Phosphorus-Mediated Optical Defect Engineering in Single-walled Carbon Nanotubes

Defect engineering of single-walled carbon nanotubes (SWCNTs) offers a versatile strategy for tailoring their optical properties through localized chemical modification. However, expanding the range of accessible defect chemistries beyond established oxygen- and aryl-based functionalities remains an important challenge. Here, we report a straightforward, light-driven strategy for introducing phosphorus-derived defects into (6,5)-enriched SWCNTs using triphenylphosphine (TPP) and a photosensitizing dye. Ultraviolet irradiation in the presence of TPP and eosin Y generates a new near-infrared emission feature centered at 1125 nm, assigned to a defect-associated emissive state (E11*), alongside the intrinsic nanotube emission at 997 nm. The formation of this state depends on the TPP concentration, photosensitizer concentration, and irradiation time, and is accompanied by attenuation of the intrinsic optical transition and an increase in the Raman D/G intensity ratio, indicating progressive structural modification of the nanotube framework. X-ray photoelectron spectroscopy reveals a phosphorus-associated chemical state and changes in the carbon chemical environment, supporting phosphorus incorporation and the formation of modified carbon sites. Control experiments further demonstrate that defect formation requires both photosensitization and irradiation, is enhanced under oxygen-depleted conditions, and is suppressed by radical scavenging, consistent with a radical-mediated photochemical pathway. Complementary density functional theory calculations support the formation of a covalent carbon–phosphorus bond, accompanied by a change in phosphorus coordination from three- to four-coordinate, and reveal a phosphorus-associated electronic trap state near the valence-band edge. These findings establish phosphorus-mediated defect engineering as a complementary approach to tailoring SWCNT optical properties and provide a foundation for investigating the relationships between defect chemistry, localized electronic states, and near-infrared emission.