Interactions of carbon nanotubes in a nematic liquid crystal. II. Experiment

Multiwall carbon nanotube (CNT) colloids with different anchoring conditions are dispersed in pentyl-cyanobiphenyl (5CB), a thermotropic liquid crystal (LC) that exhibits a room-temperature nematic phase. The experiments make use of CNTs treated for strong planar, homeotropic, or Janus anchorings. Observations with a polarizing microscope show that the CNTs placed in a uniform nematic field stabilize parallel or perpendicular to $\mathit{n}$ depending on their anchoring conditions. In the presence of a splay-bend disclination line, they are first attracted toward it and ultimately, they get trapped on it. Their orientation relative to the line is then found to be parallel or perpendicular to it, again depending on the anchoring conditions. When a sufficient number of particles are deposited on a disclination line, they form a micro- or nanonecklace in the shape of a thin thread or of a bottle brush, with the CNTs being oriented parallel or perpendicular to the disclination line according to the anchoring treatment. The system exhibits a rich versatility, even if until now the weak anchorings appear to be difficult to control. In a next step, the necklaces may be glued by means of pyrrole electropolymerization. In this manner, we realize a true materialization of the disclination lines, and we obtain nanowires capable of conducting the electricity in the place of the initial disclinations that just worked as templates. The advantage of the method is that it finally provides nanowires that are automatically connected to predesignated three-dimensional (3D) electrodes. Such a 3D nanowiring could have important applications, as it could allow one to develop electronic circuits in the third dimension. They could thus help with increasing the transistor density per surface unit, although downsizing of integrated circuits will soon be limited to atomic sizes or so. In other words, the predicted limitation to Moore's law could be avoided. For the moment, the nanowires that we obtain are not completely satisfactory, particularly because they are thickened by aggregates in some places. However, the method is far from being optimized. A few electric charges deposited on the CNTs could remedy those aggregates.

Figure 1

Splay-bend disclination line joining predesigned spots on opposite glass plates. CNTs with planar anchoring begin to condense perpendicularly on a disclination, forming an object in the shape of a bottle brush. The different gray colors correspond to the different rubbing directions that are necessary on the two plates of a sample to force a splay-bend disclination line in it (see Ref. [32]). The spacing distance between the two glass plates is about 140 μm. They are shifted by about the same distance in order to make the line more easily visible. The focus is on the CNTs about one-third of the distance from the upper plate.

Figure 2

CNT necklace in the isotropic phase projected on a glass substrate and stabilized by CNT-substrate nematic bridges.

Figure 3

SEM image of Janus CNT having one side coated with gold.

Figure 4

AFM image of a polypyrrole bridge between two CNTs after the nanowire has been extracted from the sample, washed with alcohol, and dried. The bar is 3 μm long. The maximum height of the object is 1 μm from the substrate.