Fig. 4: CM efficiencies in undoped and Mn-doped QDs.
From: Spin-exchange carrier multiplication in manganese-doped colloidal quantum dots
a, Multiexciton yields (ηXX) as a function of photon energy normalized by the bandgap (hvp/Eg). The data from the present study are shown by red (Mn-doped QDs) and black (undoped QDs) symbols; circles and squares are the PL measurements, and stars are the TA measurements. Labels 1 to 4 are sample numbers for the Mn-doped QDs. The PL and TA data were obtained using hvp = 3.1 eV and 2.41 eV, respectively. The blue triangles show the CM measurements of undoped PbSe/CdSe QDs from ref. 23 (the error bars are from the same work). CM efficiencies of core-only PbSe QDs are schematically shown by green shading (refs. 3,48,49). b, An excitonic representation of spin-exchange CM for samples Mn-1 and Mn-3 in the case of 3.1 eV excitation. This process occurs via activation of the Mn ion via exciton transfer from the CdSe shell (step 1 or 1′) followed by Mn* relaxation, which creates a biexciton in the PbSe core (step 2 or 2′). In the case of 3.1 eV excitation, the energy of a photogenerated hot exciton is sufficiently high to excite both the 4T2 and 4T1 states of the Mn ion (the 4T1 state can also be excited via capture of a band-edge exciton following hot-exciton cooling). In sample Mn-1, spin-exchange CM can be driven by both the 4T2−6A1 and 4T1−6A1 spin-flip transitions. However, in sample Mn-3, which has a higher bandgap, spin-exchange CM can be driven only by the higher-energy 4T2−6A1 transition. Due to the larger number of spin-exchange CM pathways, sample Mn-1 shows a higher CM efficiency than sample Mn-3 (a).
