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SUMMARY:The Role of Molecular Orientation in the Growth and Electronic Str
 ucture of Device Relevant Organic Films - Stephen Berkebile\, Institute of
  Physics\, Karl-Franzens-University Graz\, Austria
DTSTART:20060519T081500Z
DTEND:20060519T091500Z
UID:TALK4844@talks.cam.ac.uk
CONTACT:Dr Vittorio Fiorin
DESCRIPTION:In organic devices\, the morphology and the molecular/crystal 
 orientation of the active molecular layer are key determinants for their (
 opto-)electronic performance. For instance\, charge transport will strongl
 y depend not only on the molecular orientation due to the highly anisotrop
 ic charge carrier mobility in organic crystals\, but also on the film morp
 hology via grain boundaries.\nThe basis of controlled investigations of th
 e inorganic-on-organic and organic-on-organic interfaces are epitaxial org
 anic films with different unique orientations. Various measurements of hig
 hly oriented films of para-sexiphenyl (6P) and a-sexithiophene (6T) are pr
 esented here. The importance of the substrate and the balance between stic
 king and diffusion anisotropy on their growth is shown. Several UHV techni
 ques have been applied to characterise the films and their surfaces. In va
 lence band photoemission two distinct ionisation potentials are observed t
 hat are associated with the 6T and 6P having either planar or\, unexpected
  for the solid state\, twisted conformations. The former are shown to be a
 ssociated with films with near perpendicular orientation (6P(001) and 6T(0
 01)) while the latter are invariably observed when the molecules are orien
 ted parallel to the substrate surface (whether in lying wetting monolayers
  or in thick 6T(010)\, 6P(203)\, 6P(213) films). Our present hypothesis is
  that the twisted molecules are a result of a form of reconstruction on th
 e higher surface energy faces of organic crystals. Preliminary deliberatio
 ns to independently confirm this involving comparison of experimental and 
 calculated NEXAFS as a function of oligomer length and twist angle are pre
 sented. Calculations show that localisation arising from the core hole str
 ongly affects the NEXAFS spectra. We are unable to distinguish between dif
 ferent conformations using NEXAFS.\n
LOCATION:U203\, Chemistry\, Department of
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