Dresden 2006 – scientific programme

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O: Oberflächenphysik

O 9: Organic films I

O 9.3: Talk

Monday, March 27, 2006, 15:30–15:45, TRE Phys

Long-range order and polymerisation of sexithiophene on Ag(100) surfaces – a STM study — •Klaus Duncker, Sebastian Wedekind, Christian Hagendorf, and Wolf Widdra — Martin-Luther-Universität Halle-Wittenberg, 06120 Halle, Fb Physik

High-temperature scanning tunneling microscopy (STM) is used to study the morphology of the first few monolayers of α-sexithiophene (6T) on Ag(100) surfaces at room and elevated temperatures. The 6T molecules have been evaporated from a homebuilt Knudsen cell at a base pressure of the UHV chamber of 1×10⁻¹⁰mbar.
Several molecular superstructures coexist in different domains within the monolayer. They can be devided into two groups with either one or two molecules in the unit cell. Common to all, the flat-lying 6T molecules are oriented with their long axis parallel to [011] and [0 1 1] crystallographic directions of the Ag-substrate. As the predominant superstructure with two molecules in the unit cell we find a (9/0)(2/5) structure with a pair of chirally equivalent, parallel molecules in the unit cell. Additionally, a herring bone-like structure is found with two non-linear molecules in the unit cell forming rows in highly-ordered domains. STM images with submolecular resolution reveal flat molecules which have undergone a cis-trans isomerization exclusively between the 4th and 5th thiophene ring and which align in rows of left- and right-handed molecules.
Whereas earlier studies indicate molecular dissociation of the first monolayer during thermal desorption, we have identified by STM polymerization to long chains and polythiophene networks after annealing to about 440 K and multilayer desorption.