Observations of chemical reactions at the atomic scale: dynamics of metal-mediated fullerene coalescence and nanotube rupture.

Single-walled carbon nanotubes (SWNTs), which are graphitic tubular structures with single-atom-thick sidewalls and varying diameters, are effective containers for a wide variety of molecular species. They are particularly suitable for study by transmission electron microscopy (TEM). Chemical reactions between molecules inside nanotubes can be triggered by external stimuli (for example, the TEM electron beam), with the nanotube acting as a miniature reactor vessel, thus potentially steering a chemical reaction along a new pathway towards new products. Nanotubes are often viewed as chemically inert containers. Although their surfaces can be involved in some chemical reactions, the chemical reactivity of their interior has been considered to be very low. TEM is the only method that allows direct visualization and study of the molecules inside nanotubes. We are currently exploiting the capabilities of an aberration-corrected TEM, which can record atomic resolution images in a time much shorter than needed for the electron beam to promote structural transformations inside SWNTs. Under our imaging conditions, these transformations usually occur on a timescale of seconds, which enables us to capture atomic images of the intermediates. These images can then be combined into a movie to follow the chemical transformations as they happen. Both walls and interiors of SWNTs can be seen on TEM micrographs, such that the positions and orientations of the encapsulated molecules can be readily determined from the images (Figure 1c). Accelerated electrons interact with the