The functionalization of semiconductor surfaces with organic molecules is a necessary step in the development of hybrid organic-semiconductor structures. A significant challenge to organic layer formation is the fact that semiconducting surfaces exhibit a large number of dangling bonds, which suppress the diffusivity of adsorbed molecules and can even break the molecules apart via the formation of Si-C bonds. Recently it has been shown that these problems can be obviated by depositing the organic molecules onto a passivated surface [1].

We have studied the adsorption of brominated π conjugated tetrathienoanthracene molecules (TBTTA) onto the Si(111)-√3×√3-Ag surface. Thiophene based molecules like TBTTA are of considerable interest in organic semiconductor research due to their efficient conjugation and the chemical stability [2]. The Si(111) √3×√3-Ag surface has no Si dangling bonds and should provide a high mobility surface suitable for TBTTA adsorption. Scanning Tunneling Microscopy images reveal that at low coverage the molecules readily migrate to step edges and defects in the √3 overlayer, in fact many images show direct evidence of molecular mobility. With increasing coverage the molecules eventually form compact supramolecular structures. In terms of the √3 lattice vectors (a and b), the oblique unit cell of these structures is a_m = 3a + b, and b_m = a + 2b. The structures are quite fragile and can decompose under repeated STM imaging. Our results suggest that TBTTA is weakly bound to the √3 surface at room temperature and that the supramolecular structures are held together by weak van der Waals forces.

1. T. Suzuki et al., Phys. Chem. Chem. Phys. 11, 6498 (2009).

2. R. Gutzler et al., Nanoscale 6, 2660-2668 (2014).