Orange to red, emission tunable Mn doped two-dimensional

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Functional Inorganic Materials and Devices

Orange to red, emission tunable Mn doped two-dimensional perovskites with high luminescence and stability Chun Sun, Zhiyuan Gao, Yuchen Deng, Hanxin Liu, Le Wang, Sijing Su, Peng Li, Huanrong Li, Zi-Hui Zhang, and Wengang Bi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11665 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 23, 2019

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ACS Applied Materials & Interfaces

Orange to red, emission tunable Mn doped twodimensional perovskites with high luminescence and stability Chun Sun,* Zhiyuan Gao, Yuchen Deng, Hanxin Liu, Le Wang, Sijing Su, Peng Li, Huanrong Li, Zihui Zhang, and Wengang Bi* Dr. C. Sun, Z. Gao, H. Liu, L. Wang, S. Su, Prof. Z. Zhang, Prof. W. G. Bi. State Key Laboratory of Reliability and Intelligence of Electrical Equipment, Hebei University of Technology, 5340 Xiping Road, Tianjin, 300401, P. R. China Tianjin Key Laboratory of Electronic Materials and Devices, School of Electronics and Information Engineering, Hebei University of Technology, 5340 Xiping Road, Tianjin, 300401, P. R. China E-mail: [email protected] and [email protected] Dr. Y. Deng, Dr. P. Li, Prof. H. Li. Hebei Provincial Key Lab of Green Chemical Technology and High Efficient Energy Saving, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, P. R. China

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ABSTRACT: Lead halide perovskites are emerging as promising candidates for high-efficiency light-emitting diode (LED) applications, due to their tunable bandgaps and high quantum yield (QY). However, it remains a challenge to obtain stable red emitting materials with high QY. Herein we report a facile and convenient hot-injection strategy to synthesize Mn doped twodimensional (2D) perovskite nanosheets. The emission peak can be tuned from 597 nm to 658 nm by manipulating the crystal field strength. In particular, a QY as high as 97% for 2D perovskite is achieved. The as-prepared perovskite also possesses excellent stability, whose emission property can be maintained for almost one year. A monochrome LED is further fabricated by employing the as-prepared perovskite as phosphor, which also shows high longterm stability. We believe that these highly efficient and stable perovskites will open up new opportunities in LED applications.

KEYWORDS:

perovskite,

Mn-doping,

light-emitting

diode,

two-dimensional,

high

luminescence, high stability


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1. Introduction Recently, perovskites have been emerging as one of the most promising materials for solutionprocessable LEDs technology, in view of their outstanding optoelectronic properties, including narrow line widths, high QYs and highly tunable bandgaps throughout the entire visible range controlled by composition and crystal size.1-9 During the past few years, great progress has been made in the development of perovskite LED. Recent reports have demonstrated that perovskite LEDs with maximum external quantum efficiency (EQE) of more than 20% have been successfully fabricated.10, 11 Although great success for EQE have been achieved in less than four years,12 the problem of perovskite stability is still unsolved. The red and orange luminescent perovskites contain iodine (I), which is not stable and prone to decompose, limiting their practical applications.1, 13, 14 Doping other ions into perovskite was adopted to acquire stable red and orange emitting perovskites, which could form impurity energy level in the bandgap, such as manganese (Mn) and copper (Cu). Klimov et al. proposed incorporating Mn ions into nanocrystals of CsPbCl3 to acquire the red light, but its QY was relatively low (27%).15 Son et al. added more Mn into the solution and elevated the temperature, which made the QY enhance to 58%.16 Zhang and coworkers also prepared Mn doped CsPbCl3 with the QY over 50%.17 Although the stability of CsPbCl3 is better than the perovskites containing iodine, the CsPbCl3 can not be stored over 6 months.18 Recently, 2D layered perovskites, with the general formula L2PbX4 (L is an alkyl ammonium ligand and X = Cl, Br and I), are becoming excellent candidates as active materials, due to their high exciton binding energies and enhanced stability.19-21 The inorganic plane consisting of corner-sharing [PbX6]4- octahedra networked in two dimensions are separated by organic ligand


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layers. These 2D layered materials can be stabilized by hydrogen bonds between the ammonium headgroup and the halides of the inorganic [PbX6]4- octahedra layer, and van der Waals interactions between adjacent organic tails. Precisely because of the hydrophobicity of these long organic cation, the 2D perovskites exhibit enhanced moisture resistance over the all-inorganic ultrathin perovskite nanomaterials.22-24 However, the QYs of 2D perovskites are not high (usually 2.3 eV) due to quantum confinement, which makes it impossible for them to obtain red emission.26, 27 However, this problem can be solved by doping metal ions such as Mn and Sn into 2D perovskites. Recently, Kundu’s group has done many researches in this area, e.g. they synthesized Mn2+ doped (C4H9NH3)2PbBr4 2D perovskites using solid-state grinding methodology, which held better stability than Mn-doped CsPbCl3.28 Shortly after that, Mn doped 2D ethylenediammonium lead bromide perovskite was reported, which showed highly red-shifted Mn2+ emission peak (670 nm).29 Chen and coworkers have synthesized Sn-doped PEA2PbI4 (PEA=phenylethylammonium) perovskites, which generated broadband red-to-near-infrared emission at room temperature due to strong excitonphonon coupling.21 However, the QYs of those 2D perovskites were not high. Hence, it is necessary to develop a facile, mass-productive and reproducible procedure to preserve the high QY as well as long-term stability. Here, we present a simple method to fabricate Mn doped 2D phenylethylamine (PEA) lead bromide perovskite nanosheets, whose QY has exceeded 97%. The QY of Mn doped PEA2PbBr4 is much higher than that of Mn-doped 2D and 3D lead halide perovskites. Besides, Mn doped PEA2PbBr4 exhibited higher stability than Mn doped CsPbCl3, which preserved their QY and crystal structure during one year and could stay in water over one day. Unlike Mn doped CsPbCl3 nanocrystals,30 the content of Mn2+ has


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little effect on the Mn2+ emission peak of Mn doped 2D perovskite.28, 31-33 Hence, emission of Mn doped PEA2PbBr4 can not be tuned by Mn content, but it can be changed from orange to red through adjusting the amount of PEA. We have found that the crystal field not the Mn-Mn interaction contributed to the red-shift of Mn2+ emission band, which is more like the case of Mn doped ZnSe/CdSe/ZnSe quantum dots tuning the wavelength of the dopant emission by strain.34 2. Experiment section Chemicals: Oleic acid (90%, OA) and octadecene (90%, ODE) were obtained from Alfa Aesar. Lead acetate (99.99%, PbAc2), manganese acetate (98%, MnAc2) and phenylethylamine (98%, PEA) were obtained from Aladdin. Bromotrimethylsilane (99%) was attained from J&K. Hexane (99.5%) and toluene (99.5%) were purchased from Beijing Chemical Factory. All the chemicals were used without further purification. Synthesis of PEA2PbBr4 nanosheets: 10 mL ODE, 1mL OA, 100 μL PEA and 0.0758g PbAc2 were loaded into a 50 mL three-neck round-bottom flask. Then, the flask was evacuated and back-filled with nitrogen three times. After that, the flask was switched to N2 and the temperature was heated to 140ºC. Then, 200 μL TMSBr was quickly injected. An ice-water bath was immediately applied to cool the solution after injection. The as-prepared solution was subjected to centrifuge at 5000 rpm for 10 min. 10 ml toluene was added to the precipitate, and then centrifuged at 5000 rpm for 5 min. After that, hexane was added to wash the precipitate. Synthesis of Mn doped PEA2PbBr4 nanosheets: The synthetic procedure of Mn-doped PEA2PbBr4 nanosheets was similar to that of pure PEA2PbBr4 nanosheets, except the addition of MnAc2. The addition of MnAc2 are 0.0025g,


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0.0173g, 0.0346g and 0.058g, corresponding to x = 0.01, 0.14, 0.26 and 0.33 for the formula of PEA2MnxPb1-xBr4, respectively. LED fabrication UV LED chips (365 nm) were purchased from Sky Bright Inc. 0.02g PEA2Pb0.74Mn0.26Br4 was added in 1mL toluene/PMMA solution (5 g of PMMA mixed with 25 ml of toluene solution). The mixture was treated by ultrasonic wave for 30 minutes, and shaken for 10 minutes. Then, the mixture was coated onto the UV LED chip. Finally, this chip was dried under vacuum for 1 h. Characterizations: Fluorescence emission spectra were recorded on an Ocean Optics spectrometer (the excitation wavelength was set at 365 nm). Absorbance spectra were taken on a Shimadzu UV-3600 spectrophotometer in the range from 365 nm to 750 nm. TEM characterization was performed by a FEI Tecnai G2 Spirit TWIN operated at 200 kV. X-ray diffraction (XRD) patterns of nanosheets were recorded on a Bruker D8 Advance X-ray diffractometer (Cu Kα: λ = 1.5406 Å). The samples were prepared via putting the powder onto a clean glass slide. The PLE spectra monitored at the PL peak wavelengths were measured by an Edinburgh FLS920 fluorescence spectrometer. Time-resolved PL spectra were also carried out by an Edinburgh FLS920 fluorescence spectrometer with a time-correlated single-photon counting (TCSPC) spectrometer and the excitation wavelength was 380 nm. For the unstable sample, encapsulation method was adopted in glove box. The sample was sandwiched between two quartz plates, then UV-cured adhesive was used to seal the gap between two plates. After being exposed to UV light for 10 minutes, the quartz plates could be well packaged. The PL QYs were determined by standard procedures using the same FLS920 fluorescence spectrometer equipped with an integrating sphere with BENFLEC® coated inner face. The accuracy of this apparatus was calibrated by reference dye with known QY. For Rhodamine 101 in ethanol


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(reference QY of 100%, Exciton), the obtained PLQY was 99%. ICP elemental analysis was carried out via an Optima 8300 ICP spectrometer (PerkinElmer). The samples were dissolved in nitric acid (HNO3, 68%), and heated at 120 for 30 min. After this step, the Pb, Mn were dissolved in the acid solution. Then, the acid solution was centrifuged at 8000 rpm for 10 min to remove the organic component. At last, the supernatant was diluted using Milli-Q water and analyzed to determine the concentration of Pb2+ and Mn2+ ions. Fourier transform infrared (FTIR) spectroscopy was conducted on a Thermo-Nicole iS50 FTIR-spectrometer. Paraffin embedding method was adopted and performed in the glove box to prevent the oxidation of P-33. Small amount of P-33 was mixed with paraffin oil, then the mixture was dropped on the KBr plate. After that, another KBr plate was covered up to protect the sample. In this way, the P-33 was sandwiched by two KBr plates. X-ray photoelectron spectroscopy (XPS) was performed by an ESCALAB250 spectrometer. Electron paramagnetic resonance (EPR) measurements were carried out on powder samples (20mg) using a JEOL JES-FA200 spectrometer operated at 9.1 GHz, 19.17 mW power and 77 K temperature. 3. Results and discussion Typically, hot injection method was adopted to prepare 2D perovskites, which involved injecting halide source into the solution of organic amine and lead sources under high temperature. To obtain Mn doped products, the Mn source was needed to dissolve with organic amine and lead sources in advance. Oleic acid (OA) was used as capping agent to dissolve Pb and Mn sources. After purification, the as-prepared PEA2PbBr4 perovskites could be easily dispersed in toluene solvent which possessed chemical structures similar to the phenethyl groups.


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X-ray diffraction (XRD) pattern was measured to characterize the PEA2PbBr4 perovskite. As shown in Figure 1 and Figure S1, a series of strong (00l) periodic diffraction peaks especially the characteristic peak (001) located around 5° are observed, indicating that two-dimensional perovskite is highly oriented.35 The peaks are at regular intervals of 5.1°, which corresponds to an average interlayer spacing of 1.7 nm.25, 35-37 Atomic force microscopy (AFM) measurements confirm that the thickness of 2D PEA2PbBr4 is about 1.7 nm (Figure S2), which is in good accordance with the XRD results. Moreover, increasing the x in the PEA2Pb1-xMnxBr4 perovskites, the periodic diffraction of the perovskites is not changed. However, we observe that the all the XRD peaks monotonic shift to higher angle (the largest one is about 0.23°, which is calculated by the typical (001) peak in Figure 1 and Figure S1), representing unit cell contraction due to the substitution of larger Pb2+ (1.33 Å) by smaller Mn2+ (0.97 Å). A few low intensity diffraction peaks marked with  have also been found in the products. As Figure S4 and S5 show, these peaks do not match any peaks from the reactants, MnAc2 and PbAc2, etc. Besides, all these peaks also shift to higher angle with Mn doping concentration increasing. All these results demonstrate that these peaks belong to PEA2PbBr4 perovskite and they are homogenous. As Pradhan et al. have demonstrated,32 these peaks could be indexed to other planes of PEA2PbBr4 such as (111), (102), (112). Inductively coupled plasma mass spectrometry (ICP-MS) was performed to further confirm the presence of Mn and reveal the concentration of Mn in perovskite. Based on the ICP results, the ratios of Pb and Mn in these PEA2Pb1-xMnxBr4 perovskites are 0.99:0.01, 0.86:0.14, 0.74:0.26 and 0.67:0.33, respectively. The high doping concentrations of Mn were also confirmed by the EDX results (Figure S6, S7). At a high doping level (33%) of Mn, the intensities of the diffraction peaks are reduced, demonstrating that the lattice structure is partially influenced due to the big difference in ionic radius.


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Figure S8 and S9 show the representative transmission electron microscopy (TEM) images of the PEA2PbBr4 and PEA2Pb1-xMnxBr4. As can be seen from these figures, the nanosheets are rectangular with rounded corners, whose lateral dimensions are several hundred nanometers. The size and morphology of the 2D perovskite have nearly no change after Mn being doped into the 2D perovskite. The luminescence properties of 2D perovskites, PEA2PbBr4 and PEA2Pb1xMnxBr4,

have been characterized. The photoluminescence (PL) spectra of PEA2PbBr4 and

PEA2Pb1-xMnxBr4 are shown in Figure 2a. As can be seen, only one emission peak at 415 nm is observed for PEA2PbBr4, while two peaks centered at 415 nm and 600 nm are obtained for PEA2Pb1-xMnxBr4, with full widths at half-maximum (FWHM) of 17 and 76 nm, respectively. The former emission peak (415 nm) can be assigned to the characteristic exciton emission of the 2D perovskites,25 and the latter is mainly from 4T1−6A1 transition of Mn in perovskite.17 The broad emissions induced by Mn dopants exhibit slight red-shift (6 nm) as the doping level of Mn increases from 1% to 33%, while the narrow emissions of the exciton shift to the blue side. Usually, red-shifts of tens of nanometers can be observed in Mn doped 3D perovskites due to the Mn-Mn interaction.17, 30 This slight shift presented here is ascribed to the offset of change of crystal field and Mn-Mn interaction. As we discussed above, the lattice will contract when the doping level of Mn is high. The PEA molecules get closer to the inorganic lattice, leading to distorted structure and decreased local crystal-field symmetry. As for the change of PL QY, the PL QY of Mn dopants increase first and then decrease slightly after the doping level reaches 33% (Figure S11). The host perovskite shows a low QY value of 1%, while the QYs of Mn doped perovskites are extremely high. After doped with Mn, the best QY value of Mn doped 2D perovskite PEA2Pb0.74Mn0.26Br4 reaches up to 97%, which enhances 97 folds compared to the


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host. The reproducibility of this material is very high; over 98% of the perovskites provide the QY over 96%. As can be seen from Figure 2a inset, the perovskite solution exhibits a bright orange emission under UV light. The PEA2Pb1-xMnxBr4 perovskites can be synthesized in a large scale by simply scaling up the precursor dosages. As shown in Figure 2e, several grams of perovskite powders are obtained, which show bright orange emission under 365 nm UV irradiation. The absorption spectra of PEA2PbBr4 and PEA2Pb1-xMnxBr4 are shown in Figure 2b. An obvious absorption peak at 395 nm is found and the Mn doped PEA2PbBr4 perovskites exhibit a similar trend to PEA2PbBr4, which signifies that the optical structure is not destroyed by Mn doping. With increasing the content of Mn, a gradual blue shift about 8 nm of the bandedge absorption is observed, which indicates that the band gap increases. The conduction band minimum (CBM) of 2D perovskites is dominated by Pb 6p orbitals, while the valence-band maximum (VBM) is composed primarily of Pb 6p and Br 3p antibonding orbitals character. 27 The interaction between Pb and Br is strengthened with increasing the dopant content and the [PbBr6]4- octahedral unit contracts severely, resulting in the bandgap shifting towards higher energy.

The photoluminescence excitation (PLE) spectrum of the host material collected at 410 nm is shown in Figure 2c. As can be seen, the excitation spectrum is dominated by a sharply rising peak near the absorption edge of the host material. The excitation peak very closely resembles the absorption band of the host material, which indicates that the blue emission can be attributed to the radiative recombination of electron-hole pairs in the host perovskite material. In the Mn doped case, the PLE spectra of PEA2Pb1-xMnxBr4 monitored at 600 nm show features similar to that of the host material. The excitation spectra here also show a sharply rising peak near the


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absorption band edge of the PEA2Pb1-xMnxBr4 material, indicating that the emission at 600 nm is sensitized by band-edge absorption of the host perovskite material. In other words, the Mn ions are successfully incorporated into the lattice of host perovskite and efficient energy transfer takes place from the host perovskite to the doped Mn ions.

Time-resolved photoluminescence (TRPL) decays of PEA2Pb1-xMnxBr4 are measured to further reveal the mechanism of Mn-related emission. As for the Mn dopants emission, the decay curve shows a mono-exponential fit with a long lifetime of 0.7 ms (Figure 2d), which is due to the forbidden nature of the 4T1−6A1 internal transition.15, 17, 28, 29 Importantly, increasing the Mn2+ concentration has less effect on the lifetime of this Mn dopants emission, which is different from the Mn doped 3D counterparts.17,

30

This similar lifetime observed from different Mn

concentrations is also found in Mn doped (C4H9NH3)2PbBr4 2D perovskite.28 This can be attributed to the combined effects of Mn-Mn interaction and change of crystal field, With increasing the Mn concentration, the inorganic octahedron will shrink, which has an influence on the crystal field environment. Compared with the Mn doped 3D counterparts, the 2D perovskites are single layer structure, which is easily affected by the change of the structure. Hence, the crystal field environment of 2D perovskites has bigger influence on the lifetime than that of 3D counterparts. With increasing the Mn concentration, the Mn-Mn interaction cause the PL decay dynamics decrease. However, the change of the crystal field environment may have opposite trend. In the end, these two effects can be offset, resulting in the similar lifetime.

Different kinds of organic amine were used to analyze their effect on the optical properties of L2Pb0.74Mn0.26Br4. Octylamine (OCA) and oleylamine (OLA) with same molar mass of PEA


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were used to synthesize the L2Pb0.74Mn0.26Br4. The PL QYs of OCA2Pb0.74Mn0.26Br4 and OLA2Pb0.74Mn0.26Br4 are 21.3% and 4.9%, respectively, which are in good agreement with the PL intensities (Figure 3a). These results indicate that the PEA is beneficial to acquire high QY. The high QY of PEA2Pb0.74Mn0.26Br4 can be ascribed to distorted structure and the detailed explanation will discuss later. Besides, slight red-shift can be observed from the their PL spectra, which is in agreement with previous reports.28, 33

The influence of amine (PEA) concentration toward PEA2Pb0.74Mn0.26Br4 is also studied. The 2D PEA2Pb0.74Mn0.26Br4 perovskite samples with adding 20μL, 28μL, 33μL, 50μL, 100μL and 200μL of PEA can be donated as P-20, P-28, P-33, P-50, P-100 and P-200. As shown in Figure S12, the QY first increases as the amount of PEA increases. With adding 100 μL of PEA, the QY reaches up to the highest value (~97%), then, it decreases slightly as the PEA continues to increase. When small amount of PEA is added into the solution, the as-prepared 2D perovskite may subject to many defects due to insufficient passivation, leading to low QY. Compared to other Mn-doped 2D perovskites,28, 32, 33 the QY of as-prepared Mn doped PEA2PbBr4 is much higher than them. There are four main possible reasons to explain the high QY. Firstly, the decreased size of 2D perovskite shortens the extent of exciton diffusion in trap, thus reducing the probability of nonradiative recombination. Secondly, previous reports have already pointed that octahedral distortion is facile to acquire high QY.38-40 The distortion decreases band dispersion, which can yield a higher exciton effective mass and a smaller exciton Bohr radius, resulting in a faster radiative decay rate and higher QY in the distorted structures. It is well known that the structure of rigid aromatic hydrocarbons (PEA) is more distorted than that of flexible aliphatic hydrocarbons (butylamine, octylamine and oleylamine). Thirdly, the OA ligand can coordinate to


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Pb atom and partially passivate the surface traps (i.e. halogen vacancy) of the 2D perovskites,41, 42

leading to extra enhanced QY, which can be confirmed by the products of Mn doped 2D

perovskite with different amount of OA. As shown in Figure S15, the QY of Mn doped 2D perovskites increases with the addition of OA (0.3mL-0.7mL), and reaches to the highest value finally. The last but not least, the uniform structure can be prepared by our hot-injection method. In other words, PEA molecules are distributed uniformly in the 2D perovskite, which leaves few nonradiative traps. The emission peak exhibits nearly no change for PEA in the range of 50-200 μL. However, strong red-shift of 30 nm can be observed for the sample of P-33 and the emission band can be extended to 658 nm (P-20). Such a long wavelength of Mn2+ emission peak is unusual in the perovskite family. Only one report demonstrated a highly red-shifted emission peak at ∼670 nm in Mn doped two dimensional perovskite,29 while the majority reported Mn2+ emission locating at 578-610 nm.15-17, 28, 33 They found that the intercalated DMSO molecules affected the Mn2+ emission peak. However, further investigation of the red shift was not proceeded. The lifetime of P-33 perovskite is 0.2 ms (Figure S16), which is slight shorter than the case of P-100. This similar lifetime scale also proves that the emission stems from energy level of 4T1-6A1 transition of Mn not from self-trapped excitons, because self-trapped excitons lifetimes at room temperature in Pb-based perovskites are in the range of nanoseconds.27, 43-47 Usually, the PL spectral shift can be attributed to two aspects: the variation of the crystal field strength and the Mn-Mn exchange interaction (always takes place at higher dopant concentrations (>2%)). The concentrations of Mn for P-20, P-28 and P-33 are determined by ICP-MS, which are 25%, 25.1% and 25.7%, respectively. These Mn incorporated concentrations are closed to that of P-100, which signifies that the amount of PEA has little effect on the concentration of Mn being incorporated. Moreover, it should be noted that the emission spectrum


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exhibits little shift of less than 10 nm with Mn concentration being increased from 1% to 33%. All these results demonstrate that the large red-shift is not caused by the Mn-Mn exchange interaction.

To gain more insight into the impact of crystal field strength, electron paramagnetic resonance (EPR) measurements were conducted. EPR spectroscopy can distinguish the located lattice sites of paramagnetic ions and give insight to the local crystal-field symmetry. Unfortunately, a broad EPR line is observed for the sample of P-33 (Figure S17) at 77 K, which can not be used to analyze the changes of local crystal-field. It should be noted that this line broadening phenomenon is not caused by the amount of amine but by excess Mn, because exchange interaction between Mn-pairs can be enhanced with increasing dopant concentration.30 In order to verify this, perovskite with lowest doped concentration of Mn and 100 μL of amine i.e. PEA2Pb0.99Mn0.01Br4 (M-1) were synthesized. As can be seen from Figure S17, the line splitting is obvious. Therefore, these EPR signals can not be used to analyze the sites of Mn. However, this line broadening phenomenon can verify the high concentration of Mn in the sample of P-33. Another interesting phenomenon is that the deep red light will turn into orange after the sample being exposed to the air for a few seconds. As can be seen from Figure S18, the emission peak of P-33 shifts from 630 nm to 600 nm after being exposed to air. It's worth noting that after being exposed to air the P-33 can maintain the emission peak of 600 nm for a long time (ten months at least). The concentration of Mn can not change during this short period. Therefore, the variation of the emission peak must come from the change of the local crystal-field symmetry. Furthermore, we have observed that this phenomenon of color change is not induced by water in the air, but caused by the oxygen. We added water to the freshly prepared sample and the


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emission peak still located at 630 nm. On the other hand, the purified sample was encapsulated with polymethyl methacrylate (PMMA) in glovebox. Once it was taken outside, the color was changed immediately. Oxygen diffusion coefficient of PMMA is relatively high (3.3 × 10-9 cm2 s-1 at 22 °C),48 and the PMMA possesses good hydrophobic property. This oxidation effect can be verified by the X-ray photoelectron spectroscopy (XPS) measurements. Figure S19 shows XPS spectra of P-33 after oxidation and P-100. The Mn 2p core level spectra of P-33 after oxidation and P-100 features four main spin−orbit peaks. The peaks centered at 642.2 and 653.6 eV can be assigned to Mn 2p3/2 and Mn 2p1/2 of 2+ oxidation state for Mn,49 while the peaks centered at 646.2 and 659.0 eV can be assigned to Mn 2p3/2 and Mn 2p1/2 of 4+ oxidation state for Mn.50, 51 Besides, we can see that the intensity of Mn4+ becomes stronger in P-33 after oxidation, indicating that more Mn4+ ions are produced in this sample.

To explain why the emission peak changes with the amount of PEA, we propose a possible mechanism for Mn-related emission band induced by the change of crystal field strength. As shown in Scheme S1, large steric hindrance can be observed in the 2D perovskite when there are too many PEA ammonium cations. Large steric hindrance generally makes these PEA ammonium cations undergo further rearrangement and some of them are tilted, which force the inorganic octahedra to distort. Therefore, the symmetry of the octahedra will be reduced, leading to decreased local crystal-field symmetry. The different orientations of PEA in 2D perovskite were also reported by Mathews et al.52 In this case, the emission peak is located at 600 nm. However, it is obvious that as the amount of PEA ammonium cations decreases, the intervals between PEA ammonium cations become large (Scheme S3). Under such conditions, steric hindrance is no longer effective and the PEA ammonium cations are aligned regularly. Therefore,


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the octahedra are not under pressure and the local crystal-field symmetry increases, resulting in that the Mn-related emission peak shifts to the red side. This can be verified by the FTIR spectra because the hydrogen bond strength of this case is stronger than that of the distorted structure. As shown in Figure S20, the broad peak around 3407 cm-1 belongs to the N-H stretching peak.53 This peak becomes red-shift as the PEA ammonium cations reduce, indicating that the hydrogen bond strength increases. Decreasing the amount of PEA causes the decrease of cations, resulting in charge imbalance of 2D perovskite with regular element ratio. In order to keep the charge balance, the halide anions located in the inorganic lattice are missing. Hence, the QY of 2D perovskite will be decreased due to the missing halide anions. As the octahedra are not complete, the Mn2+ ions are easily oxidized to Mn4+ in air due to the photochemical oxidation. The O2 can directly absorb on the surface of perovskite and extract photoexcited electrons from the conduction band,54 while the holes can oxidize the Mn2+ residing in the incomplete octahedra. After oxidation, the Mn4+ has a higher effective nuclear charge, which can draw the electron of the halides, resulting in the reduction of the ability of the halides around Mn4+ giving electrons. The ammonium cations are getting closer to the nearby [PbX6]4- or [MnX6]4- octahedron (Scheme S4), because the hydrogen bonds between the ammonium headgroup and the halides around Mn4+ are impaired. Therefore, large steric hindrance between ammonium cations around the Mn2+ appears again (the green circle in Scheme S4), which makes crystal-field symmetry of these unoxidized Mn2+ decrease as well. Therefore, after the perovskite is oxidized, the emission peak of P-33 exhibits the blue shift. The shortened lifetime of the Mn related emission of these unoxidized P-33 (0.2 ms) also verifies that the symmetry of crystal-field is changed.55, 56


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In order to examine the long-term stability of Mn doped perovskites, the solid powder of P-100 was prepared and exposed to ambient air and light (placed at room temperature and under relative humidity (RH) =40%). The variation of QYs is monitored and shown in Figure 4a. As can be seen from this figure, the QY drops no more than 5% over 300 days, demonstrating excellent stability of these perovskites. The crystal structure of the Mn doped perovskite was also well preserved after stored over 300 days (Figure S23). The other major concern is the water stability, because the metal halide perovskites are easy to decompose under water. Long term stability test was also conducted in water. As shown in Figure 4b, the relative PL intensity of P100 remains about 90% after stored in water for 10 hours. The PL intensity shows 50% drop within 24 hours and totally vanishes after 72 hours. The stability is greatly improved compared to silica coated perovskites (several minutes),13 which can be comparable to the polymer coated ones. The extremely high stability can be attributed to the inherent stability of 2D perovskite. The outermost layer of 2D perovskite is composed of the organic amine cation and the hotinjection method adopted here ensures the full coverage of organic amine. Because the organic group possesses excellent hydrophobicity, the 2D perovskites exhibit excellent moisture resistance. Besides, the OA capping agent also passivates the perovskites, which gives them extra protection.

High QY and excellent stability make P-100 perovskite as alternatives of commercial phosphors, for example, YAG:Ce. Here, an orange-red emitting LED was fabricated by coating P-100 onto a commercially available 365 nm LED chip. As can be seen from Figure 5, the asprepared LED device exhibits bright orange-red emission (600 nm), whose color coordinate is (0.57, 0.42). The luminance is 71890 cd/m2. The luminous efficiency of the device reaches up to


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23 lm/W, which is higher than the Mn doped 3D perovskite QD-based LED devices.17 The longterm working stability of the device was also measured. After continuously working at 20 mA for 350 hours, emission intensity of the device is maintained at 90% of the original value (Figure 5b).

4. Conclusion In summary, we have demonstrated a facile hot-injection method to synthesize Mn doped 2D perovskite nanosheets, which exhibited extremely high QY (~97%). Mn-Mn exchange interaction has little effect on the change of emission peak while the crystal field strength mainly contributes to the shift of the emission peak. Most importantly, the PEA2Pb0.74Mn0.26Br4 perovskites possess high stability, which can preserve their QY under ambient air over 300 days. Besides, the PL of PEA2Pb0.74Mn0.26Br4 can be maintained over one day after being immersed in water. In view of the above mentioned high QY and excellent stability of the PEA2Pb0.74Mn0.26Br4 perovskites, an orange-red emitting LED has been fabricated by integrating this phosphor, which also showed high stability. Therefore, we believe that this Mn doped perovskites hold great potential for LED applications.


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FIGURES

Figure 1. XRD patterns of 2D PEA2PbBr4 and PEA2MnxPb1-xBr4.


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Figure 2. PL (a), absorption (b) and PLE spectra (c) of PEA2PbBr4 and PEA2Pb1-xMnxBr4 perovskites (inset: the photographs of solution of PEA2Pb0.74Mn0.26Br4 under ambient light and UV light). (d) Time-resolved PL decay of PEA2Pb1-xMnxBr4 perovskites. (e) The photograph of powders of PEA2Pb1-xMnxBr4 with increasing Mn doping concentration under UV light.


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Figure 3. (a) PL spectra of OCA2Pb0.74Mn0.26Br4, OLA2Pb0.74Mn0.26Br4 and PEA2Pb0.74Mn0.26Br4. (b) PL spectra of PEA2Pb0.74Mn0.26Br4 with different PEA concentration (inset: the photograph of solution with adding 33μL PEA under UV light).

Figure 4. (a) The QY of P-100 as a function of time in air. (b) The relative intensity of P-100 as a function of time in water. (c) Photographs of P-100 immersed in water under UV light.


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Figure 5. (a) PL spectrum of the LED (inset: the photograph of the LED operated at 20 mA). (b) The relative intensity of the LED measured at different working time.

ASSOCIATED CONTENT Supporting Information. TEM images, EPR spectra, XPS spectra and schematic representations are shown in Supporting Information. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Funding Sources This study was supported by Foundation of Hebei Education Department (BJ2019027), the Natural Science Foundation of Hebei Province (F2018202046) and State Key Laboratory of Reliability and Intelligence of Electrical Equipment (EERIZZ2018003). ACKNOWLEDGMENT The authors acknowledge Prof. Shu Xu and Dr. Chong Geng for the expert assistance. REFERENCES (1) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692-3696. (2) Huang, H.; Zhao, F.; Liu, L.; Zhang, F.; Wu, X.-G.; Shi, L.; Zou, B.; Pei, Q.; Zhong, H. Emulsion Synthesis of Size-Tunable CH3NH3PbBr3 Quantum Dots: An Alternative Route toward Efficient Light-Emitting Diodes. ACS Appl. Mat. Interfaces 2015, 7, 28128-28133. (3) Zhang, F.; Zhong, H.; Chen, C.; Wu, X.-G.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong, Y. Brightly Luminescent and Color-Tunable Colloidal CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. ACS Nano 2015, 9, 4533-4542. (4) Sun, C.; Gao, Z.; Liu, H.; Geng, C.; Wu, H.; Zhang, X.; Fan, C.; Bi, W. A New Method to Discover the Reaction Mechanism of Perovskite Nanocrystals. Dalton. Trans. 2018, 47, 16218-16224. (5) Li, X.; Wu, Y.; Zhang, S.; Cai, B.; Gu, Y.; Song, J.; Zeng, H. CsPbX3 Quantum Dots for Lighting and Displays: Room-Temperature Synthesis, Photoluminescence Superiorities, Underlying Origins and White Light-Emitting Diodes. Adv. Funct. Mater. 2016, 26, 2435-2445. (6) Sun, C.; Su, S.; Gao, Z.; Liu, H.; Wu, H.; Shen, X.; Bi, W. Stimuli-Responsive Inks Based on Perovskite Quantum Dots for Advanced Full-Color Information Encryption and Decryption. ACS Appl Mater Interfaces 2019, 11, 8210-8216. (7) Zhang, X.; Sun, C.; Zhang, Y.; Wu, H.; Ji, C.; Chuai, Y.; Wang, P.; Wen, S.; Zhang, C.; Yu, W. W. Bright Perovskite Nanocrystal Films for Efficient Light-Emitting Devices. J. Phys. Chem.Lett. 2016, 7, 4602-4610. (8) Chun, S.; Xinyu, S.; Yu, Z.; Yu, W.; Xingru, C.; Changyin, J.; Hongzhi, S.; Hengchong, S.; Yiding, W.; William, W. Y. Highly Luminescent, Stable, Transparent and Flexible Perovskite Quantum Dot Gels Towards Light-Emitting Diodes. Nanotechnology 2017, 28, 365601. (9) Sun, C.; Gao, Z.; Liu, H.; Wang, L.; Deng, Y.; Li, P.; Li, H.; Zhang, Z.-H.; Fan, C.; Bi, W. One Stone, Two Birds: High-Efficiency Blue-Emitting Perovskite Nanocrystals for Led and Security Ink Applications. Chem. Mater. 2019, 31, 5116-5123. (10) Cao, Y.; Wang, N.; Tian, H.; Guo, J.; Wei, Y.; Chen, H.; Miao, Y.; Zou, W.; Pan, K.; He, Y.; Cao, H.; Ke, Y.; Xu, M.; Wang, Y.; Yang, M.; Du, K.; Fu, Z.; Kong, D.; Dai, D.; Jin, Y.; Li,


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Orange to red, emission tunable Mn doped two-dimensional

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