Optically active defects in carbon nanotubes via chlorination: computational insights

Optical and electronic properties resulting from chlorine functionalization of (6,2) single-walled carbon nanotubes (SWCNTs) are explored using density functional theory. P-type doping character is evident from the generation of an unoccupied mid-gap state originating from the sp3-hybridized defect brought by neutral chlorine adducts to the sp2-hybridized lattice of the SWCNT. Such mid-gap states are realized in all binding configurations of chlorine atoms explored in this work, with the energy of the mid-gap state varying depending on the chlorine positions at the nanotube surface. A chlorine pair placed on the same hexagonal SWCNT ring shows the strongest binding to the tube sidewall while reducing the intensity of the main optical E11 band and brings new redshifted, highly optically active transitions. However, the optical intensity of the defect-introduced transitions significantly diminishes with increased distance between bound chlorines and their concentration. Functionalization with negatively charged chlorine ions is less thermodynamically stable compared to neutral chlorination, favoring only monochlorination. With the introduction of a charge, occupied mid-gap states appear near the conduction band resulting in an n-doped system. The lowest-energy, defect-associated transitions of charged systems are more redshifted but nearly optically inactive. Our calculations demonstrate that efficient emission of chlorinated SWCNTs is possible only for neutral chlorine adducts placed on the same carbon ring and at very small concentrations.