Fig. 3: CM measurements of undoped and Mn-doped PbSe/CdSe core/shell QDs.
From: Spin-exchange carrier multiplication in manganese-doped colloidal quantum dots
a, Time-resolved intensity of the NIR PbSe-core emission for the Mn-doped (Mn-1, red) and the undoped (Un, black) QDs at low (solid symbols) and high (open symbols) pump fluences (top panel; 〈Nph〉 = 0.01 and ~0.5, respectively). These dynamics were measured using pump photons with a sub-CM-threshold energy (hvp = 1.20 eV and 1.55 eV for the doped and the undoped samples, respectively). The traces are normalized so as to match the long-time tails. The symbols are raw data, and the lines are traces obtained via a deconvolution procedure to account for the 58 ps IRF. The higher pump-intensity traces develop a fast initial component that is absent in the dynamics recorded using the low pump level. The fast PL component (middle panel; symbols; isolated via subtraction of high- and low-pump-intensity traces) is due to the Auger decay of bi-excitons. It exhibits exponential dynamics with time constant τXX ≈ 170 ps (dashed line), which is the same for the doped and undoped samples. The exciton multiplicity (〈NX〉) calculated from the A/B ratio of the ‘deconvolved’ PL traces as a function of 〈Nph〉 (symbols; bottom panel) is fitted using the Poisson statistics of photon absorption events (line; bottom panel). b, A similar set of data but obtained using 3.1 eV excitation, which is above the CM threshold. As distinct from a, even the lowest intensity traces exhibit a fast bi-excitonic component (top) whose time constant (middle) is similar to that obtained using low photon excitation. Another distinction is a strong deviation of the measured 〈NX〉 (symbols; bottom panel) from the Poisson dependence (solid black line; bottom panel). In particular, the low-〈Nph〉 limit is greater than 1, a typical signature of CM. Based on these data, multiexciton yields are 48% and 75% for the undoped and doped samples, respectively. c, The PbSe-core NIR PL dynamics of doped samples Mn-3 (left) and Mn-4 (right) also exhibit a pronounced bi-excitonic component despite the use of low, sub-single-exciton pump levels (〈Nph〉 < 0.1). This points towards highly efficient CM. hvPL is the PL detection energy, which defines the bandgap of the QDs probed in the CM measurements.
