Many interesting applications, ranging in size from the nanoscale to the mesoscale, are made possible due to the special mechanical and electrical properties of carbon nanotubes. Consequently, much research has been dedicated to understanding how electrical and mechanical properties are affected by the atomic level structure of the nanotubes. To exploit these structure-dependent properties, it is essential to control nanotube synthesis, especially for single walled carbon nanotubes. Several techniques have now been developed for making single walled nanotubes. Laser sputtering of a graphite target makes ropes of straight single-walled nanotubes, which can be later separated into single nanotubes. Catalytic decomposition of acetylene by finely dispersed metal catalyst particles such as Ni and Fe on inert silica supports can also make single-walled nanotubes at considerably lower cost. However, such catalytically synthesized nanotubes are not always straight, tending to be curved with irregular diameters and structures that vary with the synthesis conditions. The defects that cause such structural irregularities also affect electrical properties. To understand how synthesis conditions affect the growth mechanisms of catalytically grown single-walled carbon nanotubes, we are studying their formation in-situ in a specially modified TEM.

We have employed a specially modified Tecnai F-20, also called an environmental scanning transmission electron microscope (ESTEM), for in-situ observations of carbon nanotube growth. Growth conditions were varied, with temperature ranging from 450C to 600C and pressures ranging from 1 to 16 mTorr of acetylene (C₂H₂). The growth rate can be estimated directly from the analysis of successive video frames. Carbon nanotubes were observed to grow by different mechanisms depending upon the growth conditions. Under some conditions, the growth speed of nanotubes, projected onto the image plane varied according to a 2-dimensional Maxwellian speed distribution. In other words, the nanotube tip growth resembles the stochastic behavior of a gas molecule. Under other growth conditions, i.e. temperature, C₂H₂ pressure, the nanotube grows fairly straight and its growth rate is less random.

The samples synthesizing during in situ TEM observations were further analyzed using high resolution TEM for (a) to evaluate electron beam effects on the nanotubes formed and (b) obtain statistical distribution of the type of CNTs formed under different temperature and pressure conditions. We did not observe any effects of the electron beam on the growth process. Low temperatures and high precursor pressure resulted in the formation of mostly multi walled tubes while samples synthesized at high temperatures (above 600C) and low precursor pressures resulted in mainly straight single walled tubes. Moreover, the growth conditions were also observed to affect the inner and the outer diameter of the tubes formed.