Tetracene Monolayer and Multilayer Thin Films on Ag(111):  Substrate-Adsorbate Charge-Transfer Bonding and Inter-Adsorbate Interaction

Temperature programmed desorption (TPD) is used for examining surface binding, intermolecular interaction, and morphology of mono- and multilayer films of tetracene on Ag(111). TPD of monolayer tetracene revealed strong inter-adsorbate repulsion caused by interaction among interface dipoles resulted from charge-transfer bonding. A modified Albano model, in which a point interface dipole is assigned to each of the aromatic rings of tetracene, is proposed to account for the interfacial dipole interaction at short range. It is found that desorption energy at the zero-coverage limit is 142 ± 7 kJ/mol. The interface dipole is determined as 8.2 ± 2.1 D, which corresponds to a partial charge transfer of 0.4 e per tetracene molecule to the Ag substrate. At full monolayer coverage, the strong inter-adsorbate repulsion reduces the desorption energy to 105 ± 14 kJ/mol. Annealing at elevated temperature (350−400 K) but below desorption temperature, on minute time scale followed by cooling, appears to produce a more stable structure. Multilayer TPD spectra show three separate half-order desorption peaks that merge into one bulk peak at higher coverage. The half-order kinetics agrees with the previously reported Stranski−Krastanov growth mode in which islands with high height-to-width ratio are formed. The desorption energies for these peaks are 100 ± 7, 110 ± 10, and 116 ± 4 kJ/mol respectively. Upon annealing, the lower energy structure transform into the higher energy ones.