Displacement and Replacement in Organic Heterostructures

Full text: In this LEED/Angle Resolved UPS study an heterostructure system is presented which shows a counter intuitive reversible replacement of two different organic molecules at the molecule substrate interface. The organic rod like molecule sexiphenyl (6P) forms a well ordered monolayer on Ag(110) which is clearly characterised by its valence band emissions and its geometric structure. Valence band tomography of the π orbitals show no evidence of backdonation to the 6P LUMO and that 6P is in its twisted confirmation with its long molecular axes aligned perpendicular to the substrate atomic rows. Upon the deposition of pentacene (5A) on it, a density of states appears at the Fermi edge. Fermi surface maps identify this intensity as the ex-LUMO of 5A filled via backdonation from the substrate. This, together with the fact that 6P emissions are still dominant, suggests that 5A has gone to the interface. Extraordinarily, on ramping the temperature the 6P features rst disappear and then reappear at the temperature of desorption of the 5A/Ag(110) monolayer. This de/re-wetting is also observed with LEED where de-wetting of 6P results in the appearance of a well ordered 5A monolayer LEED pattern which, at the 5A desorption temperature, changes completely to the 6P monolayer pattern. That the chemisorbed 5A desorbs before the physisorbed 6P is apparently counterintuitive and will be discussed in terms of the energetics of de-wetting (3D island formation) and wetting monolayers. (author)