Control of growth mechanism and optical properties of p-sexiphenyl thin films on ionic crystal substrates

Phenylene oligomers, which are low molecular weight materials of the pi-conjugated polymer of poly(p-phenylene), PPP, have been examined for their unique electrical and optical properties. Recently, the vacuum-deposited thin films of oligophenylenes were applied to light-emitting diodes (LED), non-linear optical (NLO) devices, etc. In this study, highly crystalline p-sexiphenyl (6P) thin films were prepared on an ionic crystal substrate (potassium chloride, KCI) by using organic molecular beam deposition technique. The crystal structure, molecular orientation and morphology of thin films prepared at different temperatures were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). It was confirmed that 6P rod-like and planar crystals having the preferential molecular orientations to KCI substrate surface were epitaxially grown. Then, preferential parallel orientations of 6P onto KCl surface formed at low temperatures of 14 degrees C and 50 degrees C and normal ones formed at high temperature of 150 degrees C. Further, it was confirmed that the optical properties of 6P thin films were strongly influenced by the molecular orientations in crystals formed onto KCl substrate surface. (C) 1999 Elsevier Science B.V. All rights reserved.