Carbon Nanotube Synthesis via CVD

Brief Discussion of Problems of Carbon Nanotube Synthesis via Chemical Vapor Deposition

This is a simplified, brief discussion of my remembrance of the origin of my understanding of the problem with chemical vapor deposition (CVD) synthesis of carbon nanotubes (CNTs). Generally, it has been measured that the CNT growth initially is quite rapid but quickly slows and appears to stop. Therefore, long carbon nanotubes are not produced by CVD techniques. Decades of research has not changed this situation.

In 2010 I attended the Nanotechnology Materials and Devices Workshop hosted by the University of Cincinnati’s Nanoworld and the Air Force Research Laboratory. Dr. Benji Maruyama gave an excellent presentation on the reasons that carbon nanotube (CNT) synthesis CVD ceases. It took ten years of diligent research to discover these problems.

The first insight is that carbon junk (probably amorphous carbon) is deposited onto the catalyst particles during the growth process. The explanation is that the hot carbon bearing gas, that is the heart of the CVD process, yields carbon atoms at the catalyst surface, by its molecules colliding with the catalyst surface and breaking the carbon bond within the gas molecule. Some of the free carbon atoms migrate to the CNT growth site and become a part of the nanotube. Other atoms create the aforementioned carbon junk and eventually choke off the carbon supply to the nanotube by covering the surface of the catalyst particle.

A second insight is that, at the high temperature of CVD growth, the catalyst particles tend to diffuse into the substrate upon which they are sited. This was a surprise because the silicon wafers typically used for CNT CVD growth were expected to be impermeable. The effective volume and area of the catalyst particle decreases as it sinks into the substrate thereby rendering the effective size of the catalyst particle too small to support CNT growth.

The third insight invokes the Ostwald Ripening process. In general, the catalyst particles would be more stable as one big catalyst particle sited on the substrate and so that tends to happen. At the high temperature of CVD growth, the catalyst particle atoms (iron and/or cobalt) can escape from the particle and on average are absorbed more readily by a large catalyst particle than a smaller one. As time goes on the smaller catalyst particles get smaller and the larger ones get larger. Eventually the small ones are too small to support CNT growth and the large ones are too big to support CNT growth.

One other outcome of the CVD process should be noted. The growing CNT “cooks” in the hot, carbon bearing gas atmosphere and reactions occur, through molecular “bombardment”, on the surface of the CNT. These reactions create defects that weaken and degrade the CNT’s integrity.

Limiting the length of CNTs and damaging the structure of CNTs both severely constrain the usability. Recall that individual CNTs possess tensile strength around 100 times steel and are one-sixth the density. Moreover, the electrical and thermal conductivities of CNTs are very high. To date, macroscopic assemblages of CNTs do not exhibit the spectacular physical properties of individual CNTs. Specifically the tensile strength is well below steel and the electrical conductive properties are well below copper. Imagine a world where structural materials are 100 times the strength of steel while being a fraction of the weight.

This brief discussion outlines the reason that Dr. Laubscher considers the synthesis of CNTs by CVD to be a failed technology.