Exciton Localization of Single-Walled Carbon Nanotubes Revealed by Femtosecond Excitation Correlation Spectroscopy

Photoluminescence (PL) dynamics in single-walled carbon nanotubes (SWNTs) has been studied by the femtosecond excitation correlation method with a 150 fs time resolution. The SWNT samples were synthesized by different methods and suspended in gelatin films or ${\mathrm{D}}_2\mathrm{O}$ solutions. The PL dynamics of SWNTs depends on the local environment surrounding the SWNTs rather than the synthesis methods. The very weak temperature dependence of ${\tau }_{\mathrm{PL}}$ and the environment-dependent ${\tau }_{\mathrm{PL}}$ reveal that the PL relaxation process is dominated by the interplay between free excitons and weakly localized excitons.

Figure 1

PL spectra of the $\mathrm{ACCVD}\mathrm{\text{−}}{\mathrm{D}}_2\mathrm{O}$, ACCVD-gelatin, and $\mathrm{HiPco}\mathrm{\text{−}}{\mathrm{D}}_2\mathrm{O}$ samples under 1.96 eV excitation. The inset shows the intensity dependence of PL amplitude for $(9,4)$ nanotubes in the $\mathrm{ACCVD}\mathrm{\text{−}}{\mathrm{D}}_2\mathrm{O}$ samples with the optical pulses under 1.70 eV excitation.

Figure 2

(a) Normalized FEC signals for $(9,4)$ nanotubes of $\mathrm{ACCVD}\mathrm{\text{−}}{\mathrm{D}}_2\mathrm{O}$ samples measured under 1.70 eV excitation at 5 mW. The solid line is given by the function of $A\exp (-\tau /{\tau }_{\mathrm{PL}})+B$. The inset shows the normalized FEC signals measured at different excitation intensities (5 and 10 mW at 1.70 eV). (b) Normalized fast-decay components of the FEC signals from $(9,4)$ nanotubes of the ACCVD-gelatin, HiPco-gelatin, $\mathrm{ACCVD}\mathrm{\text{−}}{\mathrm{D}}_2\mathrm{O}$, and $\mathrm{HiPco}\mathrm{\text{−}}{\mathrm{D}}_2\mathrm{O}$ samples measured with an optical pulse at 1.70 eV. The solid (ACCVD-gelatin), dotted (HiPco-gelatin), dashed ($\mathrm{ACCVD}\mathrm{\text{−}}{\mathrm{D}}_2\mathrm{O}$), and dotted-dashed ($\mathrm{HiPco}\mathrm{\text{−}}{\mathrm{D}}_2\mathrm{O}$) lines represent the normalized fast-decay components of fitting curves, $A\exp (-\tau /{\tau }_{\mathrm{PL}})$, respectively. The excitation optical pulse intensity is 5 mW. The upward direction on the vertical axis indicates a negative sign of the FEC signals.

Figure 3

PL lifetimes ${\tau }_{\mathrm{PL}}$ of the fast-decay components of the FEC signals as a function of tube diameter in the ACCVD-gelatin and $\mathrm{ACCVD}\mathrm{\text{−}}{\mathrm{D}}_2\mathrm{O}$ samples. The dotted (ACCVD-gelatin) and dashed ($\mathrm{ACCVD}\mathrm{\text{−}}{\mathrm{D}}_2\mathrm{O}$) lines represent the mean values of PL lifetimes for five nanotube species with different diameters.

Figure 4

(a) Normalized fast-decay components of the FEC signals from $(9,4)$ nanotubes of the ACCVD-gelatin samples at 10, 80, and 300 K. The solid lines represent the normalized fast-decay components of fitting curves, $A\exp (-\tau /{\tau }_{\mathrm{PL}})$, respectively. The excitation intensity is 1 mW at 1.70 eV. The upward direction on the vertical axis indicates a negative sign of the FEC signals. (b) The temperature dependence of the relaxation rate $1/{\tau }_{\mathrm{PL}}$ between 10 and 300 K for $(9,4)$ nanotubes of the ACCVD-gelatin samples. The solid line shows the relaxation rate described by Eq. (1). The inset contains a schematic sketch illustrating the dynamics of localized exciton $|e{x}_L⟩$ and free excitons $|e{x}_F⟩$ in SWNTs.