Crystallographic and morphological characterization of thin pentacene films on polycrystalline copper surfaces

Abstract

The degree of crystallinity, the structure and orientation of crystallites, and the morphology of thin pentacene films grown by vapor deposition in an ultrahigh vacuum environment on polycrystalline copper substrates have been investigated by x-ray diffraction and tapping-mode scanning force microscopy (TM-SFM). Depending on the substrate temperature during deposition, very different results are obtained: While at 77 K a long-range order is missing, the films become crystalline at elevated temperatures. From a high-resolution x-ray-diffraction profile analysis, the volume-weighted size of the crystallites perpendicular to the film surface could be determined. This size of the crystallites increases strongly upon changing temperature between room temperature and 333 K, at which point the size of individual crystallites typically exceeds 100 nm. In this temperature region, three different polymorphs are identified. The vast majority of crystallites have a fiber texture with the (001) net planes parallel to the substrate. In this geometry, the molecules are oriented standing up on the substrate (end-on arrangement). This alignment is remarkably different from that on single-crystalline metal surfaces, indicating that the growth is not epitaxial. Additionally, TM-SFM images show needlelike structures which suggest the presence of at least one additional orientation of crystallites (flat-on or edge-on). These results indicate that properties of thin crystalline pentacene films prepared on technologically relevant polycrystalline metal substrates for fast electronic applications may be compromised by the simultaneous presence of different local molecular aggregation states at all temperatures.

Sexithiophene films on ordered and disordered TiO2(110) surfaces: Electronic, structural and morphological properties

“…The importance of the growth kinetics mediated by substrate surface order can also explain recent growth studies on polycrystalline Au, Ag, and Cu substrates, where the molecules studied adopt an end-on arrangement [32][33][34] in contrast to the lying orientation that results on the single crystal substrates. Presumably, the size of the ordered domains of the substrate relative to the diffusion length of molecular is an important parameter.…”

Epitaxial growth of sexithiophene on mica surfaces

“…While the first finding is often observed for epitaxially grown organic thin films, [30][31][32][33] the latter one is a strikingly uncommon finding. 28,34,35 In order to understand this surprising observation, an in-depth analysis concerning the relationship between crystals of type S and A has been performed.…”

Orientation and electronic properties of phthalocyanines on polycrystalline substrates

“…cases on relatively ill defined, more technically relevant substrates [18][19][20][21]. Motivated by the possible application of organic semiconductors in field effect transistors increased research efforts during the last years were devoted to the growth of organic molecules on oxide substrates, in particular to silicon oxide [22][23][24].…”