碳纳米管场发射性能的研究进展

碳纳米管是一种性能优良的场发射冷阴极材料,具有低的阈值电场和高的发射电流密度,在平板显示领域具有广阔的应用前景。本文从Fow ler-Nordhe im s场发射理论出发,阐述了CNT的场发射机制,详细论述了包括CNT的结构、方向性、阵列密度、吸附气体、工作环境等诸多因素对其场发射特性的影响。研究表明,垂直取向,长径比高,密度适中,表面洁净的CNT场增强因子较大,场发射性能较好。     

化学所与索尼公司合作实现单壁碳纳米管选择性分离/富集

自单壁碳纳米管被发现以来,其优异的电学性能引起了广泛的关注。但是,现有方法制备的单壁碳纳米管都是金属性管和半导体性管的混合物,两种管的互相影响会降低彼此的器件性能。为使金属性管和半导体性管各尽其用,而不是互相影响进而降低彼此的器件性能,单壁碳纳米管的分离／富集就显得尤为重要,并成为本领域一个亟待解决的瓶颈问题。     

碳纳米管场增强因子计算模型的研究

场增强因子(β)是评价碳纳米管场发射性能的重要参数之一.本文介绍了几种计算β的模型,分析了各种因素对β的影响.通过对所得β表达式进行比较,总结了不同模型中影响场增强因子的主要因素.     

单壁碳纳米管的稳定性研究

采用化学气相沉积法(CVD),以正己烷为碳源制备单壁碳纳米管(SWNTs),研究了纯化后的单壁碳纳米管在空气中的稳定性,分别对刚纯化后和在空气中放置5d、10d和15d后的样品进行了SEM、HRTEM和TG表征,结果发现单壁碳纳米管的氧化程度随在空气中放置时间的延长而增加,15d后几乎完全转化为无定形碳.表明单壁碳管在空气中不稳定,很容易氧化,需要密闭保存.     

ZnO纳米锥场发射效应因子计算及其第一性原理研究

建立ZnO-NC(氧化锌纳米锥)数学模型,对静电场中其尖端的电势和电场进行数值计算,得到场发射效应因子表达式为β=H/8πε0·h/d(其中h和d分别为ZnO-NC的高度和尖端直径);在此基础上,采用第一性原理计算方法,进一步研究了不同高度ZnO-NC的场发射性能,结果表明:在ZnO-NC结构稳定的情况下,随着h的增加其尖端场发射效应因子β增大,根据DOS(态密度)、电子密度、Mulliken电荷、能隙及有效功函数的计算,可知h对尖端场发射性能影响显著,通过控制ZnO-NC的高度可有效提高其场发射性能.     

单壁碳纳米管的电子结构与声子谱的第一性原理计算

针对三种结构类型的单壁碳纳米管的电子结构和声子谱,采用基于密度泛函理论的第一性原理以及CASTEP软件进行了理论计算.在完成建模和结构优化的基础上,对扶手椅型(10,10)、锯齿型(6,0)和螺旋型(4,2)单壁碳纳米管的电子能带及电子态密度、声子谱及声子态密度进行了理论计算,并对计算结果进行了理论分析.结果表明:(10,10)及(6,0)单壁碳纳米管无能量禁带,它们属于金属型管;而(4,2)单壁碳纳米管有一宽度为0.85 eV的能量禁带,它属于半导体型管.以上计算结果与用其它判据给出的结论相一致.在以上三种管的声子谱中,它们依次有120条、72条和168条(其中有简并情况)色散曲线,并且它们的声子态密度依次在频率41.88 THz、23.95THz和23.43 THz处出现最大值,即在这些频率附近的格波最多.这一计算结果与物理规律是一致的.     

N-M(In,Ga,Al)共掺(6,6)型闭口氧化锌纳米管场发射第一性原理研究

运用以密度泛函理论为基础的第一性原理计算方法,研究了N-M(In,Ga,Al)共掺(6,6)型闭口氧化锌(ZnO)单壁纳米管的结构稳定性和场致发射特性.结果表明:共掺可增强体系帽端的稳定性;外加电场越大,各体系的态密度(DOS)分布越低,能隙和有效功函数越小,电荷向帽端聚集.DOS,最高占据分子轨道(HOMO)/最低未占据分子轨道(LUMO),Mulliken电荷和有效功函数分析结果一致表明,N-In共掺体系的场发射性能最优.     

单壁碳纳米管储氢密度计算与分析

借助于分子动力学方法,对单壁碳纳米管的储氢过程进行了模拟.根据得到的管内外H2分子的分布规律,计算了H2分子密度分布曲线,对其结果给出了理论分析和物理解释,提出了单壁碳纳米管储氢的多层吸附机制,定量地计算了碳纳米管储氢量(wt.;).这些为进一步研究单壁碳纳米管储氢问题提供了必要的理论依据.     

离子轰击对准直碳纳米管生长的影响

利用负偏压增强热丝化学气相沉积系统,在辉光放电的情况下制备出准直碳纳米管,并用扫描电子显微镜研究了不同负偏压对准直碳纳米管生长的影响.结果表明随着负偏压的增大,准直碳纳米管的平均直径减小,平均长度增大.由于辉光放电的产生,在衬底表面附近形成阴极鞘层,以及在阴极鞘层内形成大量的离子和在衬底表面附近形成很强的电场导致了离子对衬底表面的强烈轰击.最后,分析和讨论了离子的轰击对准直碳纳米管生长的影响.     

单壁碳纳米管中稳态机械波的激发条件研究

通过分子动力学模拟,对单壁碳纳米管中稳态机械波的激发条件进行了详细的研究.利用所施加的周期性机械外力,使单壁碳纳米管产生局部径向形变,该形变将以单壁碳纳米管的管壁为弹性媒质传播开去,从而在单壁碳纳米管中形成机械波;机械波产生的条件依赖于单壁碳纳米管波动系统能量的维持和受迫振动区域形变方向的控制;稳定机械波形成的最佳条件是碳纳米管的形变度和受迫振动频率之间的最优匹配,以及由此形成的单壁碳纳米管在周期性外力作用下产生的共振.     

不锈钢衬底的抛光处理对碳纳米管薄膜场发射性能的影响

在抛光的和未抛光的不锈钢衬底上,利用微波等离子体化学气相沉积(MPCVD)方法从甲烷和氢气的混合气体中沉积碳纳米管薄膜,并对其场发射性能进行了研究.实验发现,不锈钢衬底的机械抛光能降低碳纳米管膜的开启场强,增大它的发射电流密度.在同一场强7.5 V/μm下,衬底未抛光样品的电流密度为2.9 mA/cm2,而衬底抛光样品的电流密度达到5.5 mA/cm2.低开启场强和大发射电流密度意味着β增大,说明机械抛光能使碳纳米管膜的β增大.     

光子晶体中掺杂激活杂质的光增益研究

本文借助光子晶体中二能级原子的自发辐射理论说明缺陷态局域场存在的必然性以及局域场基本性质,为研究掺杂自发辐射的内在规律提供了理论依据,在此基础上,将自发辐射理论与数值模拟相结合,在缺陷介质中掺入和未掺入激活杂质时,数值模拟研究一维光子晶体的掺杂局域场特征以及受激辐射增强和透射率大于1现象与光子带隙边缘群速度异常和掺杂层复有效折射率负的虚部之间的内在规律,由此说明如在光子晶体的缺陷介质中掺入激活杂质,复有效折射率具有负的虚部,光子禁带中会出现品质因子非常高的杂质态,具有很大的态密度,这样便可实现自发辐射的增强,出现较强的受激辐射放大,在带隙的边缘处,光子晶体的群速度较小或群速度异常,受激辐射放大最容易出现在靠近光子带隙的边缘。     

Facile synthesis of pencil‐like ZnO nanostructures at low temperature

Pencil‐like ZnO nanostructure was synthesized by directly oxidizing granular Zn films, which was thermal deposited in a nitrogen atmosphere from Zn powder in a horizontal tube furnace. The formation of the pencil‐like structure, including a hexagonal rod and a sharp tip with diameter about 60 nm, highly depend on the thickness of the initial zinc film and the temperature of the oxidizing process. ZnO nanorods were formed in a relatively low temperature, while thicker zinc film was apt to form a dense ZnO film with tubular structures. The different structured ZnO materials showed distinguishing optical properties which indicate the intrinsic defects forming in the different growth conditions. The pencil‐like ZnO structures exhibit a relatively strong green emission attributed to the high concentrations of oxygen vacancies and its taper tip has great prospects in field‐emission devices.     

Numerical simulation of a low- and a high-electric-field photocurrent decay in a-Si:H

We report a numerical simulation of the photocurrent decay (PCD), from the steady state, in two different structure configurations based on the a-Si:H. The standard DOS of the a-Si:H is used. The high-electric-field PCD is considered in a structure configuration based on a metal/a-Si:H junction. Poisson’s and the two continuity equations are numerically solved in a one-dimensional space to calculate the current density. Two different boundary conditions of the a-Si:H film are considered. The low electric field PCD, which may occurs in a coplanar configuration, is calculated from the solution of a system of two non linear coupled rate equations which govern the free carriers concentrations and the different charges on the localized states in the gap. The calculated PCD versus time curves, for the two configurations, show a shoulder around 1 μs which separate two main regions. We can see that the initial current decay is dominated by the electron emission from the conduction-band tail and the recombination via the dangling bonds states. The second current decay is mainly due to the electrons emission from the dangling bonds. We show also that the PCD curve tends towards the PCD of the coplanar configuration when the electric field decreases.     

Enhancement of field emission in nanotip-decorated ZnO nanobottles

We report a simple fabrication process for the growth of nanotip-decorated ZnO nanobottles with an enhanced field emission property. Well-aligned ZnO nanobottles grown on ITO-coated glass substrates were synthesized by the thermal evaporation technique with the up-side down arrangement of substrate compared with the common method. The hydrothermal growth was carried out to decorate the nanobottles with ZnO nanotips. We demonstrate that the nanobottles decorated with nanotips show a much better field emission property. The improved field emission is attributed to the different local field enhancement factors at the nanobottles tops. Our results confirm the importance of the top morphology of a material in field emission.     

纳米金刚石薄膜的掺杂、表/界面调控及性能研究

本文综述了近年来国内外研究者在纳米金刚石薄膜的掺杂、导电性能、场发射性能和电化学性能等方面的工作,涉及化学气相沉积法制备n型纳米金刚石薄膜,离子注入掺杂纳米金刚石晶粒提高薄膜的n型导电性能,金属离子注入制备场发射性能良好的纳米金刚石薄膜,低剂量离子注入和晶粒表面氧终止态获得高迁移率n型电导,纳米金刚石/石墨烯复合结构的调控对其电学及电化学性能的影响,以及硼掺杂金刚石薄膜电极的微结构和电化学性能研究等。综合分析发现,晶粒掺杂和表界面协同调控可以提升薄膜的电学性能、场发射性能及电化学性能,为纳米金刚石薄膜在纳米电子器件、电化学电极等领域的应用提供了理论基础。     

Optical characteristics of m-plane InGaN/GaN multiple quantum well grown on LiAlO2 (1 0 0) by MOVPE

We investigate the optical properties of m-plane InGaN/GaN multiple quantum well grown on LiAlO₂ substrate by metal organic vapor phase epitaxy. Polarization-dependent photoluminescence and polarization-dependent photoluminescence excitation measurements have been performed at low temperature to study the optical absorption and emission characteristics. The main emission band possesses large polarization anisotropy which may be attributed to the anisotropic biaxial strain. We found the optical emission is not influenced by the polarization-induced electric field from the excitation-dependent photoluminescence measurements. From our results, we attribute the low-temperature emission band around 3.2 eV to interband transition in the quantum well. Besides, the mechanism of the main emission band is associated with interband transition and subsequent carrier localization. The realization of good-quality non-polar GaN-based devices can then be expected in near future.     

微观结构对Eu:Ti纳米材料光学性能的影响

以TiO_2∶Eu纳米颗粒为原材料,采取水热和高温(700 ℃)退火的方法合成了铕掺杂钛纳米棒(简写成Eu∶Ti 纳米棒).其结构和光学性能用场发射扫描电镜,显微共聚焦激光拉曼光谱,稳态荧光以及寿命谱进行表征.场发射扫描电镜图片显示纳米棒的直径大约为30 nm,长度大约为300~600 nm.显微共聚焦激光拉曼光谱测试结果表明纳米棒中含有部分锐钛矿结构.稳态荧光光谱显示纳米棒的发光强度明显比纳米颗粒的强度强,而且发光谱线也不同.检测Eu~(3+)的~5D_0→~7F_2跃迁(发光峰位在612 nm)的发光强度,采用394 nm激发,发现纳米棒的寿命比纳米颗粒的长120 μs.     

Wavelength-tuneable laser emission from a dye-doped achiral nematic liquid crystal dispersed into a chiral polymer scaffold

ABSTRACT

We compare the emission characteristics of a thin-film liquid crystal (LC) laser created using a polymer-stabilized, dye-doped chiral nematic LC to that of an LC laser that was fabricated using an achiral, dye-doped nematic refilled into a chiral polymer scaffold that was templated from the same chiral nematic host. Both lasers exhibit wavelength tuning upon the application of an external electric field. However, for the templated sample, tuning is found to occur across a broader wavelength-range for the same electric field amplitude. We discuss the benefits of the templated approach and how it can be used to circumvent dye bleaching that may occur during photo-polymerisation.