Encapsulated room-temperature synthesized ... - OSA Publishing

Vol. 8, No. 11 | 1 Nov 2018 | OPTICAL MATERIALS EXPRESS 3494

Encapsulated room-temperature synthesized CsPbX 3 perovskite quantum dots with high stability and wide color gamut for display YUJUN XIE,1 YAN YU,1 JUNYI GONG,1 CHENG YANG,1 PAN ZENG,1 YURONG DONG,1 BILIN YANG,1 RONGQING LIANG,1,2 QIONGRONG OU,1,2,3 1,2,4 AND SHUYU ZHANG 1

Department of Light Sources and Illuminating Engineering, Fudan University, Shanghai 200433, China 2 Engineering Research Center of Advanced Lighting Technology, Ministry of Education, Shanghai 200433, China 3 [email protected] 4 [email protected]

Abstract: Room temperature recrystallization is an intriguing method of fabricating CsPbX3 perovskite quantum dots since it does not involve high temperature or inert atmosphere, offering a promising route to the mass production of CsPbX3 quantum dots at low cost. However, their performance stability during work was seldom investigated and was far from the requirements for practical applications. Here, we demonstrate a facile and low-cost method to significantly improve the thermal, photo- and water stability of room-temperature synthesized perovskite quantum dots by effectively suppressing the unfavored grain growth and surface trap states. The fabricated quantum dots of green-emitting CsPbBr3 and redemitting CsPbBr1.2I1.8 were applied for quantum dot-converted white LEDs, which are capable of achieving a wide color gamut of 135% NTSC and 101% Rec. 2020. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction All-inorganic quantum dots (QDs) of cesium lead halide perovskites CsPbX3 (X = I, Br, Cl) have received extensive attention since Kovalenko and his colleagues first synthesized highly luminescent colloidal CsPbX3 nanocrystals in 2015 [1]. Compared with conventional chalcogenide-based QDs, CsPbX3 QDs demonstrate spectrally narrow and broadly tunable emission with extremely high photoluminescence quantum yield (PLQY) based on facile synthesis using inexpensive off-the-shelf precursors, holding great potentials in optoelectronic applications [1–10]. The most common approach to synthesizing CsPbX3 QDs is hot injection, which involves heat treatment under an inert atmosphere. The stability of these hot injection-based QDs is critically important for their long-term applications, therefore a variety of strategies to improve the stability, including ligand engineering [11,12], coating [13–20] and passivation [21–30] have been developed. Another emerging means of synthesizing high-quality CsPbX3 QDs is room-temperature (RT) recrystallization. Contrast to the hot injection method, the method of RT recrystallization does not require high temperature or inert atmosphere, so that it is capable of reducing the cost and improving the batch-to-batch reproducibility, providing a promising route towards low-cost mass-production of CsPbX3 QDs [31,32]. In 2016, Zeng’s group first reported RT synthesis of high performance CsPbX3 QDs based on supersaturated recrystallization [31]. Based on Zeng et al.’s work, Wei et al. [32] and Fang et al. [33] developed synthetic strategies for high output yield and improved stability by replacing polar solvents with weak polar or nonpolar solvents. Besides spherical QDs, the morphology of CsPbX3 colloidal nanocrystals have also been successfully engineered into nanocubes, #343125 Journal © 2018

https://doi.org/10.1364/OME.8.003494 Received 24 Aug 2018; revised 17 Oct 2018; accepted 17 Oct 2018; published 23 Oct 2018


nanorods and nanoplatelets under ambient condition by ligand-mediated reprecipitation [34] or precise control of crystallization and growth rate [35,36]. Although RT synthesized CsPbX3 QDs exhibited excellent photoluminescence (PL) properties and shape tunability, their stability under working conditions is still far behind the reported performance of those synthesized by hot injection, which severely limits their practical applications. In this paper we demonstrate a facile and low-cost route to significantly enhancing the stability of RT synthesized CsPbX3 QDs by introducing the combination of mesoporous silicate and polymer matrix for the encapsulation of QDs. We investigate the thermal, photoand water stability of our RT synthesized QDs and the underlying mechanism of stability enhancement. We also fabricate QD-converted white LEDs using green-emitting and redemitting CsPbX3 QDs and evaluate their display performance. 2. Experimental methods 2.1 Reagents All of the reagents were used directly without further purification: lead bromide (PbBr2, 99.999%, Aladdin), cesium bromide (CsBr, 99.99%, Aladdin), oleic acid (OA, Aladdin), oleylamine (OAm, Aladdin), N, N-dimethylformamide (DMF, AR, Sinopharm Chemical Reagent). Mesoporous silicates (SBA-15) with an average pore size of 15 nm were purchased from Nanjing XFNANO. Poly (methyl methacrylate) (PMMA) was purchased from Aladdin. 2.2 Synthesis of green-emitting CsPbBr3 QDs The solution preparation of CsPbBr3 QDs was based on the method of supersaturated recrystallization [31]. Briefly, the precursor solution was made by dissolving PbBr2 (0.2 mmol) and CsBr (0.2 mmol) in DMF (5 mL) and stabilized by adding OA (0.5 mL) and OAm (0.25 mL). 1 mL precursor solution was added into toluene (10 mL) under vigorous stirring at 800 rpm for 30 seconds and the color of the solution turned green. The solution was centrifuged at 10000 rpm and the precipitate was re-dispersed in a toluene/hexane solution. All above-mentioned experimental process was implemented under ambient conditions. 2.3 Synthesis of red-emitting CsPbBr1.2I1.8 QDs In this procedure, the precursor solution was made by dissolving PbBr2 (0.08 mmol), CsBr (0.08 mmol), PbI2 (0.12 mmol) and CsI (0.12 mmol) in DMF (5 mL) and stabilized by adding OA (0.5 mL) and OAm (0.25 mL). 1 mL precursor solution was added into toluene (10 mL) under vigorous stirring at 800 rpm for 2 minutes and the color of the solution turned red. The solution was centrifuged at 7000 rpm and the precipitate was re-dispersed in a toluene/hexane solution. All above-mentioned experimental process was implemented under ambient conditions. 2.4 Synthesis of SBA-15 encapsulated QDs 10 mg/mL QDs and 30 mg SBA-15 were mixed in a hexane solution and stirred at 600 rpm for 1 hour. The mixture was centrifuged at 4000 rpm and washed by hexane twice. The precipitate was dried under vacuum at around 1 kPa for 1 hour and the nanocomposite powder of SBA-15 encapsulated QDs was then obtained. 2.5 Preparation of PMMA-QD composite films 600 mg PMMA was dissolved in toluene with a concentration of 200 mg/ml and 2 mL QDs was added into the PMMA solution under vigorous stirring. The solution was stored under vacuum for 2 hours to remove the bubbles in it. To prepare a PMMA-QD film, the mixed solution (1.2 mL) was cast onto the surface of a 2 cm × 2 cm quartz substrate under ambient condition and left to dry overnight. The film thickness was 315 ± 15 μm.


2.6 Preparation of PMMA-SQD composite films The powder of SBA-15 encapsulated QDs was dispersed into the PMMA solution and the formed mixture has the same QD concentration as the PMMA-QD solution. To prepare a PMMA-SQD film, the procedures of bubble removal, casting and drying were also applied to the mixed solution and the obtained film thickness was 320 ± 15 μm. 2.7 QD characterization The morphology of neat and SBA-15 encapsulated QDs was characterized by high-resolution transmission electron microscopy (HR-TEM) (FEI Tecnai F30) operating at 200 kV. The absorption spectra were measured by an UV-visible spectroscopy (Purkinje, TU-1900). The PL spectra and PLQY were measured using a custom-made integrating sphere system equipped with a 360 nm CW laser and a fiber-coupled spectrometer (Ocean Optics, QE Pro), and the PLQY values were calculated using Suzuki’s method [37]. Time-resolved PL lifetime were measured using a fluorescence lifetime spectrometer (Edinburgh, FLS980) equipped with a picosecond pulsed laser (λ = 360 nm). An optical filter was mounted before the detector in order to remove the excitation signal. 2.8 Test of thermal stability The thermal stability of composite films was evaluated in two ways. First, the composite films were placed on a heating plate and kept for 1 minute. The corresponding PL spectra were collected under the excitation of a 365 nm LED. It is worth noting that the irradiance of the excitation source was relatively low, and the exposure time to the excitation source is within the timescale of seconds, therefore the photostability issues have negligible influence on the thermal testing results. By varying the heating temperature, we measured the relative PL intensity as a function of heating temperature ranging from 30 °C to 100 °C. In the other evaluation of thermal stability, the composite films were placed on the heating plate and continuously heated at 80 °C. The heating was ended at a pre-scheduled time and the films were cooled down to room temperature. The PLQY values and PL peak wavelengths were then measured as a function of time by varying the pre-scheduled time. During the test of thermal stability, the air humidity was kept at 40% ~60%. 2.9 Test of photostability To test the photostability, the composite films were continuously excited under UV irradiation at a wavelength of 365 nm and with a power density of 141 mW/cm2. The corresponding PLQY values and PL peak wavelengths were measured as a function of irradiation time. The air humidity was kept at 40% ~60% and the ambient temperature was kept at 20 ~25 °C. 2.10 Test of water stability To test the water stability, the composite films were kept in a dark water tank filled with deionized water. The films were taken out of the water tank at a pre-scheduled time and then fully dried for further characterization. The PLQY values and PL peak wavelengths were measured as a function of time for which the films were kept under water. The ambient temperature was kept at 20 ~25 °C.


3. Results and a discussio on

Fig. 1. a) A schemaatic diagram illusstrating the prepparation process of PMMA-SQD compo osites. TEM imag ges of b) neat and d c) SBA-15 encaapsulated CsPbBrr3 QDs. The inset figurees show the corressponding HRTEM M images. d) Photoographs of the poowder of SBA-15 encapsulated CsPbBr3 QDs Q under dayligh ht and UV illuminaation (365 nm). e) Photographs of a PMMA-SQD film under daylight and UV V excitation.

Figure 1(a) sh hows the prep paration processs of the PMM MA-SQD compposites. The Q QDs were first synthesizzed using the approach a of su upersaturated reecrystallizationn and were theen mixed with the meso oporous silicatte SBA-15 prio or to being disspersed into PM MMA. Figure 1(b) and Fig. 1(c) sho ows the TEM M and HR-TE EM images off neat CsPbB Br3 QDs and SBA-15 encapsulated CsPbBr3 QDss, respectively.. The average grain size off neat CsPbBr3 QDs is ndix), which is smaller than th the average porre size of around 12.4 ± 1.7 nm (Fig. 8 in the Appen SBA-15 (~15 nm). Althoug gh a small num mber of QDs caan only attach to the outer suurface of pore walls due to their largee grain size and d may consequuently start to fform aggregatiion, most w perfectly filled f into the pore channels of SBA-15 annd the crystal ggrains of of the QDs were the filled QD Ds were distorrted and elong gated along thhe channels. F Figure 1(d) shhows the photographs of o the powder of SBA-15 en ncapsulated C sPbBr3 QDs uunder daylight and UV illumination (365 ( nm). Thee PMMA-SQD D composite fillms were preppared by disperrsing the powder into PMMA P and caasting onto quartz substratess. The films caan be easily peeeled off from the quarrtz substrates and a form flexib ble free-standinng membranes.. Figure 1(e) shhows the photographs of a PMMA-S SQD film und der daylight aand UV excitaation. The PM MMA-QD ms were also prepared as refeerences. composite film


Fig. 2. 2 a) The normallized steady-state absorption and PL spectra of nneat and SBA-15 encapsulated QD solution. b) The time-reesolved PL spectra ra of PMMA-QD aand PMMA-SQD compo osite films. The tiime-resolved PL spectra s were recorrded at the wavellength of 520 nm with an a excitation waveelength at 360 nm.

Figure 2(aa) shows the absorption a and PL spectra off neat and SBA A-15 encapsullated QD solution. The peak emission n of neat QD solution was aaround 518 nm m with a full-w width-atm (FWHM) off 22 nm. The fluorescence fl sppectrum of SBA A-15 encapsullated QD half-maximum solution show wed a slight red d-shifted emisssion peak at 5221 nm with a F FWHM of 21 nm. The red shift of wavelength w is attributed a to th he elongated crrystal grains oof filled QDs aalong the pore channelss which reducce the quantum m confinemennt [23,38]. Thhe emission sppectra of PMMA-QD and PMMA-S SQD films sh how no channges comparedd with their solution QY values of tthese composiite films. Althoough the counterparts. We then measured the PLQ MA-SQD film ms was 77.0% ± 3.0%, crystal grainss were distorteed, the PLQY value of PMM which was sliightly higher th han that of PM MMA-QD filmss (71.4% ± 2.44%). We also m measured the time-resolved PL specttra of the PM MMA-QD and PMMA-SQD composite fillms [Fig. w a tri-expo onential functio on, the time-reesolved PL decay curves of PMMA2(b)]. Fitted with QD films and d PMMA-SQD D films have thrree componentts: a dominatinng fast componnent, and two slow com mponents. Thee lifetime of each componeent and its coorresponding ffractional intensity is su ummarized in Table T 2 in the Appendix. Thhe dominating fast componeent (τ1) is attributed to the t intrinsic ph hoton-radiativee recombinatioon, while the slower compoonent (τ2) and the slow west componen nt (τ3) is attriibuted to the shallow-levell surface trap assisted recombination n and deep-lev vel Shockley-R Read-Hall (SRH H) recombination, respectivelly. These two slow reco ombination pro ocesses are no onradiative, so they have neegative contribuutions to the PLQY vallues of composite films. Thee overall increaase in decay liffetime in PMM MA-SQD films can be attributed a to th he presence of oxide from SB BA-15 that cann possibly passsivate the CsPbBr3 QD surface via red ducing surface dangling bondds [39]. The paassivation increeases the o-radiative reco ombination byy suppressing tthe shallow- annd deepfractional inteensity of photo level nonradiaative recombin nation. The stabillity of the PM MMA-SQD com mposite films was evaluateed from three different aspects and th he PMMA-QD counterparts were w also testeed as references. The thermall stability of the composite films was first characterrized and Fig. 3(a) shows thhe relative PL intensity nction of heatin ng temperaturee ranging from 30 °C to 100 °°C. Comparedd with the value as a fun PMMA-QD film, f the PMMA A-SQD film has more resistaance to the therrmal effect. Att 100 °C, the relative in ntensity of PMM MA-SQD film decreased to 339.4%, while thhat of PMMA--QD film was only 13.0 0%. The PMM MA-SQD film also a showed a better recoverry with the rellative PL intensity risin ng back to 70.7 7% when the temperature w was cooled dow wn from 100 °°C to the room temperrature. Figure 3(b) and Fig g. 3(c) showss the PLQY values and P PL peak wavelengths as a function of time wheen the films w were continuouusly heated att 80 °C, f 16 hours, the t PMMA-SQ QD films still had a PLQY value of respectively. After heated for


48.0% withou ut any peak sh hift, while the PMMA-QD P fillms had a PLQ QY of only 144.4% and the peak was red-shifted r by more than 2 nm m.

Fig. 3. a) The relative PL P intensity as a fu unction of heatingg temperature rangged from 30 °C to C. b) The PLQY values v and c) PL peeak wavelengths aas a function of tim me when the films 100 °C contin nuously heated at 80 8 °C.

The photo ostability test of compositee films was iimplemented uunder continuuous UV irradiation at a wavelength h of 365 nm and a with a poower density oof 141 mW/cm m2 under ditions. As show wn in Fig. 4(a)), the PLQY oof PMMA-SQD D films decreased from ambient cond 77.0% to 48.5 5% after being g continuously irradiated forr 54 hours, whhile that of PM MMA-QD films was only 18.0% under the same teesting conditioons. Figure 4(bb) shows the PL peak o composite films f as a funcction of time uunder continuouus UV irradiattion. The wavelengths of PL peak of PMMA-SQD films stably stayed at 5222 nm and wass not affectedd by UV owever, that off PMMA-QD films were redd-shifted from around 518 nm m to 524 irradiation, ho nm.

Fig. 4. 4 a) The PLQY values and b) PL P peak wavelenggths as a functioon of time under 2 contin nuous UV irradiation (365 nm, 141 mW/cm m ).

We also characterized c the t moisture resistance r of tthe composite films by keeeping the samples undeer deionized waater for a preset period of tiime. Figure 5(aa) and Fig. 5(bb) shows the PLQY vaalues and PL peak p wavelengtths as a functiion of time forr which the fillms were kept under water, w respectiv vely. After on ne month, thee PLQY of PM MMA-SQD fiilms was 70.1%, which h means 91.1% % of the initial PLQY value w was remained. The PMMA-Q QD films showed slighttly weaker ressistance to mo oisture, the PL LQY of whichh was 57.6% aafter one month. A slig ght fluctuation (1 ~2 nm) in the PL peak oof PMMA-SQ D films was oobserved, while the peaak of PMMA-Q QD films was red-shifted byy around 8 nm m as the testinng period increased to one month. Apart A from thee peak waveleength shift, wee also investiggated the WHM for all thee above-mentio oned stability ttests, since the FWHM influeences the change in FW color coordinaates. Contrast to the distinctiive shift of peaak wavelength, we found thee FWHM value was alm most unchanged d and was alwaays around 21 nnm.


Fig. 5. a) The PLQY vaalues and b) PL peak wavelengths aas a function of tiime for which the films were w kept under water. w

SBA-15 an nd PMMA plaay different rolees in our strateegy of stability enhancement. SBA-15 can effectively y suppress the unfavored graain growth and aggregation of QDs inducedd by heat, which providees the composiite films with better b thermal stability. Sincee the surface liigands of perovskite QD Ds can be easilly removed by y absorption off photons [32,440], SBA-15 ccan at the same time su uppress photo--induced degraadation by preeventing the ddetachment off surface ligands under light illuminattion. Meanwhiile, oxygen wass reported to bbe detrimental tto the PL properties and d material stab bility in prior work, w particular arly with expossure over hourss to days in the presencce of light [41,4 42]. So in our case, SBA-15 also acts as a ppassivation sheell which prevents photto-oxidation du uring UV lightt irradiation [4 3,44]. We alsoo note that a feew hours of oxygen exp posure in the presence p of ligh ht may benefitt to the PLQY of PMMA-SQ QD films. This is becau use the oxygen n which binds strongly to hhalogen vacanccies in the preesence of photoexcited carriers can reemove the sub bgap trap statees associated w with halogen vvacancies i off SBA-15 exten nds the time sccale of this proocess from minnutes to a [45], and the introduction few hours. p contrib bution to the eenhancement oof thermal stabbility and Although SBA-15 has positive photostability y, it shows no n improvemeent in moistuure resistance.. We found that the fluorescence of SBA-15 en ncapsulated QDs Q was comppletely quenchhed within onee minute i moissture resistancee of compositee films is when immersed in deionizeed water. The improved uted to PMMA A. Interestingly y, we note that both films shoowed a boost iin PLQY mainly attribu within the firsst few days. Th his phenomeno on was also repported in prior w work, but mosttly in the work of organ nic-inorganic hybrid h perovsk kites [32,46]. T The water moleecules rapidly converts the surface reegion into a hy ydrate phase [45,47], [ leadinng to the formaation of an am morphous shell which reemoves the surrface trap statess [48]. Howeveer, with furtherr exposure to m moisture, the grain gro owth and enh hancement of ion diffusionn length becom mes dominant, which subsequently causes the film degradattion [45,46,499]. Although PMMA signnificantly improves the moisture resisttance of composite films, its contribution tto heat resistannce is not n temperature. as significant due to its low glass transition


Fig. 6. a) The absorbancce spectra of a) PM MMA-QD and b) P PMMA-SQD film ms before and after the stability tests. Therrmal stability test: heated at 80 °C C for 16 hours; Phhotostability test: irradiaated at 365 nm, 14 41 mW/cm2 for 54 hours; Water sttability test: immeersed in deionized water for 1 month.

Figure 6 shows the ab bsorbance specctra of PMMA A-QD and PM MMA-SQD fillms after different extreeme conditionss of stability teest. After goingg through the eextreme condittions, the PMMA-QD films f formed an excitonic absorption peeak near the band edge, w which is associated wiith the observ ved red-shift of PL peak, indicating an increased grain size. According to Huang et al.’ss work [44], it is i highly likelyy that the addittional crystal ggrowth of ng the facets off (100) and (2000). On the coontrary, the absorbance our CsPbBr3 QDs was alon f were alm most unchangged after heatt, photoexcitattion and spectra of PMMA-SQD films moisture treaatment, which is in consisttency with thhe stable posittion of PL peak. We compared the stability perfo ormance of ourr work with reeported literatuures. To the beest of our knowledge, th here is so far only o one reportt discussing thee stability of R RT synthesizedd CsPbX3 QDs, and wee summarized the comparisson results in Table 1. Ouur strategy of stability enhancement shows significcant improvem ment in all asppects comparedd with the priior work, i also comparrable to the repported perform mance of CsPbbX3 QDs and the achieeved stability is synthesized by y hot injection. In order to o explore the display d applicaation of our perrovskite compoosites, we preppared the red-emitting composite c film ms of CsPbBr1.22I1.8 QDs whichh have a peak wavelength att 656 nm with a FWHM M of 25 nm. The T white LED D was obtainedd by mountingg a green-emittting film and a red on ne onto a bluee-emitting InG GaN LED chipp. Under the ccurrent of 10 mA, the corresponding g emission specctra are shown n in Fig. 7(a). N No spectrum shhift was observved when the current waas varied. Figu ure 7(b) shows the achievablee color gamut of the white L LED. The color coordinates of blue, green g and red components arre (0.136, 0.0005), (0.142, 0.7775) and (0.722, 0.278)), respectively. A theoreticall 135% NTSC color gamut aand a 101% R Rec. 2020 color gamut can c be achieved d with proper bandpass b filterss. This color ggamut is higherr than the values achiev ved by regular phosphor LEDs (86% NTS SC) [50], cadm mium-based Q QD LEDs (104% NTSC C) [50] and otheer perovskite-b based LEDs (1002 ~127% NT TSC) [2,19,25,228,51,52] due to optimizzed peak waveelength and narrrow emission bbandwidth.


Table T 1. Summary y of stability com mparison of RT syynthesized CsPbX X3 QDs using diffeerent strategies. Thermal stability y test

Phottostability test

Water staability test

Composite

CsPbBr3 nanocrystal thin film

PMMASQD thin film

References

Condition

Reesults

Conditiions

Results

Condition

Results

T = 100 °C

15% % PL rem mained

-

-

in air for 35 days (relative humidity 40 ~60%)

~15% PL remained

32

T = 100 °C

40% % PL rem mained

365 nm, 141 mW/cm2 for 54 hou urs

64.2% relative PLQY remained

in DI water for 1 month

91.1% relative PLQY remained

This work

80 °C for 16 hours

62 2.3% rellative PL LQY rem mained

Fig. 7. a) The emission spectra and b) collor gamut of a whhite LED consistinng of CsPbBr3 and CsPbB Br1.2I1.8 QDs under the excitation off a blue LED chipp (460 nm). The ssolid line, dashed line and a dashdotted lin ne represents the coverage c of the w white LED, the R Rec. 2020 and the NTSC C, respectively.

4. Conclusio ons We demonstrrated a facile route to significantly improoving the therm mal, photo- annd water stability of RT R synthesizeed CsPbX3 QD Ds. PMMA m mainly contribbuted to the moisture resistance and d SBA-15 maiinly contributeed to the therm mal stability aand photostability. The combination of o PMMA and d SBA-15 effecctively suppresssed the unfavvored grain groowth and surface trap sttates, leading to an outstandin ng performancce of QD stabillity which is suuperior to that reported in i prior work. We W then fabriccated QD-convverted white LE EDs using CsP PbBr3 and CsPbBr1.2I1.8 QDs and such h LEDs are caapable of achiieving a wide color gamut of 135% NTSC and 10 01% Rec. 2020.


Appendices s

Fig. 8. Paarticle size distribu ution of RT syntheesized CsPbBr3 QD Ds. Tab ble 2. Fitting data a of the time-resollved PL decay currves. The PL decaay curves were fitteed by a tri-exponeential function to investigate excitoon dynamics of p erovskite QDs. The intensity-weighted d average exciton n lifetime (τavr) waas f1τ1+ f2τ2 + f3τ3, where f1, f2 and f3 is the fracttional intensity off the lifetime of τ1 , τ2 and τ3,respecctively. Composite

τ1 /ns (f ( 1)

τ2 /ns / (f2)

τ3 /ns (f3)

R2

τavr /ns

PMMA-QD

2.96 (0.540)

13.9 9 (0.370)

777.1 (0.0791)

0.99955

12.8

PMMA-SQD D

10.5 (0.796)

60.2 2 (0.236)

3378 (0.0355)

0.99574

36.0

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