Influence of cesium atoms on the field electron emission from multi-walled carbon nanotubes

It has been shown that the deposition of cesium atoms on multi-wall carbon nanotubes abruptly increases the current of the field electron emission, decreases the threshold electric field by a factor of three (to 0.8 V/m), and decreases the work function to 2.1–2.3 eV. It has been found that the flowing of the large emission current I ≥ 2 × 10^?6 A leads to a change in the current-voltage characteristics and a decrease in the emission current. This effect has been explained by escape of cesium atoms from the tips of most nanotubes into the nanotube depth due to desorption or intercalation. At the same time, the low work function is retained for some nanotubes, probably, due to the stronger bonding of Cs atoms with these nanotubes.     

Transmission electron microscopy observations of CNT morphology before and after heating in vacuum

Heating of multi-walled carbon nanotubes is often required to obtain clean patterns in the field electron emission microscope (FEEM). A transmission electron micrograph study of morphological changes in the cap structure of multi-walled carbon nanotubes due to heating in vacuum is presented. The lack of significant structural change in the cap structure when specimens were heated to 1925 K for 2.5 h determines an upper bound for the diffusivity of multi-walled carbon nanotube surface atoms, of the order of 10⁻¹⁶ cm²/s at 1925 K.     

基于碳纳米管场致发射的电子枪的初步研究

给出了基于碳纳米管场致发射电子枪的初步研究结果. 碳纳米管场致发射试验证明碳纳米管是一种很好的场致发射材料. 试验中, 在极间场强2.7MV/m的情况下得到的电流发射密度为0.5mA/cm².     

Effect of the cesium and potassium doping of multiwalled carbon nanotubes grown in an electrical arc on their emission characteristics

The effect of cesium and potassium atoms deposited onto multiwalled carbon nanotubes grown in an electrical arc on their emission characteristics was studied. The current–voltage characteristics of the field electron emission of specimens with cesium or potassium doped multiwalled carbon nanotubes of this type were revealed to retain their linear character in the Fowler–Nordheim coordinates within several orders of magnitude of change in the emission current. The deposition of cesium and potassium atoms was shown to lead to a considerable increase in the emission current and a decrease in the work function φ of studied emitters with multiwalled nanotubes. The work function was established to decrease to φ ~ 3.1 eV at an optimal thickness of coating with cesium atoms and to φ ~ 2.9 eV in the case of doping with potassium atoms. Cesium and potassium deposition conditions optimal for the attainment of a maximum emission current were found.

     

Growth of multi-walled CNTs emitters on an oxygen-free copper substrate by chemical-vapor deposition

Field emission properties of carbon nanotubes directly grown on a well-polished oxygen-free copper substrate by chemical-vapor deposition (CVD) were studied. Ni was sputtered on the copper substrate as catalyst, and the reactant gas was acetylene. From scanning electron microscopic and transmission electron microscopic images, the as-grown carbon nanotubes are seen to be bamboo structure with branches. Efficient field emission of CNTs is measured by a diode configuration, and the maximum current is 4.8 mA corresponding to a low electric field of 6.7 V/μm (the emission area is about 3.14 mm²). The diffusion between nickel and copper substrate is found to cause the loss of catalyst based on X-ray diffraction pattern of the surface of the substrate.     

碳纳米管/金刚石复合材料的场发射特性

采用微波烧结方法制备了碳纳米管/金刚石复合材料。将碳纳米管和纳米金刚石粉末混合后研磨压片,然后在微波等离子气相沉积系统中采用微波烧结。利用扫描电镜对复合材料的表面形貌和微观结构进行了分析,结果显示碳纳米管比较均匀地分散于复合材料中,并在表面形成了发射微尖。利用二极管结构在动态真空室中对复合材料的场发射特性进行了研究,复合材料有较好的场发射特性,电流密度接近15mA/cm2。     

Field emission characteristic of screen-printed carbon nanotube cathode

The fabrication of carbon nanotube emitters with excellent emission properties is described. The multi-walled carbon nanotubes (MWNTs) produced by chemical vapor deposition (CVD) method were purified with oxidation method and mixed with organic binding pastes and then screen-printed on glass substrates with ITO film. We applied anode voltage gradually to refine the emission behavior of the emitter by cleaning the top surface of screen-printed carbon nanotubes (CNTs). The density of the carbon nanotubes is about 2.5×10⁸/cm². Diode field emission experiments were performed in dynamic vacuum system to study the emission current, the emission uniformity, etc. Bright and stable character emission images were obtained in the diode structure and the emission current could approach 1 mA/cm².     

Field electron emission form single-walled carbon nanotubes with deposited cesium atoms

It has been found that deposition g of cesium atoms on single-walled carbon nanotubes covered with potassium atoms not only drastically increases emission current but also considerably changes the shape of current-voltage characteristics of field electron emission, namely, the characteristics become nonlinear in Fowler-Nordheim coordinates. It has been assumed that this effect is associated with the fact that field electron emission in these layers comes from single-walled carbon nanotubes, which have p-type conductivity after potassium treatment, while deposition of cesium leads to the formation of p-n junctions near nanotube tips. Part of the applied voltage drops in p-n junction, thus causing a nonlinearity of current-voltage characteristics.     

Field emission microscopy of adsorption and desorption of residual gas molecules on a carbon nanotube tip

Field emission of electrons from multiwall carbon nanotubes has been investigated by field emission microscopy (FEM) in ultra-high vacuum. A carbon nanotube, at the end of which at least six pentagons exist to make a closed cap, gives an FEM pattern consisting of bright pentagonal rings if the nanotube surface is clean. Adsorption of residual gas molecules is observed as bright spots in the FEM pattern, giving rise to an abrupt increase in the emission current. Adsorbed molecules seem to reside preferentially on the pentagonal sites where the strong electric field is concentrated. A heat cleaning of the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers its original clean surface. It has been revealed that the adsorption and desorption of gas molecules are responsible for stepwise fluctuation of the emission current.     

Electron field emission properties of closed carbon nanotubes

Recent experiments have shown that carbon nanotubes exhibit excellent electron field emisson properties with high current densities at low electric fields. Here we present theoretical investigations that incorporate geometrical effects and the electronic structure of nanotubes. The electric field is dramatically enhanced near the cap of a nanotube with a large variation of local field distribution. It is found that deviation from linear Fowler-Nordheim behavior occurs due to the variation of the local field in the electron tunneling region. The maximum current per tube is of the order of 10 microA. Local and microscopic aspects of field emission from nanotubes are also presented.     

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     

CNx纳米管的制备、结构观察及低场致电子发射性能研究

采用高温热解法，以乙二胺为前驱液，在沉积有铁催化剂的p型硅（111）基底上制备出了定向生长的CNx纳米管.利用扫描电子显微镜、高分辨率透射电子显微镜和拉曼光谱对CNx纳米管进行了形貌观察和表征.CNx纳米管的高度在20?μm左右，直径在50—100nm之间，具有明显的“竹节状”结构，结晶有序度较差.对CNx纳米管薄膜进行低场致发射性能测试：外加电场为1.4V/μm，观察到20?μA /cm2发射电流，外电场升至2.54V/μm时发射电流达到1.280mA/cm2，在较高外电场下，没有发现电流“饱和”.这比 关键词： CNx纳米管 高温热解 “竹节状”结构 场致发射     

不同电场下碳纳米管场致发射电流密度研究

本文运用密度泛函理论和金属电子论, 深入研究了碳纳米管场致发射电流的变化规律. 结果显示其发射电流密度取决于体系的态密度、赝能隙、管长和局域电场, 在不同范围电场下的变化规律不同. 在较低电场下, 发射电流密度随电场增强而近似线性增大(对应的宏观电场须小于18 V· μm^-1); 但在较高电场下, 发射电流密度随外电场增加呈现非周期性振荡增长趋势, 碳纳米管表现为电离发射. 本文进一步研究了金属性碳纳米管电导率在不同电场下的变化规律.     

Atomic structure of Cs layer grown on Si(0 0 1)(2 × 1) surface at room temperature

The atomic structure of Cs atoms adsorbed on the Si(0 0 1)(2 × 1) surface has been investigated by coaxial impact collision ion scattering spectroscopy. When 0.5 ML of Cs atoms are adsorbed on Si(0 0 1) at room temperature, it is found that Cs atoms occupy a single absorption site on T3 with a height of 3.18 ± 0.05 Å from the second layer of Si(0 0 1)(2 × 1) surface, and the bond length between Cs and the nearest Si atoms is 3.71 ± 0.05 Å.     

氢等离子体处理——一种有效提高碳纳米管场发射性能的方法

研究了经氢等离子体处理后多壁碳纳米管的场发射性能。测量了处理前后样品的电流电压特性和表面形貌。结果表明经氢等离子体处理后,发射性能明显改善,发射点密度由未经处理的104/cm2提高到106/cm2。发现了一种新的碳纳米管结构,称之为多结的碳纳米管,并讨论了样品发射性能提高的可能机理。这种处理提供了一种有效提高发射点密度和基于碳纳米管的平板显示器性能的方法,非常适用于低成本大面积场发射阴极的制作。     

Electron-stimulated desorption of potassium and cesium atoms from oxidized molybdenum surfaces

The electron-stimulated desorption (ESD) yields and energy distributions for potassium (K) and cesium (Cs) atoms have been measured from K and Cs layers adsorbed at 300 K on oxidized molybdenum surfaces with various degrees of oxidation. The measurements were carried out using a time-of-flight method and surface ionization detector. The ESD appearance threshold for K and Cs atoms is independent of the molybdenum oxidation state and is close to the oxygen 2s level ionization energy of 25 eV. Additional thresholds for both K and Cs atoms are observed at about 40 and 70 eV in ESD from layers adsorbed on an oxygen monolayer-covered molybdenum surface; they are associated with resonance processes involving Mo 4p and 4s excitations. The ESD energy distributions for K and Cs atoms consist of single peaks. The most probable kinetic energy of atoms decreases in going from cesium to potassium and with increasing adsorbed metal concentration; it lies in the energy range around 0.35 eV. The K and Cs atom ESD energy distributions from adlayers on an oxygen monolayer-covered molybdenum surface are extended toward very low kinetic energies. The data can be interpreted by means of the Auger stimulated desorption model, in which neutralization of adsorbed alkali-metal ions occurs after filling of holes created by incident electrons in the O 2s, Mo 4s or Mo 4p levels.     

Adsorption on the carbon nanotubes

Adsorption on single walled carbon nanotubes (SWCNTs) is a subject of growing experimental and theoretical interest. The possible adsorbed patterns of atoms and molecules on the single-walled carbon nanotubes vary with the diameters and chirality of the tubes due to the confinement. The curvature of the carbon nanotube surface enlarges the distance of the adsorbate atoms and thus enhances the stability of high coverage structures of adsorbate. There exist two novel high-coverage stable structures of potassium adsorbed on SWCNTs, which are not stable on graphite. The electronic properties of SWCNTs can be modified by adsorbate atoms and metal-semiconductor and semiconductor-semi-conductor transitions can be achieved by the doping of alkali atoms.     

Field electron emission and field desorption of cesium from graphene

Field electron emission and field desorption of cesium ions from a monatomic graphene film on Ir and graphene clusters in amorphous carbon are investigated using field electron microscopy and continuous-mode field desorption microscopy. The deposition of cesium on amorphous carbon with graphite clusters leads to inversion of the emission (i.e., emission from the emission centers disappears against the back-ground of uniform emission from the previously nonemitting surface). In both systems, ion current pulses are observed during field desorption in a stationary electric field. During field desorption from the graphene film, current pulses of Cs⁺ ions with a duration shorter than 0.1 s appear from the plane faces of the iridium point. During desorption from graphite clusters, ion current pulses form a pattern of “collapsing rings” on the screen. Possible mechanisms of the observed processes are considered using the model of cesium intercalation by graphite and by the graphene layer and the desorption of Cs atoms under the action of the electric field, as well as the “flip” of the dipole moment during the cesium intercalation.     

Fabrication of a three-dimensional complex carbon nanoneedle from carbon nanowalls

A three-dimensional complex carbon nanoneedle has been fabricated from carbon nanowalls by a direct current plasma chemical vapor deposition system. Sample grown on stainless wire substrate pretreated with the mixing powders of diamond and molybdenum exhibits novel three-dimensional complex nanostructure, the center of which is a carbon nanoneedle, and many carbon nanowalls growing from the needle. The density of unique nanostructure emitters was about 5 × 10⁷/cm². The I-V characteristic addressed an emission current density of 314 mA/cm² at the electric field of 2.5 V/μm.     

Synthesis of Cu-ZnO and C-ZnO nanoneedle arrays on zinc foil by low temperature oxidation route: Effect of buffer layers on growth, optical and field emission properties

Different densities of ZnO nanoneedle films have been prepared by pre-coated zinc foils with thin layer of copper and carbon followed by thermal oxidation at 400 °C in air. The X-ray diffraction patterns show well defined peaks, which could be indexed to the wurtzite hexagonal phase of ZnO. The scanning electron microscope images clearly reveal formation of ZnO needles on the entire substrate surface. The X-ray photoelectron spectroscopy studies indicate that Cu and C ions are incorporated into the ZnO lattice. Photoluminescence studies evaluate different emission bands originated from different defect mechanism. From the field emission studies, the threshold field, required to draw emission current density of ∼100 μA/cm², is observed to be 2.25 V/μm and 1.57 V/μm for annealed zinc foil pre-coated with copper and carbon, respectively. The annealed film with copper layer exhibits good emission current stability at the pre-set value of ∼100 μA over a duration of 4 h. The results show that buffer layer is an important factor to control the growth rate, resulting in different density of ZnO needles, which leads to field emission properties. This method may have potential in fabrication of electron sources for high current density applications.