Paper BO+AS+BI+NC-WeM5
Phase Stability of 2D and 3D Structures of Oligopyridines on HOPG Studied by Thermal Desorption Spectroscopy and Scanning Tunnelling Microscopy

Wednesday, October 22, 2008, 9:20 am, Room 201

Bis(terpyridine)derivatives (BTP) form highly ordered hydrogen bonded 2D networks on graphite (HOPG) at both the solid/liquid^1,2 and the solid/gas^3,4 interface. As found by STM at both interfaces, these adlayer structures depend on the positions of the N-atoms within the molecules, which can be varied via the synthesis process.^1,2 Complementary to the STM studies, we have performed temperature programmed desorption experiments of two different types of BTP molecules on HOPG. This revealed quantitative insights into phase stabilities of 2D (monolayer) and 3D (multilayer) phases. As to be expected from their large mass (618 amu) and correspondingly large moment of inertia, the translational and rotational degrees of freedom are found to play an important, even dominant role for the stability of more or less densely packed phases. This becomes apparent in strongly differing pre-exponential factors for desorption out of 2D and 3D phases. In agreement with STM observations at 300 K, the most stable phase (i.e., the one with the lowest chemical potential) for both molecules is not a close packed hydrogen bonded one, but a dilute 2D gas with facilitated translation and planar rotation. In this picture, ordered, hydrogen bonded structures observed at room temperature are only stabilized by the strong molecule-substrate interaction that allows enforcing higher coverages that go along with the more densely packed, ordered structures.

¹ C. Meier et al., J Phys Chem B 109 (2005) 21015
² C. Meier et al., Angew.Chem.Int.Ed. 47 (2008) 3821
³ H. E. Hoster et al., Langmuir 23 (2007) 11570
⁴ M. Roos et al., Phys. Chem. Chem. Phys. 9 (2007) 5672.