Growth and scanning tunneling spectroscopy of self-assembled π-conjugated oligomers

A novel technique for the fabrication of π-conjugated oligomers bonded to a substrate is presented. On a gold substrate, monolayers of small monomeric organic units are alternately deposited by self-assembly. In each deposition step, polymerization takes place at the interface between two monolayers through a condensation reaction. Chain growth is verified and analyzed by ellipsometry, gas chromatography, mass-spectrometry and X-ray photoelectron spectroscopy. The layers are also imaged by scanning tunneling microscopy (STM), and current–voltage spectra have been recorded by STM in interrupted feedback. The salient features for the oligomers, as compared to the bare gold substrate, can be explained by the presence of a thin (∼1 nm) layer of semiconductor with ∼2 eV energy barrier for tunneling electrons.