Exploiting Functional Impurities for Fast and Efficient Incorporation of Manganese into Quantum Dots

Abstract

The incorporation of manganese (Mn) ions into Cd­(Zn)-chalcogenide QDs activates strong spin-exchange interactions between the magnetic ions and intrinsic QD excitons that have been exploited for color conversion, sunlight harvesting, electron photoemission, and advanced imaging and sensing. The ability to take full advantage of novel functionalities enabled by Mn dopants requires accurate control of doping levels over a wide range of Mn contents. This, however, still represents a considerable challenge. Specific problems include the difficulty in obtaining high Mn contents, considerable broadening of QD size dispersion during the doping procedure, and large batch-to-batch variations in the amount of incorporated Mn. Here, we show that these problems originate from the presence of unreacted cadmium (Cd) complexes whose abundance is linked to uncontrolled impurities participating in the QD synthesis. After identifying these impurities as secondary phosphines, we modify the synthesis by introducing controlled amounts of “functional” secondary phosphine species. This allows us to realize a regime of nearly ideal QD doping when incorporation of magnetic ions occurs solely via addition of Mn–Se units without uncontrolled deposition of Cd–Se species. Using this method, we achieve very high per-dot Mn contents (>30% of all cations) and thereby realize exceptionally strong exciton-Mn exchange coupling with g-factors of ∼600.

Magnetic Transitions and Energy Transfer Processes in Sb-Based Zero-Dimensional Metal Halide Nanocrystals Doped with Manganese

“…Interestingly, as highlighted by the fitting procedure shown in Figure b for [Mn 2+ ] = 20%, a third narrow component emerges for [Mn 2+ ] > 10% and intensifies with increasing Mn 2+ content (Figure c). The correlation between the intensity of such a component and the concentration of magnetically active dopants in the NC lattice suggests that it originates from short-range exchange narrowing coupling between nearby Mn 2+ centers. − Whereas the long-range dipolar component is almost insensitive to the Mn 2+ content, the intensity of the exchange coupling signal anticorrelates with the intensity of the sextet due to nearly isolated Mn 2+ ions, eventually disappearing for [Mn 2+ ] = 40%. This suggests that a growing fraction of isolated dopant ions gradually turns into adjacent exchange-coupled centers.…”

Synthesis and Assembly of Core–Shell Nanorods with High Quantum Yield and Linear Polarization

“…It has been shown that secondary phosphines can increase the nucleation and growth rate of CdSe QDs compared with tertiary phosphines (such as TOP). [ 57,58 ] We verified that this also applies to the synthesis of CdSe NRs (Scheme S1, Supporting Information). As sketched in Figure 1f, the formation of colloidal CdSe NRs, as for nanocrystals in general, occurs at least in three stages, including monomer formation, nucleation and growth.…”

Dopant Cluster Formation and Exchange Spin Coupling as Limiting Factors for the Magnetic Activity of Mn-Doped ZnS Quantum Dots

“…The magnitudes of the -C coupling constants (−21.1 cm −1 ≡ 2.6 meV, and −8.2 cm −1 ≡ 1.0 meV) are similar to the coupling constants obtained from magnetic susceptibility measurements on Mn-doped ZnSe QDs, for which values between −19.5 cm −1 and −13.5 cm −1 were reported, with varying dopant concentration. 61 Moreover, antiferromagnetic coupling was also reported in Mn-doped ZnS 26 and CdSe; 27,62 however, no coupling constants were extracted.…”