Self-assembly of l-methionine on cu(111): Steering chiral organization by substrate reactivity and thermal activation

The self-assembly of the amino acid L-methionine on Cu(111) was investigated under ultrahigh vacuum (UHV) conditions by scanning tunneling microscopy (STM), helium atom scattering (HAS) and X-ray photoelectron spectroscopy (XPS). The system is strongly influenced by the substrate reactivity and the deposition temperature. The STM and HAS structural analysis yields that, for temperatures below 273 K, the biomolecules assemble in strings oriented with an angle of -10° with respect to the (110) axes of the substrate. For temperatures above 283 K, a regular and ordered one-dimensional (1D) phase arises following an angle of +10° with respect to the same directions. High resolution STM data of this ordered 1D arrangement evidence molecular dimerization and dimer alignment into ordered chains which are commensurate with the Cu(111) atomic lattice. XPS measurements reveal that the high temperature ordered phase consists of an exclusively anionic ensemble with a deprotonated carboxylic group and a neutral amino group, while the low temperature phase is heterogeneously composed of both zwitterionic and anionic species, depending on whether the molecules are immobilized in clusters of dimers on the free terraces or at the low-coordinated adsorption sites of the substrate step-edges. These combined results evidence a structural transformation of the supramolecular assembly which is triggered by a thermally activated process involving the underlying Cu(111) substrate and which carries the intrinsic chiral signature of the adsorbed molecular units.