Fig. 1: Auger recombination and impact ionization in undoped and Mn-doped QDs.
From: Spin-exchange carrier multiplication in manganese-doped colloidal quantum dots
a, Impact ionization (left) can be thought of as the inverse of Auger recombination (right); GS is the ground state, and X and X* are the band-edge and the hot-exciton states, respectively. During Auger recombination, the energy of one exciton is transferred to the other, which leads to the formation of a hot-exciton state (right). In the course of impact ionization, a hot exciton loses its kinetic energy by creating a new exciton (left). b, The 1S TA dynamics of the undoped CdSe QDs obtained using 2.4 eV excitation and two different pump fluences, 〈Nph〉 = 0.03 (black) and 3 (red). The higher pump intensity trace exhibits a fast initial component (~82 ps time constant) due to multi-carrier Auger recombination. The inset shows the extracted Auger dynamics obtained by subtracting tail-normalized high- and low-pump-fluence traces (δα is the difference between TA signals for the different 〈Nph〉 values). c, The same measurements as in b, applied to the Mn-doped sample (〈Nph〉 = 0.1 and 2), reveal considerably faster dynamics arising from spin-exchange Auger recombination (note a 100-fold difference in the overall time spans in c and b). Based on the ‘extracted’ Auger decay (inset), the characteristic time constant is 340 fs. d, An ‘excitonic’ representation of spin-exchange Auger recombination (left) and spin-exchange CM (right). In the first process, the energy released during spin-flip relaxation of the excited Mn ion (Mn*), EMn, is transferred to the QD band-edge exciton, leading to the formation of a hot exciton. During spin-exchange CM, the Mn ion excited via capture of a hot exciton (step 1) relaxes by generating two band-edge excitons (step 2).
