Direct observation of surface diffusion of large organic molecules at metal surfaces: PVBA on Pd(110)

The bonding and surface diffusion of 4-trans-2-(pyrid-4-yl-vinyl) benzoic acid (PVBA) on Pd(110) was investigated by variable temperature scanning tunneling microscopy at sample temperatures between 300 and 450 K. PVBA is a large organic molecule designed for nonlinear optics applications. At low coverages single PVBA molecules are randomly distributed at the surface where they bind diagonally to three neighboring Pd-rows, leading to four equivalent adsorption configurations. The “dog-bone” molecular structure could be resolved. The molecules’ surface diffusion is strictly one-dimensional along the close-packed [11̄0]-direction of the surface Pd atomic rows and obeys an Arrhenius law with an activation barrier of 0.83±0.03 eV and an attempt frequency of 1010.3±0.4 s−1.