Observation of Efficient Upconversion Photoluminescence of Single-Walled Carbon Nanotubes

Because of their potential applications in optoelectronics and bioimaging, photoluminescence (PL) properties of single-walled carbon nanotubes (SWNTs) have attracted much attention over the past decade [1-3]. Here, we report a novel aspect of the PL properties of SWNTs: availability of near-infrared PL (~950-1000 nm) by excitation with longer wavelength light (>1100-1200 nm), namely, upconversion PL. On the basis of the observations of temperature and excitation photon energy dependence, the observed phenomenon is attributed to the efficient phonon-assisted exciton upconversion processes. Furthermore, we demonstrate that strong enhancement of the upconversion PL occurs with additional chemical treatment on SWNTs. Materials exhibiting efficient upconversion PL in the near infrared range have been strongly desired for the observation of deep inside of highly scattering biological media. Our findings may thus open the door to the application of SWNTs in upconversion imaging of unprecedentedly deep inside of biological tissues.

[1] Y. Miyauchi, J. Mater. Chem. C 1, 6499 (2013). [2] Y. Miyauchi, M. Iwamura, S. Mouri, T. Kawazoe, M. Ohtsu, and K. Matsuda, Nature Photon. 7, 715 (2013). [3] M. Iwamura, N. Akizuki, Y. Miyauchi, S. Mouri, J. Shaver, Z. Gao, L. Cognet, B. Lounis and K. Matsuda, ACS Nano, asap (2014).