Abstract
A detailed understanding of the organic molecule/substrate interface is of crucial importance for the design of organic semiconducting devices, as the interface determines the contact resistance and the charge injection. Generally, two different adsorption situations are considered: physisorption and chemisorption. For small molecular adsorbates like CO or NI, the adsorption energy alone can be used as a criterion to classify the adsorption in chemisorption (adsorption energies larger than 1 eV) and physisorption (few tens of meV). This classification fails for complex pi-conjugated organic molecules. Here we discuss on the basis of a pentacene/Cu(110) model system a different set of criterio to distinguish between chemisorption and physisorption beyond the total bond energy argument. We analyze the bonding situation on the basis of density functional theory (DFT) calculations and photoelectron spectroscopy. Theory predicts (i) a significant bending of the molecule after adsorption, (ii) a buckling of the top layer Cu atoms, (iii) the emergence of new hybrid states, and (iv) a substantial charge redistribution and accompanying charge transfer. Photoemission confirms the energies of the 3 topmost molecular orbitals with an almost "half-filled" lowest unoccupied molecular orbital (LUMO). The four criteria are used to qualify the adsorption mechanism in the pentacene/Cu(110) system as chemisorption. This set of criteria is indicative of chemisorption also in the case of other noncovalently coupled large adsorbates, far beyond the pentacene/Cu(110) case.