Organic molecular nanowires: -dimethylperylene-3,4,9,10-bis(dicarboximide) on KBr(001)
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Two images of DiMe-PTCDI wires on KBr(001) (Ref. 17). During evaporation, the sample was kept at room temperature. Between the wires, atomic resolution of KBr was possible (see inset); there seems to be no wetting layer of molecules. (a) Low coverage, 0.1 ML of molecules were evaporated onto the sample, forming molecular wires. The size of the image is . The wire seems to be pinned between two KBr step edges, following the direction of the substrate; it is nearly long and less than wide. Its height is two molecular layers. (b) Higher coverage (1 ML), the size of the image is . The wires are higher and wider than at low coverage; their length is often limited by KBr step edges. As most of the steps follow the direction of the KBr substrate, the wires tend to grow perpendicular to these steps, i.e., along the  direction. Only on the large terrace in the upper right part of the image, a long wire grows in the direction. In the upper right corner, one can see a large crystallite of molecules.
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(Color online) Image shows molecular wires of DiMe-PTCDI; the size of the image is (Ref. 17). In contrast to Fig. 1, the sample was cooled down to a temperature of during evaporation of the molecules. On the cold sample, the wires are only one molecular layer high in most cases. They still tend to grow perpendicular to the step edges. One can also see that in regions where no direction is preferred due to step edges, both directions of wire growth are equally distributed. This is, e.g., the case in the lower left part of the image where the terrace is quite large and in the rectangular KBr pit on the right. Here, the edges of the pit follow the  and  directions, so neither the  nor the direction is preferred for wire growth. The figure on the lower left shows the minimum energy position of a single DiMe-PTCDI molecule on KBr(001) as found in a calculation based on the AMBER molecular force field (Ref. 18) and electrostatic interaction. The position is intuitive, as the dominating charge on the molecule is situated on the oxygen atoms due to their electronegativity; the distance between the oxygens matches the spacing between two neighboring ions in the KBr lattice. Based on this calculation and on the clearly favored  growth direction of the wires, we propose a superstructure as shown in the lower right image.
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Series of images shows molecular wires of DiMe-PTCDI on a very flat sample (Ref. 17). The size of each image is . The width of the KBr terraces is larger than ; within the images, no step edges can be seen. The images have been taken shortly after the deposition of 0.3 ML of molecules; the scanning of each image took . The distortions of the upper part of image 1 are due to piezocreep shortly after approaching the sample. One can follow the development of the molecular wire network. In image 2, a closed rectangle is formed out of the wires in image 1. Some other wires are attached to this rectangle; a small molecular island is trapped inside. In the third image, one of the attached wires has vanished, a second one in image 4. The trapped island changes its size and position throughout the images. Note that the wires are quite fuzzy in the whole series; This also indicates the high mobility of the molecules.
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