with an irregular cross section separated by partitions. This structure sets the limits on the aspect ratio of geometrical sizes and micropoint packing density in the matrix prepared by thermochemical etching.     

XPS and NEXAFS studies of aliphatic and aromatic amine species on functionalized surfaces

The chemical constitution of functionalized supports is an important parameter that determines their performance in a broad range of applications, e.g. for immobilization of biomolecules. Supports with amino functionalized surfaces are also often used for DNA microarray experiments. However, spectral data which have been reported for surfaces with amino functionalities suffer from some inconsistencies. In this article a detailed XPS (X-ray photoelectron spectroscopy) and NEXAFS (Near edge X-ray absorption fine structure) database for amino functionalized surfaces is presented. Amino-terminated surfaces prepared from aliphatic and aromatic aminosilanes or aminothiols and a field sample are considered. Effects of aging in air and damage by radiation are addressed as well.     

XPS on impregnated cathodes: Surface concentrations and thermal stability

Photoelectron spectra are presented for S, M and scandate cathodes before and after the activation procedure. Using the calculatory method and the Ba deposition method the Ba coverages for the S, M and Sc cathodes have been established as 4.0×10¹⁴, 5.8×10¹⁴ and 5.8×10¹⁴ Ba atoms cm^-2 respectively. On the Os-Ru cathode an enhancement of the Ru concentration is found which may be due to segregation. Further a short investigation was made into the thermal stability of the Ba overlayer and its regeneration. It appears that after a heating cycle to 1200 °C the S and M cathodes recover very well whereas the Sc cathode suffers from the loss of Sc which is inherently connected with the fine emission properties of this cathode.     

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:     

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:     

Processing and identification of field-emission images of metallic surfaces

Techniques and algorithms for recording and processing field-emission images have been proposed. An original method of identifying individual crystal faces and directions in the surface structure based on fieldemission images and information on the emitter lattice type is described. The results may be used for detailed investigation of field electron emission under various impacts on the metallic surface.     

Directed assembly of nanostructured carbon materials on to patterned polymer surfaces

Directed assembly of single-walled carbon nanomaterials on to polymer surfaces has been achieved. The approach relies on selective interactions of the polymer functionalities with the surface structures present on the carbon materials. The successful immobilization of the carbon structures was confirmed by scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. By generating patterned polymer surfaces with chemically distinct components through the control of polymer–polymer or polymer–substrate interactions, directed assembly of single-walled carbon nanohorns and single-walled nanotubes was demonstrated. This new type of carbon assembly might open up new avenues in the construction of functional polymer/carbon composites and flexible nanocarbon nano-electronics.     

Surface characterisation and interface studies of high-k materials by XPS and TOF-SIMS

High-k dielectric LaAlO₃ (LAO) films on Si(100) were studied by TOF-SIMS and XPS to look for diffusion processes during deposition and additional thermal treatment and for the formation and composition of possible interfacial layers. The measurements reveal the existence of SiO₂ at the LAO/Si interface. Thermal treatment strengthens this effect indicating a segregation of Si. However, thin LAO layers show no interfacial SiO₂ but the formation of a La-Al-Si-O compound. In addition, Pt diffusion from the top coating into the LAO layers occurs. Within the LAO layer C is the most abundant contamination (10²¹ at/cm³). Its relatively high concentration could influence electric characteristics. XPS shows that CO₃²⁻ is intrinsic to the LAO layer and is due to the adsorption of CO₂ of the residual gas in the deposition chamber.     

Surface energy and hybridization studies of amorphous carbon surfaces

Surface properties of a large number of amorphous carbon (a-C) films have been investigated using contact angle measurements and X-ray photoelectron spectroscopy (XPS). Dense a-C surfaces with variable sp³/(sp² + sp³) average hybridization were grown using sputtering or pulsed laser deposition (PLD) and were further chemically modified by thermal annealing, ion bombardment or covalent grafting of organic monolayers. The average carbon hybridization, impurity level and mass density, were deduced from XPS and photoelectron energy loss spectroscopy (PEELS). The depth sensitivity of the dispersive (Lifshitz–van der Waals) interaction, estimated at 1–2 nm from the dependence of γ^LW on the grafted perflorodecene molecule coverage, is much better than XPS which probes a 3–5 nm depth. The observation of a non-monotonic behavior in the correlation between surface hybridization and electron donor component of surface energy reveals that the average carbon hybridization alone does not describe the entire surface energy physics. The role of π bond clustering in the polar interactions is thus considered and some implications on surface reactivity and mutual interactions with molecular or biomolecular species are discussed.     

XPS study of residual oxide layers on p-GaAs surfaces

The total thickness and composition of a residual oxide layer after chemical etching of p-GaAs:Zn + In has been studied by X-ray photoelectron spectroscopy (XPS). The variation of the Ga to As oxides ratio along the depth has been determined. A concentration correlation of doping isovalent impurity and the dislocation density with the composition of residual oxides is looked for. The total thickness of the residual oxide layer on p- and semi-insulating GaAs is about 5–6 Å. It is found that the Ga₂O₃ quantity in the oxide bulk is greater than the same value of As₂O₃ in highly In-doped samples. In-doping in concentrations over 1.5 × 10¹⁹ cm⁻³ increases the Ga₂O₃ content and the density of the residual oxide. This influence is determined by reducing the dislocation density and changing the point defect environment. The presence of As-rich precipitates on the dislocations and in the matrix decreases the sputtering time and changes the composition of the residual oxide. The correlation between the type of high temperature dislocations revealed by Abrahams-Buiocchi (AB) etching and the oxide layer composition is shown. The results obtained could be used in the first stages of epitaxial growth, metallization and other technological processes of semiconductor device and ICs fabrication.     

XPS studies of carbon supported films formed by the resistive deposition of manganese

The surface and subsurface speciation of carbon-supported vapor-deposited films, prepared by resistively heating manganese chips in the sample chamber of an HP 5950A ESCA spectrometer, have been studied using XPS and argon ion etching. It is shown that the oxide formed in poor vacuum is MnO. Under improved experimental conditions, a mixture of Mn metal and MnO is obtained, but the metal is very reactive to oxygen. For very thick films formed in poor vacuum, the oxide covers a layer of manganese metal. The oxide is no longer evident after 60 h of etching. For the layer of manganese remaining, we have found that the Mn3s multiplet splitting is smaller than that reported in the literature. The present result is shown to be consistent with magnetic susceptibility measurements on α-Mn. For discontinuous MnO films, argon ion etching causes the Mn2p binding energy to shift to higher energy. This is inconsistent with reduction to Mn metal, as the binding energy would shift to a lower value for this case.     

等离子体增强化学气相沉积法制备含氢类金刚石膜的紫外Raman光谱和X射线光电子能谱研究

利用脉冲辉光放电的方法,在硅片上采用不同的沉积工艺制备了含氢类金刚石膜层,并采用Raman光谱和X射线光电子能谱(XPS)对膜层进行表征.用Raman光谱仪在波长为325 nm的紫外光源的激励下观察膜层的键结构.紫外Raman光谱对含氢类金刚石膜是非常有用的,它能有效避免可见光Raman光谱测量时的荧光干扰,清晰地得到膜层的D峰和G峰.同时利用XPS分析得到膜层的sp3键含量,并与Raman光谱所得数据进行比较.通过Raman光谱和XPS分析可以发现,在紫外光源的激励下,膜层的G峰峰位向高频移方向移动,G峰峰位、I(D)/I(G),G峰半高宽和sp3键含量之间存在一定的关系.     

不同方法制备的类金刚石薄膜的XPS和Raman光谱的研究

分别利用金属脉冲磁过滤真空阴极弧沉积法(FCVA)、直流磁控溅射法(SP)和脉冲辉光放电等离子体化学气相沉积法(PECVD)在硅片上沉积类金刚石膜层.并利用激光拉曼光谱法(Raman spectroscopy)和X射线光电子能谱法(XPS)对类金刚石膜层进行研究.通过研究分析发现,不同方法制备的类金刚石膜层的G峰位、D...     

Degradation of a carbon nanotube-based field-emission cathode during ion sputtering

The degradation rate of carbon nanotubes (CNTs) in an electron field emitter is calculated. The degradation mechanism is taken to be the sputtering of the CNT surface by the ions that result from the ionization of residual gas molecules by an electron impact. The degradation rate and the corresponding CNT lifetime are calculated as a function of the nanotube geometry, the applied voltage, the pressure and kind of a residual gas, the interelectrode gap, and the nanotube array density. The obtained strong dependence of the degradation rate on the applied voltage is caused by a sharp character of the I?CV emission characteristic determined by the Fowler-Nordheim relationship. The dependence of the degradation rate on the interelectrode gap is induced by the corresponding dependence of the probability of reaching the CNT surface.     

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.