Observation of molecular reorientations in vapor-deposited organic thin films during heat treatment by energy-dispersive total-reflection X-ray diffractometry

Recently-developed total-reflection X-ray diffractometers were utilized to observe temperature changes in the in-plane molecular structure of n-C₃₃H₆₈ films vapor-deposited in SiO₂ glass substrates. The X-ray diffraction profiles revealed that the n-C₃₃H₆₈ molecules reorient their molecular axes from lateral to normal directions to the substrate surface during the heating process at atmospheric pressure and behave like liquid crystals in a smectic phase. Most of the molecules were observed to sublimate during the heat treatment even under vacuum below their melting point. In addition, it was found that the molecules whose (010) plane is parallel to the surface of substrate were much more stable than those of other orientations. These results may be understood by considering the imperfection in the molecular packing arrangement, the weak interaction between as-deposited molecules and the substrates, and some influence of gas molecules on the molecular behavior during a heating process.