The intentional incorporation of impurities and defects can serve as a powerful tool for modification of the electronic and optical properties of host nanomaterials and enabling of new functionalities.¹ Introduction of sp³ defects by covalent functionalization to single-walled carbon nanotubes (SWCNTs) generates new emission states red-shifted from the optical bandgap (E₁₁) of SWCNTs by 100 to 300 meV.² With controllable emission wavelengths and high quantum yield, these defect states open up new opportunities for the applications of SWCNTs in photonics such as lasing, photon upconversion and single photon emission.^3-4

Recently, it has been shown that such doping sites are capable of emitting single photons at room-T, with single photon purity as high as 99% and emit at wavelengths from 1.1 μm to 1.55 μm covering the telecom O- and C-bands⁴. While the high single photon purity is of great importance, another critical criterial for an ideal single photon generator is photon indistinguishability, which requires the emission linewidth to be narrowed down to the Fourier transform-limitation.⁵

In this work, we investigated the environmental influences on the linewidth of defect-state emission in SWCNTs at cryogenic temperature (4K). sp³ defects were generated on the surface of surfactant and poly[9,9-dioctylfluorenyl-2,7-diyl] (PFO) wrapped (6,5)SWCNTs by aryl diazonium functionalization. Photoluminescence (PL) spectra of defect states were investigated under different environmental conditions, including surfactant wrapping, surfactant-free, PFO-wrapping tubes on polymer substrates. The result showed that the linewidth of the PL peaks is in the order of a few hundred of micro-eV for surfactant-free and PFO wrapping tubes. Furthermore, dynamic measurements revealed that for clean tubes, the PL lifetimes (T₁) are as high as a few nanoseconds, reaching to the radiative decay limitation. These results indicate that the observed linewidth is largely contributed by dephasing time (T₂). Increasing T₂ will be the critical issue for further narrowing the linewidth to the transform-limitation region.

[1] Piao, Yanmei. et al. Nature Chem. 2013, 5, 840.

[2] Hartmann, Nicolai F. et al. Nanoscale. 2015, 7,20521.

[3] Ma, Xuedan. et al. Nature Nanotech. 2015, 10, 671.

[4] Xiaowei He. et al. Nature Photonics. 2017, 11, 577.

[5] Yue Luo. et al. Nature communication. 2017, DOI: 10.1038/s41467-017-01777-w