Communication pubs.acs.org/cm
Compositionally Dependent Phase Identity of Colloidal CsPbBr3−xIx Quantum Dots Patrick Cottingham and Richard L. Brutchey* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *
ince the first report of colloidal CH3NH3PbX3 (where X = Cl−, Br−, or I−) quantum dots (QDs), this family of halide perovskite QDs has become a rapidly developing subfield within the area of halide perovskite semiconductors.1 Relative to the analogous bulk materials, perovskite QDs offer several distinct advantages, including bright photoluminescence (PL) with narrow spectral width and size-tunable band gaps. Recent work in this area has extended the colloidal synthesis to materials with the formula CsPbX3, as well as a variety of layered perovskite derivatives.2,3 Lasing materials,4 phosphors,5 and LEDs6 based on these halide perovskite QDs have already been demonstrated. Despite the clear utility of these materials, and considerable research into modifying their dimensionality and optoelectronic properties, some fundamental aspects of their crystal structure remain unresolved. Initial reports on both CH3NH3PbX3 and CsPbX3 QDs suggested that they possess a cubic Pm3̅m crystal structure based on standard laboratory Xray diffraction, which is different from their room temperature bulk structures.1,2,7 Later work using synchrotron X-ray total scattering data identified that the thermodynamically preferred Pnma orthorhombic distortion present in bulk CsPbBr3 at room temperature is also unequivocally present in ∼7 and 13 nm CsPbBr3 QDs.8 This orthorhombic distortion has also been observed in CsPbBr3 nanowires.9 Many of the remaining questions about the crystal structure of CsPbX3 QDs concern the mixed halide compositions. A continuous solid solution between CsPbBr3 and CsPbI3 has been reported for QDs based on laboratory X-ray diffraction data that appear single-phase with optical properties that are consistent with PL from a single, well-defined band-edge.2 The apparent alloying in colloidal CsPbBr3−xIx QDs is in contrast to the case for bulk CsPbBr3−xIx, which has been reported to possess a two-phase region for some range of intermediate compositions.10 Unfortunately, the analysis of X-ray diffraction (XRD) data collected on small colloidal QDs is complicated by extreme Scherrer size broadening as well as large background signals resulting from amorphous organic ligands. This is especially problematic in the analysis of low-symmetry structures and multiphase materials. As such, pair distribution function (PDF) analysis of X-ray total scattering data, which examines local structure, has proven valuable for investigating the crystal structure of nanoscale halide perovskites.8,11 To investigate the crystal structure of colloidal mixed-halide CsPbBr3−xIx QDs (where 0 ≤ x ≤ 3), a series of five materials with varying Br:I ratio was produced according to the hotinjection method of Protesescu et al.2 Figure 1a illustrates the UV−vis absorption and PL spectra of freshly prepared colloidal CsPbBr3−xIx QDs, which demonstrate a continuously increasing
S
© 2016 American Chemical Society
Figure 1. (a) UV−visible optical absorption (solid line) and photoluminescence spectra (dashed line) of QDs prepared by the hot injection method. (b) Laboratory XRD (Cu Kα = 1.54 Å) patterns of the corresponding CsPbBr3−xIx QDs. All compositions are nominal.
band edge and single band edge emission feature with increasing x, as previously reported.2 The nonlinear variation in the band gap with halide composition is commonly observed in halide perovskites and has recently been attributed to phase segregation in bulk and nanoparticulate halide perovskites.12−14 TEM images are also consistent with previous reports, showing monodisperse particles with cuboidal morphology (Supporting Information, Figure S1).2 Although the optical properties of the colloidal CsPbI3 QDs are clearly consistent with the high-temperature, black cubic phase,15 it known that these CsPbI3 QDs will gradually undergo a phase transition to the wide-gap (Eg = 2.3 eV16), yellow orthorhombic phase,2 which is also the thermodynamically preferred phase of bulk CsPbI3 at room temperature.15 Although the bulk orthorhombic structures of CsPbBr3 and CsPbI3 are both described by the Pnma space group, the relaxed structure of yellow CsPbI3 is distinct from the Pnma structure of CsPbBr3. The former consists of quasi-1D chains of PbI6 octahedra (Figure 2a), whereas the latter is a 3D perovskite derivative formed by precession of the cornersharing PbBr6 octahedra. For consistency with the solid-state literature, the orthorhombic 3D perovskite derivative phase will be referred to below as the γ-phase and the phase containing quasi-1D chains will be referred to as the δ-phase.15,17−19 Received: August 23, 2016 Revised: October 15, 2016 Published: October 19, 2016 7574
DOI: 10.1021/acs.chemmater.6b03553 Chem. Mater. 2016, 28, 7574−7577
Communication
Chemistry of Materials
Table 1. Refined Lattice Constants and Quality-of-Fit Values for Rietveld Refinements and PDF Model Fits to Synchrotron X-ray Total Scattering Data for CsPbBr3−xIx QDs Rietveld refinement composition
a (Å)
b (Å)
CsPbI3 CsPbBrI2 CsPbBr3
10.4699(1) 10.3727(1) 8.323(4)
c (Å)
4.80134(3) 17.7923(2) 4.7758(5) 17.6740(2) 11.834(6) 8.215(4) PDF model fit
Rwp
χ2
3.13 3.83 3.00
2.57 4.50 1.86
composition
a (Å)
b (Å)
c (Å)
Rw
CsPbI3 CsPbBrI2 CsPbBr1.5I1.5 δ-phase γ-phase CsPbBr2I δ-phase γ-phase CsPbBr3
10.45 10.50
4.79 4.75
17.78 17.88
21.1 17.4
10.33 8.46
4.71 12.14
17.78 8.26
19.5
10.30 8.56 8.43
4.74 11.90 11.7
17.57 8.15 8.17
18.6 13.6
(Figure 2b). The CsPbBrI2 QD composition also appears to possess the δ-structure. A high quality-of-fit was obtained by assuming that the Br atoms are distributed evenly across the three halogen sites of the δ-structure. This was modeled by refining to the δ-structure using a pseudoatom on each halide position with a scattering factor equivalent to the weighted average scattering factor of 1 Br and two I atoms. Models based on these same structures (δ-CsPbI3 and δ-CsPbBrI2 with evenly distributed halides) also provided excellent fits to the PDFs extracted from the synchrotron X-ray total scattering data (Figure 2c). Attempts to fit the diffraction and total scattering data for CsPbI3 or CsPbBrI2 with a single cubic or γ-phase with iodine substituted for bromine resulted in refinements that rapidly diverged to unphysical values. To confirm that conversion to the δ-structure was not induced by synchrotron radiation, a sample of CsPbI3 was allowed to age before measuring with a laboratory diffractometer using Cu Kα radiation. Nearly full conversion to the δ-structure was observed within 3 days (Supporting Information, Figure S3). A qualitative examination of the synchrotron XRD pattern for CsPbBr1.5I1.5 QDs suggests that it consists of a superposition of distinct diffraction maxima observed in the δstructure of CsPbI3 and CsPbBrI2 and the γ-structure of CsPbBr3. In particular, both the intense (202) reflection of CsPbBr3 at 2θ = 2.75° and the cluster of reflections between 2.3 and 2.6° 2θ in the XRD pattern of CsPbBrI2 appear to be present in the pattern of CsPbBr1.5I1.5 (Figure 3a). The slight shift of the peak cluster to higher 2θ values may be the result of further incorporation of Br into the structure of CsPbBrI2, leading to a contraction of the lattice parameters. Likewise, the slight shift of the (202) reflection of CsPbBr3 to lower 2θ values may indicate the incorporation of some I into the γ-structure of CsPbBr3, leading to an expansion of the lattice. Moreover, the synchrotron XRD pattern of the CsPbBr2I1 QDs appears to qualitatively agree with that of γ-CsPbBr3 with some degree of peak shifting to lower 2θ values with the incorporation of I (Figure 3a). Unfortunately, neither of these synchrotron XRD patterns could be quantitatively fit owing to a large number of broad, overlapping reflections.
Figure 2. (a) Cubic and δ-structures of CsPbI3. (b) Synchrotron X-ray diffraction patterns of CsPbI3 and CsPbBrI2 QDs with the results of Rietveld refinements using a single δ-phase. (c) PDFs extracted from synchrotron X-ray total scattering data for CsPbI3 and CsPbBrI2 QDs and model fits using a single, δ-phase. λ = 0.143 Å for all synchrotron experiments. All compositions are nominal.
For powder XRD measurements, the freshly synthesized QDs were washed in isopropyl alcohol and briefly dried under vacuum to remove residual high boiling solvent, which contributes to the large, diffuse background signal (i.e., the broad feature centered at 2θ ≈ 20°). These diffraction patterns are qualitatively similar to what has been previously reported;2,20 however, it is impossible to distinguish whether the crystal structure of the QDs is cubic or the lower symmetry γ-structure based on this broadened laboratory XRD data (Figure 1b). To elucidate better the structural details of the CsPbBr3−xIx QDs, room temperature synchrotron X-ray total scattering measurements were performed on these mixed-halide QDs using the 11-ID-B instrument at the Advanced Photon Source (APS) at Argonne National Laboratory. Structural parameters and the complete results of crystallographic refinements are provided in the Supporting Information. All of the synchrotron diffraction data possess a very large asymmetric background with distinct features centered at 2θ ≈ 2°(Supporting Information, Figure S2). To avoid artifacts produced by fitting background features, all Rietveld refinements were performed over the range of 3° ≤ 2θ ≤ 10°. Lattice parameters and quality-of-fit values for all Rietveld refinements are given in Table 1, where Rwp is the weighted profile R-factor and χ2 = (Rwp/Rexp)2 where Rexp is Rwp for a perfect model. The qualityof-fit values compare favorably to those previously reported for fits to 11-ID-B data collected on CsPbBr3 QDs.8 Beginning with the iodide-rich CsPbI3 and CsPbBrI2 compositions, the high-resolution synchrotron XRD data collected 1 week after synthesis indicates that the CsPbI3 QDs had fully converted to the yellow orthorhombic phase 7575
DOI: 10.1021/acs.chemmater.6b03553 Chem. Mater. 2016, 28, 7574−7577
Communication
Chemistry of Materials
Figure 4. A quasi-phase diagram for the CsPbBr3−xIx QDs. The purple region indicates the approximate composition range where two phases coexist.
Figure 3. (a) Selected area of the diffraction patterns for CsPbBr3−xIx QDs for 0 ≤ x ≤ 2, illustrating that reflections from both the δstructure and the γ-structure are present in the pattern of CsPbBr1.5I1.5. Tick marks indicate the positions and relative intensities of individual reflections belonging to each structure. (b) PDFs (black) extracted from synchrotron X-ray total scattering data collected on CsPbBr1.5I1.5 and CsPbBr2I along with two-phase model fits (blue) to the PDF. The lines at the bottom of the panels indicate the difference between the PDF and the fit. λ = 0.143 Å for all synchrotron experiments. All compositions are nominal.
state NMR suggesting that similar halide segregation occurs in bulk CH3NH3PbX3.14,21 The exact halide ratio of these phases is unclear. It is possible that the γ-structure of CsPbBr3 can accommodate some I and that the δ-structure may accommodate some additional Br, such that x < 2 for this phase in the two phase region. Alternately, competition with growing γphase domains for solvated or mobile Br ions may reduce the Br content of the δ-structure in the two-phase region. The CsPbBr3 QDs exist in the γ-phase featuring precessed PbBr6 octahedra relative to the ideal cubic perovskite structure. We specify that Figure 4 is a quasi-phase diagram because it is unclear whether the two-phase region represents the thermodynamic minimum for CsPbBr3−xIx QDs with these compositions at room temperature, or whether those compositions are kinetically trapped as two phases. Specialquasirandom-structure (SQS) calculations have predicted that the formation of bulk CsPbBrI2 is favorable at room temperature but the formation of CsPbBr1.5I1.5 and CsPbBr2I is less so, consistent with our observations for the QDs.22 However, both solid-state20 and solution23 anion exchange methods have been used to convert CsPbBr3 QDs into CsPbI3 while apparently preserving the orthorhombic or cubic CsPbBr3 crystal structure, suggesting that reorganization of metastable CsPbBr3−xIx phases can be kinetically limited in QDs. It is unclear whether CsPbBr3 QDs exhibit a tetragonal phase at moderately elevated temperature before transitioning to the high-temperature cubic structure, as is the case with bulk CsPbBr3.24 The line delineating the cubic and lower-symmetry phase regions in Figure 4 is not isothermal to reflect the higher temperature of the δ-to-cubic transition in bulk CsPbI3 (330 °C)25 compared with the γ-to-tetragonal and tetragonal-tocubic transitions in bulk CsPbBr3 (88 and 130 °C, respectively). In conclusion, we have demonstrated that QDs formed via the widely used hot injection method are two-phase over an intermediate range of the CsPbBr3−xIx composition space. This result is consistent with both first-principles calculations and Xray diffraction studies of the corresponding bulk material.10,22 Our result has implications for efforts to engineer the optoelectronic properties of CsPbBr3−xIx QDs and for understanding the solid-state photophysics of CsPbBr3−xIx QDs in the two-phase region. Moreover, the CsPbI3 QDs clearly undergo a phase transition with time from the high-temperature cubic phase to the thermodynamically preferred δ-
To gain a more quantitative understanding of the phase identity for the CsPbBr1.5I1.5 and CsPbBr2I1 QDs, we turned to PDF analysis of synchrotron X-ray total scattering data. For both the CsPbBr1.5I1.5 and CsPbBr2I QDs, the best PDF fit to the X-ray scattering data was with a two-phase model. For both compositions, a two-phase model was used that contained both the δ-structure CsPbI3 and CsPbBrI2 and the γ-structure CsPbBr3. In order to reduce the number of refining variables, the Br content in the chain-like structure was assumed to be the same as in nominal CsPbBrI2, despite the small shift in diffraction peak maxima. Attempts to fit the PDFs of CsPbBr1.5I1.5 and CsPbBr2I with a single γ- or δ-like structure either diverged to unphysical values or resulted in weighted residual (Rw) values greater than 25% (Rw < 25% is conventional criterion for a credible PDF model). Finally, Rietveld refinements and PDF analysis of synchrotron X-ray total scattering data collected on the CsPbBr3 endmember of the QD composition series were consistent with our previous report that the CsPbBr 3 QDs possess the orthorhombic γ-structure (Supporting Information, Figure S4).8 Combining the results of PDF analysis of total scattering data and Bragg diffraction, it is clear that the CsPbBr3−xIx QDs do not form a continuous solid solution over the entire composition range from 0 ≤ x ≤ 3. Using this information, it is possible to construct a qualitative, quasi-phase diagram for the CsPbBr3−xIx QDs (Figure 4). Newly synthesized QDs form at elevated temperatures (i.e., reaction temperatures of ∼160 °C) in a cubic or nearly cubic orthorhombic γ-phase. At room temperature in the solid state, the isolated iodine-rich CsPbI3 and CsPbBrI2 QDs subsequently relax into the orthorhombic δphase featuring quasi-1D chains. For some intermediate composition range, including the CsPbBr1.5I1.5 and CsPbBr2I QDs, a two-phase region exists with contributions from the γand δ-structures. This is consistent with results based on solid7576
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Chemistry of Materials
(10) Sharma, S.; Weiden, N.; Weiss, A. Phase Diagrams of Quasibinary Systems of the Typre: ABX3-A’BX3; ABX3-AB’X3; and ABX3-ABX’3; X = Halogen. Z. Phys. Chem. 1992, 175, 63−80. (11) Choi, J. J.; Yang, X.; Norman, Z. M.; Billinge, S. J. L.; Owen, J. S. Structure of Methylammonium Lead Iodide within Mesoporous Titanium Dioxide: Active Material in High-Performance Perovskte Solar Cells. Nano Lett. 2014, 14, 127−133. (12) Fu, Y.; Zhu, H.; Schrader, A. W.; Liang, D.; Ding, Q.; Joshi, P.; Hwang, L.; Zhu, X. Y.; Jin, S. Nanowire Lasers of Formamidinium Lead Halide Perovskites and Their Stabilized Alloys with Improved Stability. Nano Lett. 2016, 16, 1000−1008. (13) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S., Il. Chemical Management for Colorful, Efficient, and Stable InorganicOrganic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764−1769. (14) Rosales, B. A.; Men, L.; Cady, S. D.; Hanrahan, M. P.; Rossini, A. J.; Vela, J. Persistent Dopants and Phase Segregation in Organolead Mixed-Halide Perovskites. Chem. Mater. 2016, 28, 6848−6859. (15) Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G. Semiconducting Tin and Lead Iodide Perovskites and Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminenscent Properties. Inorg. Chem. 2013, 52, 9019−9038. (16) Brgoch, J.; Lehner, A. J.; Chabinyc, M.; Seshadri, R. Ab Initio Calculations of Band Gaps and Absolute Band Positions of Polymorphs of RbPbI3 and CsPbI3: Implications for Main-Group Halide Perovskite Photovoltaics. J. Phys. Chem. C 2014, 118, 27721− 27727. (17) Mitzi, D. B.; Dimitrakopoulos, C. D.; Kosbar, L. L. Structurally Tailored Organic-Inorganic Perovskites: Optical Properties and Solution-Processed Channel Materials for Thin-Film Transistors. Chem. Mater. 2001, 13, 3728−3740. (18) Tang, Z.; Guan, J.; Guloy, A. M. Synthesis and Crystal Structure of New Organic-Based layered Perovskites with 2,2′-biimidazolium Cations. J. Mater. Chem. 2001, 11, 479−482. (19) Chung, I.; Song, J.-H.; Im, J.; Androulakis, J.; Malliakas, C. D.; Li, H.; Freeman, A. J.; Kenney, J. T.; Kanatzidis, M. G. Semiconductor or Metal? High Electrical Conductivity and Strong Near-Infrared Photoluminescence from a Single Material. High Hole Mobility and Phase-Transitions. J. Am. Chem. Soc. 2012, 134, 8579−8587. (20) 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. (21) Roiland, C.; Trippé-Allard, G.; Jemli, K.l; Alonso, B.; Ameline, J. C.; Gautier, R.; Bataille, T.; Le Pollès, L.; Deleporte, E.; Even, J.; Katan, C. Multinuclear NMR as a Tool for Studying Local Order and Dynamics in CH3NH3PbX3 (X = Cl, Br, I) Hybrid Perovskites. Phys. Chem. Chem. Phys. 2016, 18, 27133−27142. (22) Yin, W.-J.; Yan, Y.; Wei, S.-H. Anomalous Alloy Properties in Mixed Halide Perovskites. J. Phys. Chem. Lett. 2014, 5, 3625−3631. (23) Akkerman, Q. A.; D’Innocenzo, V.; Accornero, S.; Scarpellini, A.; Petrozza, A.; Prato, M.; Manna, L. Tuning the Optical Properties of Cesium Lead Halide Perovskite Nanocrystals by Anion Exchange Reactions. J. Am. Chem. Soc. 2015, 137, 10276−10281. (24) Stoumpos, C. C.; Malliakas, C. D.; Peters, J. A.; Liu, Z.; Sebastian, M.; Im, J.; Chasapis, T. C.; Wibowo, A. C.; Chung, D. Y.; Freeman, A. J.; Wessels, B. W.; Kanatzidis, M. G. Crystal Growth of the Perovskte Semiconductor CsPbBr3: a New Material for HighEnergy Radiation Detection. Cryst. Growth Des. 2013, 13, 2722−2727. (25) Trots, D. M.; Myagkota, S. V. High-Temperature Structural Evolution of Caesium and Rubidium Triiodoplumbates. J. Phys. Chem. Solids 2008, 69, 2520−2526.
structure, which has important implications for implementation of these QDs into solid-state devices.
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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.6b03553 Synthetic details; refinement details; CsPbBr3 fits; full list of refined parameters (PDF)
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AUTHOR INFORMATION
Corresponding Author
*R. L. Brutchey. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award # DEFG02-11ER46826. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The authors gratefully acknowledge the assistance of Dr. K. Wiaderek in collecting and analyzing the synchrotron X-ray total scattering data.
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REFERENCES
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DOI: 10.1021/acs.chemmater.6b03553 Chem. Mater. 2016, 28, 7574−7577
