Covalent Defects in Carbon Nanotubes—Dependence of Exciton Energy on Defect-Defect Couplings and Configurations

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) by organic molecules creates the sp³-defect at the nanotube surface. Such sp³-defect provides the required condition for single-photon emition showing its tunability from near infrared (NIR) to telecom wavelength range achievable at 300 K. This tunability can be achieved via chemically driven manipulations of molecular structures of adducts, their interactions, and positions at the nanotube. We explore the energy splitting of the lowest excitons impacted by the interaction between two sp³-defects originated from molecular adducts with different polarities and placed at various distances in the (6,5) SWCNT. Time Dependent Density Functional (TDDFT) calculations reveal that the defect-defect interactions conform to the effective model of J-aggregates for well-spaced defects (>2 nm), leading to a redshifted and optically allowed (bright) lowest energy exciton. H-aggregate behavior providing optically inactive (dark) lowest exciton is not observed for any defect orientations. This is rationalized by the fact that for defects placed in the supposed H-type orientation, the dipole-dipole interactions are superseded by vibronic and exchange couplings due to the cylindrically diffusive character of exciton densities. Importantly, our calculations show that for the majority of inter-defect positions, the lowest exciton remains optically bright, redshifted from the main E₁₁ band, and well separated from the next optically dark state. All of these features are desirable for a strong emission in the NIR region. This implies that a selection of the defect type is not as critical for improving the emission efficiency of SWCNTs and that mixing of different types of defect pairs might be a promising direction for controlling efficient single-photon emission.