Electron emission degradation of nano-structured sp^2-bonded amorphous carbon films

The initial field electron emission degradation behaviour of original nano-structured sp^2-bonded amorphous carbon films has been observed, which can be attributed to the increase of the work function of the film in the field emission process analysed using a Fowler-Nordheim plot. The possible reason for the change of work function is suggested to be the desorption of hydrogen from the original hydrogen termination film surface due to field emission current-induced local heating. For the explanation of the emission degradation behaviour of the nano-structured sp2-bonded amorphous carbon film, a cluster model with a series of graphite （0001） basal surfaces has been presented, and the theoretical calculations have been performed to investigate work functions of graphite （0001） surfaces with different hydrogen atom and ion chemisorption sites by using first principles method based on density functional theory-local density approximation.     

Field emission properties and surface structure of nickel containing amorphous carbon

Nickel containing amorphous carbon (a-C:Ni) films have been deposited by filtered cathodic vacuum arc (FCVA) technique by introducing pure nickel into the graphite target. The field electron emission property of a-C:Ni was improved when compared to that of pure tetrahedral amorphous carbon (ta-C) by FCVA. The emission threshold field of a-C:Ni film is about 5 V μm⁻¹, whilst the threshold field of the ta-C film is about 13 V μm⁻¹. Raman spectroscopy suggests that the sp² clusters in the carbon film increase both in size and number when Ni is introduced. However, the emission was found to degrade to threshold fields beyond 20 V μm⁻¹ after the a-C:Ni film was left in ambient for a week. This observation is attributed to surface absorption of oxygen on the a-C:Ni film, as determined by X-ray Photoelectron Spectroscopy.     

Anomalous changes in electron emission during adsorption of alkali metals on a carbon film

An anomalous change was discovered in the field of electron emission during the adsorption of alkali metal atoms on the surface of an amorphous carbon film. The phenomenon involves the disappearance of electron emission from graphite nanoclusters that were local emission sources before the deposition of cesium. The observed effect is explained on the basis of surface diffusion processes of cesium atoms in a nonuniform electric field and intercalation graphite nanostructures by cesium.     

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.     

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.     

Field emission properties of a-C and a-C:H films deposited on silicon surfaces modified with nickel nanoparticles

The a-C and a-C:H films are deposited on silicon surfaces modified with and without nickel nanoparticles by using mid-frequency magnetron sputtering. The microstructures and morphologies of the films are analyzed by Raman spectroscopy and atomic force microscopy. Field emission behaviors of the deposited films with and without nickel nanoparticles modification are comparatively investigated. It is found that the hydrogen-free carbon film exhibits a high field emission current density and low turn-on electric field compared with the hydrogenated carbon film. Nickel modifying could increase the current density, whereas it has no significant effect on the turn-on electric field. The mechanism of field electron emission of a sample is discussed from the surface morphologies of the films and nickel nanoparticle roles in the interface between film and substrate.     

Field electron emission in graphite-like films

Results of investigation of carbon films deposited with the use of gas-phase chemical reactions in the plasma of a dc discharge are presented. Films obtained at different parameters of the deposition process varied widely in their structure and phase composition, from polycrystalline diamond to graphite-like material. Comparative study of the structure and phase composition of the films using Raman spectroscopy, cathodoluminescence, electron microscopy, and diffractometry, as well as the obtained field electron emission characteristics, have shown that the threshold value of the electric field strength for electron emission decreases with a decrease in the size of diamond crystallites and growth of the fraction of non-diamond carbon. The lowest threshold fields (less than 1.5 V/μm) are obtained for films consisting mainly of graphite-like material. A model based on the experimental data is proposed, which explains the mechanism of field electron emission in carbon materials.     

Effect of temperature on the electron field emission from aligned carbon nanofibers and multiwalled carbon nanotubes

Effect of temperature and aspect ratio on the field emission properties of vertically aligned carbon nanofiber and multiwalled carbon nanotube thin films were studied in detail. Carbon nanofibers and multiwalled carbon nanotube have been synthesized on Si substrates via direct current plasma enhanced chemical vapor deposition technique. Surface morphologies of the films have been studied by a scanning electron microscope, transmission electron microscope and an atomic force microscope. It is found that the threshold field and the emission current density are dependent on the ambient temperature as well as on the aspect ratio of the carbon nanostructure. The threshold field for carbon nanofibers was found to decrease from 5.1 to 2.6 V/μm when the temperature was raised from 300 to 650 K, whereas for MWCNTs it was found to decrease from 4.0 to 1.4 V/μm. This dependence was due to the change in work function of the nanofibers and nanotubes with temperature. The field enhancement factor, current density and the dependence of the effective work function with temperature and with aspect ratio were calculated and we have tried to explain the emission mechanism.     

Improvement of field emission of graphite flakes using hydrogen thermal processing

In order to improve the field electron emission properties of the graphite, the carbon nanoparticles (CNPs) are synthesized on the micrographite flakes by hydrogen thermal processing. We spin the graphite solution on the silicon wafer and desiccate it, then produce the CNPs on the graphite flakes using hydrogen thermal processing in the furnace. The processing parameters such as the processing temperature, hydrogen flow rate and processing time, were varied to find the optimal conditions for the improvement of the field emission properties of the graphite flakes. The experimental results show that the field emission properties of the graphite flakes have glaring improvement after heat treatment owing to the increase of the defect density and the CNPs on above. The turn-on field was decreased from 7.7 of the untreated sample to 4.3 V/μm of the treated sample at the optimal processing conditions.     

Field electron emission from amorphous carbon films grown in pure methane plasma

By using RF plasma-enhanced chemical vapor deposition, amorphous carbon films were grown in pure methane plasma. Field emission of the films were examined as a function of substrate temperature. It was found that the emission current from the samples prepared at substrate temperatures higher than 600 °C were considerably improved. According to the results by Raman spectroscopy, growth of graphite crystallites were promoted with high substrate temperatures. Moreover, the surface morphology was abruptly changed at high substrate temperatures over 600 °C. We discuss the field emission characteristics of the amorphous carbon films with regard to the structural features and the surface morphology.     

Properties of low-refractive-index films obtained by the close-spaced vapor transport technique under the sublimation of graphite in a quasi-closed volume

The properties of low-refractive-index carbon films obtained by close-spaced vapor transport at graphite sublimation are studied. The optical properties of the films are investigated by monochromatic multiple-angle ellipsometry, and their morphology is examined by AFM. It is found that the films have a columnar structure with a background surface roughness of about 1 nm. In addition, the surface of the film contains islands up to 50 nm in height with a footprint of ≈200 nm. A low-refractive-index carbon film deposited by close-spaced vapor transport on silicon tips is found to decrease the field emission threshold and drastically raise the current.     

Synthesis of Nano-Crystalline Graphite Films in Hollow Cathode Discharge

The results of experiments on the production of nano-crystalline graphite films in the hollow cathode discharge on Ni(111)/sapphire substrate are presented. The characteristics of the discharge for different gases and pressures are given. The emission spectra of the plasma have been measured. The layers of the deposited graphite have been characterized by the methods of Raman scattering and atomic force microscopy. The field emission of nano-crystalline graphite was also measured. The produced layers have good homogeneity and high value of the electron field emission. The presence of vertically growing graphene is revealed.     

掺杂对碳纳米管拉曼光谱及场发射性能的影响

采用高温热解法在860℃分别制备出了镓、氮以及硼和氮掺杂的碳纳米管,提纯后利用丝网印刷工艺分别将它们制备成薄膜,并测试了它们的拉曼光谱与场发射性能。测试结果表明,掺杂纳米管的缺陷密集度比纯碳纳米管明显增大,而它们的场致电子发射性能则与掺杂元素的性质密切相关。镓和氮掺杂的纳米管均具有非常优异的场发射性能,而硼和氮共掺杂的纳米管的场发射性能很差。掺杂引起碳纳米管费米能级附近能态密度的变化及功函数的降低是其具有优异场致电子发射性能的主要原因。     

Efficient field emission from coiled carbon nano/microfiber on copper substrate by dc-PECVD

Crystalline coiled carbon nano/micro fibers in thin film form have been synthesized via direct current plasma enhanced chemical vapor deposition (PECVD) on copper substrates with acetylene as a carbon precursor at 10 mbar pressure and 750 °C substrate temperature. The as-prepared samples were characterized by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). XRD pattern as well as selected area electron diffraction (SAED) pattern showed that the samples were crystalline in nature. SEM and HRTEM studies showed that as synthesized coiled carbon fibers are having average diameter ∼100 nm and are several micrometers in length. The as-prepared samples showed moderately good electron field emission properties with a turn-on field as low as 1.96 V/μm for an inter-electrode distance 220 μm. The variation of field emission properties with inter-electrode distance has been studied in detail. The field emission properties of the coiled carbon fibrous thin films are compared with that of crystalline multiwalled carbon nanotubes and other carbon nanostructures.     

沉积时间对球状微米金刚石聚晶结构的生长及其场发射特性影响

利用微波等离子体化学气相沉积法,在覆盖金属钛层的陶瓷衬底上,通过改变沉积时间制备出不同结构的类球状微米金刚石聚晶碳膜.通过扫描电子显微镜、喇曼光谱、X射线衍射谱对碳膜进行了分析测试,并研究了不同沉积时间下沉积的类球状微米金刚石聚晶薄膜的场致电子发射特性.结果显示:不同的沉积时间所制备的碳膜形貌有很大变化,场致电子发射的效果也有很大不同,从而得出了场增强因子的降低和导电通道的增长是场发射效果变差的主要原因.     

白碳纳米晶薄膜及其场致电子发射特性

利用微波等离子体化学气相沉积方法,以甲烷、氢混合气体为反应气体,具有钛镀层的玻璃作为衬底,制备了具有sp¹杂化结构的白碳纳米晶薄膜。利用X射线衍射、俄歇电子能谱,以及扫描电子显微镜对薄膜结构进行了表征。以白碳纳米晶薄膜为阴极,以镀有ITO透明导电薄膜玻璃为阳极,采用二极管结构,测试了白碳纳米晶薄膜的场致电子发射特性。开启电场为2.5 V/μm,在电场为5 V/μm时的电流密度为200μA/cm²。对白碳纳米晶薄膜生长机理,以及其场致电子发射机制进行了讨论。     

Role of the curvature of atomic layers in electron field emission from graphitic nanostructured carbon

Layers of oriented carbon nanotubes and nanometer-size plate-shaped graphite crystallites are obtained by chemical vapor deposition in a glow-discharge plasma. A structural-morphological investigation of a carbon material consisting of nanotubes and nanocrystallites is performed, and the field-emission properties of the material are also investigated. It is shown that electron field emission is observed in an electric field with average intensity equal to or greater than 1.5 V/μm. The low fields giving rise to electron emission can be explained by a decrease in the electronic work function as a result of the curvature of the atomic layers of graphitic carbon. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 5, 381–386 (10 March 1999)     

Carbon nanofibers and multiwalled carbon nanotubes from camphor and their field electron emission

Vertically aligned carbon nanofibers (CNF) and multiwalled carbon nanotubes (MWCN) have been synthesized from camphor by catalytic thermal CVD method on Co and Co/Fe thin films (for CNF) and on silicon substrates using a mixture of camphor and ferrocene (for MWCN). CNF and MWCN are studied by field emission scanning electron microscopy, high-resolution transmission electron microscopy, visible Raman spectroscopy, X-ray diffraction in order to get insight into the microstructure and morphology of these materials. Field electron emission study indicates turn-on field of about 2.56, 3.0 and 6.5 V/μm for MWCN, Co/CNF and Co/Fe/CNF films, respectively. The best performance of MWCN in field electron emission among the materials studied can be due to the highest aspect ratio, good graphitization and good density.     

A comparative study of field emission properties of carbon nanotube films prepared by vacuum filtration and screen-printing

A comprehensive comparative study of electron field emission properties of carbon nanotube (CNT) films prepared by vacuum filtration and screen-printing was carried out. Field emission performance of vacuum filtered CNT films with different filtered CNT suspension volumes was systematically studied, and the optimum electron emission was obtained with a low turn on field of ∼0.93 V/μm (at 1 μA/cm²) and a high field enhancement factor β of ∼9720. Comparing with screen-printed CNT films, vacuum filtered CNT films showed better electron emission performance, longer lifetime, and greater adhesive strength to substrates. This work reveals a potential use of vacuum filtered CNT films as field emission cathodes.     

高温退火对非晶CNx薄膜场发射特性的影响

采用射频磁控溅射方法在纯N₂气氛中沉积了非晶CN_x薄膜样品，并 在真空中退火至900 ℃.对高温退火引起的CN_x薄膜化学成分、键合结构及其场发射特性方面的变 化进行研究.用傅里叶变换红外光谱和x射线光电子能谱分析样品的内部成分及键合结构的变化，其中sp²键及薄膜中N的含量与薄膜的场发射特性密切相关.退火实验的结果表明 高温退火可以导致CN_x薄膜中N含量大量损失，并在薄膜中形成大量sp^{2< 关键词： x薄膜')" href="#">CN_x薄膜 化学键合 退火温度 场致电子发射}