碳纳米管具有完美的一维结构、密度低、导电和导热性能好等优异的物理化学性质，在纳米电子学和光电子学、真空电子学、传感器与致动器、高强度复合材料、航天工程等领域有着广泛的应用前景。对于这样一种重要的材料，将来必定会产生大规模应用。多年以来，我们研究组的目标就是实现碳纳米管的可控制生长，即人为控制合成不同管径、不同螺旋度、不同壁厚、不同长度的碳纳米管以适应不同的产业需求。而要实现这一目标，就必须研究清楚碳纳米管的生长机理，弄清楚碳纳米管是如何成核、为什么生长自动终止、如何控制管径、如何控制螺旋度等问题。基于这种认识，本论文的研究目标定位为研究清楚碳纳米管的生长机理，实现其可控制生长。论文工作的思路是实验和理论建模紧密结合，互相促进。一方面从实验探索中总结规律，进行理论建模；另一方面根据掌握的生长机理，实现可控制生长。经过几年的努力，目前主要在超顺排碳纳米管阵列的可控制生长、碳纳米管生长动力学、碳纳米管成核机理等方面取得一些阶段性成果。

1．超顺排碳纳米管阵列的可控制生长

碳纳米管阵列是一种自组织形成的有序结构，其中碳纳米管垂直于基底平行排列，并且直径分布很集中均一，长度也几乎一致。因此，碳纳米管阵列是代表高质量碳纳米管的有序结构。通过控制碳纳米管的生长速率，合成出超顺排碳纳米管阵列。该阵列区别于普通阵列的特点是：成核密度高，生长速率快，直径分布窄，平行排列的有序性更高，其中的碳纳米管表面洁净，碳纳米管之间存在较强的范德瓦尔斯力。当从超顺排碳纳米管阵列中拔出一束碳纳米管时，碳纳米管可以自组织形成一条连续的长线。在这个过程中，超顺排碳纳米管阵列起的作用如同一个蚕茧,阵列中的碳纳米管则由范德瓦尔斯力首尾相连形成连续的纯碳纳米管线。我们将这种碳纳米管线平行排列起来构成的偏振片，可以工作在紫外波段。这种碳纳米管线还可以用作白炽灯的灯丝，仅需很小的功率就可以发出白炽光。近期又针对该碳管线的粘性发展了用挥发性溶剂处理的方法，处理后的碳纳米管线无论是强度还是可操作性都得到了大幅度的提高，可以方便地构筑各种宏观物体。2005年我们将这种超顺排阵列的合成规模扩大到4英寸，可以抽出10厘米宽60米长的薄膜，这种薄膜可以很方便地用做透明导电膜和薄膜晶体管。近期又实现了这种超顺排阵列的批量化合成。

2．碳纳米管生长动力学

尽管碳纳米管可以用多种方法，多种前驱物、多种催化剂合成出来，但是人们对其生长过程还没有一个清晰的物理图像。为了研究碳纳米管的生长动力学，我们发展了一种简单易行的碳纳米管生长标记方法，用于测定不同温度下生长过程中速率的变化，从而测定整个生长反应的活化能。我们的实验结果表明，在我们实验中，碳纳米管阵列生长速率是由前驱物在催化剂表面化学反应速率决定的，并且，表面分解速率限制的生长模式有利于形成毫米级长度碳管阵列。基于这些实验结果，我们针对合成碳纳米管的CVD方法提出了一个基于VLS机制的生长模型，其中的催化剂处于液态，在生长过程中，催化剂中温度和碳原子浓度是均匀分布的，生长过程中的过饱和度可以表示为碳原子浓度和温度的函数，其数值大小决定了其成核和生长动力学。基于这个模型，我们推导出不同分压下碳纳米管稳态轴向生长速率公式，能够很好地解释生长动力学实验的结果。

3．碳纳米管成核机理

研究清楚碳纳米管的生长机理，弄清楚碳纳米管是如何成核、为什么生长自动终止、如何控制管径、如何控制螺旋度等问题,对于实现碳纳米管的大规模工业应用具有重要意义。

我们认为，无论是单壁碳纳米管还是多壁碳纳米管，无论是用电弧放电、激光蒸发还是CVD的方法，只要是利用催化剂生长碳纳米管，其遵循的生长机理一定是一样的，因为它们都源自一个液态或类液态的碳-金属过饱和溶液。我们这里提出的成核机理其出发点就是碳-金属溶液，尽管它的形成方式对三种主要的合成方法来说可能有所不同。当碳-金属溶液过冷（对应电弧放电方法和激光蒸发方法）或过饱和（对应CVD方法）时，一个小的石墨片就会在催化剂表面成核，随后碳原子在其边缘（碳原子台阶，step edge）处生长。过饱和度（是温度和碳原子浓度的函数）决定了不同的成核行为并导致不同的碳纳米管结构如单壁、双壁、多壁以及竹节状碳纳米管等。

基于以上认识，将VLS机制进一步推广，提出适用于气态、液态和固态前驱物的XLS（X-Liquid-Solid）模型。该模型的出发点是处于液态的碳-催化剂溶液，其中的碳原子浓度决定了该溶液的过饱和度，过饱和度又进一部决定了石墨单层在催化剂表面的成核方式。基于该模型，利用经典成核理论以微观键能和过饱和度为基本参数讨论了石墨单层在催化剂表面的成核机理，明确指出石墨单层初级成核的两种方式和二次成核方式，推导出各种成核方式下的临界成核尺寸和成核活化能公式，探讨了过饱和度对成核的影响。进一步利用该成核机理讨论了单根单壁碳纳米管、单壁碳纳米管束、多壁碳纳米管、竹节形多壁碳纳米管以及碳纳米纤维的成核与生长过程。并基于该机理回答了金属催化剂在生长中起什么作用、多壁管内外层是否有螺旋度的相关性、生长为何会自动终止、如何控制单壁或多壁的生长等问题。该成核机理可以普遍地应用于电弧放电、激光蒸发、CVD等方法催化合成的碳纳米管，并与目前大多数实验结果符合。

通过本论文工作，希望能够对各种碳纳米管的成核与生长过程在一个统一理论框架下得到定性和半定量的认识，期望将来进一步的工作是如何与成核、生长实验结果作定量地对比。并在理解生长机理的基础上，发展出更多可控制生长的方法，以适应不同的工业需求。

Owing to their perfect 1-D nanostructure, low density, and high conductivity of both heat and electricity, carbon nanotubes (CNTs) show great promises in nano-electronics and photonics, vacuum electronics, sensors and actuators, high strength composite materials, and aerospace engineering etc. For such an important material, there must be some large-scale applications in industry. For many years, we have aimed at achieving controlled synthesis of CNTs, i.e., synthesizing CNTs with desired diameter, chirality, wall thickness, and length to meet a variety of industrial demands. To achieve this aim, one has to studying the growth mechanisms of CNTs, finding out how CNT nucleates, why growth teminates, how to control the diameter and chirality etc. Thus the goal of this study is to clarify the growth mechanisms and achieve controlled synthesis of CNTs. The approach of this thesis is a combination of experimental investigation and theoretical modeling which are stimulative to each other. Finding out basic laws from experimental exploration will facilitate theoretical modeling. On the other hand, one can design new growth method according to the growth mechanism. After several years of efforts, we have made some progress in the controlled synthesis of super-aligned CNT arrays, the growth kinetics, and the nucleation mechanisms of CNTs.

1．Controlled synthesis of super-aligned CNT arrays

CNT array is a self-organized ordered structure, in which parallel CNTs with narrow diameter distribution and nearly identical length are regularly aligned perpendicular to the substrate. Thus CNT arrays are aggregations of high quality CNTs. By tuning the growth rate of CNT arrays, super-aligned CNT arrays are successfully synthesized. The super-aligned CNT arrays are distinguished from normal arrays by their higher nucleation density and growth rate, narrower diameter distribution, better alignment, as well as cleaner surfaces and stronger van der Waals interaction between CNTs. When trying to pick up a bundle of CNTs from the super-aligned array, a continuous yarn is obtained. Here the suer-aligned array has the similar function as a silk cocoon. Upon drawing, CNTs in it are end to end joined together by van der Waals force forming continuous yarns of pure CNTs. Optical polarizers are constructed by parallel aligning the yarns, which can work even in the UV region. These yarns can also be used as filament of light bulb, from which incandescence can be emitted at small power input. Recently a novel method was invented to process fresh yarn by passing through volatile solvent, which greatly enhanced the mechanical strength and improved the manipulability. The processed yarn is both elastic and pliable and can be freely manipulated and mold to any desired shape to construct a variety of macroscopic objects for various applications. In 2005, the synthesis was expanded to 4-inch wafer scale. A super-aligned array on 4-inch wafer can be converted to a continuous thin film of 10 centimeters wide and 60 meters long, which can be directly employed as transparent conducting film and thin film transistors. Recently we have achieved batch growth of 4-inch super-aligned arrays in a 6-inch tube furnace.

2．Growth kinetics of CNTs

Today, CNTs can be synthesized with a variety of methods by using a variety of precursors and catalysts. However, there is still no clear physical picture of the growth process. To study the growth kinetics of CNTs, we developed a simple but effective growth mark method to measure the growth rate during growth at various temperatures, from which the activation energy of the overall growth reaction can be obtained. It is found that the surface reaction is the rate-limited step in our synthesis. This surface reaction limited growth favors the growth of millimeter high CNT arrays. According the these experimental results, A model based on VLS (vapor-liquid-solid) mechanisms was proposed for CVD growth of CNTs, which involves a liquid state catalyst with homogeneous temperature and carbon concentration distribution. The growth kinetics was fully controlled by the super-saturation level which can be expressed as a function of temperature and carbon concentration in the catalyst droplet. Based on this model, the steady-state axial growth rate equation was derived, which fits very well with our experimental results of kinetic study.

3．Nucleation mechanisms of CNTs

To achieve large-scale industrial applications, it’s very important to studying the growth mechanisms of CNTs, finding out how the CNTs nucleate, why growth terminate, how to control the diameter and chirality etc.

We believe that there must be a unified mechanism for catalytic growth of CNTs, no matter what kind of catalysts and methods were used. The reason is that, the growth of CNTs, whether single-walled or mult-walled, whether arc, laser, or CVD method, all starts from the formation of a super-saturated carbon-metal solution. Here the start point of the proposed nucleation mechanism is the carbon-metal solution which will give birth to a graphene nuclei upon super-saturation or super-cooling. Then growth takes place at the edge of graphene nuclei. It’s the super-saturation level that determines the nucleation behavior and the formation of various structures such as single-walled, double-walled, multi-walled and bamboo-shaped CNTs.

Then the VLS model was further generalized to XLS (X-liquid-solid) model, in which the precursor can be in any X phase (X=gas, liquid or solid). Based on this model, we applied classical nucleation theory to the nucleation mechanisms of graphene layer over the surface of carbon-catalyst solution, in which super-saturation level and microscopic bond energies were adopted as the basic parameters. Three kinds of nucleation modes were distinguished. The critical sizes and activation energy barriers of these nucleation modes were derived, as well as the influences of super-saturation on them. The nucleation and growth processes of all kinds of CNTs and carbon nanofibers were analyzed by using this nucleation mechanism. And such questions as, what is the role of metal catalyst in nucleation and growth, is there any chirality correlation between adjacent layers of multi-walled CNTs, why growth terminates, how to control the wall thickness and diameter, etc. were answered. This nucleation mechanism can be generally applied to CNT growths via arc, laser or CVD and fit well with existing experimental results.

The work presented here enables us to understand the nucleation and growth process of CNTs in a general framework qualitatively and semi-quantitatively. In the future, quantitative descriptions are expected. Furthermore, based on the understanding of the growth mechanisms, many kinds of growth methods will be developed to meet a variety of industrial demands.