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Effect of synthesis methods on photoluminescent properties for CsPbBr3 nanocrystals: Hot injection method and conversion method
Journal Pre-proof Effect of synthesis methods on photoluminescent properties for CsPbBr3 nanocrystals: Hot injection method and conversion method Fengying Zhang, Junsheng Chen, Ying Zhou, Rongxing He, Kaibo Zheng PII:
S0022-2313(19)31721-1
DOI:
https://doi.org/10.1016/j.jlumin.2019.117023
Reference:
LUMIN 117023
To appear in:
Journal of Luminescence
Received Date: 31 August 2019 Revised Date:
16 December 2019
Accepted Date: 30 December 2019
Please cite this article as: F. Zhang, J. Chen, Y. Zhou, R. He, K. Zheng, Effect of synthesis methods on photoluminescent properties for CsPbBr3 nanocrystals: Hot injection method and conversion method, Journal of Luminescence (2020), doi: https://doi.org/10.1016/j.jlumin.2019.117023. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Effect of synthesis methods on photoluminescent properties for CsPbBr3 nanocrystals: hot injection method and conversion method Fengying Zhanga,b,c, Junsheng Chenc, Ying Zhoua, Rongxing Heb,*, Kaibo Zheng c,d,
*
a State Key Laboratory of Oil and Gas Reservoir and Exploitation and School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China. b Key Laboratory of Luminescence and Real-Time Analytical Chemistry of Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, People’s Republic of China. c Division of Chemical Physics, Department of Chemistry, Lund University, Box 124, SE-22100 Lund, Sweden. d Department of Chemistry, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.
E-mail: [email protected]; [email protected].
1
Highlights 1. Both hot injection method and conversion method were applied to prepare CsPbBr3 NCs. 2. Time-resolved photoluminescence and transient absorption dynamics of CsPbBr3 NCs synthesized by two methods were compared and analyzed. 3. Photophysical processes of CsPbBr3 NCs were proposed and discussed.
Abstract Multiple facile synthetic strategies for all inorganic perovskite CsPbBr3 nanocrystals (NCs) have been established and developed, profiting from their excellent performance and great potential applied in the field of photonic and optoelectronic. Here, CsPbBr3 NCs were synthesized by both hot injection method (method 1) and conversion method (method 2), and the discrepancy of their photophysical properties is elucidated via the complementary studies between time-resolved photoluminescence (TRPL) and transient-absorption (TA) spectroscopy. We found that CsPbBr3 NCs prepared by conversion method exhibited lower PL quantum yield (QY), which was ascribed to the larger partition of the NCs being passivated from the quenchers from the deep trap states. On the other hand, we also observed different radiative recombination rates between two samples which should be due to various trapping/detrapping times prior to the radiative recombination of the charge carriers in two samples. These results provide better guidance for the development and improvement of synthesis methodology for perovskite NCs. Keywords: perovskite nanoparticles; synthetic strategy; trap states; radiative recombination.
2
1. Introduction Hybrid lead-halide perovskites, which occurred as one of the most promising materials with prominent optoelectronic characteristics [1-4], have attracted wide attention in the past few years, especially when applied in solar cells [5-7], light-emitting diodes [8-10], photodetectors [11, 12] and lasers [13, 14]. The emerging all-inorganic counterparts (CsPbX3, X = I, Br, Cl) with replaced inorganic cation Cs+ were introduced with improved stability and charge carrier transfer properties [15, 16]. In 2015, Kovalenko et al. firstly reported the all-inorganic colloidal nanocrystals (CsPbX3 NCs), achieving high QYs up to 90% with great stability, narrow PL emission and adjustable spectra through compositional modulations of halide anions and quantum size-effects [17]. The synthesis methodologies [18-20], fundamental photophysics [21-23], as well as potential device applications have then been widely investigated [24, 25]. There exists several methods to synthesize CsPbBr3 NCs, involving 1) hot injection method (Cs-oleate precursor is injected into PbBr2 solution at high temperature, 160
200oC), 2) anti-solvent crystallization method (both ionic precursor solutions
are added into anti-solvent with low solubility, such as toluene and hexane) for spontaneous crystallization of the NCs, and 3) conversion method, where the CsPbBr3 NCs are transformed from monodisperse insulator Cs4PbBr6 with the help of excess PbBr2, and the size of CsPbBr3 NCs can be switched by adjusting the size of Cs4PbBr6 NCs [26]. Although the lattice structures and morphologies of CsPbBr3 NCs prepared via above methods seem to be identical based on the structural characterization, their optical properties and specially the photophysical behaviors could be different which has not yet been carefully discussed. Here, we compare the PL properties of CsPbBr3 NCs prepared by the hot injection method and conversion methods using both steady-state and time-resolved optical spectroscopies. The emissive state dynamics of these two types of CsPbBr3 NCs were monitored by combining TRPL and TA spectroscopy. We found that both CsPbBr3 NCs, despite of the similar size and cubic morphology, exhibited different PL QYs as well as excited state dynamics. We concluded that there are two pools in each sample. The first pool (pool A) has defects 3
as PL quenchers where the non-radiative recombination is dominated. In the other pool (pool B) of NCs, the excited state depopulation is greatly mediated by the trapping/detrapping process at shallow VBr trap states above the conduction band edge. The more pronounced trapping in VBr trap states in samples prepared via conversion method should be the origins of their lower PL QY.
2. Materials and methods Materials Cesium carbonate (Cs2CO3, 99%), lead bromide (PbBr2, 98%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), oleylamine (OLAM, 70-90%), toluene and hexane were purchased were purchased from Sigma-Aldrich. All chemicals were used as received without further purification. Method 1: hot injection method Preparation of Cs-oleate precursor. Cs2CO3 (0.814 g), OA (2.5 mL) and ODE (40 mL) were loaded into a 100 mL three-neck flask and dried under vacuum for 1 h at 120 °C. Then, the mixture was heated under N2 to 150 °C until all Cs2CO3 reacted with OA. Before usage, it needs to be preheated to make it soluble. Synthesis of CsPbBr3 NCs. Based on the method developed by Kovalenko and co-workers [17], PbBr2 (0.069 g) and ODE (10 mL) were loaded into a 50 mL three-neck flask and degassed for 1 h at 120 °C. Then, OA (0.5 mL) and OLAM (0.5 mL) mixtures were added at 120 °C under N2 atmosphere. After PbBr2 was completely dissolved, the temperature was raised to 180 °C, followed by the quick injection of 0.4 mL Cs-oleate precursor. After 5 s, the reaction mixture was cooled immediately by the ice-water bath. The crude solution was centrifuged at 1500 round per minute (rpm) for 10 min to remove large particles. Then, the supernatant was centrifuged again at 6500 rpm for 10 min, and the precipitate was collected and re-dispersed in toluene for further characterization. Method 2: conversion method Synthesis of Cs4PbBr6 NCs. Cs4PbBr6 NCs were prepared by modified hot injection method [26]. PbBr2 (0.4 mmol, 0.1468 g), ODE (20 mL), OA (0.8 mL) and 4
OLAM (1.5 mL) were loaded into a 50 mL three-neck at 120 °C. After PbBr2 was completely dissolved, the temperature was turned to 80 °C, followed by the quick injection of 3 mL Cs-oleate precursor (0.4 g Cs2CO3 dissolved in 8 mL OA in a 20 mL vial at 150 °C). After 5 s, the reaction mixture was cooled immediately by the ice-water bath. The crude solution was centrifuged at 4500 rpm for 10 min, and the precipitate was collected and redispersed in toluene or hexane (24 mL) for further characterization. Synthesis of CsPbBr3 NCs. 0.5 mL of Cs4PbBr6 NCs solution was diluted in 2 mL toluene and heated in the oil bath at 80 °C. After the temperature was stable, 50 µL PbBr2 precursor (1 mmol PbBr2 dissolved in the mixture of 2.5 mL OA, 2.5 mL OLAM and 5 mL toluene) was injected quickly. The reaction was cooled down with the ice-water bath after 45 minutes.
3. Measurements and characterizations UV-Vis absorption spectra were recorded by an Agilent 8453 UV-Vis spectrometer. Steady-state PL spectra were collected on a standard spectrofluorometer (Spex Fluorolog 1681). Powder X-ray diffraction (XRD) patterns were measured on a 4-circle Huber diffractometer in the range of 10° < 2θ < 60°, including an X-ray light source (12 keV, 0.1 nm) and a Si (111) double crystal monochromator. Transmission electron microscopy (TEM) measurement was performed using JEOL JEM-1400 microscope (80 kV). Fourier-transform infrared spectroscopy (FTIR) spectra were obtained using a BRUKER ɑ ALPHA-P FTIR Spectrometer. The streak camera (Hamamatsu, C6860) setup was used for the measurement of TRPL spectra, which contains a femtosecond laser source (800 nm, 2 kHz repetition rate, 150 fs pulse length). The laser of 400 nm generated by a BBO crystal was used to excite samples. One integrating sphere method was used for absolute quantum yield measurement. The sample was excited by a 405 nm continuous wave laser, and the integrating sphere (HORIBA, Quanta-φ, F3029) was applied to collect the overall PL and residual excitation light. The sphere output was detected by the spectrometer (AvaSpec-ULS2048-USB2-UA-50). The calibrated reference light source (Ocean 5
Optics, LS-1-CAL) was used for the correction of spectral sensitivity. Finally, the PL QY could be calculated by accounting for the numbers of the emitted and absorbed photons. TA measurements were implemented based on a femtosecond pump-probe setup. A femtosecond oscillator was used to generate the laser pulse (800 nm, 120 fs pulse length, 1 kHz repetition rate) through a regenerative amplifier (Spitfire XP Pro). The pump pulse of 400 nm was generated by a BBO crystal and to be as a second harmonic of the laser. The super-continuum probe laser we used came from a thin CaF2 plate. The polarization angle of 54.7° between pump and probe beams was achieved by placing a Berek compensator in the pump beam. To avoid photodamage of samples, they were moved constantly after each time delay point.
4. Results and discussion
Fig. 1. (a) Schematic diagram of two synthesis methods. (b) UV-Vis absorption and steady-state PL spectra, (c) XRD patterns of CsPbBr3 NCs obtained from two different synthesis methods. As shown in Fig. 1(a), CsPbBr3 NCs were prepared using both hot injection method and conversion method. Here, CsPbBr3 NCs obtained from two different synthesis methods exhibited the similar absorption emission spectra as presented in Fig. 1(b). The absorption band edge and emission peak of prepared CsPbBr3 NCs (method 1) were located at 503 nm and 511 nm, respectively, being consistent with previous reports [27, 28]. In the method 2, Cs4PbBr6 NCs plays an important role in the 6
conversion process, and their absorption spectra and PL were provided in Fig. S1. For the emission around 380 nm in the Cs4PbBr6 sample, which origins from the Pb2+ ion emissions. Once Cs4PbBr6 NCs turn into CsPbBr3 NCs, the absorption band edge changed from 315 nm to 506 nm, with the PL band moving from 378 nm to 510 nm. CsPbBr3 NCs from conversion method exhibited slightly smaller stokes shift when compared with that from hot injection method. However, we observed lower PL QY of the sample from method 2 (60%) compared with the sample from method 1 (80%) based on the integrated sphere method. To rationalize such discrepancy, we first conduct the fundamental characterization on their structures. XRD patterns in Fig. 1(c) confirmed the same typical cubic phase of CsPbBr3 NCs for two methods [17]. It is worth noting that XRD peaks marked with asterisks are attributed to the unconverted Cs4PbBr6 NCs, which can be verified by that of pre-transition Cs4PbBr6 NCs in Fig. S2. And the content of unconverted Cs4PbBr6 NCs in method 2 was roughly estimated to 15%. Besides, UV-Vis absorption spectrum presented in Fig. S3 for converted sample also confirmed the coexist of Cs4PbBr6 and CsPbBr3 NCs in method 2. While, as the Cs4PbBr6 NCs didn’t response to the visible excitation as their band edge resides below 350 nm, such Cs4PbBr6 NCs inclusion should not affect the intrinsic optical properties of CsPbBr3 NCs. The larger energy band gap of Cs4PbBr6 also exclude all the possible energy transfer or charge transfer from the CsPbBr3 QDs.
Fig. 2. TEM pictures of CsPbBr3 NCs obtained from two different synthesis methods, including the lattice spacing and size distribution. 7
We also concluded the similar size and morphology of two samples from their TEM pictures as provided in Fig. 2. Uniform dispersion cubic CsPbBr3 NCs with lattice spacing of 0.58 nm were prepared using two methods, and the mean sizes of CsPbBr3 NCs from hot injection method and conversion method were 10.3 nm and 10.6 nm, respectively. Actually, Cs4PbBr6 NCs suffered a reshaping from spherical/hexagonal shape to cubic phase of CsPbBr3 NCs in the conversion method with size decreasing from 14.4 nm to 10.6 nm (Fig. S4) [26]. This indicates any effect from size dependent quantum confinement or lattice structures on the photo-physics should be excluded. Two-phase coexistence could be clearly discerned in Fig. S5, and the lattice spacing of 0.39 nm confirmed the (300) crystal plane of rhombohedral Cs4PbBr6 NCs. As for these black dots occurred in CsPbBr3 NCs synthesized by conversion method, they have been widely accepted as the formation of Pb0 during the light illumination or electron beam exposure in the TEM, which is also highly determined by the protection of the surface capping agent [29, 30]. Easier ionization to Pb0 means the protection from surface capping agent is weaker, and this could be one of the origins from different synthesis method where the bonding strength of the capping agent could be different. FTIR spectra in Fig. S6 revealed the similar strength of the surface bonding but different number of ligands for two methods. In the next step, we investigated the emissive state dynamics of two samples via TA and TRPL studies.
8
Fig. 3. (a, b) TA spectra with 400 nm excitation (2×1013 ph/cm2/pulse) and (c, d) their respective SVD fittings for CsPbBr3 NCs obtained from two different synthesis methods. As provided in Fig. 3(a, b), there were distinct exciton bleach signals and absorption signals at ∼500 nm and ∼520 nm for early delay times, which corresponds to the band edge ground-state bleaching and hot-exciton induced red-shift, respectively [31]. With hot-exciton relaxation, the exciton-induced shift became smaller, and TA spectra were dominated by the bleach feature of ∼510 nm, which is ascribed to the state filling near the conduction band maximum and valence band minimum of CsPbBr3 NCs once excitation. The singular value decomposition (SVD) fitting for TA data gives three decay components. The fast component with lifetimes of 358 fs and 348 fs should be attributable to the cooling of hot carriers which has been extensively studied in our previous research [22, 32]. The other two components exhibit lifetimes around 190 ps and nanosecond, respectively. The fitting of TA decay at the maximum ground state bleach in Fig. S7 also give the same lifetimes of 196±0.03 ps, 4.0±0.001 ns for method 1 and 190±0.05 ps, 7.6±0.002 ns for method 2. The nanosecond components for both samples feature negative bleach of the band edge absorption together with the excited state absorption at the blue side as well as red side. This is a clear fingerprint of the recombination of band edge excitons in 9
CsPbBr3 NCs [27, 33]. On the other hand, the minor middle components exhibit only negative ground state bleach which should be attributed to the trapping of one species of charge carriers after excitation. If we assign the nanosecond components to the radiative recombination which occurs in parallel with the 190 ps trapping processes within one NC, the PLQY can be estimated using the conventional calculation:
η=
∑ ∑
i
i Ai krad
(1)
j A k i + ∑ j Aj knonrad i i rad
where krad and Ai denote the rate and amplitude of radiative recombination process. knonrad and Aj refer to the rate and amplitude of non-radiative recombination process, respectively. The calculated PL QYs were 37
(method 1) and 18.8
(method 2),
being inconsistent with the direct steady state PL measurement as mentioned above. This indicates two recombination pathways should be independent within different pools of NCs. In this scenario, the fast 190 ps trapping process should occur in those NCs with deep intrinsic trap states (most probably volume traps) where the PL would be full quenched. The similar lifetime as well as amplitude (about 10%) of these components for two samples as shown in table 1 indicated such defects should be generated regardless of synthesis methods with minor contribution. The nanosecond components, however, showed different lifetimes for two samples where the lifetime of sample 2 (7.6 ns) is longer than that of sample 1 (4.0 ns). Table 1 TA lifetimes and the respective amplitude of CsPbBr3 NCs prepared by two different methods. t1 (ps)
A1
t2 (ns)
A2
Method 1
196±0.03
0.08
4.0±0.001
0.93
Method 2
190±0.05
0.10
7.6±0.002
0.90
In TRPL measurement of two CsPbBr3 NCs, only monoexponential PL decays can be observed in both samples with the lifetime shorter than the nanosecond components in TA dynamics as shown in Fig. 4. The CsPbBr3 NCs obtained from method 1 displayed faster PL decay with lifetime of 1.96±0.47 ns. For NCs prepared 10
by conversion method, their PL lifetime was longer (2.1±0.36 ns). The similar and unchanged PL spectra during the time evolution for two samples (Fig. 4a&b) indicate the emission for both samples should be contributed from the same band edge radiative recombination. Moreover, the perfect monoexponential feature as shown in Fig. 4c manifest the single process of the radiative recombination for both samples. It should first be noticed the exciton condition (i.e. excitation wavelength is 400 nm, fluence is 2×1013 ph/cm2/pulse, etc) between TA and TRPL measurement are rather identical. The absence of fast (190 ps) TA components in TRPL decays could be due to 1) the small partition (<10%) which cannot be well resolved in TRPL due to limited time-resolution and instrument response, or 2) such trapping process does not directly quench the band edge emissive states (e.g. the initial trapping happens prior to the hot carrier cooling to the band edge states). On the other hand, the longer lifetime values of the TA nanosecond components compared with radiative PL lifetime extracted from TRPL also demonstrate that there are extra non-radiative recombination pathways within this long time-scale. Similarly, such non-radiative recombination does not directly quench the band edge emission, and hence cannot be reflected in the PL decays.
Fig. 4. (a) Time-resolved PL emissions with 400 nm excitation (2×1013 ph/cm2/pulse) for CsPbBr3 NCs. (c) Normalized PL decays integrated PL peak wavelengths. According to the FTIR results in Fig. S6, the concentration of capping agents vary between two samples. On the other hand, previous temperature dependent TRPL characterization of CsPbBr3 NCs revealed the existence of shallow trap states from Br vacancy, in which the location of shallow trap level is 0.16-0.23 eV above the conduction band edge according to the DFT calculation [34]. The fast trapping and 11
relaxation to the band edge states would delay the charge carrier recombination process. Our TA dynamics proposed a possible scenario that such trapped carrier would rather relaxed directly to the ground states with nanosecond lifetimes. And the TA long component actually combines this process with the band edge radiative recombination (1-2 ns) as shown in Fig. 5. In this scenario, the final PL QY is determined by the competition between these two processes. The longer overall lifetime in sample 2 indicates the larger contribution of non-radiative recombination from the shallow trap states which encounters for the low PLQY.
Fig. 5. Schematics of the recombination pathways for two types of CsPbBr3 NCs. Fig. 5 summarized the reasonable photophysical processes of two CsPbBr3 NCs. We believe the more contribution of VBr related trapping in sample 2 should be due to the numerous surface dangling bonds induced from the transformation process from Cs4PbBr6 NCs.
5. Conclusions In summary, we synthesized CsPbBr3 NCs based on hot injection method and conversion method, exhibiting the similar size ~10 nm and cubic morphology. Two kinds of CsPbBr3 NCs showed consistent absorption and emission spectra. However, the PL QY of sample 2 is lower than sample 1 with surprisingly longer excited state lifetime. We attribute it to the enhanced contribution of trapping process from shallow VBr trap states above the conduction band edge, which should be structurally induced from the surface dangling bonds introduced during the transformation process in 12
method 2.
Acknowledgments We gratefully acknowledge financial support from the National Natural Science Foundation of China (U1232119), China Scholarship Council (CSC No. 201706990062). Independent Research Fund Denmark-Nature Sciences (DFF-FNU, Project No DFF-7014-00302), Independent Research Fund Denmark-Sapere Aude starting grant (No. 7026-00037A) and Swedish Research Council VR starting grant (No. 2017-05337).
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Supporting Information Effect of synthesis methods on photoluminescent properties for CsPbBr3 nanocrystals:～ hot injection method and conversion method Fengying Zhanga,b,c, Junsheng Chenc, Ying Zhoua, Rongxing Heb,*, Kaibo Zheng c,d,
*
a State Key Laboratory of Oil and Gas Reservoir and Exploitation and School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China. b Key Laboratory of Luminescence and Real-Time Analytical Chemistry of Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, People’s Republic of China. c Division of Chemical Physics, Department of Chemistry, Lund University, Box 124, SE-22100 Lund, Sweden. d Department of Chemistry, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.
E-mail: [email protected]; [email protected].
17
Fig. S1. UV-Vis absorption and PL spectra of Cs4PbBr6 NCs.
18
Fig. S2. XRD patterns of prepared Cs4PbBr6 NCs.
19
Fig. S3. UV-Vis absorption spectrum for sample 2.
20
Fig. S4. (a) TEM picture and (b) size distribution of Cs4PbBr6 NCs.
21
Fig. S5. (a) TEM picture of NCs for method 2. (b) The untransformed 0D impurity in method 2.
22
Fig. S6. FTIR spectra for both NCs from different prepared methods.
23
Fig. S7. TA kinetics at the maximum ground state bleach of CsPbBr3 NCs obtained from two different synthesis methods.
24
Highlights 1. Both hot injection method and conversion method were applied to prepare CsPbBr3 NCs. 2. Time-resolved photoluminescence and transient absorption dynamics of CsPbBr3 NCs prepared by two methods were compared. 3. Photophysical processes of CsPbBr3 NCs were proposed and discussed.
1
Fengying Zhang: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing - Original Draft, Visualization. Junsheng Chen: Methodology, Formal analysis, Data Curation. Ying Zhou: Resources, Writing - Review & Editing. Rongxing He: Resources, Writing - Review & Editing, Supervision, Project administration. Kaibo Zheng: Conceptualization, Software, Formal analysis, Writing - Review & Editing, Supervision, Project administration, Funding acquisition.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0022-2313(19)31721-1
DOI:
https://doi.org/10.1016/j.jlumin.2019.117023
Reference:
LUMIN 117023
To appear in:
Journal of Luminescence
Received Date: 31 August 2019 Revised Date:
16 December 2019
Accepted Date: 30 December 2019
Please cite this article as: F. Zhang, J. Chen, Y. Zhou, R. He, K. Zheng, Effect of synthesis methods on photoluminescent properties for CsPbBr3 nanocrystals: Hot injection method and conversion method, Journal of Luminescence (2020), doi: https://doi.org/10.1016/j.jlumin.2019.117023. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Effect of synthesis methods on photoluminescent properties for CsPbBr3 nanocrystals: hot injection method and conversion method Fengying Zhanga,b,c, Junsheng Chenc, Ying Zhoua, Rongxing Heb,*, Kaibo Zheng c,d,
*
a State Key Laboratory of Oil and Gas Reservoir and Exploitation and School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China. b Key Laboratory of Luminescence and Real-Time Analytical Chemistry of Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, People’s Republic of China. c Division of Chemical Physics, Department of Chemistry, Lund University, Box 124, SE-22100 Lund, Sweden. d Department of Chemistry, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.
E-mail: [email protected]; [email protected].
1
Highlights 1. Both hot injection method and conversion method were applied to prepare CsPbBr3 NCs. 2. Time-resolved photoluminescence and transient absorption dynamics of CsPbBr3 NCs synthesized by two methods were compared and analyzed. 3. Photophysical processes of CsPbBr3 NCs were proposed and discussed.
Abstract Multiple facile synthetic strategies for all inorganic perovskite CsPbBr3 nanocrystals (NCs) have been established and developed, profiting from their excellent performance and great potential applied in the field of photonic and optoelectronic. Here, CsPbBr3 NCs were synthesized by both hot injection method (method 1) and conversion method (method 2), and the discrepancy of their photophysical properties is elucidated via the complementary studies between time-resolved photoluminescence (TRPL) and transient-absorption (TA) spectroscopy. We found that CsPbBr3 NCs prepared by conversion method exhibited lower PL quantum yield (QY), which was ascribed to the larger partition of the NCs being passivated from the quenchers from the deep trap states. On the other hand, we also observed different radiative recombination rates between two samples which should be due to various trapping/detrapping times prior to the radiative recombination of the charge carriers in two samples. These results provide better guidance for the development and improvement of synthesis methodology for perovskite NCs. Keywords: perovskite nanoparticles; synthetic strategy; trap states; radiative recombination.
2
1. Introduction Hybrid lead-halide perovskites, which occurred as one of the most promising materials with prominent optoelectronic characteristics [1-4], have attracted wide attention in the past few years, especially when applied in solar cells [5-7], light-emitting diodes [8-10], photodetectors [11, 12] and lasers [13, 14]. The emerging all-inorganic counterparts (CsPbX3, X = I, Br, Cl) with replaced inorganic cation Cs+ were introduced with improved stability and charge carrier transfer properties [15, 16]. In 2015, Kovalenko et al. firstly reported the all-inorganic colloidal nanocrystals (CsPbX3 NCs), achieving high QYs up to 90% with great stability, narrow PL emission and adjustable spectra through compositional modulations of halide anions and quantum size-effects [17]. The synthesis methodologies [18-20], fundamental photophysics [21-23], as well as potential device applications have then been widely investigated [24, 25]. There exists several methods to synthesize CsPbBr3 NCs, involving 1) hot injection method (Cs-oleate precursor is injected into PbBr2 solution at high temperature, 160
200oC), 2) anti-solvent crystallization method (both ionic precursor solutions
are added into anti-solvent with low solubility, such as toluene and hexane) for spontaneous crystallization of the NCs, and 3) conversion method, where the CsPbBr3 NCs are transformed from monodisperse insulator Cs4PbBr6 with the help of excess PbBr2, and the size of CsPbBr3 NCs can be switched by adjusting the size of Cs4PbBr6 NCs [26]. Although the lattice structures and morphologies of CsPbBr3 NCs prepared via above methods seem to be identical based on the structural characterization, their optical properties and specially the photophysical behaviors could be different which has not yet been carefully discussed. Here, we compare the PL properties of CsPbBr3 NCs prepared by the hot injection method and conversion methods using both steady-state and time-resolved optical spectroscopies. The emissive state dynamics of these two types of CsPbBr3 NCs were monitored by combining TRPL and TA spectroscopy. We found that both CsPbBr3 NCs, despite of the similar size and cubic morphology, exhibited different PL QYs as well as excited state dynamics. We concluded that there are two pools in each sample. The first pool (pool A) has defects 3
as PL quenchers where the non-radiative recombination is dominated. In the other pool (pool B) of NCs, the excited state depopulation is greatly mediated by the trapping/detrapping process at shallow VBr trap states above the conduction band edge. The more pronounced trapping in VBr trap states in samples prepared via conversion method should be the origins of their lower PL QY.
2. Materials and methods Materials Cesium carbonate (Cs2CO3, 99%), lead bromide (PbBr2, 98%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), oleylamine (OLAM, 70-90%), toluene and hexane were purchased were purchased from Sigma-Aldrich. All chemicals were used as received without further purification. Method 1: hot injection method Preparation of Cs-oleate precursor. Cs2CO3 (0.814 g), OA (2.5 mL) and ODE (40 mL) were loaded into a 100 mL three-neck flask and dried under vacuum for 1 h at 120 °C. Then, the mixture was heated under N2 to 150 °C until all Cs2CO3 reacted with OA. Before usage, it needs to be preheated to make it soluble. Synthesis of CsPbBr3 NCs. Based on the method developed by Kovalenko and co-workers [17], PbBr2 (0.069 g) and ODE (10 mL) were loaded into a 50 mL three-neck flask and degassed for 1 h at 120 °C. Then, OA (0.5 mL) and OLAM (0.5 mL) mixtures were added at 120 °C under N2 atmosphere. After PbBr2 was completely dissolved, the temperature was raised to 180 °C, followed by the quick injection of 0.4 mL Cs-oleate precursor. After 5 s, the reaction mixture was cooled immediately by the ice-water bath. The crude solution was centrifuged at 1500 round per minute (rpm) for 10 min to remove large particles. Then, the supernatant was centrifuged again at 6500 rpm for 10 min, and the precipitate was collected and re-dispersed in toluene for further characterization. Method 2: conversion method Synthesis of Cs4PbBr6 NCs. Cs4PbBr6 NCs were prepared by modified hot injection method [26]. PbBr2 (0.4 mmol, 0.1468 g), ODE (20 mL), OA (0.8 mL) and 4
OLAM (1.5 mL) were loaded into a 50 mL three-neck at 120 °C. After PbBr2 was completely dissolved, the temperature was turned to 80 °C, followed by the quick injection of 3 mL Cs-oleate precursor (0.4 g Cs2CO3 dissolved in 8 mL OA in a 20 mL vial at 150 °C). After 5 s, the reaction mixture was cooled immediately by the ice-water bath. The crude solution was centrifuged at 4500 rpm for 10 min, and the precipitate was collected and redispersed in toluene or hexane (24 mL) for further characterization. Synthesis of CsPbBr3 NCs. 0.5 mL of Cs4PbBr6 NCs solution was diluted in 2 mL toluene and heated in the oil bath at 80 °C. After the temperature was stable, 50 µL PbBr2 precursor (1 mmol PbBr2 dissolved in the mixture of 2.5 mL OA, 2.5 mL OLAM and 5 mL toluene) was injected quickly. The reaction was cooled down with the ice-water bath after 45 minutes.
3. Measurements and characterizations UV-Vis absorption spectra were recorded by an Agilent 8453 UV-Vis spectrometer. Steady-state PL spectra were collected on a standard spectrofluorometer (Spex Fluorolog 1681). Powder X-ray diffraction (XRD) patterns were measured on a 4-circle Huber diffractometer in the range of 10° < 2θ < 60°, including an X-ray light source (12 keV, 0.1 nm) and a Si (111) double crystal monochromator. Transmission electron microscopy (TEM) measurement was performed using JEOL JEM-1400 microscope (80 kV). Fourier-transform infrared spectroscopy (FTIR) spectra were obtained using a BRUKER ɑ ALPHA-P FTIR Spectrometer. The streak camera (Hamamatsu, C6860) setup was used for the measurement of TRPL spectra, which contains a femtosecond laser source (800 nm, 2 kHz repetition rate, 150 fs pulse length). The laser of 400 nm generated by a BBO crystal was used to excite samples. One integrating sphere method was used for absolute quantum yield measurement. The sample was excited by a 405 nm continuous wave laser, and the integrating sphere (HORIBA, Quanta-φ, F3029) was applied to collect the overall PL and residual excitation light. The sphere output was detected by the spectrometer (AvaSpec-ULS2048-USB2-UA-50). The calibrated reference light source (Ocean 5
Optics, LS-1-CAL) was used for the correction of spectral sensitivity. Finally, the PL QY could be calculated by accounting for the numbers of the emitted and absorbed photons. TA measurements were implemented based on a femtosecond pump-probe setup. A femtosecond oscillator was used to generate the laser pulse (800 nm, 120 fs pulse length, 1 kHz repetition rate) through a regenerative amplifier (Spitfire XP Pro). The pump pulse of 400 nm was generated by a BBO crystal and to be as a second harmonic of the laser. The super-continuum probe laser we used came from a thin CaF2 plate. The polarization angle of 54.7° between pump and probe beams was achieved by placing a Berek compensator in the pump beam. To avoid photodamage of samples, they were moved constantly after each time delay point.
4. Results and discussion
Fig. 1. (a) Schematic diagram of two synthesis methods. (b) UV-Vis absorption and steady-state PL spectra, (c) XRD patterns of CsPbBr3 NCs obtained from two different synthesis methods. As shown in Fig. 1(a), CsPbBr3 NCs were prepared using both hot injection method and conversion method. Here, CsPbBr3 NCs obtained from two different synthesis methods exhibited the similar absorption emission spectra as presented in Fig. 1(b). The absorption band edge and emission peak of prepared CsPbBr3 NCs (method 1) were located at 503 nm and 511 nm, respectively, being consistent with previous reports [27, 28]. In the method 2, Cs4PbBr6 NCs plays an important role in the 6
conversion process, and their absorption spectra and PL were provided in Fig. S1. For the emission around 380 nm in the Cs4PbBr6 sample, which origins from the Pb2+ ion emissions. Once Cs4PbBr6 NCs turn into CsPbBr3 NCs, the absorption band edge changed from 315 nm to 506 nm, with the PL band moving from 378 nm to 510 nm. CsPbBr3 NCs from conversion method exhibited slightly smaller stokes shift when compared with that from hot injection method. However, we observed lower PL QY of the sample from method 2 (60%) compared with the sample from method 1 (80%) based on the integrated sphere method. To rationalize such discrepancy, we first conduct the fundamental characterization on their structures. XRD patterns in Fig. 1(c) confirmed the same typical cubic phase of CsPbBr3 NCs for two methods [17]. It is worth noting that XRD peaks marked with asterisks are attributed to the unconverted Cs4PbBr6 NCs, which can be verified by that of pre-transition Cs4PbBr6 NCs in Fig. S2. And the content of unconverted Cs4PbBr6 NCs in method 2 was roughly estimated to 15%. Besides, UV-Vis absorption spectrum presented in Fig. S3 for converted sample also confirmed the coexist of Cs4PbBr6 and CsPbBr3 NCs in method 2. While, as the Cs4PbBr6 NCs didn’t response to the visible excitation as their band edge resides below 350 nm, such Cs4PbBr6 NCs inclusion should not affect the intrinsic optical properties of CsPbBr3 NCs. The larger energy band gap of Cs4PbBr6 also exclude all the possible energy transfer or charge transfer from the CsPbBr3 QDs.
Fig. 2. TEM pictures of CsPbBr3 NCs obtained from two different synthesis methods, including the lattice spacing and size distribution. 7
We also concluded the similar size and morphology of two samples from their TEM pictures as provided in Fig. 2. Uniform dispersion cubic CsPbBr3 NCs with lattice spacing of 0.58 nm were prepared using two methods, and the mean sizes of CsPbBr3 NCs from hot injection method and conversion method were 10.3 nm and 10.6 nm, respectively. Actually, Cs4PbBr6 NCs suffered a reshaping from spherical/hexagonal shape to cubic phase of CsPbBr3 NCs in the conversion method with size decreasing from 14.4 nm to 10.6 nm (Fig. S4) [26]. This indicates any effect from size dependent quantum confinement or lattice structures on the photo-physics should be excluded. Two-phase coexistence could be clearly discerned in Fig. S5, and the lattice spacing of 0.39 nm confirmed the (300) crystal plane of rhombohedral Cs4PbBr6 NCs. As for these black dots occurred in CsPbBr3 NCs synthesized by conversion method, they have been widely accepted as the formation of Pb0 during the light illumination or electron beam exposure in the TEM, which is also highly determined by the protection of the surface capping agent [29, 30]. Easier ionization to Pb0 means the protection from surface capping agent is weaker, and this could be one of the origins from different synthesis method where the bonding strength of the capping agent could be different. FTIR spectra in Fig. S6 revealed the similar strength of the surface bonding but different number of ligands for two methods. In the next step, we investigated the emissive state dynamics of two samples via TA and TRPL studies.
8
Fig. 3. (a, b) TA spectra with 400 nm excitation (2×1013 ph/cm2/pulse) and (c, d) their respective SVD fittings for CsPbBr3 NCs obtained from two different synthesis methods. As provided in Fig. 3(a, b), there were distinct exciton bleach signals and absorption signals at ∼500 nm and ∼520 nm for early delay times, which corresponds to the band edge ground-state bleaching and hot-exciton induced red-shift, respectively [31]. With hot-exciton relaxation, the exciton-induced shift became smaller, and TA spectra were dominated by the bleach feature of ∼510 nm, which is ascribed to the state filling near the conduction band maximum and valence band minimum of CsPbBr3 NCs once excitation. The singular value decomposition (SVD) fitting for TA data gives three decay components. The fast component with lifetimes of 358 fs and 348 fs should be attributable to the cooling of hot carriers which has been extensively studied in our previous research [22, 32]. The other two components exhibit lifetimes around 190 ps and nanosecond, respectively. The fitting of TA decay at the maximum ground state bleach in Fig. S7 also give the same lifetimes of 196±0.03 ps, 4.0±0.001 ns for method 1 and 190±0.05 ps, 7.6±0.002 ns for method 2. The nanosecond components for both samples feature negative bleach of the band edge absorption together with the excited state absorption at the blue side as well as red side. This is a clear fingerprint of the recombination of band edge excitons in 9
CsPbBr3 NCs [27, 33]. On the other hand, the minor middle components exhibit only negative ground state bleach which should be attributed to the trapping of one species of charge carriers after excitation. If we assign the nanosecond components to the radiative recombination which occurs in parallel with the 190 ps trapping processes within one NC, the PLQY can be estimated using the conventional calculation:
η=
∑ ∑
i
i Ai krad
(1)
j A k i + ∑ j Aj knonrad i i rad
where krad and Ai denote the rate and amplitude of radiative recombination process. knonrad and Aj refer to the rate and amplitude of non-radiative recombination process, respectively. The calculated PL QYs were 37
(method 1) and 18.8
(method 2),
being inconsistent with the direct steady state PL measurement as mentioned above. This indicates two recombination pathways should be independent within different pools of NCs. In this scenario, the fast 190 ps trapping process should occur in those NCs with deep intrinsic trap states (most probably volume traps) where the PL would be full quenched. The similar lifetime as well as amplitude (about 10%) of these components for two samples as shown in table 1 indicated such defects should be generated regardless of synthesis methods with minor contribution. The nanosecond components, however, showed different lifetimes for two samples where the lifetime of sample 2 (7.6 ns) is longer than that of sample 1 (4.0 ns). Table 1 TA lifetimes and the respective amplitude of CsPbBr3 NCs prepared by two different methods. t1 (ps)
A1
t2 (ns)
A2
Method 1
196±0.03
0.08
4.0±0.001
0.93
Method 2
190±0.05
0.10
7.6±0.002
0.90
In TRPL measurement of two CsPbBr3 NCs, only monoexponential PL decays can be observed in both samples with the lifetime shorter than the nanosecond components in TA dynamics as shown in Fig. 4. The CsPbBr3 NCs obtained from method 1 displayed faster PL decay with lifetime of 1.96±0.47 ns. For NCs prepared 10
by conversion method, their PL lifetime was longer (2.1±0.36 ns). The similar and unchanged PL spectra during the time evolution for two samples (Fig. 4a&b) indicate the emission for both samples should be contributed from the same band edge radiative recombination. Moreover, the perfect monoexponential feature as shown in Fig. 4c manifest the single process of the radiative recombination for both samples. It should first be noticed the exciton condition (i.e. excitation wavelength is 400 nm, fluence is 2×1013 ph/cm2/pulse, etc) between TA and TRPL measurement are rather identical. The absence of fast (190 ps) TA components in TRPL decays could be due to 1) the small partition (<10%) which cannot be well resolved in TRPL due to limited time-resolution and instrument response, or 2) such trapping process does not directly quench the band edge emissive states (e.g. the initial trapping happens prior to the hot carrier cooling to the band edge states). On the other hand, the longer lifetime values of the TA nanosecond components compared with radiative PL lifetime extracted from TRPL also demonstrate that there are extra non-radiative recombination pathways within this long time-scale. Similarly, such non-radiative recombination does not directly quench the band edge emission, and hence cannot be reflected in the PL decays.
Fig. 4. (a) Time-resolved PL emissions with 400 nm excitation (2×1013 ph/cm2/pulse) for CsPbBr3 NCs. (c) Normalized PL decays integrated PL peak wavelengths. According to the FTIR results in Fig. S6, the concentration of capping agents vary between two samples. On the other hand, previous temperature dependent TRPL characterization of CsPbBr3 NCs revealed the existence of shallow trap states from Br vacancy, in which the location of shallow trap level is 0.16-0.23 eV above the conduction band edge according to the DFT calculation [34]. The fast trapping and 11
relaxation to the band edge states would delay the charge carrier recombination process. Our TA dynamics proposed a possible scenario that such trapped carrier would rather relaxed directly to the ground states with nanosecond lifetimes. And the TA long component actually combines this process with the band edge radiative recombination (1-2 ns) as shown in Fig. 5. In this scenario, the final PL QY is determined by the competition between these two processes. The longer overall lifetime in sample 2 indicates the larger contribution of non-radiative recombination from the shallow trap states which encounters for the low PLQY.
Fig. 5. Schematics of the recombination pathways for two types of CsPbBr3 NCs. Fig. 5 summarized the reasonable photophysical processes of two CsPbBr3 NCs. We believe the more contribution of VBr related trapping in sample 2 should be due to the numerous surface dangling bonds induced from the transformation process from Cs4PbBr6 NCs.
5. Conclusions In summary, we synthesized CsPbBr3 NCs based on hot injection method and conversion method, exhibiting the similar size ~10 nm and cubic morphology. Two kinds of CsPbBr3 NCs showed consistent absorption and emission spectra. However, the PL QY of sample 2 is lower than sample 1 with surprisingly longer excited state lifetime. We attribute it to the enhanced contribution of trapping process from shallow VBr trap states above the conduction band edge, which should be structurally induced from the surface dangling bonds introduced during the transformation process in 12
method 2.
Acknowledgments We gratefully acknowledge financial support from the National Natural Science Foundation of China (U1232119), China Scholarship Council (CSC No. 201706990062). Independent Research Fund Denmark-Nature Sciences (DFF-FNU, Project No DFF-7014-00302), Independent Research Fund Denmark-Sapere Aude starting grant (No. 7026-00037A) and Swedish Research Council VR starting grant (No. 2017-05337).
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Supporting Information Effect of synthesis methods on photoluminescent properties for CsPbBr3 nanocrystals:～ hot injection method and conversion method Fengying Zhanga,b,c, Junsheng Chenc, Ying Zhoua, Rongxing Heb,*, Kaibo Zheng c,d,
*
a State Key Laboratory of Oil and Gas Reservoir and Exploitation and School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China. b Key Laboratory of Luminescence and Real-Time Analytical Chemistry of Ministry of Education, College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, People’s Republic of China. c Division of Chemical Physics, Department of Chemistry, Lund University, Box 124, SE-22100 Lund, Sweden. d Department of Chemistry, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.
E-mail: [email protected]; [email protected].
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Fig. S1. UV-Vis absorption and PL spectra of Cs4PbBr6 NCs.
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Fig. S2. XRD patterns of prepared Cs4PbBr6 NCs.
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Fig. S3. UV-Vis absorption spectrum for sample 2.
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Fig. S4. (a) TEM picture and (b) size distribution of Cs4PbBr6 NCs.
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Fig. S5. (a) TEM picture of NCs for method 2. (b) The untransformed 0D impurity in method 2.
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Fig. S6. FTIR spectra for both NCs from different prepared methods.
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Fig. S7. TA kinetics at the maximum ground state bleach of CsPbBr3 NCs obtained from two different synthesis methods.
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Highlights 1. Both hot injection method and conversion method were applied to prepare CsPbBr3 NCs. 2. Time-resolved photoluminescence and transient absorption dynamics of CsPbBr3 NCs prepared by two methods were compared. 3. Photophysical processes of CsPbBr3 NCs were proposed and discussed.
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Fengying Zhang: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing - Original Draft, Visualization. Junsheng Chen: Methodology, Formal analysis, Data Curation. Ying Zhou: Resources, Writing - Review & Editing. Rongxing He: Resources, Writing - Review & Editing, Supervision, Project administration. Kaibo Zheng: Conceptualization, Software, Formal analysis, Writing - Review & Editing, Supervision, Project administration, Funding acquisition.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: