Supporting Information

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1 Supporting Information Highly Stable Cesium Lead Halide Nanocrystals through Inorganic Passivation Ju Young Woo,, Youngsik Kim,, Jungmin Bae, # Tae Gun Kim,,# Jeong Won Kim,,# Doh C. Lee,*, and Sohee Jeong*,, Department of Chemical and Biomolecular Engineering (BK21+ Program), KAIST Institute for the NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon , Korea Nano-Convergence Research Division, Korea Institute of Machinery and Materials (KIMM), Daejeon , Korea Department of Nanomechatronics, Department of Nanoscience, Korea University of Science and Technology (UST), Daejeon , Korea # Korea Research Institute of Standards and Science (KRISS), Daejeon , Korea Experimental Section Chemicals. Cesium carbonate (Cs 2 CO 3, 99.9%, Sigma-Aldrich), lead bromide (PbBr 2, 98%, Sigma-Aldrich), lead iodide (PbI 2, %, Alfa Aesar), oleic acid (OA, technical grade, Sigma-Aldrich), 1-octadecene (ODE, technical grade, Sigma-Aldrich), oleylamine (OLA, technical grade, Sigma-Aldrich), zinc bromide (ZnBr 2, %, Sigma-Aldrich), indium bromide (InBr 3, %, Sigma-Aldrich), copper bromide (CuBr 2, 99%, Sigma-Aldrich), zinc iodide (ZnI 2, 98%, Sigma-Aldrich), anhydrous toluene (99.8%, Sigma-Aldrich), and anhydrous hexane (95%, Sigma-Aldrich) were purchased. Degassed OA and OLA were prepared by heating the OA or OLA at 110 under vacuum overnight and stored in a glovebox. Characterization. Absorption spectra were recorded using a UV-Vis spectrophotometer obtained from Thermo Scientific (Evolution 201). PL QY and PL spectra were collected using an absolute PL QY spectrometers purchased from Hamamatsu Photonics (PMA-12 S1

2 series and Quantaurus). TEM images were obtained using a Tecnai F20 field emission transmission electron microscope (200 kv, FEI). XRD patterns were recorded in theta/2theta scan using a customized X-ray diffractometer (PANalytical) and an Ultima IV (Rigaku). XRD samples were prepared by drop casting the NC solutions on glass substrates in a glovebox. XPS spectra were collected from a Multilab 2000 (Thermo Scientific) and a PHI Quantera-II (Ulvac-PHI) using an aluminum anode (Al Kα= ev). XPS samples were fabricated in a glovebox via spin coating the NCs on Au/Cr coated silicon wafers to minimize charging. Take-off angle during the XPS measurement was set to 90 otherwise noted. Highangle annular dark field (HAADF) scanning transmission electron microscopy (STEM) characterizations were performed using a Titan cubed G (FEI) operating at 300 kv. NMR spectra were collected using an AVANCE III (600 MHz, Bruker). Preparation of Cs-oleate solution. Cs-oleate solution was prepared following the method reported by Protesescu et al. 1 with some modifications g of Cs 2 CO 3, 0.5 ml of OA, and 8 ml of ODE were loaded in a 50 ml 3-neck flask and degassed for 2 h at 110. After degassing, the solution was further heated to 150 under nitrogen flow and maintained. Synthesis of pristine-cspbx 3 NCs (X is Br or I). We adopted a synthetic method reported by Protesescu et al. 1 with some modifications. For the synthesis of pristine-cspbbr 3 NCs, g of PbBr 2 (0.376 mmol), 1 ml of degassed OA, 1 ml of degassed OLA, and 10 ml of ODE were loaded in a 50 ml 3-neck flask. The mixture was degassed at 110 for 2 h. After degassing, the temperature of the solution was increased to 170 under nitrogen atmosphere, and 0.8 ml of Cs-oleate solution (150 ) was swiftly injected into the optically transparent precursor solution using a glass syringe. After 5-10 s from the injection, heating mantle was removed and the flask was rapidly quenched in an ice bath. For the synthesis of pristine-cspbi 3 NCs, g of PbI 2 (0.376 mmol) was introduced instead of PbBr 2, and other procedures were conducted in the same manner. Synthesis of metal halide-cspbx 3 NCs (X is Br or I). For the synthesis of ZnBr 2 -CsPbBr 3 NCs, g of PbBr 2 (0.376 mmol), g of ZnBr 2 (0.422 mmol), 1 ml of degassed OA, 1 ml of OLA, and 10 ml of ODE were loaded in a 50 ml 3-neck flask. The mixture solution was degassed 110 for 2 h. And then the solution was further heated to 170 under S2

3 nitrogen flow. Finally, 0.8 ml of Cs-oleate solution (150 ) was rapidly injected into the opaque precursor solution using a glass syringe s later, heating mantle was removed and the reaction flask was rapidly cooled down using an ice bath. For the synthesis of other metal bromide-cspbbr 3 NCs (e.g., InBr 3 -, CuBr 2 -, and PbBr 2 -CsPbBr 3 NCs), g of InBr 3 (0.422 mmol), g of CuBr 2 (0.422 mmol), or g of PbBr 2 (0.422 mmol) was loaded instead of ZnBr 2 and other procedures were identically performed. For the synthesis of ZnI 2 - CsPbI 3 NCs, g of PbI 2 (0.376 mmol) and g of ZnI 2 (0.422 mmol) were introduced instead of PbBr 2 and ZnBr 2, respectively, and other procedures were conducted identically. Extraction of CsPbX 3 NCs. CsPbX 3 NCs were extracted by high-speed multiple centrifugations without adding external agents. After the reaction, NC reaction solution was centrifuged for 10 min at rpm. After the centrifugation, the supernatant was discarded and NC precipitates were redispersed in 3 ml of anhydrous toluene with vigorous vortexing. After the NCs are completely dispersed in anhydrous toluene, the solution was again centrifuged for 30 min at rpm. After that, supernatant was discarded and the NC precipitates were redispersed in 4 ml of anhydrous hexane with vigorous vortexing. After complete dispersion of the NCs, the solution was centrifuged for 10 min at 6000 rpm. Finally, precipitates were discarded and supernatant was filtered using Teflon syringe filter with pore size of 0.45 µm. Temperature was set to 22 during centrifugations. It is noted that all the extraction procedures were carried out under inert conditions. Stability test under ambient condition. NCs were deposited on glass substrates or TEM grids in N 2 -filled glovebox. After initial characterizations, samples were stored under ambient condition with relative humidity of 50-60%. Characterizations (PL QY, XRD and TEM) were conducted as a function of exposure time to ambient condition. Thermal stability test. NC hexane solution was dropcast on glass substrates and the solvent was naturally dried. After deposition, the films were placed on a hot plate. During thermal stability test, the temperature of hot plate was set to 200. All the procedures were conducted in a N 2 -filled glovebox. Ligand quantification. CsPbBr 3 NCs were dispersed in 1 mm of ferrocene toluene-d 8, and then the NMR spectra were collected. Ligand was quantified by comparing the area of S3

4 surface bound ligand ( ) and ferrocene ( ) as an internal reference in NMR spectra. 2 Average size of the NCs were calculated ~12 nm based on TEM characterizations. Absorption coefficient of CsPbBr 3 NCs was taken from a literature. 3 Relative elemental quantification based on XPS. Relative elemental quantifications were conducted as follows. Integrated area of XPS spectra was divided by the number of scans and relative atomic sensitivity factor of each element. 4 The resulting corrected area of each element was compared each other. S4

5 Figure S1. Photographs of PbBr 2 reaction mixture for the synthesis of (A) pristine- and (B) ZnBr 2 -CsPbBr 3 NCs just before injection of Cs-oleate. S5

6 Figure S2. Normalized absorption and PL spectra of fresh (A) pristine- and (B) ZnBr 2 - CsPbBr 3 NCs dispersed in toluene. S6

7 Figure S3. XRD patterns of fresh (black) pristine- and (red) ZnBr 2 -CsPbBr 3 NCs. Orthorhombic CsPbBr 3 bulk patterns were taken from ICSD S7

8 Figure S4. XPS Zn 2p spectrum of ZnBr 2 -CsPbBr 3 NCs. No Zn was detected. S8

9 Figure S5. PL QYs and spectra of (A) pristine- and (B) TBABr-CsPbBr 3 NC films deposited on glass substrates before and after exposure to ambient condition. S9

10 Figure S6. (A) Low resolution HADDF-STEM image of pristine-cspbbr3 NCs after the sample was slightly exposed to air. White dashed circles highlight the region where inter-nc fusion initiates. (B) High resolution HADDF-STEM image clearly showing inter-nc fusion along [110] crystal direction. (B) was taken at white box region in (A). S10

11 Figure S7. XPS C 1s spectra of (black) pristine- and (red) ZnBr 2 -CsPbBr 3 NCs after peak calibration at ev. S11

12 Figure S8. (A) Pb 4f and (B) Br 3d XPS core level spectra of CsPbBr 3 NCs. Spectra of ZnBr 2 -CsPbBr 3 NCs show higher binding energy shift by 0.2 ev compared to those of pristine-cspbbr 3 NCs. S12

13 Figure S9. Normalized (A and B) Br/(Cs+Pb) and (C and D) Pb/Cs ratio of CsPbBr 3 NCs obtained from angle-resolved XPS measurement. For clarity, atomic ratios obtained at takeoff angle of 20 were normalized by atomic ratios obtained at take-off angle of 90. S13

14 Figure S10. NMR spectra of (A) pristine- and (B) ZnBr 2 -CsPbBr 3 NCs. Broad resonance can be attributed to surface bound ligand ( ) and oleylamine-realted species ( ). Sharp resonance can be assigned to toluene-d 8 ( ), ODE ( ), free ligands ( ), and internal reference ferrocene ( ). 5 S14

15 Figure S11. (A) Plot of normalized PL QYs of pristine-, ZnBr 2 -, and InB 3 -CsPbBr 3 NC films with various metal bromide concentrations. PL QY results were recorded as a function of time under ambient condition. (B) XRD patterns of (black) pristine-, (red) InBr 3 -, and (blue) CuBr 2 -CsPbBr 3 NCs after 5 days of exposure to ambient condition. S15

16 Figure S12. (A) Pb 4f and (B) Br 3d XPS spectra of pristine-, InBr 3, and CuBr 2 -CsPbBr 3 NCs. (C) Br/(Cs+Pb) and (D) Pb/Cs atomic ratio of CsPbBr 3 NCs obtained from XPS analysis. S16

17 Figure S13. XRD patterns of (black) pristine- and (red) PbBr 2 -CsPbBr 3 NCs after exposure to ambient condition for 8 days. S17

18 Figure S14. (A) I/(Cs+Pb) and (B) Pb/Cs atomic ratio of pristine- and ZnI 2 -CsPbI 3 NCs obtained from XPS results. S18

19 Reference (1) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX 3, X= Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, (2) Woo, J. Y.; Ko, J.-H.; Song, J. H.; Kim, K.; Choi, H.; Kim, Y.-H.; Lee, D. C.; Jeong, S. Ultrastable PbSe Nanocrystal Quantum Dots via in Situ Formation of Atomically Thin Halide Adlayers on PbSe(100). J. Am. Chem. Soc. 2014, 136, (3) De Roo, J.; Ibanez, M.; Geiregat, P.; Nedelcu, G.; Walravens, W.; Maes, J.; Martins, J. C.; Van Driessche, I.; Kovalenko, M. V.; Hens, Z. Highly Dynamic Ligand Binding and Light Absorption Coefficient of Cesium Lead Bromide Perovskite Nanocrystals. ACS Nano 2016, 10, (4) Moulder, J.; Stickle, W.; Sobol, P.; Bomben, K. Handbook of X-Ray Photoelectron Spectroscopy. Eden Prairie, MN: Perkin-Elmer Corporation (5) Moreels, I.; Fritzinger, B.; Martins, J. C.; Hens, Z. Surface Chemistry of Colloidal PbSe Nanocrystals. J. Am. Chem. Soc. 2008, 130, S19