STM Investigation on Single, Physisorbed Dendrimers

Abstract

Porphyrin dendrimers were synthesized to mimic naturally occurring proteins, which catalyze a number of biochemically important reactions. In addition, chiral dendrimers were prepared as model compounds for the study of nanoscopic chirality. The structures of these dendrimers cannot be characterized by x-ray, as most dendrimers do not crystallize. We succeeded to image single, physisorbed dendrimers on noble metal surfaces with STM. All examined dendrimers can easily be (re)moved with the STM tip, even at low scanning currents (low pA range). One possibility to avoid this, is to change the peripheral groups of the dendrimer, so that they chemisorb on the surface. We decided to study physisorbed macromolecules with unchanged molecular properties, and investigated how an other approach for chemisorbed polyamidoamin (PAMAM) dendrimers, could be adapted to our molecules. Indeed, even physisorbed molecules are stabilized by embedding them in a self-assembled monolayer of covalently bonded 1-hexadecanethiol. The so embedded dendrimers show a reduced mobility on the substrate with a conformation closer to the one in solution, permitting STM imaging at various temperatures.