Growth of Hydrogen-Bonded Molecular Aggregates on a Thin Alkali Halide Layer: Quinacridone on KCl/Ag(100)

Thef formation of self-assembled aggregates and domains of quinacridone (QA) on epitaxial KCl layers on the Ag(100) surface was investigated by scanning tunneling microscopy. If QA is deposited onto incomplete KCl layers that do not cover the complete Ag(100) surface, the QA molecules diffuse off the KCl islands and adsorb on the bare Ag surface instead. This is attributed to the high mobility of QA on KCl and the small Ehrlich-Schwoebel barrier of QA at KCl island edges on Ag(100). On KCl layers that completely cover the Ag(100) surface, the growth of QA depends on the thickness, namely, the number of monoatomic layers (ML), of the KCl layer. We find that ordered linear chains, held together by strong hydrogen bonds, and compact islands of these chains can be grown on thin KCl layers on Ag(100) for thicknesses of up to about three ML of KCl. This is attributed to attractive long-range dispersion forces between the QA molecules and the underlying Ag substrate through the KCl layer. These contribute to the adsorption energy of QA and support the adsorption of QA in a flat-lying geometry. For thicker KCl films, the attractive dispersion forces are not relevant, and hence, only nucleation of three-dimensional QA clusters occurs because the intermolecular interactions overbalance the QA/KCl interactions.