Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Crystal Structure of Individual CsPbBr3 Perovskite Nanocubes Michael C. Brennan,*,† Masaru Kuno,†,‡ and Sergei Rouvimov*,§ †
Department of Chemistry and Biochemistry, ‡Department of Physics, and §Notre Dame Integrated Imaging Facility, University of Notre Dame, Notre Dame, Indiana 46556, United States
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ABSTRACT: The atomic structure of CsPbBr3 nanocubes (NCs) was studied at the single-particle level via a high-resolution transmission electron microscopy (HRTEM) defocus-series analysis. The technique entails acquiring lattice-resolved HRTEM images of individual NCs over progressive defocus values. CsPbBr3 NC atomic structure was evaluated by comparing acquired experimental data to simulated latticeresolved images and corresponding Fourier transform patterns of both orthorhombic (Pnma) and cubic (Pm3̅m) CsPbBr3 polymorphs. Herein, CsPbBr3 NCs with average edge lengths (l) of l ∼ 10 and 5 nm are analyzed using the aforementioned technique. In the former, we find evidence for the coexistence of both cubic and orthorhombic lattices. In the latter, solely cubic character is observed, illustrating a potential size dependency to the crystal symmetry of CsPbBr3 NCs. Such structural measurements provide critical insight into elucidating the structure/(optical and electrical) function relationship of CsPbBr3 NCs.
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with average edge lengths (l) of l ∼ 10−15 nm due to the Rashba19 inversion of states. Interestingly, an orthorhombic crystal structure is necessary to observe this bright triplet ground state in CsPbBr3. The model, however, does not immediately predict other reported phenomena seen in CsPbBr3 NCs such as the existence a size-dependent Stokes shift13,14 or efficient photoluminescence upconversion under subgap excitation.16,20 Full elucidation of the crystal structure of CsPbBr3 NCs at the single-particle level is therefore paramount to improving our current understanding of the electronic structure and optical properties of this important system. CsPbBr3 adopts three distinct crystal lattices, dictated by the tilting of corner-sharing PbBr 6 octahedral building blocks.17,21,22 At room temperature, bulk CsPbBr3 assumes an orthorhombic (Pnma) phase and undergoes two successive increases in crystal phase symmetry with increasing temperature: orthorhombic → tetragonal (P4mm) at ∼88 °C and tetragonal → cubic (Pm3̅m) at ∼130 °C.17,21 In their seminal report, Protesescu et al. suggested synthetic temperatures >130 °C along with the presence of surface passivating ligands stabilize CsPbBr3 NCs in their cubic phase based on benchtop powder X-ray diffraction (XRD) measurements.6 A debate, though, exists in the literature regarding the exact crystal structure adopted by CsPbBr3 NCs, whether cubic6,23 or orthorhombic.24−26 Ensemble XRD analyses, typically used to support these structural assignments, are complicated by several important factors. They include the following: (a) size- and surface straindependent broadening of X-ray reflections due crystallite dimensions on the order of ∼10 nm (see Figure S1), (b)
INTRODUCTION Lead halide perovskites were first synthesized by Wells in 18931 and characterized by Møller in 1957.2 Since their reemergence in 2009, solar cells based on lead halide perovskites [APbX3; A = CH3NH3+, CH(NH2)2+, or Cs+; X = I−, Br−, or Cl−] have achieved great success. Today, the record certified power conversion efficiency (PCE) is ∼23%.3 This has drawn global attention to harnessing the enhanced optical properties of nanostructured lead halide perovskites.4,5 Chief among them are all-inorganic CsPbX3 perovskite nanocubes (NCs),6 which are highly attractive from both a light-emitting standpoint and for use in solar cells. The former case stems from their tunable bandgaps (∼3.2−1.7 eV), high photoluminescence quantum yields (∼40−99%), and narrow emission line widths (∼70− 100 meV).4−6 Furthermore, CsPbX3 NCs are “defect tolerant”, having optically benign low formation energy defects.4−6 In the latter case, CsPbI3 NC solar cells have demonstrated PCEs of ∼14%.7,8 We have also shown that mixed halide CsPb(Ix−1Brx)3 NCs are stable against light-induced halide segregation, suggesting their potential use in tandem solar cells.9−11 The photovoltaic utility and facile fabrication of CsPbX3 nanomaterials has consequently sparked tremendous interest in understanding their intrinsic structural, optical, and electrical properties.4−9 Of these fundamental traits, the precise crystal structure assumed by CsPbBr3 NCs remains unresolved. Knowledge of the actual lattice adopted by lead halide perovskite NCs is critical to successfully modeling their optical and electrical properties.12−16 This is especially true for CsPbBr3 (and all halide perovskites) given their long-known link between crystallographic phase and electronic structure.17,18 Recent modeling by Efros and co-workers15 has demonstrated a bright triplet exciton ground state in CsPbBr3 NCs © XXXX American Chemical Society
Received: October 31, 2018
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DOI: 10.1021/acs.inorgchem.8b03078 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry presence of amorphous oleylammonmium and oleate capping ligands, which increase background noise, (c) need to process samples at elevated temperatures or with antisolvents in order to prepare them for powder XRD measurements, which strips stabilizing surface ligands and can lead to NC aggregation/ transformation into the bulk orthorhombic phase, and (d) difficulty of differentiating cubic/orthorhombic reflections due to near-identical atomic spacing, Bragg angles, and corresponding intensities. Toward overcoming these complications, Cottingham et al.27,28 and Bertolotti et al.29 have independently studied the crystal structure of CsPbBr3 NC powders/gels [ l > 6 nm] using synchrotron XRD techniques involving Rietveld refinement and pair distribution function analyses. Notably, while Cottingham et al.27 have suggested an orthorhombic crystal structure, Bertolotti et al.29 have proposed a more dynamic structural model in which both phases appear to coexist within a single NC due to the cooperative rotation of PbBr6 octahedra. Neither study has, however, confidently identified the adopted crystal structure of CsPbBr3 NCs with l < 6 nm due to extreme size-induced peak broadening. Furthermore, given that powder XRD is an ensemble technique, only average structural information is obtained from what could potentially be a structurally heterogeneous population from either an inter- or intraparticle perspective. High-resolution transmission electron microscopy (HRTEM) defocus-series imaging is a powerful technique for investigating the crystal structure of individual CsPbBr3 NCs. In a recent illustration, Shamsi et al.26 utilized latticed resolved aberration-corrected HRTEM images and their corresponding fast-Fourier transform (FFT) patterns to survey the crystal structure of single CsPbBr3 nanosheets. Their analysis indicates that orthorhombic symmetry best describes the nanosheet crystal structure. In an additional report, Yu et al.30 performed a defocus-series image reconstruction of large area (∼100 nm), atomically thin (
