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-con jugated polymer of poly(p-phenylene), PPP, have been examined for their uni que electrical and optical properties. Recently, the vacuum-deposited thin films of oligophenylenes were applied to light-emitting diodes (LED), non-l inear optical (NLO) devices, etc. In this study, highly crystalline p-sexip henyl (6P) thin films were prepared on an ionic crystal substrate (potassiu m chloride, KCI) by using organic molecular beam deposition technique. The crystal structure, molecular orientation and morphology of thin films prepa red at different temperatures were characterized by transmission electron m icroscopy (TEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). It was confirmed that 6P rod-like and planar crystals having the preferent ial 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 opti cal properties of 6P thin films were strongly influenced by the molecular o rientations in crystals formed onto KCl substrate surface. (C) 1999 Elsevie r Science B.V. All rights reserved.