Dendritic Aggregation of Oligothiophene during Desorption of 2,5-Diiodothiophene Multilayer and Topography-Induced Alignment of Oligothiophene Nanofibers

Abstract

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.

Thermal reaction and desorption behaviors of 2,5-diiodothiophene on clean and passivated Au surfaces

“…The product of these two terms produces a peak shape more symmetric than the typical zeroth-order multilayer desorption kinetics. Although the molecular origin of this kind of clustering behavior is not known, the consequence of the clustering has been visualized in our previous study imaging photochemical reaction products [9].…”

“…Recently, we have extended the use of iodo-substituted compounds to generation of functional organic molecules or films on surfaces. For example, 2,5-diiodothiophene can be photochemically activated and coupled to produce thin films and micropatterns of oligothiophenes and polythiophenes on metal or oxide surfaces [4][5][6][7][8][9][10][11]. This approach is intriguing since it avoids the intractability problem of the unsubstituted oligothiophenes and polythiophene, i.e.…”