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Synthesis of high-efficient Mn2+ doped CsPbCl3 perovskite nanocrystals in toluene and surprised lattice ejection of dopants at mild temperature
Journal of Alloys and Compounds 806 (2019) 858e863
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Synthesis of high-efficient Mn2þ doped CsPbCl3 perovskite nanocrystals in toluene and surprised lattice ejection of dopants at mild temperature Kun Bai a, Ruosheng Zeng a, *, Bao Ke a, Sheng Cao b, ***, Xiaogang Xue a, Rihui Tan c, **, Bingsuo Zou b a School of Materials Science and Engineering, School of Life and Environmental Sciences, Guilin University of Electronic Technology, Guilin, 541004, PR China b School of Physical Science and Technology, Guangxi University, Nanning, 530004, PR China c Department of Basic Teaching, Guilin Tourism University, Guilin, 541006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 April 2019 Received in revised form 27 July 2019 Accepted 27 July 2019 Available online 27 July 2019
Doping has been established as a powerful approach for endowing novel properties to perovskite nanocrystals (NCs).However, the fast formation of perovskites makes the fundamental understanding of doping processes in these NCs very rare. In the present work, a mild temperature (<100 C) approach to produce Mn2þ doped CsPbCl3 (Mn:CsPbCl3) perovskite NCs in toluene is reported. The effects of the introduced Mn/Pb ratios in the raw materials as well as the temperature on the structure and photoluminescence (PL) properties of Mn:CsPbCl3 NCs were investigated systematically. The as-synthesized Mn:CsPbCl3 NCs shown a strong Mn2þ emission at 594 nm with a PL quantum yield (QY) of 52%, and the doping concentrations can be easily tailored from 1.8 atom% to 13.3 atom% by changed the Mn/Pb feed ratios. The study revealed that the reaction temperature plays a key role in Mn:CsPbCl3 NCs. Through elemental and microstructure characterization, it was surprisingly found that lattice ejection of Mn ions occurred at 100 C. This result greatly enhances the understanding of the “basic process” of Mn2þdoping in CsPbCl3 NCs, which provides some insights into the preparation of metal ion doped perovskite NCs. © 2019 Elsevier B.V. All rights reserved.
Keywords: Perovskite Doping Lattice ejection Optical property
1. Introduction All-inorganic perovskite nanocrystals (NCs) have attracted extensive interest due to their excellent optical properties and high carrier mobility, which have spurred their applications in lightemitting diodes, solar cells, photodetectors and so forth [1e8]. Recently, it has been discovered that doping metal ions can introduce a series of new functions in perovskite NCs, including new emission bands, improved photoluminescence (PL) quantum yield (QY), and enhanced stability, thereby the doping in perovskite NCs has become a hot topic in current research [9e16]. The doping process in NCs is generally considered to involve “lattice diffusion”
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (R. Zeng), [email protected] (S. Cao), [email protected] (R. Tan). https://doi.org/10.1016/j.jallcom.2019.07.335 0925-8388/© 2019 Elsevier B.V. All rights reserved.
and “lattice ejection” of dopant ions [17e21]. However, since perovskites are ionic crystals and the reaction grows very rapidly, their fundamentals of doping is still not widely understood [12,13,22]. Therefore, it is necessary to develop a facile synthesis procedure and more deeper understanding of the doping process in doped perovskite NCs. Among various dopants, the characters of Mn2þ emission are widely understood, and its luminescence properties show a longlived spin polarized d-d emission in the yellow-orange spectral window [23e26]. For Mn2þ doping in perovskite NCs, the compatible band alignment and efficient host exciton energy transfer to Mn2þ states are more favorable in CsPbCl3 [27e30]. Currently, there are a large number of reports on the preparation Mn2þ doped CsPbCl3 (Mn:CsPbCl3) NC, and the corresponding synthetic methods are mainly divided into the following two ways. (1) Room temperature method. Mn:CsPbCl3 NCs are synthesized through adding Mn2þ ions to the precursor by recrystallization [31,32], or via adding Mn2þ ions to the as-synthesized perovskite
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2.4. Characterization
NCs by ion exchange [33e35]. (2) High temperature hot-injection method. Mn:CsPbCl3 NCs are prepared by injecting a halogen element precursor in a reaction precursor containing Mn2þ ions and rapidly cooling to stop the growth [36e38]. At present, the high-quality of Mn:CsPbCl3 NCs are mainly obtained by high temperature hot-injection method [12,13,39], but their reproducibility still face challenges due to their demanding harsh preparation conditions that require rapid cooling processes. Hence, this demands more facile approach and in-depth study of the doping process for exploration of high-quality perovskite NCs. Herein, we report a new strategy to prepare Mn:CsPbCl3 NCs at mild temperature (<100 C) in toluene. The as-synthesized Mn:CsPbCl3 NCs showed a strong Mn2þ emission with a PL QY up to 52%, and the doping concentrations could easily be tailored from 1.8 atom % to 13.3 atom % by changing the Mn/Pb feed ratio. It is found that the reaction temperature is the most critical factor for the doping growth of Mn:CsPbCl3 NCs. The temporal evolution of Mn2þ emissions and Mn ions concentrations in Mn:CsPbCl3 NCs synthesized at 80 C and 100 C respectively was studied. With the analysis of the microstructure of Mn:CsPbCl3 NCs, the “elementary process” in these NCs was revealed from perspective of lattice ejection of dopants.
The morphology and structure of the obtained Mn:CsPbCl3 NCs were measured by transmission electron microscopy (TEM, JEOL 100 CX) and X-ray diffraction (XRD, Bruker D8 Discover). The XRD samples were prepared by depositing muddy solid NCs onto a glass slide and dried under vacuum at 60 C for 4 h. The PL decay curves were recorded at room temperature using an Edinburgh Instrument FLS980 fluorescence spectrometer (the excitation wavelength is 400 nm). The UV-vis absorption spectra were tested by using UH5300. PL spectra were collected using a Tianmei F-4600 fluorescence spectrophotometer. Elemental analysis was performed by inductively coupled plasma mass spectrometry (ICP-MS) with a Varian 720/730 series spectrometer. The PL QY was measured by Edinburgh Instrument FLS5 fluorescence spectrometer with a QY accessory. The electron paramagnetic resonance (EPR) spectrum were carried out using JEOL JES-FA200 with an X-band microwave frequency of 9.45 GHz at room temperature.Raman spectra were obtained on a Labram-010 Raman spectrometer system equipped with a 632 nm laser.
2. Experimental
3.1. Synthesis, optical and microstructure of Mn:CsPbCl3 NCs
2.1. Materials
Compared to the classic high temperature hot-injection synthesis method, the Mn:CsPbCl3 NCs here were prepared by a mild temperature synthesis, which does not require rapid cooling operation. The commonly used solvent of octadecene is also replaced by an economical and cheaper solvent of toluene. In a typical reaction, 0.1 mmol of CsAc, 0.1 mmol of PbAc2$3H2O, 0.1 mmol of MnAc2, 0.045 mL OA, 0.075 mL of OLA and 5 mL toluene were loaded in a 50 mL three-neck flask and heated to 80 C under a N2 atmosphere. 1 mL of RNH3Cl was injected into the reaction flask and keep growing for 10 min, then cooled to room temperature by air. The as-synthesized NCs were collected from the growth solution by centrifugation and re-dispersed in hexane for further characterization. Fig. 1 shows the absorption, PL, and photoluminescence excitation (PLE) spectra of undoped and Mn2þ doped CsPbCl3 NCs. Both samples have a similar absorption profile as shown in Fig. 1, but their PL have different features. The undoped sample exhibits only one exciton emission at 404 nm with a narrow full-width at half-
Cesium acetate (CsAc, 99%) and ammonium chloride acetate (NH4Cl, 99.8%) were purchased from Shanghai AccelaChemBio Co. Ltd. Lead acetate (PbAc2$3H2O, 99%), manganese acetate (MnAc2$4H2O, 99%), and oleylamine (OLA, 70%) were purchased from J&K Chemicals. Ethyl acetate (99%) and oleic acid (OA, 90%) were purchased from Aladdin Reagents. Toluene (99.5%) and hydrochloric (HCl, 37 wt% in water) were purchased from Xilong Scientific Co. Ltd. All reagents were used as-received without further purification. 2.2. Preparation of OLA-HCl (RNH3Cl) In a typical process, 10 mL of OLA and 1 mL of HCl were loaded in a 50 mL three-neck flask. The mixture was heated to 80 C with N2 flow and kept in this temperature for 1 h. Then the flask was heated to 120 C and kept for 2 h. The resulted precursor was stored for preparation of NCs at 80 C under inert atmosphere.
3. Results and discussion
2.3. Synthesis of Mn2þ doped CsPbCl3 NCs In a typical procedure, 19.7 mg of CsAc (0.1 mmol), 37.9 mg of PbAc2$3H2O (0.1 mmol), 24.5 mg of MnAc2 (0.1 mmol), 0.045 mL of OA, 0.075 mL of OLA and 5 mL of toluene were loaded in a 50 mL three-neck flask. After 10 min of nitrogen purge, the mixture was heated to a designated temperature (changed from 25 C to 100 C) and kept for 10 min until the clear solution was formed. Then 1 mL of RNH3Cl was injected into the reaction flask. Immediately, the strong yellow fluorescence was observed under the irradiation of the 365 nm ultraviolet lamp, which indicated that the Mn:CsPbCl3 NCs were formed. The reaction was stopped at the different required time. Mn:CsPbCl3 NCs with different Mn concentrations were produced by varying the Mn/Pb feed ratios in the starting mixture while keeping all other synthesis parameters fixed. The experimental results suggested that the production of the Mn:CsPbCl3NCs via the present mild temperature strategy was highly repeatable.
Fig. 1. Absorption, PL, and PL excitation (PLE) spectra of undoped (up) and Mn2þ doped (down) CsPbCl3 NCs dispersion in hexane. The excitation wavelength is 365 nm. The inset shows a digital photo of their solutions under a 365 nm ultraviolet lamp excitation.
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maximum (FWHM) of 85 meV, while the Mn2þ doped sample shows two different emission with a narrow peak at 397 nm and a broad peak at 594 nm. The narrow emission at 397 nm is attributed to the intrinsic exciton emission of CsPbCl3 NCs, while the broader orange emission peak at 594 nm with a 276 meV of FWHM is attributed to Mn2þ4T1/6A1d-d transition [13,36,40]. The PLE spectra of undoped and Mn2þ doped CsPbCl3 NCs were probed at 404 nm and 594 nm, respectively. As shown in Fig. 1, both PLE spectra follow their absorption spectra, indicating that the Mn2þ emissions origin from the energy transfer from the photoexcited CsPbCl3 NC host [12,13,37]. The digital photo of their solutions was recorded under irradiation of an ultraviolet lamp with a wavelength of 365 nm. As shown in the inset of Fig. 1, the undoped and Mn2þ doped CsPbCl3 NCs dispersed in hexane showed blue and bright orange color, respectively. The orange emission in the Mn:CsPbCl3 NCs is also a direct evidence of the successful incorporation of Mn2þ into CsPbCl3 host NC. In addition, the Mn:CsPbCl3 NCs exhibited a strong Mn2þ emission with a PL QY up to 52% (the total PL QY plus excitonic emission was 55%). The QYs of Mn2þ doped NCs is lower than that of pure NCs due to energy transfer losses from the NC host to the dopant emission center [40,41]. However, herein the QY of our NCs is comparable to the state-of-the-art of the high temperature hot-injection synthesized Mn:CsPbCl3 NCs (Table S1, Supporting Information). Fig. 2 shows the typical TEM images and XRD patterns of undoped and Mn2þ doped CsPbCl3 NCs. The TEM images in Fig. 2aeb show that the CsPbCl3 and Mn:CsPbCl3 NCs have a cubic shape with a uniform size distribution of 11.9 ± 1.8 and 11.1 ± 2.3 nm, respectively (see Fig. S1, Supporting Information). The high-resolution TEM images of their NCs (insets in Fig. 2aeb) demonstrate that these NCs are highly crystalline with a clearly lattice fringes of 0.56 nm, which is consistent with the nature of perovskite CsPbCl3 [31,42,43]. The XRD patterns of CsPbCl3 and Mn:CsPbCl3 NCs in Fig. 2c shows that both samples are wellcrystallized, and are structurally identical to tetragonal phase CsPbCl3 structure (PDF#18-0366) [44,45]. Compared with the pristine NCs, the magnify (002) diffraction of tetragonal
Mn:CsPbCl3NCs shows a clearly peak shift to a higher angle compared to the undoped sample, which clearly indicates that lattice doping of Mn occurs. The absence of other diffractions indicates no significant presence of crystalline impurities, and that the tetragonal crystal structure is largely unaffected by Mn2þ doping. These measurements show that the mild temperature approach is an effective way to obtain high-quality Mn:CsPbCl3 NCs. 3.2. Effects of Mn/Pb ratio on the structure and optical properties of Mn:CsPbCl3 NCs The mild temperature synthesis approach was used to yield a series of Mn:CsPbCl3 NCs with different Mn/Pb feed ratios (i.e., 0, 0.1, 0.5, 1, and 1.5). Table S2 (Supporting Information) shows the positive correlation between the feed Mn/Pb ratios and actual Mn contents. The Mn doping concentrations (relative to Pb content) could be tailored from 1.8 atom % to 13.3 atom % with the feed Mn/ Pb molar ratios adjusted from 0.1 to 1.5. All XRD patterns of Mn:CsPbCl3 NCs in Fig. 3a show a well-crystallized tetragonal phase structure without crystalline impurities. The detailed optical properties of Mn:CsPbCl3 NCs are shown in Fig. 3bed. All samples show an obvious first absorption band at about ~400 nm, and this absorption band shift slightly continuously to the blue as the ratio of Mn/Pb increases (Fig. 3b). The similar blue tendency was also observed in their excitonic PL emission (Fig. 3c), which is likely due to the effect of Mn2þ ions changing the host electronic band structure [33,37,43]. The PL intensity of Mn2þemission increases proportionately with the increases the Mn/Pb feed ratio and reaches the maximum PL QY up to 52% (Fig. 3d) at the Mn/Pb ratio of 1.0, then decrease with the further increase of the Mn/Pb feed ratios. It is noted that the PL QY of CsPbCl3 host exciton decreases monotonously with the increase the Mn/Pb feed ratios, which indicates that the PL QY of Mn2þ emission increase first is due to the fact that as the Mn/Pb ratio increases, more excitation energy is transferred from the CsPbCl3 host exciton to the Mn2þ emission acceptors [37,41,46]. When the Mn/Pb feed ratio exceeds 1.0, their PL QY exhibits a drop trend, which may be attributable to the formed Mn2þ-Mn2þ pairs and enhanced energy migration to traps at elevated Mn/Pb ratios [36,37,40]. 3.3. Effects of reaction temperature on the optical properties of Mn:CsPbCl3 NCs
Fig. 2. Typical TEM images of (a) undoped and (b) Mn2þ doped CsPbCl3 NCs. The insets show their corresponding HRTEM image. (c) XRD patterns from the undoped and Mn2þ doped CsPbCl3 NCs, the right panel magnifies the (002) diffraction to show the peak shifts.
Fig. 4 shows the optical properties of Mn:CsPbCl3 NCs prepared at a fixed Mn/Pb feed ratio of 1.0 and different reaction temperatures of 25, 40, 60, 80 and 100 C. The PL spectra in Fig. 4a show that all samples have two PL peaks around at ~400 nm and ~590 nm, which correspond to their excitonic and Mn2þ ion emission, respectively. Fig. 4b shows the PL image of their solution under the radiation of a UV lamp with a wavelength of 365 nm. By changing the synthesis temperature, these NCs represent different colors, suggesting that their PL emissions could be profoundly tailored by adjusting the reaction temperature of Mn:CsPbCl3 NCs. Fig. 4ced show the PL QYs and decay curves of Mn2þ emissions in Mn:CsPbCl3 NCs synthesized at different reaction temperature, respectively. Specifically, the PL QY of Mn2þ emission increases as the synthesis temperature of Mn:CsPbCl3 NCs increases, reaches a maximum of 52% at a reaction temperature of 80 C, and then decreases as the reaction temperature further increases. The PL lifetime of Mn:CsPbCl3 NCs prepared at 80 C was obtained to be about 1.7 m, which is similar to others [37,40]. This means that the PL lifetimes of Mn2þ emissions in Mn:CsPbCl3 NCs obtained at various temperatures
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Fig. 3. (a) Powder XRD patterns, (b) PL spectra, (c) PL QY, and (d) UV-vis absorption spectra of Mn2þ doped CsPbCl3 NCs synthesized at 80 C with different feed ratios of Mn/Pb. The excitation wavelength is 365 nm “x10” refers to magnified 10 times.
Fig. 4. (a) PL spectra and (b) PL QY of Mn2þ doped CsPbCl3 NCs synthesized at different reaction temperatures with a Mn/Pb feed ratio of 1.0. The excitation wavelength is 365 nm. (c) Typical digital photo of their NCs dispersed in hexane under a 365 nm ultraviolet lamp excitation. (d) Excited state Mn2þ emission decay curves (y-axis in log scale) of Mn2þ doped CsPbCl3 NCs.
do not have a clear change, showing a good agreement with the previous report [40]. It is found that the doping concentration of Mn ions increases with the increase of reaction temperature (Table S2, Supporting Information), improving the PL QY of Mn:CsPbCl3 NCs [37,40].
3.4. The “lattice ejection” of dopants at mild temperature Fig. 5a shows the temporal evolution of Mn2þ emission in Mn:CsPbCl3 NCs with a Mn/Pb feed ratio of 1.0 synthesized at 80 and 100 C, respectively (the corresponding PL spectra shown in
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Fig. 5. Temporal evolution of Mn2þ PL QYs (a) and Mn doping concentrations (b) of Mn2þ doped CsPbCl3 NCs with a Mn/Pb feed ratio of 1.0 prepared at 80 and 100 C, respectively.
Fig. S2 at Supporting information). Under the reaction condition of 80 C, the PL QYs of the Mn2þ emission reached a maximum in about 10 min, and then a slow decrease occurred. Under the reaction condition of 100 C, the PL QYs of the Mn2þ emission possessed a sharp maximum at 2 min. It is reported that the PL intensity of Mn2þ emission exhibits a positive correlation with the dopant ions diffusion in Mn2þ doped ZnSe NCs [17,18]. To reveal this point in our case, we purified the aliquot NCs taken at different reaction time for these samples and performed component analysis. The ICP-MS measurement result in Fig. 5b shows a significant increase in the Mn doping concentration in these samples during the first 2 min of the reaction, indicating the presence of diffusion of Mn ions during this process. As the reaction time increased, the concentration of Mn ions in the NCs separated at the reaction temperature of 80 C did not change significantly. However, a sharp drop in the concentration of Mn ions was observed in the NC isolated at the reaction temperature of 100 C. This phenomenon that the Mn doping concentration decreases with the increase of reaction time is similar to that of Mn doped chalcogenide NCs, which is usually related to “lattice ejection” of dopant [17,18,47]. To the best of our knowledge, this is the first example to demonstrate the “elementary process” of “lattice ejection” in Mn2þ doping CsPbCl3 NCs. It is noted that the critical temperature of “lattice ejection” in our
Mn:CsPbCl3 NCs synthesis procedure is 80 C, once this temperature is exceeded, the “lattice ejection” of Mn ions occur very rapidly. This may explain why the reaction needs to be quickly terminated in the high temperature hot-injection synthesis method. To further confirm that lattice ejection of Mn ions does occur at 100 C, further XRD and EPR analysis were carried out for the Mn2þ doped CsPbCl3 NCs synthesized at different times. As shown in Fig. 6a, the XRD spectra of NCs synthesized with different reaction time are almost the same, and no obvious narrowing or broadening of peaks was observed. This indicates that the phase structure of NCs has not been changed significantly at 100 C, which also excludes the possibility of the change of the PL QYs resulted from the change of phase. The EPR spectra of all the Mn:CsPbCl3 NCs in Fig. 6b show a well-defined sextet hyperfine splitting pattern (hyperfine constant A ¼ 86 G), which is consistent with the reported results [30]. The EPR spectrum of 30 min indicates the well-isolated Mn-ions are uniformly distributed in host lattice compared to that of 5s. That is so because Mn ions in the host lattice were carried sufficient lattice diffusion at 100 C. As indicated in Fig. 6b, there is a slightest decrease in dipolar broadening with time which suggests Mn ions in host move apart probably due to lattice diffusion [16]. Combined with the results of elemental analysis, the lattice ejection in Mn:CsPbCl3 NCs at 100 C is well confirmed.The lattice ejection was also monitoring by Raman spectrum (Fig. S3 in Supporting information), Mn-induced vibrational modes of the crystal 190 cm1 was observed to a few wavenumbers shifted, which further confirms the lattice diffusion of dopants [48].
4. Conclusion In conclusion, we reported a new strategy to obtain Mn:CsPbCl3 NCs at mild temperature in economic toluene. The as-synthesized Mn:CsPbCl3 NCs shown a strong Mn2þ emission with a PL QY up to 52%, and the doping concentrations can be easily tarried from 1.8 atom % to 13.3 atom % by changed the Mn/Pb feed ratios. The reaction temperature was the most critical factor for the doping growth of Mn:CsPbCl3 NCs. It not only influences on the degree of crystallinity of the Mn:CsPbCl3NCs, but also effects the “lattice diffusion” and “lattice ejection” of the Mn2þ ions; and finally determines the optical performance of Mn2þ in Mn:CsPbCl3 NCs. This work may provide some new strategy and perceptive for the preparation and study of the doping process in doped perovskite NCs.
Fig. 6. Temporal evolution of powder XRD patterns (a) and room temperature EPR spectrum of Mn2þ doped CsPbCl3 NCs synthesized at 100 C.
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Conflicts of interest There are no conflicts to declare. Acknowledgements This work was financial supported by the National Natural Science Foundation of China (Grant No. 21661010) and Guangxi Natural Science Foundation (Grant No. 2017GXNSFGA198005). S. Cao appreciates the special fund of “Guangxi Bagui Scholar”. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.07.335. References , M.V. Kovalenko, L. Manna, Genesis, challenges and [1] Q.A. Akkerman, G. Raino opportunities for colloidal lead halide perovskite nanocrystals, Nat. Mater. 17 (2018) 394e405. [2] P. Ramasamy, D.-H. Lim, B. Kim, S.-H. Lee, M.-S. Lee, J.-S. Lee, All-inorganic cesium lead halide perovskite nanocrystals for photodetector applications, Chem. Commun. 52 (2016) 2067e2070. [3] L. Protesescu, S. Yakunin, M.I. Bodnarchuk, F. Krieg, R. Caputo, C.H. Hendon, R.X. Yang, A. Walsh, M.V. Kovalenko, Nanocrystals of cesium lead halide perovskites (CsPbX3, X ¼ Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut, Nano Lett. 15 (2015) 3692e3696. [4] J. Song, J. Li, X. Li, L. Xu, Y. Dong, H. Zeng, Quantum dot light-emitting diodes based on inorganic perovskite cesium lead halides (CsPbX3), Adv. Mater. 27 (2015) 7162e7167. [5] J. Li, L. Xu, T. Wang, J. Song, J. Chen, J. Xue, Y. Dong, B. Cai, Q. Shan, B. Han, H. Zeng, 50-fold EQE improvement up to 6.27% of solution-processed allinorganic perovskite CsPbBr3 QLEDs via surface ligand density control, Adv. Mater. 29 (2017) 1603885. [6] Z. Bai, H. Zhong, Halide perovskite quantum dots: potential candidates for display technology, Sci. Bull. 60 (2015) 1622e1624. [7] X.-g. Wu, J. Tang, F. Jiang, X. Zhu, Y. Zhang, D. Han, L. Wang, H. Zhong, Highly luminescent red emissive perovskite quantum dots-embedded composite films: ligands capping and caesium doping-controlled crystallization process, Nanoscale 11 (2019) 4942e4947. [8] X. Chen, F. Zhang, Y. Ge, L. Shi, S. Huang, J. Tang, Z. Lv, L. Zhang, B. Zou, H. Zhong, Centimeter-sized Cs4PbBr6 crystals with embedded CsPbBr3 nanocrystals showing superior photoluminescence: nonstoichiometry induced transformation and light-emitting applications, Adv. Funct. Mater. 28 (2018) 1706567. [9] F. Li, Z. Xia, Y. Gong, L. Gu, Q. Liu, Optical properties of Mn2þ doped cesium lead halide perovskite nanocrystals via a cation-anion co-substitution exchange reaction, J. Mater. Chem. C 5 (2017) 9281e9287. [10] Z.-J. Yong, S.-Q. Guo, J.-P. Ma, J.-Y. Zhang, Z.-Y. Li, Y.-M. Chen, B.-B. Zhang, Y. Zhou, J. Shu, J.-L. Gu, L.-R. Zheng, O.M. Bakr, H.-T. Sun, Doping-enhanced short-range order of perovskite nanocrystals for near-unity violet luminescence quantum yield, J. Am. Chem. Soc. 140 (2018) 9942e9951. [11] M. Liu, G. Zhong, Y. Yin, J. Miao, K. Li, C. Wang, X. Xu, C. Shen, H. Meng, Aluminum-doped cesium lead bromide perovskite nanocrystals with stable blue photoluminescence used for display backlight, Adv. Sci. 4 (2017) 1700335. [12] Y. Zhou, J. Chen, O.M. Bakr, H.-T. Sun, Metal-doped lead halide perovskites: synthesis, properties, and optoelectronic applications, Chem. Mater. 30 (2018) 6589e6613. [13] A.K. Guria, S.K. Dutta, S. Das Adhikari, N. Pradhan, Doping Mn2þ in lead halide perovskite nanocrystals: successes and challenges, ACS Energy Lett 2 (2017) 1014e1021. [14] S. Hou, M.K. Gangishetty, Q. Quan, D.N. Congreve, Efficient blue and white perovskite light-emitting diodes via manganese doping, Joule 2 (2018) 2421e2433. [15] L. Fei, X. Yuan, J. Hua, M. Ikezawa, R. Zeng, H. Li, Y. Masumoto, J. Zhao, Enhanced luminescence and energy transfer in Mn2þ doped CsPbCl3-xBrx perovskite nanocrystals, Nanoscale 10 (2018) 19435e19442. [16] S. Zou, Y. Liu, J. Li, C. Liu, R. Feng, F. Jiang, Y. Li, J. Song, H. Zeng, M. Hong, X. Chen, Stabilizing cesium lead halide perovskite lattice through Mn(II) substitution for air-stable light-emitting diodes, J. Am. Chem. Soc. 139 (2017) 11443e11450. [17] D. Chen, R. Viswanatha, G.L. Ong, R. Xie, M. Balasubramaninan, X. Peng, Temperature dependence of “elementary processes” in doping semiconductor nanocrystals, J. Am. Chem. Soc. 131 (2009) 9333e9339. [18] R. Zeng, M. Rutherford, R. Xie, B. Zou, X. Peng, Synthesis of highly emissive Mn-doped ZnSe nanocrystals without pyrophoric reagents, Chem. Mater. 22 (2010) 2107e2113.
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Xu, F. Li, F. Lin, Y. Chen, Z. Cai, Y. Wang, X. Chen, Synthesis of CsPbCl3-Mn nanocrystals via cation exchange, Adv. Opt. Mater. 5 (2017) 1700520. [36] H. Liu, Z. Wu, J. Shao, D. Yao, H. Gao, Y. Liu, W. Yu, H. Zhang, B. Yang, CsPbxMn1exCl3 perovskite quantum dots with high Mn substitution ratio, ACS Nano 11 (2017) 2239e2247. [37] W. Liu, J. Zheng, S. Cao, L. Wang, F. Gao, K.-C. Chou, X. Hou, W. Yang, Mass production of Mn2þ-doped CsPbCl3 perovskite nanocrystals with high quality and enhanced optical performance, Inorg. Chem. Front. 5 (2018) 2641e2647. [38] Y.H. Song, W. Bin Kim, J.S. Yoo, S.H. Choi, W.K. Park, Y. Kim, W.S. Yang, D.H. Yoon, H.S. Jung, Design of orange-emitting CsPb0.77Mn0.23Cl3 perovskite film for application in optoelectronic device, Chem. Eng. J. 331 (2018) 803e808. [39] X. Zhang, L. Li, Z. Sun, J. Luo, Rational chemical doping of metal halide perovskites, Chem. Soc. Rev. 48 (2019) 517e539. [40] X. Yuan, S. Ji, M.C. De Siena, L. Fei, Z. Zhao, Y. Wang, H. Li, J. Zhao, D.R. Gamelin, Photoluminescence temperature dependence, dynamics, and quantum efficiencies in Mn2þ-doped CsPbCl3 perovskite nanocrystals with varied dopant concentration, Chem. Mater. 29 (2017) 8003e8011. [41] D. Rossi, D. Parobek, Y. Dong, D.H. Son, Dynamics of excitoneMn energy transfer in Mn-doped CsPbCl3 perovskite nanocrystals, J. Phys. Chem. C 121 (2017) 17143e17149. [42] Q. Wang, X. Zhang, Z. Jin, J. Zhang, Z. Gao, Y. Li, S.F. Liu, Energy-down-shift CsPbCl3:Mn quantum dots for boosting the efficiency and stability of perovskite solar cells, ACS Energy Lett 2 (2017) 1479e1486. [43] W. Liu, Q. Lin, H. Li, K. Wu, I. Robel, J.M. Pietryga, V.I. Klimov, Mn2þ-doped lead halide perovskite nanocrystals with dual-color emission controlled by halide content, J. Am. Chem. Soc. 138 (2016) 14954e14961. [44] D. Cai, D. Zhu, X. Yuan, G. Wei, H. Li, J. Zhao, W. Han, Thermally stable luminescence of Mn2þ in Mn doped CsPbCl3 nanocrystals embedded in polydimethylsiloxane films, J. Lumin. 202 (2018) 157e162. [45] M. He, Y. Cheng, L. Shen, C. Shen, H. Zhang, W. Xiang, X. Liang, Mn-doped CsPbCl3 perovskite quantum dots (PQDs) incorporated into silica/alumina particles used for WLEDs, Appl. Surf. Sci. 448 (2018) 400e406. [46] S. Cao, J. Zheng, C. Dai, L. Wang, C. Li, W. Yang, M. Shang, Doping concentration-dependent photoluminescence properties of Mn-doped Zn-InS quantum dots, J. Mater. Sci. 53 (2018) 1286e1296. [47] R. Buonsanti, D.J. Milliron, Chemistry of doped colloidal nanocrystals, Chem. Mater. 25 (2013) 1305e1317. [48] T. Guner, B. Akbali, M. Ozcan, G. Topcu, M.M. Demir, H. Sahin, Monitoring the doping and diffusion characteristics of Mn dopants in cesium lead halide perovskites, J. Phys. Chem. C 122 (2018) 11543e11549.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis of high-efficient Mn2þ doped CsPbCl3 perovskite nanocrystals in toluene and surprised lattice ejection of dopants at mild temperature Kun Bai a, Ruosheng Zeng a, *, Bao Ke a, Sheng Cao b, ***, Xiaogang Xue a, Rihui Tan c, **, Bingsuo Zou b a School of Materials Science and Engineering, School of Life and Environmental Sciences, Guilin University of Electronic Technology, Guilin, 541004, PR China b School of Physical Science and Technology, Guangxi University, Nanning, 530004, PR China c Department of Basic Teaching, Guilin Tourism University, Guilin, 541006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 April 2019 Received in revised form 27 July 2019 Accepted 27 July 2019 Available online 27 July 2019
Doping has been established as a powerful approach for endowing novel properties to perovskite nanocrystals (NCs).However, the fast formation of perovskites makes the fundamental understanding of doping processes in these NCs very rare. In the present work, a mild temperature (<100 C) approach to produce Mn2þ doped CsPbCl3 (Mn:CsPbCl3) perovskite NCs in toluene is reported. The effects of the introduced Mn/Pb ratios in the raw materials as well as the temperature on the structure and photoluminescence (PL) properties of Mn:CsPbCl3 NCs were investigated systematically. The as-synthesized Mn:CsPbCl3 NCs shown a strong Mn2þ emission at 594 nm with a PL quantum yield (QY) of 52%, and the doping concentrations can be easily tailored from 1.8 atom% to 13.3 atom% by changed the Mn/Pb feed ratios. The study revealed that the reaction temperature plays a key role in Mn:CsPbCl3 NCs. Through elemental and microstructure characterization, it was surprisingly found that lattice ejection of Mn ions occurred at 100 C. This result greatly enhances the understanding of the “basic process” of Mn2þdoping in CsPbCl3 NCs, which provides some insights into the preparation of metal ion doped perovskite NCs. © 2019 Elsevier B.V. All rights reserved.
Keywords: Perovskite Doping Lattice ejection Optical property
1. Introduction All-inorganic perovskite nanocrystals (NCs) have attracted extensive interest due to their excellent optical properties and high carrier mobility, which have spurred their applications in lightemitting diodes, solar cells, photodetectors and so forth [1e8]. Recently, it has been discovered that doping metal ions can introduce a series of new functions in perovskite NCs, including new emission bands, improved photoluminescence (PL) quantum yield (QY), and enhanced stability, thereby the doping in perovskite NCs has become a hot topic in current research [9e16]. The doping process in NCs is generally considered to involve “lattice diffusion”
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (R. Zeng), [email protected] (S. Cao), [email protected] (R. Tan). https://doi.org/10.1016/j.jallcom.2019.07.335 0925-8388/© 2019 Elsevier B.V. All rights reserved.
and “lattice ejection” of dopant ions [17e21]. However, since perovskites are ionic crystals and the reaction grows very rapidly, their fundamentals of doping is still not widely understood [12,13,22]. Therefore, it is necessary to develop a facile synthesis procedure and more deeper understanding of the doping process in doped perovskite NCs. Among various dopants, the characters of Mn2þ emission are widely understood, and its luminescence properties show a longlived spin polarized d-d emission in the yellow-orange spectral window [23e26]. For Mn2þ doping in perovskite NCs, the compatible band alignment and efficient host exciton energy transfer to Mn2þ states are more favorable in CsPbCl3 [27e30]. Currently, there are a large number of reports on the preparation Mn2þ doped CsPbCl3 (Mn:CsPbCl3) NC, and the corresponding synthetic methods are mainly divided into the following two ways. (1) Room temperature method. Mn:CsPbCl3 NCs are synthesized through adding Mn2þ ions to the precursor by recrystallization [31,32], or via adding Mn2þ ions to the as-synthesized perovskite
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2.4. Characterization
NCs by ion exchange [33e35]. (2) High temperature hot-injection method. Mn:CsPbCl3 NCs are prepared by injecting a halogen element precursor in a reaction precursor containing Mn2þ ions and rapidly cooling to stop the growth [36e38]. At present, the high-quality of Mn:CsPbCl3 NCs are mainly obtained by high temperature hot-injection method [12,13,39], but their reproducibility still face challenges due to their demanding harsh preparation conditions that require rapid cooling processes. Hence, this demands more facile approach and in-depth study of the doping process for exploration of high-quality perovskite NCs. Herein, we report a new strategy to prepare Mn:CsPbCl3 NCs at mild temperature (<100 C) in toluene. The as-synthesized Mn:CsPbCl3 NCs showed a strong Mn2þ emission with a PL QY up to 52%, and the doping concentrations could easily be tailored from 1.8 atom % to 13.3 atom % by changing the Mn/Pb feed ratio. It is found that the reaction temperature is the most critical factor for the doping growth of Mn:CsPbCl3 NCs. The temporal evolution of Mn2þ emissions and Mn ions concentrations in Mn:CsPbCl3 NCs synthesized at 80 C and 100 C respectively was studied. With the analysis of the microstructure of Mn:CsPbCl3 NCs, the “elementary process” in these NCs was revealed from perspective of lattice ejection of dopants.
The morphology and structure of the obtained Mn:CsPbCl3 NCs were measured by transmission electron microscopy (TEM, JEOL 100 CX) and X-ray diffraction (XRD, Bruker D8 Discover). The XRD samples were prepared by depositing muddy solid NCs onto a glass slide and dried under vacuum at 60 C for 4 h. The PL decay curves were recorded at room temperature using an Edinburgh Instrument FLS980 fluorescence spectrometer (the excitation wavelength is 400 nm). The UV-vis absorption spectra were tested by using UH5300. PL spectra were collected using a Tianmei F-4600 fluorescence spectrophotometer. Elemental analysis was performed by inductively coupled plasma mass spectrometry (ICP-MS) with a Varian 720/730 series spectrometer. The PL QY was measured by Edinburgh Instrument FLS5 fluorescence spectrometer with a QY accessory. The electron paramagnetic resonance (EPR) spectrum were carried out using JEOL JES-FA200 with an X-band microwave frequency of 9.45 GHz at room temperature.Raman spectra were obtained on a Labram-010 Raman spectrometer system equipped with a 632 nm laser.
2. Experimental
3.1. Synthesis, optical and microstructure of Mn:CsPbCl3 NCs
2.1. Materials
Compared to the classic high temperature hot-injection synthesis method, the Mn:CsPbCl3 NCs here were prepared by a mild temperature synthesis, which does not require rapid cooling operation. The commonly used solvent of octadecene is also replaced by an economical and cheaper solvent of toluene. In a typical reaction, 0.1 mmol of CsAc, 0.1 mmol of PbAc2$3H2O, 0.1 mmol of MnAc2, 0.045 mL OA, 0.075 mL of OLA and 5 mL toluene were loaded in a 50 mL three-neck flask and heated to 80 C under a N2 atmosphere. 1 mL of RNH3Cl was injected into the reaction flask and keep growing for 10 min, then cooled to room temperature by air. The as-synthesized NCs were collected from the growth solution by centrifugation and re-dispersed in hexane for further characterization. Fig. 1 shows the absorption, PL, and photoluminescence excitation (PLE) spectra of undoped and Mn2þ doped CsPbCl3 NCs. Both samples have a similar absorption profile as shown in Fig. 1, but their PL have different features. The undoped sample exhibits only one exciton emission at 404 nm with a narrow full-width at half-
Cesium acetate (CsAc, 99%) and ammonium chloride acetate (NH4Cl, 99.8%) were purchased from Shanghai AccelaChemBio Co. Ltd. Lead acetate (PbAc2$3H2O, 99%), manganese acetate (MnAc2$4H2O, 99%), and oleylamine (OLA, 70%) were purchased from J&K Chemicals. Ethyl acetate (99%) and oleic acid (OA, 90%) were purchased from Aladdin Reagents. Toluene (99.5%) and hydrochloric (HCl, 37 wt% in water) were purchased from Xilong Scientific Co. Ltd. All reagents were used as-received without further purification. 2.2. Preparation of OLA-HCl (RNH3Cl) In a typical process, 10 mL of OLA and 1 mL of HCl were loaded in a 50 mL three-neck flask. The mixture was heated to 80 C with N2 flow and kept in this temperature for 1 h. Then the flask was heated to 120 C and kept for 2 h. The resulted precursor was stored for preparation of NCs at 80 C under inert atmosphere.
3. Results and discussion
2.3. Synthesis of Mn2þ doped CsPbCl3 NCs In a typical procedure, 19.7 mg of CsAc (0.1 mmol), 37.9 mg of PbAc2$3H2O (0.1 mmol), 24.5 mg of MnAc2 (0.1 mmol), 0.045 mL of OA, 0.075 mL of OLA and 5 mL of toluene were loaded in a 50 mL three-neck flask. After 10 min of nitrogen purge, the mixture was heated to a designated temperature (changed from 25 C to 100 C) and kept for 10 min until the clear solution was formed. Then 1 mL of RNH3Cl was injected into the reaction flask. Immediately, the strong yellow fluorescence was observed under the irradiation of the 365 nm ultraviolet lamp, which indicated that the Mn:CsPbCl3 NCs were formed. The reaction was stopped at the different required time. Mn:CsPbCl3 NCs with different Mn concentrations were produced by varying the Mn/Pb feed ratios in the starting mixture while keeping all other synthesis parameters fixed. The experimental results suggested that the production of the Mn:CsPbCl3NCs via the present mild temperature strategy was highly repeatable.
Fig. 1. Absorption, PL, and PL excitation (PLE) spectra of undoped (up) and Mn2þ doped (down) CsPbCl3 NCs dispersion in hexane. The excitation wavelength is 365 nm. The inset shows a digital photo of their solutions under a 365 nm ultraviolet lamp excitation.
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maximum (FWHM) of 85 meV, while the Mn2þ doped sample shows two different emission with a narrow peak at 397 nm and a broad peak at 594 nm. The narrow emission at 397 nm is attributed to the intrinsic exciton emission of CsPbCl3 NCs, while the broader orange emission peak at 594 nm with a 276 meV of FWHM is attributed to Mn2þ4T1/6A1d-d transition [13,36,40]. The PLE spectra of undoped and Mn2þ doped CsPbCl3 NCs were probed at 404 nm and 594 nm, respectively. As shown in Fig. 1, both PLE spectra follow their absorption spectra, indicating that the Mn2þ emissions origin from the energy transfer from the photoexcited CsPbCl3 NC host [12,13,37]. The digital photo of their solutions was recorded under irradiation of an ultraviolet lamp with a wavelength of 365 nm. As shown in the inset of Fig. 1, the undoped and Mn2þ doped CsPbCl3 NCs dispersed in hexane showed blue and bright orange color, respectively. The orange emission in the Mn:CsPbCl3 NCs is also a direct evidence of the successful incorporation of Mn2þ into CsPbCl3 host NC. In addition, the Mn:CsPbCl3 NCs exhibited a strong Mn2þ emission with a PL QY up to 52% (the total PL QY plus excitonic emission was 55%). The QYs of Mn2þ doped NCs is lower than that of pure NCs due to energy transfer losses from the NC host to the dopant emission center [40,41]. However, herein the QY of our NCs is comparable to the state-of-the-art of the high temperature hot-injection synthesized Mn:CsPbCl3 NCs (Table S1, Supporting Information). Fig. 2 shows the typical TEM images and XRD patterns of undoped and Mn2þ doped CsPbCl3 NCs. The TEM images in Fig. 2aeb show that the CsPbCl3 and Mn:CsPbCl3 NCs have a cubic shape with a uniform size distribution of 11.9 ± 1.8 and 11.1 ± 2.3 nm, respectively (see Fig. S1, Supporting Information). The high-resolution TEM images of their NCs (insets in Fig. 2aeb) demonstrate that these NCs are highly crystalline with a clearly lattice fringes of 0.56 nm, which is consistent with the nature of perovskite CsPbCl3 [31,42,43]. The XRD patterns of CsPbCl3 and Mn:CsPbCl3 NCs in Fig. 2c shows that both samples are wellcrystallized, and are structurally identical to tetragonal phase CsPbCl3 structure (PDF#18-0366) [44,45]. Compared with the pristine NCs, the magnify (002) diffraction of tetragonal
Mn:CsPbCl3NCs shows a clearly peak shift to a higher angle compared to the undoped sample, which clearly indicates that lattice doping of Mn occurs. The absence of other diffractions indicates no significant presence of crystalline impurities, and that the tetragonal crystal structure is largely unaffected by Mn2þ doping. These measurements show that the mild temperature approach is an effective way to obtain high-quality Mn:CsPbCl3 NCs. 3.2. Effects of Mn/Pb ratio on the structure and optical properties of Mn:CsPbCl3 NCs The mild temperature synthesis approach was used to yield a series of Mn:CsPbCl3 NCs with different Mn/Pb feed ratios (i.e., 0, 0.1, 0.5, 1, and 1.5). Table S2 (Supporting Information) shows the positive correlation between the feed Mn/Pb ratios and actual Mn contents. The Mn doping concentrations (relative to Pb content) could be tailored from 1.8 atom % to 13.3 atom % with the feed Mn/ Pb molar ratios adjusted from 0.1 to 1.5. All XRD patterns of Mn:CsPbCl3 NCs in Fig. 3a show a well-crystallized tetragonal phase structure without crystalline impurities. The detailed optical properties of Mn:CsPbCl3 NCs are shown in Fig. 3bed. All samples show an obvious first absorption band at about ~400 nm, and this absorption band shift slightly continuously to the blue as the ratio of Mn/Pb increases (Fig. 3b). The similar blue tendency was also observed in their excitonic PL emission (Fig. 3c), which is likely due to the effect of Mn2þ ions changing the host electronic band structure [33,37,43]. The PL intensity of Mn2þemission increases proportionately with the increases the Mn/Pb feed ratio and reaches the maximum PL QY up to 52% (Fig. 3d) at the Mn/Pb ratio of 1.0, then decrease with the further increase of the Mn/Pb feed ratios. It is noted that the PL QY of CsPbCl3 host exciton decreases monotonously with the increase the Mn/Pb feed ratios, which indicates that the PL QY of Mn2þ emission increase first is due to the fact that as the Mn/Pb ratio increases, more excitation energy is transferred from the CsPbCl3 host exciton to the Mn2þ emission acceptors [37,41,46]. When the Mn/Pb feed ratio exceeds 1.0, their PL QY exhibits a drop trend, which may be attributable to the formed Mn2þ-Mn2þ pairs and enhanced energy migration to traps at elevated Mn/Pb ratios [36,37,40]. 3.3. Effects of reaction temperature on the optical properties of Mn:CsPbCl3 NCs
Fig. 2. Typical TEM images of (a) undoped and (b) Mn2þ doped CsPbCl3 NCs. The insets show their corresponding HRTEM image. (c) XRD patterns from the undoped and Mn2þ doped CsPbCl3 NCs, the right panel magnifies the (002) diffraction to show the peak shifts.
Fig. 4 shows the optical properties of Mn:CsPbCl3 NCs prepared at a fixed Mn/Pb feed ratio of 1.0 and different reaction temperatures of 25, 40, 60, 80 and 100 C. The PL spectra in Fig. 4a show that all samples have two PL peaks around at ~400 nm and ~590 nm, which correspond to their excitonic and Mn2þ ion emission, respectively. Fig. 4b shows the PL image of their solution under the radiation of a UV lamp with a wavelength of 365 nm. By changing the synthesis temperature, these NCs represent different colors, suggesting that their PL emissions could be profoundly tailored by adjusting the reaction temperature of Mn:CsPbCl3 NCs. Fig. 4ced show the PL QYs and decay curves of Mn2þ emissions in Mn:CsPbCl3 NCs synthesized at different reaction temperature, respectively. Specifically, the PL QY of Mn2þ emission increases as the synthesis temperature of Mn:CsPbCl3 NCs increases, reaches a maximum of 52% at a reaction temperature of 80 C, and then decreases as the reaction temperature further increases. The PL lifetime of Mn:CsPbCl3 NCs prepared at 80 C was obtained to be about 1.7 m, which is similar to others [37,40]. This means that the PL lifetimes of Mn2þ emissions in Mn:CsPbCl3 NCs obtained at various temperatures
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Fig. 3. (a) Powder XRD patterns, (b) PL spectra, (c) PL QY, and (d) UV-vis absorption spectra of Mn2þ doped CsPbCl3 NCs synthesized at 80 C with different feed ratios of Mn/Pb. The excitation wavelength is 365 nm “x10” refers to magnified 10 times.
Fig. 4. (a) PL spectra and (b) PL QY of Mn2þ doped CsPbCl3 NCs synthesized at different reaction temperatures with a Mn/Pb feed ratio of 1.0. The excitation wavelength is 365 nm. (c) Typical digital photo of their NCs dispersed in hexane under a 365 nm ultraviolet lamp excitation. (d) Excited state Mn2þ emission decay curves (y-axis in log scale) of Mn2þ doped CsPbCl3 NCs.
do not have a clear change, showing a good agreement with the previous report [40]. It is found that the doping concentration of Mn ions increases with the increase of reaction temperature (Table S2, Supporting Information), improving the PL QY of Mn:CsPbCl3 NCs [37,40].
3.4. The “lattice ejection” of dopants at mild temperature Fig. 5a shows the temporal evolution of Mn2þ emission in Mn:CsPbCl3 NCs with a Mn/Pb feed ratio of 1.0 synthesized at 80 and 100 C, respectively (the corresponding PL spectra shown in
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Fig. 5. Temporal evolution of Mn2þ PL QYs (a) and Mn doping concentrations (b) of Mn2þ doped CsPbCl3 NCs with a Mn/Pb feed ratio of 1.0 prepared at 80 and 100 C, respectively.
Fig. S2 at Supporting information). Under the reaction condition of 80 C, the PL QYs of the Mn2þ emission reached a maximum in about 10 min, and then a slow decrease occurred. Under the reaction condition of 100 C, the PL QYs of the Mn2þ emission possessed a sharp maximum at 2 min. It is reported that the PL intensity of Mn2þ emission exhibits a positive correlation with the dopant ions diffusion in Mn2þ doped ZnSe NCs [17,18]. To reveal this point in our case, we purified the aliquot NCs taken at different reaction time for these samples and performed component analysis. The ICP-MS measurement result in Fig. 5b shows a significant increase in the Mn doping concentration in these samples during the first 2 min of the reaction, indicating the presence of diffusion of Mn ions during this process. As the reaction time increased, the concentration of Mn ions in the NCs separated at the reaction temperature of 80 C did not change significantly. However, a sharp drop in the concentration of Mn ions was observed in the NC isolated at the reaction temperature of 100 C. This phenomenon that the Mn doping concentration decreases with the increase of reaction time is similar to that of Mn doped chalcogenide NCs, which is usually related to “lattice ejection” of dopant [17,18,47]. To the best of our knowledge, this is the first example to demonstrate the “elementary process” of “lattice ejection” in Mn2þ doping CsPbCl3 NCs. It is noted that the critical temperature of “lattice ejection” in our
Mn:CsPbCl3 NCs synthesis procedure is 80 C, once this temperature is exceeded, the “lattice ejection” of Mn ions occur very rapidly. This may explain why the reaction needs to be quickly terminated in the high temperature hot-injection synthesis method. To further confirm that lattice ejection of Mn ions does occur at 100 C, further XRD and EPR analysis were carried out for the Mn2þ doped CsPbCl3 NCs synthesized at different times. As shown in Fig. 6a, the XRD spectra of NCs synthesized with different reaction time are almost the same, and no obvious narrowing or broadening of peaks was observed. This indicates that the phase structure of NCs has not been changed significantly at 100 C, which also excludes the possibility of the change of the PL QYs resulted from the change of phase. The EPR spectra of all the Mn:CsPbCl3 NCs in Fig. 6b show a well-defined sextet hyperfine splitting pattern (hyperfine constant A ¼ 86 G), which is consistent with the reported results [30]. The EPR spectrum of 30 min indicates the well-isolated Mn-ions are uniformly distributed in host lattice compared to that of 5s. That is so because Mn ions in the host lattice were carried sufficient lattice diffusion at 100 C. As indicated in Fig. 6b, there is a slightest decrease in dipolar broadening with time which suggests Mn ions in host move apart probably due to lattice diffusion [16]. Combined with the results of elemental analysis, the lattice ejection in Mn:CsPbCl3 NCs at 100 C is well confirmed.The lattice ejection was also monitoring by Raman spectrum (Fig. S3 in Supporting information), Mn-induced vibrational modes of the crystal 190 cm1 was observed to a few wavenumbers shifted, which further confirms the lattice diffusion of dopants [48].
4. Conclusion In conclusion, we reported a new strategy to obtain Mn:CsPbCl3 NCs at mild temperature in economic toluene. The as-synthesized Mn:CsPbCl3 NCs shown a strong Mn2þ emission with a PL QY up to 52%, and the doping concentrations can be easily tarried from 1.8 atom % to 13.3 atom % by changed the Mn/Pb feed ratios. The reaction temperature was the most critical factor for the doping growth of Mn:CsPbCl3 NCs. It not only influences on the degree of crystallinity of the Mn:CsPbCl3NCs, but also effects the “lattice diffusion” and “lattice ejection” of the Mn2þ ions; and finally determines the optical performance of Mn2þ in Mn:CsPbCl3 NCs. This work may provide some new strategy and perceptive for the preparation and study of the doping process in doped perovskite NCs.
Fig. 6. Temporal evolution of powder XRD patterns (a) and room temperature EPR spectrum of Mn2þ doped CsPbCl3 NCs synthesized at 100 C.
K. Bai et al. / Journal of Alloys and Compounds 806 (2019) 858e863
Conflicts of interest There are no conflicts to declare. Acknowledgements This work was financial supported by the National Natural Science Foundation of China (Grant No. 21661010) and Guangxi Natural Science Foundation (Grant No. 2017GXNSFGA198005). S. Cao appreciates the special fund of “Guangxi Bagui Scholar”. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.07.335. References , M.V. Kovalenko, L. Manna, Genesis, challenges and [1] Q.A. Akkerman, G. Raino opportunities for colloidal lead halide perovskite nanocrystals, Nat. Mater. 17 (2018) 394e405. [2] P. Ramasamy, D.-H. Lim, B. Kim, S.-H. Lee, M.-S. Lee, J.-S. Lee, All-inorganic cesium lead halide perovskite nanocrystals for photodetector applications, Chem. Commun. 52 (2016) 2067e2070. [3] L. Protesescu, S. Yakunin, M.I. Bodnarchuk, F. Krieg, R. Caputo, C.H. Hendon, R.X. Yang, A. Walsh, M.V. Kovalenko, Nanocrystals of cesium lead halide perovskites (CsPbX3, X ¼ Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut, Nano Lett. 15 (2015) 3692e3696. [4] J. Song, J. Li, X. Li, L. Xu, Y. Dong, H. Zeng, Quantum dot light-emitting diodes based on inorganic perovskite cesium lead halides (CsPbX3), Adv. Mater. 27 (2015) 7162e7167. [5] J. Li, L. Xu, T. Wang, J. Song, J. Chen, J. Xue, Y. Dong, B. Cai, Q. Shan, B. Han, H. Zeng, 50-fold EQE improvement up to 6.27% of solution-processed allinorganic perovskite CsPbBr3 QLEDs via surface ligand density control, Adv. Mater. 29 (2017) 1603885. [6] Z. Bai, H. Zhong, Halide perovskite quantum dots: potential candidates for display technology, Sci. Bull. 60 (2015) 1622e1624. [7] X.-g. Wu, J. Tang, F. Jiang, X. Zhu, Y. Zhang, D. Han, L. Wang, H. Zhong, Highly luminescent red emissive perovskite quantum dots-embedded composite films: ligands capping and caesium doping-controlled crystallization process, Nanoscale 11 (2019) 4942e4947. [8] X. Chen, F. Zhang, Y. Ge, L. Shi, S. Huang, J. Tang, Z. Lv, L. Zhang, B. Zou, H. Zhong, Centimeter-sized Cs4PbBr6 crystals with embedded CsPbBr3 nanocrystals showing superior photoluminescence: nonstoichiometry induced transformation and light-emitting applications, Adv. Funct. Mater. 28 (2018) 1706567. [9] F. Li, Z. Xia, Y. Gong, L. Gu, Q. Liu, Optical properties of Mn2þ doped cesium lead halide perovskite nanocrystals via a cation-anion co-substitution exchange reaction, J. Mater. Chem. C 5 (2017) 9281e9287. [10] Z.-J. Yong, S.-Q. Guo, J.-P. Ma, J.-Y. Zhang, Z.-Y. Li, Y.-M. Chen, B.-B. Zhang, Y. Zhou, J. Shu, J.-L. Gu, L.-R. Zheng, O.M. Bakr, H.-T. Sun, Doping-enhanced short-range order of perovskite nanocrystals for near-unity violet luminescence quantum yield, J. Am. Chem. Soc. 140 (2018) 9942e9951. [11] M. Liu, G. Zhong, Y. Yin, J. Miao, K. Li, C. Wang, X. Xu, C. Shen, H. Meng, Aluminum-doped cesium lead bromide perovskite nanocrystals with stable blue photoluminescence used for display backlight, Adv. Sci. 4 (2017) 1700335. [12] Y. Zhou, J. Chen, O.M. Bakr, H.-T. Sun, Metal-doped lead halide perovskites: synthesis, properties, and optoelectronic applications, Chem. Mater. 30 (2018) 6589e6613. [13] A.K. Guria, S.K. Dutta, S. Das Adhikari, N. Pradhan, Doping Mn2þ in lead halide perovskite nanocrystals: successes and challenges, ACS Energy Lett 2 (2017) 1014e1021. [14] S. Hou, M.K. Gangishetty, Q. Quan, D.N. Congreve, Efficient blue and white perovskite light-emitting diodes via manganese doping, Joule 2 (2018) 2421e2433. [15] L. Fei, X. Yuan, J. Hua, M. Ikezawa, R. Zeng, H. Li, Y. Masumoto, J. Zhao, Enhanced luminescence and energy transfer in Mn2þ doped CsPbCl3-xBrx perovskite nanocrystals, Nanoscale 10 (2018) 19435e19442. [16] S. Zou, Y. Liu, J. Li, C. Liu, R. Feng, F. Jiang, Y. Li, J. Song, H. Zeng, M. Hong, X. Chen, Stabilizing cesium lead halide perovskite lattice through Mn(II) substitution for air-stable light-emitting diodes, J. Am. Chem. Soc. 139 (2017) 11443e11450. [17] D. Chen, R. Viswanatha, G.L. Ong, R. Xie, M. Balasubramaninan, X. Peng, Temperature dependence of “elementary processes” in doping semiconductor nanocrystals, J. Am. Chem. Soc. 131 (2009) 9333e9339. [18] R. Zeng, M. Rutherford, R. Xie, B. Zou, X. Peng, Synthesis of highly emissive Mn-doped ZnSe nanocrystals without pyrophoric reagents, Chem. Mater. 22 (2010) 2107e2113.
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