Visualization of ordering phenomena in di‐fluorinated sexiphenyl by
HRTEM
Authors: Kirmse Holm , Sparenberg Mino , Sadofev Sergey , Blumstengel Sylke , Koch Christoph T 
Published: December 2016

Asymmetrically, di‐fluorinated para‐sexiphenyl (6P‐F2) can be used as a model system of a polar conjugated molecule for the growth on an inorganic semiconductor forming a hybrid inorganic/organic system (HIOS) for opto‐electronic applications. The two fluorine atoms are positioned terminally at only one end of the 6P molecule. This asymmetry triggers a dipole moment causing strong interaction with the electric field of the polar crystal structure of ZnO serving as inorganic substrate. The molecule/substrate interaction is expected to be much stronger than that for pure 6P or for symmetrically fluorinated 6P‐F4 [1].

Based on the findings for non‐fluorinated 6P which was found to form a well ordered crystalline structure when grown on ZnO(10‐10) [2], structure models of the 6P‐F2/ZnO interface are created to predict the high‐resolution TEM image contrast. In a first approach, only potential 6P‐F2 bulk structures derived from the β‐6P structure were considered. 6P molecules were replaced by 6P‐F2 molecules in three different ways (see structure models of Fig. 1a) to c) (left column)). In Fig. 1a) a tip‐to‐end ordering is considered where the fluorinated tip of the molecule points always in the same direction possibly towards the substrate surface. In Fig. 1b) an alternating ordering is realized. Only every second molecule points downwards the others point upwards. The alternating arrangement is realized for two directions being the horizontal direction and the direction normal to the projection plane. The third type of ordering is a tip‐to‐tip and end‐to‐end one (see Fig. 1c). The difference between these 3 types is highlighted by the green arrow markers symbolizing the fluorinated tips. For comparison, the crystal structure of pure 6P is seen in Fig. 1d).

HRTEM image contrast simulations were performed with the JEMS software package [3]. The results are given in the 3
rd
and 4
th
column of Fig. 1. According to the findings published in [2] a large defocus df up to 1000 nm underfocus was applied for gaining high phase contrast for the low spatial frequency details of the 6P‐F2 structure. Moreover, the sample thickness t was varied up to 100 nm in steps of 25 nm. Here the results for t = 100 nm are given.

A clear indication for ordering is only seen in Fig. 1c viz. for the tip‐to‐tip / end‐to‐end alignment. The other types (Fig. 1a and b) show fringes almost identical to those of pure 6P (Fig. 1d). Also, for df = ‐1000 nm the image contrast seems directly interpretable in terms of basic structural features. The dark lines trace the position of the center of mass of the next neighbor 6P‐F2 or 6P molecules.

Having a more careful look now at Fig. 1c where a clear indication for ordering is seen. The contrast transfer for tip‐to‐tip / end‐to‐end alignment simulated for df = ‐500 nm promotes two dark fringes for the end‐to‐end position of the molecules (see yellow arrows in Fig. 1c) whereas the fluorinated tips do not exhibit any fringes. For df = ‐1000 nm a faint fringe appears at the tip‐to‐tip position (cf. red arrows) while the distance between the two dark fringes increases.

Conclusively, only the tip‐to‐tip / end‐to‐end alignment of 6P‐F2 can be clearly identified by HRTEM imaging at large underfocus and the defocus condition strongly influences the visibility of structural details and interpretability of the HRTEM image contrast.

With respect to HIOS applications, the arrangement of 6P‐F2 on either the polar (0001) or the nonpolar (10‐10)ZnO surface will be studied in further detail. A potential influence of the polar/nonpolar substrate surface on the arrangement of the polar 6P‐F2 molecules will be considered.