[i] Y. Ando, Chemical Vapor Deposition of Carbon Nanotubes. A Review on Growth Mechanism and Mass Production.   http://www.researchgate.net/profile/Yoshinori_Ando/publication/42804843_Chemical_Vapor_Deposition_of_Carbon_Nanotubes_A_Review_on_Growth_Mechanism_and_Mass_Production/links/0fcfd50809726e590a000000.pdf

The CVD of carbon and carbon –related compounds is very well developed.

This includes for examples the CVD preparation of carbon nanotubes (CNT), graphene, diamond thin films., al well as various carbon-doped materials (such us C-doped GaAs, AlGaAs, etc.). Numerous reviews have been published [[i]]

            Below some examples:

CNT Precursors

            Various classes for carbon-contaning compounds are used as C precursors for CNT growth (see detailed description below.).

For synthesizing CNTs, typically, nanometer-size catalyst particles are required to enable hydrocarbon decomposition at a lower temperature than the spontaneous decomposition temperature of the hydrocarbon. (the prerequisite for the catalyst selection is:  (i) high solubility of C at high temperatures; (ii) high carbon diffusion rate; these are for example: metals( generally Fe, Co, Ni, but also other metals, such as Cu, Au, Ag, Pt, Pd), solid organometallocenes (ferrocene, cobaltocene, nickelocene) releasing metal catalysts in situ. [Kauppinen 57]

Multi-wall carbon nanotubes (MWCNTs).

 Most commonly used carbon precursors for the growth of CNT are methane,22[1] 23 ethylene,24[1] 25 acetylene,26 benzene,27 xylene,28 and carbon monoxide.29 ., benzene (pyrolysis at 1100

C) [Endo et al.30–32], acetylene (helical MWCNTs at 700 °C), [ Jose- Yacaman et al.33], cyclohexane 34[1] 35, fullerene (MWCNTs) [36,37].

Single-wall carbon nanotubes (MWCNTs).

SWCNTs were first produced by Dai et al.[38] from disproportionation of carbon

monoxide at 1200

C, in the presence of molybdenum. nanoparticles.

Later, SWCNTs were also produced from benzene,39 acetylene,40 ethylene,41 methane,[23-42] , cyclohexane,43 fullerene44 etc. by using various catalysts.

In 2002 Maruyama et al. reported the low-temperature synthesis of high-purity SWCNTs from alcohol on Fe–Co-impregnated zeolite support;45 and since then, ethanol EtOH became the most popular CNT precursor in the CVD method worldwide.46–48 The significant advantage of ethanol is that ethanol-grown CNTs are almost free from amorphous carbon, owing to the etching effect of OH radical.[49]

Later, vertically-aligned SWCNTs were also grown on Mo-Co-coated quartz and silicon substrates.50[1] 51 Recently, Maruyama’s group has shown that intermittent supply of acetylene in ethanol CVD significantly assists ethanol in preserving the catalyst’s activity and thus enhances the CNT growth rate.[52].

The molecular structure of the precursor is crucial for determining the morphology of the grown CNTs. Linear hydrocarbons such as methane, ethylene, acetylene thermally decompose into atomic carbons or linear dimers/trimers of carbon, and generally produce straight hollow CNTs.

On the other hand, cyclic hydrocarbons such as benzene, xylene, cyclohexane, fullerene, produce relatively curved/hunched CNTs with the tube walls often bridged inside.36[1] 37.

General experience is that low-temperature CVD (600–900

C) yields MWCNTs, whereas high-temperature (900–1200

C) reaction favors SWCNT growth. This indicates that SWCNTs have a higher energy of formation (presumably owing to small diameters; high curvature bears high strain energy). Perhaps that is why MWCNTs are easier to grow (than SWCNTs) from most of the hydrocarbons, while SWCNTs grow from selected hydrocarbons

(viz. carbon monoxide, methane, etc. which have a reasonable stability in the temperature range of 900–1200

C).

Commonly efficient precursors of MWCNTs (viz. acetylene, benzene, etc.) are unstable at higher temperature and lead to the deposition of large amounts of carbonaceous compounds other than the nanotubes.

In 2004, using ethylene CVD, Hata et al. reported water-assisted highly-efficient synthesis of impurity-free SWCNTs on Si substrates.53 They proposed that controlled supply of steam into the CVD reactor acted as a weak oxidizer and selectively removed amorphous carbon without damaging the growing CNTs. Balancing the relative levels of ethylene and water was crucial to maximize the catalyst’s lifetime. However, very recently, Zhong et al. have shown that a reactive etchant such as water or hydroxyl radical is not required at all in cold-wall CVD reactors if the hydrocarbon activity is low.54 These studies emphatically prove that the carbon precursor plays a crucial role in CNT growth. Therefore, by proper selection of CNT precursor and its vapour pressure, both the catalyst’s lifetime and the CNT-growth rate can be significantly increased; and consequently, both the yield and the quality of CNTs can be improved.