Using steric constraints to template an organic array on Si(111)-7 x 7

We show that the Si(111)-7 x 7 surface reconstruction can be used to template an ordered array of 1,3,5-trimethyl benzene (mesitylene) molecules. The disorder that normally derives from the multiplicity of admissible adsorption geometries, for small aromatic molecules on 7 x 7, is suppressed, and the molecules are found to occupy both halves of the 7 x 7 unit cell with equal probability. It is argued that a steric interaction, associated with the methyl groups, hinders nearest neighbor adsorption and this leads to the formation of an array that has the molecules at the corners of the 7 x 7 half unit cells. To understand the ordering kinetics we used: scanning tunneling microscopy to study site occupancy as a function of coverage, ab initio total energy calculation to study the stability of the attachment sites and kinetic Monte Carlo modeling to investigate the emergence of translational order in the overlayer. [GRAPHICS] When two mesitylene molecules adsorb in nearest neighbor bridging geometries on the 7 x 7 reconstruction, the hydrogen atoms located on the methyl groups interact. To reduce the interaction energy, the molecules form an ordered array that places each molecule at the corner of the half cell. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim