碳纳米管的变温场发射

考察了温度变化对沉积在钨丝针尖上的碳纳米管场发射的影响，发现碳纳米管场发射电流随温度升高而增大，场发射电流的稳定性基本没有变化. 多根碳纳米管的场发射特性随温度变化出现偏离Fowler-Nordheim理论的现象，这种现象可能来自碳纳米管的不均匀性.     

碳纳米管场致电子发射新机制

基于对长达 1μm的 (5 ,5 )碳纳米管的量子力学计算 ,作者发现使碳纳米管具有优异场致电子发射特性的因素除了人们预期的尖端场增强之外 ,电荷在纳米管尖端的积累造成有效功函数 (真空势垒 )的非线性下降也起了非常重要的作用 .对外加电场Vappl=10— 14V/ μm下的碳纳米管进行了计算 ,得到与实验结果相近的发射电流     

钛碳化物改性碳纳米管的场发射性能

通过在碳纳米管(CNTs)表面沉积钛薄膜并经过高温真空退火处理,在CNTs表面形成了低功函数的钛碳化物.研究了钛碳化物改性CNTs的场发射性能,并利用X射线光电子能谱(XPS)对改性碳管进行了结构表征.实验结果表明,高温真空退火可使沉积在CNTs表面的钛原子与碳原子发生化学反应生成钛碳化物;经钛碳化物改性处理的CNTs的场发射性能明显改善,开启电场由改性前的121降低到104V/μm,当电场强度为234V/μm时,场发射电流密度由改性前的23增大到改性后的13.5mA/cm²,同时,CNTs的表面抗离子轰击能力增强,发射稳定性改善.对钛碳化物改性增强CNTs薄膜场发射性能的机理进行了分析. 关键词： 碳纳米管 钛碳化物 场发射 结构表征     

锥顶碳纳米管的结构稳定性与场致发射性能

运用密度泛函理论研究了锥顶碳纳米管的结构稳定性与电子场致发射性能.结果表明:在外电场作用下,该体系的结构稳定性明显优于碳纳米锥体、C₃₀半球封口的碳纳米管,且电子发射性能与锥角大小、锥顶构型密切相关,特别是锥角38.9°及棱脊型顶部的cone₁@(6,6)综合性能最优,用其作为场致发射源的阴极时可显著提高发射电流密度并延长器件的使用寿命. 关键词： 锥顶碳纳米管 电子场致发射 结构稳定性 密度泛函理论     

纳米碳管阵列场发射电流密度的理论数值优化

以纳米碳管阵列为研究对象,利用镜像悬浮球模型及Fowler-Nordheim电流密度公式,对纳米碳管阵列的场发射电流密度进行计算,进而综合考虑场发射增强因子及场发射电流密度对纳米碳管阵列场发射性能进行定量优化.参考碳管阵列场发射电流密度最大值及场发射增强因子,表明当纳米碳管阵列间距为碳管高度十分之一时,纳米碳管阵列的场发射性能得到优化.与以前的理论估算结果相比,优化的阵列间距进一步减小.当纳米碳管间距过大,场发射增强因子增加,而场发射电流密度会在更大程度上减小;当纳米碳管密度较大时,场发射增强因子受到静电 关键词： 纳米碳管 场发射 增强因子 电流密度     

Mechanism of field electron emission from carbon nanotubes

Field electron emission (FE) is a quantum tunneling process in which electrons are injected from materials (usually metals) into a vacuum under the influence of an applied electric field. In order to obtain usable electron current, the conventional way is to increase the local field at the surface of an emitter. For a plane metal emitter with a typical work function of 5 eV, an applied field of over 1 000 V/μm is needed to obtain a significant current. The high working field (and/or the voltage between the electrodes) has been the bottleneck for many applications of the FE technique. Since the 1960s, enormous effort has been devoted to reduce the working macroscopic field (voltage). A widely adopted idea is to sharpen the emitters to get a large surface field enhancement. The materials of emitters should have good electronic conductivity, high melting points, good chemical inertness, and high mechanical stiffness. Carbon nanotubes (CNTs) are built with such needed properties. As a quasi-one-dimensional material, the CNT is expected to have a large surface field enhancement factor. The experiments have proved the excellent FE performance of CNTs. The turn-on field (the macroscopic field for obtaining a density of 10 μA/cm²) of CNT based emitters can be as low as 1 V/μm. However, this turn-on field is too good to be explained by conventional theory. There are other observations, such as the non-linear Fowler-Nordheim plot and multi-peaks field emission energy distribution spectra, indicating that the field enhancement is not the only story in the FE of CNTs. Since the discovery of CNTs, people have employed more serious quantum mechanical methods, including the electronic band theory, tight-binding theory, scattering theory and density function theory, to investigate FE of CNTs. A few theoretical models have been developed at the same time. The multi-walled carbon nanotubes (MWCNTs) should be assembled with a sharp metal needle of nano-scale radius, for which the FE mechanism is more or less clear. Although MWCNTs are more common in present FE applications, the single-walled carbon nanotubes (SWCNTs) are more interesting in the theoretical point of view since the SWCNTs have unique atomic structures and electronic properties. It would be very interesting if people can predict the behavior of the well-defined SWCNTs quantitatively (for MWCNTs, this is currently impossible). The FE as a tunneling process is sensitive to the apex-vacuum potential barrier of CNTs. On the other hand, the barrier could be significantly altered by the redistribution of excessive charges in the micrometer long SWCNTs, which have only one layer of carbon atoms. Therefore, the conventional theories based upon the hypothesis of fixed potential (work function) would not be valid in this quasi-one-dimensional system. In this review, we shall focus on the mechanism that would be responsible for the superior field emission characteristics of CNTs. We shall introduce a multi-scale simulation algorithm that deals with the entire carbon nanotube as well as the substrate as a whole. The simulation for (5, 5) capped SWCNTs with lengths in the order of micrometers is given as an example. The results show that the field dependence of the apex-vacuum electron potential barrier of a long carbon nanotube is a more pronounced effect, besides the local field enhancement phenomenon.     

Effects of Cs treatment on field emission properties of capped carbon nanotubes

Within the methodology [M. Khazaei, A.A. Farajian, Y. Kawazoe, Phys. Rev. Lett. 95 (2005) 177602] based on first-principles electronic structure calculations, the effects of Cs treatment on current emissions and emission patterns of capped carbon nanotubes are considered at low deposition densities when the nanotubes are under an electric field 0.2 V/Å. The results show that the current emission from the cap with one adsorbed Cs is 3.4 times larger than the cap without any Cs. It is 9.6 times larger in the cap with two adsorbed Cs atoms. After Cs deposition the emission patterns become asymmetric (current emission from the carbon atoms located at the topmost pentagon ring close to Cs atoms is larger than the other atomic sites). There are very few localized states on Cs atoms. Hence, although the tunneling probability of electron emission from Cs atoms is significant, there is no current from Cs atoms. In addition, the effect of Cs on work function reduction of nanotubes is explained in terms of Cs deposition densities and the surface dipole moments.     

碱金属掺杂碳纳米管储氢量差异的理论分析

采用巨正则蒙特卡罗方法模拟锂掺杂单壁碳纳米管阵列（SWCNTA－Single Walled Carbon Nanotube Array）和钾掺杂SWCNTA的物理吸附储氢。重点研究了碱金属原子的种类和掺杂位置对SWCNTA储氢的影响。通过分析碱金属掺杂SWCNTA与氢分子间相互作用的差异及其变化的原因,给出了碱金属掺杂SWCNTA储氢量差异的理论解释,并对今后的研究工作提出了新的建议。     

碳纳米管在大气压环境中的场致发射特性

研究了在大气压环境下,单根碳纳米管作为场致发射阴极,与阳极间距为100—200 nm时的场致发射特性.对比了碳纳米管在不同阴阳极间距和不同气体环境中的场致发射电流和噪声的特点. 关键词： 碳纳米管 场致发射 大气压     

Peierls相变与磁场中碳纳米管的场发射

应用紧束缚模型和WKB方法研究碳纳米管的out-of-plane型Peierls相变,及其对碳纳米管的场发射的影响.结果发现Peierls相变会在室温出现,并使碳纳米管费米面附近出现能隙,导致碳纳米管发生金属—半导体转变,从而抑制碳纳米管的场发射.磁场也会抑制Peierls形变,Peierls相变和磁场相互竞争影响碳纳米管的能带结构,从而影响碳纳米管的场发射. 关键词： 场发射 碳纳米管 Peierls相变     

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     

Enhanced field emission properties of screen-printed doubled-walled carbon nanotubes by polydimethylsiloxane elastomer

Field emission (FE) properties of double-walled carbon nanotubes (DWCNTs) treated by polydimethylsiloxane (PDMS) elastomer with different heating temperature have been systematically studied. The current density of treated DWCNT films decreases with the increase of heating temperature. The screen-printed DWCNTs treated by PDMS elastomer with drying temperature 150 °C for 20 min have the best FE performance with a marvelous field enhancement factor (β = 20194). The optimized FE performance is attributed to the morphological change of DWCNT films after PDMS elastomer treatment and the change of separation energy for the CNT-substrate interface. It is proved that the PDMS treatment is a facile and effective method for field emission display (FED) application, especially for low-temperature FED preparation.     

Field emission of carbon nanotube array with normal-gate cold cathode

A hexagon pitch carbon nanotube (CNT) array vertical to the normal gate of cold cathode field emission displayer (FED) is simulated by solving the Laplace equation. The calculated results show that the normal gate causes the electric field around the CNT tops to be concentrated and emission electron beam become a column. The field enhancement factor and the emission current intensity step up greatly compared with those of diode structure. Emission current density increases rapidly with the decrease of normal-gate aperture. The gate voltage exerts a critical influence on the emission current.     

氮掺杂及水分子吸附碳纳米管电子场发射第一性原理研究

对闭口碳纳米管(CNT)顶端分层掺氮及吸附不同数目水分子体系,运用第一性原理研究了有电场存在时的电子场发射性能.结果表明：掺氮并吸附水分子的CNT结构稳定;外电场愈强、水分子数愈多,体系态密度(DOS)向低能端移动幅度愈大且最高分子占据轨道(HOMO)/最低分子空轨道(LUMO)能隙愈小.吸附能,DOS/LDOS,HOMO/LUMO及其能隙分析一致表明,第三层氮掺杂CNT吸附不同数目水分子体系的场发射性能最佳. 关键词： 氮掺杂 水吸附 密度泛函理论 电子场发射     

掺硼水吸附碳纳米管电子场发射性能的第一性原理研究

运用第一性原理研究了掺硼碳纳米管(BCNT)顶端吸附水分子后的电子场发射性能.结果表明：掺B及吸附H₂O的碳纳米管(BCNT+H₂O)端部形成电子聚集的原子尺度微区,其电子态密度(DOS)在费米能级(E_f)附近有很大提高.根据计算的电子DOS,HOMO/LUMO及Mulliken电荷分布等可知BCNT+H₂O比CNT+H₂O有更好的场发射性能. 关键词： 掺硼碳纳米管 吸附 密度泛函理论 电子场发射     

Effects of various defects on the electronic properties of single-walled carbon nanotubes: A first principle study

The geometries,formationenergies and electronic band structures of （8, 0） and （14, 0） singlewailed carbon nanotubes （SWCNTs） with various defects, inehlding vaeaney, Stone-Wales defect, and octagon pentagon pair defect, have been investigated within the framework of the density- huictional theory （DFT）, and the influence of the concentration within the same style of deflect on the physical and chenfical properties of SWCNTs is also studied. The results suggest that the existeilcc of vacancy and octagon-pentagon pair deflect both reduce the band gap, whereas the SW- defect induces a band gap opening in CNTs. More int, erestingly, the band gaps of （8, 0） and （14, 0） SWCNTs eonfigurations with two octagon pentagon pair defect presents 0.517 eV and 0.163/eV, which arc a little smaller than the perfectt CNTs. Furthermore, with the concentration of defects increasing, there is a decreasing of band ga.p making the two types of SWCNTs change from a semiconductor to a metallic conductor.     

Field emission properties of carbon nanotubes film grown on NiCr alloy films

Carbon nanotubes (CNTs) are prepared on NiCr alloy films by low pressure thermal chemical vapor deposition at 600 °C. NiCr alloy films are deposited by magnetic co-sputtering method, and the various thickness and Ni/NiCr ratios are controlled by sputtering power. The diameter and length of CNTs, as well as the roughness of the CNTs films, mainly depend on the Ni/NiCr ratio. The field emission current density of the CNTs film increases with the increasing Ni/NiCr ratio from 65 wt% to 83 wt%, and decreases when the Ni/NiCr ratio is more than 87 wt% in the alloy film.     

利用场发射显微镜研究O2对单壁碳纳米管场发射的影响

利用场发射显微镜和四极质谱计研究了充入高纯O2的四极质谱和O2对单壁碳纳米管场发射的影响.单壁碳纳米管经过约1000℃的热处理得到清洁态场发射像后，充入O2，分别测量了O2吸附和脱附后场发射的I V特性.实验观测到在单壁碳纳米管上O2的吸附使场发射电流减小，说明逸出功增加.在10-4Pa的O2压强下对单壁碳纳米管进行约1000℃的热处理，可以产生氧化刻蚀作用，观测到场发射像的变化，并测量了氧化刻蚀产生的I V特性变化. 关键词： 单壁碳纳米管 场发射显微镜 场发射 四极质谱     

碳纳米管端口的场蒸发

将单根多壁碳纳米管(multi-walled carbon nanotube,MWCNT)组装在W针尖上并送入超高真空场发射/场离子显微镜(Ultrahigh Vacuum Field-emission/Field-Ion microscope,UHV-FEM/FIM)进行场蒸发及场发射研究.结果表明,场蒸发可以降低MWCNT的逸出功,从而增强其场发射能力.估算MWCNT的蒸发场低于1.3×10⁸V·cm^-1,且在此场强下的平均蒸发速率为9.4nm·min^-1.定性讨论了MWCNT的蒸发场大大低于C的理论值的原因.首先,通过场解吸获得的清洁端口上有较多悬挂键,平均每个C原子的配位数较小,所以升华热较低.其次,可能存在于MWCNT中的H原子会在强场下碰撞端口的C原子,使其更易蒸发.以上结果显示了利用场蒸发剪短碳纳米管从而改善其场发射特性的可行性. 关键词： 碳纳米管 场蒸发 场发射     

Field emission patterns with atomic resolution of single-walled carbon nanotubes by field emission microscopy

Electron emission properties of single-walled carbon nanotubes (SWCNTs) assembled on a tungsten tip were investigated using field emission microscopy (FEM). The transmission electron microscopy (TEM) micrograph confirmed the existence of an SWCNT bundle on the W tip. Under appropriate experimental conditions,a series of FEM patterns with atomic resolution were obtained. These patterns arose possibly from the field emission of the open end of an individual (16,0) SWCNT protruding from the SWCNT bundle. The magnification factor and the resolution under our experimental conditions were calculated theoretically. If the value of the compression factor β was set at β= 1.76, the calculated value of the magnification factor was in agreement with the measured value. The resolving powerof FEM was determined by the resolution equation given by Gomer. The resolutionof 0.277 nm could be achieved under the typical electric field of 5.0×107 V/cm, which was close to the interatomic separation 0.246 nm between carbon atoms along the zigzag edge at the open end for the (16, 0) SWCNT. Consequently, our experimental results were further supported by our theoretical calculation.