We have studied the structure and epitaxial properties of films of the aromatic molecules p-sexiphenyl and POPOP grown on atomically well-ordered KCl(001) substrates. These electroluminescent molecules have potential applications in optoelectronic devices such as organic light-emitting diodes.

Samples of p-sexiphenyl and POPOP on KCl(001) were grown at Kobe University by vapour deposition in vacuum. Figure 44 shows a fluorescence micrograph of a POPOP film. The film consists of needle-like crystallites which show clearly a preferential orientation. Structure and epitaxial properties of these samples were characterised on the ID10B 2+2 diffractometer using grazing-incidence diffraction. Grazing-incidence geometry proved to be crucial for the study of these films, in order to limit radiation damage.

Figure 45a shows an in-plane scan along the [110] high-symmetry direction of the KCl substrate. Three peaks labelled A*, 2A* and B*' according to their in-plane scattering vectors can be identified as due to the POPOP film. 2A* occurs at twice the q-value of A*, whereas B*' is not related to the other two reflections. At these in-plane positions, rod scans along the surface normal were performed, as shown in Figure 45b. The rod scans at A* and 2A* display split peaks with the splitting for 2A* increasing to twice the value at A*. The splitting of the A* reflections indicates an oblique crystal lattice. The two components are due to crystallites rotated by 180°. The B*' rod features single peaks and can be identified as 2b*, where b* is the reciprocal lattice vector along the special direction of the monoclinic bulk structure. The B*' rod can be identified as due to POPOP crystallites rotated by 90° with respect to the crystallites giving rise to the A* reflections.

From the spacing of the reflections along the rods we found that the POPOP crystallites grow with the (102) plane parallel to the surface and in fact all observed POPOP reflections could be indexed according to the bulk structure. The (102) planes consist of layers of molecules with the long molecular axes in the plane forming a herringbone arrangement (Figure 46). These layers are oriented parallel to the substrate with the crystallographic b axis parallel to the [110] direction of the substrate to within 0.7°. The good epitaxial alignment indicates that the film structure persists all the way to the interface with the substrate. In future studies we want to investigate the structure and growth mode of the first few layers.

Authors
D-M. Smilgies (a), N. Boudet (a), B. Struth (a), Y. Yamada (b), H. Yanagi (b).

(a) ESRF
(b) Faculty of Engineering, Kobe University (Japan)