Abstract
We investigated the adsorption of 4-methoxy-4′-(3-sulfonatopropyl)stilbazolium (MSPS) on different ionic (001) crystal surfaces by means of noncontact atomic force microscopy. MSPS is a zwitterionic molecule with a strong electric dipole moment. When deposited onto the substrates at room temperature, MSPS diffuses to step edges and defect sites and forms disordered assemblies of molecules. Subsequent annealing induces two different processes: First, at high coverage, the molecules assemble into a well-organized quadratic lattice, which is perfectly aligned with the <110> directions of the substrate surface (i.e., rows of equal charges) and which produces a Moiré pattern due to coincidences with the substrate lattice constant. Second, at low coverage, we observe step edges decorated with MSPS molecules that run along the <110> direction. These polar steps most probably minimize the surface energy as they counterbalance the molecular dipole by presenting oppositely charged ions on the rearranged step edge.
Highlights
The adsorption of organic molecules on a crystalline substrate surface is governed by a delicate balance between the molecule–molecule (MM) and the molecule–substrate (MS) interaction
We observe a rearrangement of the KCl substrate surface, which will be discussed at the end of this section
For the moment we would just like to mention that the diffusion of MSPS, with its strong electric dipole, on an ionic surface can Figure 2: 0.2 ML of MSPS evaporated onto KCl. (a) displays the Noncontact atomic force microscopy (NC-AFM) topography after deposition at RT (Δf = −59 Hz, A0 = 7 nm), (b) shows the surface after annealing to 110 °C for 15 min (Δf = −40 Hz, A0 = 7 nm)
Summary
The adsorption of organic molecules on a crystalline substrate surface is governed by a delicate balance between the molecule–molecule (MM) and the molecule–substrate (MS) interaction. In order to completely understand the self-organization of MSPS on KCl there are two additional points that must be clarified: First, is the observed Moiré pattern an effect of coincidences between the quadratic lattices of molecules and substrates, and can the molecular lattice be regarded as being incompressible (i.e., the intermolecular interactions are much stronger than the molecule–substrate interactions)?
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