One-dimensional aggregates of the organic dye quinacridone on metallic and dielectric surfaces

Abstract

The present work deals with the question to what extent the motif of intermolecular hydrogen bonds (H-bonds) can be used to generate isolated one-dimensional (1D) structures, i.e. "chains", of organic molecules on surfaces through self-assembly. For this purpose, the structures of quinacridone (QA) were investigated using LEED and STM. Of particular interest was the question of whether such molecular chains can be formed not only on metallic substrates, but also on thin layers of insulating materials, where they may be electronically decoupled from the underlying metallic substrate. <br/> First, the self-assembled structures of QA on the Ag(100) and Cu(111) surfaces were investigated. It was found that QA forms homochiral molecular chains that are held together by intermolecular H-bonds. These chains are metastable and are stabilized by a kinetic barrier. During annealing, the molecular chains on both surfaces transform into heterochiral structures. These heterochiral structures contain fewer H-bonds per molecule, which is overcompensated by stronger bonds between the molecules and the substrates. The kinetic barriers are given by the breaking of the intermolecular H-bonds and presumably by a slight reconstruction of the first layers of the metal substrates. <br/> The second step consisted of electronically decoupling the molecular chains of QA from the underlying metal substrates. For this purpose, QA was evaporated onto thin films of insulating materials, namely KCl on Ag(100) and hBN on Cu(111), where the interactions between substrate and adsorbate are usually weaker. Interestingly, molecular chains of QA with different azimuthal orientations were observed on both thin KCl layers and a single hBN layer, which are stabilized by additional van der Waals interactions with the substrate. On thicker KCl layers, 3D clusters are formed instead of molecular chains, since the additional stabilization by the metal substrate is not given there. <br/> In addition, it was investigated whether the QA structures on KCl layers can be stabilized by introducing a larger number of steps into the system, which can serve as favorable nucleation sites. For this purpose, the growth of epitaxial KCl layers on a vicinal Ag(100) surface was investigated. The KCl layer grows over the step edges of the vicinal surface in a carpet-like growth, which leads to a slight deformation of the KCl lattice. This has an effect on the growth of QA structures on the KCl layer. The deformation of the KCl lattice leads to more possible adsorption configurations for QA molecules and thus to a wider range of azimuthal chain orientations. <br/> Overall, the studies in the present work show that the formation of intermolecular H-bonds is a powerful motif that can be used to create one-dimensional organic structures on metal surfaces and thin films of insulating materials. However, the substrate also plays an important role, as all observed structures are the result of a delicate balance between the intermolecular and substrate-adsorbate interactions.