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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Evolution of Morphology, Phase Composition and Photoluminescence of Cesium Lead Bromine Nanocrystals with Temperature and Precursors Meng Li, Xiao Zhang, Tao Dong, Peng Wang, Katarzyna Matras-Postolek, and Ping Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10200 • Publication Date (Web): 28 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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The Journal of Physical Chemistry

Evolution of Morphology, Phase Composition and Photoluminescence of Cesium Lead Bromine Nanocrystals with Temperature and Precursors Meng Li,a Xiao Zhang,b Tao Dong,a Peng Wang,a Katarzyna Matras-Postolek,c and Ping Yanga* a

School of Material Science & Engineering, University of Jinan, No. 336,

Nanxinzhuangxi Rd, Jinan, 250022, PR China, E-mail: [email protected] b

Fuels and Energy Technology Institute and Department of Chemical

Engineering, Curtin University, Perth WA6845, Australia. c

Chemical Engineering and Technology, Cracow University of Technology,

Warszawska 24 St., 31-155 Krakow, Poland.

ABSTRACT: Compared with CsPbBr3 nanocrystals (NCs), the study of the structure and physical properties of Cs4PbBr6 and CsPb2Br5 NCs is not sufficient. In this paper, CsPbBr3, CsPb2Br5, and Cs4PbBr6 NCs were prepared by a hot-injection method using oleylamine (OAm) and oleic acid (OA) without adding other ligands. The evolution of phase composition, morphology and photoluminescence (PL) property were investigated. It is found that rhombohedral Cs4PbBr6 was created at low temperature with low Pb/Cs ratios and short reaction time. CsPbBr3 phase was then obtained with increasing Pb/Cs ratios at high 1

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temperature through the reaction of Cs4PbBr6 and PbBr2. The evolution of phase composition occurred with time to create CsPbBr3, CsPb2Br5, and Cs4PbBr6 NCs. For a Pb/Cs molar ratio of 3, CsPbBr3 were firstly obtained at 180 °C. However, resulting sample is CsPb2Br5 phase after 120 min. The excesses PbBr2 is a key for such phase change because no similar phenomenon was observed in the case of molar ratio of Pb/Cs of 2. At low temperature (e.g. 140 and 160 °C), rhombohedral Cs4PbBr6 phase was obtained and then reacted with PbBr2 to fabricate cubic CsPbBr3 nanosheets with sizes of several hundred nanometers. With changing phase composition, cubic, rod, rhombohedral morphologies were created. The PL properties of the NCs depended strongly on the phase composition. As a result, CsPbBr3 NCs reveal highly bright PL with narrow and symmetrical PL spectra (PL peak at 520 nm). In contrast, No PL was observed for Cs4PbBr6 and CsPb2Br5 phases. The results provide a possibility to well control the growth for the application of cesium lead halide NCs.

INTRODUCTION Because of unique optical and electronic properties including high absorption coefficients, easily tunable photoluminescence (PL) over the whole visible range, high PL quantum yield (PLQY), and long carrier diffusion length, inorganic cesium-lead-halide perovskite (CsPbX3) nanocrystals (NCs), have gained considerable attention for the applications in perovskite solar cells, light-emitting diodes, lasers, and photodetectors.1-5 CsPbX3 NCs, as a kinds of typical cesium lead bromine NCs, have been studied for high PL. For example, Protesescu et al prepared the NCs by a hot-injection method with oleylamine (OAm) and oleic acid (OA) as organic ligands.6 Up to now, efforts have been reported about the synthesis of CsPbBr3 NCs. In terms of morphologies, CsPbX3 NCs have been synthesized as cubes, nanoplatelets, nanowires, and nanocages.6-10 For composition adjustment, the study focused on ion exchange reactions including anions (Cl-, Br-, I-), some isovalents (Sn2+, Mn2+, Cd2+, Ba2+, 2


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Ge2+) and aliovalent cations (Rb+, Ag+, Cu+, Bi3+).11-13 The ion exchange resulted in the change of morphology and phase. As a result, the properties of NCs are strongly depended on composition and preparation conditions. Compared with CsPbBr3 NCs, the research on other cesium lead bromine perovskite derivatives (e.g. CsPb2Br5 and Cs4PbBr6) lagged behind. Owing to the diversity of structure and morphology, CsPb2Br5 and Cs4PbBr6 would possess particular optical or optoelectronic properties. CsPbBr3 is described as a 3-dimensional structure owing to the corner-shared PbX64- octahedrons.14 Cs4PbBr6 is a kind of zero-dimensional (0D) structure which PbX64octahedrons separated in it.15 In general, CsPb2Br5 NCs reveal a layered structure. As a kind of anisotropic two-dimensional (2D) nanomaterials, CsPb2Br5 NCs possess fantastic structure-dependent physical and chemical properties, and have great potential in the fields of sensing, catalysis, electro-chemistry, and optoelectronics.16,17 Recent years, some results indicated that CsPb2Br5 NCs performed a good performance in the field of photoelectronic devices, such as light-emitting diodes, laser, and photodetectors.18-20 Although 2-dimensional CsPb2Br5 nanosheets possess great potential in many fields, the report on the controllable synthesis of CsPb2Br5 and Cs4PbBr6 is still rare. Recently, Jiang and co-workers synthesized CsPb2Br5 nanosheets by a hot-injection method using octylamine to control the formation of nanosheets.21 Deng et al studied the phase evolution process from CsPbBr3 to CsPb2Br5 with controllable morphology by using alkyl-thiols at room temperature.22 However, it has to accurately tune the ratio of alkyl-thiol, alkyl-amines, and alkyl-acids ligands for the formation of CsPb2Br5 nanosheets.22 Prashant et al reported that DDAB controlled the phase evolution process between CsPbBr3 and CsPb2Br5 NCs.23 Thus, the synthesis of CsPb2Br5 nanosheets required additional ligands. Because of complicated composition and rich in crystal structures, the study of phase composition evolution is helpful for the potential formation mechanism and growth 3



controlling of Cs-Pb-X-based NCs. For Cs-Pb-Br NCs, reports have indicated the phase formation process. For example, Alivisatos et al using amine initiated the phase evolution from CsPbBr3 to Cs4PbBr6.24 Jiang et al explored the shape and phase evolution from CsPbBr3 to CsPb2Br5.21 Manna et al achieved reversible post synthesis transformations between CsPbBr3 and Cs4PbBr6.25 However, these studies just focused on the phase evolution of two phases in a single reaction system. No report indicated the systematically phase evolution and property evolution of Cs4PbBr6, CsPbBr3, and CsPb2Br5 NCs. The phase formation is depended strongly on the preparation condition and parameters. Normally, CsPbBr3 NCs revealed high PLQY with bright green-emitting. However, the PL of Cs4PbBr6 and CsPb2Br5 NCs is argument because their PL properties are related to the bandgap, phase composition, morphology, and size.6 For example, some literature reported that Cs4PbBr6 revealed high green PL, while others believe the green emission is just from the CsPbBr3 impurity and the easily formed defect level during synthesis.26-28 As to CsPb2Br5 NCs, it was suggested that CsPb2Br5 nanosheets have strong luminescence emission.29 However, some opposite views indicate that CsPb2Br5 NCs are indirect bandgap semiconductor and have high nonradiative Auger recombination efficiency which resulted in CsPb2Br5 NCs without PL.21, 30 Thus, it is still desired challenge to discuss systematically the phase composition and properties of Cs4PbBr6, CsPbBr3, and CsPb2Br5 NCs. In this work, CsPbBr3, Cs4PbBr6, and CsPb2Br5 NCs were prepared using OAm and OA via a hot-injection method without additional other organic ligands. The phase evolution was systematically tested for CsPbBr3, Cs4PbBr6, and CsPb2Br5 by changing time, temperature, and the molar ratios of Pb/Cs. Up to now, a large amount of reports researched the influence of OA and OAm ligands on the morphologies of CsPbBr3 NCs.3,7 Because of the using of OA during the synthesis process of Cs-OA precursor, additional OA would be introduced along with the addition of Cs-OA precursor solution. As a result, for the purpose of eliminating the 4


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influence of ligands, we tuned the added amount of OA during the dissolution process of PbBr2, so that the total amount of OA was 1 mL (the same volume as OAm). In addition, the PL properties and morphology evolution of Cs4PbBr6 and CsPb2Br5 were studied.

EXPERIMENTAL SECTION Chemicals. Lead bromide (PbBr2, 99%) and oleylamine (OAm, 90%) were purchased from Aladdin. Cesium carbonate (Cs2CO3, 99.9%), Octadecene (ODE, technical grade of 90%), and oleic acid (OA) were obtained from Sigma-Aldrich. Hexane and ethanol were taken from Tianjin Chemical Reagent Company. All chemicals were used without further purification. Synthesis and Purification of Cs-Pb-Br NCs. Samples were synthesized by a hot-injection method with slight modifaction.6 To prepare cesium oleate (Cs-OA) precursor solutions, 0.16 g of Cs2CO3, 2.5 mL of OA, and 6 mL of ODE were loaded into a 50 mL 3-neck flask. And then, the solution was heated to 120 °C and kept for 30 min. Finally, the temperature was raised to 150 oC in N2 until Cs2CO3 completely reacted with OA. The concentration of Cs+ in Cs-OA precursor solution was 0.1155 M. After that, ODE (5 mL) and PbBr2 (0.188 mmol) were loaded in a 100 mL 4-neck flask. And then, the solution was dried in N2 for 1 h at 120 °C, OAm (1 mL) and OA were injected. The total amount of OA and OAm were controlled in 1 mL. The temperature was kept at 120 oC until the complete dissolution of PbBr2. The temperature was raised to reaction temperature and Cs-OA solution was quickly injected. After certain time, the resulting solution was immediately cooled down to room temperature with an ice bath. The synthesized samples were centrifuged at 9000 rpm for 4 min. The NCs were re-suspended in hexane. The preparation conditions (the amount of OA and Cs-OA precursors, temperature and time) of samples were illustrated in Table S1. Here, we used the form of “Pb/Cs molar ratio-reaction temperature-order of sampling” to 5



name the samples. Different order of sampling corresponds to the different reaction time, which shown detailedly in Table S1. Characterization. The absorption spectra of samples were recorded by a Hitachi U-4100 spectrometer. A Hitachi F-4600 spectrometer was used for PL measurements. The crystal structure of samples was identified by a Bruker D8 X-ray powder diffractometer (XRD) using a Cu Ka target. The sample was drip-coated on a glass sheet to conduct XRD measurement. Transmission electron microscopy (TEM) images were recorded on a JEM-2010 microscope operated at 120 kV. PL lifetime measurements were carried out using the time-correlated single-photon-counting spectrofluorometer system (λex= 370 nm, Fluorocube-01, JY-IBH, Horiba). The high-resolution transmission electron microscopy (HRTEM) images of samples were measured using a JEM-2100F microscope operated at 200 kV. Scanning electron microscopy (SEM) images were performed on QUANTA 250 FEG microscope. An energy dispersive X-ray spectroscopy (EDS) detector coupled with a QUANTA 250 FEG scanning electron microscope (FEI) was used for elemental analysis. X-ray photoelectron spectroscopy (XPS) analysis was conducted by a Thermo Scientific K-Alpha Al-Kα X-ray source (hv=1486.6 eV) with a 400 um spot size and 20 eV pass energy. Atomic force microscopy (AFM) images were recorded by Bruker Multimode 8.

RESULTS AND DISCUSSION Using a hot-injection method, the formation process of Cs4PbBr6, CsPbBr3, and CsPb2Br5 NCs was systematically studied. The NC formation included nucleation, growth and Oswald ripening process.31,32 CsPbBr3 NCs, tetragonal CsPb2Br5 nanosheets, and hexagonal Cs4PbBr6 have been reported.30,33,34 Meanwhile, most of reports focused on CsPbBr3 NCs. Few reports related on CsPb2Br5 and Cs4PbBr6 phases. In general, the synthesis of CsPb2Br5 nanosheets required additional ligands. There is no report about the synthesis of CsPb2Br5 NCs by 6


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Figure 1. TEM images of samples 3-180-1 (a), 3-180-2 (b), 3-180-3 (c), 3-180-4 (d). SEM images of samples 3-180-5 (e) and 3-180-7 (f). HRTEM images of samples 3-180-1 (g) and 3-180-7 (h). The insets in (a) to (f) show the pictures of samples under room light (left) and 365 nm UV light (right). Insets in (g) and (h) are corresponding fast Fourier transform (FFT) patterns.

traditional hot-injection method only with the assistance of OA and OAm. CsPbBr3 NCs crystallize as orthorhombic, tetragonal, and cubic phases. The cubic phase with the highest symmetry is usually formed at high temperature.9,35 The preparation of perovskite NCs was 7



carried out through the reaction of PbBr2 and Cs-OA precursors. The reaction equation is as follows.35

2Cs ― OA + 3 Pb𝐵𝑟2 2 𝐶𝑠𝑃𝑏𝐵𝑟3 +𝑃𝑏 ―𝑂𝐴2

(1)

Sample 3-180-1 to sample 3-180-7 were prepared with the molar ratio of Pb/Cs was at 3. In order to track the growth and shape evolution process, the reaction was terminated for 0.17, 30, 80, 100, 116 and 120 min. Figure 1 shows the TEM and SEM images of samples 3-180-1, 3-180-2, 3-180-3, 3-180-4, 3-180-5, and 3-180-7. The insets in Figure 1 show the pictures of samples under room light (left) and 365 nm UV light (right). At beginning of reaction, the morphology of sample 3-180-1 is irregular cube as shown in Figure 1 (a). With increasing time to 30 min, more regular cubic structures were observed in Figure 1 (b). In Figure 1 (d), large nanosheets were observed, but many small particles still retained. The images in Figure 1 (d-f) shown that small particles gradually disappeared with prolonging time. With time to 120 min, small particles completely disappeared and all of the NCs became into the large sheet structure (Figure 1 (f)). No PL was observed for sample 3-180-7. The PL spectra show the PL intensity increased at beginning for sample 3-180-1 to sample 3-180-2, and then decreased gradually, finally disappeared (Figure S1. The bandgap of CsPbBr3 is 2.3 eV. 6,21,25 It has been reported that the valent band and conduct band of CsPbBr3 NCs originates from Pb(6s)-Br(4p) and Pb(6p)-Br(4p) antibonding interactions.36 Cs+ ions do not contribute directly to the valence band or conduction band in CsPbBr3 NCs.36 It was generally convinced that CsPbBr3 performed bright PL. The PL spectra demonstrated that no PL was observed for CsPb2Br5 NCs. The structure of sample 3-180-1 was also demonstrated by HRTEM measurement (Figure 1 (g)). The lattice fringe is 4.13 Å, corresponding the (110) facet of CsPbBr3 monoclinic phase. The diffraction spots in FFT pattern assigned to the (110) 8


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crystal planes of CsPbBr3 monoclinic phase as shown in the inset in Figure 1(g). The HRTEM image in Figure 1 (h) shows the lattice fringes are 3.04 and 3.8 Å, corresponding the (213) and (210) facet of Cs2PbBr5 tetragonal phase, respectively. The FFT pattern displayed the single-crystalline structure of the CsPb2Br5 nanosheets. The diffraction spots could be assigned to the (213) and (210) crystal planes (Figure 1 (h)). The thickness of sample 3-180-7 as nanosheets was verified about 15 nm by an AFM image (Figure S2).

Figure 2. XRD patterns of sample 3-180-1 to 3-180-7.

Figure 2 shows the XRD patterns of sample 3-180-1 to 3-180-7. The result revealed sample 3-180-1 was CsPbBr3 with monoclinic structure. With increasing time, the peaks at 15.21, 21.498, and 30.698° gradually weakened, and disappeared at 116 min (sample 3-180-6). Meanwhile, the intensity of peaks at 11.665, 23.39, 35.437 and 47.86° gradually increased, which correspond to diffractions from (002), (210), (312), (420) planes of tetragonal CsPb2Br5. EDS element mapping was shown in Figure 3 for sample 3-180-7. Figure 3 (b-d) shows the elements of Cs, Pb, and Br with uniformly distribution. The EDS element quantification analysis demonstrates that the molar ratio of Cs/Pb/Br is at 8.48/20.03/71.48, which is closed to 1:2:5 related to the element ratio of CsPb2Br5 (Table S2). 9



Figure 3. (a) SEM image of sample 3-180-7. Inset is the HRTEM image of sample 3-180-7. (b, c, d) EDS element mapping images of sample 3-180-7.

The basic structural unit of CsPbBr3 is PbBr6 octahedral structure which surrounded by Cs+. PbBr64- octahedrals are corner-shared, and Cs+ ions fill in the octahedral vacancies. Tetragonal CsPb2Br5 possess an obviously different crystal structure from CsPbBr3 and Cs4PbBr6. For CsPb2Br5, one Pb2+ coordinated with eight Br-. Cs+ is sandwiched between two Pb-Br coordination polyhedrons, and its coordination number is 10.21 Compared with CsPbBr3 and Cs4PbBr6 NCs, the study of the structure and physical properties of CsPb2Br5 is not sufficient. Using a Pb/Cs ratio of 3, pure CsPb2Br5 could be prepared at 180 °C after reaction for 120 min. It is found that CsPb2Br5 is derived from CsPbBr3. With increasing time, the phase evolution process from CsPbBr3 to CsPb2Br5 occurred. Different from our result, CsPbBr3 nanowires were reported by Yang et al. by prolonging the reaction time in hot injection method. The different resulting phase mainly comes from the different molar ratio 10


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of Pb/Cs. In our research, CsPb2Br5 phase was prepared by extending the reaction time with the molar ratio of Pb/Cs at 3. However, after calculating, the molar ratio of Pb/Cs is at 2 in the synthesis process of CsPbBr3 nanowires reported by Yang et al. Owing to the phase evolution is resulted from the unreacted PbBr2, the lower molar ratio of Pb/Cs isn’t favor of the formation of CsPb2Br5 phase. We assumed that CsPb2Br5 NCs is acquired based on the following reaction mechanism.29

CsPb𝐵𝑟3 +𝑃𝑏𝐵𝑟2 𝐶𝑠𝑃𝑏2𝐵𝑟5

(2)

Figure 4. Schematic illustration of phase evolution process from CsPbBr3 to CsPb2Br5.

The phase evolution process from CsPbBr3 to CsPb2Br5 was ascribed the broken of PbBr6 octahedrons firstly. Pb2+ and Br- were then rearranged to form PbBr8 as shown in Figure 4. Some reports have shown that the organic ligands can cause the phase evolution process.23 To demonstrate the phase evolution is resulted from the unreacted PbBr2 not related to the organic ligands, we conduct the contrast experiments. Firstly, sample 3-180-1 was synthesized. And then, the sample 3-180-1 was purified to remove the unreacted PbBr2 and organic ligands. After that, the purified sample was added into the same reaction system which contained 5 mL ODE, 1 mL OA, and 1 mL OAm, to continue reaction for 60 min and

11



120 min. The XRD results demonstrated that the sample which was purified can’t undergo phase evolution process (Figure S3). The phase evolution process with time was also illustrated with the molar ratio of Pb/Cs of at 4. Figure S4 shows the XRD patterns of samples 4-180-1, 4-180-3, and 4-180-5. At beginning of reaction, CsPbBr3 phase was firstly created (sample 4-180-1). Due to the excess amount of PbBr2, the phase composition of Cs-Pb-Br NCs would be changed with time. Namely, their morphologies are also changed because of structure difference of phases. Sample 4-180-1 exhibited CsPbBr3 phase with monoclinic structure. With increasing time to 60 min, sample 4-180-3 still maintained CsPbBr3 structure except for a weak peak at 2θ=11.665o, which correspond to diffractions from the (002) lattice plane. With further increasing time, sample 4-180-5 revealed a higher XRD peaks at 11.665, 24.032, 29.355, and 33.343o corresponding to CsPb2Br5 phase. TEM images indicated the morphology change from cubic to nanosheet for sample 4-180-1 to 4-180-5 (Figure S5). Sample 4-180-1 revealed a regular cubic morphology with an average size of 14.74 nm (Figure S5 (a)). With increasing time, the size increased and the morphology became irregular (Figure S5 (b, c)). A large sheet-like shape was observed for sample 4-180-4 (Figure S5 (d)). When the reaction time was further extended to 120 min (Figure S5 (e, f)), all specimen became into large nanosheets and small particle specimen were almost disappeared. This is ascribed to the dissolution of the cubic CsPbBr3 phase and the growth of CsPb2Br5 phase. Figure S6 shown the PL spectra of samples 4-180-1, 4-180-2, 4-180-4, 4-180-5. At same sample concentrations, sample 4-180-1 revealed the brightest PL. This is ascribed the PL properties of CsPbBr3 NCs (Figure S6). The PL intensity gradually decreased with the increasing time. After reacted 120 min, the PL intensity decreased to 25% of sample 4-180-1.

12


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Figure 5. (a) XRD patterns of samples 3-120-1, 3-130-1, 3-160-1. (b) XRD patterns of samples 3-120-2, 3-130-2, 3-160-2. Diamonds represent the CsPb2Br5 phase (PDF#25-0211). Next, we investigated the influence of temperature and molar ratio of Pb/Cs on the phase of Cs-Pb-Br NCs. At a Pb/Cs molar ratio of 3, samples were prepared at 120, 130, and 160 oC. In the case of 10 s, the XRD patterns indicate that samples 3-120-1, 3-130-1, and 3-160-1 exhibited Cs4PbBr6 phase with rhombohedral structure (Figure 5 (a)). However, it is worth to illustrate that a weaken peak at 34.3o appeared in the XRD pattern of sample 3-160-1, corresponding to the CsPbBr3 monoclinic phase. Thus, related high temperature is beneficial to the formation of CsPbBr3. Sample 3-120-2 was Cs4PbBr6 phase. After reaction for 120 min at 120°C, no phase changing occurred, in which samples 3-120-1 and 3-120-2 revealed similar phase composition. The most of XRD peaks of sample 3-130-2 were corresponded to Cs4PbBr6 except for a weak peak at 34.3o, which was related to CsPbBr3 monoclinic phase. The XRD pattern of sample 3-160-2 shows that the peak intensity of Cs4PbBr6 phase was very weak, and the peaks corresponding to CsPbBr3 phase became generally high (Figure 5(b)). In addition, it is pointed that several weak peaks of CsPb2Br5 phase appeared for sample 3-130-2. As a result, within short reaction time (10 s), Cs4PbBr6 NCs were formed easily compared with CsPbBr3 using a Pb/Cs molar ratio of 3. With increasing time to 120 min, CsPbBr3 phase created because PbBr2 reacted with Cs4PbBr6 when temperature was 13



more than 130 oC. In addition, with further increasing temperature, by-product CsPb2Br5 was formed. If temperature raised to 180 oC, pure CsPb2Br5 phase was fabricated at 120 min as shown in Figure 2. The phase evolution process from Cs4PbBr6 to CsPbBr3 is as follows.37

𝐶𝑠4𝑃𝑏𝐵𝑟6 +3𝑃𝑏𝐵𝑟2 4𝐶𝑠𝑃𝑏𝐵𝑟3

(3)

Figure 6. Schematic illustration of phase evolution process from Cs4PbBr6 to CsPbBr3.

Figure 6 shows the schematic illustration of phase evolution process from Cs4PbBr6 to CsPbBr3. In Cs4PbBr6 structure, PbBr6 octahedrons are separated by Cs+ and not shared Brwith each other. PbBr6 octahedrons exist as independent octahedrons.14, 15, 26, 28 The unit cell of Cs4PbBr6 is not cubic, but is slightly compressed along the direction of body diagonal. Each cell has an individual PbBr6 octahedral structure, and each Cs+ is shared by two unit cells.28 CsPbBr3 has a typical cubic structure. PbBr64- octahedrals are corner-shared, and Cs+ ions fill in the octahedral vacancies. Cs4PbBr6 is inclined to generate at lower temperature and lower Pb/Cs molar ratio. When increasing the temperature or prolonging the reaction time, it is beneficial to form a structure with higher symmetry. Therefore, the Cs4PbBr6 phase continues to react with unreacted PbBr2 to form the CsPbBr3 phase. Our result is the same with the former report, which was demonstrated by Tang et al. that the extra PbBr2 can cause the phase evolution from Cs4PbBr6 to CsPbBr3 phase.38 14


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Figure 7. TEM images of sample 3-120-1 (a), sample 3-120-2 (b), sample 3-130-1 (c), sample 3-130-2 (d), sample 3-140-1 (e), sample 3-140-2 (f), sample 3-160-1 (g), and sample 3-160-2 (h). 15



The TEM images of samples are shown in Figure 7. At beginning, rhombohedral Cs4PbBr6 NCs were formed as shown in Figure 7 (a), (c), and (e). The size of the NCs increased with time. At 160 °C, large nanosheets were observed except for rhombohedral NCs shown in Figure 7 (g). Although some cubic specimen appeared in Figure 7 (b), the XRD pattern of the sample did not revealed the existence of CsPbBr3 NCs. After 120 min at 130 oC, cubic nanosheets and a few nanowires were observed in Figure 7 (d). At 140 and 160 oC, both samples 3-140-2 and 3-160-2 changed into cubic nanosheets (Figure 7 (f) and (h)). Compared with sample 3-140-2, the size of sample 3-160-2 dramatically increased. The formation of Cs-Pb-Br nanosheets included a nucleation and growth processes. The nucleation occurred immediately after the injection of Cs-OA precursor solution. Because of OAm in solutions, the growth on (110) facets was inhibited and promoted the formation of anisotropic platelet structure.37,39 Finally, large nanosheets were created after 120 min.40 To observed the morphology evolution from sample 3-160-1 to sample 3-160-2, samples are prepared with different time for 10, 30, 60, and 90 min (Figure S7). Rhombohedral NCs also existed after 10 min. The sizes of rhombohedral NCs and nanosheets increased. After 30 min, rhombohedral NCs disappeared (Figure S7 (b)). From 30 to 120 min, the morphology of nanosheets became more regular (Figure S7 (b-d)). Two kinds of nanosheets with high and low crystallinity were observed for sample 3-160-2.

The HRTEM image of sample 3-160-120 is shown in Figure 8. The lattice fringe is 4.13 Å, corresponding the (110) facet of CsPbBr3 phase with well crystallinity. The inset in Figure 8 shows the profile of the lattice fringes. In addition, a few nanosheets with voids were also observed (Figure S8). Because the protonated OAm attached on the surface of nanosheets, OAm caused the falling off of Cs+. Thus, nanosheets became void when the nanosheet was

16


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thin.41 CsPbBr3 phase reveals high PL properties while the PL of Cs4PbBr6, CsPb2Br5 is very weak.

Figure 8. HRTEM image of sample 3-160-2. The inset shows the profile of the lattice fringes (The area is indicated by a green arrow).

The PL spectra of samples (Figure S9 (b)) show samples 3-120-2, 3-130-2, 3-140-2, and 3-160-2 revealed different PL peaks. This confirms samples with CsPbBr3 phase. With increasing time, CsPbBr3 NCs gradual appeared. Even if at low temperature (120 oC), the CsPbBr3 phase inevitably appeared after reaction for 120 min. The time-resolved PL decay curves of samples 3-120-2, 3-130-2, 3-140-2, and 3-160-2 were measured (Figure S9 (c)). Absorption spectra of sample 3-120-2 revealed two peaks at 237 and 314 nm, which came from the Cs4PbBr6 (Figure S10 (a)).24 Decay curves were fitted by bi-exponential model (Figure S9 (c)). Time constants, fast and slow components, and average lifetime were shown in Table S2. Furthermore, the molar ratio of Pb/Cs was increased to 2. The result in Figure 9 (a) confirms that samples 2-90-1, 2-120-1, 2-140-1, and 2-160-1 are Cs4PbBr6. After 120 min, samples 2-90-2 and sample 2-120-2 did not undergo phase evolution process (Figure 9(b)). Sample 2-140-2 was CsPbBr3 phase, which undergone phase evolution process from sample

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2-140-1. Sample 2-160-2 is the mixture of CsPbBr3 and CsPb2Br5. The XRD peaks at 11.625, 23.390, and 29.355o are derived from tetragonal CsPb2Br5.

Figure 9 (a) XRD patterns of samples 2-90-1, 2-120-1, 2-140-1, and 2-160-1. (b) XRD patterns of samples 2-90-2, 2-120-2, 2-140-2, 2-160-2. Grey diamonds represent CsPb2Br5 phase (PDF#25-0211).

Figure 10 displays the TEM images of samples. In Figure 10 (a, c, e, g), rhombus NCs were observed, corresponding to Cs4PbBr6. Some cubic NCs were also observed in Figure 10 (e, g), which comes from CsPbBr3 NCs. Thus, related high temperature is beneficial for the formation of CsPbBr3. Figure 10 (b, d, f, h) show the morphology changing after reacting for 120 min. Sample 2-90-2 and 2-120-2 maintained rhombus morphology. Sample 2-140-2 revealed regular cubic structure which is CsPbBr3 from the XRD pattern. Different from Sample 2-140-2, Yang et al. prepared CsPbBr3 nanowires at 150 oC with a molar ratio of Pb/Cs of 2 by prolonging the reaction time in a hot-injection method.9 The different morphology from sample 2-140-2 mainly comes from the different using amount of OAm and OA ligands. Here, we give a more detailed explanation. While synthesize CsPbBr3 nanowires, the used amount of OA is more than OAm in Yang’s Procedure. Many reports shown the same results that more OA was in favor of the formation of CsPbBr3 nanowires. 18


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Figure 10. TEM images of samples 2-90-1 (a), 2-90-2 (b), 2-120-1 (c), 2-120-2 (d), 2-140-1 (e), 2-140-2 (f), 2-160-1 (g), and 2-160-2 (h).

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42-44

Meanwhile, the less amount of OA is in favor of the formation of CsPbBr3 nanosheets.3

At 180 oC, large nanosheets came out, corresponded to CsPbBr3 and CsPb2Br5. The TEM image and XRD pattern of sample 2-90-2 are shown in Figure S10. A lattice spacing of 6.86 Å was clearly observed, corresponding to the (110) plane of Cs4PbBr6 (Figure S10 (a)). The inset in Figure S10 (a) shown the FFT pattern, which indicated the diffraction spots of (110) crystal planes. After Rietveld refinement, the low value (＜15%) of Rwp provides good fits to the raw data of sample 2-90-2 (Figure S10 (b)). The PL measurement indicated that just samples 2-140-2 and 2-160-2 revealed PL (Figure S11 (a)). The adsorption spectrum of sample 2-160-2 revealed an enhanced absorption edge from scattering of large particles (Figure S11 (a)). The time-resolved PL decay curves of samples 2-140-2 and 2-160-2 were measured (Figure S11 (b)). The time constants (τ1 and τ2), fast and slow components (B1, B2), and the average lifetime were listed (Table S3). No PL was observed for Cs4PbBr6 phase. The valence band and conduct band of Cs4PbBr6 NCs mainly originate from Pb(6p) orbit and Pb(6p) orbit, respectively.28 It is still controversies on the luminescence of Cs4PbBr6. A few literature indicated that Cs4PbBr6 with a high PLQY.15,41,45 However, our results proved Cs4PbBr6 did not show any PL over the visible spectral region. The reported PL may come from the inevitably formed CsPbBr3 NCs during the synthesis procedure of Cs4PbBr6. In addition, it has been confirmed the band gap of Cs4PbBr6 is around 3.9 eV.14,46 The basic structural unit of Cs4PbBr6 and CsPbBr3 are same. The difference of PL property is ascribed to the difference of octahedron connection. XPS analysis was used to indicate the changing of binding energy. All XPS spectra were calibrated using C 1s peak at 285.0 eV (Figure S12). It is reported that Br- in cesium lead halide NCs were in two chemical environments, the higher band energy regions and the lower band energy regions, which corresponded to the Pb-Br and Cs-Br complexes, 20


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respectively.47,48 Compared with Cs4PbBr6, the Br 3d peaks of CsPbBr3 shifted from 69.3 and 68.25 to 69.2 and 68.15 (Figure. 11 (a, b)). CsPb2Br5 revealed a same binding energy with Cs4PbBr6 in higher band energy regions. But the higher binding energy shift was shown in lower band energy regions Figure. 11 (a, c). Figure 11 (d) indicates that Br 3d peak in CsPbBr3 has the highest intensity. The highest intensity is related to the tightly packed structure of CsPbBr3. Pb2+ is also in two chemical environments. The higher band energy region and the lower band energy region correspond to the Pb-Br and Pb-OA complexes, respectively. CsPb2Br5 possesses the highest binding energy, which corresponds to the sandwiched structure (Figure S13). The lower binding energy shift of Pb 4f orbit in CsPbBr3 demonstrated the increase of Pb-OA complexes, which is ascribed to the stabilizing effect of organic ligands.

Figure 11. Peak fitting of Br 3d spectra of Cs4PbBr6 NCs (a), CsPbBr3 NCs (b), and CsPb2Br5 NCs (c). (d) Br 3d spectra of Cs4PbBr6, CsPbBr3, and CsPb2Br5 NCs.

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Theoretically, according to eq. 1, CsPbBr3 NCs are produced with the molar ratio of Pb/Cs at 1.5. XRD patterns in Figure S12 indicated sample 1.5-120-2 with a ratio of Cs to Pb of 1.5 was Cs4PbBr6. Both samples 1.5-120-1 and 1.5-120-2 are rhombohedral Cs4PbBr6 (Figure S14). With continued decreasing the molar ratio of Pb/Cs to 1, XRD patterns confirmed sample 1-130-1 and 1-180-1 are rhombohedral Cs4PbBr6 (Figure S14). TEM images in Figure S15 (c-f) verified samples 1-120-1, 120-2, 1-130-1, and 1-130-2 with regular rhombus morphology with uniform size distribution. Although a few cubic NCs were observed in Figure S15 (g), the XRD pattern confirmed the main phase was also Cs4PbBr6. After 120 min, sample 1-180-3 is a mixed phase of Cs4PbBr6 and CsPbBr3 (Figure S15 (h)). In low Pb/Cs ratios, related high temperature is necessary for the phase evolution from Cs4PbBr6 to CsPbBr3. The PL properties of samples were recorded. Sample 1-180-2 and 1-180-3 revealed bright PL (Figure S16 (a)). The high temperature is in favour of the formation of CsPbBr3 NCs. The lifetime curves and parameters are shown in Figure S16 (b) and Table S3.

CONCLUSIONS

The phase evolution and morphology of Cs4PbBr6, CsPbBr3, and CsPb2Br5 NCs were systematically studied. By regulating temperature, time, and Pb/Cs ratios, CsPbBr3, CsPb2Br5, and Cs4PbBr6 phases were created without additional ligands. It is found that except for temperature, the molar ratios of Pb/Cs and reaction time have a great influence on the phase evolution process of the NCs. The rhombohedral Cs4PbBr6 was easily formed at low temperature, low Pb/Cs ratios, and short time. With increasing temperature, Pb/Cs ratios, or prolonging time, Cs4PbBr6 phase reacted with unreacted PbBr2 to form CsPbBr3 phase. At high temperature (more than 160 oC) and high molar ratios of Pb/Cs (≥3), resulting samples 22


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undergone a phase evolution from Cs4PbBr6 to CsPbBr3 with extending time, and eventually to form CsPb2Br5 nanosheets with few microns. The PL properties of three phases indicate just CsPbBr3 phase with high PL, narrow and symmetrical PL peak around 520 nm. No PL was observed for CsPb2Br5 and Cs4PbBr6 phases. The results promote the well growth controlling of Cs4PbBr6, CsPbBr3, and CsPb2Br5 NCs and explore the phase evolution processes.

ASSOCIATED CONTENT Supporting Information. Details of the preparation conditions and supplementary measurement information of samples: Preparation conditions and the resulting phase of samples; PL spectra of samples 3-180-0, 3-180-30, 3-180-60, 3-180-80, 3-180-100, 3-180-116, and 3-180-120; AFM image of sample 3-180-7; The result of EDS element quantification analysis of sample 3-180-7; XRD patterns of sample 3-180-1 and the contrast samples; XRD patterns of samples 4-180-1, 4-180-3, and 4-180-5; TEM images of sample 4-180-1, 4-180-2, 4-180-3, 4-180-4, and 4-180-5, SEM image of sample 4-180-5; PL spectra of samples 4-180-1, 4-180-2, 4-180-4, 4-180-5; TEM images of samples 10 min, 30 min, 60 min, 90 min; voids in TEM image of sample 3-160-2; Absorption and PL spectra as well as Time-resolved PL decay curves of samples 3-120-2, 3-130-2, 3-140-2, 3-160-2. 2-120-2; Components B1 and B2, time constants τ1 and τ2, and average lifetime τ of samples; HRTEM image of sample 2-90-2, XRD rietveld refinement of sample 2-90-2; absorption and PL spectra, and time-resolved PL decay curves of samples 2-140-2 and 2-160-2; XPS C 1s spectra of Cs4PbBr6, CsPbBr3, and CsPb2Br5 after peak calibration at 285.0 eV; Pb 4f spectra of Cs4PbBr6, CsPbBr3, and CsPb2Br5 NCs; XRD patterns of sample 1-120-2, 1-130-2, 1-180-1, and 180-3; TEM images of samples 1.5-120-1,

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1.5-120-2, 1-120-1, 1-120-2, 1-130-1, 1-130-2, 1-180-1, 1-180-2; PL spectra and time-resolved PL decay curves of samples 1-180-2 and 1-180-3 ACKBOWLEDGEMENTS This work was supported by the projects from National Natural Science Foundation of China (grant no. 51572109, 51772130, and 51501071), the program for Taishan Scholars.

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