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Mechanochemical synthesis and characterization of Mn-doped CsPbCl3 perovskite nanocrystals
Journal Pre-proof Mechanochemical synthesis and characterization of Mn-doped CsPbCl3 perovskite nanocrystals Jianing Cheng, Yanan Li, Wei Qu, Min Sun, Yiwen Liu, Wangzhou Shi, Weijie Du, Yiwen Zhang PII:
S0925-8388(19)34861-3
DOI:
https://doi.org/10.1016/j.jallcom.2019.153615
Reference:
JALCOM 153615
To appear in:
Journal of Alloys and Compounds
Received Date: 8 October 2019 Revised Date:
23 December 2019
Accepted Date: 30 December 2019
Please cite this article as: J. Cheng, Y. Li, W. Qu, M. Sun, Y. Liu, W. Shi, W. Du, Y. Zhang, Mechanochemical synthesis and characterization of Mn-doped CsPbCl3 perovskite nanocrystals, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153615. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Graphical abstract Mn-doped CsPbCl3 nanocrystals fabricated by a solid-state mechanochemical synthesis exhibit tunable color and a high photoluminescence quantum yield of 40.7%.
Mechanochemical synthesis and characterization of Mn-doped CsPbCl3 perovskite nanocrystals Jianing Cheng, Yanan Li, Wei Qu, Min Sun, Yiwen Liu, Wangzhou Shi, Weijie Du*, and Yiwen Zhang*
Key Laboratory of Optoelectronic Materials and Devices, Mathematics and Science College, Shanghai Normal University, Shanghai 200234, China
*Corresponding author. Tel.: +86 21 64321622; Fax: +86 21 6432 8968.
E-mail address: [email protected], [email protected].
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ABSTRACT
Mn-doped CsPbCl3 nanocrystals are considered to be a promising material for luminescence and display applications owing to Mn doping provides a way to introduce new optical properties and reduce the use of toxic Pb. Complicate ion-exchange and hot-injection are commonly used for the fabrication of the nanocrystals. Developing a fast and efficient synthesis method remains a challenge so far. Herein, we developed a simplified solid-state mechanochemical approach to synthesize Mn-doped CsPbCl3 nanocrystals and investigated the structure and luminescence properties of the as-synthesized nanocrystals. Cubic morphology of Mn-doped CsPbCl3 nanocrystals with average size of 13 nm have been obtained by this approach. Mn was successfully incorporated in CsPbCl3 perovskite nanocrystals, resulting in a high photoluminescence quantum yield of 40.7% with an improvement of stability. Emission peak of 594 nm that can be assigned to the luminescence of Mn2+ d-d transition was observed under 365 nm UV excitation. The color of the luminescence changed from blue to warm white to yellow depended on the amount of Mn doping, exhibiting a tunable-color property. It is expected that this approach provides an effective route that would help for synthesis of perovskite nanocrystals doped with different dopants.
KEYWORDS: Mn-doped CsPbCl3; nanocrystals; mechanochemical synthesis; photoluminescence; tunable-color.
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1. Introduction Lead halide perovskite nanocrystals (NCs), with the general formula APbX3, where A is a large cation and X is a halide (Cl, Br, I), are considered to be a promising emitting material for luminescence and display applications owing to their great merits of narrow emission half width, long minority carrier lifetime, and high charge-carrier mobility [1-8]. Since the pioneer work by Friend and co-workers in 2014 using CH3NH3PbI3–xClx as emitting materials to fabricate light emitter diode (LED), the illumination application of lead halide perovskite has become a subject of intense research [9, 10]. Organic-inorganic hybrid perovskite (CH3NH3PbX3) and all-inorganic perovskite (CsPbX3) materials have been reported for the preparation of LED devices [11-18]. Although lead halide perovskite has the potential to be a promising material for LED application, lead is toxic and its solubility in water leads to insufficient long-term stability. Some efforts have been devoted to replace lead with other elements, including Mn, Yb, Er, Ce, Sm, Sn, and Bi [19-25]. Among these elements, Mn has been found to have a remarkable effect in improving the stability and photoluminescence quantum yield (PLQY) of CsPbCl3 NCs [26-33]. New coupled electronic states between the exciton and Mn is considered to be the reason behind the optical properties improvement of CsPbX3 NCs. In the previous reports, Mn-doped NCs were usually obtained using solution based chemical formulation including ion-exchange and hot-injection methods. There is an inherent problem however with the wet chemical route for preparing Mn-doped NCs. Different solubility of the manganese and lead precursors in the same solvent restrict the precise 3
regulation of manganese doping. Furthermore, hot-injection and ion-exchange methods require complex synthesis conditions, high reaction temperatures, and protective atmosphere of inert gas for the elements doping. For ion-exchange approach, additional purification needs to be performed after the ion exchange reaction in order to obtain pure NCs, which easily leads to some changes in optical and electrical properties [20, 33]. New doping strategies for fast and efficient synthesis remains a challenge today.
In this study, we designed a solvent-free mechanochemical approach to synthesize Mn-doped CsPbCl3 perovskite NCs (Scheme is shown in Fig. 1). Mechanochemical synthesis is a solid-state reaction capable of bypassing solubility issues [34-36]. It provides a fast reaction in a short time by a simple milling process. The large amount of heat released during the milling is beneficial to facilitate the formation of a melting to assist in the doping of elements into the CsPbCl3 lattice. Our work demonstrated that Mn-doped CsPbCl3 perovskite NCs with a mean size of 13 nm were successfully synthesized by mechanochemical approach. Tunable color can be achieved by controlling the amount of Mn doping. The doped CsPbCl3 NCs with the Mn/(Mn + Pb) feed ratio of 0.1 showed a highest PLQY of 40.7% and an improvement of stability.
2. Experimental 2.1. Chemicals
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PbCl2 (99%, Aladdin), CsCl (99%, Aladdin), MnCl2·4H2O (98.5%, Shanghai runjie chemical reagent), hexane (99%, Shanghai runjie chemical reagent), oleylamine (OAm, 80%~90%, Aladdin) were used as purchased without further purification.
2.2. Synthesis of Mn-doped nanocrystals
In a typical synthesis of NCs with Mn/(Mn + Pb) feed ratio of 0.1, CsCl (2 mmol), PbCl2 (1.8 mmol), MnCl2· 4H2O (0.2 mmol) were added in a stainless steel vial bottle (100 ml) and mixed with stainless steel balls of 5, 8, and 10 mm in diameter under Ar environment. The weight ratio of the balls to the raw materials was 100. The ball milling speed was determined to be 580 rpm and milled for 30 min at room temperature. Considering that surface defects of the NCs will cause luminescence quenching [37, 38], 1.5 ml of oleylamine was added to the mixture under Ar environment for surface passivation and ball milled for another 30 min. The as-synthesized material was dissolved in hexane and subsequently was centrifuged at 10000 rpm for 5 min. The supernatant was decanted and the solid was dissolved in hexane again. The process of washing the nanocrystals with hexane and centrifugation was repeated 3 times. The products after washing were dispersed in hexane for further analysis. The effects of different Mn/(Mn + Pb) feed ratios of 0, 0.03, 0.05, 0.1, 0.15, 0.2, 0.4, 0.6, and 1.0 on the structure and optical performance of CsPbCl3 NCs were studied.
2.3. Measurement and characterization 5
A variety of techniques were used to characterize the doped CsPbCl3 NCs. The structure of the NCs were determined by X-ray diffraction (XRD) analysis with Cu Kα (λ = 0.15406 nm) radiation (Bruker, D8 Advance). Transmission electron microscope (TEM, JEOL JEM2100F) with an acceleration voltage of 200 kV was used to analyze the morphology and microstructure of the NCs. The Mn/(Mn + Pb) ratios of the NCs were measured using an energy dispersive X-ray spectrometer (EDS). Absorption spectra of the NCs dissolved in hexane were obtained using a UV-vis spectrophotometer (Mapada, UV-3200). Photoluminescence (PL) spectra were taken using a fluorescence spectrophotometer (Lengguang, F98). Time-resolved PL and PLQY measurements were performed using a Fluorolog-3 Horiba Jobin–Yvon spectrofluorimeter equipped with an integrating sphere.
3. Results and discussion XRD patterns of the Mn-doped CsPbCl3 NCs with different Mn/(Mn + Pb) feed ratios from 0 to 1 are compared in Fig. 2(a). A diffraction pattern that can be assigned to the cubic phase of CsPbCl3 (JCPDS: 18-0366, a = b = c =5.605 Å) was observed on the undoped sample. When the Mn/(Mn + Pb) ratio was between 0.03 and 0.15, the (101) peak slightly shifted to higher 2θ angle with the increase of Mn content, as shown in Fig. 2(b). The ionic radius of Mn2+ is 0.97 Å, which is smaller than that of Pb2+ (1.33 Å). This decrease of cation size caused the (101) peak to shift to a higher 2θ values, suggesting that Mn was incorporated in the lattice of the host CsPbCl3 NCs. For the Mn doping ratio higher than 0.20, several diffraction peaks that could 6
not be attributed to the structure of CsPbCl3 appeared. The intensity of the peaks (101) and (200) became significantly weak and some irregular diffraction peaks were observed when the ratio of Mn/(Mn + Pb) was higher than 0.40, indicating the collapse of the host of CsPbCl3. CsMnCl3 structure has been reported to be ascribed to a trigonal phase [39, 40]. A high Mn doping amount means that the crystal structure changed from a cubic phase to a trigonal phase. We propose that this is the reason behind the pure cubic phase cannot be obtained by the mechanochemical synthesis in the case of high amount of Mn doping. In order to obtain pure Mn-doped CsPbCl3 NCs, it is need to control the Mn/(Mn + Pb) feed ratio below 0.15.
TEM was used to further analyze the morphology and microstructure of the obtained NCs. Fig. 3 presents the TEM images and size distribution of three samples with the Mn/(Mn + Pb) feed ratios of 0, 0.03, and 0.10, respectively. For the undoped sample, cubic-shaped NCs were observed. The size distribution calculated from the 100 NCs showed an approximately average size of 13 nm. The corresponding high-resolution TEM (HR-TEM) image demonstrated that these NCs have high crystallinity with a d-spacing of 0.39 nm, which can be ascribed to (101) crystal plane of the cubic perovskite crystal phase. When the Mn doping ratio was between 0.03 and 0.10, the doped NCs were almost identical in shape and size as that of the undoped sample. The Mn-doped NCs maintained high crystallinity and same d-spacing of 0.39 nm, as shown in the corresponding HR-TEM images. The TEM observation demonstrated that high crystallinity NCs similar to that fabricated by hot-injection or ion-exchange method were successfully achieved by using 7
mechanochemical approach [41-43]. EDS analysis was carried out to determine the actual Mn2+ concentration in the NCs washed by hexane, as summarized in Table 1. The Mn/(Mn + Pb) ratios of the doped NCs were almost the same as the feed ratio. It is suggested that the doping concentration of Mn can be easily controlled by modifying the feed ratio in the starting materials.
The absorption spectra of the obtained NCs were measured using UV-vis instrument (Fig. 4). The undoped sample exhibited an absorption peak at 400 nm, which is a typical optical characteristic for CsPbCl3 NCs [44, 45]. The energy bandgaps calculated from plots (Ahν)2 versus hν is 3.02 eV, which are presented in Fig. S1 (Supporting Information), where A is absorbance value, h is Planck's constant, v is frequency. It was larger than that of bulk CsPbCl3 (2.96 eV) [46]. This was attributed to a quantum confinement of charge carrier in CsPbCl3 NCs. When the Mn/(Mn + Pb) ratio was below 0.15, the energy bandgap remained unchanged at 3.02 eV, suggesting the slight lattice contraction shown in XRD measurements did not change the bandgap. Mn doping introduces an energy level within the bandgap, therefore, there is no change in the energy band upon a small doping. For the sample with Mn content of 0.20, a slight blue shift in the absorption peak was detected, which may be due to the existence of the secondary phases. When the doping ratio of Mn increased to 0.4, the intensity of the absorption peak became weak. No absorption peak was observed for the sample with Mn/(Mn + Pb) ratio of 0.6, indicating that the collapse of the CsPbCl3 structure. These results were consistent with the XRD
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measurements, namely, the cubic CsPbCl3 structure began to collapse when the Mn/(Mn + Pb) ratio was higher than 0.4.
One of the most attractive properties of perovskite NCs is their strong PL emission. The PL spectra of the obtained NCs excited at 365 nm ultraviolet (UV) are shown in Fig. 5(a). The undoped NCs exhibited one emission peak at 412 nm. The doping of Mn induced a broad emission peak near 594 nm, which can be attributed to the ligand field transition (4T1–6A1) of Mn2+ ions. Similar results were observed for that obtained by ion-exchange and hot-injection methods [43, 47]. The increase of Mn-doping amount caused an enhancement of the Mn emission peak accompanied by a decrease in the CsPbCl3 emission intensity. It demonstrates that the efficient transfer of energy from the CsPbCl3 host to the Mn ions. In addition, the Mn emission shifted to long wavelength with an increase in the Mn content. When the Mn/(Mn + Pb) ratio increased from 0.03 to 0.15, a shift of 15 nm was observed. The shift was presumed to be due to the lattice shrinkage indicated in the XRD measurements, resulting in the change of the energy level between Mn emission states [47]. Commission Internationale de L'Eclairage 1931 (CIE 1931) color coordinates of the obtained NCs at the different Mn contents are shown in Fig. 5(b). The corresponding CIE coordinates were (0.164, 0.076), (0.332, 0.268), (0.396, 0.294), (0.464, 0.350), and (0.520, 0.404) for the Mn/(Mn + Pb) ratios of 0, 0.03, 0.05, 0.10, and 0.15, respectively. The color changed from purple to warm white to orange, indicating that the color can be adjusted by modifying the Mn concentration. It was interesting that the color coordinates close to warm white were obtained with Mn content of 0.03 and 9
0.05. It is suggested that white lighting can be available by the furthermore optimization such as different elements doping to change emission peak. The emission of white light from a single material is considered to be a promising research for illumination applications.
The PLQYs of the samples are summarized in Table 1. Previous studies have reported that the PLQY of CsPbCl3 was relatively low [30]. The low PLQY was attributed to the defect states in the wide band gap CsPbCl3, resulting in an increase in non-radiative deactivation. The PLQY of the NCs were 13.3%, 22.1%, 28.8%, 40.7%, and 17.9% for the Mn doping of 0, 0.03, 0.05, 0.10, and 0.15, respectively. Mn doping enhanced the PLQY of NCs compared to that of the undoped sample. The highest PLQY of 40.7% was achieved for the sample with Mn/(Mn + Pb) ratio of 0.10, beyond which a decrease was observed. The energy level of Mn d-d states, which is introduced by Mn doping, is less sensitive to the non-radiative centers and thus enhanced the PLQY[19]. For the sample with the Mn doping content of 0.15, the enhancement of Mn–Mn exchange interaction and a consequent increase in the nonradiative deactivation rate are supposed to be the reason of the decrease in PLQY. The PL decay profiles of the excitonic emission at 412 nm are shown in Fig. 6(a). The profiles can be fitted with a biexponential decay and the average lifetimes calculated from the fitted curves were 3.695, 5.875, 4.364, and 1.999 ns corresponding to the Mn/(Mn + Pb) feed ratios of 0, 0.05, 0.10, and 0.15, respectively. PL decay of the Mn emission at 590 nm exhibited a very long lifetime of approximately 1.6 ms owing to the spin-forbidden nature of the 4T1−6A1 transition, as shown in Fig. 6(b). This was 10
considered to be one of the reasons for the improvement of PLQY. The change of the Mn/(Mn + Pb) ratio in the range between 0.05 to 0.15 did not have a significant influence on the lifetime of the Mn d-electron emission. Furthermore, the fit of PL decay showed a single exponential decay behavior, which was supposed to be the characteristic of a homogeneous lattice doped by Mn2+ ions. These results demonstrated that low defects with a homogenous Mn doping of CsPbCl3 NCs were achieved by the mechanochemical synthesis, resulting in the improvement of PLQY.
The effect of Mn doping on the stability of the NCs were investigated. The as-prepared NCs with different Mn/(Mn + Pb) feed ratios were coated on the surface of glasses and then placed for 0 and 30 days. The PL images of the glasses under UV irradiation are presented in Fig. 7. After 30 days of placement, the undoped NCs coated glass exhibited a darker PL image than that placed for 0 day. For the samples coated from the NCs with Mn/(Mn + Pb) feed ratios of 0.05 and 0.10, no obvious difference in the PL images was observed even placed for 30 days. It revealed that the Mn doping improved the stability of CsPbCl3 NCs.
4. Conclusions In conclusion, a solid-state mechanochemical method was developed to synthesize Mn-doped CsPbCl3 NCs. We have demonstrated this approach is an efficient method for preparing Mn-doped CsPbCl3 NCs. It provides a route to precisely control the Mn content and eliminate the problem of using complex synthesis condition in the hot-injection and ion-exchange methods but without 11
altering the material performance. Mn was successfully introduced into CsPbCl3 NCs by the mechanochemical synthesis. When the Mn/(Mn + Pb) ratio was between 0.03 and 0.10, cubic NCs of approximately 13 nm with high crystallinity were obtained. The Mn doping amount can be tuned by the feed ratio of starting materials. The colors of purple, warm white, and orange were realized by the Mn content modification. A high PLQY of 40.7% was achieved by controlling the Mn/(Mn + Pb) ratio to 0.10. This approach provide a possibility to easily and effectively synthesize CsPbCl3 NCs doped with different dopants.
Acknowledgements The authors would like to thank Dr. M. Jia for assistance with the PL lifetime measurements. This work was supported by the Science and Technology Commission of Shanghai Municipality (Grant No. 18070502800, No. 18590780100), the National Natural Science Foundation of China (Grant No. 61704108), the Innovation Program of
Shanghai
Municipal
Education
Commission
(Grant
No.
2019-01-07-00-02-E00032), the Natural Science Foundation of Shanghai (Grant No. 17ZR420600), the Program for the Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.
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Absorption Coefficient of Cesium Lead Bromide Perovskite Nanocrystals, ACS Nano 10 (2016) 2071-2081. [38] A. Pan, B. He, X. Fan, Z. Liu, J.J. Urban, A.P. Alivisatos, L. He, Y. Liu, Insight into the Ligand-Mediated Synthesis of Colloidal CsPbBr3 Perovskite Nanocrystals: The Role of Organic Acid, Base, and Cesium Precursors, ACS Nano 10 (2016) 7943-7954. [39] J. Goodyear, D.J. Kennedy, The crystal structure of CsMnCl3, Acta Cryst. 29 (1973) 744-748. [40] T. Li, G.D. Stucky, G.L. McPherson, The crystal structure of CsMnCl3 and a summary of the structures of RMX3 compounds, Acta Cryst. 29 (1973) 1330-1335. [41] M. He, Y. Cheng, R. Yuan, L. Zhou, J. Jiang, T. Xu, W. Chen, Z. Liu, W. Xiang, X. Liang, Mn-Doped cesium lead halide perovskite nanocrystals with dual-color emission for WLED, Dyes Pigments 152 (2018) 146-154. [42] G.H. Ahmed, J.K. El-Demellawi, J. Yin, J. Pan, D.B. Velusamy, M.N. Hedhili, E. Alarousu, O.M. Bakr, H.N. Alshareef, O.F. Mohammed, Giant Photoluminescence Enhancement in CsPbCl3 Perovskite Nanocrystals by Simultaneous Dual-Surface Passivation, ACS Energy Lett. 3 (2018) 2301-2307. [43] D. Chen, S. Zhou, G. Fang, X. Chen, J. Zhong, Fast Room-Temperature Cation Exchange Synthesis of Mn-Doped CsPbCl3 Nanocrystals Driven by Dynamic Halogen Exchange, ACS Appl. Mater. Inter. 10 (2018) 39872-39878. [44] R. Ahumada-Lazo, J.A. Alanis, P. Parkinson, D.J. Binks, S.J.O. Hardman, J.T. Griffiths, F. Wisnivesky Rocca Rivarola, C.J. Humphrey, C. Ducati, N.J.L.K. Davis, Emission Properties and Ultrafast Carrier Dynamics of CsPbCl3 Perovskite Nanocrystals, J. Phys. Chem. C (2019) 2651-2657. [45] M. Chen, Y. Zou, L. Wu, Q. Pan, D. Yang, H. Hu, Y. Tan, Q. Zhong, Y. Xu, H. Liu, B. Sun, Q. Zhang, Solvothermal Synthesis of High-Quality All-Inorganic Cesium Lead Halide Perovskite Nanocrystals: From Nanocube to Ultrathin Nanowire, Adv. Funct. Mater. 27 (2017) 1701121. [46] Q.A. Akkerman, S.G. Motti, A.R.S. Kandada, E. Mosconi, V. D'Innocenzo, G. Bertoni, S. Marras, B.A. Kamino, L. Miranda, F. De Angelis, A. Petrozza, M. Prato, L. Manna,
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Tables Table 1. PLQY and Mn/(Mn + Pb) ratios of the Mn-doped CsPbCl3 NCs Samples
Mn/(Mn + Pb)
Mn/(Mn + Pb)
feed ratios
ratios in the NCs
PLQY
−
−
13.3%
CsPbCl3:Mn0.03
0.03
0.04
22.1%
CsPbCl3:Mn0.05
0.05
0.06
28.8%
CsPbCl3:Mn0.10
0.10
0.10
40.7%
CsPbCl3:Mn0.15
0.15
0.14
17.9%
CsPbCl3
19
Fig. 1. Schematic of mechanosynthesis of Mn-doped CsPbCl3 NCs
20
Fig. 2. (a) XRD patterns of the NCs with different Mn/(Mn + Pb) feed ratios of (A) 0, (B) 0.03, (C) 0.05, (D) 0.10, (E) 0.15, (F) 0.20, (G) 0.40, (H) 0.60, and (I) 1.0. (b) Expanded view of the (101) peak between 22 and 23º, indicating that doping of Mn caused a shift toward higher 2θ degree.
21
Fig. 3. TEM images of the NCs with Mn/(Mn + Pb) feed ratios of (a) 0, (b) 0.03, and (c) 0.10. The corresponding HR-TEM images of the NCs with Mn/(Mn + Pb) feed ratios of (d) 0, (e) 0.03, and (f) 0.10. The size distribution analyzed from 100 NCs with Mn/(Mn + Pb) feed ratios of (g) 0, (h) 0.03, and (i) 0.10.
22
Fig. 4. UV–vis absorbance spectra of the NCs with different Mn/(Mn + Pb) feed ratios of (A) 0, (B) 0.03, (C) 0.05, (D) 0.10, (E) 0.15, (F) 0.20, (G) 0.40, (H) 0.60, and (I) 1.0.
23
Fig. 5. (a) PL spectra and (b) CIE color coordinates of the Mn-doped NCs with different Mn/(Mn + Pb) feed ratios of (A) 0, (B) 0.03, (C) 0.05, (D) 0.10, and (E) 0.15. The corresponding CIE coordinates are A (0.164, 0.076), B (0.332, 0.268), C (0.396, 0.294), D (0.464, 0.350), and E (0.520, 0.404).
24
Fig. 6. (a) PL decay dynamics of excitonic emission of the NCs with emission at 412 nm after excitation with a picosecond pulse LED source at 340 nm. (b) Decay dynamics of Mn d-emission from Mn-doped CsPbCl3 NCs with emission at 590 nm.
25
Fig. 7. PL images of the glasses coated with the as-prepared NCs and placed for 0 and 30 days.
26
Highlights Mn-doped
CsPbCl3
nanocrystals
were
synthesized
by
a
solid-state
mechanochemical approach for the first time. Cubic morphology of Mn-doped CsPbCl3 nanocrystals with a size of approximately 13 nm were obtained. Doping of Mn increased the photoluminescence quantum yield to 40.7% and enhanced the stability of nanocrystals. Tunable photoluminescence color was achieved by regulating the Mn content.
Author Contribution Statement Jianing Cheng: Conceptualization, Investigation. Yanan Li: Investigation. Wei Qu: Investigation. Min Sun: Investigation. Yiwen Liu: Investigation. Wangzhou Shi: Methodology. Weijie Du: Writing - Original Draft, Supervision. Yiwen Zhang: Conceptualization, Writing - Original Draft, Writing - Review & Editing, Supervision.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(19)34861-3
DOI:
https://doi.org/10.1016/j.jallcom.2019.153615
Reference:
JALCOM 153615
To appear in:
Journal of Alloys and Compounds
Received Date: 8 October 2019 Revised Date:
23 December 2019
Accepted Date: 30 December 2019
Please cite this article as: J. Cheng, Y. Li, W. Qu, M. Sun, Y. Liu, W. Shi, W. Du, Y. Zhang, Mechanochemical synthesis and characterization of Mn-doped CsPbCl3 perovskite nanocrystals, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2019.153615. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Graphical abstract Mn-doped CsPbCl3 nanocrystals fabricated by a solid-state mechanochemical synthesis exhibit tunable color and a high photoluminescence quantum yield of 40.7%.
Mechanochemical synthesis and characterization of Mn-doped CsPbCl3 perovskite nanocrystals Jianing Cheng, Yanan Li, Wei Qu, Min Sun, Yiwen Liu, Wangzhou Shi, Weijie Du*, and Yiwen Zhang*
Key Laboratory of Optoelectronic Materials and Devices, Mathematics and Science College, Shanghai Normal University, Shanghai 200234, China
*Corresponding author. Tel.: +86 21 64321622; Fax: +86 21 6432 8968.
E-mail address: [email protected], [email protected].
1
ABSTRACT
Mn-doped CsPbCl3 nanocrystals are considered to be a promising material for luminescence and display applications owing to Mn doping provides a way to introduce new optical properties and reduce the use of toxic Pb. Complicate ion-exchange and hot-injection are commonly used for the fabrication of the nanocrystals. Developing a fast and efficient synthesis method remains a challenge so far. Herein, we developed a simplified solid-state mechanochemical approach to synthesize Mn-doped CsPbCl3 nanocrystals and investigated the structure and luminescence properties of the as-synthesized nanocrystals. Cubic morphology of Mn-doped CsPbCl3 nanocrystals with average size of 13 nm have been obtained by this approach. Mn was successfully incorporated in CsPbCl3 perovskite nanocrystals, resulting in a high photoluminescence quantum yield of 40.7% with an improvement of stability. Emission peak of 594 nm that can be assigned to the luminescence of Mn2+ d-d transition was observed under 365 nm UV excitation. The color of the luminescence changed from blue to warm white to yellow depended on the amount of Mn doping, exhibiting a tunable-color property. It is expected that this approach provides an effective route that would help for synthesis of perovskite nanocrystals doped with different dopants.
KEYWORDS: Mn-doped CsPbCl3; nanocrystals; mechanochemical synthesis; photoluminescence; tunable-color.
2
1. Introduction Lead halide perovskite nanocrystals (NCs), with the general formula APbX3, where A is a large cation and X is a halide (Cl, Br, I), are considered to be a promising emitting material for luminescence and display applications owing to their great merits of narrow emission half width, long minority carrier lifetime, and high charge-carrier mobility [1-8]. Since the pioneer work by Friend and co-workers in 2014 using CH3NH3PbI3–xClx as emitting materials to fabricate light emitter diode (LED), the illumination application of lead halide perovskite has become a subject of intense research [9, 10]. Organic-inorganic hybrid perovskite (CH3NH3PbX3) and all-inorganic perovskite (CsPbX3) materials have been reported for the preparation of LED devices [11-18]. Although lead halide perovskite has the potential to be a promising material for LED application, lead is toxic and its solubility in water leads to insufficient long-term stability. Some efforts have been devoted to replace lead with other elements, including Mn, Yb, Er, Ce, Sm, Sn, and Bi [19-25]. Among these elements, Mn has been found to have a remarkable effect in improving the stability and photoluminescence quantum yield (PLQY) of CsPbCl3 NCs [26-33]. New coupled electronic states between the exciton and Mn is considered to be the reason behind the optical properties improvement of CsPbX3 NCs. In the previous reports, Mn-doped NCs were usually obtained using solution based chemical formulation including ion-exchange and hot-injection methods. There is an inherent problem however with the wet chemical route for preparing Mn-doped NCs. Different solubility of the manganese and lead precursors in the same solvent restrict the precise 3
regulation of manganese doping. Furthermore, hot-injection and ion-exchange methods require complex synthesis conditions, high reaction temperatures, and protective atmosphere of inert gas for the elements doping. For ion-exchange approach, additional purification needs to be performed after the ion exchange reaction in order to obtain pure NCs, which easily leads to some changes in optical and electrical properties [20, 33]. New doping strategies for fast and efficient synthesis remains a challenge today.
In this study, we designed a solvent-free mechanochemical approach to synthesize Mn-doped CsPbCl3 perovskite NCs (Scheme is shown in Fig. 1). Mechanochemical synthesis is a solid-state reaction capable of bypassing solubility issues [34-36]. It provides a fast reaction in a short time by a simple milling process. The large amount of heat released during the milling is beneficial to facilitate the formation of a melting to assist in the doping of elements into the CsPbCl3 lattice. Our work demonstrated that Mn-doped CsPbCl3 perovskite NCs with a mean size of 13 nm were successfully synthesized by mechanochemical approach. Tunable color can be achieved by controlling the amount of Mn doping. The doped CsPbCl3 NCs with the Mn/(Mn + Pb) feed ratio of 0.1 showed a highest PLQY of 40.7% and an improvement of stability.
2. Experimental 2.1. Chemicals
4
PbCl2 (99%, Aladdin), CsCl (99%, Aladdin), MnCl2·4H2O (98.5%, Shanghai runjie chemical reagent), hexane (99%, Shanghai runjie chemical reagent), oleylamine (OAm, 80%~90%, Aladdin) were used as purchased without further purification.
2.2. Synthesis of Mn-doped nanocrystals
In a typical synthesis of NCs with Mn/(Mn + Pb) feed ratio of 0.1, CsCl (2 mmol), PbCl2 (1.8 mmol), MnCl2· 4H2O (0.2 mmol) were added in a stainless steel vial bottle (100 ml) and mixed with stainless steel balls of 5, 8, and 10 mm in diameter under Ar environment. The weight ratio of the balls to the raw materials was 100. The ball milling speed was determined to be 580 rpm and milled for 30 min at room temperature. Considering that surface defects of the NCs will cause luminescence quenching [37, 38], 1.5 ml of oleylamine was added to the mixture under Ar environment for surface passivation and ball milled for another 30 min. The as-synthesized material was dissolved in hexane and subsequently was centrifuged at 10000 rpm for 5 min. The supernatant was decanted and the solid was dissolved in hexane again. The process of washing the nanocrystals with hexane and centrifugation was repeated 3 times. The products after washing were dispersed in hexane for further analysis. The effects of different Mn/(Mn + Pb) feed ratios of 0, 0.03, 0.05, 0.1, 0.15, 0.2, 0.4, 0.6, and 1.0 on the structure and optical performance of CsPbCl3 NCs were studied.
2.3. Measurement and characterization 5
A variety of techniques were used to characterize the doped CsPbCl3 NCs. The structure of the NCs were determined by X-ray diffraction (XRD) analysis with Cu Kα (λ = 0.15406 nm) radiation (Bruker, D8 Advance). Transmission electron microscope (TEM, JEOL JEM2100F) with an acceleration voltage of 200 kV was used to analyze the morphology and microstructure of the NCs. The Mn/(Mn + Pb) ratios of the NCs were measured using an energy dispersive X-ray spectrometer (EDS). Absorption spectra of the NCs dissolved in hexane were obtained using a UV-vis spectrophotometer (Mapada, UV-3200). Photoluminescence (PL) spectra were taken using a fluorescence spectrophotometer (Lengguang, F98). Time-resolved PL and PLQY measurements were performed using a Fluorolog-3 Horiba Jobin–Yvon spectrofluorimeter equipped with an integrating sphere.
3. Results and discussion XRD patterns of the Mn-doped CsPbCl3 NCs with different Mn/(Mn + Pb) feed ratios from 0 to 1 are compared in Fig. 2(a). A diffraction pattern that can be assigned to the cubic phase of CsPbCl3 (JCPDS: 18-0366, a = b = c =5.605 Å) was observed on the undoped sample. When the Mn/(Mn + Pb) ratio was between 0.03 and 0.15, the (101) peak slightly shifted to higher 2θ angle with the increase of Mn content, as shown in Fig. 2(b). The ionic radius of Mn2+ is 0.97 Å, which is smaller than that of Pb2+ (1.33 Å). This decrease of cation size caused the (101) peak to shift to a higher 2θ values, suggesting that Mn was incorporated in the lattice of the host CsPbCl3 NCs. For the Mn doping ratio higher than 0.20, several diffraction peaks that could 6
not be attributed to the structure of CsPbCl3 appeared. The intensity of the peaks (101) and (200) became significantly weak and some irregular diffraction peaks were observed when the ratio of Mn/(Mn + Pb) was higher than 0.40, indicating the collapse of the host of CsPbCl3. CsMnCl3 structure has been reported to be ascribed to a trigonal phase [39, 40]. A high Mn doping amount means that the crystal structure changed from a cubic phase to a trigonal phase. We propose that this is the reason behind the pure cubic phase cannot be obtained by the mechanochemical synthesis in the case of high amount of Mn doping. In order to obtain pure Mn-doped CsPbCl3 NCs, it is need to control the Mn/(Mn + Pb) feed ratio below 0.15.
TEM was used to further analyze the morphology and microstructure of the obtained NCs. Fig. 3 presents the TEM images and size distribution of three samples with the Mn/(Mn + Pb) feed ratios of 0, 0.03, and 0.10, respectively. For the undoped sample, cubic-shaped NCs were observed. The size distribution calculated from the 100 NCs showed an approximately average size of 13 nm. The corresponding high-resolution TEM (HR-TEM) image demonstrated that these NCs have high crystallinity with a d-spacing of 0.39 nm, which can be ascribed to (101) crystal plane of the cubic perovskite crystal phase. When the Mn doping ratio was between 0.03 and 0.10, the doped NCs were almost identical in shape and size as that of the undoped sample. The Mn-doped NCs maintained high crystallinity and same d-spacing of 0.39 nm, as shown in the corresponding HR-TEM images. The TEM observation demonstrated that high crystallinity NCs similar to that fabricated by hot-injection or ion-exchange method were successfully achieved by using 7
mechanochemical approach [41-43]. EDS analysis was carried out to determine the actual Mn2+ concentration in the NCs washed by hexane, as summarized in Table 1. The Mn/(Mn + Pb) ratios of the doped NCs were almost the same as the feed ratio. It is suggested that the doping concentration of Mn can be easily controlled by modifying the feed ratio in the starting materials.
The absorption spectra of the obtained NCs were measured using UV-vis instrument (Fig. 4). The undoped sample exhibited an absorption peak at 400 nm, which is a typical optical characteristic for CsPbCl3 NCs [44, 45]. The energy bandgaps calculated from plots (Ahν)2 versus hν is 3.02 eV, which are presented in Fig. S1 (Supporting Information), where A is absorbance value, h is Planck's constant, v is frequency. It was larger than that of bulk CsPbCl3 (2.96 eV) [46]. This was attributed to a quantum confinement of charge carrier in CsPbCl3 NCs. When the Mn/(Mn + Pb) ratio was below 0.15, the energy bandgap remained unchanged at 3.02 eV, suggesting the slight lattice contraction shown in XRD measurements did not change the bandgap. Mn doping introduces an energy level within the bandgap, therefore, there is no change in the energy band upon a small doping. For the sample with Mn content of 0.20, a slight blue shift in the absorption peak was detected, which may be due to the existence of the secondary phases. When the doping ratio of Mn increased to 0.4, the intensity of the absorption peak became weak. No absorption peak was observed for the sample with Mn/(Mn + Pb) ratio of 0.6, indicating that the collapse of the CsPbCl3 structure. These results were consistent with the XRD
8
measurements, namely, the cubic CsPbCl3 structure began to collapse when the Mn/(Mn + Pb) ratio was higher than 0.4.
One of the most attractive properties of perovskite NCs is their strong PL emission. The PL spectra of the obtained NCs excited at 365 nm ultraviolet (UV) are shown in Fig. 5(a). The undoped NCs exhibited one emission peak at 412 nm. The doping of Mn induced a broad emission peak near 594 nm, which can be attributed to the ligand field transition (4T1–6A1) of Mn2+ ions. Similar results were observed for that obtained by ion-exchange and hot-injection methods [43, 47]. The increase of Mn-doping amount caused an enhancement of the Mn emission peak accompanied by a decrease in the CsPbCl3 emission intensity. It demonstrates that the efficient transfer of energy from the CsPbCl3 host to the Mn ions. In addition, the Mn emission shifted to long wavelength with an increase in the Mn content. When the Mn/(Mn + Pb) ratio increased from 0.03 to 0.15, a shift of 15 nm was observed. The shift was presumed to be due to the lattice shrinkage indicated in the XRD measurements, resulting in the change of the energy level between Mn emission states [47]. Commission Internationale de L'Eclairage 1931 (CIE 1931) color coordinates of the obtained NCs at the different Mn contents are shown in Fig. 5(b). The corresponding CIE coordinates were (0.164, 0.076), (0.332, 0.268), (0.396, 0.294), (0.464, 0.350), and (0.520, 0.404) for the Mn/(Mn + Pb) ratios of 0, 0.03, 0.05, 0.10, and 0.15, respectively. The color changed from purple to warm white to orange, indicating that the color can be adjusted by modifying the Mn concentration. It was interesting that the color coordinates close to warm white were obtained with Mn content of 0.03 and 9
0.05. It is suggested that white lighting can be available by the furthermore optimization such as different elements doping to change emission peak. The emission of white light from a single material is considered to be a promising research for illumination applications.
The PLQYs of the samples are summarized in Table 1. Previous studies have reported that the PLQY of CsPbCl3 was relatively low [30]. The low PLQY was attributed to the defect states in the wide band gap CsPbCl3, resulting in an increase in non-radiative deactivation. The PLQY of the NCs were 13.3%, 22.1%, 28.8%, 40.7%, and 17.9% for the Mn doping of 0, 0.03, 0.05, 0.10, and 0.15, respectively. Mn doping enhanced the PLQY of NCs compared to that of the undoped sample. The highest PLQY of 40.7% was achieved for the sample with Mn/(Mn + Pb) ratio of 0.10, beyond which a decrease was observed. The energy level of Mn d-d states, which is introduced by Mn doping, is less sensitive to the non-radiative centers and thus enhanced the PLQY[19]. For the sample with the Mn doping content of 0.15, the enhancement of Mn–Mn exchange interaction and a consequent increase in the nonradiative deactivation rate are supposed to be the reason of the decrease in PLQY. The PL decay profiles of the excitonic emission at 412 nm are shown in Fig. 6(a). The profiles can be fitted with a biexponential decay and the average lifetimes calculated from the fitted curves were 3.695, 5.875, 4.364, and 1.999 ns corresponding to the Mn/(Mn + Pb) feed ratios of 0, 0.05, 0.10, and 0.15, respectively. PL decay of the Mn emission at 590 nm exhibited a very long lifetime of approximately 1.6 ms owing to the spin-forbidden nature of the 4T1−6A1 transition, as shown in Fig. 6(b). This was 10
considered to be one of the reasons for the improvement of PLQY. The change of the Mn/(Mn + Pb) ratio in the range between 0.05 to 0.15 did not have a significant influence on the lifetime of the Mn d-electron emission. Furthermore, the fit of PL decay showed a single exponential decay behavior, which was supposed to be the characteristic of a homogeneous lattice doped by Mn2+ ions. These results demonstrated that low defects with a homogenous Mn doping of CsPbCl3 NCs were achieved by the mechanochemical synthesis, resulting in the improvement of PLQY.
The effect of Mn doping on the stability of the NCs were investigated. The as-prepared NCs with different Mn/(Mn + Pb) feed ratios were coated on the surface of glasses and then placed for 0 and 30 days. The PL images of the glasses under UV irradiation are presented in Fig. 7. After 30 days of placement, the undoped NCs coated glass exhibited a darker PL image than that placed for 0 day. For the samples coated from the NCs with Mn/(Mn + Pb) feed ratios of 0.05 and 0.10, no obvious difference in the PL images was observed even placed for 30 days. It revealed that the Mn doping improved the stability of CsPbCl3 NCs.
4. Conclusions In conclusion, a solid-state mechanochemical method was developed to synthesize Mn-doped CsPbCl3 NCs. We have demonstrated this approach is an efficient method for preparing Mn-doped CsPbCl3 NCs. It provides a route to precisely control the Mn content and eliminate the problem of using complex synthesis condition in the hot-injection and ion-exchange methods but without 11
altering the material performance. Mn was successfully introduced into CsPbCl3 NCs by the mechanochemical synthesis. When the Mn/(Mn + Pb) ratio was between 0.03 and 0.10, cubic NCs of approximately 13 nm with high crystallinity were obtained. The Mn doping amount can be tuned by the feed ratio of starting materials. The colors of purple, warm white, and orange were realized by the Mn content modification. A high PLQY of 40.7% was achieved by controlling the Mn/(Mn + Pb) ratio to 0.10. This approach provide a possibility to easily and effectively synthesize CsPbCl3 NCs doped with different dopants.
Acknowledgements The authors would like to thank Dr. M. Jia for assistance with the PL lifetime measurements. This work was supported by the Science and Technology Commission of Shanghai Municipality (Grant No. 18070502800, No. 18590780100), the National Natural Science Foundation of China (Grant No. 61704108), the Innovation Program of
Shanghai
Municipal
Education
Commission
(Grant
No.
2019-01-07-00-02-E00032), the Natural Science Foundation of Shanghai (Grant No. 17ZR420600), the Program for the Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning.
12
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Tables Table 1. PLQY and Mn/(Mn + Pb) ratios of the Mn-doped CsPbCl3 NCs Samples
Mn/(Mn + Pb)
Mn/(Mn + Pb)
feed ratios
ratios in the NCs
PLQY
−
−
13.3%
CsPbCl3:Mn0.03
0.03
0.04
22.1%
CsPbCl3:Mn0.05
0.05
0.06
28.8%
CsPbCl3:Mn0.10
0.10
0.10
40.7%
CsPbCl3:Mn0.15
0.15
0.14
17.9%
CsPbCl3
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Fig. 1. Schematic of mechanosynthesis of Mn-doped CsPbCl3 NCs
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Fig. 2. (a) XRD patterns of the NCs with different Mn/(Mn + Pb) feed ratios of (A) 0, (B) 0.03, (C) 0.05, (D) 0.10, (E) 0.15, (F) 0.20, (G) 0.40, (H) 0.60, and (I) 1.0. (b) Expanded view of the (101) peak between 22 and 23º, indicating that doping of Mn caused a shift toward higher 2θ degree.
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Fig. 3. TEM images of the NCs with Mn/(Mn + Pb) feed ratios of (a) 0, (b) 0.03, and (c) 0.10. The corresponding HR-TEM images of the NCs with Mn/(Mn + Pb) feed ratios of (d) 0, (e) 0.03, and (f) 0.10. The size distribution analyzed from 100 NCs with Mn/(Mn + Pb) feed ratios of (g) 0, (h) 0.03, and (i) 0.10.
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Fig. 4. UV–vis absorbance spectra of the NCs with different Mn/(Mn + Pb) feed ratios of (A) 0, (B) 0.03, (C) 0.05, (D) 0.10, (E) 0.15, (F) 0.20, (G) 0.40, (H) 0.60, and (I) 1.0.
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Fig. 5. (a) PL spectra and (b) CIE color coordinates of the Mn-doped NCs with different Mn/(Mn + Pb) feed ratios of (A) 0, (B) 0.03, (C) 0.05, (D) 0.10, and (E) 0.15. The corresponding CIE coordinates are A (0.164, 0.076), B (0.332, 0.268), C (0.396, 0.294), D (0.464, 0.350), and E (0.520, 0.404).
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Fig. 6. (a) PL decay dynamics of excitonic emission of the NCs with emission at 412 nm after excitation with a picosecond pulse LED source at 340 nm. (b) Decay dynamics of Mn d-emission from Mn-doped CsPbCl3 NCs with emission at 590 nm.
25
Fig. 7. PL images of the glasses coated with the as-prepared NCs and placed for 0 and 30 days.
26
Highlights Mn-doped
CsPbCl3
nanocrystals
were
synthesized
by
a
solid-state
mechanochemical approach for the first time. Cubic morphology of Mn-doped CsPbCl3 nanocrystals with a size of approximately 13 nm were obtained. Doping of Mn increased the photoluminescence quantum yield to 40.7% and enhanced the stability of nanocrystals. Tunable photoluminescence color was achieved by regulating the Mn content.
Author Contribution Statement Jianing Cheng: Conceptualization, Investigation. Yanan Li: Investigation. Wei Qu: Investigation. Min Sun: Investigation. Yiwen Liu: Investigation. Wangzhou Shi: Methodology. Weijie Du: Writing - Original Draft, Supervision. Yiwen Zhang: Conceptualization, Writing - Original Draft, Writing - Review & Editing, Supervision.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: