Anisotropic light transmission of aligned carbon nanotube sheets coated substrates

Abstract

Abstract-We have performed optical investigations of layersformed by sheets of aligned multi-wall carbon nanotubes (MWCNTs). Nanotubes have extraordinary mechanical, electrical and optical properties when they are unidirectionally oriented. Free-standing CNT sheets drawn mechanically from vertically-grown MWCNT forests can be deposited on glass substrates, with nanotube bundles mostly oriented in the drawing direction. These CNT sheet layers have optical anisotropy, which can be detectable even for a single CNT sheet. We have studied the optical anisotropy in transmission, finding a similar value at different length scales indicating similarities in the orientational order of the nanotubes, thus preservation of their average degree of order. Finally, light transmission was observed when the samples were at 45 deg between crossed polarizers.The great interest in carbon nanotubes (CNTs) triggered by their discovery by Iijima [1] has led to significant scientific, engineering, and medical research for revealing the properties of these materials and ways to apply them. Carbon nanotubes are long cylinders from perfect honeycomb lattice walls of covalently bonded carbon atoms. Multi-wall carbon nanotubes (MWNTs), consist of two or more concentric cylindrical shells of graphene sheets coaxially arranged around a central hollow core. CNTs have extraordinary electrical and mechanical properties. MWNTs and single-wall CNTs are flexible and resilient [2] hollow tubular structures. The tubes that we have used here have diameters of about 20nm and lengths of 200-300 micrometers [3]. The extraordinarily high anisotropy is not limited to the shape of the tubes but it is also in their properties and, in particular, in the absorption of light.Due to their highly anisotropic optical characteristics, sheets formed by aligned CNTs are promising as linear optical polarizers. Lei Ren et al. reported a highly aligned SWCNT film of a thickness of 2 micrometers, deposited on a sapphire substrate, transparent to terahertz (THz) radiation polarized perpendicular to the nanotube axis with an extinction ratio of about 10dB [4]. Later, Kyoung et al., by winding MWCNT sheets on a U-shaped polyethylene frame, obtained free-standing CNT layers of 9 micrometer thickness with an extinction ratio ~37dB in a spectral region from 0.1-2THz [5].Herein we explore the optical properties of highly aligned MWNT sheets on glass substrate in the visible wavelength range finding a clear anisotropy in the optical transmission as an effect of the CNT alignment.Since, CNT sheets are attractive transparent electrodes due to their electrical conductivity and optical transparency, the study of their anisotropic properties and the dependence on the structural organization is of great importance for developing optical devices such as liquid crystal displays. In particular, the quality of the alignment of CNT strands is expected to be relevant for the properties of the sheets since it determines the anisotropy in the characteristics and, as consequence, the performan...

Alignment of Nematic Liquid Crystal by Carbon Nanotube Sheets on Substrates with Hexamethyldisilazane and Silicon Dioxide Co‐Adjuvant Layers

“…More detailed information on the degree of alignment of the CNTs in sheets can be derived by their optical absorption since the alignment of CNTs in the sheets results in macroscopic anisotropy in the light transmission . One single layer of our CNT sheets, deposited on glass substrate, can transmit about 80–82% of light with polarization perpendicular to the CNT alignment axis and about 64–68% of light with parallel polarization, measured avaraging over an area of about 0.5 cm 2 .…”

“…One single layer of our CNT sheets, deposited on glass substrate, can transmit about 80–82% of light with polarization perpendicular to the CNT alignment axis and about 64–68% of light with parallel polarization, measured avaraging over an area of about 0.5 cm 2 . Similar values can be found when measuring over an area of about 0.38 mm 2 , indicating no substantial variations between average value from almost overall CNT sheet area and more local measurements on areas looking with fairly good alignment . Variations of the degree of alignment of the tubes on a smaller scale and on areas with better and worse alignment are clearly prsent.…”