Efficient Mn²⁺ Doping in Non-Stoichiometric Cesium Lead Bromide Perovskite Quantum Dots

Abstract

Doping magnetic transition metal ions (e.g., Mn²⁺) into colloidal quantum dots endows novel optical and magnetic properties to the host materials. CsPbBr₃ quantum dots (QDs) are emerging light-emitting materials with high structural and chemical flexibility in the visible spectral regime. However, efficiently doping Mn²⁺ ions in CsPbBr₃ QDs remains challenging, especially when size confinement and ensemble uniformity are needed for understanding the underexplored exciton-dopant exchange interaction. Here, we introduce a doping mechanism based on electrostatic surface Mn²⁺ adsorption that enables efficient Mn²⁺ incorporation in strongly confined CsPbBr₃ QDs. The resultant QDs are found to have a Cs-deficient stoichiometry compared to their undoped counterparts. A redox reaction-based purification method was developed to remove Mn²⁺ cations that are tightly adsorbed on the surface to determine the concentration of lattice-incorporated Mn²⁺. Our synthesis enables a Mn²⁺ doping/alloying concentration of up to ∼44% with a Mn²⁺ photoluminescence efficiency exceeding 90%. This allows for the determination of the intrinsic exciton-to-dopant energy transfer rate.

Introduction

Incorporating impurities such as manganese­(II) in colloidal quantum dots (QDs) has been demonstrated as a versatile way to impart new optical and magnetic properties to the host material. − Over the past decade, lead halide perovskite QDs have been explored as a new family of host materials for their highly efficient photoluminescence (PL) and facile synthesis. − In the all-inorganic CsPbX₃ (X = Cl, Br, I) family, Mn²⁺-doped CsPbCl₃ QDs were first demonstrated. , Although CsPbCl₃ QDs often have low photoluminescence quantum yield (PLQY), benefiting from the fast exciton-to-Mn Dexter-type energy transfer, Mn²⁺-doped CsPbCl₃ nanocrystals exhibit intense and broad emission from ⁴T_1g to ⁶A_1g d-d transitions rather than weak and sharp emission from excitons. − The improved emission efficiency in the visible spectral range has enabled the application of CsPbCl₃ for solar concentrators and down-converters. −

Compared to CsPbCl₃, the band gap of CsPbBr₃ falls into the visible spectral regime and is more suitable for solar energy harvesting and light-emitting applications. However, direct doping of Mn²⁺ ions into CsPbBr₃ perovskite nanocrystals has been surprisingly more challenging than CsPbCl₃ nanocrystals. Alternatively, Mn²⁺ doping in mixed-halide (Cl/Br) perovskite nanocrystals can be obtained either by introducing multiple halides in precursors during the synthesis or by converting presynthesized Mn-doped CsPbCl₃ into CsPb­(Br/Cl)₃ through postsynthesis Br^– exchange. Such approaches often result in the loss of Mn²⁺ dopants and their PL emissions when the Br^– composition increases. Recent studies have also revealed that Mn–Cl bonds, rather than Mn–Br bonds, are preferred in mixed-halide nanocrystals. The uncertainties in material composition and the chemical environment of Mn²⁺ add additional barriers to understanding the Mn incorporation mechanisms and exciton-Mn interaction in pure CsPbBr₃ lattices.

Another method to introduce Mn²⁺ into the host lattices is cation exchange. While the approach has not been very fruitful for 0D CsPbBr₃ QDs, strong Mn²⁺ PL emission can be obtained by cation-exchange in anisotropic CsPbBr₃ nanoplatelets (NPLs), nanowires (NWs), and 2D nanoclusters. − The large surface area and surface defects facilitate the Mn²⁺ attachment and the diffusion of Mn²⁺ into the lattices. In addition, confinement effects due to reduced dimensionality increase the bandgap, promoting exciton-to-dopant energy transfer for enhanced Mn emission. However, such postsynthesis treatments often yield significant variations in particle lateral sizes, which can be intensified by Ostwald ripening during the treatment. Additionally, the Mn²⁺ dopant can still migrate or be expelled from the nanoparticles. It is worth noting that the one-pot synthesis of Mn-doped low-dimensional nanostructures at room temperature has also been demonstrated. ,− Despite the successes of Mn²⁺ doping in these anisotropic nanostructures, the lack of ensemble uniformity and uncertainties on dopant distribution in each particle have hindered the understanding of exciton-dopant exchange interaction from the ensemble-averaged optical properties.

Direct hot-injection synthesis of 0D Mn-doped CsPbBr₃ QDs usually produces weakly quantum-confined or irregularly shaped nanoparticles. The relatively small host bandgap leads to a low Mn²⁺ emission efficiency. The bandgap of CsPbBr₃ QDs can be expanded to ∼ 2.7 eV by reducing the QD size. − Furthermore, strong 3D confinement forces the spatial overlap between the wave functions of the Mn dopants and the exciton, promoting Mn²⁺ PL efficiencies. , Unfortunately, there have been limited attempts to directly dope Mn²⁺ in strongly confined CsPbBr₃ QDs adopting hot-injection synthesis for high ensemble uniformity. A pioneering study suggests that the high Mn–Br bond dissociation energy makes the decomposition of Mn–Br precursors difficult, subsequently reducing doping efficiency. In addition, incorporating Mn²⁺, a hard Lewis acid, into QD lattices composed of soft Lewis acid and base (Pb²⁺ and Br^–) will be less preferred than doping Mn²⁺ in CsPbCl₃ with a hard Lewis base (Cl^–). The thermodynamics of Mn-X bonds have certainly affected the ability to synthesize Mn²⁺-doped CsPbBr₃ QDs. To date, improved Mn²⁺ PL emission in directly hot-injection synthesized Mn²⁺-doped CsPbBr₃ QDs (∼6.5 nm) has been demonstrated as a side product through the conversion of intermediate Mn²⁺-doped 2D L₂PbBr₄ plates into Mn²⁺-doped QDs and NPLs. Most recently, room temperature Mn²⁺ doping in 5 nm CsPbBr₃ QDs has also been demonstrated. Nevertheless, the size regulation effect and Mn²⁺ incorporation efficiency or dopant density are still limited. Therefore, a generalized doping method is needed to produce monodispersed QDs with high doping efficiency in order to understand the photophysics of exciton-Mn interaction in CsPbBr₃ QDs.

In this work, we developed a synthesis under a bromide-rich environment with high Mn²⁺ ionic strength for efficient Mn²⁺ doping in size-confined CsPbBr₃ QDs. The resulting Mn²⁺-doped CsPbBr₃ QDs (∼4 nm) exhibit efficient (90 ± 10% PLQY) Mn²⁺ emission with a doping/alloying concentration up to ∼44%. These Mn²⁺-doped QDs are highly nonstoichiometric, featuring Cs-deficient regions near the QD surface. The Mn²⁺ doping efficiency increases with the extent of the Cs deficiency. Nuclear magnetic resonance (NMR) spectroscopy and elemental analysis suggest that a large quantity of Mn²⁺ ions is tightly adsorbed on the QD surface, which can be thoroughly removed by chemical redox-reaction-based purification using H₂O₂ and HBr without affecting the Mn²⁺ ions incorporated in the QD lattices. Finally, the exciton-to-dopant energy transfer rate is measured by transient absorption (TA) spectroscopy. Our study provides a facile way to dope Mn²⁺ in perovskite QDs, providing new insights to apply perovskite QDs to light-harvesting, spintronics, and hot electron production applications.

Results

Mn²⁺-doped CsPbBr₃ QDs were prepared using manganese­(II) acetate tetrahydrate as the Mn²⁺ precursor in a one-pot synthesis (details in Experimental Section). The bromide-rich environment is created by adding hydrobromic acid (HBr aqueous solution) to the system, leading to a bromide-terminated QD surface. , HBr also reacts with Mn acetate tetrahydrate (Mn­(Ac)₂·4H₂O) to facilitate the decomposition of Mn²⁺ precursors by producing acetic acid that can be removed from the system after extended evacuation. The Mn²⁺ incorporation efficiency can be tuned by solely varying the loading amount of Mn­(Ac)₂·4H₂O/HBr (more details of the synthesis conditions and their effects on Mn²⁺ doping are provided in the Discussion section, Supporting Information Table S1, Supporting Information Note 1, and Figure S1).

Absorption and PL spectra of the Mn²⁺-doped and undoped (control) QDs at room temperature are plotted in Figure a and b. The overall PLQY of Mn²⁺-doped CsPbBr₃ QDs (PL emissions from both exciton and Mn) is generally above 90% (Figure S2a), and the size of both Mn²⁺-doped and undoped QDs is ∼4 nm (Figure c and d, Figure S3) with a tight size distribution (±7.4% and ± 6%, respectively). Compared to undoped QDs, Mn²⁺-doped QDs tend to be more irregularly shaped. The Mn²⁺-doped QDs show an almost completely quenched exciton PL. By integrating the PL peaks from Mn²⁺ (∼605 nm), the highest Mn²⁺ emission PLQY is determined to be >90%. The exciton PL exhibits a blueshift of ∼50 meV in doped QDs compared to undoped QDs sharing similar sizes (Figures S4 and S5). This phenomenon has been observed in II–VI QDs and lead halide perovskite NCs, which can be attributed to the possible increase in bandgap resulting from Mn²⁺ alloying and local lattice periodicity breaking. − Additionally, Mn²⁺incorporation significantly blueshifts (∼186 meV) the absorption spectrum of the QDs (see Figure a) and changes the profile of higher-order exciton absorptions (black vertical arrows), suggesting that Mn²⁺ alloying may have resulted from efficient dopant incorporation. No anisotropic nanostructures, such as NPLs and NWs, are produced during the synthesis, implying a doping mechanism different from the previously reported monolayer perovskite-mediated doping.

1.

(a) Absorption spectra of Mn²⁺-doped and undoped CsPbBr₃ QDs measured at room temperature. Vertical arrows denote higher-order exciton peaks. (b) PL spectra of Mn²⁺-doped and undoped CsPbBr₃ QDs (Room temperature, excited using a 385 nm LED). (c) High-angle Annular Dark-field Scanning Transmission Electron Microscopy (HAADF-STEM) images of Mn²⁺-doped CsPbBr₃ QDs (purified by the GPC-chemical-GPC approach, as discussed in the main text and details given in the Experimental Section), and (d) undoped CsPbBr₃ QDs.

Inductively coupled plasma mass spectrometry (ICP-MS) elemental analysis is typically used to determine the chemical composition and doping concentration of QDs. Accurate determination of chemical composition in perovskite QDs is usually challenging due to the inefficient removal of unreacted precursors. Following previous reports, we purified the Mn²⁺-doped QD colloids using gel permeation chromatography (GPC) columns , after traditional antisolvent precipitation/resuspension cycles (Figure S6). The cationic composition (atomic ratios, normalized to Pb²⁺) of QDs is then obtained from ICP-MS (Supporting Information Table S2). The Br/Pb ratio of QD was determined using energy-dispersive X-ray spectroscopy (EDS) (Figure S7 and S8) and X-ray photoelectron spectroscopy (XPS). The chemical composition (not including Mn²⁺) is Cs_0.58–0.66PbMn _x Br_3.5–4.8 for doped QD and Cs_0.73–0.9PbBr_3.2–4 for undoped QDs. The bromide-rich QD composition in both QDs is expected since an anion-rich synthesis environment is used.

The Cs/Pb ratio of Mn²⁺-doped QDs with various Mn²⁺ PLQYs and undoped QDs is plotted in Figure a. The Cs/Pb stoichiometry of undoped QDs (0.7–0.9) matches that of the reported model structure with the QD’s surface terminated by [PbBr₂/ABr] (A = cationic ligand) (Supporting Information Note 2). In this model, surface undercoordinated Cs⁺ ions were replaced by cationic ligands, such as oleylammonium, resulting in a Cs/Pb atomic ratio of less than one as the QD size decreases. The oleylammonium bromide surface passivation is supported by the NMR spectrum (Figure S9). In contrast, the Cs/Pb ratio in Mn²⁺-doped QDs is consistently reduced to approximately 0.6 (as low as 0.55) with increased Mn²⁺ doping, indicating a Cs deficiency in the QD lattices. Note that the sizes of all Mn²⁺-doped QDs with high Mn PLQYs (>40%) are similar (4–4.5 nm). Therefore, the size effect on the Cs/Pb ratio in these QDs is negligible. For doped QDs with lower Mn PLQY (<30%), the size can be slightly larger, which may contribute to a slightly higher Cs/Pb ratio. It is also worth noting that a considerable number of Pb²⁺ ions are replaced by Mn²⁺ in heavily doped QDs. Therefore, the actual Cs⁺ loss can be underestimated in heavily doped QDs by calculating the Cs/Pb atomic ratios.

2.

(a) Mn²⁺ PLQY of Mn²⁺-doped and undoped CsPbBr₃ QDs with respect to the Cs/Pb ratio. Decreasing the Cs/Pb atomic ratio enhances the nonstoichiometry and Mn²⁺ emission (All QDs have overall PLQYs > 70%). Additional photographs are provided in Figure S11. (b) Schematic representation of the distribution of Mn²⁺ ions and ligands in relation to the amount of Cs. (c) Zeta-potential (ζ-potential) and (d) Dynamic light scattering (DLS) intensity-based hydrodynamic size distribution (nm) for Mn²⁺-doped and undoped CsPbBr₃ QD colloids (purified by antisolvent only). DLS correlation functions are provided in Supporting Information Figure S12. (e) Expanded region of ¹H NMR spectrum of Mn²⁺-doped and undoped CsPbBr₃ QDs in chloroform-d (CDCl₃). (f) ¹³³Cs NMR signals for Mn²⁺-doped CsPbBr₃ QDs and their fits showing core (green) and intermediate (purple) Cs⁺ species, alongside undoped CsPbBr₃ QDs display core (green), intermediate (purple), and surface (yellow) Cs⁺ species.

Elemental analyses also showed that the Mn/Pb atomic ratio is 10–50 in almost all Mn²⁺-doped QD samples, even after GPC purification. Given the sophisticated purification processes, such an unexpectedly high Mn²⁺ concentration in QD colloids cannot be simply attributed to free-standing Mn²⁺ precursors. Instead, it suggests that many Mn²⁺ ions are tightly attached to QD surfaces, and some of them are probably bound to the surface as Z-type ligands. The extremely large excess Mn²⁺ concentration (up to ∼17,000 Mn²⁺ per QD) is beyond the capacity of typical surface ligand coverage, suggesting the majority of the excess Mn²⁺ ions are physically adsorbed on the surface of the QD. EDS and XPS measurements also confirm the large Mn/Pb ratio in the doped QDs (Figures S8 and S10). Additionally, the XPS results indicate that the doped QD sample contains a mixture of long-chain hydrocarbon moieties (oleylammoniums and oleates), with their quantity larger than that of the undoped QDs. This suggests that many excess Mn²⁺ ions exist as Mn carboxylates, such as Mn-oleates. As illustrated in Figure b, an electric double layer structure is proposed, in which QDs are charged due to the nonstoichiometry, and Mn²⁺ ions can electrostatically adsorb on the charged QD surface, forming a diffusion layer that balances the charge on the QDs and stabilizes the QD colloids.

The electric double layer structure is evidenced by a ζ-potential of ∼21 mV in Mn²⁺-doped QD colloids (Figure c). The positive ζ-potential provides further evidence that many surface-physiosorbed Mn²⁺ ions are firmly attached to QDs and cannot be easily removed by conventional purification methods. In comparison, the undoped QDs have a negligible ζ-potential value (Figure c). To better study the colloidal structure of our Mn²⁺-doped QDs, we measured their hydrodynamic diameters (D _h) using dynamic light scattering (DLS). For undoped QDs, D _h is slightly smaller but close to the physical size of QD-ligands (∼4 nm, Figure d). The difference between D _h and the size measured using STEM imaging is attributed to the uncertainties in refractive indices when estimating the size distribution using the correlation function shown in Figure S12. In stark contrast, the D _h of Mn²⁺-doped QDs is ∼15 nm with a small population at ∼130 nm. The increased D _h is attributed to electric double layers and QD aggregations induced by the QD surface charges and lack of direct organic ligand passivation in Mn²⁺-doped QDs. STEM images also confirm that the Mn²⁺-doped QDs can aggregate (Figure S13). The surface Mn²⁺ adsorption on QDs plays an important role in facilitating Mn²⁺ incorporation in CsPbBr₃ lattices, given that perovskite QD surface ions are often under dynamic solubility equilibria. Details of the proposed mechanism for efficient Mn²⁺ doping are described in the Discussion section.

¹H NMR spectroscopy was used to study the chemistry of Mn²⁺-doped QD surfaces. Figure e (blue) shows that undoped QDs are capped by alkylammonium cations, which exhibit broadened peaks (indicated by the dashed vertical lines) at 3.34 ppm (alpha protons) and 6.87 ppm (ammonium protons) from oleylammoniums, in good agreement with previous studies. − However, the characteristic peak of alkylammonium alpha-protons nearly disappears in the ¹H NMR spectrum of Mn²⁺-doped QDs. Additionally, the peak from the alkenyl protons at 5.4 ppm in oleate anions and oleylammonium cations is significantly broadened (Figure e (red)). The line broadening is attributed to the effect of the paramagnetic Mn²⁺ ions adsorbed on the QD’s surface, forming ion pairs with organic oleates. The oleates are counterions for surface-adsorbed Mn²⁺, imparting the colloidal stability of doped QDs in organic solvents such as toluene and hexanes. The interaction of the paramagnetic Mn²⁺ ions with surrounding molecules shortens the spin–spin relaxation time (T₂), which in turn leads to the broadening of the peaks (given that the line width is inversely proportional to T₂ ( ${\upsilon }_{1/2}=\frac{1}{{\pi \mathrm{T}}_2}$ ). The NMR results agree well with our QD surface structural model.

The nonstoichiometry of Mn²⁺-doped QDs is further supported by the ¹³³Cs NMR spectrum (Figure f, top). In the undoped QD (Figure f, bottom), the ¹³³Cs signal is broadened because Cs⁺ ions are distributed in different regions in the QDs with various lattice disordering. The spectrum can be fitted with a minimum of three Gaussian peaks, corresponding to three groups of Cs⁺ ions depending on their locations in the QDs and on the degree of lattice disorder: the lattices in the core (least disordered), the lattices on or very close to the surface (most disordered), and the lattices in the intermediate region. This is in good agreement with a previous study using well-passivated CsPbBr₃ QDs. The ¹³³Cs signal of the Mn²⁺-doped QDs is much narrower (Figure f, top) due to, in particular, the absence of the downfield (yellow-shaded) component associated with the Cs⁺ ions located in the surface lattices. This suggests that the nonstoichiometric Mn²⁺-doped QD has Cs deficient lattices on and near the surface, promoting surface Mn²⁺ adsorption. The ¹H and ¹³³Cs NMR spectra of Mn²⁺-doped QDs with lower Mn²⁺ PLQY show less peak narrowing and a surface Cs component smaller than undoped QDs (Figure S14). The line width changes observed in the ¹³³Cs NMR spectrum corroborate the correlation between the extent of Cs deficiency and Mn²⁺ doping efficiency.

Excess Mn²⁺ ions in the QD colloid must be removed to accurately quantify the Mn²⁺ incorporation. Since the surface-adsorbed Mn²⁺ is not incorporated into the crystal lattices, we employed a chemical purification method by oxidizing Mn²⁺ using H₂O₂ and removing the generated MnO _x using hydrobromic acid (Details in the Experimental Section). Figure a shows the absorption and PL spectra of a Mn²⁺-doped QD sample before chemical purification, after H₂O₂ oxidation, and after HBr treatment. No noticeable spectral shifts or Mn²⁺ PL intensity changes are found, indicating that the surface-adsorbed Mn²⁺ ions are not emissive, and the lattice incorporated Mn²⁺ ions are not oxidized. However, after H₂O₂ treatment, the QD solution becomes darker in color (Figure b) due to the generation of MnO _x , which tends to be detached from the QD surface. HBr aqueous solution was then added to the QD solution to reduce MnO _x and extract the produced Mn²⁺ into the aqueous phase. After cleaning, the QD solution returns to its light-yellow color. It is worth noting that extensive iterations of H₂O₂/HBr purification will decrease the Mn²⁺ emission intensity and compromise the QD colloidal stability (Figure S15). After chemical purification, the QDs were purified again using GPC to remove any possible free-standing Mn²⁺.

3.

(a) Absorption and PL spectra of Mn²⁺-doped QD and surface purification with H₂O₂ and HBr. The intensity of the PL spectra in (a) is normalized with respect to the samples’ absorbance values at 385 nm. (b) Schematic diagrams illustrating Mn²⁺-doped QDs before, during the oxidation of Mn²⁺, and after the removal of surface-adsorbed Mn²⁺ ions, accompanied by photographs taken in daylight (left column) and under UV light (right column). (c) XRD patterns of the Mn²⁺-doped (red curve) and undoped (blue curve) CsPbBr₃ QDs. A noticeable shift of the (200) XRD peak to higher 2θ values results from lattice contraction as Mn²⁺ ion concentration increases, expected from substituting larger Pb²⁺ ions with smaller Mn²⁺ ions. (d) EPR spectra of Mn²⁺-doped CsPbBr₃ QDs before and after GPC-chemical-GPC purifications at room temperature.

Electron paramagnetic resonance (EPR) spectroscopy was also employed to investigate the different chemical environments of Mn²⁺ ions in the Mn²⁺-doped QD colloids (Figure d). The EPR spectrum of Mn²⁺-doped QD colloids after only antisolvent purifications exhibits strong EPR signal with a full width at half-maximum (fwhm) of ∼235 G (Figure d, red curve). No hyperfine structure was observed. Based on the elemental analyses above, the EPR response of Mn²⁺-doped QD samples before purification is likely dominated by the surface-adsorbed Mn²⁺. In fact, a very similar EPR spectrum is obtained from a Mn-oleate solution (Figure S16), also aligning with a previous report. Interestingly, after GPC-chemical-GPC purification, the EPR signal of the Mn²⁺-doped QD colloids almost vanished (Figure d, green curve). The absence or weakness of EPR responses from lattice-incorporated Mn²⁺ in Mn²⁺-doped CsPbBr₃ NCs was attributed to strong antiferromagnetic coupling in linear Mn–Br–Mn bonds. Given the high lattice incorporated Mn²⁺ density in our Mn²⁺-doped QDs, it is reasonable that the average distance between Mn²⁺ ions is very small, leading to very broad or nearly nondetectable EPR signals. In lightly doped QDs where dopants are more separated, weak and broad EPR responses with a fwhm of ∼600 G were found after decoupling the sharp EPR signals of surface-adsorbed Mn²⁺ from the overall EPR responses (this sample is not chemically purified due to the low concentration of surface-adsorbed Mn²⁺, which makes it unstable in harsh chemical environments, Figure S17). In addition, we note that similar broad EPR signals were reported for 0.5% and 3% Mn-doped CsPbI₃ QD samples, which were also attributed to substantial antiferromagnetic coupling of the Mn²⁺ ions.

To test the effectiveness of physiosorbed Mn²⁺ removal through the GPC-chemical-GPC purification, QD samples synthesized using the highest Mn-precursor loading ratio were used for low-temperature EPR analysis (Figure S18). After the GPC-chemical-GPC purification, QDs exhibit continuously undetectable EPR responses even at low temperatures (120 K). In contrast, similar QDs, only after antisolvent precipitation-resuspension cycles, show strong EPR signals at both room and low temperatures (Figure S18). It has also been reported that reasonably solvated Mn²⁺ species should show readily detectable EPR signals at low temperatures. The EPR results strongly suggest the thorough removal of surface-adsorbed Mn²⁺ species. This is also corroborated by ICP-MS analysis of GPC-chemical-GPC purified QDs, which shows a significantly decreased Mn/Pb atomic ratio of less than 1 compared to 40–45 before the GPC-chemical-GPC purification. The Cs/Pb ratio remained almost unchanged (0.58), indicating that the structure of the QDs remains nearly intact after purification. Furthermore, we have quantified the spin concentration in the QD samples before GPC-chemical-GPC purification (Figure S19). The resultant 2.9 × 10²⁰ spins/mL corresponded to a Mn²⁺ concentration that matches very well with the concentration of removed Mn²⁺ in this sample determined by ICP-MS (details in Supporting Information Note 3). These results corroborated the conclusion that almost all spins in the EPR spectrum came from surface physiosorbed Mn²⁺, which were nearly all removed by the GPC-chemical-GPC purification.

Knowing that the surface physiosorbed Mn²⁺ can be fully removed, a correlation between Mn²⁺ PLQY and doping concentration in Mn²⁺-doped QDs was established by performing ICP-MS analyses on a series of Mn²⁺-doped QD samples purified by the GPC-chemical-GPC cycles (Figure S20). The doping/alloying concentrations of QDs, assuming only Pb²⁺ ions were replaced by Mn²⁺, range from ∼6.5% to ∼44%. The Mn²⁺ PLQY increases sublinearly with the doping concentration, implying Mn²⁺ dopants are not optically equivalent when the doping/alloying concentration is high. The efficient incorporation of Mn²⁺ in CsPbBr₃ QDs was also confirmed using X-ray diffraction (XRD) measurements. As shown in Figure c, the (200) reflection from the XRD pattern of Mn²⁺-doped QDs experienced a 0.60° shift to higher angles due to the lattice constraint induced by replacing Pb²⁺ with smaller Mn²⁺. According to Vegard’s law (Supporting Information Note 4), the Mn²⁺ doping concentration is estimated to be 19%, falling into the range of the estimated doping concentration from the ICP-MS analysis.

Surface-adsorbed Mn²⁺ plays an important role in the luminescence stability of QDs. Studies show that lattice-incorporated Mn²⁺ tends to diffuse to the surface and leave the crystal. The PL spectra of an Mn²⁺-doped QD colloid (Figure a) show virtually no intensity drops after five iterations of precipitation-resuspension purification cycles using methyl acetate as the antisolvent. In comparison, an undoped QD colloid experiences a PLQY efficiency drop from ∼80% to <20% after two iterations of purification (Figure S21a). Additionally, the Mn²⁺-doped QD showed no noticeable PLQY decrease over 120 days of storage (Figure S21b). The outstanding Mn²⁺ PL stability can be attributed to the protection of surface Mn²⁺ adsorption, which suppresses the diffusion loss of doped Mn²⁺. Indeed, after GPC-chemical-GPC purification, Mn²⁺-doped QDs exhibit reduced but adequate Mn²⁺ PL stability (Figure S22a and S22b). To better reveal the function of surface-adsorbed Mn²⁺ and the stability of lattice-incorporated Mn²⁺ ions, we have studied the thermal stability of Mn²⁺ PL emission using QDs before and after GPC-chemical-GPC purifications. Specifically, the PL spectra of QD colloids were monitored over time at 80 °C (Figure S22c and S22d). A QD solution with surface-adsorbed Mn²⁺ exhibits an ∼20% Mn²⁺ PL intensity drop at 600 nm, whereas purified QD experienced a ∼50% Mn²⁺ PL intensity drop. While the PL drop can be partially attributed to heat-induced nonradiative processes that compete with exciton-to-dopant ET, the additional PL loss in the purified sample can be explained by the loss of Mn²⁺, presumably located in the surface lattices (or close to) of the QDs. Interestingly, the Mn²⁺ PL intensity drop of purified QDs became very slow after 5 min, indicating that some Mn²⁺ ions incorporated in QD lattices are more stable than others. The two categories of Mn²⁺ dopants coincide with the two different types of lattices in QDs: surface lattices with Cs deficiency and center/intermediate lattices with normal stoichiometry. Further thoughts on the potential correlations between Mn²⁺ doping locations and their chemical environments are given in the Discussion section.

4.

(a) PL spectra of Mn²⁺-doped QDs after 1×, 2×, and 5× antisolvent precipitation-dissolution iterations using methyl acetate as the antisolvent. (b) Absorption (dotted red curve) and PLE spectra of Mn²⁺-doped QD were monitored at 582 nm (green curve) and 473 nm (blue curve), respectively. (c) The decay curve of exciton emission for Mn²⁺-doped CsPbBr₃ QDs. (d) Phosphorescence decay curves recorded at 600 nm show the Mn²⁺ emission from Mn²⁺-doped CsPbBr₃ QDs (Mn²⁺ PLQY ∼75%). The solid lines in (c) and (d) are fits using the biexponential decay functions. (e) 2D contour plot of TA spectra of Mn²⁺-doped CsPbBr₃ QDs with pump intensities of ∼18.8 μJ/cm². (f) Bleach recovery dynamics in TA spectra of undoped (blue, monitored at 470 nm) and Mn²⁺-doped (orange, monitored at 450 nm) CsPbBr₃ QDs.

Photoluminescence excitation (PLE) spectra obtained at two different wavelengths (473 and 582 nm) from the exciton and Mn²⁺ PL overlap with the absorption spectrum of Mn²⁺-doped QDs, suggesting all Mn²⁺ emission bands are sourced from excitons in QDs (Figure b). To understand the recombination dynamics of excitons and Mn²⁺ dopants in CsPbBr₃ QDs, we performed time-resolved PL intensity measurements. The PL decay trace of the exciton emission can be fitted by a biexponential decay function with a ∼370 ps fast component (convoluted with the instrument response function) and a 5.9 ns slow component (Figure c). Given the high PLQY of Mn²⁺-doped QDs, we attribute the fast decay to energy transfer (ET) or possible electron transfer, and the slower decay to exciton radiative recombination from a small fraction of undoped QDs or QDs with insufficient dopant incorporation in the ensemble. The temporal evolution of the Mn²⁺ emission intensity is plotted in Figure d. Considering the two categories of Mn²⁺ in lattices near the surface and in the core, the Mn²⁺ PL decay curve was fit by a biexponential decay function with time constants of 188 and 386 μs (the fitting parameters and a single exponential fit are provided in Figure S23 and Table S3). Such a biexponential PL decay is more evident in QDs with lower doping levels (Figure S24). The two decay components are tentatively attributed to Mn²⁺ dopants in two regions with different degrees of interdopant interaction. In comparison, the Mn²⁺ emission lifetime can be as long as ∼2 ms and 300–400 μs when the dopants are diluted in methylammonium (MA) lead bromide single crystals and CsPbBr₃ NPLs, respectively. ,, Given that the heavy atom effect should be similar in these two crystals, the shortened lifetime can be attributed to the Mn–Mn couplings. Indeed, reducing the Mn²⁺ doping level leads to an overall slower Mn²⁺ PL decay (Figure S24). However, two decay components can still be observed, indicating that the two possible categories of Mn²⁺ local environments are likely inherited from the doping mechanism.

Although reported for Mn²⁺-doped CsPbCl₃ NCs, intrinsic ET rates remain elusive in 0D Mn²⁺-doped CsPbBr₃ QDs. TA spectroscopy was used to determine the ET rate in Mn²⁺-doped QDs. Figure e and f show the TA spectra and the bleach recovery dynamic traces monitored at the peak position for undoped and Mn²⁺-doped CsPbBr₃ QDs, respectively. Both samples exhibit a PLQY greater than 75%, indicating that the exciton trapping process has a minimal impact on the bleach recovery dynamics. The fast bleach recovery in Mn²⁺-doped QDs is therefore attributed to the exciton-to-dopant ET. It is worth noting that possible exciton-to-dopant charge transfer (CT) is not ruled out in this study. CT from excitons to Mn²⁺ has been discovered in Mn-doped lead halide perovskites. ,, We will, however, focus our discussions on ET since a recent study has revealed that CT is more preferred in lead chloride perovskites. The apparent ET rate is then extracted using reported methods to obtain a time constant of ∼94 ps. Unlike large NCs and 2D NPLs with uncertain degrees of exciton Mn²⁺ wave function overlapping, all lattice-incorporated Mn²⁺ are presumably able to couple with the exciton in these strongly confined QDs. From the estimated doping concentration of the sample (103–188 Mn²⁺ ions per QD, considering uncertainties in PLQY measurements, see Figure S20), the intrinsic exciton-to-Mn ET rate in Mn²⁺-doped CsPbBr₃ is calculated to be 0.06–0.1 ns^–1 per Mn²⁺. This rate, even enhanced by strong quantum confinement, is 30–50 times slower than that found in Mn²⁺-doped CsPbCl₃ and >100 times slower than that in Mn²⁺-doped CdS and CdSe QDs. ,,

The relatively slow ET rate in Mn²⁺-doped CsPbBr₃ QDs is unlikely to be caused by Mn-to-exciton back ET, given the large difference between the exciton and Mn²⁺ PL energies (∼630 meV). Although the ET rate can be influenced by the high ionicity of Mn²⁺ in perovskite lattices and the lower exciton energy, the significantly slower ET rate in Mn²⁺-doped CsPbBr₃ QDs compared to that in Mn²⁺-doped CsPbCl₃ QDs still suggests that the Mn-exciton exchange interaction is relatively weak in Mn²⁺-doped CsPbBr₃ QDs. A previous study also indicates that the Mn-exciton exchange coupling in Mn²⁺-doped CsPbI₃ QDs is unexpectedly weak. Several factors can contribute to the slow average ET rate in CsPbBr₃ QDs. First, the potential Mn²⁺ dopant clustering can also introduce MnBr₄ ^2–-rich domains inside the QD, promoting exciton localizations and decreasing exciton-Mn wave function overlapping. Second, the recently reported , lattice periodicity-breaking effect in Mn²⁺-doped CsPbBr₃ QDs can also localize excitons to nondopant sites. Last but not least, Mn²⁺ dopants experience different lattice stoichiometries, and they may not share the same exciton-dopant interaction. To provide some insights on this, the ET rate is also determined for Mn²⁺-doped CsPbBr₃ QDs exhibiting ∼53% and ∼7% Mn²⁺ PLQYs (Figure S25). Interestingly, a faster per Mn²⁺ ET rate is found in the lightly doped sample. This observation is qualitatively consistent with the observation that the Mn²⁺ PLQY increases sublinearly with doping concentration (Figure S20), which suggests that Mn²⁺ in different lattice environments may not exhibit the same ET kinetics. (More details are provided in Supporting Information Note 5.)

Discussion

The doping of QDs is inherently hindered by the “self-purification” mechanism, whereby impurities are repelled toward the surface during nanocrystal growth. Self-purification makes smaller QDs even harder to dope, given that the impurity formation energy increases with increasing quantum confinement. In the case of perovskites, highly ionic lattices lead to large ion migration mobilities. , This will accelerate the exclusion of Mn²⁺ dopant in perovskites. It has been well-established that the doping efficiency of QDs highly depends on the surface adsorption of dopant ions. To incorporate cationic substitutional impurities, an anion-rich host will facilitate surface adsorption of cationic impurities for enhanced doping efficiency. While demonstrated in conventional II–VI QDs, such a surface-adsorption facilitated doping process has not been well understood in perovskite QDs, presumably due to their dynamic surfaces that are unlikely to stabilize surface-adsorbed ions.

We propose a doping mechanism considering the surface adsorption of Mn²⁺ and stoichiometry deviations in our Mn²⁺-doped QDs. As illustrated in Figure , the bromide-rich surface will attract cations near the surface at the early stage of QD growth. Previous studies show that Pb²⁺ tends to form anionic complexes such as PbBr₃ ^– and PbBr₄ ^2–. , Therefore, Mn²⁺ and Cs⁺ are likely the main cationic species in the reaction mixture. When the divalent Mn²⁺ ions are readily available in large quantities (Mn²⁺/Cs⁺ ∼3.6, molar ratio), their surface adsorption is electronically favored, and the Mn²⁺ incorporation becomes efficient. In contrast, monovalent Cs⁺ surface adsorption is not favored. Therefore, further growth of the QD will result in Cs-deficient lattices close to the surface of QDs, as evidenced by ¹³³Cs NMR spectroscopy. This Cs deficiency will subsequently encourage the surface physisorption of Mn²⁺ for charge neutrality. This can also explain why our QDs have firmly adsorbed Mn²⁺ ions that are >10-fold equivalent to the Pb²⁺ ions that constitute the QDs.

5.

Diagram showing the suggested growth mechanism of Mn²⁺-doped CsPbBr₃ QDs. Schemes are not in-scale.

To provide some experimental insights into the proposed doping mechanism, we have systematically investigated the effect of changes in synthesis conditions on the structural and optical properties of the obtained Mn²⁺-doped QDs (Table S1). First, the Mn­(Ac)₂·4H₂O and HBr reaction is crucial for achieving efficient doping. Reducing the amount of Mn­(Ac)₂·4H₂O or withdrawing the 230 °C heating process (Figure S26) will decrease the Mn²⁺ doping efficiency. This is consistent with the surface adsorption-driven mechanism of Mn²⁺ incorporation. Second, reducing the bromide concentration (HBr) will lead to low Mn²⁺ incorporation, which indicates the necessity of bromide-rich conditions. Third, increasing the Cs precursor concentration will not significantly compromise the Mn²⁺ doping and the degree of Cs deficiency in the Mn²⁺-doped QDs. This is expected in the proposed electrostatic surface adsorption model since Mn²⁺ possesses a higher charge density than Cs⁺. Additionally, reducing the Cs precursor concentration will only decrease the reaction yield of the synthesis but not the Mn²⁺ incorporation. This insensitivity to the amount of Cs precursors supports the Mn²⁺ surface adsorption model. Finally, the QD growth can be suppressed by loading additional Mn­(Ac)₂·4H₂O/HBr during synthesis. This allows for the synthesis of Mn²⁺-doped QDs with a size of only 3 nm (±0.3 nm) (Figure S27). This strongly suggests the efficient surface Mn²⁺ adsorption, which could cover the surface and suppress the QD growth if additional Mn²⁺ ions are introduced.

The Mn²⁺ surface adsorption model suggests that Mn²⁺ ions incorporated into lattices near the QD surface may experience a different chemical environment. Due to the lack of Cs⁺, increased lattice distortion is expected, which could affect the Mn coordination environment. This hypothesis can explain the above observation that some Mn²⁺ ions are less thermally stable than others, as well as the revelation that there are two different Mn²⁺ PL decay time constants. Given that the nonstoichiometric surface lattices can offer additional flexibility in Mn²⁺ incorporation, it should be noted that the possibility of Mn²⁺ doping through occupying Cs⁺ sites or even interstitial doping cannot be ruled out. In fact, in Figure c, the (110) XRD peak of doped QDs broadens and shifts toward smaller angles compared to that of undoped QDs. This suggests crystal structural changes, such as increased lattice inhomogeneity and lattice expansion induced by possible interstitial dopants. , It is also worth noting that the interstitially doped Mn²⁺ can also exhibit the typical d-d PL emission. The potential Cs⁺-substitutional or interstitial Mn²⁺ doping can lead to a higher apparent dopant concentration estimated by elemental analysis. Finally, this doping strategy can also be applied to CsPbCl₃ QDs. Although successful Mn²⁺ doping has been demonstrated in CsPbCl₃ QDs, our method can dope CsPbCl₃ QDs more efficiently, manifested as a significantly red-shifted Mn²⁺ PL band (from ∼609 to 650 nm) due to strong Mn–Mn coupling (Figure S28). In principle, the nonstoichiometric perovskite QDs can lift the barrier of the highly dynamic surface of perovskite QDs for a large variety of impurity doping.

Conclusion

In conclusion, we have demonstrated that size-confined CsPbBr₃ QDs can be efficiently doped with Mn²⁺. The Mn²⁺-doped QDs exhibit 90 ± 10% Mn²⁺ emission PLQY and extraordinary PL stability. These doped QDs are nonstoichiometric as they are Cs-deficient. Detailed structural characterization shows that the surface of the doped QDs is covered with a large concentration of Mn²⁺ that cannot be removed using traditional purification methods such as antisolvent precipitation and GPC column. A chemical redox reaction-based purification is developed to thoroughly remove the surface-adsorbed Mn²⁺, enabling the detailed quantification of lattice-incorporated Mn²⁺ in CsPbBr₃ QDs. Using QDs with simultaneously high PLQY and quantum confinement, the exciton-to-dopant energy transfer rate was measured for the first time in Mn²⁺-doped CsPbBr₃ QDs. The ET rate of ∼0.06 ns^–1 suggests an intrinsic suppressed exchange interaction between the excitons and Mn²⁺ in CsPbBr₃ compared to CsPbCl₃. The nonstoichiometric QD facilitated surface adsorption promises facile and efficient doping and alloying cationic impurities into traditionally “almost undopable” CsPbBr₃ QDs.

Experimental Section

Materials

The following chemicals were used as received: cesium carbonate (Cs₂CO₃, puratronic, 99.994% metals basis, Alfa Aesar), lead­(II) bromide (PbBr₂, puratronic, 99.999% metals basis, Alfa Aesar), manganese acetate tetrahydrate (Mn­(CH₃COO)₂·4H₂O, ACROS Organics), oleylamine (OAm, technical grade, 70%, Sigma-Aldrich), oleic acid (OA, technical grade, 90%, Sigma-Aldrich), 1-octadecene (ODE, technical grade, 90%, Sigma-Aldrich), hydrobromic acid (HBr, 48 wt % in water, Acros), hydrogen peroxide solutions (H₂O₂, 30 wt., ACS grade, Sigma-Aldrich), nitric acid (HNO₃, 67–70% w/w, VWR Chemicals), rhodamine 6G perchlorate (99%, Aldrich Chemistry), acetone (certified ACS, Fisher), hexanes (HPLC grade, Millipore), toluene (ACS grade, Fisher), methyl acetate (ReagentPlus, 99%, Sigma-Aldrich) acetonitrile (C₂H₃N, >99.9%, Sigma-Aldrich) and tetrabutylammonium hexafluorophosphate (TBAPF₆, Sigma-Aldrich). Bio-Beads S-X1 GPC medium was sourced from Bio-Rad Laboratories.

Cs-Precursor Preparation

The Cs-oleate precursor was prepared by dissolving 300 mg of Cs₂CO₃ in a mixture of 1.2 mL of OA and 3.2 mL of ODE in a 50 mL three-necked round-bottom flask. This mixture was degassed under a vacuum at room temperature for 3–5 min while stirring vigorously. The mixture was then heated to 130 °C under vacuum. Following this, the flask was filled with nitrogen and cooled to 120 °C for subsequent use.

Mn²⁺-Doped CsPbBr₃ QD Synthesis

60 mg of PbBr₂ (0.16 mmol) and 255 mg of Mn (CH₃COO)₂·4H₂O (Mn­(Ac)₂·4H₂O, 1.04 mmol) were added in a three-neck round-bottom flask, followed with 1 mL of OA, 1 mL of OAm, 5 mL of ODE, and 0.5 mL of HBr. The mixture was heated at 150 °C under vacuum for 40 min. The flask was then filled with nitrogen, and an additional 1 mL of dried OA and 1 mL of dried OAm were injected to solubilize the unreacted solids. To obtain dried OA and OAm, both ligands were dried under vacuum in separate flasks at 150 °C for 15 min. The solution was next heated to 200 °C for 40 min, after which the temperature was increased to 230 °C and held for 5–10 min until stabilized. Then, the reaction mixture was allowed to decrease to 173 °C, and 0.7 mL of the Cs-precursor solution (containing 0.3 mmol CsOA) was injected. Following a short reaction time of ∼5 s, the reaction was rapidly quenched with an ice bath. The crude solution was centrifuged at 7800 rpm for 7 min, and the precipitate was discarded. The QDs were precipitated from supernatant by adding acetone in a 4:1 volume ratio, followed by centrifugation at 7800 rpm for 7 min. The supernatant was discarded, and the precipitate was then resuspended in toluene. QDs with various Mn²⁺ PLQYs can be achieved by varying the amount of Mn­(CH₃COO)₂·4H₂O and injection temperature; details can be found in Table S1. Undoped QDs were prepared by adopting a previously reported method. For synthesizing QDs with Mn²⁺ PLQY above ∼50%, the doping could be tuned by simply changing the amount of Mn­(Ac)₂·4H₂O/HBr precursors (up to 255 mg and 0.5 mL, respectively) without adjusting other conditions. For synthesizing QDs with lower Mn²⁺ PLQYs, the injection temperature was increased (up to 200 °C), and the amount of Mn­(Ac)₂·4H₂O/HBr precursors was decreased.

Antisolvent Purification

Mn²⁺-doped QDs colloid was then purified by adding methyl acetate in a 3:1 volume ratio and centrifuged at 7800 rpm for 7 min to reprecipitate QDs. The collected precipitate was redispersed in hexanes or toluene for storage and further characterizations. QDs with various Mn²⁺ PLQYs can be achieved by varying the amount of Mn­(CH₃COO)₂·4H₂O and injection temperature; details can be found in Table S1. Undoped QDs were prepared by adopting a previously reported method.

GPC Purification of Mn²⁺-Doped QDs

Mn²⁺-doped QDs were purified by GPC using the method reported by B. Greytak et al. Briefly, 4–5 g of Bio-Beads were washed three times and soaked overnight using toluene before use. Next, ∼5 mL of toluene was added to a glass column, which was carefully blocked with glass wool to prevent any bead leakage. The soaked Bio-Beads were then transferred to the column to reach a bed height of 10 cm. Once the column was fully packed, it was thoroughly rinsed with toluene until no free polystyrene was detected in the eluent, as confirmed by UV–vis absorption and fluorescence. Then, 1 mL of QD colloid was carefully loaded into the GPC column. The collected QDs were loaded into another GPC column after chemical purification for further purification iterations. The eluted QDs were collected and stored for elemental analysis and spectroscopic measurements.

Chemical Purification

20 μL of H₂O₂ solution (30 wt %) was added to the Mn²⁺-doped CsPbBr₃ QD toluene solution. The solutions were then shaken to initiate Mn²⁺ oxidation. The Mn²⁺-doped CsPbBr₃ QD solution will turn a dark brown color and start bubbling. The organic phase was collected, 20 μL of HBr was added, and the container was shaken for 15 s. After the organic phase became clear, the solution was centrifuged to accelerate phase separation, and the organic phase (QD colloid) was collected for further analysis. It is worth noting that the Mn²⁺ PL stability can vary depending on the relative amount of QDs and H₂O₂ solutions.

Elemental Analyses

The composition of QDs was determined using ICP-MS (Agilent 7850). The QD solutions using various purification methods were digested in concentrated nitric acid for ICP-MS analyses. The Br content in the halide perovskite was determined by STEM and EDS using Titan Themis 300 S/TEM. QD samples for XPS analysis were prepared by drop-casting the purified QD solution onto clean silicon substrates. An Omicron DAR 400 equipped with a CN10 charge neutralizer was used to collect the XPS data.

Structural Characterizations

Electron microscope images of all nanocrystals were obtained using a Titan Themis 300 TEM microscope operated at 300 kV. DLS analysis of CsPbBr₃ NCs and Mn²⁺-doped CsPbBr₃ QDs suspended in hexanes was recorded using a pUNk v1.0.0.3 DLS system equipped with a red laser (660 nm). The light scattering intensity was measured as a photon count rate in units of kilocounts per second (kcps). To account for varying scattered light intensities from nanoparticles of different sizes, the instrument automatically adjusted the incident laser beam power to achieve an optimal photon count rate. A built-in attenuator was used to set the laser power to specific levels as required. ζ-potential measurements were performed using the Anton Paar Litesizer 500. A suspension of Mn²⁺-doped or undoped CsPbBr₃ nanocrystals (NCs) in toluene was introduced into a Univette equipped with a quartz cuvette containing 700 μL acetonitrile and 5–20 μL of 10 mM tetrabutylammonium hexafluorophosphate (TBAPF₆) in acetonitrile as the supporting electrolyte. XRD patterns for CsPbBr₃ NCs and Mn²⁺-doped CsPbBr₃ NCs were collected on a Rigaku SmartLab X-ray diffractometer with a Cu–Kα source. The samples were prepared by drop-casting purified NCs onto a zero-background sample holder to ensure precise detection of XRD signals. Measurements were performed at room temperature with scans recorded in the 10–40° (2θ) range. Data analysis was conducted using Rigaku’s PDXL2 software package.

NMR Experiments

The ¹H and ¹³³Cs NMR spectra were acquired at a probe temperature of 20 °C on a 500 MHz JEOL ECZL NMR instrument (operating at 500.16 MHz for proton and 65.60 MHz for cesium) equipped with a 5 mm gradient inverse broadband Royal probe. The 1H NMR spectrum was recorded with single pulse excitation, a spectral width of 7508 Hz, and a pulse width of 3.95 μs (45° flip angle). A scan number of 8 transients and a repetition rate of 4.19 s (3.19 s for the acquisition time and 1 s for the relaxation delay) were used. The raw data (FIDs) for protons were processed without any apodization function prior to Fourier transformation. The ¹³³Cs NMR spectrum was recorded with a spectral width of 19723.9 Hz with 40K data points and a pulse width of 7.02 μs (45° flip angle). A scan number of 4096 transients and a repetition rate of 3.66 s (1.66 s for the acquisition time and 2 s for the relaxation delay) were used. Exponential weighting with a line-broadening function of 25 Hz was applied before the Fourier transformation. ¹³³Cs is the only naturally occurring stable isotope of cesium with a nonzero nuclear spin (I) of 7/2. Since it has a very small quadrupole moment (−3 × 10^–3) (eQ) × 10^–28 C m^–2, a high natural abundance (100%), and a low magnetogyric ratio (3.5277 × 10⁷ rad T^–1 s^–1) and has an excellent receptivity relative to the value of 269 for ¹³C. ¹H and ¹³³C NMR chemical shifts for protons are reported in parts per million (ppm) and are referenced to residual protons in the solvent peak CHCl₃ at 7.26 ppm. ¹³³Cs NMR chemical shifts are referenced using the cesium peak from Cs₂CO₃ at 0.0 ppm as the external standard.

EPR Experiments

The EPR spectra were acquired at room temperature using a commercial Bruker Biospin EMX spectrometer operating at a frequency of 9.51 GHz. The Mn²⁺-doped CsPbBr₃ NCs were suspended in an EPR-inactive hexane or toluene solvent, and a few milliliters of the solution were put in 4 mm OD low-loss quartz tubes. The tubes were then inserted in the middle of the cylindrical microwave cavity for the EPR measurements. Typical microwave powers of 1–2 mW with 3 G modulation amplitude and 100 kHz field modulation were employed for these experiments. The Zeeman splitting g-values were calibrated using a DPPH (2,2-diphenyl-1-picrylhydrazyl) standard. The magnetic field resonance values and FHWM line widths were determined from fits to the spectra using Lorentzian line shapes (i.e., the first derivative of the Lorentzian-shaped microwave absorption curves).

Low-temperature EPR spectra were recorded with a Bruker/ColdEdge ESR900 WaveGuide cryostat, operating at a frequency of 9.35 GHz. This setup included a liquid helium flow system, allowing temperature control between 3 and 300 K. The Mn²⁺-doped CsPbBr₃ NCs were dissolved in an EPR-inactive toluene solvent, with several milliliters of the mixture placed into 4 mm OD thin-wall Suprasil EPR tubes. The experiments were conducted using standard microwave power levels ranging from 0.2 to 2 mW, featuring a modulation amplitude of 3 G and a field modulation frequency of 100 kHz. Calibration of the Zeeman splitting g-values was performed using BDPA (α,γ-bisdiphenylene-β-phenylallyl) standard.

Optical Spectroscopy Measurements

An Ocean Insight Maya 2000 spectrometer was used to record absorption and PL spectra. A 385 nm LED was used to excite the samples for PL measurements. The PLQY of QDs was measured with respect to rhodamine 6G standard (PLQY 95%) as well as a reference QD sample (Mn²⁺-doped CdS/ZnS, PLQY 60.4% (Figure S2b), both excited with OBIS LX SF 405 nm continuous-wave laser (Coherent). QDs and Rhodamine 6G were suspended/dissolved in hexane and ethanol, respectively, and the difference in solvent refractive indices was accounted for when calculating the PLQY. Absolute PLQY was measured using an integrating sphere (Labsphere) coupled with a computer-controlled spectrometer (Ocean Optics QE Pro). The light sources used were a 405 nm laser diode (LDM405, Thorlabs, 4.0 mW) and a 415 nm fiber-coupled LED excitation source (M415F3, Thorlabs, 14.4 mW). PLE measurements were performed on colloidally suspended, highly Mn²⁺-doped CsPbBr₃ nanocrystals in hexanes, using a quartz cuvette as the sample holder. The measurements were conducted with a HORIBA Jobin Yvon Fluorolog-3 spectrofluorometer equipped with a xenon lamp. Data were collected using the two-curve method over a wavelength range of 250–750 nm.

Pump–probe TA measurements were conducted at room temperature using a HELIOS TA spectrometer from Ultrafast Systems. The 405 nm, 2 kHz output from an Apollo-Y Optical Parametric Amplifier (OPA) served as the pump beam, with a fluence of ∼18.8 μJ/cm². The probe beam, with a white-light spectrum, was generated in a sapphire crystal within the HELIOS spectrometer by focusing the 1064 nm, 2 kHz output from a Hyperion femtosecond amplified laser. Both the pump and probe pulses had a duration of approximately 350 fs.

Time-Resolved Photoluminescence (TRPL) Spectroscopy

The exciton PL lifetimes were measured using a time-correlated single-photon counting technique. The colloidal sample in a cuvette was excited with a 405 nm pulsed laser (Picoquant LDH-D-C-405) driven by a Picoquant Sepia PDL828 module at a 5 MHz repetition rate. The PL emission from the Mn²⁺-doped CsPbBr₃ NCs was collected with an achromatic lens, sent through a set of long-pass (425 nm, Edmund Optics #84-742) and bandpass (466 ± 20 nm, Edmund Optics #86-352) filters to remove the scattered laser and the Mn²⁺ emission, and detected using a single-photon avalanche photodiode (Hamamatsu C11202-100). The photon arrival time was recorded using a Picoquant HydraHarp 400 correlator. The Mn²⁺ PL lifetimes were measured by exciting the sample using the 375 nm, 80 MHz output of a pulsed laser (Beckl & Hickl). The average excitation power was 30 μW, focused to a beam diameter of around 200 μm. PL was collected in free-space, backscattering geometry, and spectrally resolved with an HRS-300 Acton Spectrometer coupled to a Teledyne PIXIS400 CCD camera. The temporal resolution of the PL decay was achieved using a Time-Correlated Single Photon Counting (TCSPC) system by Becker & Hickl, comprising an IDQ-id100 fast avalanche photodiode and an SPN 130, capable of simultaneously measuring both nanosecond (fluorescence) and millisecond (phosphorescence) decay components. The phosphorescence decay time is measured in a triggered accumulation multichannel scaler (TA-MCS) mode, where the high-frequency laser output is modulated at lower frequencies. For an in-depth description of the electronics behind the TA-MCS mode, please refer to the Beckl & Hickl available online free of charge.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.5c12086.

• Additional optical measurements (UV–vis, PL, PLQY, TRPL, TA), elemental analyses (EDS, ICP-MS, XPS), STEM imaging, NMR, EPR, synthesis conditions, additional photographs of QD samples, and TRPL fitting parameters. (PDF)

The authors declare no competing financial interest.