Discotic columns DOI: 10.1002/smll.200700939 Nanocontrolled Bending of Discotic Columns by Spiral Networks** Pierre-Olivier Mouthuy, Sorin Melinte, Yves Henri Geerts, Bernard Nysten, and Alain Marie Jonas* Controlling the orientation of supramolecular assemblies such as polymer chains and liquid-crystalline phases is crucial as it allows the full exploitation of their electro-optic properties for use in various emergent photonic devices [1] and semiconduct- ing structures. [2] This is particularly true for one-dimensional anisotropic semiconductors like columnar mesophases, since the transport occurs only along the stacks of their p-conjugated molecular units. [3] As each column can be assimilated to an individual nanowire, [4] both integration and design possibi- lities of devices should benefit from a nanoscale method enabling not only precise alignment and confinement, but also control of the deformation of columns. However, the rigidity of the columns will naturally limit the available range of imposable deformations. While several techniques exist for determining the bending modulus of inorganic semiconducting nanowires, [5] there is no comparable work on columns of discotics owing to their softness. Up to now, the mechanical constants of the columns were thus derived from calculations [6] or macroscopic experiments, principally by the transmission of strains through the bulk material. [7] The difficulty actually lies in finding a technique able to bend a limited number of columns with a measurable force. This challenging task requires that first the columns are confined, and then that the local fields are used to align them. Magnetic fields, [8] dip-coating, [9] and epitaxial layers [10] are global methods for thin-film alignment. Only chemical nanografting or nanopatterning, [11] and exposure to infrared polarized laser [12] were reported to successfully control the local orientation of columns but the molecules have still to be confined. On the other hand, confinement was realized in the pores of an alumina matrix [13] and should be possible by nanoimprintating [14] but these two techniques do not allow the estimation of local aligning forces. Here, we exploit the fact that columnar mesophases exhibit anisotropic interfacial tension to attain nanocontrolled bending and confinement. By simply inserting discotics in a spiral network of nanogrooves, the anisotropic interfacial tension dictates the columns to follow a tangential alignment, thereby giving rise to confined columns of both controlled orientation and curvature. In addition to this technological achievement, the increase of bending energy for decreasing curvature radius of the columns directly leads us toward a method for estimating the bending modulus of the columns. Our alignment technique can be easily extended to any shape of nanopatterns, provided that they are carefully designed. The molecule used in this study is the 3-nm-diameter tetrasubstituted discotic phthalocyanine (Pc) shown in Figure 1a. [15] This compound showed interesting semiconduct- ing properties in previous theoretical [3] and experimental [4] studies. In nanogrooves with submicrometer depth and width dimensions, Pc columns align perpendicularly to the sidewalls (Figure 1b). For crisscrossed nanogrooves, they adopt a uniform alignment imposed by the longest nanogroove (Figure 1c). [16] This simply results from the minimization of surface energy since interfacial tensions are anisotropic: g // Pc/wall > g ? Pc/wall . Here, g // Pc/wall and g ? Pc/wall stand for the interfacial tensions between mesophase and nanogroove walls when columns are parallel and perpendicular to the interface, respectively. In the following, the longer nano- groove will be called the orienter, while the shorter nanogroove parallel to columns axes will be called the follower. The key role of orienters is to locally align the columns in the direction perpendicular to their sidewalls. Therefore, by fabricating a network of crisscrossed nano- grooves, it becomes possible to guide columns over long distances, with one set of nanogrooves (orienters) controlling the orientation and the other set of perpendicular nanogrooves (followers) confining the columns. From the various nano- patterns, a developed spiral of followers should lead to a practical way to highlight, in one step, the ability to locally align discotic columns following different curvatures. It is known that columnar systems spontaneously form develop- able domains; [17] hence, a controlled alignment respecting this tendency should thus be theoretically possible. The orienters must thus be radial, and the resulting network should look as shown in Figure 1d. For design simplification however, the spiral as decomposed into consecutive concentric arcs with increasing radius from the center. The lowest radius was fixed at 400 nm, with increments of 200 nm at each 90 8-angle step. In a given quadrant, the spacing between two consecutive followers is thus L o ¼ 800 nm. Unlike follower arcs, spacing between radial orienters is not constant and care must be taken when designing them. The minimum possible spacing between two orienters is L f,min ¼ 300 nm; this spacing ranges from L f,min to L f,max ¼ 600 nm. When the spacing reaches L f,max from the center, a new orienter is thus inserted. Both orienters and followers have the same width W. If columns are oriented in accordance with our expectations, the elementary cell of such a network should look like in Figure 1e, with DF the angular communications [  ] P.-O. Mouthuy, Dr. S. Melinte, Prof. B. Nysten, Prof. A. Jonas CeRMIN, Universite ´ Catholique de Louvain Louvain-la-Neuve, B-1348 (Belgium) Fax: (þ32) 10-451-593 E-mail: jonas@poly.ucl-ac.be Prof. Y. H. Geerts Laboratoire de Chimie des Polyme `res, CP206/1 Universite ´ Libre de Bruxelles Boulevard du triomphe, 1050 Bruxelles (Belgium) [  ] We thank D. Serban, A. Vlad, S. Faniel, L. Gence and B. Hackens for suggestions and discussions. We are grateful to the WINFAB technical team, P. Lipnik, P. Viville, P. Damman for help with experiments. This work was partially supported by the ARC-Dynanomove, ARC-Nanomol, RW Nanotic-Feeling and Cite, IUAPS FS 2 , and FRFC. P.-O. Mouthuy is a research fellow of the Belgian F.R.S.-FNRS. 728 ß 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2008, 4, No. 6, 728–732