Self-Assembly of l-Tryptophan on the Cu(001) Surface

Adsorption and assembly of l-tryptophan on the Cu(001) surface have been studied by means of scanning tunneling microscopy. It has been found that l-tryptophan adsorbs on the surface in its anionic form, the same as the seven amino acids studied so far, and that at low coverage not only the carboxylate group but also the indole group of the adsorbates is attached to the surface, different from the seven studied. Five superstructures of the adsorbates have been found to exist at different coverage, which are (7 7, −7 7), (6 4, −5 4), (4 × 4), (4 0, 1 2), and (8−1, −1 2), and models have been proposed for these superstructures, for further investigation. The driving forces of these superstructures are suggested to be the intermolecular hydrogen bonds along with, for the high coverage superstructures, π-stacking interactions between the indole groups of neighboring molecules. We believe what has been demonstrated here is the flexibility and potential of large and nonplanar molecules with several different functional groups in forming different ordered superstructures on metal surfaces.