Ultrathin Films of Pentacene on Ag(111): Charge-Transfer Bonding and Inter-Adsorbate Interactions

Abstract

Temperature programmed desorption (TPD) was used to examine the surface binding and intermolecular interactions of mono- and multi-layer thin films of the polycyclic aromatic acene, pentacene, deposited on a Ag(111) surface. The TPD spectra of sub-monolayer cov- erages revealed the presence of three distinct phases (denoted as α1, α2, and α3). The α1 phase was attributed to adsorption on step sites, while the α2 and α3 phases were assigned to adsorption on terrace sites under different local molecular densities. A physical model was constructed to describe the desorption kinetics from each of the three monolayer phases, including intermolecular repulsion from interfacial dipoles produced as a result of charge transfer bonding between pentacene and the Ag substrate. Fit analysis of the sub-monolayer spectra revealed desorption energies in the zero-coverage limit of 218±8, 166±8, and 162±9 kJ/mol for the α1, α2, and α3 phases, respectively. The interface dipoles of the α2 and α3 terrace adsorption sites were found to be effectively invariant (within error) and deduced as 18±7 and 23±10 D, respectively. These values suggest a partial charge transfer of 0.6 to 0.7 electrons from each pentacene molecule to the Ag substrate and is equivalent to 0.13 electrons per aromatic ring. The TPD spectra from the multilayer films also exhibited three phases. Leading edge analysis of the lowest temperature multilayer peak yielded a desorption energy of 121±15 kJ/mole, while simulations predicted desorption energies ca. 10-15 kJ/mole higher for the higher temperature phases. The three multilayer phases were assigned, from lowest to highest temperature, as an amorphous bulk film, a thin film, and polycrystalline structures.