Supporting Information. Direct-Indirect Nature of the Bandgap in Lead-Free Perovskite Nanocrystals

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1 Supporting Information Direct-Indirect Nature of the Bandgap in Lead-Free Perovskite Nanocrystals Yuhai Zhang, Jun Yin, Manas R. Parida, Ghada H. Ahmed, Jun Pan, Osman M. Bakr, Jean-Luc Brédas, # Omar F. Mohammed *, King Abdullah University of Science and Technology, KAUST Solar Center, Division of Physical Sciences and Engineering, Thuwal , Kingdom of Saudi Arabia # New permanent address: School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia , USA Corresponding Author omar.abdelsaboor@kaust.edu.sa S1

2 Materials. All reagents were used without any purification: BiI3 (bismuth triiodide, 99%, Alfa Aesar), BiCl3 (bismuth trichloride, 99%, Alfa Aesar), BiBr3 (bismuth tribromide, 99%, Alfa Aesar), Cs 2 CO 3 (cesium carbonate, 99%, Sigma Aldrich), OA (oleic acid, 90%, Sigma Aldrich), ODE (1- octadecene, 90%, Sigma Aldrich), OLA (oleylamine, 90%, Sigma Aldrich), and anhydrous toluene (99.98%, Sigma Aldrich). Synthesis of Cs 3 Bi 2 X 9 Nanocrystals (X = Cl-, Br-, or I-). Cs 3 Bi 2 X 9 nanocrystals were synthesized using a modified hot-injection method. 1 In a typical procedure, the BiI 3 precursor and the Cs-oleate precursor were synthesized separately. First, a mixture of 1.5 ml of OA, 1.5 ml of OLA, 15 ml of ODE and 330 mg of BiI 3 was stirred in a 50-mL flask and degassed at 90 C under vacuum for 1 hour to form a yellow colloidal BiI 3 precursor. Second, a mixture of 0.45 g of Cs 2 CO 3, 1.5 ml of OA and 20 ml of ODE was stirred and degassed at 130 C under vacuum for 1 hour to generate a clear colloidal Cs-oleate precursor. In a typical synthesis, 2 ml of the BiI 3 precursor was loaded into a glass vial set on a hot plate at 100 C; 5 min later, 0.1 ml of the Cs-oleate precursor was injected. A color change from yellow to deep orange was observed, suggesting the formation of Cs 3 Bi 2 I 9 nanocrystals. The assynthesized nanocrystals were collected via centrifugation at 12,000 rpm for 3 min, followed by dispersal in 2 ml of toluene for further characterization. Steady-State Measurements of Photoluminescence and Absorption. The as-prepared Cs 3 Bi 2 X 9 nanocrystals prepared via hot injection were diluted by factors of 4 and 40 in toluene for steady-state measurements of photoluminescence and absorption, respectively. We note that the samples in the original ODE solution were much more stable colloids than the purified samples, and therefore, the original samples were used for optical characterization. A Cary 5000 UV-vis spectrometer (Agilent Technologies) was used for absorption measurements in the range from 300 nm to 700 nm. A FluoroMax-4 spectrofluorometer (Horiba Scientific; a slit width of 10 nm and a scan rate of 500 nm/min) was used to record the photoluminescence spectra. The excitation wavelengths used for the Cs 3 Bi 2 X 9 nanocrystals were different for the different halides: Cs 3 Bi 2 I 9 was excited at 470 nm, Cs 3 Bi 2 Br 9 at 370 nm, and Cs 3 Bi 2 Cl 9 at 340 nm. Temperature-dependent photoluminescence measurements S2

3 were conducted using the same instrument but with a sample holder whose temperature was controlled by means of heated water circulation. Transmission Electron Microscopy. TEM images were acquired using a Tecnai transmission electron microscope with an acceleration voltage of 120 kev. HRTEM images were acquired using the same instrument. It is worth noting that the Cs 3 Bi 2 X 9 nanocrystals were not stable under irradiation with electron beams and tended to decompose within a short period of time (< 5 s). X-ray Diffraction Measurements. Powder X-ray diffraction was performed using a Bruker AXS D8 diffractometer with Cu-Ka radiation (λ = Å). The samples were prepared via the drop casting of the nanocrystal suspension onto a clean glass slide, followed by drying at room temperature. Nanosecond Transient Absorption. Nanosecond transient absorption measurements were collected using an Ultrafast Systems EOS transient absorption spectroscopy system, with a time resolution of 200 ps and a detection limit of 400 µs. For the experimental setup, a two-channel probe (probe-reference) method was used. In this method, a standard probe reference channel is attached, from which the probe beam is split into two before passing through the sample. While the beam from one arm travels through the sample, the other is sent directly to a reference spectrometer, which monitors the intensity fluctuations of the probe beam. The main advantage of this technique is that it allows the user to achieve a specified signal-to-noise ratio with fewer laser pulses on average. The pump pulses at 532 nm were generated by a Helios Laser System (Coherent). The pump and probe beams were focused on the sample, and the transmitted probe light from the sample was collected and focused on the broadband UV-Vis detector to record the time-resolved transient absorption spectra. Raman Spectrum. A LabRam Aramis Raman spectrometer (Horiba Jobin Yvon) equipped with a 477-nm Argon ion laser excitation source was employed to acquire vibrational Raman spectra of Cs 3 Bi 2 I 9 thin- S3

4 film. The Raman signal was acquired using an integration time of 10 s. Raman bands were observed in the fingerprint region from 100 to 600 cm -1 at a spectral resolution of 1.1 cm -1. Density Functional Theory Calculations. Density functional theory (DFT) calculations were used to provide additional details on the optical inter-band transitions. After optimizing the crystal structure of Cs 3 Bi 2 I 9 based on its experimental lattice parameters (a=b=8.30 Å and c=21.00 Å), we calculated the electronic bands and the density of states. As shown in Figure S7 of the Supporting Information, the general gradient approximation (GGA)/Perdew-Burke-Ernzerhof (PBE) functional without spin-orbit coupling (SOC) proved to predict the bandgap of Cs 3 Bi 2 I 9 (2.14 ev) well because of error cancellation, whereas the PBE functional with SOC retained the band curvature but resulted in more conduction band degeneracies and a greatly reduced bandgap (1.70 ev). Thus, we used the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional to correct the bandgap based on the calculated results to account for SOC effects, as shown in Figure 2d. We found that Cs 2 Bi 3 I 9 shows an indirect bandgap of 2.10 ev between the K and Γ symmetry points and a slightly larger direct bandgap of 2.23 ev at the Γ point of the Brillouin zone, in very good agreement with the experimental results deduced from the absorption and photoluminescence spectra. The projected density of states shows that the dominant contributions to the top of the valence band are from Bi-6s and I-5p, whereas the bottom of the conduction band mainly consists of Bi-6p, indicating that the absorption continuum might be due to inter-band electronic transitions around the Γ point, with the main contribution being from Bi 3+ (6s)I - (5p) Bi 3+ (6p). Computational Methods. Density functional theory (DFT) calculations were performed to optimize the crystal structures of Cs 3 Bi 2 I 9 using the projector-augmented wave (PAW) method as implemented in the VASP code. 2,3 The experimental hexagonal structures at room temperature, which belonged to the P63/mmc space group, were used as the initial guess. The Perdew-Burke-Ernzerhof (PBE) functional with and without spin-orbit coupling (SOC) as well as the hybrid functional Heyd- Scuseria-Ernzerhof (HSE) were used to obtain the electronic band structures and the density of states. The plane-wave cutoff energy was set to ev, and a Γ-centered K-mesh in S4

5 the Brillouin zone was employed. The crystal structure was fully relaxed when the total force on each atom was < 0.01 ev/å 1. A molecular graphics viewer (VESTA) was used to plot the crystal structure of Cs 3 Bi 2 I 9. S5

6 Figure S1. (a) The BiI 3 precursor before and after the injection of Cs-oleate. The color change from yellow to deep orange suggests the formation of Cs 3 Bi 2 I 9 nanocrystals. (b) The process of the formation of Cs 3 Bi 2 I 9 nanocrystals as the reaction time advances, indicating a LaMer nucleation mechanism. S6

7 Figure S2. (a) X-ray diffraction patterns of a Cs 3 Bi 2 I 9 thin film and a Cs 3 Bi 2 I 9 single crystal. (b) Absorption and photoluminescence spectra of the Cs 3 Bi 2 I 9 thin film, showing absorption and photoluminescence profiles similar to those of the Cs 3 Bi 2 I 9 nanocrystals. However, the photoluminescence measurements of the thin-film sample suffered from severe scattering problems. The inset shows a photograph of the as-prepared Cs 3 Bi 2 I 9 thin film on a glass substrate. S7

8 Figure S3. (a) Photoluminescence spectra of Cs 3 Bi 2 I 9 nanocrystals recorded at increasing temperatures with a step size of 10 C. Note that the samples were synthesized at 100 C. (b) The variation in the ratios of the intensities of the 605-nm peak and the 635-nm peak over that of the 580-nm peak. The 635-nm peak shows a temperature dependence similar to that of the 605-nm peak. S8

9 Figure S4. Photoluminescence spectra of 18-nm Cs 3 Bi 2 I 9 nanocrystals recorded during the heating (left panel) and cooling (right panel) processes. The essentially identical shapes of the spectra suggest a reversible temperature dependence, further confirming that the 605-nm peak stems from an indirect transition instead of from post-growth of Cs 3 Bi 2 I 9 particles or phase separation. S9

10 Figure S5. Photoluminescence profiles of Cs 3 Bi 2 I 9 nanocrystals with different concentrations. The multi-spectral feature essentially kept unaltered after dilution, indicating that the characteristics of photoluminescence are not relevant to the concentration-induced particle segregation. The slight shifting in PL peak may be due to the size decrease of particle after dilution. S10

11 Figure S6. (a) Symmetric k-points (A, H, L, Γ, M, and K) in the Brillouin zone and (b) the band structures of Cs 3 Bi 2 I 9 as calculated using the PBE functional without and with SOC effects. S11

12 Figure S7. Raman spectrum of Cs 3 Bi 2 I 9 thin-film, indicating a phonon mode of 18.6 mev in this material. The indirect transition requires at least one phonon participation to obey momentum conservation law. S12

13 Figure S8. Absorption and emission spectra of the Cs 3 Bi 2 I 9 nanocrystals and the Coumarin 6 reference dye. Based on these data, the quantum yield of the Cs 3 Bi 2 I 9 nanocrystals was estimated to be 0.017% using a reference method. 4 S13

14 Figure S9. Absorption and emission spectra of Cs 3 Bi 2 I 9 nanocrystals of different sizes. No quantum confinement effect was observed even when the size was reduced to 3 nm, indicating that the excitons have a strong binding energy and an extremely small Bohr radius. These 3-nm nanocrystals were obtained by decreasing the injection temperature to 75 C. S14

15 Figure S10. (a) Power dependent PL spectra of Cs 3 Bi 2 I 9 nanocrystals. (b) PL intensity at 580, 605, and 635 nm were plotted against power density. We speculate that 635-nm peak may originates from the same indirect bandgap transition of 605-nm peak but with assistance of different number of phonons. (c) Fitting results of the relationship between PL intensity and power density indicate a good linear correlation, suggesting the PL are originated from the exciton recombination, instead of defects. (d) The measurement was conducted in the spectrometer chamber (Agilent Technologies) with slight modification. Excitation was set at 470 nm from a Xe lamp with slit of 5 nm. S15

16 Figure S11. (a) Temperature dependent absorption spectra of CsPbBr 3 NCs and Cs 3 Bi 2 I 9 NCs. We used the CsPbBr 3 NCs as a standard direct-bandgap material as a comparison to Cs 3 Bi 2 I 9 NCs under investigation. (b) The optical densities of CsPbBr 3 NCs and Cs 3 Bi 2 I 9 NCs are plotted vs. temperature, which highlights the different evolutions of the transition probabilities under thermal activation. (c) Normalized absorption spectra show an unaltered absorption profile of CsPbBr 3 NCs and a red-shifted profile of Cs 3 Bi 2 I 9 NCs. The arrows denote an increase in optical density (OD) for the indirect-transition band at longer wavelength and a decrease in OD for the direct-transition band at shorter wavelength. S16

17 Figure S12. (a) The transient absorption profile at 1.02 ns is aligned with the steady-state absorption profile to illustrate the components from the direct-bandgap transition (green shadow) and indirect-bandgap transition (pink shadow). (b) Transient absorption spectra in the time scale ranging from 1.02 ns to 48 ns, showing a large overlap between the indirect-bandgap transition and excited-state absorption (ESA2). This overlap can be the reason why the indirect transition is not observed in TA measurements. S17

18 References (1) Liu, W.; Lin, Q.; Li, H.; Wu, K.; Robel, I.; Pietryga, J. M.; Klimov, V. I. Mn 2+ -Doped Lead Halide Perovskite Nanocrystals with Dual-Color Emission Controlled by Halide Content. J. Am. Chem. Soc. 2016, 138, (2) Kresse, G.; Furthmuller, J. Comput. Mater. Sci. Efficiency of ab-initio Total Energy Calculations for Metals and Semiconductors using a Plane-wave Basis Set. 1996, 6, (3) Kresse, G.; Joubert, D. Phys. Rev. B. From Ultrasoft Pseudopotentials to the Projector Augmented-wave Method. 1999, 59, (4) Leng, M.; Chen, Z.; Yang, Y.; Li, Z.; Zeng, K.; Li, K.; Niu, G.; He, Y.; Zhou, Q.; Tang, J. Lead-Free, Blue Emitting Bismuth Halide Perovskite Quantum Dots. Angew. Chem. Int. Ed. 2016, 55, S18