Structural effects on electrical conduction of conjugated molecules studied by scanning tunneling microscopy

We have studied electrical conduction of conjugated molecules with phenyl r ings embedded into alkanethiol self-assembled monolayers (SAMs), to investi gate the molecular structural effect on the electrical conduction. Scanning tunneling microscope (STM) images of this surface revealed that the conjug ated molecules with phenyl rings adsorbed mainly on defects and domain boun daries of the pre-assembled alkanethiol (nonanethiol C9) SAM and formed con jugated domains. in the case of conjugated molecules with one or three meth ylene groups between the sulfur and phenyl rings, the measured height of th e conjugated molecular domains depended on their lateral sizes, while a str ong dependence was not observed in the case of conjugated molecules without a methylene group. By analyzing size dependence on the height of the conju gated molecular domain, we could evaluate the electronic conductivity of th e molecular domains. As a result of the analysis, to increase the vertical conduction of the molecular domains, one methylene group was found to be ne cessary between the sulfur and aromatic phenyl rings. Local barrier heights on the conjugated molecular domains in all the cases were larger than on t he C9 SAM surface, suggesting that the increase in the vertical conductivit ity is not likely to be due to the lowering of the local barrier height, bu t can be attributed to the conjugated molecular adsorption. X-ray photoelec tron spectra (XPS) and ultraviolet Light photoelectron spectra (UPS) reveal ed that the carrier density among conjugated molecular SAR;Is does not depe nd on the number of methylene groups between the sulfur and phenyl rings, s uggesting that the higher vertical conduction of conjugated molecules with one methylene group can probably be attributed to higher transfer probabili ty of carriers during the STM measurements.