Bright Single-Wall Carbon Nanotubes with Luminescent Aryl and Oxygen Defects— Functionalization and Characterization Methods

Single-wall carbon nanotubes (SWCNTs) are a promising emitter for in-vivo bioimaging, optical sensing, and optoelectronic devices in the near-infrared (nIR). While SWCNTs show remarkable photostability and high sensitivity towards various analytes, their performance as sensors and nIR emitters is limited by their low photoluminescence quantum yield (PLQY) of 0.5 – 1 % in dispersion. One strategy to overcome this problem is the covalent functionalization of SWCNTs with luminescent defects, leading to red-shifted emission in the nIR and increased PLQYs. The characteristics of the defect emission depend on the type of defect and its binding configuration on the nanotube lattice. Hence, several synthetic strategies have been developed to control the emission wavelength and defect functionality [<i>ACS Nano </i><b>2019</b>, <i>13</i>, 9259, <i>ACS Nano </i><b>2021</b>, <i>15</i>, 5174, <i>Nat. Commun.</i> <b>2021</b>, <i>12</i>, 2119].<br/><br/>To achieve a fundamental understanding of the structure-property relationships of luminescent defects, knowledge of their density and distribution along the nanotube is crucial. To this end, we demonstrate a combined approach of Raman spectroscopy and PLQY measurements, which enables the absolute quantification of the density of luminescent defects in SWCNTs with different chiralities and diameters [<i>J. Phys. Chem. Lett. </i><b>2022</b>, <i>13</i>, 3542, <i>ACS Nano</i> <b>2023</b>, <i>17</i>, 21771]. We now use this metric to compare different types of luminescent defects. Supported by photoluminescence measurements of individual SWCNTs at cryogenic temperature, fundamental differences between oxygen and aryl defects are revealed. Irrespective of the functionalization method, aryl defects appear to act as isolated exciton trapping sites, whereas oxygen defects show a tendency to form local clusters on the nanotube lattice. We combine these findings with insights on the interplay of reaction parameters to obtain oxygen-functionalized SWCNTs with exceptionally high PLQYs (&gt; 3%), thus pushing the limits of nanotube brightness for high-contrast nIR bioimaging beyond the state of the art.