Friday, February 26, 2010

Massive Growth of Metal Free Carbon Nanotubes on Pure Silicon and Germanium Demonstrated by EU Researchers

U.S. Patent Application 20100047152, figure 6 (below) shows massive growth of metal free carbon nanotubes on pure silicon substrate by chemical vapor deposition achieved by European researchers. 

COST Science Officer Materials, Physical and Nanosciences Caroline Whelan and (Hanret, BE) and Santiago Cruz Esconjauregui (Leuven, BE) found  a method for massive carbon nanotube (CNT) growth by using metal-free catalyst nanoparticles, for example silicon (Si) or germanium (Ge) nanoparticles and a hot filament wire. 

According to U.S. Patent Application 20100047152, the method uses the step of decomposing a carbon source gas to form carbon fragments which then recombine at the metal-free catalyst nanoparticles to grow carbon nanotubes. The method yields carbon nanotubes which do not contain metal impurities.

Before performing CNT growth, the substrates with nanoparticles on were etched in HF (2%) for 1 min. at room temperature in order to remove a possibly present native oxide from the Si nanoparticles. The samples were placed in the CVD reactor chamber with temperatures ranging between 600 degree C. and 9000 degree. C., under reducing atmosphere in N2:H2 (4:2 l/min.) for 5 minutes at atmospheric pressure.

A wire was used as a hot filament  and was located above the substrate  comprising the catalyst nanoparticles. A flow of 0.1 l/min. of acetylene, ethylene or methane in addition to the other gases (N2:H2) was flown over the hot filament such that the Carbon source gas was cracked. The gas composition used for this experiment was N2:H2:C at a ratio 4:2:0.1 l/min. The Carbon source used was either one of acetylene, ethylene or methane. The CNTs were grown for half an hour at atmospheric pressure. 

FIG. 4, (below) illustrates a scanning electron microscopy picture after growth of CNTs onto Si nanoparticles

FIG. 5 and FIG. 6 illustrate SEM pictures after growth of CNTs onto the Si nanoparticles. Massive growth of CNTs is observed, i.e. CNTs grew much closer to each other when compared to FIG. 4. The carbon nanotubes were grown by chemical vapor deposition (CVD). 

A more intensive growth was observed in the area where the Si nanoparticles are closer to the hot filament  (see upper row of CNTs in FIG. 5 below. The hot filament may be a metallic filament such as a W filament or a Ta filament. The hot filament is maintained at a temperature suitable for decomposing or cracking the carbon source gas. For example when a hot filament is used for decomposing the carbon source gas, the filament may be kept at a temperature of 950.degree. C

The most commonly accepted growth mechanism for CNTs is based on catalytic decomposition of a carbon source on a surface of a metal nanoparticle which acts as catalyst in the CNT synthesis. According to this growth mechanism, the hydrocarbon source decomposes on front-exposed surfaces of the metal nanoparticle thereby releasing hydrogen and carbon, which dissolves in the nanoparticle. The dissolved carbon then diffuses through the metal nanoparticle and is precipitated to initiate formation of CNTs.

One of the key issues in the growth mechanisms described in the prior art is the need for a metal catalyst particle to initiate the carbon nanotube growth. A disadvantage thereof is that the metal catalyst particles can lead to the presence of impurities in the grown CNTs. Before the CNTs can be used in many applications, these impurities have to be removed. A variety of chemical and thermal oxidative treatments are usually required to remove the unwanted metal impurities from the CNTs. For example, a multi-step purification procedure may be used which involves the use of nitric acid reflux and thermal oxidation.

Catalyst-free growth of CNTs has been achieved previously by using laser ablation and arc discharge CNT growth. However, these methods require very high temperatures, i.e. temperatures of above 3000.degree. C. Due to these high required temperatures, these methods are not suitable for in-situ CNT growth and consequently require an ex-situ approach. Furthermore, these methods may give low production yields compared to CVD methods that can be performed at relatively low temperatures (450-1100.degree. C.), can be in-situ or ex-situ, and give mass production yields. 

COST is an intergovernmental framework for European Cooperation in Science and Technology, allowing the coordination of nationally-funded research on a European level. COST contributes to reducing the fragmentation in European research investments and opening the European Research Area to cooperation worldwide

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