Paper NS+NC-FrM6
Structure and Electron Transport within Self-Assembled Monolayers (SAMs) of Discotic Molecules on Au-substrates

Friday, October 24, 2008, 10:00 am, Room 311

Hexa-peri-hexabenzocoronene (HBC) and HBC-derivatives can self-assemble in form of columnar film structures, which have recently attracted a significant amount of attention with regard to one-dimensional charge transport. The HBC molecules are known to form the large ordered columnar rows studied by various methods. In previous work the preparation of vertically oriented columnar rows has been achieved by employing a zone casting technique.¹ Here, we demonstrate another approach to fabricate such columnar system by an immersion process, where one-dimensional columns are formed by the self organization of discotic HBC molecules modified by thiol (-SH) anchors adsorbed on Au(111) substrates. The molecular arrangements and charge transport of such columnar films is investigated by scanning tunneling microscopy (STM) and current imaging tunneling spectroscopy (CITS). The HBC self-assembled monolayer (SAMs) are found to consist of long range ordered domains consisting of equidistant paired rows. Each row consists of a one-dimensional column of closely packed stacked HBC units. The periodicity between these rows amounts to be twice than the diameter of the HBC-molecules. From the STM data and the molecular orientation obtained from near-edge x-ray absorption spectroscopy (NEXAFS) a structural model is derived. The current-voltage (I-V) characteristics measured by locally positioned STM tip above the HBC-SAM at 298 K and at 80 K exhibit a pronounced temperature dependence, indicates the presence of an additional charge transport mechanism including a hopping between the adjacent HBC disks aside from direct tunneling

¹Simpson, C. D.; Wu, J.; Watson, M. D.; Müllen, K., From graphite molecules to columnar superstructures - an exercise in nanoscience. Journal of Materials Chemistry 2004, 14, (4), 494-504.