Colloidal Mn-Doped Cesium Lead Halide Perovskite Nanoplatelets Wasim J. Mir, Metikoti Jagadeeswararao, Shyamashis Das, Angshuman Nag,, * Department of Chemistry, and Centre for Energy Science, Indian Institute of Science Education and Research (IISER), Pune, 411008, India. Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India. *Corresponding authors e-mails: AN: angshuman@iiserpune.ac.in Supporting Information Experimental Section: Chemicals: Lead (II) chloride (PbCl 2, 99.99%, Aldrich), manganese (II) chloride tetrahydrate (MnCl 2.4H 2 O, 98.0%, Aldrich), cesium carbonate (Cs 2 CO 3, 99.9%, Aldrich), lead (II) bromide (PbBr 2, 99.999%, Aldrich), oleic acid (OA, 90%, Aldrich), oleylamine (OLA, technical grade 70%, Aldrich), 1-octadecene (ODE, technical grade 90%, Aldrich), chloroform-d (CDCl 3, 99.8 atom % D, Aldrich), hydrochloric acid (HCl, 33.4%, Rankem), nitric acid (HNO 3, 72.0%, Rankem), N,N-dimethylformamide (DMF, 99.5%, SDFCL), dimethyl sulfoxide (DMSO, 99.8%, Rankem), toluene (99.5%, Aldrich), hexane (99.9%, Rankem), acetone (99.0%, FINAR), ethyl acetate(99.5%, Rankem). All the chemicals and solvents were used as such without any further purification. 1
Preparation of Cs-oleate stock solution: Cs 2 CO 3 (1 mmol, 350 mg) was loaded in three neck round bottom flask (RB) along with 20 ml 1-octadecene (ODE) and 1.25 ml oleic acid (OA). The reaction mixture was degassed at 100 o C and then temperature was raised to 150 o C forming Cs-oleate solution, which was then stored at room temperature. Preparation of PbCl 2 solution with and without Mn 2+ molecular precursor: PbCl 2 (2 mmol, 556.22 mg) was dissolved at room temperature in a mixture of solvents containing 2 ml DMSO, 3 ml DMF, 0.25 ml HCl and 0.25 ml HNO 3, making the total volume of solution as 5.5 ml. This stock solution is then divided into five parts with 1.0 ml in each. For doping Mn 2+ in CsPbCl 3 nanoplatelets, additionally x mol% MnCl 2.4H 2 O with respect to Pb 2+ was added to the 1.0 ml PbCl 2 solution. Synthesis of Mn-doped CsPbCl 3 nanoplatelets (NPLs): All syntheses were carried out at room temperature under ambient atmosphere. Mn-doped CsPbCl 3 nanoplatelets (NPLs) were prepared after modifying the synthesis of CsPbBr 3 NPLs reported in ref. 1 Preheated (~100 o C) Cs-oleate solution (0.1 ml) was mixed with a solution containing 1.25 ml ODE, 0.125 ml OA, and 0.125 ml OLA in a test tube at room temperature under vigorous stirring. 0.2 ml PbCl 2 solution, with or without Mn 2+ precursor, was swiftly injected to the above mixture, followed by quick addition of 5 ml acetone. Appearance of white precipitate upon addition of acetone as anti-solvent signifies the formation of CsPbCl 3 NPLs. The supernatant is carefully thrown out and white precipitate of CsPbCl 3 NPLs is dispersed in hexane or toluene. Further washing is carried out carefully by using ethyl acetate as anti-solvent and centrifuged at 4000 rpm followed by re-dispersion of the NPL precipitate in hexane. These colloidal CsPbCl 3 NPLs show tendency to aggregate within a few days, however colloidal dispersion of CsPbCl 3 NPLs in hexane show prolonged colloidal stability up to a week under refrigerator. 2
Anion Exchange: The anion exchange reactions were carried out at room temperature to prepare undoped and Mn-doped CsPbBr 3 NPLs. Typically 0.2 ml crude solution of CsPbCl 3 NPLs was diluted by adding 1 ml hexane in a glass vial under vigorous stirring and to this diluted solution 1ml PbBr 2 stock solution (prepared by mixing 261 mg PbBr 2 in 1ml OA, 1ml OLA and 8 ml ODE at 130 o C) was swiftly injected. As obtained CsPbBr 3 NPLs were washed once using ethyl acetate, centrifuged at 6000 rpm, followed by re-dispersion in hexane. Characterization: Transmission electron microscopy (TEM) images and high resolution transmission electron microscope (HRTEM) images were collected on a UHR FEG-TEM, JEOL JEM-2100F electron microscope using a 200 kv electron source. Atomic force microscopy (AFM) data were collected using Keysight atomic force microscope (model: AFM 5500) by using tapping mode technique. The sample was prepared by drop casting dilute dispersion of CsPbCl 3 nanoplatelets in hexane on a silicon wafer followed by flushing with nitrogen and then further drying under vacuum for about 6 hours. Powder x-ray diffraction (XRD) patterns were recorded using Bruker D8 Advance x-ray diffractometer using Cu Kα radiation (1.54 Å). XRD patterns were from films of NPL prepared by drop casting concentrated colloidal dispersion. X-band electron paramagnetic resonance (EPR) measurements were performed on JEOL JES-FA200 ESR spectrometer. Inductively coupled plasma optical emission spectroscopy (ICP-OES) data were obtained by employing ARCOS M/s. Spectro, Germany. Thermo scientific (Evolution 300) UV/Vis spectrometer was used for UV-visible absorption measurements. Steady state photoluminescence (PL) and PL decay dynamics were recorded on FLS 980 (Edinburgh Instruments) using microsecond flash lamp with power 100 W. For Mn emission decay dynamics, the sample was excited at 300 nm using microsecond flash lamp, whereas, excitonic PL decay was recorded using 340 nm picosecond pulsed LED laser 3
source. PL quantum yield measurements were obtained for fresh samples. For excitonic emission quinine sulfate dye solution in 0.5 M HCl was used as reference standard and for Mn-emission rhodamine-6g dye in distilled water was used as reference standard. Fourier transform infrared spectroscopy (FTIR) studies in solid state using KBr disc were obtained using NICOLET 6700 FTIR spectrometer. 1 H-NMR studies of colloidal dispersion in deuterated chloroform were performed on Bruker 400 MHz NMR spectrometer. Thermogravimetric analysis (TGA) measurements were recorded using Perkin Elmer STA 6000. The samples were heated in the range of 30-800 C at the heating rate of 10 C/min. Differential Scanning Calorimeter (DSC) measurements were carried out on powder samples using TA Q20 DSC at heating and cooling rate of 10 o C/min. Figure S1: Surface characterization of CsPbCl 3 NPLs using (a) FTIR spectrum of powder sample embedded in KBr pellet, and (b) 1 H NMR spectrum of colloidal CsPbCl 3 NPLs in CDCl 3 (298 K, 400 MHz). The assigned peaks correspond to bound oleic acid in deprotonated form and protonated oleylamine. The unassigned sharp peaks in the 1 H-NMR spectrum are from solvent CDCl 3, acetone and ODE. 4
100 TGA plot CsPbCl 3 NPL 80 % weight loss 60 40 20 0 100 200 300 400 500 600 700 800 Temperature ( o C ) Figure S2: TGA plot of CsPbCl 3 NPLs in the temperature range of 30 to 800 o C. The plot depicts first major loss in weight at 300 o C due to organic capping part and 2 nd major weight loss ~630 o C because of inorganic part. 5
Figure S3: (a) DSC curve of CsPbCl 3 NPL showing a broad dip in the temperature range of 38 to 65 o C suggesting structural phase transition. (b) Variable temperature XRD diffraction pattern in the temperature range of 30 to 150 o C, reveals no major temperature dependent change in XRD pattern, probably because of insignificant difference in cubic and tetragonal structure of CsPbCl 3. 6
Figure S4: Schematics of unit cell for (a) cubic CsPbCl 3 and (b) tetragonal CsPbCl 3. Both cubic and tetragonal unit cell are having similar lattice parameter, where the tetragonal one exhibit c/a ratio of 1.007. x % Mn CsPbCl 3 Norm. absorbance 0% 0.1% 0.2% 0.8% 2% 300 400 500 600 Wavelength (nm) Figure S5: UV-visible absorption spectra of Mn-doped CsPbCl 3 NPLs. Spectra were normalized at the band-edge excitonic peak for a better comparison of the peak energy (wavelength) at different dopant concentrations. 7
Mn-doped CsPbBr 3 NPLs Intensity (a.u.) Bulk CsPbBr 3 0.8% Mn-doped CsPbCl 3 NPLs BulkCsPbCl 3 20 30 40 50 60 2 theta (degree) Figure S6: Powder XRD pattern of 0.8% Mn-doped CsPbCl 3 and Mn-doped CsPbBr 3 NPLs along with their bulk references. Figure S7: UV-visible absorption and PL spectra of anion exchange reaction for (a) undoped CsPbCl 3 to CsPbBr 3 NPLs and (b) 0.8% Mn-doped CsPbCl 3 to CsPbBr 3 NPLs. Clearly the weak Mn-emission was observed from Mn-doped CsPbBr 3 NPLs, and some reaction show absence of Mn emission. 8
1 (a) 1 (b) PL intensity (a.u.) 0.1 Emission @ 495 nm PL Intensity (a.u.) 0.1 0.01 1E-3 1E-4 Emission @ 586 nm 0 20 40 60 80 Time (ns) 1E-5 0 2 4 6 8 Time (ms) Figure S8: PL decay dynamics of Mn-doped CsPbBr 3 NPLs, (a) with emission at 495 nm after excitation with picosecond pulse LED source at 340 nm, and (b) with emission at 586 nm after excitation with a microsecond flash lamp at 300 nm. Reference: 1. Akkerman, Q. A.; Motti, S. G.; Srimath Kandada, A. R.; Mosconi, E.; D Innocenzo, V.; Bertoni, G.; Marras, S.; Kamino, B. A.; Miranda, L.; De Angelis, F.; Petrozza, A.; Prato, M.; Manna, L., Solution Synthesis Approach to Colloidal Cesium Lead Halide Perovskite Nanoplatelets with Monolayer-Level Thickness Control. J. Am. Chem. Soc. 2016, 138, 1010-1016. 9