DFT Investigation of Oligothiophenes on a Si(001) Surface

The nature of the interaction of a series of alpha-oligothiophenes (1T up to 6T) with a Si(001) surface was studied by means of density functional theory (DFT) structural optimizations. For IT, we found that the products of the [4 + 2] cycloaddition are more thermodynamically stable than those of the [2 + 2] cycloaddition, in agreement with previous cluster approach calculations. Since there are many possible ways of placing the all-trans nT molecule on a Si(001) surface, we have tested a number of possible conformations; in particular, starting from simple geometric considerations, we built the 6T molecule from IT as a result of [2 + 2] cycloadditions. The binding energy of oligomers with n from 2 to 6 displays a steady drop, attributable to an increased deformation energy. Our study sheds light on the chemisorption of oligothiophenes on Si(001) showing, in particular, that molecular chemisorption is possible for oligothiophenes with up to four thiophene units. For 6T, we estimated the van der Waals contribution to the binding energy, being of the order of 1 eV for molecule-surface distances approximate to 4 angstrom, in agreement with the experimental evidence of the presence of 6T physisorbed states on the Si(111) surface. Finally, the calculated work function for 6T on Si(001) is in very good agreement with the experimental estimate.