单壁碳纳米管由于具备纳米尺寸的原子结构和良好的电学性质，被认为是构筑未来芯片和柔性器件的最佳材料之一。一直以来，直接制备的单壁碳纳米管都是金属性和半导体性碳纳米管的混合物，这为碳纳米管的实际应用带来巨大困难。据估计，要想使碳纳米管进入到商业化应用的阶段，其不仅要具备非常高密度和良好的品质，同时还要具有高达99.9999％的半导体纯度。虽然金属性和半导体性碳纳米管的电学性质完全不同，但是它们在结构上的差异十分微小，甚至可以仅相差一个手性指数。然而，基于当前对碳纳米管生长机理的认识，直接制备的半导体性碳纳米管阵列纯度难以有所突破。本论文以一次偶然发现作为切入点，详细介绍了超长碳纳米管在生长过程中会有负电荷自发积累这一新现象。通过对超长碳纳米管上自发积累的电荷进行系统表征，以及一系列原位电学测量，确认了化学气相沉积法制备碳纳米管时电荷产生和转移的普遍规律，提出了碳纳米管生长的电化学原理。随后开展的定量理论分析，不仅对碳纳米管生长的这一新机理提供了有力的支撑，还预测了金属性和半导体性碳纳米管在生长过程中对外加电场会产生不同的响应。基于碳纳米管生长的电化学原理，发现并提出了场效应催化原理。利用金属性碳纳米管和半导体性碳纳米管电子态密度的巨大差异，通过外加电场给催化剂区域充大量负电荷，人为扩大了不同类型碳纳米管生长时的自由能差异，减小了半导体性碳纳米管的重新成核势垒。基于上述场效应催化原理，利用脉冲电场实现了大于99.9% 纯度的高质量半导体性碳纳米管水平阵列的稳定制备，并进一步通过控制碳纳米管同时成核，实现了1英寸晶元尺寸半导体性碳纳米管水平阵列的稳定制备。理论分析表明，在限制催化剂颗粒尺寸的前提下，利用电致手性扭转的方法，原则上能够获得满足半导体工业需求的高纯度的半导体性碳纳米管水平阵列。对化学气相沉积过程中电化学原理和场效应催化原理的系统性研究，不仅实现了直接制备高纯度半导体性碳纳米管水平阵列，为碳纳米管在半导体工业的实际应用铺平了道路，同时也将为其他新型纳米材料的可控制备提供新的思路，有望对半导体工业的发展产生积极影响。
Single-walled carbon nanotubes (SWCNTs) are anticipated as the most promising material to construct high-performance chip and flexible devices in the future for their nanostructures and extraordinary electronic properties. A prerequisite of practical applications of SWCNTs in electronics is directly synthesizing pure s-CNT arrays with high quality. However, as-grown SWCNTs, usually, are the admixture of metallic SWCNTs (m-CNTs) and semiconducting SWCNTs (s-CNTs), which has been one of the biggest hurdle for their industrial applications. Although the difference in electronic density of state between s-CNTs and m-CNTs is dramatic, the difference in structures between them is really small. The growth of highly-pure s-CNTs is impeded by the structural similarity of SWCNTs. To eliminate this roadblock, growth mechanism of SWCNTs should be further studied.In this paper, we started the discussion of growth mechanism and controlled growth of CNTs with an unexpected discovery that growing ultralong CNTs are negatively charged during the growth. By ex-situ measuring the charge distribution of as-grown ultralong CNTs and in-situ monitoring the charge generation during the growth of CNTs, we confirmed that the charge generation and transfer generally happened in a catalytic growth of CNTs and proposed an electrochemical mechanism to illustrate this process. With more detailed and quantitative analysis, the different growth behavior of m-CNTs and s-CNTs under applied electric field is predicted. Based on the electrochemical mechanism of the growth of CNTs, we further reported a concept of field-effect catalysis (FEC). Using the dramatic difference in the electronic density of state between m-CNTs and s-CNTs, the free energy difference between the growth of m-CNTs and s-CNTs is amplified by negatively charging the catalyst. As a result, m-CNTs tend to be twisted to s-CNTs with an energy barrier through the formation of a chiral junction. To selective grow s-CNTs, carefully designed electric field pulse is used to negatively charge the catalyst and to introduce a perturbation to overcome aforementioned energy barrier. Based on this method, high quality s-CNT arrays are obtained stably with the purity up to 99.9%. Moreover, with the control of nucleation time of CNTs in inch-size area, stable growth of wafer-scale highly-pure s-CNT arrays is realized. Calculation of FEC shows that direct synthesis of s-CNTs with any purity to meet the requirement of semiconductor industry can be achieved if the diameter of CNTs is further confined.The study of electrochemical mechanism and FEC in CVD system not only realize the growth of highly-pure s-CNT arrays that paves the way for the industrial application of CNT electronics, but also provides a new way of thinking about selective synthesis of other nanomaterials and promotes the development of semiconductor industry.
