Journal
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS
Volume 246, Issue 1, Pages 122-126Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/j.nimb.2005.12.008
Keywords
organic semiconductors; thin films; X-ray reflectivity; structure
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In this work we present an X-ray diffraction study of the early stages of growth of an organic semiconductor (sexithiophene, T-6) thin film prepared by high vacuum sublimation. Specular reflectometry and grazing incidence X-ray diffraction were used to monitor the formation of T-6 films on silicon oxide. Our results show that T-6 grows as a crystalline layer from the beginning of the evaporation. The reflectometry analysis suggests that, in the range of rates and temperatures studied, the growth is never layer by layer but rather 3D in nature, In-plane GIXD has allowed us to observe for the first time a thin film phase of T-6 formed of molecules standing normal to the substrate and arranged in a compressed unit cell with respect to the bulk, i.e. the unit cell parameters b and c are relatively smaller. We have followed the dynamics of formation of this new phase and identified the threshold of appearance of the bulk phase, which occurs above approximate to 5-6 monolayers. These results are relevant to the problem of organic thin film transistors, for which we have previously demonstrated experimentally that only the first two monolayers of T-6 films are involved in the electrical transport. The layers above the second one do not effectively contribute to charge mobility, either because they are more disordered or because of a screening of the gate field. (c) 2006 Elsevier B.V. All rights reserved.
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