Improved Stability and Photoluminescence Yield of Mn 2+ -Doped CH 3 NH 3 PbCl 3 Perovskite Nanocrystals

: Organic–inorganic CH 3 NH 3 PbCl 3 perovskite nanocrystals (PNCs) doped with Mn 2+ , CH 3 NH 3 Pb x Mn 1 − x Cl 3 , have been successfully prepared using a reprecipitation method at room temperature. Structural and morphological characterizations reveal that the CH 3 NH 3 Pb x Mn 1 − x Cl 3 PNCs with cubic phase transforms from particles to cubes and increases in size from 16.2 ± 4.4 nm in average diameter to 25.3 ± 7.2 nm in cubic length after the addition of Mn 2+ precursor. The CH 3 NH 3 Pb x Mn 1 − x Cl 3 PNCs exhibit a weak exciton emission at ~405 nm with a low absolute quantum yield (QY) of around 0.4%, but a strong Mn 2+ dopant emission at ~610 nm with a high QY of around 15.2%, resulting from efﬁcient energy transfer from the PNC host to the Mn 2+ dopant via the 4 T 1 → 6 A 1 transition. In addition, the thermal and air stability of CH 3 NH 3 Pb x Mn 1 − x Cl 3 PNCs are improved due to the passivation with (3-aminopropyl) triethoxysilane (APTES), which is important for applications such as light emitting diodes (

In general, hot injection approach has been applied in the preparation of Mn 2+ -doped lead halide PNCs because of the homogeneous products and relatively simple purification process [32,33]. However, the highly strict conditions of this method and poor stability of the resulting PNCs greatly hinder their applications. In contrast, organic-inorganic hybrid CH 3 NH 3 PbCl 3 PNCs can be easily prepared via solution-based reprecipitation approach at room temperature when (3-aminopropyl) triethoxysilane (APTES) is used as a capping ligand [34,35]. Additionally, the SiO 2 shell resulting from the hydrolysis of APTES capping ligand can greatly improve the product yield and stability of CH 3 NH 3 PbCl 3 PNCs towards water and air [34].
In this work, we demonstrate that Mn 2+ can be doped into CH 3 NH 3 PbCl 3 PNCs via a reprecipitation method. By adjusting the doping concentration of Mn 2+ , CH 3 NH 3 Pb x Mn 1−x Cl 3 PNCs with a strong dopant emission located at 610 nm were obtained. Besides the greatly improved overall QY of CH 3 NH 3 Pb x Mn 1−x Cl 3 PNCs (from~0.4% to~15.6%) upon Mn 2+ doping, the thermal and air stability have also been enhanced by the SiO 2 shell formed from hydrolysis of APTES. The combined stability and high PL QY make these PNCs potentially useful for various photonics applications including LEDs.
Mn 2+ -doped CH 3 NH 3 PbCl 3 PNCs: CH 3 NH 3 Cl was prepared according to a reported procedure [18]. The Mn 2+ -doped CH 3 NH 3 PbCl 3 PNCs were synthesized following a method reported previously with some small changes [1]. 1.5 mL precursor solution of PNCs was prepared by dissolving 0.05 mmol CH 3 NH 3 Cl, 0.05 mmol (PbCl 2 + MnCl 2 ), 0.05 mmol APTES or octylamine and 40 µL oleic acid in 1.5 mL DMF solvent and ultrasonicated until the solution became transparent. Then, 10 µL, 25 µL, 50 µL and 100 µL of the precursor solution were injected slowly into 5 mL toluene to form CH 3 NH 3 PbCl 3 PNCs solution. Mn 2+ -doped CH 3 NH 3 PbCl 3 PNCs with different doping concentration of Mn 2+ were prepared by varying the ratio between PbCl 2 and MnCl 2 . The solid samples were obtained by adding ethyl acetate in toluene to precipitate CH 3 NH 3 PbCl 3 PNCs and centrifuged at 4000 rpm subsequently, then re-dispersed in toluene. These steps were repeated three times. At last, the dry samples were collected by placing the samples at 50 • C oven overnight.
Mn 2+ -doped CH 3 NH 3 PbCl 3 PNCs films: The films were prepared by dropping purified solution on microslides and dried at room temperature.

Characterization
Absorption and photoluminescence properties of the samples were collected with circular dichroism spectra (MOS450, Biologic Science Instruments, Seyssinet-Pariset, France) and fluorescence spectra (PTI QM-TM, Photon Technology International, Ontario, Canada), respectively. Absolute QY was recorded on HAMAMATSU C11347 spectrometer (Hamamatsu Photonics Instruments, Hamamatsu City, Japan) by testing samples in toluene. Luminescence lifetime measurements were collected on EDINBURGH INSTRUMENTS (FLS920, Edinburgh Instruments Ltd., Livingston, UK). X-Ray diffraction (XRD, MiniFlex 600, Rigaku Corporation, Tokyo, Japan) analysis was used to obtain the crystalline phase. The scanning angle range was 10-50 • (2θ) with a rate of 3 • /min. Temperature-dependent Powder XRD (Ultima IV, Rigaku Corporation, Tokyo, Japan) were studied to indicate the thermal stability of samples. Transmission electron microscopy (TEM, Tecnai G2 F20, FEI Technologies, Hillsboro, OR, USA) were carried out to obtain the morphology and interlayer spacing of samples at an accelerating voltage of 200 kV. The elemental analysis was conducted on inductively coupled plasma atomic emission spectroscopy (ICP-AES, Agilent 720, Agilent Technologies, Santa Clara, CA, USA).

Structural and Morphological Characterizations
The structure and morphology of the prepared samples were determined by XRD and TEM measurements, respectively. For the nominal doping concentration of Mn 2+ in 0~75 at.% range, the XRD pattern ( Figure 1) shows a series of diffraction peaks that can all be attributed to cubic phase CH 3 NH 3 PbCl 3 perovskite structure (space group: Pm3m), demonstrating that the incorporation of Mn 2+ has little effect on the crystal structure of CH 3 NH 3 PbCl 3 [18]. With the addition of 90 at. % MnCl 2 , some diffraction peaks attributed to CH 3 NH 3 Cl and MnCl 2 precursors could be clearly observed due to the excess amount of precursors. Note that the broad band ranging from 15 • to 38 • is characteristic of amorphous silica resulting from the hydrolysis of APTES, which has been clearly elucidated in our previous work [34]. To further determine the concentration of Mn 2+ dopant, ICP-AES was used, with results summarized in Table S1. In the synthesis of Mn 2+ -doped CH 3 NH 3 PbCl 3 PNCs, it was found that APTES would react with MnCl 2 to form a puce complex at a high concentration of Mn 2+ precursor, as shown in Figure S1. The color of CH 3 NH 3 Pb x Mn 1−x Cl 3 PNCs capped with APTES under room light is deeper than that of CH 3 NH 3 Pb x Mn 1−x Cl 3 PNCs passivated with octylamine owing to the adsorption of Mn 2+ -APTES complex, indicating that the detected concentration in Table S1 are higher than the actual concentration of Mn 2+ in the crystal lattice. Tecnai G2 F20, FEI Technologies, Hillsboro, OR, USA) were carried out to obtain the morphology and interlayer spacing of samples at an accelerating voltage of 200 kV. The elemental analysis was conducted on inductively coupled plasma atomic emission spectroscopy (ICP-AES, Agilent 720, Agilent Technologies, Santa Clara, CA, USA).

Structural and Morphological Characterizations
The structure and morphology of the prepared samples were determined by XRD and TEM measurements, respectively. For the nominal doping concentration of Mn 2+ in 0~75 at.% range, the XRD pattern ( Figure 1) shows a series of diffraction peaks that can all be attributed to cubic phase CH3NH3PbCl3 perovskite structure (space group: Pm3 _ m), demonstrating that the incorporation of Mn 2+ has little effect on the crystal structure of CH3NH3PbCl3 [18]. With the addition of 90 at. % MnCl2, some diffraction peaks attributed to CH3NH3Cl and MnCl2 precursors could be clearly observed due to the excess amount of precursors. Note that the broad band ranging from 15° to 38° is characteristic of amorphous silica resulting from the hydrolysis of APTES, which has been clearly elucidated in our previous work [34]. To further determine the concentration of Mn 2+ dopant, ICP-AES was used, with results summarized in Table S1. In the synthesis of Mn 2+ -doped CH3NH3PbCl3 PNCs, it was found that APTES would react with MnCl2 to form a puce complex at a high concentration of Mn 2+ precursor, as shown in Figure S1. The color of CH3NH3PbxMn1−xCl3 PNCs capped with APTES under room light is deeper than that of CH3NH3PbxMn1−xCl3 PNCs passivated with octylamine owing to the adsorption of Mn 2+ -APTES complex, indicating that the detected concentration in Table S1 are higher than the actual concentration of Mn 2+ in the crystal lattice. To determine the morphology evolution and size distribution, TEM images were recorded, as shown in Figure 2.  To determine the morphology evolution and size distribution, TEM images were recorded, as shown in Figure 2. For undoped CH 3 NH 3 PbCl 3 PNCs (Figure 2a with the increase of Mn 2+ precursor concentration, the lower concentration of APTES will result in large cubic PNCs, which is consistent with the TEM results of this work. High-resolution TEM (HRTEM, Figure 2f) reveals that the lattice space of undoped PNCs is around 0.28 nm, corresponding to the (200) crystal plane of cubic CH 3 NH 3 PbCl 3 . formation of small spherical PNCs due to the large steric hindrance of branched APTES [34]. Likewise, with the increase of Mn 2+ precursor concentration, the lower concentration of APTES will result in large cubic PNCs, which is consistent with the TEM results of this work. High-resolution TEM (HRTEM, Figure 2f) reveals that the lattice space of undoped PNCs is around 0.28 nm, corresponding to the (200) crystal plane of cubic CH3NH3PbCl3.

Optical Properties
Similar to Mn 2+ -doped all inorganic CsPbCl3 PNCs, new emission band may be introduced in Mn 2+ -doped organic-inorganic CH3NH3PbCl3 PNCs, as shown in the PL spectra in Figure 3, where the electronic absorption spectra are also shown. For undoped CH3NH3PbCl3 PNCs, the absorption spectrum exhibits a sharp rise starting around 400 nm and an excitonic peak at ~373 nm. In comparison, the excitonic peak positions of all the Mn 2+ -doped CH3NH3PbCl3 PNCs shift 4-10 nm towards long wavelength owing to a significant increase in average particle size. For the PL spectra, Mn 2+ -doped CH3NH3PbCl3 PNCs feature a broad emission band peaked at 610 nm, attributed to the spin-forbidden 4 T1→ 6 A1 transition of Mn 2+ [31], along with a narrow luminescence band located at ~400 nm, which is assigned to intrinsic excitonic emission of the CH3NH3PbCl3 host. Compared to the Mn 2+ dopant emission (~590 nm) in CsPbCl3 PNCs [28,31], Mn 2+ dopant emission in the CH3NH3PbCl3 system significantly red shifts by ~20 nm, possibly caused by the crystal lattice difference between cubic CH3NH3PbCl3 and cubic/tetragonal CsPbCl3. The lattice parameters of cubic CH3NH3PbCl3 (space group: Pm3 _ m, a = b = c = 5.685 Å) are a little larger than that of cubic CsPbCl3 (a = b = c = 5.605 Å) and tetragonal CsPbCl3 (a = b = 5.590 Å, c = 5.630 Å) [18,36], resulting in the smaller energy difference (ΔO) of Mn 2+ d-orbitals.
In addition, all excitonic emissions of Mn 2+ -doped CH3NH3PbCl3 PNCs show a slight red-shift (~4 nm) with respect to that of undoped CH3NH3PbCl3 PNCs, consistent with red-shifted absorption spectra, attributed to the increase in size of PNCs. Intriguingly, with the increased concentration of

Optical Properties
Similar to Mn 2+ -doped all inorganic CsPbCl 3 PNCs, new emission band may be introduced in Mn 2+ -doped organic-inorganic CH 3 NH 3 PbCl 3 PNCs, as shown in the PL spectra in Figure 3, where the electronic absorption spectra are also shown. For undoped CH 3 NH 3 PbCl 3 PNCs, the absorption spectrum exhibits a sharp rise starting around 400 nm and an excitonic peak at~373 nm. In comparison, the excitonic peak positions of all the Mn 2+ -doped CH 3 NH 3 PbCl 3 PNCs shift 4-10 nm towards long wavelength owing to a significant increase in average particle size. For the PL spectra, Mn 2+ -doped CH 3 NH 3 PbCl 3 PNCs feature a broad emission band peaked at 610 nm, attributed to the spin-forbidden 4 T 1 → 6 A 1 transition of Mn 2+ [31], along with a narrow luminescence band located at~400 nm, which is assigned to intrinsic excitonic emission of the CH 3 NH 3 PbCl 3 host. Compared to the Mn 2+ dopant emission (~590 nm) in CsPbCl 3 PNCs [28,31], Mn 2+ dopant emission in the CH 3 NH 3 PbCl 3 system significantly red shifts by~20 nm, possibly caused by the crystal lattice difference between cubic CH 3 NH 3 PbCl 3 and cubic/tetragonal CsPbCl 3 . The lattice parameters of cubic CH 3 NH 3 PbCl 3 (space group: Pm3m, a = b = c = 5.685 Å) are a little larger than that of cubic CsPbCl 3 (a = b = c = 5.605 Å) and tetragonal CsPbCl 3 (a = b = 5.590 Å, c = 5.630 Å) [18,36], resulting in the smaller energy difference (∆ O ) of Mn 2+ d-orbitals. emission will be quenched upon the incorporation of Mn 2+ . With the addition of a higher Mn 2+ concentration, the transfer of excitonic energy from the host to Mn 2+ dopants becomes more dominant, giving rise to the decrease of excitonic emission intensity and enhancement of dopant emission intensity. Importantly, the overall PL QY was highly improved from 0.4% (undoped) to 15.6% (75 at. % Mn 2+ -doped), indicating efficient excitons-to-Mn 2+ energy transfer, which is desired for light emitting applications. To demonstrate that the Mn 2+ dopant emission does not originate from the Mn 2+ -APTES complex, contrast test was conducted as shown in Figure S3. Without the formation of CH3NH3PbCl3 perovskite, no absorption and emission band can be observed. In the range of 380-700 nm, no emission can be seen for MnCl2 precursor, indicating that the Mn 2+ dopant emission results from the host absorption. In addition, PLE spectra are collected to further illustrate the emission mechanism of Mn 2+ -doped CH3NH3PbCl3 PNCs. As shown in Figure S4, a sharp onset located at ~400 nm can be clearly observed for all Mn 2+ -doped CH3NH3PbCl3 PNCs, consistent with the absorption of undoped CH3NH3PbCl3 PNCs. These results display that the emission band centered at 610 nm originates from the absorption of CH3NH3PbCl3 PNCs, manifesting the energy transfer from host to dopants.
The volume ratio between precursor solution and anti-solvent (toluene) is an important factor in the synthesis of CH3NH3PbCl3 PNCs when adopting the reprecipitation strategy. Figure S5 displays the PL spectrum of Mn 2+ -doped CH3NH3PbCl3 PNCs solution prepared with different In addition, all excitonic emissions of Mn 2+ -doped CH 3 NH 3 PbCl 3 PNCs show a slight red-shift (~4 nm) with respect to that of undoped CH 3 NH 3 PbCl 3 PNCs, consistent with red-shifted absorption spectra, attributed to the increase in size of PNCs. Intriguingly, with the increased concentration of Mn 2+ dopant, the PL intensity of host increases first and then decreases, which is highly unusual. On one hand, the Mn 2+ dopant ions may remove the preexisting structural defects and enhance the excitonic emission of hosts [25,37]. At the low doping concentration, both the intensity of excitionic emission and dopant emission are increased after 25 at. % Mn 2+ doping, attributed to the dominant defect passivation of Mn 2+ and exciton-to-Mn 2+ energy transfer. On the other hand, the excitonic emission will be quenched upon the incorporation of Mn 2+ . With the addition of a higher Mn 2+ concentration, the transfer of excitonic energy from the host to Mn 2+ dopants becomes more dominant, giving rise to the decrease of excitonic emission intensity and enhancement of dopant emission intensity. Importantly, the overall PL QY was highly improved from 0.4% (undoped) to 15.6% (75 at. % Mn 2+ -doped), indicating efficient excitons-to-Mn 2+ energy transfer, which is desired for light emitting applications.
To demonstrate that the Mn 2+ dopant emission does not originate from the Mn 2+ -APTES complex, contrast test was conducted as shown in Figure S3. Without the formation of CH 3 NH 3 PbCl 3 perovskite, no absorption and emission band can be observed. In the range of 380-700 nm, no emission can be seen for MnCl 2 precursor, indicating that the Mn 2+ dopant emission results from the host absorption. In addition, PLE spectra are collected to further illustrate the emission mechanism of Mn 2+ -doped CH 3 NH 3 PbCl 3 PNCs. As shown in Figure S4, a sharp onset located at~400 nm can be clearly observed for all Mn 2+ -doped CH 3 NH 3 PbCl 3 PNCs, consistent with the absorption of undoped CH 3 NH 3 PbCl 3 PNCs. These results display that the emission band centered at 610 nm originates from the absorption of CH 3 NH 3 PbCl 3 PNCs, manifesting the energy transfer from host to dopants.
The volume ratio between precursor solution and anti-solvent (toluene) is an important factor in the synthesis of CH 3 NH 3 PbCl 3 PNCs when adopting the reprecipitation strategy. Figure S5 displays the PL spectrum of Mn 2+ -doped CH 3 NH 3 PbCl 3 PNCs solution prepared with different volume ratios. The Mn 2+ dopant emission greatly increases in the range of 50-200 volume ratio after normalizing to excitonic emission, implying that more Mn 2+ ions are incorporated into the crystal lattice of CH 3 NH 3 PbCl 3 . However, the intensity of dopant emission is diminished when reaching 500 toluene/precursor volume ratio, owing to the poor crystallinity of the host at high Mn 2+ dopant concentration, thereby lower the transfer efficiency of photo-induced excitons between the host and dopants due to the competition of charge transfer to trap states [28].
To investigate the PL decay of CH 3 NH 3 Pb x Mn 1−x Cl 3 PNCs, time-resolved PL spectra are obtained. All the PL decay kinetics were fitted with a single exponential curve, as shown in Figure 4. An expected decrease of excitonic lifetime from 2.40 ns to 2.04 ns can be observed with increasing the Mn 2+ concentration from 0% to 75% (Figure 4a), due to the energy transfer from the host to Mn 2+ dopants. Interestingly, the small change of exciton emission lifetime indicates the insignificant effect of dopant states on the overall exciton decay in the CH 3 NH 3 PbCl 3 PNCs. Since the introduction of Mn 2+ dopants provides a new and efficient decay pathway of the excitons, which leads to the radiative decay of Mn 2+ via the 4 T 1 → 6 A 1 transition, the overall PL QY of the doped PNCs was improved [28]. volume ratios. The Mn 2+ dopant emission greatly increases in the range of 50-200 volume ratio after normalizing to excitonic emission, implying that more Mn 2+ ions are incorporated into the crystal lattice of CH3NH3PbCl3. However, the intensity of dopant emission is diminished when reaching 500 toluene/precursor volume ratio, owing to the poor crystallinity of the host at high Mn 2+ dopant concentration, thereby lower the transfer efficiency of photo-induced excitons between the host and dopants due to the competition of charge transfer to trap states [28].
To investigate the PL decay of CH3NH3PbxMn1−xCl3 PNCs, time-resolved PL spectra are obtained. All the PL decay kinetics were fitted with a single exponential curve, as shown in Figure 4. An expected decrease of excitonic lifetime from 2.40 ns to 2.04 ns can be observed with increasing the Mn 2+ concentration from 0% to 75% (Figure 4a), due to the energy transfer from the host to Mn 2+ dopants. Interestingly, the small change of exciton emission lifetime indicates the insignificant effect of dopant states on the overall exciton decay in the CH3NH3PbCl3 PNCs. Since the introduction of Mn 2+ dopants provides a new and efficient decay pathway of the excitons, which leads to the radiative decay of Mn 2+ via the 4 T1→ 6 A1 transition, the overall PL QY of the doped PNCs was improved [28]. PL decay curve of Mn 2+ dopant emission located at 610 nm was also studied and presented in Figure 4b. A very long lifetime of 1.2 ms was observed resulting from the spin-forbidden nature of Mn 2+ 4 T1→ 6 A1 transition, suggesting the successful incorporation of Mn 2+ [28,38].

Stability Investigation
Although moisture stability of CH3NH3PbCl3 has been greatly improved, their practical applications are still restricted by their low decomposition temperature, which is determined by their low formation energies [39]. A recent study found that the formation energy of Mn 2+ -doped CsPbBr3 is calculated to be ~5% larger than that of pure CsPbBr3 using DFT simulation, suggesting the better thermal stability of CsPbBr3 after Mn 2+ doping with a specific concentration [39]. In this work, the thermal stability of pure CH3NH3PbCl3 and CH3NH3PbxMn1−xCl3 PNCs has also been studied using temperature-dependent XRD, as presented in Figure 5. Without the addition of MnCl2 precursor, CH3NH3PbCl3 PNCs capped with octylamine ( Figure 5a) show poor thermal stability. Some diffraction peaks due to impurities could be observed when the temperature was increased up to 170 °C. CH3NH3PbxMn1−xCl3 PNCs passivated with octylamine were relatively stable under 170 °C, but still to be decomposed at 180 °C (Figure 5b). In contrast, both CH3NH3PbCl3 PNCs and CH3NH3PbxMn1−xCl3 PNCs passivated with APTES show better thermal stability, as shown in Figure 5c,d. The decomposition temperature was significantly improved by 20-30 °C compared to PNCs passivated with octylamine. The improved thermal stability of CH3NH3PbCl3 PNCs PL decay curve of Mn 2+ dopant emission located at 610 nm was also studied and presented in Figure 4b. A very long lifetime of 1.2 ms was observed resulting from the spin-forbidden nature of Mn 2+ 4 T 1 → 6 A 1 transition, suggesting the successful incorporation of Mn 2+ [28,38].

Stability Investigation
Although moisture stability of CH 3 NH 3 PbCl 3 has been greatly improved, their practical applications are still restricted by their low decomposition temperature, which is determined by their low formation energies [39]. A recent study found that the formation energy of Mn 2+ -doped CsPbBr 3 is calculated to be~5% larger than that of pure CsPbBr 3 using DFT simulation, suggesting the better thermal stability of CsPbBr 3 after Mn 2+ doping with a specific concentration [39]. In this work, the thermal stability of pure CH 3 NH 3 PbCl 3 and CH 3 NH 3 Pb x Mn 1−x Cl 3 PNCs has also been studied using temperature-dependent XRD, as presented in Figure 5. Without the addition of MnCl 2 precursor, CH 3 NH 3 PbCl 3 PNCs capped with octylamine ( Figure 5a) show poor thermal stability. Some diffraction peaks due to impurities could be observed when the temperature was increased up to 170 • C. CH 3 NH 3 Pb x Mn 1−x Cl 3 PNCs passivated with octylamine were relatively stable under 170 • C, but still to be decomposed at 180 • C (Figure 5b). In contrast, both CH 3 NH 3 PbCl 3 PNCs and CH 3 NH 3 Pb x Mn 1−x Cl 3 PNCs passivated with APTES show better thermal stability, as shown in Figure 5c,d. The decomposition temperature was significantly improved by 20-30 • C compared to PNCs passivated with octylamine. The improved thermal stability of CH 3 NH 3 PbCl 3 PNCs passivated with APTES may be due to the SiO 2 coating layer, which maintains the morphology and crystal structure of CH 3 NH 3 PbCl 3 as a result of the higher thermal stability of SiO 2 .
Crystals 2018, 8,4 7 of 10 passivated with APTES may be due to the SiO2 coating layer, which maintains the morphology and crystal structure of CH3NH3PbCl3 as a result of the higher thermal stability of SiO2. The air stability of CH3NH3PbxMn1−xCl3 PNCs films was also studied, as shown in Figure S6. The PL intensity CH3NH3PbxMn1−xCl3 PNCs passivated with APTES and octylamine drop ~9% and ~73% after one week, indicating the better air stability of CH3NH3PbxMn1−xCl3 PNCs capped with APTES.

Conclusions
Doping of Mn 2+ into organic-inorganic CH3NH3PbCl3 PNCs has been demonstrated using a versatile reprecipitation approach. The PL QY of the obtained samples are greatly improved after Mn 2+ doping as a result of efficient energy transfer from the host to dopants. Furthermore, both thermal and air stability are enhanced due to the presence of SiO2 on the PNC surface. Importantly, compared to the Mn 2+ dopant emission (~590 nm) in all inorganic CsPbCl3 system, the larger crystal lattice of CH3NH3PbCl3 results in a lower energy splitting of Mn 2+ d-orbitals, thereby emitting at a longer wavelength (~610 nm). In this manner, Mn 2+ dopant emission may be tuned by modulating the crystal lattice through substitution of different components in perovskites.  The air stability of CH 3 NH 3 Pb x Mn 1−x Cl 3 PNCs films was also studied, as shown in Figure S6. The PL intensity CH 3 NH 3 Pb x Mn 1−x Cl 3 PNCs passivated with APTES and octylamine drop~9% and~73% after one week, indicating the better air stability of CH 3 NH 3 Pb x Mn 1−x Cl 3 PNCs capped with APTES.

Conclusions
Doping of Mn 2+ into organic-inorganic CH 3 NH 3 PbCl 3 PNCs has been demonstrated using a versatile reprecipitation approach. The PL QY of the obtained samples are greatly improved after Mn 2+ doping as a result of efficient energy transfer from the host to dopants. Furthermore, both thermal and air stability are enhanced due to the presence of SiO 2 on the PNC surface. Importantly, compared to the Mn 2+ dopant emission (~590 nm) in all inorganic CsPbCl 3 system, the larger crystal lattice of CH 3 NH 3 PbCl 3 results in a lower energy splitting of Mn 2+ d-orbitals, thereby emitting at a longer wavelength (~610 nm). In this manner, Mn 2+ dopant emission may be tuned by modulating the crystal lattice through substitution of different components in perovskites. APTES precursors (black line), and using MnCl 2 , APTES, PbCl 2 and CH 3 NH 3 Cl precursors (red line). (c) PL spectra of MnCl 2 (λ ex = 360 nm); Figure S4: PLE spectra of Mn doped CH 3 NH 3 PbCl 3 PNCs with different doping concentration (λ em = 610 nm); Figure S5: PL spectra of 50 at.% Mn 2+ -doped CH 3 NH 3 PbCl 3 PNCs solutions with different toluene/precursor volume ratios; Figure S6: Air stability test of Mn 2+ -doped CH 3 NH 3 PbCl 3 PNCs passivated with octylamine and APTES; Table S1. Elemental analysis of CH 3 NH 3 Pb x Mn 1-x Cl 3 PNCs passivated with APTES under different doping concentration using ICP-AES.