LONG-RANGE ORDERED, COMMENSURATE SUPERSTRUCTURES OF LARGE ORGANIC ADSORBATES - A LEED AND STM STUDY OF EC4T AG(111)/

The adsorption behaviour and especially the geometric structure of a l arge organic molecule, end-capped quaterthiophene (EC4T), on Ag(111) h as been studied using TPD, LEED and STM. Three different peaks in the TPD spectra of the undissociated molecule represent the desorption of the multilayer, the bilayer and a small fraction of the monolayer duri ng a structural phase transition. The majority of the monolayer cannot be desorbed thermally due to a covalent bonding of EC4T to the Ag sub strate. This site-specific bonding is also responsible for the occurre nce of a 2D-commensurate superstructure for coverages at and below 90% of a saturated monolayer and for the reversible phase transition whic h leads to a unidirectionally compressed, 1D-commensurate superstructu re for a saturated monolayer. (C) 1997 Elsevier Science B.V.