Ligand Assisted Transformation of Cubic CsPbBr3 Nanocrystals into 2

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Ligand Assisted Transformation of Cubic CsPbBr3 Nanocrystals into Two-Dimensional CsPb2Br5 Nanosheets Subila K. Balakrishnan and Prashant V. Kamat* Radiation Laboratory, Department of Chemistry & Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

O

copy confirmed the 1:1:3 atomic ratios for the prepared CsPbBr3 sample (EDX spectra and atomic composition is included in the Supporting Information, Figure S1). The chemical transformation of perovskite NCs through the ligand exchange was initiated with the addition of DDAB (250 μM) to CsPbBr3 NCs (100 μM) dispersed toluene. The changes in the absorption and emission spectra were monitored for 10 min (Figures 1B,C). Within the first minute, a marked change is seen in the absorption spectrum with decreased absorbance in the 400−520 nm region and increased absorbance at 320 nm. This initial transformation seen upon addition of DDAB represents the first step of ligand exchange. This change is then followed by gradual blue shift in the absorption shoulder (500 to 490 nm) with time (the magnified view of the absorption change can be seen in the inset of Figure 1B). Concurrently a blue-shift was also observed in the emission spectra (Figure 1 C) accompanied by a significant decrease in the emission quantum yield (from 70% to 4%). A closer look at the absorption changes in the 300−380 nm wavelength region reveals the existence of two distinct species (Figure 1D). In addition to the 320 nm band observed immediately after the addition of DDAB, a new band at 345 nm emerges with increasing time. The absorption at the 320 nm band corresponds to the [PbBr3]− complex21 and represents initial product of chemical transformation induced by DDAB within the first minute. Thus, the interaction between DDAB and CsPbBr3 results in quick exfoliation as Cs+ ions are exchanged with the long cationic alkyl chain leading to the formation of Pb complexes. With increasing time we observe the formation of [Pb2Br5]− complex with its broad absorption around 340 nm.22,23 As the system equilibrates, the lead halide complex reorganizes to form 2-D CsPb2Br5 perovskite in solution. A blue shift was observed in the emission spectra with time during the same time frame of system equilibration. It is known that the position of the valence band maximum (VBM) and conduction band minimum (CBM) in lead halide perovskite (ABX3) structure is governed by the corner sharing BX6 (e.g., PbBr6) in the perovskite structure.24,25 The extended sharing of PbBr6 octahedra in the lattice of the perovskite contributes to the band structure. As the DDAB ligand exfoliates the PbBr6 octahedra of CsPbBr3, we observe a blue shift in the absorption and band edge emission. At longer times we observe an

wing to their excellent photoluminescence and optoelectronic properties, metal halide hybrid perovskite nanocrystals (NCs) are gaining importance in high efficiency solar cells and light emitting diodes.1−3 The intrinsically soft structure and chemical instability renders these perovskite structures to undergo transformations with the exchange of cations or anions within the lattice. Such chemical exchanges have often been utilized to tune their photophysical properties.4−8 For example, halide ion exchange reactions produce mixed halide perovskites with a tunable band gap covering the entire visible spectrum.4,9−11 Additionally, the cation exchange in methylammonium lead halide (MAPbX) perovskite nanocrystals (NCs) with long alkyl chain cations can produce hybrid two-dimensional (2-D) perovskites.12−15 By controlling the concentration and composition of ligands (e.g., oleic acid (OA) and oleylamine (OM)), it is also possible to alter the shapes of perovskite structure, from zero-dimensional quantum dots to 2D nanoplatelets or nanosheets.16−18 We now present dodecyl dimethylammonium bromide (DDAB) induced transformation of CsPbBr3 NCs into crystalline 2-D CsPb2Br5 nanosheets and the mechanistic insights associated with morphological changes. CsPbBr3 perovskite nanocrystallites exhibit three different phases: orthorhombic (Phnm), tetragonal (P4/mbm), and cubic (Pm3m).19,20 The cubic phase obtained at higher temperatures (≥130 °C) exhibits good stability. We employed a hot injection method to obtain cubic CsPbBr3 NCs by reacting Cs-oleate with Pb(II)-bromide at 170 °C. The PbBr2 (0.19 mmol) was dissolved in octadecene (high boiling point solvent, 5 mL) containing OM (1.52 mmol) and OA (1.58 mmol) as ligands under an argon atmosphere. Cs-oleate (0.046 mmol) was then quickly injected at 170 °C, and the NCs were precipitated using a butanol−acetone (1:2) mixture followed by centrifugation. The purified NCs were then redispersed in the toluene (synthetic details are included in the Supporting Information, section 3). The absorption and emission spectra of colloidal CsPbBr3 in toluene are presented in Figure 1A. The CsPbBr3 NCs in toluene exhibit band edge (absorption shoulder) around 500 nm and emission wavelength maximum at 507 nm. The CsPbBr3 NCs exhibit bright photoluminescence (quantum yield = 70%) with a full width at half-maximum (fwhm) of 22 nm indicating the NCs to be monodisperse. The transmission electron microscopy (TEM) analysis further confirms that the NCs are monodisperse with an estimated size of 7 ± 2 nm (Figure 2A). The d-spacing analysis indicates that the nanocrystals possess (001) plane with a cubic structure. The elemental analyses by energy dispersive X-ray (EDX) spectros© 2017 American Chemical Society

Received: September 29, 2017 Revised: November 9, 2017 Published: November 9, 2017 74

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Figure 1. (A) Absorption (a, black trace) and emission (b, red trace) spectra of CsPbBr3 nanocrystals in toluene (3 mL). (B) Absorption and (C) normalized emission spectra (excitation wavelength 380 nm) recorded with a time interval of 1 min after the addition of DDAB (250 μM) to CsPbBr3 NCs in toluene: (a) before adding DDAB and (b−k) after addition of DDAB. The magnified region of absorption at the band edge is shown in the inset of B. (D) Zoomed section of absorption spectra in the 300−360 nm wavelength region during the reaction with DDAB.

Figure 2. TEM and HRTEM images of CsPbBr3 NCs (Panels A, B, C) and DDAB treated CsPbBr3 sample (Panels D, E, F) at different magnifications. The d-spacing value is inserted in the HRTEM image of a single CsPbBr3. d-spacing analysis was carried out with single crystal of CsPbBr3 (Panel C) and a small area of 2-D sheet (Panel F). The selected area diffraction pattern collected during HRTEM analysis is included in the insets of panels B and E.

as well as optical properties.23,30−32 The blank experiments showing the ability of Pb2+ to complex with Br− in a solution containing DDAB and spectral changes associated with the formation of [PbBr3]− and [PbBr4]2− complexes are shown in the Supporting Information (Figure S4). A decrease in the 285 nm band of PbBr2 (1 mM) with a concurrent increase in the absorption at 310 nm indicated the formation of PbBr3− complex following the addition of 250 μM of DDAB. Upon increasing the concentration of DDAB to 500 μM, a new peak at 360 nm corresponding to [PbBr4]2− emerges. These control experiments indicate the formation of lead bromide complex in a Pb2+ system containing excess bromide (DDAB). However, when DDAB is added to presynthesised CsPbBr3 NCs we initially observe formation of the PbBr3− complex. But this

evolution of second emission band with maximum at 523 nm (Figure 1C,k). This lower energy emission band seen at 523 nm is rather weak, making a smaller contribution to the overall quantum yield of 4%. The existence of dual emissions have been noted for other metal halide nanocrystals. Jiang and coworkers reported CsPb2Br5 nanosheets with large indirect band gap and a PL inactive behavior.26 On the other hand Han et al. synthesized green emissive CsPb2Br5 nanoplatelets.27 The emission properties of CsPb2Br5 have yet to be established fully because of the existence of direct and indirect transitions.28,29 We have earlier studied the complexation of Pb2+ and halide ions in the precursor solution to understand the evolution of the overall perovskite structure in achieving desired electronic 75

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Figure 3. Schematic representation of ligand-assisted exfoliation and transformation of cubic CsPbBr3 perovskite nanocrystals to tetragonal CsPb2Br5 nanosheets.

complex soon reorganizes to form a stable [Pb2Br5]− species with absorption at wavelength ∼345 nm. The presence of isosbestic points at 320 and 345 nm further indicates evolution of CsPb2Br5 from initially formed DDA+[PbBr3−] complex (Reactions 1 and 2).

of the 2-D structure. The diffraction spots associated with the (220) and (200) crystal planes could be indexed to the [001] zone axis of tetragonal CsPb2Br5.26,33 The d-spacing analysis carried out on the HRTEM images of nanosheets show a value of 0.306, matching with the value of the (220) plane of the tetragonal structure. Note that this value differs from the value of 0.576 nm, corresponding to the (001) plane of cubic perovskite. The HRTEM analysis indicates a tetragonal structure (Supporting Information, Figures S2 and S3) for the newly derived 2-D nanosheets. The XRD spectrum of CsPbBr3 nanocrystals (Supporting Information Figure S9) showed a peak at 2θ = 14.9° and 30.2° corresponding to (100) and (200) planes, respectively, as per the cubic crystal structure (ICSD 29073). The DDAB treated sample shows the disappearance of the XRD peak at 2θ = 14.9° (100) and the appearance of new diffraction peaks at 2θ = 12, 24, 29.3. These peaks are assigned to (002), (202), and (213) planes, consistent with the standard PXRD patterns of tetragonal CsPb2Br5 (PDF no. 25-0211). We also probed the influence of alkyl chain length on the selective transformation of CsPbBr3 nanocrystals. The absorption spectra recorded following the addition of butylammonium bromide (BAB) and methylammonium bromide (MAB) to CsPbBr3 NCs suspension are shown in the Supporting Information (Figure S10). The butyl ammonium cation (BAB, 250 μM) initiates the exfoliation of perovskite NCs by forming PbBr3− complex as seen from the appearance of the absorption band at 320 nm. However, additional spectral changes that can be associated with CsPb2Br5 formation could not be seen upon leaving the solution for equilibration. The TEM analysis also failed to show the presence of 2-D nanosheets in BAB treated samples. The treatment of CsPbBr3 NCs with MAB (500 μM) failed to exhibit noticeable spectral changes, an indication that CsPbBr3 is not susceptible to ligand exchange with MAB. Thus, the length of the alkyl chain becomes important as it drives the reorganization to form 2D structures. This process is analogous to the formation of layered double hydroxides.36 Thus, we need ammonium salt with long alkyl chain (e.g., DDAB) to induce the exfoliation followed by reorganization (Figure 3). The structural sensitivity to different chemical environments has been tested by other research groups. The synthesis of green luminescent tetragonal CsPb2Br5 nanoplatelets was reported by Guopeng Li et al., who studied the formation of CsPb2Br5 nanosheets from the CsPbBr3 nanocubes by reacting with the excess PbBr2 precursor.26 They showed that it is thermodynamically favorable to undergo reorganization of

nCsPbBr3 + DDA+ → (n − x)CsPbBr3 + x DDA+[PbBr3]− + xCs+

(1)

2x DDA+[PbBr3]− + xCs+ → x[CsPb2 Br5] + x DDA+Br − + x DDA+

(2)

The selection of the ligand cation is important for inducing exfoliation of the perovskite lattice. If we replace dodecyl ammonium bromide (DDAB) with dodecyl amine (DDA) we fail to observe similar transformations. The spectral behavior monitored over 10 min upon addition of DDA to CsPbBr3 nanocrystal suspension did not show any significant changes in the band edge absorption (Supporting Information, Figure S5). The perovskite nanocrystals failed to exhibit morphological changes as evident from the TEM analysis (Supporting Information, Figure S6). The above results indicate that the ammonium cation of the long alkyl chain is essential to trigger the ligand-assisted exfoliation. The morphological changes associated with the DDAB interaction with perovskite NCs were analyzed through the TEM analysis. Figure 2 shows the TEM images of NCs, before and after 10 min of DDAB treatment. Transformation of cubic CsPbBr3 NCs into two-dimensional nanosheets with lateral sizes on the micrometer scale could be seen in these images. The shape and size of these 2-D crystallites are significantly different than those of the CsPbBr3 NCs (7 nm) prior to the DDAB treatment. Owing to their large lateral dimension (in micrometers), these nanosheets tend to lie flat on the TEM grid. On the other hand, the TEM images of the intermediate sample confirm dissolution of perovskite NCs (Supporting Information Figure S1B). The TEM images of samples corresponding to 1 min of DDAB treatment did not show any NCs or 2-D sheets. These results suggest that the transformation to obtain 2-D structure is not instantaneous but requires additional time (∼10 min) to achieve the stabilized configuration. The time dependent TEM analysis and spectral changes presented here confirm the two step mechanism of dissolution and reorganization of perovskite nanocrystals (Reactions 1 and 2) into 2-D nanosheets. The selected area diffraction (SAED) pattern obtained for the nanosheet (Panel F, Figure 2) exhibits the crystalline nature 76

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(2) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High-Performance Photovoltaic Perovskite Layers Fabricated through Intramolecular Exchange. Science 2015, 348, 1234−1237. (3) Kamat, P. V. Organometal Halide Perovskites for Transformative Photovoltaics. J. Am. Chem. Soc. 2014, 136, 3713−3714. (4) Hoffman, J. B.; Schleper, A. L.; Kamat, P. V. Transformation of Sintered CsPbBr3 Nanocrystals to Cubic CsPbI3 and Gradient CsPbBrxI3−x through Halide Exchange. J. Am. Chem. Soc. 2016, 138, 8603−8611. (5) van der Stam, W.; Geuchies, J. J.; Altantzis, T.; van den Bos, K. H. W.; Meeldijk, J. D.; Van Aert, S.; Bals, S.; Vanmaekelbergh, D.; de Mello Donega, C. Highly Emissive Divalent-Ion-Doped Colloidal CsPb1−xMxBr3 Perovskite Nanocrystals through Cation Exchange. J. Am. Chem. Soc. 2017, 139, 4087−4097. (6) Kuang, X.; Claridge, J. B.; Price, T.; Iddles, D. M.; Rosseinsky, M. J. Isolation of Two-Dimensional 2:1 Cation-Ordered Perovskite Units by Anion Vacancy Ordering in Ba6Na2Nb2P2O17. Inorg. Chem. 2008, 47, 8444−8450. (7) Giorgi, G.; Fujisawa, J.-I.; Segawa, H.; Yamashita, K. Cation Role in Structural and Electronic Properties of 3D Organic−Inorganic Halide Perovskites: A DFT Analysis. J. Phys. Chem. C 2014, 118, 12176−12183. (8) Swarnkar, A.; Ravi, V. K.; Nag, A. Beyond Colloidal Cesium Lead Halide Perovskite Nanocrystals: Analogous Metal Halides and Doping. ACS Energy Lett. 2017, 2, 1089−1098. (9) Nedelcu, G.; Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Grotevent, M. J.; Kovalenko, M. V. Fast Anion-Exchange in Highly Luminescent Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, I). Nano Lett. 2015, 15, 5635−5640. (10) Jang, D. M.; Park, K.; Kim, D. H.; Park, J.; Shojaei, F.; Kang, H. S.; Ahn, J.-P.; Lee, J. W.; Song, J. K. Reversible Halide Exchange Reaction of Organometal Trihalide Perovskite Colloidal Nanocrystals for Full-Range Band Gap Tuning. Nano Lett. 2015, 15, 5191−5199. (11) Balakrishnan, S. K.; Kamat, P. V. Au−CsPbBr3 Hybrid Architecture: Anchoring Gold Nanoparticles on Cubic Perovskite Nanocrystals. ACS Energy Lett. 2017, 2, 88−93. (12) Cao, D. H.; Stoumpos, C. C.; Farha, O. K.; Hupp, J. T.; Kanatzidis, M. G. 2D Homologous Perovskites as Light-Absorbing Materials for Solar Cell Applications. J. Am. Chem. Soc. 2015, 137, 7843−7850. (13) Stoumpos, C. C.; Cao, D. H.; Clark, D. J.; Young, J.; Rondinelli, J. M.; Jang, J. I.; Hupp, J. T.; Kanatzidis, M. G. Ruddlesden−Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors. Chem. Mater. 2016, 28, 2852−2867. (14) Sun, S.; Yuan, D.; Xu, Y.; Wang, A.; Deng, Z. Ligand-Mediated Synthesis of Shape-Controlled Cesium Lead Halide Perovskite Nanocrystals via Reprecipitation Process at Room Temperature. ACS Nano 2016, 10, 3648−3657. (15) Kumar, S.; Jagielski, J.; Yakunin, S.; Rice, P.; Chiu, Y.-C.; Wang, M.; Nedelcu, G.; Kim, Y.; Lin, S.; Santos, E. J. G.; Kovalenko, M. V.; Shih, C.-J. Efficient Blue Electroluminescence Using QuantumConfined Two-Dimensional Perovskites. ACS Nano 2016, 10, 9720−9729. (16) Liang, Z.; Zhao, S.; Xu, Z.; Qiao, B.; Song, P.; Gao, D.; Xu, X. Shape-Controlled Synthesis of All-Inorganic CsPbBr3 Perovskite Nanocrystals with Bright Blue Emission. ACS Appl. Mater. Interfaces 2016, 8, 28824−28830. (17) Amgar, D.; Aharon, S.; Etgar, L. Inorganic and Hybrid OrganoMetal Perovskite Nanostructures: Synthesis, Properties, and Applications. Adv. Funct. Mater. 2016, 26, 8576−8593. (18) Sichert, J. A.; Tong, Y.; Mutz, N.; Vollmer, M.; Fischer, S.; Milowska, K. Z.; García Cortadella, R.; Nickel, B.; Cardenas-Daw, C.; Stolarczyk, J. K.; Urban, A. S.; Feldmann, J. Quantum Size Effect in Organometal Halide Perovskite Nanoplatelets. Nano Lett. 2015, 15, 6521−6527. (19) Zhang, X.; Xu, B.; Zhang, J.; Gao, Y.; Zheng, Y.; Wang, K.; Sun, X. W. All-Inorganic Perovskite Nanocrystals for High-Efficiency Light

PbBr6 octahedrons to PbBr8 capped-triangular prisms and form metastable tetragonal CsPb2Br5 structures. Recently, Alivisatos and co-workers studied the role of ligands such as amines and thiols in transforming CsPbBr3 NCs to Cs4PbBr6 NCs.34,35 The atomic ratio of the nanocrystals was calculated from the XPS analysis (details are included in the Supporting Information (Figures S7 and S8, Table S1)). The ratios of Pb/Cs and Br/Pb obtained for CsPbBr3 nanocrystals were 1.1 and 2.85, respectively, and they are in good agreement with the expected empirical ratio of 1:1:3. Interestingly, the ratios of Pb/ Cs and Br/Pb for 2D nanosheets (viz., after DDAB treatment) were 2.12 and 2.48, respectively, indicating a composition of 1:2:5. The atomic weight percentage estimated from TEM-EDS analysis of the nanosheets also shows the ratio of Pb:Cs to be 2 (EDS data is tabulated in the Supporting Information). These results further support the transformation of CsPbBr3 NCs to CsPb2Br5 nanosheets. In summary, ligand-assisted transformation of CsPbBr3 NCs to 2-D CsPb2Br5 nanosheets offers a simple technique to alter the morphology of perovskite structures. Such structural transformations are likely to be important for surface treatment to passivate defect sites and design 3-D/2-D interfaces.37 By subjecting CsPbBr3 films to controlled surface treatment with DDAB it should be possible to create such interfaces. As demonstrated recently, creation of the 3-D/2-D interface improves the photovoltaic and optoelectronic properties of perovskite solar cells.38,39

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04142. Experimental details of CsPbBr3NC synthesis, instruments and techniques employed, TEM images and EDX analysis, X-ray photoelectron spectroscopy (XPS), and XRD analysis control experiment with BAB and MAB (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(P.V.K.) E-mail: [email protected]. ORCID

Prashant V. Kamat: 0000-0002-2465-6819 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research described herein was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Basic Energy Sciences, Office of Science, U.S. Department of Energy, through Grant No. DE-FC02-04ER15533. This is document no. NDRL 5188 from Notre Dame Radiation Laboratory. We would like to thank Dr. Sergei Rouvimov of Notre Dame Integrated Imaging Facility Advanced Electron Microscopy Core for his assistance in TEM and EDX measurements and Steven Kobosko for his help in XPS measurements

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REFERENCES

(1) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542−546. 77

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Chemistry of Materials Emitting Diodes: Dual-Phase CsPbBr3-CsPb2Br5 Composites. Adv. Funct. Mater. 2016, 26, 4595−4600. (20) Wang, K.-H.; Wu, L.; Li, L.; Yao, H.-B.; Qian, H.-S.; Yu, S.-H. Large-Scale Synthesis of Highly Luminescent Perovskite-Related CsPb2Br5 Nanoplatelets and Their Fast Anion Exchange. Angew. Chem., Int. Ed. 2016, 55, 8328−8332. (21) Oldenburg, K.; Vogler, A. Electronic-Spectra and Photochemistry of Tin(II), Lead(II), Antimony(III), and Bismuth(III) Bromide Complexes in Solution. Z. Naturforsch., B: J. Chem. Sci. 1993, 48, 1519−1523. (22) Nitsch, K.; Dušek, M.; Nikl, M.; Polák, K.; Rodová, M. Ternary Alkali Lead Chlorides: Crystal Growth, Crystal Structure, Absorption and Emission Properties. Prog. Cryst. Growth Charact. Mater. 1995, 30, 1−22. (23) Yoon, S. J.; Stamplecoskie, K. G.; Kamat, P. V. How Lead Halide Complex Chemistry Dictates the Composition of Mixed Halide Perovskites. J. Phys. Chem. Lett. 2016, 7, 1368−1373. (24) Ravi, V. K.; Markad, G. B.; Nag, A. Band Edge Energies and Excitonic Transition Probabilities of Colloidal CsPbX3 (X = Cl, Br, I) Perovskite Nanocrystals. ACS Energy Lett. 2016, 1, 665−671. (25) Umebayashi, T.; Asai, K.; Kondo, T.; Nakao, A. Electronic Structures of Lead Iodide Based Low-Dimensional Crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 155405. (26) Li, G.; Wang, H.; Zhu, Z.; Chang, Y.; Zhang, T.; Song, Z.; Jiang, Y. Shape and Phase Evolution from CsPbBr3 Perovskite Nanocubes to Tetragonal CsPb2Br5 Nanosheets with an Indirect Bandgap. Chem. Commun. 2016, 52, 11296−11299. (27) Han, C.; Li, C.; Zang, Z.; Wang, M.; Sun, K.; Tang, X.; Du, J. Tunable Luminescent CsPb2Br5 Nanoplatelets: Applications in LightEmitting Diodes and Photodetectors. Photonics Res. 2017, 5, 473−480. (28) Dursun, I.; De Bastiani, M.; Turedi, B.; Alamer, B.; Shkurenko, A.; Yin, J.; El-Zohry, A. M.; Gereige, I.; AlSaggaf, A.; Mohammed, O. F.; Eddaoudi, M.; Bakr, O. M. CsPb2Br5 Single Crystals: Synthesis and Characterization. ChemSusChem 2017, 10, 3746−3749. (29) Zhang, Y.; Yin, J.; Parida, M. R.; Ahmed, G. H.; Pan, J.; Bakr, O. M.; Brédas, J.-L.; Mohammed, O. F. Direct-Indirect Nature of the Bandgap in Lead-Free Perovskite Nanocrystals. J. Phys. Chem. Lett. 2017, 8, 3173−3177. (30) Yoon, S. J.; Draguta, S.; Manser, J. S.; Sharia, O.; Schneider, W. F.; Kuno, M.; Kamat, P. V. Tracking Iodide and Bromide Ion Segregation in Mixed Halide Lead Perovskites during Photoirradiation. ACS Energy Lett. 2016, 1, 290−296. (31) Manser, J. S.; Reid, B.; Kamat, P. V. Evolution of Organic− Inorganic Lead Halide Perovskite from Solid-State Iodoplumbate Complexes. J. Phys. Chem. C 2015, 119, 17065−17073. (32) Stamplecoskie, K. G.; Manser, J. S.; Kamat, P. V. Dual Nature of the Excited State in Organic-Inorganic Lead Halide Perovskites. Energy Environ. Sci. 2015, 8, 208−215. (33) Beck, H. P.; Clicqué, G.; Nau, H. A Study on AB2 × 5 Compounds (A: K, In, Tl; B: Sr, Sn, Pb; X: Cl, Br, I). Z. Anorg. Allg. Chem. 1986, 536, 35−44. (34) Sakai, N.; Haghighirad, A. A.; Filip, M. R.; Nayak, P. K.; Nayak, S.; Ramadan, A.; Wang, Z.; Giustino, F.; Snaith, H. J. SolutionProcessed Cesium Hexabromopalladate(IV), Cs2PdBr6, for Optoelectronic Applications. J. Am. Chem. Soc. 2017, 139, 6030−6033. (35) Liu, Z.; Bekenstein, Y.; Ye, X.; Nguyen, S. C.; Swabeck, J.; Zhang, D.; Lee, S.-T.; Yang, P.; Ma, W.; Alivisatos, A. P. Ligand Mediated Transformation of Cesium Lead Bromide Perovskite Nanocrystals to Lead Depleted Cs4PbBr6 Nanocrystals. J. Am. Chem. Soc. 2017, 139, 5309−5312. (36) Ma, R.; Liu, Z.; Takada, K.; Iyi, N.; Bando, Y.; Sasaki, T. Synthesis and Exfoliation of Co2+−Fe3+ Layered Double Hydroxides: An Innovative Topochemical Approach. J. Am. Chem. Soc. 2007, 129, 5257−5263. (37) ten Brinck, S.; Infante, I. Surface Termination, Morphology, and Bright Photoluminescence of Cesium Lead Halide Perovskite Nanocrystals. ACS Energy Lett. 2016, 1, 1266−1272. (38) Grancini, G.; Roldán-Carmona, C.; Zimmermann, I.; Mosconi, E.; Lee, X.; Martineau, D.; Narbey, S.; Oswald, F.; De Angelis, F.;

Graetzel, M.; Nazeeruddin, M. K. One-Year Stable Perovskite Solar Cells by 2D/3D Interface Engineering. Nat. Commun. 2017, 8, 15684. (39) Rodríguez-Romero, J.; Hames, B. C.; Mora-Seró, I.; Barea, E. M. Conjugated Organic Cations to Improve the Optoelectronic Properties of 2D/3D Perovskites. ACS Energy Lett. 2017, 2, 1969−1970.

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