2.3.2 Chemical Vapor Deposition

The growth process involves a catalyst material at high temperature in a tube furnace and a hydrocarbon gas flowing through the tube reactor for a period of time. Materials grown over the catalyst are collected upon cooling the system to room temperature. The key parameters in CVD growth of CNTs are the hydrocarbons, catalysts and growth temperature. The active catalytic species are typically transition-metal nano-particles formed on a support material such as alumina. The general CNT growth mechanism in a CVD process involves the dissolution and saturation of carbon atoms in the metal nano-particles. The precipitation of carbon from the saturated metal particle leads to the formation of tubular carbon solids in $ sp^2$ structure. Tubular formation is favored over other forms of carbon such as graphitic sheets with open edges. This is because a tube contains no dangling bonds and, therefore, is in a low energy form.

Recent interests in CVD growth of CNTs are also due to the idea that aligned and ordered CNT structures can be grown on surfaces in a controlled manner that is not possible with arc-discharge or laser ablation techniques [25].

Methods developed to obtain MW-CNTs include CVD growth of tubes in the pores of mesoporous silica. DAI and coworkers have devised growth strategies for ordered MW-CNTs and SW-CNTs by CVD on a catalytically patterned substrate [25]. They found that MW-CNTs can self-assemble into aligned structures as they grow, and the driving force for self-alignment is the VAN DER WAALS interaction between CNTs [26]. The growth approach involves catalyst patterning and rational design of the substrate to enhance catalyst-substrate interaction and control the catalyst particle size. Figure 2.4 shows scanning electron microscopy (SEM) images of regularly positioned arrays of CNT towers grown from patterned iron squares on a porous silicon substrate.

Ordered SW-CNT structures can be directly grown by methane CVD on catalytically patterned substrates. A method has been devised to grow suspended SW-CNT networks with directionality on substrates containing lithographically patterned silicon pillars [27,28]. Contact printing is used to transfer catalyst materials onto the pillar's tops selectively. CVD of methane using these substrates leads to suspended SW-CNTs forming nearly ordered networks with the CNT orientations directed by the pattern of the pillars (Fig. 2.5).

Figure 2.4: Self-aligned MW-CNT arrays grown by CVD on a catalytically patterned porous silicon substrate a) SEM image of tower structures consisting of aligned CNTs and b) SEM image of the side view of the towers [25].
Figure 2.5: Directed growth of suspended SW-CNT a) and b) square of CNTs and c) CNT power-line structure [28].
M. Pourfath: Numerical Study of Quantum Transport in Carbon Nanotube-Based Transistors