PhD thesis - Evans Research Group - University of Wisconsin-Madison

32Fig. 2.2: Atomic force microscope images of pentacene island formation on SiO 2substrates at pentacene coverages of (a) 0.30 ML, (b) 0.60 ML, (c) 0.75 ML, (d) 0.92 ML,(e) 1.15 ML, and (f) 1.24 ML.The heights of the islands are 1.58±0.06 nm. These heights are measured using linescans from the AFM images. This value is comparable to the interplanar spacing ofpentacene molecules grown on SiO 2 , where pentacene molecules are slightly inclined withrespect to the normal of the substrate. In XRD experiments interplanar spacing of thesingle crystal pentacene was 14.5 Å. For pentacene films deposited in vacuum the heightwas 15.4 Å [26-28].The charge transport in the submonolayer regime was studied by analyzing the currentvoltagecharacteristic curves at two submonolayer coverages (0.72 ML, 0.9 ML), duringpauses in the deposition. The results of the electrical measurements are shown in Fig. 2.3.Figure 2.3(a) shows the drain current as a function of pentacene coverage during this