Electrical characterization of thin single crystals of sexithiophene

Extremely thin, single crystals of sexithiophene (6T), 2-14 nm thick and 2-5 μm in length and width, can be grown on flat gold substrates by thermal evaporation. The thickness dimension corresponds to 1-6 monolayers (ML) of 6T molecules arranged with their long axes nearly perpendicular to the substrate. We have measured the current-voltage (I-V) characteristics through the thickness of these crystallites, after doping them with iodine, using conducting probe atomic force microscopy (CPAFM). The I-V traces are linear in the ±50 mV regime. The conductance (I/V) of the doped 6T crystals does not decrease monotonically with increasing thickness as might be expected, but instead has a maximum at 3 ML thickness, and we discuss several possible explanations for this observation.