The study of the self-assembly of organic molecules on solid surfaces is an ever increasing field in surface science due to the possibility of developing new nanostructured materials using the bottom-up approach. However, the theoretical study of these systems has been hindered by the fact that, usually, the large size of the molecules forbids the use of first-principle calculations that include both the surface and the molecule since, in general, the final arrangement is the result of a combination between molecule-molecule and molecule-substrate interaction. In some cases, however, the molecule-substrate interaction is so weak that the 2D ordering comes mainly dictated by the interaction between the external functional groups of the molecules. In this case, theoretical calculations could help in explaining the properties and details of the different mechanisms leading to the final arrangement.
In this contribution we describe STM measurement and DFT calculations based on the B3LYP formalism of the self-assembly properties for the high-coverage phase (2/3 of a ML) of PCBM ([6,6]-phenyl C61 butyric acid methyl ester, a C60 derivative) on Au(111). The STM measurements show that the PCBM molecules order forming double rows running parallel to the close-pack directions of the surface and separated around 20 Å. While the arrangement of the molecules within a row is due to the van der Waals forces between the C60 cages, the calculations predict the 2D arrangement between rows and demonstrate that it is a consequence of the interactions (mostly weak hydrogen bonding) between the "tails" of the PCBM molecule. This example demonstrate the importance of the theory for explaining the fine details of the different interactions that determine the final 2D self-assembly. |