3.7 Conclusions

In conclusion, time-resolved, variable temperature STM investigations of the C60 monolayer deposited on the WO2/W(110) surface reveal many interesting phenomena. The C60 molecules self-assemble into a well-ordered molecular layer in which they form a close-packed hexagonal structure with a unit cell parameter equal to 0.95 0.05 nm. The nucleation of the C60 layer starts at the substrate’s inner step edges.

By using the WO2/W(110) surface as a preformed nanostructured template, it was shown that the “dim” C60 molecules follow the oxide nanorows of the substrate, occupy the grooves between them, and, as a result, are situated slightly lower than the others (“bright” C60).

At room temperature the molecules rotate faster than the time-resolution of the STM and so appear as featureless spheres, and at 78 K their orbital structure is frozen. Their movement is suppressed, but their orbital ordering depends on the cooling regime. When quickly quenched, the molecules’ orbitals are randomly arranged with no correlation between their orientations; however after slow cooling the “stripes” of their orbitals are mostly aligned along or perpendicular to a close-packed direction of the monolayer, due to intermolecular interactions.

Between the two extremes of room temperature fast rotation and low temperature freezing, two separate phase transitions are demonstrated: a rotational phase transition at 259 K and a kinetic glassy transition at 220 K. The temperature of the rotational phase transition is identical to that of 3D C60 crystals (259 K) and the temperature of the kinetic transition is substantially higher (220 K) than the bulk (90 K).

Different mechanisms of molecular nanomotion, such as rotation, spinning and switching between different orientations have been observed. The total energy of the molecular interaction in the film is 48 meV, as estimated from mean field theory. This was found to be approximately a factor of two greater than the energy level separation of an individual C60 molecule, as determined from time-resolved STM experiments.

The measurements of the phase transition temperature TC in the film, combined with the argument of the reduced coordination number suggests a strong contribution from the interaction between the molecules and the substrate. The energy of such interactions was estimated to be 24 meV. The observation of a glassy transition at 220 K reveals a non-exponential relaxation in the C60 monolayer. The Kauzmann temperature was estimated to be 45 K.

Individual C60 molecules in the monolayer have also been observed to switch between neutral and negatively charged states. The charging of the C60 causes changes in the local density of electron states and consequently a variation in tunneling current. The negatively charged C60 state results from the acceptance of a tunneling electron from the STM tip or the substrate depending on the bias applied.

It was found that molecular movement accompanies the molecule’s switching between these states. The results obtained shed light on the switching of C60 between different charge states and provide important information for the further development of nanoscale molecular devices and molecular switch concepts.

These experiments have demonstrated the effectiveness of STM in the investigation of phase and kinetic transitions in the vicinity of critical temperatures. This is an unexpected result because, in general, the characteristic times of molecular and atomic motion are on the order of picoseconds and the characteristic frequency cut-off of an STM is much slower at 10 kHz ( 100s). However, as the dynamics are highly temperature-dependent, one can find a temperature range where the fluctuations happen on a time scale able to be resolved by STM and thus the temperature of the kinetic transition can be extracted.