Visualization and manipulation of individual dopant states in single conjugated oligomers.

Nonlinear excitations associated with dopant states play fundamental roles for charge transport in conjugated polymers. Here we report on real-space visualization of individual dopant states in single conjugated oligomers of poly-para-phenylene using cryogenic scanning tunneling microscopy and spectroscopy. We have found that these states exhibit a typical spatial extension of 4 nm along the oligomers. In particular, these states create a shallow level inside the band gap of the parent oligomers. The origin of these states is traced to a novel doping mechanism of dehydrogenation of the phenylene moiety. Furthermore, we use a scanning tunneling microscope tip to charge/discharge the dopant states and measure their lifetimes. The present results demonstrate a strategy to characterize and manipulate individual dopant states in conjugated polymers with subnanometer resolution.