Nonradiative Quenching of Fluorescence in a Semiconducting Carbon Nanotube: A Time-Domain Ab Initio Study

As shown experimentally, strong nonradiative decay channels exist in carbon nanotubes (CNT) and are responsible for low fluorescence yields. The decay of the electronic excitation to its ground state is simulated in the (6,4) semiconducting CNT with surface hopping in the Kohn-Sham representation, providing a unique time-domain atomistic description of fluorescence quenching. The decay in the ideal CNT is estimated to occur on a 150 ps time scale and is only weakly dependent on temperature. Vibrationally induced decoherence strongly influences the electronic relaxation. Defects decrease the excited state lifetime to tens of picoseconds, rationalizing the multiple decay time scales seen in experiments.

Figure 1

Defects imposed on the (6,4) CNT studied in this work. The 7557 defect (right column) is produced by insertion of a C-C dimer across a hexagonal carbon cell. The SW defect (left column) appears due to rotation of one of the C-C bonds. The bottom panels show the transition densities corresponding to the lowest energy excitations generated in the CNT by the defects with the red and blue colors representing positive and negative density changes.

Figure 2

Phonon-induced fluctuation of the lowest excitation energy of the CNTs used in this study. Note the different energy scales in each panel.

Figure 3

Fourier transforms of the excitation energies shown in Fig. 2.

Figure 4

Population of the ground electronic state in the CNTs studied as a function of time. $T=300\mathrm{K}$, except for the dotted line.