Researchers have been able to confine small molecules inside larger molecules for a number of years and, more recently, they have even been able to watch the movement of the smaller molecules. However, it has not been possible to control this motion or measure the forces causing it. Now, Makoto Ashino of the University of Hamburg and co-workers have provided new insights into such systems by measuring how individual metallofullerene molecules confined inside carbon nanotubes respond to the tip of an atomic force microscope (see page 337).

Ashino and co-workers started by encapsulating dysprosium atoms inside carbon-82 molecules to form Dy@C82, and then inserting these metallofullerene molecules into single-walled carbon nanotubes to form (Dy@C82)@SWNT 'peapod' structures. These molecules within molecules were then deposited onto an insulating surface and probed with dynamic non-contact atomic force microscopy.

In addition to studying the surface topography of these peapods, Ashino and his co-workers — who are based in Hamburg, the Max Planck Institute for Solid State Research in Stuttgart, Eindhoven University of Technology, the Hong Kong University of Science and Technology, and Nottingham University — also simultaneously measured the energy lost by the vibrating tip of the AFM as it moved over the surface of the (Dy@C82)@SWNT structures. Not surprisingly, the presence or absence of a Dy@C82 molecule inside the nanotube influenced both the shape of the surface and the energy-loss images (see Fig. 2 on page 338). The highly elastic nature of nanotubes meant that there was no energy-loss signal for those that did not contain any smaller molecules. Moreover, the team were able to show that the maximum energy loss for filled nanotubes occurred directly above the sites of the Dy@C82 molecules.

The image here shows the surface topography of an empty nanotube (left) and a peapod structure, with the height represented by different colours (black corresponds to 0 nm, white to 2 nm), and the scale bar representing 1 nm in both horizontal directions. The atomic-scale corrugations on the surface of the empty nanotube, along with its helicity, can be clearly seen in the left part of the image, whereas the surface undulations (which have an amplitude of 56 ± 5 pm in the vertical direction) caused by the Dy@C82 molecules are clearly visible for the nanotube on the right.