Balance of Forces in Self-Assembled Monolayers

Self-assembly represents a typical example of bottom-up nanotechnology where the formation of macroscopic structures is driven by forces operating at the molecular level. Under situations where several forces of comparable strength are competing with one another, the stable structure achieved depends on the fine balance of all the forces. A good example is the formation of self-assembled monolayers (SAMs) of non-branched alkanethiol molecules, CH3-(CH2)-SH, on the surface of gold, where the interaction between the S headgroup and the gold substrate competes with the force acting between the alkane tails. Here we demonstrate that, if the tail is short, n <= 2, the S-Au interaction dictates the structure of the SAM. As the tail becomes longer than n = 2, the enhanced tail-tail interaction is able to displace the S headgroup. Our finding highlights the importance of the "weak" van der Waals interaction for molecular assembly even in the presence of chemical bonding.