Ferromagnetic excited-state Mn2+dimers in Zn1−xMnxSe quantum dots observed by time-resolved magnetophotoluminescence

Abstract

Colloidal Mn 2+ -doped semiconductor nanocrystals are solution processable analogs of classic phosphor and diluted magnetic semiconductor materials with promising applications ranging from fluorescence microscopy to spintronic information processing. At doping levels of only a few cation mole percent, Mn 2+ dimers form in appreciable concentration and cause shortened photoluminescence decay times and reduced luminescence circular polarization under applied magnetic fields. Here, we show that these differences allow the use of timeresolved magnetophotoluminescence measurements to investigate the magnetic properties of the luminescent dimer excited state in Zn 1−x Mn x Se nanocrystals. These measurements reveal that Mn 2+ -Mn 2+ dimers are coupled ferromagnetically in their luminescent excited state, in contrast with the antiferromagnetic coupling of their ground state. We find that Mn 2+ -Mn 2+ dimers also luminesce with much purer circular polarization than Mn 2+ monomers under applied magnetic fields. These results are explained well by perturbation theory and density functional theory analyses of the microscopic orbital exchange interactions within the photoexcited Mn 2+ -Mn 2+ dimers. This discovery of photoswitchable dimer magnetism (from S = 0 to S = 4) with strong associated circularly polarized luminescence raises intriguing possibilities for optical spin manipulation in doped semiconductors.

Quantum confinement effects on optical transitions in nanodiamonds containing nitrogen vacancies

“…This technique has shown very promising results in our lab for the theoretical characterization of the excited states in doped semi-conductor quantum dots. 28,[41][42][43][44][45][46] Using this technique on a broad range of NV-diamond QD sizes, we explore quantum confinement effects on the highly localized mid-gap transitions, as well as the higher energy CT states. Because CT transitions are broad and show strong spectral overlap with many other optical features, this theoretical method is strongly motivated by the desire to disentangle the full range of electronic transitions in NV-doped nanodiamond.…”

Mn doped AIZS/ZnS nanocrystals: Synthesis and optical properties

“…The small τ 1 and A 2 for 0.025mmol-Mn:AIZS/ZnS NCs could be related to the low Mn level in the hosts. Interestingly, according to the literature [33–34], a lower Mn level in host NCs should produce a longer PL lifetime probably because the Mn-Mn distance is long. Such a conflict indicates that the Mn emission with Mn-Mn coupling could be existing but not be the major PL mechanism for the fast decay (corresponding to τ 1 ) in Mn:AIZS/ZnS NCs.…”

“…It is suggested that the lifetimes in the range of tens of microseconds may result from radiative decay through trap states or be related to the emission of Mn dopants (diffused from MnS surface to ZnS shell) with enhanced overlap between 3d and sp host states caused by lattice strain due to the spatial distribution of Mn dopants in host NCs. Other authors reported that the spatial distribution of Mn in ZnSe or ZnS NCs affects the PL decay significantly [33–34]. Homogenous Mn dopants in ZnSe or ZnS NCs result in single exponential decay.…”

“…More recent work have been focused on ZnSe or ZnS NCs as hosts for Mn doping because of the wide bandgaps of ZnSe or ZnS. Nearly pure and highly bright (quantum yield (QY) > 50%) Mn dopant emission has been achieved in Mn:ZnS, Mn:ZnSe, and Mn:ZnSeS NCs [22–34]. Some studies have also disclosed that the spin coupling of Mn-Mn in ZnSe or ZnS could be critical for the PL decay dynamics or lifetimes [33–34].…”

“…Although achieving homogenous distribution of Mn in ZnSe or ZnS NCs involves complex precursors and synthesis protocols, these studies do be valuable in providing insights of the Mn roles in tuning the optical properties of Mn-doped NCs. It should also be noted that the Mn-doped ZnSe or ZnS NCs usually are prepared at a high temperature up to 280 ~ 320°C or in a hot-injection reaction (hard to scale up), and highly toxic precursor selenium-tributylphosphine were usually adopted in ZnSe based synthesis [22–34]. For biomedical applications, heat-up syntheses of high quality NCs with all less toxic precursors and mild synthetic temperature are desired because such approaches are apt to scale up for the mass production of NCs in a single and safe synthetic reaction, which will sustain biomedical research for reliable experimental data collection and also levitate synthesis costs/skills [35].…”