Biexciton Auger Recombination in Colloidal Graphene Quantum Dots

We measure biexciton Auger recombination (AR) in colloidal graphene quantum dots (GQDs) by transient absorption spectroscopy. AR is reflected in GQDs with 132 and 168 ${sp}^2$-hybridized C atoms as a decay with a $\sim 0.3\mathrm{ps}$ time constant and an amplitude depending superlinearly on pump fluence. Despite the orders-of-magnitude difference in size between GQDs and carbon nanotubes, their biexciton AR rates are similar. This similarity is a result of strong carrier interactions in $s{p}^2$-hybridized carbon nanostructures and suggests that GQDs may hold promise in the context of carrier multiplication.

Figure 1

Ground-state absorption cross section of C132 (blue line) and C168 (red line). The molecular structures of C132 and C168 are inset; hydrogen atoms are not shown. $S_1$ and $S_2$ refer to the two lowest-energy singlet excitons. $\beta$ denotes the prominent low-energy absorption band derived from the two degenerate (nondegenerate) bright singlet excitons in C168 (C132). The lower right inset illustrates the pump-probe scheme used in Fig. 3.

Figure 2

$-\mathrm{\Delta }\alpha L$ as a function of probe energy and delay for C132 pumped at 1.94 eV and fluences of (a) $1.3\times {10}^{16}$ and (b) $1.4\times {10}^{14}{\text{photons}\mathrm{cm}}^{-2}$ per pulse. The color scale corresponds to the data for $t\le 5.0\mathrm{ps}$ in (a). The data in the other three quadrants are multiplied by the factors shown to match the scales.

Figure 3

$-\mathrm{\Delta }\alpha (\mathrm{\Phi },t)L$ versus probe delay for C132 pumped at 1.94 eV and probed at 2.34 eV. For the pump fluence corresponding to a particular scan, see the abscissa of Fig. 4; data for the same fluence share the same color symbols in both figures. Solid curves are fits described in the text. The dashed curve is the instrument response function.

Figure 4

Amplitudes $A_i$ from Eq. (1) describing the fits of $-\mathrm{\Delta }\alpha (\mathrm{\Phi },t)L$ in Fig. 3. The inset shows the quantity $A_{f^{\prime }}$ defined in the text and associated with multiexcitons and uses the horizontal scale of the main figure. Curves show the fits described in the text.