Molecular nanostructures with strong dipole moments on the Si(111) 5 × 2-Au surface.

Switchable organic molecules adsorbed on a silicon surface combine the flexibility and the low cost of molecular electronic devices with the sophistication of modern silicon technology. The first step towards creating such hybrid devices is the formation of regular, ordered patterns of molecules on a silicon surface. A stepped Si surface passivated by a sub-monolayer of gold is found to provide a useful substrate for forming ordered molecular patterns. Molecules with strong dipole moments, such as fluorophenols, form a one-dimensional molecular array on such a substrate by adsorbing on top of the step edges. Local barrier height measurement by scanning tunneling spectroscopy demonstrates the possibilities to detect the direction of the dipole moment of an individual molecule. Polarization-dependent x-ray absorption spectroscopy reveals an oriented adsorption in both the azimuthal and polar directions.