Nonlinear photoluminescence properties of trions in hole-doped single-walled carbon nanotubes

We studied the excitation density dependence of photoluminescence (PL) spectra of excitons and trions (charged excitons) in hole-doped single-walled carbon nanotubes. We found that the PL intensity of trions exhibited a strong nonlinear saturation behavior as the excitation density increased, whereas that of excitons exhibited a weak sublinear behavior. The strong PL saturation of trions is attributed to depletion of doped holes that are captured by excitons in the formation processes. Moreover, the effective radiative lifetime of a trion was evaluated to be approximately 20 ns.

Figure 1

Photoluminescence spectra of (7,5) SWNTs hole-doped with various ${\mathrm{F}}_4$TCNQ concentrations. The (7,5) $E_{11}$ exciton and trion peaks are denoted as X and X${}^{+}$, respectively. The excitation photon energy is 1.55 eV. The inset shows the PL spectrum of nondoped (7,5) SWNTs with an excitation photon energy of 1.55 eV.

Figure 2

(a) Excitation density dependence of the PL spectra of nondoped (7,5) SWNTs. The $E_{11}$ exciton peak is denoted as X. (b) Excitation density dependence of the PL spectra of (7,5) SWNTs hole-doped with a ${\mathrm{F}}_4$TCNQ concentration of 300 μg/mL. The $E_{11}$ exciton and trion peaks are denoted as X and X${}^{+}$, respectively.

Figure 3

(a) Excitation density dependence of the integrated PL intensity of the $E_{11}$ exciton (X) peak (1.19 eV, solid circles) of nondoped SWNTs. The dotted line represents the linear behavior normalized with experimental data around the weak excitation density conditions. (b) Excitation density dependence of the integrated PL intensity of the $E_{11}$ exciton (X) peak (1.19 eV, black solid circles) and the trion (X${}^{+}$) peak (1.01 eV, red solid circles) of SWNTs hole-doped with ${\mathrm{F}}_4$TCNQ concentration of 300 μg/mL. The solid circles, solid curves, and dotted lines represent the experimental data, the fitting of the experimental data to the rate equations, and the linear behavior normalized with experimental data around the weak excitation density conditions, respectively. The data points and fitted lines of the $E_{11}$ exciton (X) and trion (X${}^{+}$) peaks are denoted as black and red, respectively. The inset shows a schematic of the three-level system of hole-doped SWNTs consisting of the exciton (X), trion (X${}^{+}$), and ground (gr) states.

Figure 4

Excitation density dependence of the integrated PL intensity of the trion (X${}^{+}$) peak of SWNTs hole-doped with various ${\mathrm{F}}_4$TCNQ concentrations. The experimental data are normalized around the weak excitation density conditions, and the dotted line represents the linear behavior normalized around the weak excitation density conditions.