**1. Introduction**

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Carbon nanotubes (CNTs) are fascinating materials with outstanding mechanical, optical, thermal, and electrical properties [1-4]. CNTs also have a huge aspect ratio and a large sur‐ face area to volume ratio. Because of their unique properties, vertically aligned centimeter long CNT arrays have generated great interest for environmental sensors, biosensors, spin‐ ning CNT into yarn, super-capacitors, and super-hydrophobic materials for self-cleaning surfaces [5-11]. Yun et al. studied a needle-type biosensor based on CNTs to detect dopa‐ mine. Their results showed advantages of using CNT biosensors for detecting neurotrans‐ mitters [11]. Most of the envisioned applications require CNTs with high quality, a long length, and well aligned vertical orientation. Although many researchers have studied the synthesis of vertically aligned CNT arrays, the CNT growth mechanism still needs to be bet‐ ter understood. In addition, CNT lengths are typically limited to a few millimeters because the catalyst lifetime is usually less than one hour [12- 16]. Many groups have studied the ki‐ netics of CNT growth trying to improve CNT properties. Different observation methods [17-22] were used to determine the effect of the catalyst, buffer layers, carbon precursor, and deposition conditions on nanotube growth. One of the suggested growth mechanisms pos‐ tulates several steps [23]. First, the carbon source dissociates on the surface of the substrate. Next, the carbon atoms diffuse to the molten catalyst islands and dissolve. The metal-carbon solution formed reaches a supersaturated state. Finally, the carbon nanotubes start to grow from the carbon- catalyst solution. *In situ* observation of CNTs during their nucleation and growth is a useful method to understand the growth mechanism, which might help to over‐ come the limitation of the short length of nanotubes, and to control array growth and quali‐ ty. Various remarkable approaches of *in situ* observation have been performed to affirm the growth mechanism of vertically aligned CNTs and also to obtain kinetics data such as

© 2013 Cho et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Cho et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

growth rate and activation energy [12, 24-27]. Puretzky *et al*. studied the growth kinetics of CNT arrays using *in situ* time-lapse photography and laser irradiation under diffusion-limit‐ ed growth conditions [28]. *In situ* transmission electron microscopy (TEM) was used by Kim *et al*. to study the dynamic growth behavior of CNT arrays [29]. Additionally, a pseudo *in situ* monitoring method was used to investigate the kinetics of CNT array growth by creat‐ ing marks on the side of the CNT array during the growth. Using this method, several groups demonstrated root growth for their catalyst systems. However, their studies were limited to short lengths and also required *ex-situ* observation with SEM to obtain the growth length as a function of time [25, 27, 30, 31]. Most reported methods are designed to operate and monitor the growth length with time for relatively short (a few millimeters long) CNT arrays and also do not provide kinetics data for growing centimeter long CNT arrays.

wafers using e-beam evaporation. After the deposition, the substrates were annealed for several hours at 400 ºC in Air. All the experiments were performed using the following optimized recipe for centimeter long CNT arrays: 560 mmHg of argon, 60 mmHg of hydro‐ gen, and 140 mmHg of ethylene as a carbon precursor. The water concentration in the reac‐ tor was near 900 ppm measured by a quadrupole mass spectrometer (QMS). The total pressure was kept at one atmosphere during the growth and the temperature varied from 690 ºC to 840 ºC. Real-time images of the CNT array growth were recorded from the moment that ethylene was introduced into the reactor. The images were used to study the growth mecha‐ nism and kinetics of the CNT growth. Scanning electron microscopy (SEM, Phillips XL30 ESEM), high resolution transmission electron microscopy (HRTEM, JOEL 2000 FX) and Micro-Raman spectroscopy (Renishaw inVia Reflex Micro-Raman) were employed to characterize

Kinetics of Growing Centimeter Long Carbon Nanotube Arrays

http://dx.doi.org/10.5772/50837

225

Real-time photography was used to study the growth mechanism and kinetics of centime‐ ter long CNT array growth. The digital camera provided clear images showing details relat‐ ed to the dynamic changes of the array shape during the growth. This was achieved by controlling the intervals for taking pictures from a few seconds to several hours depending

**Figure 2.** Real-time images of the centimeter long CNT array growth evolution with time during CVD at 780 ºC: (a)

Fig. 2 illustrates sequential images of vertically aligned centimeter long CNT arrays grown at 780 ºC. As can be seen from Fig. 2, it is easy to distinguish the substrate from the CNT array. Arrow 1 in Fig. 2a points to a side view of the substrate. Arrow 2 in Fig. 2f shows the side view of the CNT array. The growth length can be obtained as a function of the deposi‐ tion time from the images. Changes in the array shape can also be observed during the entire growth time. In Fig. 2f, the growth length was 12.47 mm and the catalyst lifetime was 450 min. This experiment was repeated several times at the same deposition conditions and the results were reproducible. Hence, Fig. 2 demonstrates the real-time images which allow

Side image of the substrate at zero growth time, (b) to (f) Images of CNT arrays grown for different times.

the CNT morphology.

**3. Results and discussion**

on the experimental conditions.

**3.1. Growth evolution by real-time photography**

Recently, we have developed new catalyst systems which are able to grow over 1 centimeter long vertically aligned CNT arrays [6]. In this paper we examined the growth mechanism and kinetics of centimeter long carbon nanotube arrays using a real-time photography tech‐ nique and the effect of growth temperature and growth time on morphology of CNTs.
