Nanotechnology’s Wonder Material: Synthesis of Carbon Nanotubes - Hierarchical Nanostructures for Energy Devices - page 30

microfabrication processes for electronics applications. The high growth
temperatures of CNTs (600-900 1C) remain an obstacle to full process inte-
gration, but researchers keep lowering the growth temperature for nanotube
CVD growth. 16 It is safe to say that CVD remains the most plausible way to
incorporate CNTs in industrial manufacturing processes and bottom-up
fabrication strategies. The next section will introduce several kinds of CVD
methods for CNT growth and discuss their technical features.

d n 3 r 4 n g | 6

3.2.1.1 Chemical Vapor Deposition for CNT Growth
As the name indicates, CVD relies on vapor or a gaseous carbon feedstock.
Common CVD precursors include a number of hydrocarbons or alcohols
that differ in their chemical reactivity and compatibility with gas delivery
systems. CVD apparatus usually contains equipment to control and monitor
gas flow and process pressure. As the sample size gets bigger, the heating
uniformity takes center stage and the machines get increasingly sophisti-
cated and acquire additional features such as multiple heating zones.
CVD machines also fall roughly into three principal designs: (1) hot-wall
reactor, such as a tube furnace, where hot walls are the primary means of
heating the process gases, (2) cold-wall reactor where only the sample gets
heated, and the process gas cracks upon contact with the sample, and (3)
hot-filament reactor where the gas cracks on a hot filament situated close to
the sample surface. As spatial uniformity of the mass transport process
becomes paramount to achieve uniform growth, the shower-head type cold-
wall CVD reactors are by far the most practical type of reactor to obtain
uniform growth on samples larger than 10 cm. However, due to cost con-
siderations as well as historical reasons, the hot-wall reactors (tube furnaces)
dominate the carbon nanotube growth literature, and most of the results
that we discuss were obtained in a hot-wall reactor. We also note paren-
thetically that it is one of the major reasons that the results from the re-
search literature are often dicult to translate to industrial scale growth
processes.
The process used to deliver the cracking energy also distinguishes several
forms of CVD (Figure 3.2); mostly, they are categorized into thermal CVD,
which utilizes Joule heating, or plasma enhanced CVD (PECVD). Both pro-
cesses can be implemented in cold-wall and hot-wall reactors.
In order to enhance the energy eciency of thermal CVD, the hot-wall
reactor furnace is insulated by ceramic walls, and the chemical reactions for
growth occur inside a leak-free quartz reactor (Figure 3.2A). The thermal
energy in this case is delivered to the growth substrate by thermal con-
duction from the heating element, or by radiation of infrared (IR) light from
the hot walls. Each thermal contribution depends on the mechanical design
of the furnace system and spectral characteristics of the quartz glasses in IR
ranges at growth temperatures.
In a cold-wall reactor, the heated area is confined to the immediate vicinity
of the substrate. In addition, reduced thermal capacitance of the cold-wall

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