Dendritic aggregation of oligothiophene during desorption of 2,5-diiodothiophene multilayer and topography-induced alignment of oligothiophene nanofibers.

The multilayer desorption behavior of 2,5-diidothiophene and the dendritic aggregation of photochemical reaction products during the desorption of 2,5-diiodothiophene multilayers have been studied. Like many other aromatic compounds, 2,5-diiodothiophene shows a multilayer desorption behavior different from the typical zeroth-order kinetics, a metastable desorption peak growth at approximately 220 K followed by a thick multilayer peak growth at approximately 235 K. Traditionally, these desorption behaviors have been attributed to the formation of three-dimensional clusters. This paper provides the direct evidence of this clustering process by producing nondesorbing photoreaction products in the multilayer and by imaging their clusters after the multilayer desorption. Oligothiophene species are produced via photochemical reactions of 2,5-diiodothiophene during the multilayer deposition at approximately 180 K in ultrahigh vacuum (UHV). Upon heating the multilayer to room temperature, the oligothiophene species forms into fibrous aggregates with a fractal dimension varying from 1.37 to 1.81 depending on their surface concentration. Using a topographical alteration of the substrate with a repeating pattern, these oligothiophene fibers can be aligned to a certain direction. This may allow in-situ fabrication of aligned conjugated polymer fibers directly on a target substrate.