Zero-Dimensional Cs 4 PbBr 6 Perovskite Nanocrystals

Transcription

1 Supporting Information Zero-Dimensional Cs 4 PbBr 6 Perovskite Nanocrystals Yuhai Zhang 1, Makhsud I. Saidaminov 1, Ibrahim Dursun 1, Haoze Yang 1, Banavoth Murali 1, Erkki Alarousu 1, Emre Yengel 1, Buthainah A. Alshankiti 1, Osman M. Bakr 1 *, Omar F. Mohammed 1 * 1 King Abdullah University of Science and Technology, KAUST Solar Center, Division of Physical Sciences and Engineering, Thuwal , Kingdom of Saudi Arabia Corresponding Authors omar.abdelsaboor@kaust.edu.sa osman.bakr@kaust.edu.sa S1

2 Materials. All reagents were used without any purification: Cs 2 CO 3 (cesium carbonate, 99%, Sigma- Aldrich), OA (oleic acid, 90%, Sigma-Aldrich), OLA (oleylamine, 90%, Sigma-Aldrich), DMF (N,N-dimethylformamide, 99.8%, Sigma-Aldrich), 1,2-dichlorobenzene (anhydrous, 99%, Sigma-Aldrich) and anhydrous n-hexane (99.98%, Sigma-Aldrich). Synthesis of Cs 4 PbBr 6 Nanocrystals. Cs 4 PbBr 6 nanocrystals were synthesized using a modified reverse microemulsion method. 1 In a typical procedure, the PbBr 2 precursor and the Cs-oleate precursor were synthesized separately. First, a mixture of 2.25 g of Cs 2 CO 3 and 21.5 ml of OA was stirred and degassed at 130 C under vacuum for 1 hour to generate a yellowish stock of Cs-oleate precursor. Second, 0.2 ml Cs-oleate precursor, 10 ml n-hexane, 5 ml OA were loaded in a 50-mL three-neck flask, followed by mild degassing and nitrogen purging. Third, into the flask, a mixture of PbBr 2 (0.03 M, DMF, 1 ml), HBr (48 wt%, 15 L), 0.1 ml OA, and 0.05 ml OLA was swiftly injected under vigorous stirring. A color change from pale-white to green was observed in 10 min, suggesting the formation of Cs 4 PbBr 6 nanocrystals. The as-synthesized nanocrystals were collected via centrifugation at 8000 rpm for 3 min (one-centrifugation-only purification process), followed by dispersion in 2 ml of toluene for further characterization. Synthesis of CsPbBr 3 Nanocrystals. CsPbBr 3 nanocrystals were synthesized using a modified precipitation method. 2 In a typical experiment, a mixture of CsPbBr 3 (0.04 M in DMF, 100 L), OA (10 L), and OLA (5 L) was injected into 2 ml toluene under vigorous stirring. The reaction was allowed to proceed for 10 min before purification. The as-synthesized nanocrystals were used for optical characterizations and anion exchange experiment without any purification procedure. Anion Exchange Experiment. Anion exchange was conducted in toluene at room temperature. The iodine precursor of OAmI was prepared through a literature method. 3 Dried OAmI was dissolved in toluene to form a stock solution of 0.11 M. In a typical experiment, 208 L of CsPbBr 3 nanocrystal and 3 ml of toluene was loaded in a 4-mL quartz cuvette with a stirring bar. To initiate the anion exchange, 10 L of S2

3 OAmI was added. The anion exchange reaction completed in 20 s, followed by in situ PL measurement. Steady-State Measurements of Photoluminescence and Absorption. The as-prepared Cs 4 PbBr 6 nanocrystals were diluted for 100 times in n-hexane for steady-state measurements of photoluminescence and absorption, respectively. A Cary 5000 UV-vis spectrometer (Agilent Technologies) was used for absorption measurements in the range from 450 nm to 700 nm. A FluoroMax-4 spectrofluorometer (Horiba Scientific; a slit width of 2 nm and a scan rate of 500 nm/min) was used to record the photoluminescence spectra. The excitation wavelength used for the Cs 4 PbBr 6 nanocrystals were set at 400 nm. Transmission Electron Microscopy. TEM images were acquired using a Tecnai transmission electron microscope with an acceleration voltage of 120 kev. HRTEM images were acquired using the same instrument. It is worth noting that the Cs 4 PbBr 6 nanocrystals were not stable under irradiation with electron beams and tended to decompose within a short period of time (< 5 s). X-ray Diffraction Measurements. Powder X-ray diffraction was performed using a Bruker AXS D8 diffractometer with Cu-Ka radiation ( = Å). The samples were prepared via the drop casting of the nanocrystal suspension onto a clean glass slide, followed by drying at room temperature. Experimental Section for Bandgap Measurement of Thin-film. The Cary 6000i UV-Vis-NIR spectrophotometer was used to record the absorption of the Cs 4 PbBr 6 NCs in an external diffused reflectance accessory (DRA) integrating sphere equipped with the system. Firstly the 100% correction of base line was performed that is known to mirror correction for a specular reflectance measurement. The absorbance spectra were recorded from the two ordinate modes namely in 1) Absorbance and 2) Absorbance using reflectance Log F(R) mode. For the absorbance measurements, the thin film of Cs 4 PbBr 6 NCs onto the Si reference substrate was placed perpendicular to the light beam in the center of the closed integrating sphere. S3

4 However, for the reflectance measurements, the Cs 4 PbBr 6 NCs /Si film was placed outside the DRA accessory using a small area substrate holder accessory. The absorbance/reflectance was baseline calibrated using the Si reference in both the cases. The plot of ( h ) 2 vs. h (Tauc plot) with extrapolation of the intercept onto the energy axis calculated the band gap and were obtained to be identical. PESA Measurement: Photoelectron spectroscopy in air (PESA) measurement was carried out on the thin films of Cs 4 PbBr 6 NCs drop casted on to the substrate using Riken Photoelectron Spectrometer (Model AC-2). The drop casted films were then transferred to an ultra-high vacuum chamber to remove any solvent in the Cs 4 PbBr 6 NCs sample. The UV lamp intensity was fixed at 50 nw, which was pre-calibrated for the light correction coefficient. The HOMO level obtained from the onset energy value is e.v. The estimated Tauc plot band gap calculated is 2.37 ev. Hence, to deduce the LUMO level, it is the summation of HOMO + Eg= LUMO Time-Resolved Photoluminescence (TRPL) Measurement. Time-resolved photoluminescence (TRPL) spectra were collected using a high-resolution streak camera (Hamamatsu C10910) where the pump beam is generated with the second harmonic (410 nm) of a Spectra-Physics MaiTai ehp and Inspire HF-100 OPO. APE Pulse Select pulse picker is used to select the repetition rate of the pulse beam to 4 MHz. Measurements were performed at room temperature with the excitation fluence of 0.1 µj/cm 2. Temperature-dependent Photoluminescence Spectrum S4

5 The temperature-dependent photoluminescence spectra were characterized using a Horiba JY LabRAM Aramis spectrometer with an Olympus 50x lens in a Linkam THMS600 stage. A 473- nm laser was used as the excitation source. Thermogravimetric Analysis (TGA) and Difference Thermogravimetric (DTG) Measurements Thermogravimetric Analysis (TGA) and Difference Thermogravimetric (DTG) Measurements were performed with simultaneous thermal analyzer NETZSCH STA 449 at a temperature rate of 10 K/min. References (1) Chen, D.; Wan, Z.; Chen, X.; Yuan, Y.; Zhong, J. Large-Scale Room-Temperature Synthesis and Optical Properties of Perovskite-Related Cs 4 PbBr 6 Fluorophores. J. Mater. Chem. C 2016, 4, (2) Li, X. M.; Wu, Y.; Zhang, S. L.; Cai, B.; Gu, Y.; Song, J. Z.; Zeng, H. B. CsPbX 3 Quantum Dots for Lighting and Displays: Room-Temperature Synthesis, Photoluminescence Superiorities, Underlying Origins and White Light-Emitting Diodes. Adv. Funct. Mater. 2016, 26, (3) Nedelcu, G.; Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Grotevent, M. J..; Kovalenko, M. V. Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX 3, X = Cl, Br, I). Nano Lett. 2015, 15, S5

6 Figure S1. Schematic showing the crystal structure of Cs 4 PbBr 6 NCs. Every single PbBr 6 4- octahedra was isolated by 4 Cs + ions from neighboring PbBr 6 4- octahedrons, making a perfect 0- dimensional perovskite crystal lattice. S6

7 Figure S2. Absorption spectra of Cs 4 PbBr 6 NCs (a) and CsPbBr 3 NCs (b) as a function of concentration in terms of monomers. (c) Absorption spectra of Cs 4 PbBr 6 NCs and CsPbBr 3 NCs show a common absorption peak at 315 nm, indicating that this transition may stem from Pb 2+ ions. (d) Molar absorption coefficients calculated based on the absorption data in (a) and (b). S7

8 Figure S3. (a) Absorption spectrum of Cs 4 PbBr 6 NCs colloidal, recorded in a reflectance spectrometer equipped with an integrated sphere. (b) Tauc plot for absorption of Cs 4 PbBr 6 NCs colloidal, revealing an optical bandgap at 2.33 ev. (c) Tauc plots for absorption of Cs 4 PbBr 6 NCs thin-film. Two positions (1 and 2) on the film were measured in the absorbance mode and reflectance mode, respectively. (d) Photo-Electron Spectroscopy in Air (PESA) measurement of Cs 4 PbBr 6 NCs on silicon substrate, indicating the valence band maximum locates at ev below vacuum level. S8

9 Figure S4. TEM images of Cs 4 PbBr 6 NCs synthesized with varied amount of oleic acid (2, 5, 10 ml). The corresponding synthetic yield and average particle size were listed above. S9

10 Figure S5. Photoluminescence quantum yield (PLQY) measurements of Cs 4 PbBr 6 NCs in the form of colloidal and thin-film, respectively. The measurement was conducted in an integrated sphere with an excitation wavelength at 480 nm. Samples usually have an OD about S10

11 Figure S6. PL lifetime measurement of thin-film sample of Cs 4 PbBr 6 NCs, showing very close values to its colloidal counterpart. S11

12 Figure S7. PL spectral evolution of CsPbBr 3 NCs colloidal upon the addition of iodine precursor (OAmI), showing a clear redshift besides a PLQY decrease. In contrast, Cs 4 PbBr 6 NCs only show a PLQY decrease. S12

13 Figure S8. Thermogravimetric analysis (TGA) and difference thermogravimetric (DTG) measurements show that 43.8 wt% of the final product come from the tethering ligands OA (37.9%) or leftover solvent DMF (5.9%), suggesting a reaction yield of 85%. S13