Lab Supplies "RX" for Carbon Nanotube Growth Published Sept. 14, 2010 By Heyward Burnette Materials and Manufacturing WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- The Air Force Research Laboratory headed a research team internationally recognized for its groundbreaking discovery of chemical vapor deposition-based growth mechanisms for single-wall carbon nanotubes, which are key elements for an ever-increasing number of important technical applications. The Materials and Manufacturing Directorate-led team was the first to connect catalyst changes to carbon nanotube growth mechanisms, a breakthrough expanding scientific understanding of the growth process and subsequently prompting dramatic improvements in SWNT length, yield, performance, and cost. Specifically, this effort revealed how SWNTs stop growing as a result of catalyst ripening and, consequently, established the role of the substrate in carbon nanotube carpet growth. By inhibiting catalyst ripening, the researchers were able to explain the rapid (10 min) growth of dense carpets of high-purity SWNTs to heights (lengths) up to 2.5 mm. In addition to elevating AFRL's status as a key international contributor to the scientific knowledge of SWNT growth, the finding completely transformed the understanding of nanotube synthesis and resulted in a highly cited, three-paper series on dynamic catalyst evolution. Carbon nanotubes are cylindrical carbon molecules with novel properties (e.g., extraordinary strength, efficient thermal conductivity) rendering them useful for many applications in nanotechnology, electronics, optics, structures, and other areas of materials science. Initiating CVD-based growth of SWNTs involves preparing a substrate with a layer of metal catalyst particles. The size of these particles dictates the respective diameters of the SWNTs, each of which grows at a specific catalyst site (with the particle staying at either the tip or the base of the growing structure). Scientists have previously been able to produce dense carpets of millimeter-long nanotubes using a super-growth, water-assisted CVD process (which enhances both the activity and the lifetime of the catalyst). While the ultralong SWNTs attained more recently are promising, the creation of such longer structures has historically received little attention and held limited appeal. This lack of enthusiasm was due to insufficient understanding of carbon nanotube growth and kinetics, a knowledge shortfall that inhibited potential applications and precipitated costly, random growth experiments. Of particular concern was the growth termination that occurs once micron-long nanotubes have been achieved. This early-ending nanotube synthesis created process variability, short lengths, compromised purity, and lower yields. Compounding this problem was the equally deficient understanding of carpet growth, itself a complicated process. AFRL scientists responded to these obstacles by first conceiving of the possible "culprit" mechanism responsible for CNT growth termination and then assembling a high-powered multidisciplinary, multiuniversity team to investigate this mechanism. As noted, the team uncovered the role of dynamic catalyst ripening in CVD-based SWNT growth and, further, explained the "supergrowth" effect by demonstrating the inhibition of catalyst ripening. This work establishes the mechanistic understanding fundamental to optimizing the creation--and, ultimately, the use--of SWNTs. In clarifying the collective termination behavior of carpet growth and thus defining a way forward for CNT producibility, these research results provide both a rational basis for designing new catalyst systems for nanotube growth and a foundation for achieving formerly unattainable catalyst life. Improved catalyst design will, in turn, translate to better process stability, longer lengths, enhanced purity, and higher yields.