Formation of 2D Self-Assembled Layers of Allyl Cyanide on Pd(111)

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We present a mechanistic study on the interaction of allyl cyanide (AC) with Pd(111) resulting in the formation of different types of self-organized 2D layers. We employ a combination of scanning tunneling microscopy with infrared reflection absorption spectroscopy to obtain complementary information on the real space distribution of AC molecules in the self-organized 2D layers and on their chemical structure as a function of surface temperature. Specifically in the temperature range of 230–250 K, AC was found to form a self-organized 2D layer involving two different types of surface species adsorbed in trans and cis configurations. Each of these species forms identical individual (8 × 5) sublattices, which are shifted with respect to each other by three Pd atoms. Both functional groups of AC─the CN and C═C bonds─are oriented nearly parallel with respect to the metal surface. With decreasing temperature, strong changes in the layer structure were observed: the AC species agglomerate in irregular concave nonagons comprising nine molecules with the nearest AC–AC distance of three Pd atoms. These individual concave nonagons form a self-assembled 2D layer with the (13 × 10) overstructure on the extended areas of Pd(111). Below 210 K, the degree of order in the AC overlayer drastically decreases and only little structured features can be found. They comprise mainly hexagons embedded into a disordered 2D layer and some parallel rows. The nearest AC–AC distance is close to three Pd atoms in these structures. For all observed AC layers, we provide detailed atomistic-level models and accurate distances between the individual species, which were found to strongly depend on the surface temperature. With the increasing surface temperature, a noticeable increase in the characteristic distances between the interacting neighboring species was observed, which we refer to as changes in the nature of intermolecular interactions.