First-principles study of the dipole layer formation at metal-organic interfaces

摘要

We study the dipole layer formed at metal-organic interfaces by means offirst-principles calculations. Interface dipoles are monitored by calculatingthe work function change of Au, Ag, Al, Mg and Ca surfaces upon adsorption of amonolayer of PTCDA (3,4,9,10-perylene-tetra-carboxylic-di-anhydride), peryleneor benzene molecules. Adsorption of PTCDA leads to pinning of the work functionfor a range of metal substrates. It gives interface dipoles that compensate forthe difference in the clean metal work functions, leading to a nearly constantwork function. In contrast, adsorption of benzene always results in a decreaseof the work function, which is relatively constant for all metal substrates.Both effects are found in perylene, where adsorption on low work functionmetals gives work function pinning, whereas adsorption on high work functionmetals gives work function lowering. The work function changes upon adsorptionare analyzed and interpreted in terms of two competing effects. If the moleculeand substrate interact weakly, the molecule pushes electrons into the surface,which lowers the work function. If the metal work function is sufficiently lowwith respect to the unoccupied states of the molecule, electrons are donatedinto these states, which increases the binding and the work function.