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Cs4PbBr6 perovskite composite
Optik - International Journal for Light and Electron Optics 208 (2020) 164579
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
One-pot synthesis of CsPbBr3/Cs4PbBr6 perovskite composite Xiaogai Penga, Jin Chena,*, Fengchao Wanga,*, Canyun Zhanga, Bobo Yanga,b a b
T
College of Sciences, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, China Institute of Future Lighting, Academy for Engineering and Technology, Fudan University, 220 Handan Road, Shanghai 200433, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Perovskite CsPbBr3/Cs4PbBr6 composite Quantum dots One-pot route
In current study, a facile one-pot approach was introduced for the preparation of CsPbBr3/ Cs4PbBr6 composite. The formation mechanism behind this proposed approach was investigated in detail. As the results shown, the structure observations by XRD and HRTEM suggested the obtained sample, prepared under an optimal condition of (90 ℃, 20 min), was in a miscible phase of CsPbBr3/Cs4PbBr6. The morphology analysis by TEM illustrated the sample monodispersed as a clear cubic shape with an average size of ∼10 nm. The composition examination by EDS revealed a Pb-poor and Br-rich state which was favorable for the Cs4PbBr6 formation. Particularly, the synthesized sample had a narrow line-width of ∼22 nm, a photoluminescence quantum yield of ∼57 %, and a color purity of 88 %. This indicates the obtained sample is a promising candidate in lighting and display field.
1. Introduction Semiconductor quantum dots (QDs) are a new type of broadband emission spectral material, which are widely used in the field of light-emitting [1], display [2], and biosensing [3] due to their ability to tune optical bandgap from violet to near infrared region. Besides, these QDs can absorb high-energy photons and per photon can generate multiple electron-hole pairs (multi-exciton effects). Compared with bulk semiconductor materials, the bandgap of QDs is wider and the electronic states become discrete [4], which is crucial for obtaining high-efficient photoelectric devices. In recent decade, the perovskite QDs with ABX3 (A = CH3NH3+, CH (NH2)2+ and Cs+, B = Pb2+, Sn2+ and Ge2+, X = Cl−, Br− and I−) structure have been well investigated, because of their high photoluminescence quantum yield (PLQY), facile bandgap tunability, and good carrier mobility, particularly in all-inorganic CsPbX3 QDs [5]. Since the pioneered work reported by Kovalenko and his co-workers about inorganic colloidal CsPbX3 nanocrystals [6], more enthusiasms have been attracted in this field. Different technologies have been involved into the fabrication for this material, such as hot-injection [7], recrystallization [8], microwave [9]. Hot-injection method is a prevalent one for the QDs synthesis, whereas this method is restricted in the nucleation homogeneity and complicated operation [10]. As another popular one, recrystallization route is insufficient yield. Compared to these methods, one-pot route is more favor to the simplicity and homogeneity. Thus, this approach is a promising candidate for the scale-up production of perovskite QDs. As well known, CsPbX3 QDs feature a fragile and unstable structure composed of [PbX6]4− octahedron with Cs+ cations filling. This structure has poor tolerance under moisture and heating because of the easy losing of halide anions. Diverse strategies have been involved to overcome above concerns, such as ligand passivation route [11], core-shelling method [12,13]. Recently, some studies reported that Cs4PbX6, a derived member of Cs-Pb-X compounds, could play an in situ passivation for the stability improvement of
⁎
Corresponding authors. E-mail addresses: [email protected] (J. Chen), [email protected] (F. Wang).
https://doi.org/10.1016/j.ijleo.2020.164579 Received 1 February 2020; Accepted 16 March 2020 0030-4026/ © 2020 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 208 (2020) 164579
X. Peng, et al.
CsPbX3 material [14,15]. CsPbX3 is allowed and protected to grow in the Cs4PbX6 matrix. In addition, some evidences also showed that the miscible phase of CsPbX3/Cs4PbX6 is beneficial for the luminescent elevation which may result from the improvement of the radiative recombination of excited carriers [16–19]. However, the studies on the facile and simultaneous synthesis of this composite are still scarce. In this study, a facile one-pot approach was introduced for the synthesis of the CsPbBr3/Cs4PbBr6 composite. To our knowledge, similar work is reported rarely to date. The formation mechanism of the proposed approach was deeply analyzed. The properties of the obtained sample, including photoluminescence (PL) emission, structure, morphology, and composition, were investigated in detail. These properties were determined by fluorescence spectrometer, X-ray diffractometer (XRD), transmission electron microscope (TEM), and energy-dispersive X-ray spectroscopy (EDS), respectively. 2. Experimental details 2.1. Synthesis of CsPbBr3/Cs4PbBr6 QDs Typically, 1 mmol Cs2CO3 (cesium carbonate, 99 %, AR), 3 mmol PbBr2 (lead bromide, 99 %, AR), 0.25 ml OA (oleic acid, 90 %, AR), 0.25 ml OLA (oleylamine, 70 %, AR) and 10 ml ODE (1-octadecene, 90 %) were successively added into the three-neck flask with magnetic stirring. Then, the solution was heated to allow the crystals to grow under the protected atmosphere of nitrogen. The investigated reaction temperature varied in the range of 60–110 ℃ with an increment of 10 ℃, while the reaction time was 20 min. After reaction, ice bath was employed to cool the solution immediately. The as-obtained products were collected via centrifugation and washing processes for several times. Finally, the precipitates were dispersed in the n-hexane solution for next using. 2.2. Characterization for as-obtained samples PL emission spectra were determined by a spectrofluorometer (FS5, Edinburgh, UK). The crystal structure was measured by X-ray diffraction (XRD) (Ultima IV, Rigaku, Japan) with a Cu-Kα radiation (40 kV, 40 mA). The morphology of as-synthesized QDs were studied by the transmission electron microscopy (TEM) (TECNAI G2 F20, FEI, USA) equipped with an accelerating voltage of 200 kV. The absorption spectrum was investigated by a UV–vis (UV–vis) spectrometer (UH4150, Hitachi, Japan). The surface composition of QDs was analyzed by an energy-dispersive X-ray spectroscopy (EDS) (Quanta 200 FEG, FEI, USA). The PL decay was detected by highresolution spectrofluorometer (FLS920, Edinburgh Instruments, UK). 3. Results and discussion Fig. 1a shows the schematic diagram of one-pot approach for the preparation of CsPbBr3/Cs4PbBr6 QDs. As seen, the whole synthesis employs the injection process no longer. This simplifies the operation effectively compared to the well-known hot-injection route. To observe the influences of reaction temperature on the performances of obtained samples, the value of the temperature was varied from 60 to 110 ℃ with increments of 10 ℃. The reaction time was set as a constant of 20 min. Fig. 1b shows the photoluminescence of the synthesized samples prepared at different reaction temperatures. The 365 nm wavelength was carried out for the excitation. As observed, the color of these samples changed from cyan to green with the increasing temperature. This indicated the emission of the obtained samples could be tuned with the temperature changing. It may stem from the growing of QDs size as temperature increased which has been suggested by previous study [20]. The PL emission spectra are shown in Fig. 2a. As seen, the PL emission intensity of the obtained samples prepared in 60–80 ℃ range varied slightly while that of 90 ℃ sample increased dramatically. Interestingly, the PL intensity decreased when the temperature was over 90 ℃. This is further showed in Fig. 2b. It is noteworthy that the emission peak of 90 ℃ sample featured a symmetric shape with a narrow line-width of ∼22 nm near the value in previous report [21]. The evolutions of the PL emission intensity may be caused by the different growing stages of the QDs. In the initial solution, Cs+ and Pb2+ cations would combine with OA to form Cs-OA and Pb-OA complexes, while the Br− anions from PbBr2 reactant acted with OAm to form Br-OAm complexes, as suggested by Zhang et al. [22]. When the reaction solution was heated, these complexes released Cs+, Pb2+, and Br− ions again within the reaction solution. Then these ions integrated to form CsxPbyBrz products. The schematic diagram of the synthesis is shown in Fig. 3. In the 60–80 ℃ range, the crystal nuclei were primarily formed in the solution. However, because of the low solubility of reactants
Fig. 1. a) the scheme of synthesis procedure and b) the luminescence image of obtained samples synthesized at different temperatures under 365 nm excitation. 2
Optik - International Journal for Light and Electron Optics 208 (2020) 164579
X. Peng, et al.
Fig. 2. a) PL emission spectrum, b) emission intensity, and c) central wavelength of the samples prepared at different temperatures.
Fig. 3. Schematic diagram of the synthesis of the proposed one-pot approach.
in ODE, a small number of Cs+, Pb2+, and Br− ions were produced to allow the crystal grow, this may attribute to the low PL emission intensity. As the temperature increased over 80 ℃, the solubility of these reactants increased to provide more cations and anions. This accelerated the processes of crystallization and formation of CsxPbyBrz products. Thus, the PL peak presented a high intensity at 90 ℃. Increasing the temperature further, Ostwald ripening occurred during the crystal growing resulted in the crystal size becoming larger. This decreased the quantum confinement effect of the synthesized products. So, the PL emission decreased in 100∼110 ℃ range. Noteworthily, the emitting wavelength of the obtained samples red-shifted from 496 to 515 nm with the increasing temperature. This may originate from the size growing of the crystals. Additionally, the photoluminescence quantum yield (PLQY) of the obtained QDs prepared at different temperatures were also determined by Edinburgh Instruments FLS 920. Among these, the sample obtained at 90 ℃ had the highest value of ∼57 %. Thus, the various properties of this 90 ℃ sample were further investigated. To understand the structure of the obtained sample prepared at 90 ℃, the X-ray diffraction (XRD) analysis was employed in current study. Fig. 4 shows the XRD patterns. As identified by the standard JCPDS cards of No. 54-0752 and No. 73-2478, the obtained sample consisted of cubic CsPbBr3 (a = b = c = 5.830 Å) and trigonal Cs4PbBr6 (a = b =13.730 Å, c =17.320 Å) composites. Typically, the diffraction peaks located at 15.19°, 21.55°, 26.48°, 30.64°, 34.37°, 37.77°, 46.69° could be attributed to (100), 3
Optik - International Journal for Light and Electron Optics 208 (2020) 164579
X. Peng, et al.
Fig. 4. XRD patterns of the obtained sample prepared at 90 ℃.
(110), (111), (200), (210), (211), (300) planes of cubic CsPbBr3, respectively. The peaks located at 12.64°, 20.08°, 22.41°, 25.43°, 27.51°, 28.60°, 30.27° were ascribed to (012), (113), (300), (024), (131), (214), (223) planes of trigonal Cs4PbBr6, separately. The composites of CsPbBr3/Cs4PbBr6 may derive from the lower solubility of Pb2+ than that of Cs+ and Br− in the reaction solution, which was preferable to form Cs4PbBr6 products. As studied by previous report [23], Cs4PbBr6 could be seen as the intermediate phase for CsPbBr3. With more Pb2+ emerging in the reaction, the Cs4PbBr6 would transform to be CsPbBr3. The transmission electron microscopy (TEM) was employed to investigate the morphology of the obtained sample. As seen in Fig. 5a, the QDs dispersed homogeneously without aggregation. The QDs’ shapes featured a clear cubic structure. The size was ranging from 7 to 15 nm, and the average value was ∼10 nm. These QDs’ structure was further analyzed via the high resolution TEM (HRTEM) as shown in Fig. 5b. It was found that there were two typical lattice structures. The interplanar distances of 2.9 Å and 4.2 Å could be attributed to the (200) and (100) plans of cubic CsPbBr3, respectively. The lattice spacings of 7.0 Å and 3.0 Å were corresponding to the (110) (312) plans of trigonal Cs4PbBr6, separately. Above could be also directly proved via selected area fast Fourier transformation (FFT) analysis. This indicated the obtained sample composed of CsPbBr3/Cs4PbBr6 composites, which kept steps with the XRD analysis well. Additionally, the clear lattice fringes suggested that the synthesized sample had a good crystallinity. Fig. 6a displays the absorption spectrum of the prepared sample. As seen, there were two characteristic absorption peaks locating at 314 and 515 nm could be ascribed to Cs4PbBr6 and CsPbBr3, respectively [24]. This was quiet consistent with the above XRD and HRTEM analysis. The bandgap energy Eg of this composite was extracted from the (Ahν)2 versus (hν) plots, where A is the absorbance, h is the Planck constant, and ν is the photon frequency. Here, the Eg value was around of 2.85 eV, which were tuned by the coexistence of Cs4PbBr6 (3.25 eV) and CsPbBr3 (2.0 eV). The chemical composition of the obtained sample is shown in Fig. 6b. The atomic percent of Cs:Pb:Br was 3:1:5, which matched the mixed state of CsPbBr3/Cs4PbBr6 well. The Pb/(Cs + Br) ratio was 0.13, which suggested a Pb-poor state of the synthesized sample resulting from the poor solubility of Pb source in the reaction solution. This was responsible for the formation of Cs4PbBr6 product. Noteworthily, the Br/(Pb + Cs) ratio was 1.23, which meant a Br-rich state. As suggested by previous report [25], the richness of halogen may play a self-passivation effect because the redundant halogen ions on the surface will connect with cations. This facilitates the reducing of surface trap-defects and non-radiative recombination centers. Fig. 6c shows the time-resolved PL decay curve of the sample synthesized at 90 ℃. This PL curve could be well fitted with a biexponential decay function as shown in Eq. (1) [26]. Where A1, A2 are constants, t is time. τ1 and τ2 are the short-lived lifetime and long-lived lifetime, respectively. As observed, τ1 was 4.71 ns (A1 = 34.8 %) and τ2 was 37.20 ns (A2 = 65.2 %).
Fig. 5. a) TEM image and b) HRTEM image of the obtained sample prepared at 90 ℃. 4
Optik - International Journal for Light and Electron Optics 208 (2020) 164579
X. Peng, et al.
Fig. 6. a) the absorption spectra and PL spectra, b) chemical composition, c) PL decay curve, and d) CIE coordinate of CsPbBr3/Cs4PbBr6 composites.
A(t) = A1 exp(-t/τ1) + A2 exp(-t/τ2)
(1)
τave = (A1τ1 + A2τ2 ) / (A1τ1 + A2τ2)
(2)
2
2
The average lifetime (τave) was calculated by Eq. (2) [27]. It was found that the τave value was 35.14 ns. This indicated a high ratio of radiative to nonradiative recombination and less transition of detect states. As noted, the excluding of trap-defect and nonradiative recombination, caused by Br-rich self-passivation on QDs surface, may be prefer to result in the long lifetime. As shown in Fig. 6d, the CIE coordinate of the CsPbBr3/Cs4PbBr6 composites was located at (0.0617, 0.708). The calculated color purity was 88 %, which was accordance with the narrow PL line-width of ∼22 nm. This suggests that the obtained sample is a promising candidate for the application in wide color-gamut display.
4. Conclusions In summary, the CsPbBr3/Cs4PbBr6 composite was facilely and successfully synthesized by a facile one-step approach. Meanwhile, the mechanism behind this approach was analyzed in detail. The results showed the reaction condition of (90 ℃, 20 min) was favorable for the sample synthesis. XRD and HRTEM analysis confirmed the miscible phase of CsPbBr3/Cs4PbBr6 in obtained sample. TEM image showed that the sample monodispersed well and had a clear cubic shape with an average size of ∼10 nm. EDS observation revealed a Pb-poor and Br-rich composition. Noteworthily, Pb-poor state was preferable for the Cs4PbBr6 formation, Br-rich state was favorable for a self-passivation role to reduce the surface trap-defects. Remarkably, the synthesized sample had a narrow line-width of ∼22 nm, a photoluminescence quantum yield of ∼57 %, and a color purity of 88 %. This suggests the obtained sample is a promising candidate for the application of photoelectric devices.
Declaration of Competing Interest The authors declare no conflicts of interest.
Acknowledgements The project was sponsored by Shanghai Sailing Program, China (No. 18YF1422500) and Research start-up project of Shanghai Institute of Technology (No. YJ2018-9). 5
Optik - International Journal for Light and Electron Optics 208 (2020) 164579
X. Peng, et al.
References [1] J. Song, J. Li, X. Li, L. Xu, Y. Dong, H. Zeng, Nanocrystals: quantum dot light‐emitting diodes based on inorganic perovskite cesium lead halides (CsPbX3), Adv. Mater. 27 (2015) 7162–7167. [2] H. Li, H. Liu, H. Zhang, G. Zhu, L. Wang, B. Yang, W. Li, Application of quantum dots labeling to the interaction between reconstructed-phage display peptide and insulin receptor, Chem. Res. Chinese U. 25 (2004) 982–984. [3] J. Yuan, N. Gaponik, A. Eychmüller, Application of polymer quantum dot-enzyme hybrids in the biosensor development and test paper fabrication, Anal. Chem. 84 (2012) 5047–5052. [4] P. Cozzoli, T. Pellegrino, L. Manna, Synthesis, properties and perspectives of hybrid nanocrystal structures, Chem. Soc. Rev. 35 (2006) 1195–1208. [5] Q. Shan, J. Li, J. Song, Y. Zou, L. Xu, J. Xue, Y. Dong, C. Hou, J. Chen, B. Han, H. Zeng, All-inorganic quantum-dot light-emitting diodes based on perovskite emitters with low turn-on voltage and high humidity stability, J. Mater. Chem. C Mater. Opt. Electron. Devices 5 (2017) 4565–4570. [6] L. Protesescu, S. Yakunin, M.I. Bodnarchuk, F. Krieg, R. Caputo, C.H. Hendon, R. Yang, A. Walsh, M.V. Kovalenko, Nanocrystals of cesium lead halide perovskites (CsPbX3, X =Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut, Nano Lett. 15 (2015) 3692–3696. [7] L. Wang, B. Liu, X. Zhao, H. Demir, Solvent-assisted surface engineering for high-performance all inorganic perovskite nanocrystal light-emitting diodes, ACS. Appl. Mater. Inter. 10 (2018) 19828–19835. [8] C. Gao, Y. Zhang, X. Ma, F. Yu, Y. Jia, Y. Lei, P. Chen, W. Sun, Z. Xiong, A method toards 100% internal quantum efficiency for all-inorganic cesium halide perovskite light-emitting diodes, Org. Electron. 58 (2018) 88–93. [9] Q. Pan, H. Hu, Y. Zou, M. Chen, L. Wu, D. Yang, X. Yuan, J. Fan, B. Sun, Q. Zhang, Microwave-assisted synthesis of high-quality “all-inorganic’’ CsPbX3 (X = Cl, Br, I) perovskite nanocrystals and their application in light emitting diodes, J. Mater. Chem. C Mater. Opt. Electron. Devices 5 (2017) 10947-10945. [10] K. Lee, J. Lee, H. Kang, B. Park, Y. Kwon, H. Ko, C. Lee, J. Lee, H. Yang, Over 40 cd/A efficient green quantum dot electroluminescent device comprising uniquely large-sized quantum dots, ACS Nano 8 (2014) 4893–4901. [11] J. Li, L. Xue, T. Wang, J. Song, 50-fold EQE improvement up to 6.27% of solution-processed all-inorganic perovskite CsPbBr3 QLEDs via surface ligand density control, Adv. Mater. 29 (2016) 7162–7167. [12] B. Qiao, P. Song, J. Cao, S. Zhao, Z. Shen, D. Gao, Z. Liang, Z. Xu, D. Song, X. Xu, Water-resistant, monodispersed and stably luminescent CsPbBr3/CsPb2Br5 coreshell like structure lead halide perovskite nanocrystals, Nanotechnology 28 (2017) 445602. [13] Q. Zhong, M. Cao, H. Hu, D. Yang, One-pot synthesis of highly stable CsPbBr3@SiO2 core-shell nanoparticles, ACS Nano 12 (2018) 8579–8587. [14] L. Xu, J. Li, F. Tao, Y. Zhao, Synthesis of stable and phase-adjustable CsPbBr3@Cs4PbBr6 nanocrystals via novel anion–cation reactions, Nanoscale Adv. 1 (2019) 980–988. [15] 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. [16] L. Quan, R. Quintero-Bermudez, O. Voznyy, G. Walters, Highly emissive green perovskite nanocrystals in a solid state crystalline matrix, Adv. Mater. 29 (2017) 1605945. [17] Y. Ling, L. Tan, X. Wang, Y. Zhou, Composite perovskites of cesium lead bromide for optimized photoluminescence, J. Phys. Chem. Lett. 8 (2017) 3266–3271. [18] J. Xu, W. Huang, P. Li, D. Onken, C. Dun, Y. Guo, K. Ucer, C. Lu, H. Wang, S. Geyer, R. Williams, D. Carroll, Imbedded nanocrystals of CsPbBr3 in Cs4PbBr6: kinetics, enhanced oscillator strength, and application in light-emitting diodes, Adv. Mater. 29 (2017) 1703703. [19] B. Kang, K. Biswas, Exploring polaronic, excitonic structures and luminescence in Cs4PbBr6/CsPbBr3, J. Phys. Chem. Lett. 9 (2018) 830–836. [20] G. Peng, H. Wang, T. Zhang, L. Mi, Y. Zhang, Z. Zhang, W. Zhang, Y. Jiang, Solvent-polarity-engineered controllable synthesis of highly fluorescent cesium lead halide perovskite quantum dots and their use in white light-emitting diodes, Adv. Funct. Mater. 26 (2016) 8478–8486. [21] 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. [22] J. Zhang, L. Fan, J. Li, X. Liu, R. Wang, L. Wang, G. Tu, Growth mechanism of CsPbBr3 perovskite nanocrystals by a co-precipitation method in a CSTR system, Nano Res. 12 (2019) 121–127. [23] Y. Li, H. Huang, Y. Xiong, S. Kershaw, A. Rogach, Reversible transformation between CsPbBr3 and Cs4PbBr6 nanocrystals, Cryst. Eng. Comm. 20 (2018) 4900–4904. [24] S. Kondo, K. Amaya, T. Saito, Localized optical absorption in Cs4PbBr6, J. Phys-Condens. Mat. 14 (2002) 2093–2099. [25] X. Li, Y. Wu, S. Zhang, B. Cai, Y. Gu, J. Song, H. Zeng, CsPbX3 quantum dots for lighting and displays: room‐temperature synthesis, photoluminescence superiorities, underlying origins and white light‐emitting diodes, Adv. Funct. Mater. 26 (2016) 2584-2584. [26] D. Bi, C. Yi, J. Luo, J.D. Decoppet, Polymer-templated nucleation and crystal growth of perovskite films for solar cells with efficiency greater than 21%, Nat. Energy 1 (2016) 16142. [27] Z. Liang, S. Zhao, Z. Xu, Shape-controlled synthesis of all-inorganic CsPbBr3 perovskite nanocrystals with bright blue emission, ACS. Appl. Mater. Inter. 8 (2016) 28824–28830.
6
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
One-pot synthesis of CsPbBr3/Cs4PbBr6 perovskite composite Xiaogai Penga, Jin Chena,*, Fengchao Wanga,*, Canyun Zhanga, Bobo Yanga,b a b
T
College of Sciences, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, China Institute of Future Lighting, Academy for Engineering and Technology, Fudan University, 220 Handan Road, Shanghai 200433, China
A R T IC LE I N F O
ABS TRA CT
Keywords: Perovskite CsPbBr3/Cs4PbBr6 composite Quantum dots One-pot route
In current study, a facile one-pot approach was introduced for the preparation of CsPbBr3/ Cs4PbBr6 composite. The formation mechanism behind this proposed approach was investigated in detail. As the results shown, the structure observations by XRD and HRTEM suggested the obtained sample, prepared under an optimal condition of (90 ℃, 20 min), was in a miscible phase of CsPbBr3/Cs4PbBr6. The morphology analysis by TEM illustrated the sample monodispersed as a clear cubic shape with an average size of ∼10 nm. The composition examination by EDS revealed a Pb-poor and Br-rich state which was favorable for the Cs4PbBr6 formation. Particularly, the synthesized sample had a narrow line-width of ∼22 nm, a photoluminescence quantum yield of ∼57 %, and a color purity of 88 %. This indicates the obtained sample is a promising candidate in lighting and display field.
1. Introduction Semiconductor quantum dots (QDs) are a new type of broadband emission spectral material, which are widely used in the field of light-emitting [1], display [2], and biosensing [3] due to their ability to tune optical bandgap from violet to near infrared region. Besides, these QDs can absorb high-energy photons and per photon can generate multiple electron-hole pairs (multi-exciton effects). Compared with bulk semiconductor materials, the bandgap of QDs is wider and the electronic states become discrete [4], which is crucial for obtaining high-efficient photoelectric devices. In recent decade, the perovskite QDs with ABX3 (A = CH3NH3+, CH (NH2)2+ and Cs+, B = Pb2+, Sn2+ and Ge2+, X = Cl−, Br− and I−) structure have been well investigated, because of their high photoluminescence quantum yield (PLQY), facile bandgap tunability, and good carrier mobility, particularly in all-inorganic CsPbX3 QDs [5]. Since the pioneered work reported by Kovalenko and his co-workers about inorganic colloidal CsPbX3 nanocrystals [6], more enthusiasms have been attracted in this field. Different technologies have been involved into the fabrication for this material, such as hot-injection [7], recrystallization [8], microwave [9]. Hot-injection method is a prevalent one for the QDs synthesis, whereas this method is restricted in the nucleation homogeneity and complicated operation [10]. As another popular one, recrystallization route is insufficient yield. Compared to these methods, one-pot route is more favor to the simplicity and homogeneity. Thus, this approach is a promising candidate for the scale-up production of perovskite QDs. As well known, CsPbX3 QDs feature a fragile and unstable structure composed of [PbX6]4− octahedron with Cs+ cations filling. This structure has poor tolerance under moisture and heating because of the easy losing of halide anions. Diverse strategies have been involved to overcome above concerns, such as ligand passivation route [11], core-shelling method [12,13]. Recently, some studies reported that Cs4PbX6, a derived member of Cs-Pb-X compounds, could play an in situ passivation for the stability improvement of
⁎
Corresponding authors. E-mail addresses: [email protected] (J. Chen), [email protected] (F. Wang).
https://doi.org/10.1016/j.ijleo.2020.164579 Received 1 February 2020; Accepted 16 March 2020 0030-4026/ © 2020 Elsevier GmbH. All rights reserved.
Optik - International Journal for Light and Electron Optics 208 (2020) 164579
X. Peng, et al.
CsPbX3 material [14,15]. CsPbX3 is allowed and protected to grow in the Cs4PbX6 matrix. In addition, some evidences also showed that the miscible phase of CsPbX3/Cs4PbX6 is beneficial for the luminescent elevation which may result from the improvement of the radiative recombination of excited carriers [16–19]. However, the studies on the facile and simultaneous synthesis of this composite are still scarce. In this study, a facile one-pot approach was introduced for the synthesis of the CsPbBr3/Cs4PbBr6 composite. To our knowledge, similar work is reported rarely to date. The formation mechanism of the proposed approach was deeply analyzed. The properties of the obtained sample, including photoluminescence (PL) emission, structure, morphology, and composition, were investigated in detail. These properties were determined by fluorescence spectrometer, X-ray diffractometer (XRD), transmission electron microscope (TEM), and energy-dispersive X-ray spectroscopy (EDS), respectively. 2. Experimental details 2.1. Synthesis of CsPbBr3/Cs4PbBr6 QDs Typically, 1 mmol Cs2CO3 (cesium carbonate, 99 %, AR), 3 mmol PbBr2 (lead bromide, 99 %, AR), 0.25 ml OA (oleic acid, 90 %, AR), 0.25 ml OLA (oleylamine, 70 %, AR) and 10 ml ODE (1-octadecene, 90 %) were successively added into the three-neck flask with magnetic stirring. Then, the solution was heated to allow the crystals to grow under the protected atmosphere of nitrogen. The investigated reaction temperature varied in the range of 60–110 ℃ with an increment of 10 ℃, while the reaction time was 20 min. After reaction, ice bath was employed to cool the solution immediately. The as-obtained products were collected via centrifugation and washing processes for several times. Finally, the precipitates were dispersed in the n-hexane solution for next using. 2.2. Characterization for as-obtained samples PL emission spectra were determined by a spectrofluorometer (FS5, Edinburgh, UK). The crystal structure was measured by X-ray diffraction (XRD) (Ultima IV, Rigaku, Japan) with a Cu-Kα radiation (40 kV, 40 mA). The morphology of as-synthesized QDs were studied by the transmission electron microscopy (TEM) (TECNAI G2 F20, FEI, USA) equipped with an accelerating voltage of 200 kV. The absorption spectrum was investigated by a UV–vis (UV–vis) spectrometer (UH4150, Hitachi, Japan). The surface composition of QDs was analyzed by an energy-dispersive X-ray spectroscopy (EDS) (Quanta 200 FEG, FEI, USA). The PL decay was detected by highresolution spectrofluorometer (FLS920, Edinburgh Instruments, UK). 3. Results and discussion Fig. 1a shows the schematic diagram of one-pot approach for the preparation of CsPbBr3/Cs4PbBr6 QDs. As seen, the whole synthesis employs the injection process no longer. This simplifies the operation effectively compared to the well-known hot-injection route. To observe the influences of reaction temperature on the performances of obtained samples, the value of the temperature was varied from 60 to 110 ℃ with increments of 10 ℃. The reaction time was set as a constant of 20 min. Fig. 1b shows the photoluminescence of the synthesized samples prepared at different reaction temperatures. The 365 nm wavelength was carried out for the excitation. As observed, the color of these samples changed from cyan to green with the increasing temperature. This indicated the emission of the obtained samples could be tuned with the temperature changing. It may stem from the growing of QDs size as temperature increased which has been suggested by previous study [20]. The PL emission spectra are shown in Fig. 2a. As seen, the PL emission intensity of the obtained samples prepared in 60–80 ℃ range varied slightly while that of 90 ℃ sample increased dramatically. Interestingly, the PL intensity decreased when the temperature was over 90 ℃. This is further showed in Fig. 2b. It is noteworthy that the emission peak of 90 ℃ sample featured a symmetric shape with a narrow line-width of ∼22 nm near the value in previous report [21]. The evolutions of the PL emission intensity may be caused by the different growing stages of the QDs. In the initial solution, Cs+ and Pb2+ cations would combine with OA to form Cs-OA and Pb-OA complexes, while the Br− anions from PbBr2 reactant acted with OAm to form Br-OAm complexes, as suggested by Zhang et al. [22]. When the reaction solution was heated, these complexes released Cs+, Pb2+, and Br− ions again within the reaction solution. Then these ions integrated to form CsxPbyBrz products. The schematic diagram of the synthesis is shown in Fig. 3. In the 60–80 ℃ range, the crystal nuclei were primarily formed in the solution. However, because of the low solubility of reactants
Fig. 1. a) the scheme of synthesis procedure and b) the luminescence image of obtained samples synthesized at different temperatures under 365 nm excitation. 2
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Fig. 2. a) PL emission spectrum, b) emission intensity, and c) central wavelength of the samples prepared at different temperatures.
Fig. 3. Schematic diagram of the synthesis of the proposed one-pot approach.
in ODE, a small number of Cs+, Pb2+, and Br− ions were produced to allow the crystal grow, this may attribute to the low PL emission intensity. As the temperature increased over 80 ℃, the solubility of these reactants increased to provide more cations and anions. This accelerated the processes of crystallization and formation of CsxPbyBrz products. Thus, the PL peak presented a high intensity at 90 ℃. Increasing the temperature further, Ostwald ripening occurred during the crystal growing resulted in the crystal size becoming larger. This decreased the quantum confinement effect of the synthesized products. So, the PL emission decreased in 100∼110 ℃ range. Noteworthily, the emitting wavelength of the obtained samples red-shifted from 496 to 515 nm with the increasing temperature. This may originate from the size growing of the crystals. Additionally, the photoluminescence quantum yield (PLQY) of the obtained QDs prepared at different temperatures were also determined by Edinburgh Instruments FLS 920. Among these, the sample obtained at 90 ℃ had the highest value of ∼57 %. Thus, the various properties of this 90 ℃ sample were further investigated. To understand the structure of the obtained sample prepared at 90 ℃, the X-ray diffraction (XRD) analysis was employed in current study. Fig. 4 shows the XRD patterns. As identified by the standard JCPDS cards of No. 54-0752 and No. 73-2478, the obtained sample consisted of cubic CsPbBr3 (a = b = c = 5.830 Å) and trigonal Cs4PbBr6 (a = b =13.730 Å, c =17.320 Å) composites. Typically, the diffraction peaks located at 15.19°, 21.55°, 26.48°, 30.64°, 34.37°, 37.77°, 46.69° could be attributed to (100), 3
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Fig. 4. XRD patterns of the obtained sample prepared at 90 ℃.
(110), (111), (200), (210), (211), (300) planes of cubic CsPbBr3, respectively. The peaks located at 12.64°, 20.08°, 22.41°, 25.43°, 27.51°, 28.60°, 30.27° were ascribed to (012), (113), (300), (024), (131), (214), (223) planes of trigonal Cs4PbBr6, separately. The composites of CsPbBr3/Cs4PbBr6 may derive from the lower solubility of Pb2+ than that of Cs+ and Br− in the reaction solution, which was preferable to form Cs4PbBr6 products. As studied by previous report [23], Cs4PbBr6 could be seen as the intermediate phase for CsPbBr3. With more Pb2+ emerging in the reaction, the Cs4PbBr6 would transform to be CsPbBr3. The transmission electron microscopy (TEM) was employed to investigate the morphology of the obtained sample. As seen in Fig. 5a, the QDs dispersed homogeneously without aggregation. The QDs’ shapes featured a clear cubic structure. The size was ranging from 7 to 15 nm, and the average value was ∼10 nm. These QDs’ structure was further analyzed via the high resolution TEM (HRTEM) as shown in Fig. 5b. It was found that there were two typical lattice structures. The interplanar distances of 2.9 Å and 4.2 Å could be attributed to the (200) and (100) plans of cubic CsPbBr3, respectively. The lattice spacings of 7.0 Å and 3.0 Å were corresponding to the (110) (312) plans of trigonal Cs4PbBr6, separately. Above could be also directly proved via selected area fast Fourier transformation (FFT) analysis. This indicated the obtained sample composed of CsPbBr3/Cs4PbBr6 composites, which kept steps with the XRD analysis well. Additionally, the clear lattice fringes suggested that the synthesized sample had a good crystallinity. Fig. 6a displays the absorption spectrum of the prepared sample. As seen, there were two characteristic absorption peaks locating at 314 and 515 nm could be ascribed to Cs4PbBr6 and CsPbBr3, respectively [24]. This was quiet consistent with the above XRD and HRTEM analysis. The bandgap energy Eg of this composite was extracted from the (Ahν)2 versus (hν) plots, where A is the absorbance, h is the Planck constant, and ν is the photon frequency. Here, the Eg value was around of 2.85 eV, which were tuned by the coexistence of Cs4PbBr6 (3.25 eV) and CsPbBr3 (2.0 eV). The chemical composition of the obtained sample is shown in Fig. 6b. The atomic percent of Cs:Pb:Br was 3:1:5, which matched the mixed state of CsPbBr3/Cs4PbBr6 well. The Pb/(Cs + Br) ratio was 0.13, which suggested a Pb-poor state of the synthesized sample resulting from the poor solubility of Pb source in the reaction solution. This was responsible for the formation of Cs4PbBr6 product. Noteworthily, the Br/(Pb + Cs) ratio was 1.23, which meant a Br-rich state. As suggested by previous report [25], the richness of halogen may play a self-passivation effect because the redundant halogen ions on the surface will connect with cations. This facilitates the reducing of surface trap-defects and non-radiative recombination centers. Fig. 6c shows the time-resolved PL decay curve of the sample synthesized at 90 ℃. This PL curve could be well fitted with a biexponential decay function as shown in Eq. (1) [26]. Where A1, A2 are constants, t is time. τ1 and τ2 are the short-lived lifetime and long-lived lifetime, respectively. As observed, τ1 was 4.71 ns (A1 = 34.8 %) and τ2 was 37.20 ns (A2 = 65.2 %).
Fig. 5. a) TEM image and b) HRTEM image of the obtained sample prepared at 90 ℃. 4
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Fig. 6. a) the absorption spectra and PL spectra, b) chemical composition, c) PL decay curve, and d) CIE coordinate of CsPbBr3/Cs4PbBr6 composites.
A(t) = A1 exp(-t/τ1) + A2 exp(-t/τ2)
(1)
τave = (A1τ1 + A2τ2 ) / (A1τ1 + A2τ2)
(2)
2
2
The average lifetime (τave) was calculated by Eq. (2) [27]. It was found that the τave value was 35.14 ns. This indicated a high ratio of radiative to nonradiative recombination and less transition of detect states. As noted, the excluding of trap-defect and nonradiative recombination, caused by Br-rich self-passivation on QDs surface, may be prefer to result in the long lifetime. As shown in Fig. 6d, the CIE coordinate of the CsPbBr3/Cs4PbBr6 composites was located at (0.0617, 0.708). The calculated color purity was 88 %, which was accordance with the narrow PL line-width of ∼22 nm. This suggests that the obtained sample is a promising candidate for the application in wide color-gamut display.
4. Conclusions In summary, the CsPbBr3/Cs4PbBr6 composite was facilely and successfully synthesized by a facile one-step approach. Meanwhile, the mechanism behind this approach was analyzed in detail. The results showed the reaction condition of (90 ℃, 20 min) was favorable for the sample synthesis. XRD and HRTEM analysis confirmed the miscible phase of CsPbBr3/Cs4PbBr6 in obtained sample. TEM image showed that the sample monodispersed well and had a clear cubic shape with an average size of ∼10 nm. EDS observation revealed a Pb-poor and Br-rich composition. Noteworthily, Pb-poor state was preferable for the Cs4PbBr6 formation, Br-rich state was favorable for a self-passivation role to reduce the surface trap-defects. Remarkably, the synthesized sample had a narrow line-width of ∼22 nm, a photoluminescence quantum yield of ∼57 %, and a color purity of 88 %. This suggests the obtained sample is a promising candidate for the application of photoelectric devices.
Declaration of Competing Interest The authors declare no conflicts of interest.
Acknowledgements The project was sponsored by Shanghai Sailing Program, China (No. 18YF1422500) and Research start-up project of Shanghai Institute of Technology (No. YJ2018-9). 5
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References [1] J. Song, J. Li, X. Li, L. Xu, Y. Dong, H. Zeng, Nanocrystals: quantum dot light‐emitting diodes based on inorganic perovskite cesium lead halides (CsPbX3), Adv. Mater. 27 (2015) 7162–7167. [2] H. Li, H. Liu, H. Zhang, G. Zhu, L. Wang, B. Yang, W. Li, Application of quantum dots labeling to the interaction between reconstructed-phage display peptide and insulin receptor, Chem. Res. Chinese U. 25 (2004) 982–984. [3] J. Yuan, N. Gaponik, A. Eychmüller, Application of polymer quantum dot-enzyme hybrids in the biosensor development and test paper fabrication, Anal. Chem. 84 (2012) 5047–5052. [4] P. Cozzoli, T. Pellegrino, L. Manna, Synthesis, properties and perspectives of hybrid nanocrystal structures, Chem. Soc. Rev. 35 (2006) 1195–1208. [5] Q. Shan, J. Li, J. Song, Y. Zou, L. Xu, J. Xue, Y. Dong, C. Hou, J. Chen, B. Han, H. Zeng, All-inorganic quantum-dot light-emitting diodes based on perovskite emitters with low turn-on voltage and high humidity stability, J. Mater. Chem. C Mater. Opt. Electron. Devices 5 (2017) 4565–4570. [6] L. Protesescu, S. Yakunin, M.I. Bodnarchuk, F. Krieg, R. Caputo, C.H. Hendon, R. Yang, A. Walsh, M.V. Kovalenko, Nanocrystals of cesium lead halide perovskites (CsPbX3, X =Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut, Nano Lett. 15 (2015) 3692–3696. [7] L. Wang, B. Liu, X. Zhao, H. Demir, Solvent-assisted surface engineering for high-performance all inorganic perovskite nanocrystal light-emitting diodes, ACS. Appl. Mater. Inter. 10 (2018) 19828–19835. [8] C. Gao, Y. Zhang, X. Ma, F. Yu, Y. Jia, Y. Lei, P. Chen, W. Sun, Z. Xiong, A method toards 100% internal quantum efficiency for all-inorganic cesium halide perovskite light-emitting diodes, Org. Electron. 58 (2018) 88–93. [9] Q. Pan, H. Hu, Y. Zou, M. Chen, L. Wu, D. Yang, X. Yuan, J. Fan, B. Sun, Q. Zhang, Microwave-assisted synthesis of high-quality “all-inorganic’’ CsPbX3 (X = Cl, Br, I) perovskite nanocrystals and their application in light emitting diodes, J. Mater. Chem. C Mater. Opt. Electron. Devices 5 (2017) 10947-10945. [10] K. Lee, J. Lee, H. Kang, B. Park, Y. Kwon, H. Ko, C. Lee, J. Lee, H. Yang, Over 40 cd/A efficient green quantum dot electroluminescent device comprising uniquely large-sized quantum dots, ACS Nano 8 (2014) 4893–4901. [11] J. Li, L. Xue, T. Wang, J. Song, 50-fold EQE improvement up to 6.27% of solution-processed all-inorganic perovskite CsPbBr3 QLEDs via surface ligand density control, Adv. Mater. 29 (2016) 7162–7167. [12] B. Qiao, P. Song, J. Cao, S. Zhao, Z. Shen, D. Gao, Z. Liang, Z. Xu, D. Song, X. Xu, Water-resistant, monodispersed and stably luminescent CsPbBr3/CsPb2Br5 coreshell like structure lead halide perovskite nanocrystals, Nanotechnology 28 (2017) 445602. [13] Q. Zhong, M. Cao, H. Hu, D. Yang, One-pot synthesis of highly stable CsPbBr3@SiO2 core-shell nanoparticles, ACS Nano 12 (2018) 8579–8587. [14] L. Xu, J. Li, F. Tao, Y. Zhao, Synthesis of stable and phase-adjustable CsPbBr3@Cs4PbBr6 nanocrystals via novel anion–cation reactions, Nanoscale Adv. 1 (2019) 980–988. [15] 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. [16] L. Quan, R. Quintero-Bermudez, O. Voznyy, G. Walters, Highly emissive green perovskite nanocrystals in a solid state crystalline matrix, Adv. Mater. 29 (2017) 1605945. [17] Y. Ling, L. Tan, X. Wang, Y. Zhou, Composite perovskites of cesium lead bromide for optimized photoluminescence, J. Phys. Chem. Lett. 8 (2017) 3266–3271. [18] J. Xu, W. Huang, P. Li, D. Onken, C. Dun, Y. Guo, K. Ucer, C. Lu, H. Wang, S. Geyer, R. Williams, D. Carroll, Imbedded nanocrystals of CsPbBr3 in Cs4PbBr6: kinetics, enhanced oscillator strength, and application in light-emitting diodes, Adv. Mater. 29 (2017) 1703703. [19] B. Kang, K. Biswas, Exploring polaronic, excitonic structures and luminescence in Cs4PbBr6/CsPbBr3, J. Phys. Chem. Lett. 9 (2018) 830–836. [20] G. Peng, H. Wang, T. Zhang, L. Mi, Y. Zhang, Z. Zhang, W. Zhang, Y. Jiang, Solvent-polarity-engineered controllable synthesis of highly fluorescent cesium lead halide perovskite quantum dots and their use in white light-emitting diodes, Adv. Funct. Mater. 26 (2016) 8478–8486. [21] 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. [22] J. Zhang, L. Fan, J. Li, X. Liu, R. Wang, L. Wang, G. Tu, Growth mechanism of CsPbBr3 perovskite nanocrystals by a co-precipitation method in a CSTR system, Nano Res. 12 (2019) 121–127. [23] Y. Li, H. Huang, Y. Xiong, S. Kershaw, A. Rogach, Reversible transformation between CsPbBr3 and Cs4PbBr6 nanocrystals, Cryst. Eng. Comm. 20 (2018) 4900–4904. [24] S. Kondo, K. Amaya, T. Saito, Localized optical absorption in Cs4PbBr6, J. Phys-Condens. Mat. 14 (2002) 2093–2099. [25] X. Li, Y. Wu, S. Zhang, B. Cai, Y. Gu, J. Song, H. Zeng, CsPbX3 quantum dots for lighting and displays: room‐temperature synthesis, photoluminescence superiorities, underlying origins and white light‐emitting diodes, Adv. Funct. Mater. 26 (2016) 2584-2584. [26] D. Bi, C. Yi, J. Luo, J.D. Decoppet, Polymer-templated nucleation and crystal growth of perovskite films for solar cells with efficiency greater than 21%, Nat. Energy 1 (2016) 16142. [27] Z. Liang, S. Zhao, Z. Xu, Shape-controlled synthesis of all-inorganic CsPbBr3 perovskite nanocrystals with bright blue emission, ACS. Appl. Mater. Inter. 8 (2016) 28824–28830.
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