Dresden 2009 – wissenschaftliches Programm
O 19.9: Vortrag
Dienstag, 24. März 2009, 12:30–12:45, SCH A215
Temperature-induced phase transition in self-assembled supramolecular monolayers — •Rico Gutzler1, Thomas Sirtl1, Michael Schmittel2, Wolfgang M. Heckl1,3, and Markus Lackinger1 — 1Center for NanoScience (CeNS) and Ludwig-Maximilians-University — 2University Siegen — 3Deutsches Museum
We present self-assembled molecular monolayers at the liquid-solid interface, whose morphology can be switched reversibly between two crystallographically different structures by heating above a temperature threshold. At room temperature, STM experiments reveal a non-densely packed six-fold honeycomb pattern at the interface between solution and graphite. The solute molecule with C3 symmetry and carboxyl-terminated lobes adsorbs in a planar manner when dissolved in nonanoic acid. In this adsorption geometry, all of the three carboxylic groups are involved in two-fold intermolecular hydrogen bonds. Raising the temperature of the sample induces a phase transition which is accompanied by a substantial change in the monolayer morphology. The rearrangement results in a row structure comprised of densely packed, nearly upright standing molecules. The reversibility is demonstrated by subsequent cooling to room temperature which yields the original hexagonal monolayer. This unambiguously proves that the room temperature structure is not only a kinetically trapped metastable state which is just converted to the thermodynamically equilibrium structure by supplying thermal energy. Since in the row structure one carboxylic group per molecule points off the surface, the phase transition is accompanied by a change in surface affinity.