Molecular Orientation and Film Morphology of Pentacene on Native Silicon Oxide Surface

Abstract

The growth morphology and mechanism of pentacene films on native Si oxide surface have been studied by using high-resolution electron energy loss spectroscopy (HREELS), X-ray diffraction (XRD), and atomic force microscopy (AFM). Despite the good agreement between our own and the reported XRD results, the previous XRD interpretation that the pentacene molecules are tilt-standing on the substrate cannot explain our HREELS data. The HREELS results show that a substantial portion of the first two layers of pentacene molecules are tilted-standing or randomly oriented, whereas the upper-layer molecules are mostly lying flat to the substrate. AFM reveals that the first two layers of molecules form a flat and smooth surface, but the upper layers show a rough terrace structure with a mean-square roughness equal to the average thickness (without counting the first two layers). This relationship is explained by a theoretical model which assumes the pentacene molecules to remain on a particular molecule layer after arrival. The observed film growth morphology may have significant implication on the performance of electronic devices based on pentacene thin films. A plausible explanation was proposed for the discrepancy between the HREELS-indicated and the XRD-derived molecular orientations.

Phase separation in vacuum codeposited pentacene/6,13-pentacenequinone thin films

“…This is in contrast to the Stranski-Krastanov-type growth reported for P thin films on SiO 2 , where at least one complete layer is being formed on which island are built up. 40 The PQ submonolayer film exhibits an area coverage of only 10% ͑due to the instant formation of multilayer islands͒ and an island density ͑N͒ of 1.4 m −2 . For comparison, a P film of same nominal thickness had a coverage of 50% and N = 5.5 m −2 .…”

Structural origin of the mobility enhancement in a pentacene thin-film transistor with a photocrosslinking insulator

“…This is consistent with previous results. [18][19][20] It is interesting to note that the measured d 001 is somewhat smaller than the length of a single pentacene molecule ͑1.60 nm͒. This suggest that the long axes of the pentacene molecules are slightly tilted.…”

Polymorphism in pentacene thin films on SiO2 substrate

“…15,16 Therefore, investigation and control of the polymorphism and film structures are demanded to improve device performance of OTFTs. Several studies on the structure and the growth mechanism of pentacene films have been carried out; [17][18][19][20][21][22][23][24] however, the mechanism of the transformation from the thin-film phase to the bulk phase is not fully clarified.…”