Exciton Storage by Mn²⁺ in Colloidal Mn²⁺-Doped CdSe Quantum Dots

Abstract

Colloidal Mn2+-doped CdSe quantum dots showing long excitonic photoluminescence decay times of up to τexc = 15 μs at temperatures over 100 K are described. These decay times exceed those of undoped CdSe quantum dots by ∼103 and are shown to arise from the creation of excitons by back energy transfer from excited Mn2+ dopant ions. A kinetic model describing thermal equilibrium between Mn2+ 4T1 and CdSe excitonic excited states reproduces the experimental observations and reveals that, for some quantum dots, excitons can emit with near unity probability despite being ∼100 meV above the Mn2+ 4T1 state. The effect of Mn2+ doping on CdSe quantum dot luminescence at high temperatures is thus completely opposite from that at low temperatures described previously.

Carrier Dynamics and Magneto-Optical Properties of Cd1-xMnxS Nanoparticles

“…The characteristic emissions 4 T1 → 6 A1 between levels of Mn 2+ ions, labelled as E(Mn 2+ ) in Figs. 6b and 6c and with transition energy ~2.12 eV, also confirm that these magnetic impurities were substitutionally incorporated in the Cd1-xMnxS NPs [1,22,23]. This incorporation of Mn 2+ ions in NPs has also been proved by electron paramagnetic resonance (EPR) measurements and simulations in other samples which were synthesized by the same method used in this work [17,20].…”

“…This result can take into account two effects caused by the increasing x-concentration of Cd1-xMnxS QDs: (i) the increasing energy gap that was observed in OA spectra of Fig. 1a; and (ii) possible density amplification of virtual levels associated to shallow defects of QDs [23]. Therefore, the combination of these effects in the electronic structure of Cd1-xMnxS QDs (x ≠ 0) explains the nearly constant values obtained for the EA activation energy.…”

Exciton storage in CdSe/CdS tetrapod semiconductor nanocrystals: Electric field effects on exciton and multiexciton states

“…Consideration of the dynamics presented here for tetrapods in addition to our past work on nanorods, 8,23 leads us to propose that the primary states responsible for the formation of these charge-separated excitons in the presence of an external electric field are the localized trap states, 3,6,[43][44][45] as opposed to the quantum-confined band states of the nanocrystal. The conduction and valence band offsets of CdSe and CdS are such that, to a first approximation, a quasi type-II heterostructure is formed [ Fig.…”

Thermometry at the nanoscale

“…It has been found that the relative intensity of excitonic emission increases with temperature. A discussion of the thermal population mechanisms and its effect on photoluminescence was presented by Beaulac et al 104 and Vlaskin et al 80 The maximum relative sensitivity values reported near room temperature for these QDs are 1.3% K À1 at 318 K (ref. 80) ( Table 2 and Fig.…”