**2. CVD versus PECVD: Morphology control issues**

#### **2.1. CVD and PECVD: General**

The term 'chemical vapor deposition', or 'CVD', is commonly used for describing the processes and chemical reactions which occur in a solid material deposited onto a heated substrate using a gaseous precursor. However, more complicated process than the 'common' CVD takes place [1,2] during the growth of CNTs. In this case, the carbon-containing gaseous precursors (e.g., CH4, C2H4, C2H2, CO) firstly dissociate into atomic or molecular carbon species on the surface of catalyst nanoparticles, and then the nucleation occurs as these carbon species diffuse into the cat‐ alyst nanoparticles, reach a supersaturated state, and then segregate from the surface of nano‐ particles to form a nanotube cap. Subsequently, the growth of nanotubes is sustained by the continuous incorporation of carbon atoms *via* bulk and/or surface diffusion. Figure 2 shows the SEM image of randomly-oriented SWCNTs with a unique 'bridging' morphology in catalytic CVD. This array was grown by using Ar/H2/CH4 gas mixture on Fe/Al2O3 catalyst. By tuning the growth condition (e.g., temperature and pressure), it was demonstrated that the SWCNTs could be of a high quality (a high IG/ID in the Raman spectra) and could contain a significantly higher content of metallic nanotubes as compared to the 'standard' metallic nanotube content of 33% (1/3 metallic and 2/3 semiconducting) produced in many CVD processes [3].

growth morphology (see Figure 1). Besides, the possibility of creating the 3D structures of car‐

The fabrication methods involved are the conventional CVD utilizing various gases such as methane, ethane, acetylene, argon, and hydrogen; plasma-enhanced CVD based on induc‐ tively-coupled low-temperature plasma reactor; microwave PECVD. The advantages of the plasma-based CVD process will be shown and discussed with a special attention. We will also discuss the influence of the process parameters such as process temperature, pressure, gas composition, discharge power etc. on the morphology of the nanotube arrays and networks, and demonstrate that the proper selection of the parameters ensures very high level of the process controllability and as a result, sophisticated control and tailoring of the growth

Characterization technologies used are scanning and transmission electron microscopy (SEM and TEM), as well as atomic force microscopy (AFM), Raman and X-ray photoelectron spectroscopy techniques. The results of the numerical simulations will also be used to support

The term 'chemical vapor deposition', or 'CVD', is commonly used for describing the processes and chemical reactions which occur in a solid material deposited onto a heated substrate using a gaseous precursor. However, more complicated process than the 'common' CVD takes place [1,2] during the growth of CNTs. In this case, the carbon-containing gaseous precursors (e.g., CH4, C2H4, C2H2, CO) firstly dissociate into atomic or molecular carbon species on the surface of catalyst nanoparticles, and then the nucleation occurs as these carbon species diffuse into the cat‐ alyst nanoparticles, reach a supersaturated state, and then segregate from the surface of nano‐ particles to form a nanotube cap. Subsequently, the growth of nanotubes is sustained by the continuous incorporation of carbon atoms *via* bulk and/or surface diffusion. Figure 2 shows the SEM image of randomly-oriented SWCNTs with a unique 'bridging' morphology in catalytic

bon nanotubes through post-processing of the arrays by liquids will be discussed.

structure and morphology of the carbon nanotube arrays.

20 Syntheses and Applications of Carbon Nanotubes and Their Composites

the growth models and proposed growth mechanisms.

**Figure 1.** Morphologies of the representative CNT arrays grown by CVD (a) and PECVD (b).

**2. CVD versus PECVD: Morphology control issues**

**2.1. CVD and PECVD: General**

On the other hand, PECVD refers to the CVD process that uses plasma environment as an extra dimension to control the growth of CNTs. Plasma by definition contains ionized species and is generally considered as the fourth state of matter along with solid, liquid and gas. Recent advan‐ ces in the plasma-based nanofabrication offer unprecedented control over the structure and sur‐ face functionalities of a range of nanomaterials [4]. One of the major advantages, as compared to the conventional CVD processes, is that nanostructures can grow vertically-aligned due to the electrical field in the vicinity of surface [5]. Another benefit of using plasma is that the tempera‐ ture required to dissociate carbon feedstock could be greatly reduced [6]. Figure 3 illustrates the isolated CNTs grown in a PECVD system using Ni/SiO2 as the catalyst, C2H2/NH3 as the gas pre‐ cursors, and a DC glow discharge. It can be seen clearly that these nanotubes are aligned vertical‐ ly to the substrate surface, due to the plasma sheath-directed growth. These freestanding nanotubes could give many opportunities to custom-design novel functional devices.

**Figure 2.** Typical randomly-oriented SWCNT networks with a unique "bridging" morphology grown in catalytic CVD [3].

**Figure 3.** Low- and high-resolution SEM images of the typical arrays of vertically-aligned CNTs grown in PECVD proc‐ ess with a glow discharge. The growth followed the 'tip-growth' mode as the catalyst nanoparticles are noticeable on the top of each nanotube [4].

#### **2.2. Morphologies of nanotube arrays**

In general, there are three types of morphologies observed in the directly-grown nanotube arrays: entangled, horizontally aligned, and vertically aligned. Each of these morphologies has their specific functionalities and can be desirable for different applications. In this work, we will briefly describe the first two morphologies and then pay the most attention to the arrays of vertically aligned nanotubes.

**2.3. Vertically-aligned arrays of carbon nanotubes**

differentiation and proliferation of these stem cells [12].

wetting (this will be discussed in more detail in the next section).

**Figure 5.** Dense array of vertically aligned single-walled carbon nanotubes.

supporting SiO2 layer [14].

**2.4. Entangled arrays of carbon nanotubes**

Vertically-aligned CNTs not only preserve the excellent intrinsic properties of individual nanotubes, but also show a high surface-to-mass ratio owing to their three-dimensional microstructure. Moreover, the surface of the vertically-aligned CNTs could be easily function‐ alized. These advantageous features have placed the vertically-aligned CNT arrays among the most promising materials for a variety of applications ranging from field emitters, heat sinks, nanoelectrochemical systems, gas- and bio-sensors, drug delivery systems, to molecular/ particular membranes. For example, Wu et al. used the functionalized vertically-aligned CNTs to deliver nicotine for therapeutic purposes [11]; Han et al. studied the release behaviors of bone morphogenetic protein-2 (BMP-2; a growth factor for human mesenchymal stem cells) on the vertically-aligned CNTs with different surface wettability, in attempting to control the

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23

Growth of the vertically-aligned CNTs can be easily obtained in PECVD. Figure 5 shows SEM images (high and low magnification) of the vertically-aligned nanotubes grown in the lowtemperature plasma [13]. These CNTs have a diameter of 50-200 nm, a height of several micrometers, and followed a "tip-growth" mechanism. Interesting, they collapse upon liquid

The the vertically-aligned CNTs grown using CVD process are much denser and longer, and have more uniform distribution of diameters. In such arrays, very strong Van der Waals forces are present. The CVD process is therefore suitable for mass production of CNTs, and may contribute to lowering the price of CNTs. We have recently demonstrated that highly uniform and dense arrays of SWCNTs with more than 90% population of thick nanotubes (>3 nm in diameter) could be obtained by tailoring the thickness and microstructure of the catalyst

Networks of entangled nanotubes consist of randomly-oriented nanostructures. The thickness of the entangled array may vary from sum-monolayer to a few monolayers. Advantages of

The horizontally-aligned CNT arrays were usually grown on the quartz wafers using CVD. The alignment could in such arrays be controlled by two factors: gas flow direction and sub‐ strate lattice. These arrays could have a very high density (up to 50 SWNT/µm) over large area. These nanotubes have also a large diameter and good electrical properties desirable for the nanoelectronic applications [7].

On the other hand, the vertically aligned CNTs could be grown using both CVD and PECVD. *Hata et al.* demonstrated that by using Fe/Al2O3 as the catalysts, C2H4 as the feed‐ stock and a trace amount of water vapor (100 – 300 ppm) as the growth enhancer, highyield, milli-meter long vertically aligned SWCNTs could be produced [8]. Water in this process was used to etch the possible amorphous carbon phase deposited onto the catalyst during the growth, therefore enhancing the lifetime and activity of the catalyst. The vertical alignment was supported by the collective van der Waals' interactions among the nanotubes [9]. In contrast, the CNTs grown in PECVD process do not require such growth enhancer to align them vertically, since the electrical field in the plasma-surface sheath at the vicinity of the substrate could easily direct the growth.

The third type of CNT arrays is the entangled network consisting of interconnected random‐ ly-oriented nanotubes. In some cases, these networks are not entirely 'random'; instead, they may form certain unique features such as the 'Y-junctions', as well as 'knotted' and 'bridging' structures. *Sun et al.* demonstrated that by using a porous membrane filter to collect the nanotubes at room temperature, a unique 'Y-junction' with high electronic performance could be induced in an aerosol CVD process [10]. Similar to VACNTs, they can be produced in both CVD and PECVD processes. Figure 4 illustrates both the horizontally and vertically aligned morphologies obtained by our group.

**Figure 4.** Highly uniform, dense array of vertically aligned single-walled carbon nanotubes (SWCNTs) grown on trilay‐ ered Fe/Al2O3/SiO2 catalyst (a). Horizontally-aligned nanotubes (b).

#### **2.3. Vertically-aligned arrays of carbon nanotubes**

**2.2. Morphologies of nanotube arrays**

22 Syntheses and Applications of Carbon Nanotubes and Their Composites

arrays of vertically aligned nanotubes.

the nanoelectronic applications [7].

the substrate could easily direct the growth.

aligned morphologies obtained by our group.

ered Fe/Al2O3/SiO2 catalyst (a). Horizontally-aligned nanotubes (b).

In general, there are three types of morphologies observed in the directly-grown nanotube arrays: entangled, horizontally aligned, and vertically aligned. Each of these morphologies has their specific functionalities and can be desirable for different applications. In this work, we will briefly describe the first two morphologies and then pay the most attention to the

The horizontally-aligned CNT arrays were usually grown on the quartz wafers using CVD. The alignment could in such arrays be controlled by two factors: gas flow direction and sub‐ strate lattice. These arrays could have a very high density (up to 50 SWNT/µm) over large area. These nanotubes have also a large diameter and good electrical properties desirable for

On the other hand, the vertically aligned CNTs could be grown using both CVD and PECVD. *Hata et al.* demonstrated that by using Fe/Al2O3 as the catalysts, C2H4 as the feed‐ stock and a trace amount of water vapor (100 – 300 ppm) as the growth enhancer, highyield, milli-meter long vertically aligned SWCNTs could be produced [8]. Water in this process was used to etch the possible amorphous carbon phase deposited onto the catalyst during the growth, therefore enhancing the lifetime and activity of the catalyst. The vertical alignment was supported by the collective van der Waals' interactions among the nanotubes [9]. In contrast, the CNTs grown in PECVD process do not require such growth enhancer to align them vertically, since the electrical field in the plasma-surface sheath at the vicinity of

The third type of CNT arrays is the entangled network consisting of interconnected random‐ ly-oriented nanotubes. In some cases, these networks are not entirely 'random'; instead, they may form certain unique features such as the 'Y-junctions', as well as 'knotted' and 'bridging' structures. *Sun et al.* demonstrated that by using a porous membrane filter to collect the nanotubes at room temperature, a unique 'Y-junction' with high electronic performance could be induced in an aerosol CVD process [10]. Similar to VACNTs, they can be produced in both CVD and PECVD processes. Figure 4 illustrates both the horizontally and vertically

**Figure 4.** Highly uniform, dense array of vertically aligned single-walled carbon nanotubes (SWCNTs) grown on trilay‐

Vertically-aligned CNTs not only preserve the excellent intrinsic properties of individual nanotubes, but also show a high surface-to-mass ratio owing to their three-dimensional microstructure. Moreover, the surface of the vertically-aligned CNTs could be easily function‐ alized. These advantageous features have placed the vertically-aligned CNT arrays among the most promising materials for a variety of applications ranging from field emitters, heat sinks, nanoelectrochemical systems, gas- and bio-sensors, drug delivery systems, to molecular/ particular membranes. For example, Wu et al. used the functionalized vertically-aligned CNTs to deliver nicotine for therapeutic purposes [11]; Han et al. studied the release behaviors of bone morphogenetic protein-2 (BMP-2; a growth factor for human mesenchymal stem cells) on the vertically-aligned CNTs with different surface wettability, in attempting to control the differentiation and proliferation of these stem cells [12].

Growth of the vertically-aligned CNTs can be easily obtained in PECVD. Figure 5 shows SEM images (high and low magnification) of the vertically-aligned nanotubes grown in the lowtemperature plasma [13]. These CNTs have a diameter of 50-200 nm, a height of several micrometers, and followed a "tip-growth" mechanism. Interesting, they collapse upon liquid wetting (this will be discussed in more detail in the next section).

**Figure 5.** Dense array of vertically aligned single-walled carbon nanotubes.

The the vertically-aligned CNTs grown using CVD process are much denser and longer, and have more uniform distribution of diameters. In such arrays, very strong Van der Waals forces are present. The CVD process is therefore suitable for mass production of CNTs, and may contribute to lowering the price of CNTs. We have recently demonstrated that highly uniform and dense arrays of SWCNTs with more than 90% population of thick nanotubes (>3 nm in diameter) could be obtained by tailoring the thickness and microstructure of the catalyst supporting SiO2 layer [14].

#### **2.4. Entangled arrays of carbon nanotubes**

Networks of entangled nanotubes consist of randomly-oriented nanostructures. The thickness of the entangled array may vary from sum-monolayer to a few monolayers. Advantages of such morphology, as compared to individual nanotubes, are scalability, stability, reproduci‐ bility, and low cost of the CNT-based devices. They are therefore widely used as thin film transistors, transparent conductive coatings, solar cells, gas and biosensors. The electrical resistivity in entangled SWCNTs is determined by the nanotube-nanotube junctions in the network, and the nanotube-metal junctions at the electrodes (so-called Schottky barrier). The intrinsic resistance of the nanotubes usually plays a minor role if the array density is not far away from the percolation threshold [15]. In addition, it is generally perceived that for the CNT-based device to deliver outstanding performance, chirality-selected growth of CNT is a pre-requisite. However, for entangled SWCNTs, this stringent requirement may be avoided if the density is within a certain range (usually 1–3 nanotubes/µm2 ) [16,17,18].

or degradation of the nanotube ordering or orientation (in particular, post-processing deteri‐ orates the vertical orientation of the nanotubes), damages the substrate structure in high temperature annealing process, etc. Thus, removal of the metallic catalyst by after-growth postprocessing is feasible only for limited small-scale experimental production [21]. Hence, the development of the catalyst-free methods for growing arrays of high quality, dense vertically aligned nanotubes is a pressing demand now. The metal-free nucleation and growth of carbon nanotubes is possible, yet with the use of other catalytic material, and with a low quality outcome. For example, the nucleation and growth on semiconductor nanoparticles in CVD process was recently reported [22,23,24]. In these works, the nanotubes were catalyzed and grown without metal catalyst, but those nanotubes are not vertically aligned but highly tangled, tousled, and the surface density is quite low. Therefore, obtaining high quality arrays

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**Figure 7.** a) Three typical process configurations: localized plasma, remote plasma, gas environment; (b) nanotubes growth on Si substrate contacting with plasma: dense nanotubes as-grown on a doted spot; (c) photo of the plasma above substrate and (d) photo of the microwave reactor; (e) complete experiment matrix, which indicates the sub‐ strate condition (for scratched or non-scratched surface), and the process environment condition; remote gas/plasma and contacting gas/plasma. Among all possible 6 variants tested, only localized plasma process have produced nano‐

tubes on substrate [25].

of CNTs on a catalyst-free silicon substrate still remains elusive.

There are many parameters of the CVD process that should be controlled to grow entangled CNTs with some special patterns. For example, the length of the nanotubes could be deter‐ mined by the exposure time of the carbon feedstocks. Recently, we have demonstrated that the density of entangled SWCNTs, which is a critical factor in device performance, could be controlled over 3-order-of-magnitude in acetylene-modulated CVD processes (Figure 6a) [2]. In addition, we also obtained a special 'knotted' morphology of the CNT network by using porous silica as the catalyst-supporting layer (Figure 6b) [19]. In contrast to this morphology, a much lower density of nanotubes was observed on flat silica surface.

**Figure 6.** Representative arrays of entangled carbon nanotubes [2,19].
