6.2 Rolling molecules on surfaces under stm imaging  (Page 2/3)

Although the power of STM imaging has been demonstrated, imaging of molecules themselves is still often a difficult task. The successful imaging of the IBM work was attributed to selection of a heavy atom. Other synthetic organic molecules without heavy atoms are much more difficult to be imaged under STM. Determinations of the mechanism of molecular motion is another. Besides imaging methods themselves, other auxiliary methods such as DFT calculations and imaging of properly designed molecules are required to determine the mechanism by which a particular molecule moves across a surface.

Herein, we are particularly interested in surface-rolling molecules, i.e., those that are designed to roll on a surface. It is straightforward to imagine that if we want to construct (and image) surface-rolling molecules, we must think of making highly symmetrical structures. In addition, the magnitudes of interactions between the molecules and the surfaces have to be adequate; otherwise the molecules will be more susceptible to slide/hop or stick on the surfaces, instead of rolling. As a result, only very few molecules are known can roll and be detected on surfaces.

Surface rolling of molecules under the manipulation of stm tips

As described above, rolling motions are most likely to be observed on molecules having high degree of symmetry and suitable interactions between themselves and the surface. C ₆₀ is not only a highly symmetrical molecule but also readily imageable under STM due to its size. These properties together make C ₆₀ and its derivatives highly suitable to study with regards to surface-rolling motion.

The STM imaging of C ₆₀ was first carried out at At King College, London. Similar to the atom positioning experiment by IBM, STM tip manipulation was also utilized to achieve C ₆₀ displacement. The tip trajectory suggested that a rolling motion took into account the displacement on the surface of C ₆₀ . In order to confirm the hypothesis, the researchers also employed ab initio density function (DFT) calculations with rolling model boundary condition ( [link] ). The calculation result has supported their experimental result.

The results provided insights into the dynamical response of covalently bound molecules to manipulation. The sequential breaking and reforming of highly directional covalent bonds resulted in a dynamical molecular response in which bond breaking, rotation, and translation are intimately coupled in a rolling motion ( [link] ), but not performing sliding or hopping motion.