Solving the Long-Standing Controversy of Long-Chain Alkanethiols Surface Structure on Au(111)

The determination of the amount of gold adatoms and vacancies present at the thiol-gold interface of the (root 3 x root 3)-R30 degrees lattice on Au(111), usually observed for long-chain alkanethiols, is crucial to complete our knowledge on the species resulting from the thiol-substrate interaction. While this issue has been addressed for short and intermediate alkanethiols where Au adatom-complexes of the type RS-Au-ad-SR, known as staples, are formed, this issue is still under discussion for long-chain alkanethiols where different and contradictory models have been proposed. In this report we present X-ray diffraction data of the (root 3 x root 3)-R30 degrees structure for hexadecanethiol, allowing us to establish such basic structural features as the existence of Au adatoms and vacancies, the thiol adsorption-induced reconstruction of the surface, and the structure of the interface. These results allow us to reject the notion that vacancies and adatoms coexist within the same unit cell, and allow us to note that over the whole thiol-Au(111) interface the two exist in equal proportions. Vacancies appear on the gold surface as vacancy islands, and gold-adatoms appear flanked by two sulfur atoms, as observed on thiol-protected gold clusters and short thiols on Au(111). The coexistence of radicals and staples in the same (root 3 x root 3)-R30 degrees cell is consistent with experiment and theory.