Growing carbon nanotubes in a microscope

Abstract

Great advances have been achieved recently in producing carbon nanotubes (CNTs). By the same token, precisely controlling the growth of both multiwalled and single-walled carbon nanotubes (MWNTs and SWNTs) to generate desired structures remains a major challenge. The importance of this deficiency cannot be underestimated because some of the most promising applications of CNTs in next-generation electronics and optical devices depend strongly on nanotube diameter size and specific chirality (i.e., whether the tube winds left or right). For this reason, better understanding of the initial stages and mechanisms of CNT growth could be key to developing practical, large-scale synthesis of CNTs. Generating nanotubes involves reacting a gaseous carbon source onmetal catalyst particles in a process known as chemical vapor deposition (CVD). Observation of the gas–solid interactions is possible using an ‘environmental’ transmission electron microscope (TEM) fitted out with gas injectors and cells. Indeed, several groups have carried out in situ observations of carbon nanofiber and nanotube growth.1, 2 But nucleation—the first and most important step of nanotube production—is still a puzzle, mainly because the environmental TEM images are blurred by vibrations caused by the chemical reaction or electrical currents. In order to get around this problem, we observed the formation of CNTs in an entirely condensed phase (solid) process. Moreover, we were able to monitor, at atomic resolution, the birth of both SWNTs and MWNTs in a standard TEM.3 To grow a CNT inside the TEM, we subject anMWNT containing a catalytically active metal particle core to electron irradiation. This causes carbon from the graphitic shells of the MWNT to be injected into the body of the metal core and subsequently to emerge as an SWNT or MWNT inside the host nanotube (see Figure 1). This process differs from previous in situ studies because it does not involve a gaseous source of carbon, and elecFigure 1. The sequence of images shows the growth of an MWNT from a metal catalyst particle. The process is driven by the injection of carbon atoms, produced by electron irradiation, into the metal particle.