Carbon nanotubes possess unique electronic properties that are very useful for building electro-optical devices at nanometer scales. In optoelectronic applications, a large number of carbon nanotubes will be assembled in a desired form.

Single-walled carbon nanotubes (SWCNTs) are self-assembled in a triangular lattice in bundles, strands, or mats. The photonic devices based on carbon nanotubes can take advantage of the strong anisotropy of optical properties of SWCNTs under polarized light. This can be achieved by aligning the SWCNTs in a certain direction to form carbon nanotube mats. In this work, the aim was to study the optical properties of aligned carbon nanotube mats (CNTMs) using ab initio density functional calculations. The computational method of the linear combination of atomic orbital (LCAO) formalism was utilized. The optical properties of various CNTMs that were constructed from semiconducting SWCNTs were calculated. As a further pursuit of the applications of nanodevices, it is demonstrated that the aligned carbon nanotube mats will have relevant properties for photonic applications.

The carbon nanotube mat was constructed utilizing SWCNT (16, 0) as the basic building block. The diameter of individual SWCNT (16, 0) is 1.25 nm. The electronic structure of individual SWCNT (16, 0) was calculated. There are 64 atoms in the tube unit cell of SWCNT (16, 0). The large number of atoms per unit cell that enter into the ab initio calculations presents some technical challenges. A real space approach of LCAO calculations was utilized.

The physical factors for the strong anisotropy in the optical properties of the aligned carbon nanotube mat were studied further. Particularly, a strong anisotropy is observed in the first peaks of the imaginary parts of the dielectric function when the polarization of the light is parallel or perpendicular to the tube axis. It is noted that the first absorption peak in E2, in the aligned CNTM, is associated with the van Hove singularities that are located just above and below the Fermi level. A joint density of states (JDOS) was calculated that is based on the same calculation formula as that of the computation of E2, but without the inclusion of the electron excitation matrix elements in the calculation.

The optical properties of the aligned carbon nanotube (16, 0), (10, 0), and (8, 4) mats were studied for photonic device applications. Ab initio density functional potentials were studied, and the linear combination of atomic orbital formalism was utilized. The electronic structure of the carbon nanotube mats and the real and imaginary parts of the dielectric functions as functions of the photon energy were calculated. The calculated dielectric functions of the aligned carbon nanotube mats show a strong anisotropy when the electric field of the light is parallel or perpendicular to the tube axes. Especially, there are strong peaks in the imaginary part of the dielectric function near the absorption edges, when the electric field of the light is parallel to the carbon nanotube axes. The unusual optical properties of the semiconducting carbon nanotube mats present an opportunity for applications in electro-optical devices in the infrared energy region.

This work was done by G. L. Zhao and D. Bagayoko of Southern University and A & M College, and L. Yang of NASA Ames Research Center for the Office of Naval Research. ONR-0023


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Optical properties of aligned carbon nanotube mats for photonic applications

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This article first appeared in the June, 2011 issue of Defense Tech Briefs Magazine.

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