Supporting Information. Intrinsic Lead Ion Emissions in Zero-dimensional Cs 4 PbBr 6 Nanocrystals

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1 Supporting Information Intrinsic Lead Ion Emissions in Zero-dimensional Cs 4 PbBr 6 Nanocrystals Jun Yin, 1 Yuhai Zhang, 1 Annalisa Bruno, 2 Cesare Soci, 2 Osman M. Bakr, 1 Jean-Luc Brédas, 3,* Omar F. Mohammed 1,* 1 KAUST Solar Center, Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal , Kingdom of Saudi Arabia 2 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics (COPE), Georgia Institute of Technology, Atlanta, Georgia , United States *Corresponding Authors: omar.abdelsaboor@kaust.edu.sa; jean-luc.bredas@chemistry.gatech.edu

2 Experimental Details Materials All reagents were used without any purification: CsBr (99.999%, Sigma-Aldrich), OA (oleic acid, 90%, Sigma-Aldrich), OLA (oleylamine, 90%, Sigma-Aldrich), DMF (N,N-dimethylformamide, 99.8%, Sigma-Aldrich), anhydrous toluene (99.98%, Sigma-Aldrich). Synthesis of emissive Cs 4 PbBr 6 nanocrystals Emissive Cs 4 PbBr 6 nanocrystals were synthesized using a precipitation method at room temperature. In a typical experiment, a mixture of CsPbBr 3 (0.04 M in DMF, 200 µl), OA (10 µl), and OLA (10 µl) was injected into 2 ml toluene under vigorous stirring. The reaction was allowed to proceed for 10 min before centrifugation and decanting. The product can be well dispersed in toluene, or drop cast into films for further characterization. Synthesis of non-emissive Cs 4 PbBr 6 nanocrystals We note that the addition amount of ligands (OA and OLA) is the key to control the Cs 4 PbBr 6 nanocrystal transformation from visible emissive to non-emissive. The non-emissive Cs 4 PbBr 6 nanocrystals were synthesized using a similar protocol as described above except for the addition amount of OA (20 µl) and OLA (20 µl). Steady-state photoluminescence measurements The non-emissive and emissive samples were mounted into a liquid nitrogen-cooled Linkam Stage (FTIR 600) that allows operating temperatures from 300 K down to 77 K. The steady-state photoluminescence spectra were recorded by a Fluorolog-3, (HORIBA Jobin Yvon) spectrofluorometer employing a Xe lamp as excitation source and a Synapse CCD camera as detector. The wavelength resolution is 0.5 nm. 2

3 Computational Details DFT calculations were performed to optimize the crystal structures of CsPbBr 3 and Cs 4 PbBr 6 using the PWSCF code as implemented in the Quantum ESPRESSO (QE) package. 1 The CsPbBr 3 lattice is the orthorhombic phase (Pnam) and Cs 4 PbBr 6 shows the rhombohedral phase (R3 c). Starting from the experimental lattice parameters of CsPbBr 3 (a = 8.24 Å, b = Å, and c = 8.20 Å) and Cs 4 PbBr 6 (а = b = Å and с = Å), the structures have been optimized by relaxing both the cell parameters and the atomic positions at the generalized gradient approximation (GGA)/Perdew-Burke-Ernzerhof (PBE) level. The resulting lattice constants for CsPbBr 3 are: a = 8.54 Å, b = Å and c = 8.21 Å; and for Cs 4 PbBr 6 : а = b = Å and с = Å. Ultrasoft pseudopotentials were used without and with consideration of spin-orbit coupling (SOC). Plane-wave basis set cutoffs for the wavefunctions and charge density were set at 50 and 300 Ry, respectively. The crystal structures were fully relaxed until the total force on each atom was less than 0.01 ev/å. A uniform grid of k-mesh in the Brillouin zone was employed to obtain the electronic band structures and projected density of states for Cs 4 PbBr 6 supercells before and after a Pb 2+ ion replacement. The Raman intensities (at the Γ point of the first Brillouin zone) were calculated on both CsPbBr 3 and Cs 4 PbBr 6 using the Phonon code as implemented in the QE package. 1 The local density approximation (LDA) exchange-correlation functional with norm-conserving pseudopotentials was used. The plane-wave expansion cutoff for the wavefunctions was set at 90 Ry. Uniform grids of (CsPbBr 3 ) and (Cs 4 PbBr 6 ) Monkhorst-Pack scheme were used for the k-point sampling together with self-consistency threshold of Ry. The spinorbit coupling was not included in the Raman calculations since it plays a less significant role than geometry to describe the vibrational properties of heavy-metal based perovskite systems. 3

4 Figure S1. XRD pattern of emissive and non-emissive Cs 4 PbBr 6 NCs with an identical phase in crystallography (space group 167: R3 c). 4

5 Figure S2. Steady-state absorption spectra of emissive and non-emissive Cs 4 PbBr 6 NCs in hexane solvent. 5

6 Figure S3. Temperature-dependent steady-state photoluminescence spectra of non-emissive Cs 4 PbBr 6 NCs with fitted high-energy (blue dashed lines) and low-energy (pink dashed lines) emission bands. 6

7 Figure S4. High-energy and low-energy emission maxima as a function of temperature. Figure S5. (a) Optimized crystal structure of Cs 4 PbBr 6 and first Brillouin zone for Cs 4 PbBr 6 with space group of R3 c, in which the high-symmetry k points and paths are labelled; (b) band structures of Cs 4 PbBr 6 calculated at the PBE and PBE+SOC levels. 7

8 Figure S6. Steady-state photoluminescence spectra of emissive Cs 4 PbBr 6 NCs in the temperature range from 198 to 298 K. 8

9 Figure S7. Temperature-dependent steady-state photoluminescence spectra of Cs 4 PbBr 6 NCs using 470 nm excitation. References (1) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I., et al. Quantum Espresso: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21,