Supramolecular structures of halogenated oligothiophenes on the Si(111)-√3 ×√3-Ag surface

Abstract

We have studied the adsorption of brominated tetrathienoanthracene (TBTTA) molecules onto the Si(111)-√3×√3-Ag surface at room temperature. The two-dimensional √3 silver adlayer acts to passivate the silicon surface and provides a high-mobility template for TBTTA adsorption. Scanning tunneling microscopy (STM) images reveal that at low coverage, the molecules readily migrate to step edges and defects in the √3 overlayer. With increasing coverage, the molecules eventually form compact supramolecular structures. In terms of the hexagonal √3 lattice vectors (a√3 and b√3), the oblique unit cell of these structures can be defined by lattice vectors am=3a√3+2b√3, and bm=−a√3+b√3. The structures are quite fragile and can decompose under repeated STM imaging. This is particularly true at higher bias and suggests an electric field-induced dissociation in these instances. With increasing molecular dose, the size and stability of the structures increases. At higher coverage, the spatial extent of the supramolecular structures is often limited by defects in the underlying √3 layer. Our results suggest that the √3-Ag surface provides a relatively inert substrate for the adsorption of TBTTA molecules, and that the supramolecular structures are held together by relatively weak intermolecular forces.