Supporting Information Enhancing the Stability of CH 3 NH 3 PbBr 3 Quantum Dots by Embedding in Silica Spheres Derived from Tetramethyl Orthosilicate in Waterless Toluene Shouqiang Huang, Zhichun Li, Long Kong, Nanwen Zhu, Aidang Shan, and Liang Li*, School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, China. E-mail: liangli117@sjtu.edu.cn. Tel.: +86 21 54747567; Fax: +86 21 54747567. S-1
Experimental methods Chemicals and materials PbBr 2 (99%, Aladdin), methylamine (CH 3 NH 2, 33% in methyl alcohol, Aladdin), hydrobromic acid (HBr, 48%, Aladdin), hydriodic acid (HI, 47%, Aladdin), n-octylamine ( 99.5%, Aladdin), N-dimethylformamide (DMF, analytical grade, Sinopharm Chemical Reagent Co. Ltd, China), oleic acid (OA, 90%, Aldrich), Tetrahydrofuran (THF, 99.5%, Aladdin), toluene (analytical grade, H 2 O content 0.018%, Sinopharm Chemical Reagent Co. Ltd, China), tetramethyl orthosilicate (TMOS, 98%, Aldrich), tetraethyl orthosilicate (TEOS, 99.99%, Aldrich), and poly(methyl methacrylate) (PMMA, Aladdin) were used as received without further purification. Preparation of CH 3 NH 3 Br CH 3 NH 3 Br was synthesized by the reaction of methylamine and HBr with the molar ratio of 1: 1. 30 ml of methylamine (33% in methyl alcohol) was cooled to 0 C with continuous stirring, and the HBr was added slowly. After stirring for 2 h, the precipitate was obtained by rotary evaporation at 50 C, followed by washing and centrifuging with diethyl ether three times, and then dried under vacuum. CH 3 NH 3 I was synthesized according to the same procedures except to the addition of HI. Preparation of the colloidal MAPB-QD solution In a typical procedure of MAPB-QDs, 0.64 mmol CH 3 NH 3 Br (MABr), 0.64 mmol PbBr 2, 100 μl n-octylamine, and 2 ml oleic acid were dissolved into 20 ml of DMF to form a transparent solution. Then, 10 ml of the above solution was slowly (0.1 ml/min) added into 100 ml toluene under nitrogen protection. After stirring 24 h, the green colloidal S-2
MAPB-QD solution (the supernatant) was collected through centrifugation at 10000 rpm for 10 min, and the yellow precipitates at the bottom of the centrifuge tube were discarded. The chemical yield of MAPB-QDs is calculated to be 43.7% by ICP-OES analysis. The pure MAPB-QD powders were obtained by freeze drying (FreeZone 4.5, Labconco, Kansas City, USA). For the preparation of the colloidal MAPI-QD solution, 0.16 mmol CH 3 NH 3 I, 0.16 mmol PbI 2, 40 μl n-octylamine, and 2 ml oleic acid were initially dissolved into 10 ml THF, and then 2 ml of the obtained solution was slowly added into 50 ml toluene under nitrogen protection. Preparation of the MAPB-QDs/SiO 2 powders 100 μl TMOS was introduced into a 50 ml three-necked flask containing 20 ml of the colloidal MAPB-QD toluene solution (0.64 mg/ml, H 2 O content 0.0623%) with sealing plugs. The sealed three-necked flask was placed in the temperature and humidity chamber with the temperature of 25 C and relative humidity (RH) of 60%. After stirring 36 h, the precipitates were collected through centrifugation at 10000 rpm for 10 min, and the MAPB-QDs/SiO 2 powders were obtained by freeze drying. The MAPI-QDs/SiO 2 powders were also prepared following the same procedures by adding 100μL TMOS into 20 ml MAPI-QD toluene solution with the stirring time of 30 h. Preparation of the MAPB-QDs/SiO 2 /PMMA film 0.8 g PMMA solid particles were added into 2 ml of toluene. After 3 days, all the PMMA solid particles were dissolved to form a transparent toluene solution, followed by the addition of 15 mg MAPB-QDs/SiO 2 powders. With the mechanical vibration process, all the MAPB-QDs/SiO 2 were dispersed homogeneously in the PMMA-toluene solution. Then, the S-3
resultant mixture was dripped on a clear glass through a vacuuming procedure to form the MAPB-QDs/SiO 2 /PMMA film with 18 mm in diameter and 1 mm in thickness. For comparison purpose, the pure MAPB-QDs/PMMA film was also prepared following the same method mentioned above. Characterization Powder X-ray diffraction (XRD) patterns were performed on a Bruker D8 Advance X-ray Diffractometer at 40 kv and 40 ma using Cu K α radiation (λ=1.5406 Å). The morphologies were investigated by the JEOL JSM-7800F field emission scanning electronmicroscope (FESEM), and the JEM-2100F and JEM-ARM200F transmission electron microscope (TEM) instruments. The surface composition was determined by the X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD ), and all the binding energies were calibrated with the C 1s peak at 284.8 ev. Fourier transform infrared (FTIR) spectra were measured by using a Nicolet 6700 spectrometer (USA). Photoluminescence (PL) and UV-vis absorption spectra were recorded on a Hitachi F-7000 fluorescence spectrophotometer and a Hitachi U-3900 spectrophotometer, respectively. The total luminescence spectra were also characterized in the form of excitation-emission matrix (EEM) (F-7000, Hitachi) with the scanning emission spectra varied from 300 to 550 nm by increasing the excitation wavelength from 300 to 550 nm at 5 nm increments. The PL decay curves were collected with a time-resolved fluorescence spectrofluorometer (QM/TM/IM, PTI, USA). The absolute PL quantum yields of the colloidal QD solutions were detected using a fluorescence spectrometer with an integrated sphere excited at the 450 nm LED light source. Photostability assays S-4
The photostability measurements of the colloidal QD solutions were analysized using two bottom-transparent, airtight, quartz colorimetric cuvettes containing 2 ml of the colloidal MAPB-QD solutions (0.64 mg/ml). 10 μl TMOS was added into one of the MAPB-QD solution. Prior to illumination, two of the colloidal MAPB-QD solutions were kept in a glovebox (dark environment) for 4 h, and the hydrolysis reaction of TMOS could be happened. Every periodic interval, the PL and UV-vis absorption spectra of the colloidal QD solutions after illuminating with 450 nm LED light (175 mw/cm 2, RXN-605D DC, China) were recorded. The photostability measurements of the MAPB-QD and MAPB-QD/SiO 2 powders and their PMMA films were analysis in a temperature and humidity chamber (25 C, RH 60%) using a 470 nm LED light (21 mw/cm 2 ) provided by Ocean Optics LS-450. The PL spectra were in-situ real-time recorded on a computer. S-5
(a) (b) Figure S1. (a) SEM and (b) TEM images of the bulk MAPB precipitates obtained by centrifugation. S-6
(a) (b) Percentage (%) 40 35 30 25 20 15 10 5 0 1.5 2.0 2.5 3.0 3.5 Diameter (nm) Figure S2. (a) TEM image of pure MAPB-QDs and (b) the corresponding size distribution analysis. S-7
PL counts 1000 MAPB-QD solution MAPB-QDs/SiO 2 solution, 4h later MAPB-QD solution, 4 h later 100 0 20 40 60 80 Time (ns) Figure S3. Time-resolved PL decays and the fitted curves for the colloidal MAPB-QD and MAPB-QDs/SiO 2 solutions detected at 494 nm with excitation of 420 nm. Figure S3 shows the fluorescence decay curve (black) of the colloidal MAPB-QD solution at 494 nm after excitation of 420 nm. The curve can be well fitted by a biexponential function, and the obtained short-lived PL lifetime (τ 1 ) is 6.6 ns with a percentage of 75.0% (Table S1), which corresponds to the radiative recombination of the electron-hole pairs. 1 The long-lived PL lifetime (τ 2 = 24.2 ns) with a percentage of 25.0% is assigned to the surface state-related nonradiative recombination. 2 S-8
Table. S1 PL quantum yields (PLQY) and the fitting parameters of the decay curves for (I) MAPB-QDs and (II) MAPB-QDs/SiO 2. Samples PLQY (%) A 1 (%) τ 1 (ns) A 2 τ 2 (ns) τ avg (ns) MAPB-QD solution 87 75.0 6.6 25.0 24.2 16.3 MAPB-QDs/SiO 2 solution, 4 h later 89 78.2 8.7 21.8 27.5 17.5 MAPB-QD solution, 4 h later 86 78.2 6.3 21.8 22.7 14.5 Note: A and τ are the normalized amplitude and decay time constant, respectively. τ avg (τ average ) is calculated as τ avg = 2 1 1 A1 τ1 A τ + + A τ A τ 2 2 2 2 2 S-9
PL peak positions 540 MAPB-QD solution MAPB-QDs/SiO 2 solution 520 500 480 460 Light off 11 h Light on Light off Light on 21 h 43 h 33 h 0 5 10 15 20 25 30 35 40 45 50 Illumination time (h) Figure S4. PL emission peak positions of the colloidal QD solutions as a function of the illumination time. Figure S4 shows the varied PL emission peak positions, and the blueshifts were observed in both solutions during the light-on time, which possibly could be attributed to the photo-etching of the MAPB-QDs. 3 When the light was off, the PL peaks redshifted several nanometers. After two illumination cycles, the PL peak of the MAPB-QDs/SiO 2 solution appears to have stabilized around the initial position of 494 nm. However, the PL peak of the MAPB-QDs solution continuously redshifted to 502 nm in the third cycles of illumination, and some yellow precipitates were formed in the cuvette (Figure 2a), which indicated the growth of MAPB-QDs after the whole test process. It should be noted that these PL peak position shifts are complicated, which need to be further studied. S-10
1.2 Remnant PL 1.0 0.8 0.6 0.4 0.2 0.0 MAPB-QD solution (dark, open, RH=60%) MAPB-QDs/SiO 2 solution (dark, open, RH=60%) MAPB-QD solution (dark, open, RH=80%) MAPB-QDs/SiO 2 solution (dark, open, RH=80%) MAPB-QD solution (dark, sealed) MAPB-QDs/SiO 2 solution (dark, sealed) 0 1 2 3 4 5 6 7 8 9 10 11 Storage time (h) Figure S5. Remnant PL emissions of the MAPB-QD and (b) MAPB-QDs/SiO 2 solutions stored in the dark with different moistures (RH 60% and 80%). As shown in Figure S5, the remnant PL emissions of the MAPB-QD and MAPB-QDs/SiO 2 solutions in the sealed system (dark) after 11 h were 88.09% and 100%, respectively. The declined PL of the pure MAPB-QD solution in the dark was originated from the degradation caused by the residual water in toluene and the unstable structure of MAPB. In comparison to the corresponding remnant PL emissions (81.70% and 89.95%) driven by continuous illumination of 11 h (Figure 2b), the introduction of illumination promoted the PL reduction. However, this reduction is not so obvious at the initial illumination time of 11 h, since the photoactivation phenomenon can be happened easily, which is beneficial to the improvement of PL. To accelerate the degradation of the MAPB-QD and MAPB-QDs/SiO 2 solutions by moisture, the open systems exposed to RH S-11
60% and 80% were also designed. When exposed to RH 60%, the remnant PL emissions of the MAPB-QD and MAPB-QDs/SiO 2 solutions were sharply decreased to 39.33% and 56.41%, respectively. When the moisture was increased to RH 80%, both of the solutions were completely decomposed with the storage time of 11 h. Thus, the moisture has great influence on the stability of MAPB-QDs. S-12
(a) 3.2 Absorbance (a.u.) 2.4 1.6 0.8 0 h 11 h 21 h 33 h 43 h 49 h 0.0 320 360 400 440 480 520 560 600 (b) 3.2 Absorbance (a.u.) 2.4 1.6 0.8 Wavelength (nm) 0.0 320 360 400 440 480 520 560 600 Wavelength (nm) Figure S6. UV-vis absorption spectra of the colloidal (a) MAPB-QD and (b) MAPB-QDs/SiO 2 solutions as a function of the illumination time. 0 h 11 h 21 h 33 h 43 h 49 h The UV-vis absorption spectra of the solutions are shown in Figure S6. The absorption redshifts and the fast decreased intensities are clearly present for the MAPB-QD solution as the illumination extended to 49 h (Figure S6a), whereas those of the MAPB-QDs/SiO 2 solution change slowly (Figure S6b). S-13
100 μl TMOS (a) (b) Centrifugation (c) 20 ml colloidal MAPB-QD solution Stirring for 36 h Figure S7. (a) Schematic illustration of a three-necked flask containing 20 ml of the colloidal MAPB-QD solution (H 2 O content 0.0623%) and 100 μl of TMOS. (b) The obtained green gel precipitates after stirring of 36 h and (c) the corresponding centrifuged samples. S-14
Figure S8. TEM image of MAPB-QDs/SiO 2 with the stirring time of 12 h. S-15
(a) (b) (c) (d) 003 Figure S9. (a) SEM, (b) TEM, and (c, d) HRTEM images of MAPB-QDs/SiO 2 after reaction of 36 h (Insets: the corresponding fast Fourier transform patterns). S-16
(a) (b) Figure S10. TEM images of MAPB-QDs/SiO 2 with different RH values: (a) 60% and (b) 80% in the opened three-necked flask (25 C) after stirring of 1 h. S-17
PL (a.u.) 0 h stirring 1 h, RH 60% stirring 2 h, RH 60% stirring 1 h, RH 80% stirring 2 h, RH 80% 320 400 480 560 640 720 800 Wavelength (nm) Figure S11. PL emission spectra of the MAPB-QDs/SiO 2 solutions with different RH values in the opened three-necked flask (25 C) after stirring different times (excited at 450 nm). Note: We suspect that the initial increased PL intensities (1 h) are caused by light scattering of silica composites. S-18
(a) MAPB-QDs MAPB-QDs/SiO 2 C 1s (b) 103.5 ev Si 2p Intensity (a. u.) O KLL Pb 4s Pb 4p 3/2 Pb 3p 1/2 O 1s Pb 4d 3/2 Pb 4d N 1s 5/2 Br 3s Br 3p Si 2s 3/2 Pb 4f Si 2p Br 3d Pb 5p 3/2 Pb 5d Intensity (a. u.) (c) 900 750 600 450 300 150 0 Binding Energy (ev) 110 108 106 104 102 100 98 96 Binding Energy (ev) 532.9 ev O 1s (d) 1650 Intensity (a. u.) Transmission MAPB-QDs MAPB-QDs/SiO 2 795 952 540 538 536 534 532 530 528 526 Binding Energy (ev) 1073 3500 3000 2500 2000 1500 1000 Wavelength (nm) Figure S12. (a) Survey-scan XPS spectra of MAPB-QDs and MAPB-QDs/SiO 2. XPS spectra of MAPB-QDs/SiO 2 : (b) Si 2p and (c) O 1s. (d) FTIR spectra of MAPB-QDs and MAPB-QDs/SiO 2. The XPS spectra of MAPB-QDs and MAPB-QDs/SiO 2 are shown in Figure S12. The survey scan XPS spectrum of MAPB-QDs (Figure S12a) confirms the elements of Pb, Br, C, O, and N with the atomic concentrations of 2.39%, 5.20%, 81.61%, 8.08%, and 2.72%, respectively. The detected O element may be contributed by the residual oleic acid. The atomic concentrations of Pb, Br, C, Si, O, and N for MAPB-QDs/SiO 2 are 1.52%, 2.14%, 50.60%, 12.78%, 31.66%, and 1.30%, respectively, indicating the dominant components of SiO 2 is present in the outermost surface layers. The deviation of the ratio of Pb: Br compared to the nominal atomic ratio may be caused by the unstable structure of CH 3 NH 3 PbBr 3, and some of them are decomposed in the XPS detection. S-19
Excitation Wavelength (nm) 500 400 300 300 400 500 Emission Wavelength (nm) 0.000 500.0 1000 Figure S13. EEM spectrum of the MAPB-QDs/SiO 2 powders. S-20
Figure S14. Schematic illustration of the photostability test instrument over the MAPB-QDs/SiO 2 powders and the MAPB-QDs/SiO 2 /PMMA film illuminated with 470 nm LED light (21 mw/cm 2 ) provided by Ocean Optics LS-450. S-21
Figure S15. Schematic illustrations of the SiO 2 formation process in the analytical grade-toluene and the protection of MAPB-QDs by SiO 2 spheres from degradation due to moisture. S-22
Figure S16. The green MAPB-QDs/SiO 2 LED operating at 100 ma. S-23
(a) Intensity (a.u.) (c) 500 600 700 800 900 Emission Wavelength (nm) (b) 1.6 1.4 Remnant PL 1.2 1.0 0.8 0.6 0.4 0.2 (d) MAPI-QDs MAPI-QDs/SiO 2 0 20 40 60 80 100 120 Illumination time (min) Figure S17. (a) PL emission spectrum of the MAPI-QD solution (inset: optical image of the colloidal MAPI-QD solution). (b) Photostabilities of pure MAPI-QD and MAPI-QDs/SiO 2 powders under illumination of 470 nm LED light. TEM images of (c) pure MAPI-QDs and (d) MAPI-QDs/SiO 2. Figure S17a shows the PL emission spectrum of the red colloidal MAPI-QD solution (inset of Figure S17a) with an emission peak at 628 nm. The TEM image of pure MAPI-QDs is shown in Figure S17c, and the average diameter of MAPI-QDs is calculated to be 4.2 nm through statistical analysis. After the generation of SiO 2 layers from TMOS, most of MAPI-QDs are embedded into SiO 2 aggregates (Figure S17d), which greatly improve the photostability of MAPI-QDs with 470 nm LED illumination (Figure S17b). S-24
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