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Self-organized liquid crystals for carbon nanotube alignment

Final Activity Report Summary - SOLCANTA (Self-organized liquid crystals for carbon nanotube alignment)

Carbon atoms arranged in a honey-comb lattice forming a sheet can roll up in nanometrically thin but very long (compared to the diameter) cylinders, referred to as carbon nanotubes (CNTs). Their mechanical, electrical and thermal properties are exceptional but for many uses a large-scale organization of the otherwise disordered systems of tubes is required. The project has explored the use of liquid crystals (LCs) to impose such an organization via a transfer of order between a host (LC) and guests (CNTs). Liquid crystals are formed by molecules that tend to assume a common long-range orientation yet keeping a fluid nature. Many different systems can show liquid crystalline phases: some do it as pure compounds (these are used in displays), others need a solvent, like some viruses or DNA.

The collective uniform orientation of liquid crystal molecules is used to bring CNTs along, in the same direction. While a few earlier experiments indirectly have suggested that such order transfer from LC to CNTs works, a central part of the present project has been the development of a method based on polarised Raman spectroscopy, capable of providing an unambiguous proof of the LC-induced CNT alignment. As a general most important achievement, I have employed this method to demonstrate the feasibility of using LCs of several different kinds for dispersing and aligning unsupported CNTs, also of a variety of types, even allowing a dynamic response in some cases.

The proof of the alignment has been obtained in a very clear, direct way using the Raman spectra of nanotubes, a sort of fingerprint of their presence, with the employment of the polarisation of light that provides directional sensitivity. Different types of nanotubes have been used and their effect on the composites studied in terms of dispersion and effect on the properties of the liquid crystal. Using glass cells with transparent conductive electrodes it is possible to monitor the CNT orientation while imposing with an electric field a change of orientation of the liquid crystal molecules, from an alignment parallel to the substrates to a vertical orientation, parallel to the applied field. We have demonstrated in this work that the orientation of CNTs (if well dispersed) follows that of the LC throughout the switching process, opening for a dynamic control of the nanotube orientation.

The structure of the LC molecules has an important contribution to the formation of the LC-CNT composite. As part of this project, we showed that the presence of aromatic rings in the structure is responsible for interaction between the LC molecules and the carbon nanotubes. This was demonstrated investigating both components of the composite: observing a shift in the peak position of the Raman modes compared to pristine nanotubes and on the other side, for the liquid crystals an increase of local order due to the nanotubes.

The organising properties are not related to a unique LC type but we demonstrated that both single component LCs and surfactants forming liquid crystalline phases in water can align carbon nanotubes. In particular, two different types of surfactant phases have been used and for both it was possible to demonstrate an alignment transfer onto the CNTs. With a tailored combination of surfactants it was possible to have very high nanotube concentration while keeping the transfer of alignment, resulting in an effect of the CNT alignment that became macroscopic and easily detectable by eye when observing the sample with polarised light.