Scanning tunneling microscopy study of imaging change induced by electric field change of bipyridine derivatives in self-assembled monolayers

We observed image changes induced by the polarity change of electric fields, by scanning tunneling microscopy (STM), for pre-assembled alkanethiol self-assembled monolayers (SAMs) embedded with small amounts of the bipyridine moiety. In the case of bipyridine dithiol molecules, STM experiments revealed that the apparent thickness of the molecular protrusions was larger under the positive bias. In this case, it was considered that the image change was caused by a conductance difference with different polarities of the tip bias. However, for the bromine-terminated bipyridine molecules, the thickness of the molecular protrusions was larger under negative bias, contrary to the case of dithiol type molecule. We considered that this image change was caused by an actual molecular conformational change because of the strong dipole moment of bromine-terminated bipyridine molecule.