面向实用的新颖碳基点电子源的TEM原位构建

Carbon nanotubes are widely accepted as the ideal materials for field emission due to the superiority of their structures and properties. They are even excepted as very promising candidates to take the place of tungsten cathodes in many advanced instruments. To date, however, the CNT-based point electron sources for real applications are not yet available, mainly attributed to the structural weakness of CNT itself, as well as the difficulties during the materials fabrication and processing. This project aims to solve the problems mentioned above via the newly-developed in-situ transmission electron microscopy (TEM) techniques, so as to achieve the real application of carbon point electron sources. In-situ TEM technique provides not only the direct correlation between structures and properties, but a powerful tool for nanostructure fabrication. By using this technique, we have and will create some novel carbon-based field emitters with combined structures: single-walled nanocone/ multiwalled nanotube, carbon-onion/W-tip, and few-layer graphene encapsulating a W tip, etc. All these new structure share the same feature: they possess the most crucial part of a CNT,i.e. the highly curved closed end consisting of sp2 C-C bonds, but the CNT shank has been modified or even completely removed. These structures are supposed to be more stable, and the field emission stability and the capability to carry large emission current should be remarkably improved. This is actually the key issues for a practical electron source with high brightness and better coherence. We are planning to carry out thorough tests to investigate the fabrication techniques, formation mechanism, and field emission properties of these novel structures. Through structural tailoring of these carbon electron sources, better performance will be achieved to make them the practical field emitters.

碳纳米管因其优越的结构和性能被公认为理想的场发射材料，并有望取代钨阴极成为各种先进仪器的场发射点电子源。然而迄今为止，真正实用的碳管点电子源尚未出现，这主要归因于碳管本身还存在的结构弱点以及加工技术方面的困难。本项目拟采用新兴的原位透射电镜技术来解决这些难题，以推动碳基点电子源的实用化。原位电镜技术不仅可以实现性能和结构的直接对应，还是强大的加工制备手段。利用此技术，我们已经并还将再构造出几种新颖的碳基场发射复合尖端结构：单壁纳米锥/多壁管、巴基葱/钨针尖、少层石墨烯包裹的钨针尖等。这些结构的共同特点是：保留碳管最关键部位，即由sp2碳键构成的高曲率闭合末端，改造或是消除碳管径部。它们的结构更加稳固，场发射稳定性和大电流的承受力显著提高，这也是碳材料成为实用高亮相干电子源的关键。我们将对这些结构的制备工艺、形成机理和场发射性能做深入系统的测试和研究,并通过结构调控使其更接近实用场发射阴极。

本项目旨在利用原位透射电镜技术，构造出若干新颖的碳基场发射复合尖端结构，并对其制备工艺、形成机理和性能调控进行系统研究，从而为推动碳基点电子源的实用化打下基础。已取得的主要成果包括：1）利用电子束辐照和焦耳热的交替作用，实现碳纳米管的晶态/非晶结构的可控循环，从而高精度地调节碳管的机械和导电性能，并首次实现了碳管场发射性能的可逆调控（Nano Lett. 2016；Carbon 2018, 封面）。2）发展了一系列碳纳米管的长度与直径的高精度结构控制技术。首次实现两个不同直径、层数和手性的多壁碳管的无缝焊接，从而实现碳管的可控加长甚至碳管和石墨烯的无缝互连（Materials Horizons, 2019，封面）；利用电子束辐照并同时辅以高温退火，实现单根或多根碳管直径和长度的无损缩减和高精度调控，并提出了一种基于位错攀爬的碳原子对有序蒸发机制来解释此新现象。3）对于金属电极/碳异质纳米结构在大电流作用下的界面反应进行系统研究，发现不同重金属对石墨烯表现出不同的“碳饥渴”行为，被吸入的碳再以弯曲石墨烯的形式在金属表面析出，并提出了周期性横向扩张/纵向增厚的石墨烯边沿生长模型；作为对比，轻金属则正好相反，它们反纳米碳结构吸入，经过体内扩散后在碳表面沉积，且此过程可逆；这些为制备稳定的金属/碳发射阴极或电源负极提供重要参考（ACS Nano 2017, Nano Energy 2017）。4）此外，还发展了一种高度可控的石墨烯剪裁技术：利用一种碳/钨“纳米刀”对石墨烯边缘原子进行可控蒸发，从而实现对单片石墨烯的形状、尺寸以及边缘手性的任意控制；再通过加热处理，其边缘粗糙度可由1-2纳米进一步提升至原子级平整度，从而获得所期待的石墨烯点电子源结构，并以此来调控其场发射性能。这些碳管和石墨烯结构控制工作（部分待发表）不仅为精密加工sp2碳基场发射阴极提供重要技术基础，更重要的是，我们以此提出和材料生长过程相反的一种“有序质量减法”过程来控制结构的新概念，为碳管和石墨烯的制备及电子学方面的应用提供了独特思路。