Research at CMMPEDevices & applications — Hybrid LC carbon nanotube devices

 

Multiwall carbon nanotubes (MWCNTs) are normally grown as tangled masses by laser ablation or arc discharge. However, using plasma enhanced chemical vapour deposition (PECVD) techniques it is possible to grow dense aligned mats of ‘grass-like’ MWCNTs as well as individual nanotubes in sparse arrays through the use of ebeam patterning of the catalyst. The fact that they exhibit very high conductivity and aspect ratio means that we can use them as electron sources, as has been demonstrated in field emission displays, and as microwave sources. Conducting MWCNTs can also be used as electrode structures in optically anisotropic media such as liquid crystals, as potential alignment layers and making novel micro-optical components possible.

Figure 2. SEM of the patterned MWCNT electrodes.

Their ability to appear as large (with respect to the size of the liquid crystal molecules) structures within a liquid crystal device means that there is a strong interaction between the nanotubes and the liquid crystal material. This interaction can then be interpreted as an optical interaction through the optical anisotropy of the liquid crystal. Hence nano-structures can be used to form defect centres in liquid crystal materials which can then be manipulated by applying an external electric field. On the other hand, considering the fact that the diameter of MWNT is from tens of nanometres to a hundred nanometres, the interaction between nanotubes and liquid crystal is restricted to the micron scale, which is much smaller compared than with current liquid crystal devices.

Current research is into understanding how these devices actually work and what their potential applications are.

• How does the LC align to the MWCNTs?
• What is the exact refractive index profile?
• What is the optimal device configuration?
• What are the suitable applications of these devices?

Further reading:

Additional videos of hybrid liquid crytstal carbon nanotube devices.

Sparse multiwall carbon nanotube electrode arrays for liquid-crystal photonic devices
T.D. Wilkinson, X. Wang, K.B.K. Teo, W.I. Milne
Advanced Materials, 20(2), 363-366, (2008)

Characterization of carbon nanotube-thermotropic nematic liquid crystal composites
O. Trushkevych, N. Collings, T. Hasan, et al.
J. Phys. D: Appl. Phys. 41, 125106, (2008)

Patterned multiwall carbon nanotube electrode arrays for liquid crystal photonic devices
T.D. Wilkinson, X. Wang, K. Teo, W.I. Milne
Proc. SPIE, 6988, 69881E-1, (2008)

Figure 1. Simulation of the electric field surrounding a single MWCNT in vacuum.

We have recently demonstrated an electrically switchable micro-optical component based on a sparse array of MWCNTs grown on a silicon surface, forming half of a liquid crystal cell. The nanotubes act as individual electrode sites, which spawn an electric field profile, dictating the refractive index profile of the liquid crystal cell. The refractive index profile then acts as a series of graded index profiles which form a simple lens structure. By changing the electric field applied it is possible to tune the properties of this graded index structure and hence the optical structure.

Figure 3. Microscope figure showing the LC-MWCNT array switching up to 2.2V/m.

Figure 4. LC-MWCNT array dynamic switching.

 

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