M. Picher, P.A. Lin, J. Winterstein, S. Blankenship and R. Sharma
Carbon nanotubes (CNTs) provide two particularly exciting prospects. Firstly, the versatility of their properties according to their structure and dimensions makes them objects of fundamental interest. Secondly, their outstanding mechanical strength, electrical and thermal conductivity and opto-electronic properties offer many opportunities for future nanotechnology applications. The combination of these exceptional and versatile properties might lead to the development of electronic systems where both active devices and interconnects are based on the same material. However, in spite of longstanding efforts, no major electronics application involving CNTs is available in the market yet. The main obstacle to the development of a CNT-based technology is that many aspects of their growth mechanisms remain obscure. In particular, the relationship between the nucleating/growing nanotube and the catalyst nanoparticle is not well understood. Despite some remarkable improvements in the control of CNT features at the synthesis stage during the last decade, the optimization of CNT growth conditions remain mostly empirical and nanotube samples are frequently a mixture of different structures (number of walls, defects density, length, diameter, chiral angle) and morphologies (straight, bundled or entangled).
The first stages of nanotube life, e.g. the activation of the catalytic nanoparticle, followed by the segregation and the lift-off of a hemispherical cap are obviously critical steps which strongly influence the final yield and the type of structures grown. We have employed in situ high-resolution transmission electron microscopy (HRTEM) combined with electron energy loss (EELS) and Raman spectroscopies in order to probe the first stages of growth. An original pulsed gas introduction protocol is used in order to artificially increase the time resolution of the study. We report on the critical chemical and physical changes occurring at the nanoparticle scale for different catalytic systems.