In the framework of tight-binding theory, we investigate modulation of the electronic and optical properties of single-walled carbon nanotubes helically wrapped with single-stranded DNA. We develop a model to simulate DNA-carbon nanotube hybrids with a variety of nanotube structures and DNA wrapping geometries. We determine band structure changes due to the charged helical wrap for semiconducting and metallic nanotubes. For metallic nanotubes, DNA induces a small bandgap; for semiconducting tubes, the gap typically decreases. To quantify the robustness of the hybrid structures, we compute the polarization component of the energy of cohesion. A typical value of this energy is 0.5 eV per DNA base, evidence of the marked stability of the structures. Optical absorption calculations predict that nanotube symmetry breaking in the Coulomb potential of the ionized DNA backbone can lift optical selection rules, qualitatively changing the one-electron absorption spectrum for light polarized across the SWNT. In addition, circular dichroism is predicted for DNA wrapped single-walled carbon nanotubes, even when the nanotube itself is achiral. These changes may be used for experimental determination of the presence of DNA wrapping.
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