ELSEVIER Synthetic M&ids 84 (1997) 605-506 Substrate Dependent Orientation and Structure of Sexithiophene Thin Films P. Lang?, M. El Ardhaouil , J.C. Wittmann2, J.P. Dallas3, G. Horowitzl, B. Lot.z2, F. Garnierl, C. Straupe2 1:Laboratoir.e des Mat&faux Mol~culaires; CNRS 2 rue H.Dunant 94320 THIAlS, France 2 :Institut Charles Sadron, ,6 rue Boussingault, 67083 STRASBOURG, France 3: C.E.C.M, 15 rue Georges Urbain 94400 VITRY, France Abstract The use of different kinds of substrates ( Si02, SiH/Si, HOPG) allows to induce orientation and crystalline organization of sexithiophene organic films. This substrate behavior IS related to its ability to develop interactions with the oligothiophene .Electron diffraction pattern and TEM analysis demonstrate the existence of an epitaxial relationship between the film organization and the substrate ( HOPG, SiH) . Keywords: polythiophene and derivatives, organic /inorganic interfaces, orientation Vacuum evaporated oligothiophenes lead to crystallized semiconducting films which can be used in FET, LED... The optical and charge transport properties are strongly dependent on the mesoscopic organization and molecular orientation of these films, which can be varied through the temperature of substrate, the chemical structure of the molecule and finally the surface during evaporation state of the substrate [1,2] . In this paper, we discuss the influence of the substrate on the orientation and on the cristalline organization of the organic films. We analyze the film properties deposited on several kinds of substrates, that interact in different ways with the oligothiophene molecule. Deposition conditions, surface preparation of Si02 and SiH/Si(lll) and spectroscopic techniques used have already been described [2]. The X-ray diffraction was recorded at a grazing angle of 0.5” or 0.25” with a Co cathode bl.79OG . I Comparison of SiOT and SiH/Si(lll) substrates Fig.1 shows the s-polarized infrared ATR spectra in the C=C stretching region of 6T/ Si02 and 6T / SiH/Si(lll) (thickness: e = 20nm). The band at 1490cm-1 is L-polarized (L is the long axis of the molecule) , whereas the other ones at 1426 and 1440 cm-l are M-polarized, i.e perpendicular to L and in the molecular plane . The ratio 1 of the absorbances of these L and M polarized bands Abs (L)/Abs(M) is larger for SiH/Si(lll) substrate than both for SiO2 substrate and for 6T in KBr pellet. This indicates that 6T molecules on SiH/Si substrate are preferentially oriented with L axis parallel to the substrate . The proportion of these molecules is estimated as shown in ref. [3], from the ratio c estimated both for the 6T film and for the 6T in KBr pellet. Calculations show that about 85% of molecules have their L axis parallel to the substrate surface . In the case of SiO2 substrate, the band at 1490 cm-* is very weak, indicating that the molecules are mainly oriented normal to the surface: (= 95% ). This is also the case for common substrates that interact little with 6T such as glass, saphire, quartz... [1,2,]. The grazing reflected X-rays diffraction pattern confirms (Fig. 2) these conclusions . When using SiO2 , the peaks (hO0) (where a,Lc are the unit cell dimension, a is the long dimension) at d=25.18A and d=12.60& 03796779/97/$17.00 Q 1997 Elsetier Sqience S.A. AlI rights reserved PII SO379-6779(96)04072-6 io 10350 1400 1450 1500 1550 wavenumbers/cm-1 Fi .l: ATR s-polarized spectra of 6TlSiW SI f 111) (-0-j and CT/SiOz C-o-1; (- - 4 GT/KBr -5 10 15 e2yo, 25 30 35 2 Fig.2: Grazing X-ray diffraction curve (i=O.25’) for GT/SiWSi(lll) : (-o- ) and 6T/SiO2( - )