New Directions in Molecular Self-Assembly


Self-assembled monolayers (SAMs) have been thoroughly investigated due to their utility in many fields such as sensing, device assembly, molecular electronics and microelectromechanical systems.  Thiol SAMs are by far the most extensively studied in the literature due to their robustness and the ability to control their assembly in the dimension perpendicular to the surface of the metal.  There are, however, problems with thiol SAMs because they contain structural defects like etch pits and rotational domain boundaries that make them more susceptible to oxidation and displacement.
Thioethers are similar to thiols, but have two alkyl tails that cause them to lie parallel to the surface instead of arranging nearly perpendicular like thiols. This adsorption geometry allows for control over the packing density in the dimensions parallel to the surface by simply adjusting side chain length.  Thioether SAMs may also be preferable to thiol SAMs for many uses because they are more resistant to oxidation.  Despite these advantages, there has only been very limited research conducted on the self-assembly of thioethers on metals.
We have shown that a range of thioethers self-assemble on both Cu and Au surfaces.(1-4) As with thiols, thioethers form well-ordered monolayers, however, due to the slightly weaker molecule-metal bond, the coverage and temperature-dependent behavior is very different than that of alkane thiols. Adsorption is sensitive to the different regions of the Au{111} herringbone reconstruction. Thioethers lie parallel to the surface and form well-ordered chains in domains that preferentially bind first in fcc regions, then hcp and finally on soliton walls. The sulfide-Au interaction is strong enough to disrupt the native herringbone reconstruction of Au, however, unlike thiols, dibutyl sulfide adsorption does not result in etch pit formation. Monolayers have very low defect density as compared to thiol-SAM based systems. This low defect density hints at the possibility of using a thioether moiety as a basis for a self-assembled system free of typical defects like etch pits, which allow attack and degradation of the monolayer. Upon annealing the surface with a high molecular coverage the system returns to an intermediate-coverage structure, thereby demonstrating the reversibility of the assembly. Elevating the temperature further causes the entire monolayer to desorb, and the original herringbone structure of Au returns. We postulate that it is this reversibility, coupled with the high rate of concerted rearrangements, that allows this thioether SAMs to reach their very high level of order.
We have recently extended this work to include phosphines and selenoethers.(1-3) We demonstrated that the simple hard/soft rules of inorganic chemistry can be used to rationalize the observed trend of molecular interaction strengths with the soft gold surface, i.e., P > Se > S. The structure of the monolayers can be explained by the geometry of the molecules in terms of dipolar, quadrupolar or van der Waals interactions between neighboring species driving the assembly of distinct ordered arrays. As these studies directly compare one element with another in simple systems, they may serve as a guide for the design of self-assembled monolayers with novel structures and properties.

1) "Effect of Head Group Chemistry on Surface-Mediated Molecular Self-Assembly" A. D. Jewell, S. V. Kyran, D. Rabinovich and E. C. H Sykes - Chemistry A European Journal 2012, 18, 7169-7178. (link)

2) "Molecular-Scale Surface Chemistry of a Common Metal Nanoparticle Capping Agent: Triphenylphosphine on Au(111)" - A. D. Jewell, E. Charles H. Sykes and G. Kyriakou - ACS Nano, 2012, 6, 3545-3552. (link)

3) "Gently Lifting Gold's Herringbone Reconstruction: Trimethylphosphine on Au(111)" A. D. Jewell, H. L. Tierney and E. C. H. Sykes - Physical Review B 2010, 82, 205401. (link)

4) "Adsorption, Assembly, and Dynamics of Dibutyl Sulfide on Au{111}" D. O. Bellisario, A. D. Jewell, H. L. Tierney, A. E. Baber and E. C. H. Sykes - Journal of Physical Chemistry C 2010, 114, 14583-14589. (link)