Unidirectionally Oriented Multi-Layered Carbon Nanotubes Film for Polarizer

Abstract

Nanowire-grid polarizer (WGP) that is generally used for a linearly polarized UV light that required for photo-alignment requires to have a very short pitch of about 100 nm to make, which is highly cost process due to nanopattern of metals. Therefore, we propose multi-layered CNT (Carbon nanotube) sheet that can replace the conventional WGPs. In this regard, unidirectionally oriented CNT bundles are proposed which shows a strong optical activity and anisotropic behavior with visible and ultraviolet wavelengths. In addition, a film prepared by oriented CNT bundles shows a high transparency at shorter wavelengths of 380 nm and even less. Furthermore, oriented CNT film exhibits 75-80% of DOP in the region of 350nm ~ 800 nm, especially ~91% in ultraviolet region, and provides wide absorption spectrum due to the π -plasmon of graphite. Moreover, the DOP and optical activity of aligned SWCNT can be further enhanced by improving the degree of alignment of CNT by stretching the polyvinyl alcohol substrate. We prepare a multi-layer CNT polarizer by depositing CNT layers one over another on a Silicon substrate by chemical vapor deposition technique. And then a bundle of CNTs are unidirectionally stretched to make the CNT sheet by using a mechanical rotation tool. The long axis of CNT rods in each sheet is orient parallel to the stretching direction. Then the stacked CNT sheet is transferred to a glass substrate as indicated in Fig 1. (a).The final multi-layered CNT polarizer consists of 3, 5, 8 and 12 CNT sheets stacked together with matching orientations. The relative transmittance of CNT polarizer within visible region is decreasing with increasing number of CNT layers. The incident light with its electric vector parallel (perpendicular) to the stretched CNT axis is effectively absorbed (transmitted) by the multi-layered CNT polarizers. Wavelength-dependent transmittance is showing a good correlation with optical microscopic results, as shown in Fig 1. (b). The transmittance of CNT polarizer at 365 nm is measured to be >10%, 6.5%, 1.0% and 0.5% for 3, 5, 8 and 12 CNT layers, respectively, whereas the iodine polarizer is exhibiting almost zero transmittance. Moreover, the proposed CNT polarizer is capable to extend the absorption of incident light up to deep UV region providing a wider operating wavelength range from 300 nm to400 nm and all multi-layered CNT polarizers exhibit high stability against long time UV exposure. To measure the multi-layer CNT polarizer’s performance, we have employed fringe field switching (FFS) cell with 3.4 μm with film spacers (passivation layer = 0.5 µm / gap between pixel electrodes = 4.5 µm / pixel electrode width = 3.5 µm). As shown in Fig. 1(c), obtained multi-layer CNT polarizer exhibits higher DOP of 91% by matching orientation of stacked CNTs in each layer. Inserted left image in the graph is the FFS cells exposed with 3 CNT layers. It exhibits slight nonuniformity in brightness of low grey scales called mura at 3 V, whereas the cells exposed with 12 CNT layers (right insert image of Fig. 1 (c)) do not show such mura. The proposed CNT polarizer film is expected to be applied as photomask with low-cost. we believe it has strong potentiality to utilize in future photonic and electro-optic devices with broad band gap from UV to infrared. Figure 1