Finite (6,6) Single-Walled Carbon Nanotubes with Organic Surface Defects as Candidate Single Photon Emitters

We used density functional theory (DFT) calculations to investigate the possibility of using finite (6,6) "armchair" single-walled carbon nanotubes (SWCNT), which are metallic in the limit of infinite tube length, as hosts for organic color centers (OCC) made by sidewall functionalization of the nanotubes, as has been done in recent years with chiral semi-conducting SWCNTs. Our ground-state calculations show that appreciable HOMO-LUMO gaps exist in finite segments with lengths of several nanometers. This suggests a possibility of creating in finite (6,6) SWCNTs the localized exciton states responsible for enhanced photoluminescence in chiral SWCNTs via the introduction of organic defects to the nanotube wall, changing the local bonding configuration from the native sp² to sp³. The problem then becomes a question of which defects are likely to produce optically allowed "bright" exciton states with lower energy than the "dark" states prevalent in the native SWCNTs. To this end, we employ TDDFT calculations to compare the nature of low-lying excited states of defect-free segments to segments with various organic defects. Preliminary results suggest identifying appropriate defect molecules for finite (6,6) SWCNTs is not a straightforward mapping of what works for the chiral semi-conducting SWCNTs.