Mechanisms of the Adsorption and Self-Assembly of Molecules with Polarized Functional Groups on Insulating Surfaces

The adsorption and self-assembly of several pentahelicene molecules with polarized and rigid functional groups have been studied on the Suzuki (001) surface in ultrahigh vacuum by noncontact AFM and KPFM, with the assistance of DFT calculations. It is shown that the adsorption strongly depends not only on the functional groups but also on the ionic substrate lattice. In particular, the local dipole of the functional groups, the adsorption geometry with respect to the cation lattice, and the ionic structure of the surface play a dominant role with a considerably large impact on the self-assembly of the helicene molecules. The work provides a detailed insight into the interaction of functionalized molecules with ionic insulator surfaces and into self-assembly. ■ INTRODUCTION Ordered organic thin films are believed to be important surface systems for a variety of applications, in particular, for conducting devices. For optimal properties, the films must be 2-D and well-ordered (crystalline), which is realized by supramolecular self-assembly in most cases. Insulating substrates are of particular importance because the intrinsic electronic properties of the molecules can be decoupled from the substrate, which is essential in microelectronics. To tailor organic thin films and optimize their properties on insulating surfaces, the adsorption and self-assembly of molecules needs to be understood on the molecular scale. This can be best achieved on model surfaces with selected molecules by noncontact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM). In the past few years, the (001) surfaces of alkali halides have been shown to be adequate substrates because they are almost atomically flat, providing easy access for nc-AFM and KPFM, even if molecules are supported on them. An important aspect is the adsorption and diffusion of the molecules and, in particular, the influence of the ionic sublattices of the alkali halide substrate. A common finding is that the adsorption of molecules is relatively weak on such surfaces, resulting in a high mobility of the molecules. The molecules either form molecular nanowires at surface steps or self-assemble into large 2-D ordered films. The current tentative picture for the adsorption considers not only the van der Waals but also electrostatic contribution in the molecule−surface interaction: van der Waals forces are not strongly site-specific, and the molecules can occupy almost any position on the surface. However, as soon as a molecule includes polar groups or atoms carrying a partial charge, the molecule can directly bind via a stronger electrostatic interaction with the ionic surface lattice (charge matching). In view of the increasing amount of work, the detailed adsorption mechanisms of the molecule−surface interaction are still under debate, and work including a comparison between experiment and theory is very limited. Furthermore, not much is known with respect to the impact of adsorption on selfassembly (and vice versa), and the precise mechanisms of selfassembly have not been considered much in the past. By a combination of nc-AFM/KPFM experiments and density functional theory (DFT), we give detailed insights into the adsorption and self-assembly mechanisms of molecules with rigid polar functional groups on a nanostructured insulating surface. We select racemic [5]helicene derivatives (pentahelicene) as model molecules because they are chiral, helical polyaromatic compounds that can have enhanced chiral and optoelectronic properties for future applications. Most importantly, such molecules can be functionalized and have the tendency to self-assemble into larger structures due to π−π orbital interactions of their benzenoid rings. Furthermore, helicene molecules, in general, can be vacuum-deposited onto surfaces, as shown on metal and even insulating surfaces. We study a large spectrum of functionalizations by considering either one or two rigidly bound bromine atoms or cyano groups (Figure 1), which are all electroattractive with Received: February 18, 2014 Revised: April 9, 2014 Published: April 9, 2014 Article