Supporting Information for:

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1 Supporting Information for: Post-Synthesis Transformation of Insulating Cs 4 PbBr 6 Nanocrystals Into Bright Perovskite CsPbBr 3 Through Physical and Chemical Extraction of CsBr Francisco Palazon, Carmine Urso, Luca De Trizio, Quinten Akkerman, Sergio Marras, Federico Locardi, Ilaria Nelli, Maurizio Ferretti, Mirko Prato and Liberato Manna * Nanochemistry Department, and Materials Characterization Facility, Istituto Italiano di Tecnologia, Via Morego 30, Genova, Italy Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Via Dodecaneso 31, Genova, Italy EXPERIMENTAL DETAILS Materials Lead (II) bromide (PbBr 2, % trace metals basis), cesium carbonate (Cs 2 CO 3, reagentplus, 99%), octadecene (ODE, technical grade, 90%), oleic acid (OAc, 90%), and Fe III 4[Fe II (CN) 6 ] 3 (Prussian Blue; PB, 99.9%) were purchased from Sigma-Aldrich. Toluene (anhydrous, 99.8%) was purchased from Carlo Erba reagents. Oleylamine (OA, 80-90%) was purchased from Acros Organics and filtered through a 0.45 µm PTFE filter before use. All other chemicals were used without any further purification. Synthesis of Cs 4 PbBr 6 NCs Cs 4 PbBr 6 NCs were synthesized as previously described. 1 The synthesis was performed in air and without any predried chemicals or solvents. In a typical synthesis, PbBr 2 (0.1 mmol) was dissolved in 5 ml ODE, 0.2 ml OA and 1.5 ml OLAM in a 20 ml vial on a hotplate set at 150 C. After the PbBr 2 was completely dissolved (around 100 ºC), the vial was moved to a hotplate kept room temperature (RT), and the solution was allowed to cool. When the temperature reached 80 ºC, 0.75 ml of Cs-OA (0.4 g Cs 2 CO 3 dissolved in 8 ml OA in a 20 ml vial on a hotplate set to 150 ºC) was swiftly injected. After about 30 seconds the mixture turned turbid white and, depending on the required size, the vial was quickly cooled down after 0-10 min to RT by immersion in a cold water bath. The NCs were directly washed by centrifugation (at 4500 rpm for 10 minutes), followed by redispersion in 6 ml TOL (or HEX for optical measurements). Transformation with Prussian Blue in solution PB powder (ca. 100 mg) was added to a colloidal dispersion of Cs 4 PbBr 6 in TOL (ca. 2 ml) and left to react for one day at room temperature without stirring to yield CsPbBr 3 NCs. Subsequently, the NCs were isolated by centrifugation at 1000 rpm for 1 minute and filtration through PTFE filters with 0.2 µm pore size. XRD characterization High Temperature X-ray diffraction analysis (HTXRD) from RT to 400 C, under vacuum was performed using a Rigaku Smartlab system equipped with a 9 kw CuKα rotating anode (operating at 40kV and 150mA) and an Anton Paar DHS 900 domed hot stage. DTA/TG - GC/MS characterization. The DTA/TG investigation (LabsysEvo 1600 Setaram) was performed from 35 to 1000 C, heating at 5 C/min in fluent He (20ml/min). The sample, ~ 80 µl, was put into an open alumina crucible and dried under vacuum for five minutes, after which was weight and insert in the instrument. S1

2 Every minute the autoinjector set at 280 C (Automation) collects for 10 seconds, in a loop of 1 ml, the evolved gases from the output of TG. The collected molecules were injected in the GC (TraceGC Ultra ThermoFisher) operating in the following conditions: oven and inlet temperature respectively 140 C and 280 C, carrier gas He 1.2 ml/min, split ratio 1:10 and split flow 12 ml/min, column Mega-5 (5%Phenyl 95% Methyl polysiloxane). The GC was coupled to a MS quadrupole (DSQ I ThermoFisher) operating in the EI mode (70eV), ion source at 250 C, transfer line at 280 C. The MS scansion was performed in the m/z range Steady state UV-Vis extinction spectroscopy Optical extinction spectra of dilute toluene dispersions of NCs were recorded in quartz cuvettes of 1 cm path-length employing a Varian Cary 300 UV-Vis spectrophotometer. SEM characterization. High Resolution Scanning Electron Microscopy (HRSEM) analysis was carried out using a JEOL JSM 7500FA scanning electron microscope, equipped with a cold field emission gun. Photoluminescence spectroscopy on film. PL spectra were recorded with a Nikon A1 microscope with a laser excitation of 405 nm. Scheme S1. 3D models of CsPbBr 3, Cs 4 PbBr 6, and CsPb 2 Br 5 structures. S2

3 Figure S1. Differential thermal analysis (DTA) in the ºC range. An endothermic peak at 507 ºC is seen, assigned to the melting of Cs 4 PbBr 6. S3

4 Figure S 2. Transmittance percentage of films of pristine Cs 4 PbBr 6 NCs (black curve) and annealed at 200 ºC (blue curve). Inset shows a zoom of the signal in the nm range. S4

5 Figure S3. XRD of samples annealed ex-situ under nitrogen at ambient pressure. All diffractograms are acquired in air at room temperature after cooling from the indicated temperature. Figure S4. Normalized XRD diffractograms of pristine sample as well as annealed at 200 ºC and 400 ºC. As the temperature is raised the peak shifts towards lower angles due to thermal expansion of the unit cell and becomes thinner as a consequence of crystal sintering. S5

6 Figure S 5. Size distribution of initial Cs 4 PbBr 6 NCs and resulting CsPbBr 3 NCs after reaction with Prussian Blue. The statistics are performed on 200 Cs 4 PbBr 6 NCs and 250 CsPbBr 3 NCs. The size of the NCs is reduced from 9.8 +/- 1.1 nm (Cs 4 PbBr 6 NCs, before reaction) to 6.3 +/- 1.4 nm (CsPbBr 3 NCs, after reaction). S6

7 Figure S 6. XRD signal of 3D NCs sample left for 3 days to react with Prussian Blue (PB). Peaks marked with a star correspond to PB while the other peaks correspond to the CsPbBr 3 NCs. Noteworthy, no signal at 11.6 º is seen, as would be expected from CsPb 2 Br 5. S7

8 Figure S 7. TEM (a-b) and optical characterization of cesium lead iodide NCs before (a, c-black line) and after (b, c- red lines) reaction with Prussian Blue. References 1. Akkerman, Q. A.; Park, S.; Radicchi, E.; Nunzi, F.; Mosconi, E.; De Angelis, F.; Brescia, R.; Rastogi, P.; Prato, M.; Manna, L., Nearly Monodisperse Insulator Cs 4 PbX 6 (X = Cl, Br, I) Nanocrystals, Their Mixed Halide Compositions, and Their Transformation into CsPbX 3 Nanocrystals. Nano Lett. 2017, 17, S8