Tuning Blinking Behavior of Highly Luminescent Cesium Lead Halide

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Tuning Blinking Behavior of Highly Luminescent Cesium Lead Halide Nanocrystals through Varying Halide Composition Aidi Zhang, Chaoqing Dong, and Jicun Ren J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 5, 2017

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The Journal of Physical Chemistry

Tuning Blinking Behavior of Highly Luminescent Cesium Lead Halide Nanocrystals through Varying Halide Composition Aidi Zhang, Chaoqing Dong*, and Jicun Ren* College of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China *Corresponding author: Dr. & Prof. Jicun Ren E-mail: [email protected] Tel: 0086-21-54746001 Fax: 0086-21-54741297

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ABSTRACT: Colloidal cesium lead halide nanocrystals (CsPbX3, X = Cl, Br, I) are newcomer optoelectronic materials and have been successfully utilized as bright light source and high efficiency photovoltaics due to their facial solution processability. However, detailed understanding of their fundamental chemistry and photophysics behavior are urgently required for utilizing the full potential of the perovskite nanocrystals. In this work, we described a simple and efficient approach for regulating the blinking behavior of the CsPbX3 nanocrystals through varying halide composition. We systematically investigated the ensemble and single-particle optical properties of the mixed-halide CsPbBrxI3-x and CsPbBr3-yIy nanocrystals via two different anion-exchange routes (Br- ions → I- ions and I- ions → Br- ions). The power-law fitting results demonstrated that both the “on” and “off” events of their fluorescence exhibited similar powerlaw rule to those of the cadmium chalcogenide nanocrystals (e.g. CdSe nanocrystals). Under optimum conditions, the percentage of “non-blinking” CsPbBr3-yIy nanocrystals (the “on time” fraction > 99% of the observation time) was about 71%. Furthermore, we observed that the blinking behavior of the perovskite nanocrystals also could be regulated by directly mixing two different perovskite nanocrystals. These findings will provide new insights in realizing the full potential of these substances in photovoltaic and optoelectronic applications.


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INTRODUCTION Colloidal nanocrystals (NCs) are of interest for applications in multimodal imaging sensing, lighting, and integrated photonics.1-6 In particular, the hybrid organic-inorganic lead halide perovskite NCs (MAPbX3, MA = CH3NH3, X = Cl, Br, I), and all-inorganic cesium lead halide perovskite NCs (CsPbX3, X = Cl, Br, I) have emerged as a class of very promising materials for solar cell and LED displays,7-9 due to its excellent photovoltaic enabling properties resulting in rapid increase in device efficiency over the past three years. The high performance of perovskite based devices is mainly ascribed to their broad absorption spectra, long diffusion length of electron and holes, and slow carrier recombination, etc.10-11 Especially, the photoexcited carrier dynamics have been gained more attentions because the fully understanding of the photoexcited carrier dynamics is of critical importance for improving the power conversion efficiencies of the perovskite based solar cells.12-14 Although great efforts have been carried out, detailed understanding of their fundamental chemistry and photophysics properties are still far from fully being understood.10, 12, 15-16 Photoluminescence (PL) blinking is an interesting phenomenon, in which the PL intensity of single emitters randomly switching between bright, emissive state (“on” state), and dark, nonemissive state (“off” state) under continuous excitation.17-24 Blinking causes some troubles in application fields of quantum dots (QDs) requiring continuous excitation and emission, such as single-dot light source,25-26 and single-molecular tracking,27 and light-emitting diodes.28 Over the last two decades, intensive efforts have been devoted to investigate or suppress the PL blinking and Auger recombination (nonradiative recombination) process in semiconductor NCs.29-38 Unfortunately, the underlying physical mechanism is still under debate. So far, effective suppression of both blinking and Auger recombination has been realized in several specific


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nanocrystal structures. The first approach is the “giant” QDs. These dot-in-bulk CdSe/CdS QDs comprising a quantum-confined CdSe core embedded into an ultrathick (7-9 nm) CdS shell.33, 36 Experimental results demonstrated that a larger shell volume appeared to be a necessary condition for controlling the blinking behavior. However, these “giant” QDs often have poor size distribution, moderate PL QYs, and usually need more time to be coated with a thick shell.36 The second approach is changing the solution environment or the surface state of the QDs by introduction of the so-called “anti-blinking” agents (small molecule compounds or functional polymer).35, 39-42 This method could not play a permanent suppression effect, and the blinking exacerbated again when these “anti-blinking” agents was displaced by other compounds or lost to the environment. The third approach is introducing an alloyed gradient structure. Efros et al. demonstrated that the abruptness of the hetero-interfaces or bounding surfaces would facilitate the Auger recombination of QDs.43 They proposed to soften the confinement potential using materials with graded interfaces. Klimov and coworkers found that CdSe QDs coated with a few of CdS monolayers significantly suppressed the Auger process,44 and revealed that these unique properties originated from a thinner alloy layer at the core/shell interfaces, which significantly smoothed the confinement potential of the QDs. Ion-exchange reaction is a postsynthetic chemical approach of colloidal NCs, and has been proven to be a simple and versatile approach for preparing new nanostructured materials not readily accessible by current methods.45 For the cation-exchange, the II-VI, IV-VI, and III-V semiconductor NCs have been the widely studied systems,46-54 involving complete or partial exchange of cation from their parent NCs. Recently, anion exchange in perovskite NCs has been reported as an effective method for obtaining NCs with compositional and optical tunability,45, 5556

which is attributed to the high halide ion mobility and the rigid nature of the cationic sublattice


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of halide perovskites. To the best of our knowledge, there are few reported examples of successfully tuning NCs blinking via regulating the anion composition of the NCs.57-60 In this study, we proposed a simple method to regulate the blinking behavior of colloidal CsPbX3 NCs (X = Br, I) via postsynthetic reactions with halide precursors, and demonstrated that varying the halide composition was a simple and versatile approach for regulating the blinking behavior of perovskite NCs with preservation of shape and crystal structure of the initial NCs. EXPERIMENTAL SECTION Chemicals and Materials. Cesium carbonate (Cs2CO3, 99.9%), 1-octadecene (ODE, 90%), oleic acid (OA, 90%), oleylamine (OAm, 70%), lead iodide (PbI2, 99.999%), lead bromide (PbBr2, 99.999%), and poly (methyl methacrylate) (PMMA, Average Mw ～ 15000 by GPC) were purchased from SigmaAldrich. All chemicals were used as received without further purification. Synthesis of CsPbX3 NCs. Preparation of Cs-oleate solution: 0.814 g Cs2CO3 and 2.5 mL OA was mixed with 40 mL ODE in a 100 mL three-neck flask. The mixture was degassed and dried at 120 oC under vacuum for 60 min, and then heated to 150 oC under argon gas until all Cs2CO3 was reacted with OA. Because the Cs-oleate precipitates out of ODE at room temperature, it has to be reheated to 100 o

C and obtain a colorless clear solution before injection.

Synthesis of CsPbX3 NCs: 0.2 mmol PbX2 (0.092 g PbI2 or 0.073 g PbBr2) were put into a 50 mL flask with 5 mL ODE, 0.5 mL OAm, and 0.5 mL OA. The mixture was dried at 120 oC under


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vacuum for 60 min. After the complete solubilisation of a PbX2 salt, the flask was refilled with argon gas. Then, the temperature was increased to 180 oC, followed by the rapid injection of 0.5 mL of Cs-oleate solution. Five seconds later, the reaction mixture was cooled by the ice-water bath. Without purification, the crude solution was used for further anion exchange reactions. Anion-Exchange Reactions with OAm-X. The anion exchange reactions were conducted in a Schlenk line under argon flux. For the synthesis of CsPbBrxI3-x NCs (0 < x < 3), 100 uL of crude CsPbBr3 NCs solution were dispersed in 5 mL toluene in a 25 mL three-neck flask, and different quantities (50, 100, 150, 200, 250 uL) of 0.1 mol/L OAm-I solution (OAm-I was dissolved in toluene) were swiftly injected and vigorously stirred. The final color of the NCs solution was dependent on the amount of the halide precursor solution. The synthesis of CsPbBr3-yIy NCs (0 < y < 3) was conducted using the crude CsPbI3 NCs solution and OAm-Br precursor solution with the same protocol. Inter-NCs Anion-Exchange Reactions. The inter-NCs anion exchange reactions were conducted in a Schlenk line under argon flux. For the synthesis of CsPb(Br/I)3 NCs, 100 uL of crude CsPbBr3 NCs solution were dispersed in 5 mL toluene in a 25 mL three-neck flask, and different quantities (100, 125, 150, 175, 200, 250 uL) of crude CsPbI3 NCs solution were swiftly injected and vigorously stirred. The final color of the NCs solution was dependent on the amount of the CsPbI3 NCs solution. Characterization of Perovskite NCs. For detailed optical, morphology, and the single-particle characterization of the perovskite NCs, please see the Supporting Information.


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RESULTS AND DISCUSSION Preparation and Characterization of Parent and Mixed-Halide Perovskite NCs. Colloidal CsPbBr3 and CsPbI3 NCs with oleylamine and oleic acid as surface ligands were synthesized following a reported procedure with slight modification.9 In this work, the CsPbBr3 and CsPbI3 NCs possessed PL peaks of 510 nm and 683 nm (Figure 1a). The corresponding full widths of half-maxima (FWHM) of their PL spectra were 27 nm and 51 nm. Their PL QYs were about 90% and 35%. Figure 1b and 1c display the TEM images of the perovskite NCs, and the inserted are their corresponding HRTEM images. It is clearly seen that the CsPbBr3 and CsPbI3 NCs showed the cubic shape, and the NCs were uniformly arranged on the carbon-coated Cu grid. The d-spacing values of 0.58 nm were also measured in agreement with reported values of cubic CsPbBr3 unit cells.7 All their average sizes were about 8.5 ± 1.2 nm for CsPbBr3 NCs and 14.0 ± 1.8 nm for CsPbI3 NCs, based on the measurement of more than 200 NCs. The size distribution is shown in Figure S1. In this study, the halide precursors in the exchange reaction process were oleylammonium halides (abbreviated as OAm-Br and OAm-I), which were obtained by reacting oleylamine with HCl or HI.55-56 OAm-X (X = Br, I) was one of the halide precursors which can trigger the anion exchange.56 The reason for choosing of OAm-X as halide precursors is that, the perovskite NCs synthesized in this study were already coated with oleylamine molecules as surface ligands. The OAm-X acting as the reactant will decrease the interference to the surface organic shell around the NCs.


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Figure 1. (a) Optical absorption and PL spectra of CsPbBr3 and CsPbI3 NCs dispersed in toluene. (b) TEM image of CsPbBr3 NCs (8.5 nm). The inserted bar is 5 nm. (c) TEM image of CsPbI3 NCs (14.0 nm). The inserted bar is 5 nm. (d) Overview of the reaction routes and precursors for the anion exchange process on CsPbX3 NCs (X = Br, I). ______________________________________________________________________________ Figure 1d depicts the routes for the anion exchange reactions on CsPbX3 NCs (X = Br, I). The operations were conducted in dry toluene as a solvent, by mixing a certain amount of the desired halide sources and perovskite NCs solution. Figure S2 demonstrates the in situ PL studies of the two different anion-exchange routes (Br- ions → I- ions and I- ions → Br- ions). The anionexchange reactions were rapidly finished in one minute, which originated from the high halide ion mobility and the rigid nature of the cationic sublattice of the halide perovskites NCs. The rapid movement of the PL spectra reflected the continuous formation of a homogeneous mixedhalide NCs solution, because any compositional inhomogeneities within the nanocrystal would lead to broad or multiple peaks.55-56


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Figure 2. (a), (b) The evolution of emission colors (UV excitation, wavelength = 320 nm) upon forming mixed-halide CsPbBrxI3-x and CsPbBr3-yIy nanocrystals. The dosages of OAm-Br or OAm-I were 0, 50, 100, 150, 200, and 250 uL, respectively. (c), (e) The evolution of the PL and optical absorption spectra of CsPbBr3 NCs after treatment with various quantities of OAm-I. (d), (f) The evolution of the PL and optical absorption spectra of CsPbI3 NCs after treatment with various quantities of OAm-Br. ______________________________________________________________________________


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Figure 3. (a)-(e) TEM images of CsPbBr3 NCs with various degrees of conversion with iodide anions (OAm-I). (f)-(j) TEM images of CsPbI3 NCs with various degrees of conversion with bromide anions (OAm-Br). The scale bars are 50 nm. ______________________________________________________________________________ Video S1 recorded an exchange process for the CsPbBr3 NCs after gradually introducing different amounts of OAm-I precursors solutions. By adjusting the halide precursors, the optical emission and absorption spectra of the mixed-halide NCs can be tuned almost over the entire visible spectral region (Figure 2). During the PL spectra evolution, all the anion exchange discussed here either led to a red shift, from 510 nm to 662 nm (Br- ions → I- ions), or led to a blue shift, from 683 nm to 540 nm (I- ions → Br- ions). The PL spectra of such ion-exchanged NCs were Stokes-shifted with respect to the optical absorption spectra, and remarkably bright fluorescence of all mixed-halide NCs was characterized by high QYs of 30-80% (Figure S3). Table S1 demonstrates the FWHM of the PL spectra extracted from the Figure 2c and 2d. It was an interesting phenomenon that the FWHM evolution of the PL spectra showed converse trends in these two anion exchange process. Specifically, for the Br- ions → I- ions exchange route, the


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FWHM of the CsPbBrxI3-x NCs showed a gradual increase and then reached equilibrium at about 52 nm. As for the reverse case (I- ions → Br- ions), the FWHM of the CsPbBr3-yIy NCs continuously decreased until arrived at 27 nm. The data also reflected that the mixed-halide perovskite NCs containing more Br- ions and less I- ions displayed narrower FWHM. During the ion exchange process (Br- ions → I- ions), the average diameters of the CsPbBrxI3-x NCs increased slightly (Figure 3), their average sizes were 8.9 ± 1.1 nm, 9.4 ± 1.3 nm, 10.0 ± 1.0 nm, 10.3 ± 1.5 nm, and 10.5 ± 1.1 nm, respectively (Figure 3a-3e, Figure S4). The average size of the CsPbBr3-yIy NCs showed a certain decrease with the addition of OAm-Br precursor (Figure 3f-3j, Figure S5). In details, their average sizes were 13.5 ± 1.5 nm, 12.9 ± 1.6 nm, 12.0 ± 1.5 nm, 11.8 ± 1.4 nm, and 11.3 ± 1.6 nm, respectively. These mixed-halide perovskite NCs possess very good crystallinity, which is preferred for improving the luminescence efficiency and the LED device performance. Blinking Behavior of Parent and Mixed-Halide Perovskite NCs. The procedures for the single-particle measurements included four steps, sample preparation, collecting the fluorescence imaging, extracting the fluorescent trajectories, counting the percentages of “on time” fractions and fitting the power-law exponents. Specifically, a dilute solution of the perovskite NCs was mixed with a 2% chloroform solution of PMMA and spincoated onto a coverslip. Then they were illuminated by evanescent field excitation through the edge of the objective with a 488 continuous-wave argon ion laser, using a total internal reflection fluorescence microscopy (TIRFM). Subsequently, the fluorescence from the NCs was collected by an electron-multiplying charge coupled device (EM-CCD) camera (Figure 4a). Different emission lights of the perovskite NCs were separately collected by choosing suitable emission filters. Image acquisition was performed using the ImageJ software. Six successive frames of the


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TIRFM images with exposure time of 30 ms were shown in Figure 4b. In this fluorescence image, all the NCs were well separated and no aggregation was observed. The fluorescence trajectories of NCs were extracted based on the ImageJ software and obvious fluorescence intermittency (blinking) was observed in each of the perovskite NCs (Figure 4c). For clearly distinguish the “on” and “off” events, the threshold was set to three times the standard deviation above the average background signal intensity. ______________________________________________________________________________

Figure 4. Schematic illustration for assessing the blinking behaviour of perovskite NCs. (a) Schematic diagram of the TIRFM setup. (b) Consecutive TIRFM image sequences of perovskite NCs embedded in a PMMA matrix on a glass substrate (512 × 512 pixel). The exposure time is 30 ms. (c) Representative fluorescence-intensity trajectories of five random individual perovskite NCs with a temporal resolution of 30 ms and their corresponding count rate histograms. The dashed red line is the chosen value as the threshold between “on time” and “off time”. (d) The “on time” distribution histograms for the CsPbBr3 and CsPbI3 NCs. ______________________________________________________________________________


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Two parameters, the “on time” distribution and power-law exponents were used to assess the blinking behavior as the quantitative and qualitative approaches. The fraction of “on time” represents that the percentages of the fluorescence emission of a NC in the total acquisition time (120 s). A “non-blinking” NC is defined that when the “on time” fraction of a NC is over 99% in the overall acquisition time.61-63 Figure 4d shows the “on time” fractions distribution of the CsPbBr3 and CsPbI3 NCs. The percentage of CsPbBr3 NCs with the “on time” < 10% was about 59% and no NCs demonstrated “non-blinking” behavior (“on time” fraction > 99%). That meant the CsPbBr3 NCs had long periods of “off time” and severe blinking (Video S2 in the Supporting Information). For the CsPbI3 NCs, the percentage of the “on time” fraction < 10% was about 13%, and the value of “non-blinking” NCs were about 3%. These results above illustrated that the CsPbI3 NCs possessed less blinking than the CsPbBr3 NCs. To investigate the effect of the dosages of the halide precursors on the blinking behavior of the parent perovskite NCs, five different dosages of the halide precursors (50, 100, 150, 200, and 250 uL) were chosen for assessing the blinking behavior of the mixed-halide CsPbBrxI3-x and CsPbBr3-yIy NCs. The total “on time” fractions (the accumulated percentage of PL emission states) and the detailed distribution of the “on time” fractions of mixed-halide perovskite NCs were systematically investigated, in order to quantify the blinking degree of these NCs. With the addition of the OAm-I precursor, the total “on time” fraction of the CsPbBrxI3-x NCs exhibited an increase at the initial period, and then reached the equilibrium. Their detailed values were 29%, 42%, 57%, 62%, and 66%, respectively (Figure 5a). For the CsPbBr3-yIy NCs, their total “on time” fractions were 47%, 65%, 83%, 89%, and 80%, respectively. These results indicated that the introduction of halide precursors had a significant influence on the blinking behavior of their parent perovskite NCs.


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Figure 5. (a) The total (accumulated) “on time” fraction of upon forming mixed-halide CsPbBrxI3-x and CsPbBr3-yIy NCs. (b) “On time” distribution histograms for CsPbBrxI3-x NCs. (c) “On time” distribution histograms for CsPbBr3-yIy NCs. ______________________________________________________________________________ Figure 5b-5c shows the detailed distribution of the “on time” fractions of the mixed-halide CsPbBrxI3-x and CsPbBr3-yIy NCs. The percentages of “non-blinking” CsPbBrxI3-x NCs (“on time” fraction > 99%) were 2%, 14%, 19%, 26%, and 33%, respectively. These results indicated that the increase of the OAm-I precursor was beneficial for suppressing the blinking of CsPbBr3 NCs. For appropriate dosages of OAm-Br precursor (50-200 uL), the percentages of CsPbBr3-yIy NCs with “on time” > 99% were 6%, 28%, 49%, 71% (Video S3 in the Supporting Information), and this value decreased to about 52% when the dosage was 250 uL. These data illustrated that the “on time” fraction of the CsPbBr3-yIy NCs was sensitive to the amounts of OAm-I precursor.


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For comparison, we also prepared mixed-halide perovskite NCs by directly varying precursors’ dosage, and further investigated their blinking. The specific molar ratios of [PbBr2]/[PbI2] were 0.15:0.05, 0.10:0.10, and 0.05:0.15. The percentages of these CsPbBrxI3-x NCs with the “on time” fraction < 10% were 41%, 32%, and 22%. The percentages of “non-blinking” NCs were about 0%, 4.1%, and 2.0% (Figure S6 in the Supporting Information). These results demonstrated that mixed-halide CsPbBrxI3-x NCs containing high iodine content exhibited suppressed blinking. ______________________________________________________________________________ Table 1. On/off power-law exponents extracted from the data that shown in Figure S7 and S8. The values in parentheses are the standard deviations of the fit mon 1.94 (0.04) 1.71 (0.06) 1.58 (0.05) 1.48 (0.08) 1.39 (0.07) 1.35 (0.03) 1.32 (0.05) 1.26 (0.08) 1.07 (0.06) 0.77 (0.04) 1.23 (0.07) 1.43 (0.06)

CsPbBr3 NCs CsPbBr3 NCs + 50 uL OAm-I CsPbBr3 NCs + 100 uL OAm-I CsPbBr3 NCs + 150 uL OAm-I CsPbBr3 NCs + 200 uL OAm-I CsPbBr3 NCs + 250 uL OAm-I CsPbI3 NCs CsPbI3 NCs + 50 uL OAm-Br CsPbI3 NCs + 100 uL OAm-Br CsPbI3 NCs + 150 uL OAm-Br CsPbI3 NCs + 200 uL OAm-Br CsPbI3 NCs + 250 uL OAm-Br

moff 1.29 (0.03) 1.22 (0.07) 1.21 (0.08) 1.24 (0.05) 1.21 (0.04) 1.24 (0.07) 0.95 (0.06) 1.11 (0.04) 1.22 (0.05) 1.40 (0.08) 1.28 (0.06) 0.95 (0.04)

______________________________________________________________________________ The “on time” and “off time” distributions of the parent perovskite NCs and the mixed-halide perovskite NCs were further analyzed by the log-log plot of the probability densities.17, 33, 64 Both the “on” and “off” events followed the power-law distributions,

P (ton / off ) = Bt

− mon / off

(1)


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Here, t is the time fractions, P(ton) or P(toff) is the probability distribution for the “on time” or “off time” fractions, mon or moff is the power-law exponent for the “on time” or “off time” fractions, and B is a constant. The power-law fitting statistics from about 80 NCs were plotted on a log-log scale, and the results were shown in Figure S7-S8, and Table 1. The fitting results demonstrated that both the “on” and “off” events of the fluorescence exhibited similar power-law to those of the cadmium chalcogenide QDs (e.g. CdSe QDs).42, 65-69 Compared with CsPbI3 NCs, the CsPbBr3 NCs possessed a bigger mon and a smaller moff value. These results implied that the CsPbBr3 NCs displayed frequent fluorescence intermittency and shorter “on time” fraction than the CsPbI3 NCs. The underlying variations of power-law exponents of the probability density during the two different anion-exchange routes (Br- ions → I- ions and I- ions → Br- ions) were particularly studied. The mon exponents of the mixed-halide CsPbBrxI3-x NCs were 1.71 (0.06), 1.58 (0.05), 1.48 (0.08), 1.39 (0.07), and 1.35 (0.03), respectively. These moff exponents were statistically identical. The values were 1.22 (0.07), 1.21 (0.08), 1.24 (0.05), 1.21 (0.04), 1.24 (0.07), respectively. This reflected that these NCs showed an increase of the long “on” events of the NCs at the initial period, and then approximately trended to be stable. The mon exponents of the mixed-halide CsPbBr3-yIy NCs were 1.26 (0.08), 1.07 (0.06), 0.77 (0.04), 1.23 (0.07), and 1.43 (0.06), respectively. Furthermore, the moff exponents were also sensitive to the amounts of OAm-Br precursor, these data were 1.11 (0.04), 1.22 (0.05), 1.40 (0.08), 1.28 (0.06), and 0.95 (0.04), respectively. Both the evolution of the mon and moff exponents demonstrated that the CsPbBr3-yIy NCs showed suppressed blinking with moderate OAm-Br precursor (50, 100, and 150 uL), and then the blinking were aggravated again with more OAm-Br precursor (200 and 250 uL).


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Figure 6. (a) PL spectra of inter-particle anion-exchange reactions in CsPbX3 NCs systems. (b) PL QYs and the total “on time” fraction for the CsPb(Br/I)3 NCs. (c) “On time” distribution histograms for the CsPb(Br/I)3 NCs. (d) Power-law exponents for the CsPb(Br/I)3 NCs. ______________________________________________________________________________ Blinking Behavior of Inter-Particle Anion Exchange Process. The inter-particle anion exchange reaction is a uniquely dynamic process that takes place in solution between different perovskite NCs.55-56 Herein, we also designed experiments to investigate the optical properties during the inter-particle anion exchange process (for detailed experimental operation, please see the Supporting Information). Under continuous stirring, CsPb(Br/I)3 NCs with different emission peaks were obtained by simply mixing a certain amount


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of CsPbBr3 NCs solution with different dosages of CsPbI3 NCs solution. It is worthy of note that the perovskite NCs themselves can serve as halide sources for each other during this exchange process. The halides ions moved from NCs to NCs in the solution phase, and reached the equilibrium state in about 10-20 minutes. After the completion of inter-particle anion exchange, the obtained CsPb(Br/I)3 NCs possessed narrow PL spectra region and the PL peaks ranged between those of their parent NCs (Figure 6a). These NCs still showed high PL QYs (Figure 6b), which is equivalent to those obtained from direct synthesis or in-direct anion exchange reactions. Four different molar ratios of [CsPbI3]/[CsPbBr3] (1:1, 1:1.5, 1:2, and 1:2.5) were chosen for further assessing the evolution of the blinking behavior. The total “on time” fraction of the CsPb(Br/I)3 NCs showed a gradual increase, from 31%, to 52%, 65% and 68% (Figure 6b). Figure 6c shows the detailed distribution of the “on time” fractions of the CsPb(Br/I)3 NCs. The percentages of “non-blinking” NCs (“on time” fraction > 99%) for the molar ratios of 1:1 and 1:1.5 were 6% and 7%. In the case of 1:2 and 1:2.5, the values of NCs with the “on time” fraction > 99% approximately reached at 28% and 30%. Meanwhile, the values of the “on time” fraction < 10% for the selected ratios were 5%, 14%, 8%, and 13%, respectively. The power-law fitting process and their corresponding power-law exponents of the “on time” and “off time” fractions of the final CsPb(Br/I)3 NCs are shown in Figure S9 and Figure 6d. The mon values showed a gradual decrease with the addition of the CsPbI3 NCs, from 1.66 (0.06) to 1.56 (0.09), 1.47 (0.07) and 1.39 (0.08). Meanwhile, the moff values were 1.37 (0.08), 1.44 (0.06), 1.54 (0.10), and 1.55 (0.09), respectively. Both of the data showed that the introduction of CsPbI3 NCs reduced the duration time of the “off” events, and these results also indicated that the interparticle blinking behavior of the perovskite NCs could be regulated by controlling the composition of the NCs.


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Discussion. So far, several theoretical models or mechanism were proposed to elaborate the blinking behavior of the NCs.29, 31-33, 70-71 Of particular, the most widely accepted explanation is that the classical charging/discharging model.17, 72 For an uncharged NC, a photon excites the NC and creates an electron-hole pair (exciton). When the electron and hole recombines, the NC emits another photon and brings about the photoluminescence. This is known as the radiative recombination. If the NC is charged, the extra carrier triggers a process known as Auger recombination, during which the exciton energy is acquired by an extra electron or hole. Because the rate of Auger recombination is orders of magnitude faster than that of radiative recombination, and the PL is completely ‘quenched’.70-71 Some research found that the blinking can be suppressed if the Auger recombination is forbidden43 or if their corresponding lifetime is made longer than the fluorescence lifetime.33, 73 In our work, the halide ion exchange by adding OAm-X (X = Br, I) precursors or directly mixing two different perovskite NCs is basically the same, and both will result in mixed-halide perovskite NCs. This halide ion exchange process may eliminate the interior traps of the perovskite NCs. It can be concluded that the mixed-halide perovskite NCs may possess an alloyed structure, which can reduce the rate of Auger recombination and further suppress the Auger process. To get deeper insight into the nature of the emitting states of the perovskite NCs, we analyzed the PL lifetime using the steady state/life time fluorescence spectrometer. Results showed that the PL lifetime became bigger after introducing the OAm-I (Figure S10 in the Supporting Information). The prolonged lifetime for the mixed-halide CsPbBrxI3-x NCs was a feature for the suppression of the Auger recombination. It can be concluded that the nonradiative recombination rate was decreased or weakened by varying the halide composition. The insights


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gained from these above surveys indicated that the observed PL blinking was in accordance with the proposed charging blinking models. This result was consistent with Klimov’s recent research,16 in which they demonstrated that the PL blinking behavior of the CsPbX3-based perovskite NCs resulted from random charging/discharging of the NCs driven by photoionization. CONCLUSIONS In summary, we have demonstrated a fast and simple postsynthesis anion exchange approach for tuning the blinking behavior of the cesium lead halide perovskite NCs (CsPbX3, X = Br, I), with preservation of the shape and crystal structure of their parent perovskite NCs. We systematically investigated the ensemble and single particle optical properties of the mixedhalide CsPbBrxI3-x and CsPbBr3-yIy NCs via two different anion-exchange routes (Br- ions → Iions and I- ions → Br- ions). The PL peaks of the obtained mixed-halide NCs could be tuned from 510 nm to 683 nm while maintaining high PL QYs of about 30-80% and narrow FWHM of 26-52 nm. The power-law fitting results demonstrated that both the “on” and “off” events of their fluorescence exhibited similar power-law rule to those of the cadmium chalcogenide QDs. Under optimum conditions, we fabricated “non-blinking” mixed-halide CsPbBr3-yIy nanocrystals (“on time” fraction > 99% of the observation time) of about 71%. These finding provides unique insights into the photophysics process of the perovskite NCs, and is of crucial importance for improving the potential of the perovskite NCs in various application fields. ASSOCIATED CONTENT Supporting Information.


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The following files are available free of charge. Synthesis of OAm-Br and OAm-I, characterization of the NCs, particle distribution, in-situ PL measurement, PL QYs and lifetimes, power-law fitting statistics, and FWHM values. (PDF) Fast anion-exchange of CsPbBr3 NCs with the addition of OAm-I precursor, blinking behavior of CsPbBr3 NCs and CsPbBr3-yIy NCs. (AVI) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: 0086-21-54746001. Fax: 0086-21-54741297. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 21135004, 21327004 and 21475087), and China Postdoctoral Science Foundation (No. 2016M591661). REFERENCES (1) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Semiconductor Nanocrystals as Fluorescent Biological Labels. Science 1998, 281, 2013-2016. (2) Chan, W. C. W.; Nie, S. Quantum Dot Bioconjugates for Ultrasensitive Nonisotopic Detection. Science 1998, 281, 2016-2018. (3) Kamat, P. V. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737-18753. (4) Zhang, H.; Wang, L.; Xiong, H.; Hu, L.; Yang, B.; Li, W. Hydrothermal Synthesis for HighQuality CdTe Nanocrystals. Adv. Mater. 2003, 15, 1712-1715. (5) Reiss, P.; Carrière, M.; Lincheneau, C.; Vaure, L.; Tamang, S. Synthesis of Semiconductor Nanocrystals, Focusing on Nontoxic and Earth-Abundant Materials. Chem. Rev. 2016, 116, 10731-10819.


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Figure 1. (a) Optical absorption and PL spectra of CsPbBr3 and CsPbI3 NCs dispersed in toluene. (b) TEM image of CsPbBr3 NCs (8.5 nm). The inserted bar is 5 nm. (c) TEM image of CsPbI3 NCs (14.0 nm). The inserted bar is 5 nm. (d) Overview of the reaction routes and precursors for the anion exchange process on CsPbX3 NCs (X = Br, I). 205x105mm (300 x 300 DPI)


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Figure 2. (a), (b) The evolution of emission colors (UV excitation, wavelength = 320 nm) upon forming mixed-halide CsPbBrxI3-x and CsPbBr3-yIy nanocrystals. The dosages of OAm-Br or OAm-I were 0, 50, 100, 150, 200, and 250 uL, respectively. (c), (e) The evolution of the PL and optical absorption spectra of CsPbBr3 NCs after treatment with various quantities of OAm-I. (d), (f) The evolution of the PL and optical absorption spectra of CsPbI3 NCs after treatment with various quantities of OAm-Br. 172x183mm (300 x 300 DPI)



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Figure 3. (a)-(e) TEM images of CsPbBr3 NCs with various degrees of conversion with iodide anions (OAmI). (f)-(j) TEM images of CsPbI3 NCs with various degrees of conversion with bromide anions (OAm-Br). The scale bars are 50 nm. 290x135mm (300 x 300 DPI)


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Figure 4. Schematic illustration for assessing the blinking behaviour of perovskite NCs. (a) Schematic diagram of the TIRFM setup. (b) Consecutive TIRFM image sequences of perovskite NCs embedded in a PMMA matrix on a glass substrate (512 × 512 pixel). The exposure time is 30 ms. (c) Representative fluorescence-intensity trajectories of five random individual perovskite NCs with a temporal resolution of 30 ms and their corresponding count rate histograms. The dashed red line is the chosen value as the threshold between “on time” and “off time”. (d) The “on time” distribution histograms for the CsPbBr3 and CsPbI3 NCs. 172x135mm (300 x 300 DPI)



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Figure 5. (a) The total (accumulated) “on time” fraction of upon forming mixed-halide CsPbBrxI3-x and CsPbBr3-yIy NCs. (b) “On time” distribution histograms for CsPbBrxI3-x NCs. (c) “On time” distribution histograms for CsPbBr3-yIy NCs. 161x126mm (300 x 300 DPI)


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Figure 6. (a) PL spectra of inter-particle anion-exchange reactions in CsPbX3 NCs systems. (b) PL QYs and the total “on time” fraction for the CsPb(Br/I)3 NCs. (c) “On time” distribution histograms for the CsPb(Br/I)3 NCs. (d) Power-law exponents for the CsPb(Br/I)3 NCs. 194x161mm (300 x 300 DPI)


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