Ordering and dynamics of oligo(phenyleneethynylene) self-assembled monolayers on Au(111)

Self-assembled monolayers of 4-[4'-(phenylethynyl)-phenylethynyl]-benzenethiolate (PPB-S) molecules on flame annealed Au(111) have been investigated by ultra-high vacuum scanning tunneling microscopy. We have found a hitherto unknown self-assembled monolayer phase. This phase consists of a fine-striped pattern that is aligned along the close packed < 1 (1) over bar0 > directions of the Au(111) surface. The fine-striped pattern has root 3 periodicity and is separated by narrow vacancy lines which are oriented perpendicular to the fine-striped pattern. Surprisingly, the herringbone reconstruction of the underlying Au(111) substrate is not fully lifted by the self-assembled monolayer. Spatially resolved current-time scanning tunneling microscopy experiments revealed that the PPB-S molecules at the edges of the vacancy lines exhibit dynamic behavior and frequently jump back and forth between neighboring stripes.