Supporting Information

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1 Electronic Supplementary Material (ESI) for Nanoscale. This journal is The Royal Society of Chemistry Supporting Information Pressure response to the structure and optical properties in two-dimensional perovskite-like CsPb 2 Br microplates Zhiwei Ma, a Fangfang Li, b Guangyu Qi, a Lingrui Wang, a Chuang Liu, a Kai, Wang, a Guanjun Xiao, a, * and Bo Zou, a a State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 3002, China b Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics Jilin University, Changchun, 3002, China. * xguanjun@jlu.edu.cn

2 Experimental Section 2 Materials: PbBr 2 (Aladdin,.%), Cs 2 CO 3 (Sigma-Aldrich,.%), oleylamine (OLA Aladdin, 0-0%), 3 oleic acid (OA, Sigma-Aldrich, 0%), octadecylene (ODE, Aladdin, 0%) and toluene (.%, Sinopharm Chemical Reagent Co., Ltd., China). All regents were used directly without further purification. Preparation of Cs-oleate (Cs-OA): In a typical experiment, Cs 2 CO 3 (0. g), OA (0. ml) and ODE (3. ml) were added into a 0 ml 3-neck flask, degassed at room temperature for 30 min and then dried for h at ºC under N 2 until all Cs 2 CO 3 reacted with OA. Preparation of CsPb 2 Br Microplates: CsPb 2 Br MPs were synthesized according to a modified 0 synthetic approach. In the typical synthesis of CsPb 2 Br MPs, ODE ( ml) and PbBr 2 (0.3 g) were added into a 0 ml 3-neck flask. The mixture was degassed and heated to ºC. OLA (0. ml) and 2 OA (0. ml) were added into the flask and then the mixture was heated to 0 ºC under N 2 until all 3 PbBr 2 dissolved. Cs-OA (0. ml) was injected into the flask under 0 ºC. The reaction was terminated at h and the crude solution was quenched by toluene. The as-synthesized MPs were precipitated by centrifugation at 000 rpm and redispersed in toluene. After three to four centrifugations, the supernatant was discarded and the precipitate was dispersed in toluene for further characterization. Characterization and High Pressure Generation: SEM images were observed by JSM-00F, JAPAN. A symmetrical diamond anvil cell (DAC) was employed to generate high pressure. A stainless steel gasket was preindented to 0 μm in thickness followed by laser-drilling the central part to form a 2 0 μm diameter hole to serve as the sample compartment. The CsPb 2 Br MPs and a small ruby ball were loaded together in the sample compartment. In high pressure optical absorption, PL and XRD experiments, we used silicone oil as pressure-transmitting medium the pressures were determined 2 by the ruby fluorescence method. All of the measurements were performed at room temperature. 2 In Situ High-Pressure Optical Experiments: The excitation source, a 3 nm line of a UV DPSS laser 2 with the power of.0 mw, was used for PL measurements. The high-pressure evolution of steady- 2 state PL spectra of CsPb 2 Br MPs was collected by a modified spectrophotometer (Ocean Optics, 2 QE000) with the data-collection time of s. The laser beam passed through tunable filter and 2 was focused onto the sample with m spot UV Plan apochromatic objective. Each new 2

3 acquisition was carried out several minutes later after elevation of the pressure, aiming to restrain 2 any kinetic factor during the measurements. PL micrographs of the samples were obtained using a 3 camera (Canon Eos D mark II) equipped on a microscope (Ecilipse TI-U, Nikon). The camera can record the photographs under the same conditions including exposure time and intensity. Absorption spectra were measured in the exciton absorption band region using a Deuterium- Halogen light source. High-pressure absorption and PL spectra of CsPb 2 Br MPs were recorded with an optical fiber spectrometer (Ocean Optics, QE000). The in situ high pressure angle-dispersive XRD experiments were carried out at the W2 High Pressure Station in Beijing Synchrotron Radiation Facility. A focused monochromatic X-ray beam with about μm in diameter and 0 wavelengths of 0. Å was used for the diffraction experiments. CeO 2 was used as the standard sample to do the calibration. The diffraction pattern were recorded by a two-dimensional area Mar- 2 CCD detector and integrated into one-dimensional profile with the Fit2D program. All the high- 3 pressure experiments were conducted at room temperature. Computation: Geometry optimization was calculated using the plane-wave pseudopotential method with the generalized gradient approximation (GGA) based on density functional theory with CASTEP package. The starting structure was obtained from the Cambridge Structure Database. The kinetic energy cutoff of 0 ev and the Brillouin zone sampling for geometry optimizations was performed using a 3 Monkhorst-Pack k-point mesh. The convergence thresholds were set at values of.0 e- ev/atom for energy and 0.03 evå - for force

4 2 Figure S. EDS of the CsPb 2 Br MPs

5 2 Figure S2. DFT calculated total and partial DOS projected onto Cs, Pb, and Br atoms

6 2 Figure S3. The photographs in the sample chamber at selected pressures irradiated by a 3 nm 3 laser

7 Refinements methods and procedures 2 Pawley and Rietveld refinements of XRD patterns were accomplished using the Reflex module 3 combined in Materials Studio.0 program. 2- All Pawley and Rietveld refinements were performed using four refinement cycles fine convergence criteria. First, the pattern was indexed by means of the peak picking option of the software package. Potential solutions for cell parameters were found using the TREOR 0 and DICVOL methods. Then, a Pawley profile-fitting procedure was applied to refine cell parameters and search space group. The final Rietveld refinement (including Pawley refined parameters, Pb and halide atomic positions, preferred orientations and overall isotropic factor) were performed to obtain the crystal structural parameters. The quality of the fitting 0 between the experimental and calculated profile is assessed by the various R parameters like R p (profile factor) and R wp (weighted profile factor). 2 3 { R p = i I obs i I cal i I obs i } i { R wp = i w i(i obs i I cal i ) 2 i w i (I obs i ) 2 I obs i I cal } 2 Where, i and i indicates the experimental, calculated and total number of points respectively. And the w i is the reciprocalof the variance of observation I obs i

8 2 3 Figure S. Pressure dependence of angle dispersive synchrotron X-ray diffraction peak positions for CsPb 2 Br MPs in the range (a) from to and (b) from to

9 2 3 Figure S. (a) SEM image of the obtained CsPb 2 Br MPs when the pressure was completely released to ambient conditions. (b) Element mapping images for Cs, Pb, and Br, respectively. Elemental mapping indicates that the elements of Cs, Pb and Br are homogeneously distributed throughout the whole sample, which suggests high purity of the final products. (c) The PL spectra of the obtained CsPb 2 Br MPs at ambient condition and quenched phase upon completely releasing the pressure to ambient conditions 0 2 3

10 2 Table S. Refinement cell parameters and refinement statistics at ambient pressure and 2. GPa 3 for CsPb 2 Br MPs. atm 2. GPa Temperature (K) 23 (3) 23 (3) Crystal system Tetragonal Tetragonal Space group I/mcm I/mcm a=b (Å)..32 c (Å)..3 α=β=γ ( ) 0 0 Volume (Å 3 ) Z range ( ) - - Wavelength (Å) Number of refined peaks 2 2 R wp (%) R p (%)

11 2 Table S2. The lattice parameters and unit cell volume for structures of CsPb 2 Br MPs at different 3 pressures. Pressure (GPa) a=b (Å) c (Å) V (Å 3 ) I/mcm I/mcm

12 2 Table S3. The three Pb-Br bonds and two Br-Pb-Br angles within lead-bromide pentahedra for 3 CsPb 2 Br MPs as function of pressure. Pressure (GPa) Pb-Br bonds L (Å) Pb-Br bonds L 2 (Å) Br-Pb-Br angles A (degrees) Br-Pb-Br angles A 2 (degrees)

13 2 Table S. Comparison of the band gap and crystal stability between CsPbBr 3 and CsPb 2 Br 3 Initial bandgap (ev) Slope of change in band gaps de(gap)/dp (ev/gpa) Phase transition pressure (GPa) CsPb 2 Br ~. CsPbBr ~.2 3

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