Increasing the concentration of single-wall carbon nanotubes (SWCNTs) is an effective method for enhancing their luminescence intensity. However, an increase in the concentration of SWCNTs would inevitably increase their reabsorption effect, degrading their luminescence efficiency. Herein, we systematically investigated variations in the photoluminescence (PL) intensity of (6,5) single-chirality SWCNTs while increasing their concentration. The results show that the PL intensity first increased to a maximum and then decreased with increasing concentration. Numerical analysis indicates that the concentration boundary corresponding to the maximum PL intensity was strongly dependent on the ratio of the optical absorbances of the SWCNTs at their excitation and emission wavelengths. According to this, statistical analysis by experimentally measuring the optical absorption spectra of 18 kinds of single-chirality SWCNTs shows that the concentration boundaries of SWCNTs were dependent upon their Types and diameters. The concentration boundary of Type I SWCNTs was higher than that of Type II SWCNTs, and the concentration boundaries of both Types increased with increasing diameter. These results provide important guidance for spectral characterization and applications in bioimaging and photoelectronic devices.

In this work, we quantitatively studied the intertube coupling of different (n, m)-sorted semiconducting single-wall carbon nanotubes (SWCNTs) on their photoluminescence (PL) efficiencies by precisely tuning the ratio of (9, 4) and (6, 5) SWCNTs in the mixture. A significant decrease in the PL intensity of (9, 4) SWCNTs was observed after mixing with (6, 5) species when fixing the (9, 4) concentration, which was confirmed to be caused by the absorption of incident photons and reabsorption of the emitted photons by the added (6, 5) species. By contrast, a similar decrease in the PL intensity of (6, 5) SWCNTs was also observed after mixing with the larger-diameter (9, 4) species. Different from that of (9, 4) SWCNTs, the PL decrease of (6, 5) SWCNTs was found to originate not only from photon absorption and reabsorption by the (9, 4) species but also from one-way exciton energy transfer (EET) from the (6, 5) SWCNTs to the larger-diameter (9, 4) SWCNTs. Both the experimental results and numerical simulations further demonstrated that increasing the concentration of mixed (9, 4) SWCNTs would enhance the effects of photon absorption and reabsorption and EET on the PL intensity of (6, 5) SWCNTs quantified by the decrease ratio of the (6, 5) PL intensity. Meanwhile, the influence of EET was found to be always weaker than that of photon absorption and reabsorption. We proposed that the observed EET between isolated SWCNTs in a surfactant solution is derived from their proximity due to Brownian motion.