Evolution of epitaxial order in para-sexiphenyl on KCl(100)

This work focuses on the structural and morphological studies of the evolution of epitaxially grown para-sexiphenyl (p-6P) thin films on freshly cleaved KCl(100) surfaces. The film growth is varied using growth times between 5sec and 60min and substrate temperatures in the range of 80–150°C. The study is based on a combination of X-ray diffraction to derive structural information on the molecular scale, and atomic force as well as electron microscopy to obtain morphological information. The initial growth orientation of p-6P crystallites is identified to be (203¯)-oriented, which consists of edge-on molecules aligned parallel to 〈011〉KCl forming needles that are also parallel to these directions. Furthermore it is shown that surface step edges do not affect the needle growth, but those needles even overgrow step edges. The subsequent epitaxial growth turned out to be crucially dependent on the substrate temperature with respect to the quantitative ratios of material in different growth orientations, their in-plane alignment, the size of crystallites and the mosaicity of the films. It is shown that some crystallites start their growth using the initially grown needles as seeds, resulting in a different crystal orientation. For such crystallites a realignment of needles during their growth is identified, which is found to be strongly dependent on the growth temperature. Furthermore the heights of needles show distinct temperature dependence. In addition well-aligned terraced islands formed by standing molecules were observed. Finally a growth model for this system can be elucidated.