Enhanced Luminescence and Stability of Cesium Lead Halide

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Enhanced Luminescence and Stability of Cesium Lead Halide Perovskite CsPbX Nanocrystals by Cu -Assisted Anion Exchange Reactions 3

2+

Yi-Chia Chen, Hung-Lung Chou, Jou-Chun Lin, Yi-Cheng Lee, Chih-Wen Pao, Jeng-Long Chen, Chia-Che Chang, Ruei-Yu Chi, Tsung-Rong Kuo, Chin-Wei Lu, and Di-Yan Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11535 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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The Journal of Physical Chemistry

Enhanced Luminescence and Stability of Cesium Lead Halide Perovskite CsPbX3 Nanocrystals by Cu2+-Assisted Anion Exchange Reactions Yi-Chia Chen,1# Hung-Lung Chou,2# Jou-Chun Lin,1 Yi-Cheng Lee,1 Chih-Wen Pao,3 Jeng-Long Chen,3 Chia-Che Chang,1 Ruei-Yu Chi,4 Tsung-Rong Kuo,5 Chin-Wei Lu,4 Di-Yan Wang1* 1

Department of Chemistry, Tunghai University, Taichung 40704, Taiwan

2

Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 10607, Taiwan

3

National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

4

Department of Applied Chemistry, Providence University, Taichung, 43301, Taiwan.

5

Graduate Institute of Nanomedicine and Medical Engineering, College of Biomedical Engineering, Taipei Medical University, Taipei 11031, Taiwan.

#

Yi-Chia Chen and Hung-Lung Chou contributed equally to this work.

Corresponding author: [email protected]

Abstract Inorganic CsPbX3 perovskite nanocrystals (NCs) have exhibited great optical properties, such

as

tunable

emission

wavelength,

narrow

emission

line-widths,

and

high

photoluminescent quantum yields. However, the unstable crystal structure of perovskite CsPbX3 NCs lead to a deterioration in optical performance. In this work, it is demonstrated that inorganic perovskite NCs, including CsPbCl3 and CsPbBr3-xClx NCs with excellent photoluminescence quantum yield and optical stability can be improved via anion exchange reaction treated with a new halide precursor consisting of copper halide (CuX2)-oleylamine (OLA) complexes. Unlike traditional perovskite synthesized processes for better crystalline structures operated at high temperatures, this work offers an economical method operable at the room temperatures. The treated CsPbX3 perovskite nanocrystals were characterized by in situ photoluminescence (PL) spectra and in-situ X-ray diffraction (XRD) and exhibited stable crystalline structures and enhanced photoluminescence. Cu2+ ions were only absorbed on the surface of perovskite NCs confirmed by the X-ray absorption spectroscopy (XAS) analysis. Density functional theory calculation explained that the origin of high stability and good crystallinity for treated perovskite NCs stemmed from adsorption of CuCl2 on perovskite’s surface to passivate defect sites during the recrystallization process.

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Introduction Inorganic cesium lead-halide perovskite CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) have been considered as promising semiconductor materials for low cost and highly efficient light emitting diode and solar cells.1-5 Additionally, their bandgap energies could be tuned through compositional modulations and quantum size effects.6-8 The CsPbX3 NCs exhibited higher durability toward oxygen and moisture in comparison with primary organic-inorganic perovskite CH3NH3PbX3 materials.9-10 Recently, many researches demonstrated an achievement in controlling tunable PL emission of CsPbX3 NCs by adjusting their morphology,11-12 size,13 or halide ratio1, 14 by using traditional re-precipitation methods under high temperature condition. The CsPbX3 (X=Br to I) exhibited high photoluminescent quantum yields (PLQY) up to 80% from green to red spectral regions but the CsPbCl3 showed low PLQY at blue spectral region.15-16 Also, the structural instability of CsPbX3 NCs, such as phase transition, sensitive in air/moisture and fast anion exchange, still remains important issues for further optoelectronic applications. To stabilize the phase structure and reduce the photodegradation of CsPbX3 NCs, various protection treatments have been utilized in literature. Several reports indicated that the high-quality and phase-stable CsPbI3 NCs could be obtained by using an alkyl phosphinic acid17 and bidentate ligand18 as surfactants to replace traditional oleic acid, which prevented the phase transformation from the cubic phase to the orthorhombic phase or enhanced the bonding strength in between the inorganic surface and the long-chain capping ligands. In addition, by exposure in X-ray flux, an organic ligand shell with intermolecular C=C bonding was grown on the surface of CsPbX3 NCs, leading to enhanced stability of the NCs in air and moisture.19 Surface modification of CsPbX3 NCs with specific organic ligand was effective to reduce the phase transformation rate and stabilize the crystal structure. For example, no obvious defects of CsPbBr3 NCs were seen by using a thiocyanate salt for surface treatment, which removed excess lead from the NCs surface, resulting in shallow traps elimination and near-unity green emitters.20 Near-unity PLQYs of the perovskite NCs can also be achieved through engineering the local order of lattice via nickel ion doping.21 A post-synthesis route, called an anion exchange method has been utilized to modify the surface of CsPbX3 (X = Cl, Br, I) NCs with accurate exchange ratio of halides.14, 22-24 Previous exchange reaction for different CsPbX3 NCs were performed by using a oleylamine-halide precursor, which needed a complicated pretreatment under Schlenk line at high temperatures.1 Recently, it was reported that the anion or cation exchange reaction of CsPbX3 NCs or nanowire could be achieved at the room temperature by using metal halides as exchange sources to replace the halide and Pb ions in the pristine perovskite NCs.25-27 For example, the structure of CsPbBr3 perovskite nanowires would fragment into low aspect-ratio CsPbX3


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(X=Cl, Br and I) nanorods during halide ion exchange reaction with PbX2–ligand solution.27 The halogen defect on the surface of CsPbX3 QDs can be improved during a ZnX2/hexane solution post-treatment, resulting in strong photoluminescence.28 The surface defects of 2D CsPbBr3 nanoplatelets can be repaired by addition of a PbBr2-ligand solution, resulting in PLQY enhancement.29 One report also indicated that the cubic phase of an emitting CsPbCl3 NCs easily transformed to non-emitting tetragonal CsPb2Cl5 phase irreversibly due to several purifications with introduction of non-polar solvents.30 Although the CsPbX3 NCs with different emission light could be tuned via these exchange method, the instability issue was yet resolved. Therefore, it is still a challenge to develop a novel method to stabilize perovskite crystal structures and unveil the anion exchange mechanism during the transformation process. In this work, a new precursor consisting of copper halide (CuX2)-oleylamine (OLA) complex was found to provide a good effect on structural stabilization and crystallinity improvement at room temperature for inorganic perovskite NCs, including CsPbCl3 and CsPbBr3-xClx NCs via anion exchange reaction. The PLQY and crystallinity of the resulting CsPbCl3 and CsPbBr3-xClx NCs were influenced strongly by the precursor of copper halide complex with different oxidation states of copper ions. The detailed exchange mechanism in CsPbX3 was proposed and investigated by in-situ PL spectra and in-situ X-ray diffraction (XRD). Overall results indicated that the precursor with Cu2+ ions exhibited better protection efficiency than that with Cu+ ions, resulting in a 6~30-fold improvement of PLQY for CsPbCl3 and CsPbBr3-xClx NCs. X-ray absorption spectroscopy (XAS) and Density functional theory (DFT) calculation was performed to understand the recrystallization mechanism of Cu halide complex through the exchange reaction. Results and Discussion To investigate the changes of optical property of CsPbCl3 after treated with CuCl-OLA, CuCl2-OLA, CuBr-OLA and CuBr2-OLA complex, the PL spectra were measured and shown in Figure 1. The related absorption spectra of all samples are included in the supporting information. When pristine CsPbCl3 NCs are treated with CuCl-OLA and CuCl2-OLA complex, the peaks of PL spectra of treated CsPbCl3 have a slight blue shift around few nanometer (403nm for CuCl2, 401 nm for CuCl) in comparison with untreated CsPbCl3 (405 nm) (Figure 1a), which could be ascribed to the dielectric constant change of treated CsPbCl3 NCs because of additional existence of Cu2+ or Cu+ ions in the nanocrystal solution. The full width at half maximum (FWHM) for three samples are all ~12 nm. The PLQY of CsPbCl3 treated with CuCl is still similar to that of pristine CsPbCl3, but most importantly, the PLQY of CsPbCl3 treated with CuCl2 is significantly increased around 6-fold. Figure 1(b) shows that after treated with CuBr and CuBr2, the CsPbCl3-xBrx NCs are successfully formed with blue



emission wavelength at 460 nm. The PLQY of CsPbCl3-xBrx treated with CuBr2 (92.6%) is much higher than that treated with CuBr (22.3%) and PbBr2 (see supporting information). Also, the inset images of Figure 1 (a) and 1 (b) represent that the CsPbX3 treated with CuX2 exhibits higher emission intensity than that treated with CuX under illumination of UV light. Overall results indicate that Cu2+ could play an important role on anion exchange process to improve crystallinity for higher PLQY. To investigate the detailed structural differences of CsPbCl3 perovskite NCs after reacted with different copper halides, transmission electron microscopy (TEM) were employed. The images are shown in Figure 2. It is seen that the pristine CsPbCl3 NCs (untreated) exhibit an imperfect and incomplete structure with a random size distribution of 10 ± 3 nm (Figure 2(a)). After treated with Cu halide complex, the crystallinity of CsPbCl3 and CsPbCl3-xBrx NCs (Figure 2 (b) to 2 (e)) are improved in comparison with the pristine one and no aggregation is found. The size of treated CsPbCl3 NCs (10 ± 1nm) is similar to that of the pristine one but the size of treated CsPbCl3-xBrx NCs increases slightly (12 ± 2 nm). Moreover, it is noted that the NCs treated by Cu2+-halide complex exhibit the cubic shape with more clear edges than that treated by Cu+ -halide complex. High resolution TEM image (Figure 2(f)) shows that the lattice distance of 0.31 nm is found in the representative CuCl2 treated CsPbCl3 NCs, which is related to (200) crystal plane of the perovskite cubic phase. The inset shows that Fast Fourier transforms (FFT) of treated CsPbCl3 reveals several index facets, including (101), (100) and (200). Also, the XRD pattern (Figure 3) indicates that all untreated CsPbCl3 NCs (a = 5.605 Å, space group Pm3̅m, ICSD 29072), and treated CsPbCl3, CsPbCl3-xBrx NCs remain the cubic phase in agreement with previous reports.1 Obviously, anion exchange reactions by using Cu halide would not change the crystal phase of the NCs. The XRD peaks of CsPbCl3 NCs reacted with the bromide precursor are left-shifted to small angles, representing that Cl ions are successfully exchanged by Br ions in the crystal structure of perovskite NCs. To study the transformation mechanism of perovskite NCs during anion exchanging process, the in-situ PL of CsPbCl3 reacted with CuCl2 and CuBr2 were measured to monitor their photo emission evolutions. In Figure 4 (a), it is found that when CsPbCl3 NCs react with CuBr2, the emission wavelength of CsPbCl3 NCs shifts from 410 nm to 460 nm instantly, meaning that some Cl ions are replaced by Br ions to form CsPbCl3-xBrx in few seconds. In contrast, the PL intensity of CsPbCl3 NCs treated with CuCl2 dramatically decreases at the beginning but gradually increase during an ongoing reaction (Figure 4 (b)). Eventually, the PL intensity is saturated in 60 minutes, which is higher than that of pristine CsPbCl3 NCs without any treatment. The results suggest that the anion exchange rate of CsPbCl3 with Br ions is much faster than that of CsPbCl3 with Cl ions, which can be ascribed


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to the lower activation energy (0.25 eV) of CsPbBr3 formation compared to that (0.29 eV) of CsPbCl3.22 To investigate the structural transformation of CsPbCl3 NCs during anion exchanging reaction, in-situ XRD was performed. In Figure 4(c), the results show that the cubic phase of pristine CsPbCl3 NCs is disappeared immediately when CsPbCl3 NCs react with CuCl2. In minutes, the CsPbCl3 NCs with cubic phase reappear in the same reaction solution. The broaden peak of 2~20o-30o is attributed to the silica materials of capillary container. The similar phenomenon is also seen in CsPbBr3 NCs and CuBr2 system (see supporting information, Figure S1). During the anion exchanging process, the mechanism is proposed that the cubic structure of perovskite NCs could be broken into pieces due to unstable crystallinity of the perovskite NCs. Then, the small pieces of perovskite structure subsequently become new nucleation seeds for recrystallization, resulting in better crystal structure formation of CsPbCl3 NCs with few defects as shown in Figure 4 (d). Cu ions are not involved in the crystal structure of CsPbX3 perovskite NCs confirmed by X-ray photoemission spectrum (Figure S2). Our finding is different from the previous literature report which indicated the cation exchange reactions of divalent transition metal halides, including Sn2+, Cd2+, and Zn2+, in the perovskite NCs by van der Stam, et al.31 To demonstrate the stability of copper halide complex-treated CsPbX3 NCs, the PL emission was tracked over 30 weeks (Figure 5 (a)). The results indicate that the PL emission of untreated CsPbCl3 NCs is dropped quickly to 50% of original intensity in a week. The CsPbCl3 and CsPbCl3-xBrx NCs treated with Cu2+-halide complex preserve a higher PL emission with a value of 90% and 99% after 20 weeks, which are much higher than 5% for CsPbCl3 and 3% for CsPbCl3-xBrx treated with Cu+-halide complex, respectively. Besides, no obvious changes in PL spectra for treated CsPbCl3 and CsPbCl3-xBrx NCs (after 30 weeks) are found in comparison with that of original treated or pristine NCs (Figure 5 (b)). The corresponding PL spectra of other samples are shown in the supporting information. Moreover, based on XRD analysis, there is no obviously phase changes observed in the treated NCs (see supporting information). To understand the role of Cu2+ ion in the CsPbCl3 NCs during anion exchanging reaction, the Cu K- and Pb L3-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were performed to investigate the bonding environment around Cu and Pb atoms. All samples were washed and centrifuged to remove additional organic surfactant and free Cu2+ ions. Figure 6 (a) shows the Pb L3-edge XANES spectra of pristine CsPbCl3 NCs, CsPbCl3 NCs treated with CuCl2 and CuBr2 (representing the electronic transitions from 2p core level to 5d unoccupied states) were close to the references of Pb(NO3)2 (Pb2+), suggesting that the oxidation states of Pb were +2 in all in perovskite NCs. The k2-weighted Pb L3-edge EXAFS spectra of those three samples are



shown in Figure 6 (b). A similar oscillation was found in pristine CsPbCl3 NCs and CsPbCl3 NCs treated with CuCl2, indicating that there is similar local coordination environment and local order around Pb atoms in the pristine CsPbCl3 and CsPbCl3 treated with CuCl2. This result also expresses that good crystal structure of perovskite NCs remains still even after a recrystallization process. This results are agreed with the analysis of XRD. To investigate the role of Cu2+ ions in the representative CsPbCl3 NCs treated with CuBr2, the Cu K-edge XAS spectra were obtained. No signal was found in transmission mode and it is only observed by fluorescence yield mode using 7-element silicon drift detector (Figure 6 (c)). This is ascribed to the few residues of Cu ion in the washed sample (less than 10 ppm). The k2-weighted Cu K-edge EXAFS data of CsPbCl3 NCs treated with CuBr2 is shown in Figure 6 (d). It’s found that the EXAFS oscillation of Cu K-edge is quite different from that of Pb L3-edge, indicating that no Cu2+ ions were doped into the crystal structure of CsPbCl3 NCs treated with CuBr2, meaning that the signal of Cu2+ ions is originated from that of Cu2+ ions adsorbed on the surface of the perovskite nanocrystals. To understand the important absorbing role of Cu2+ in stabilizing perovskite NCs crystal structure during anion exchanging reaction, density functional theory (DFT) was performed to simulate the adsorption energy of copper ion with different oxidation number on the surface of perovskite NCs. In general, it is believed that the capping agent effectively passivate the surface defect leading to PL quantum yield enhancement. Figure 7 shows that the representative models of DFT simulations are constructed by the CuCl2 adsorbed on defect-CsPbCl3(100) slab. The defect CsPbCl3(100) slab is modeled without one Cl atom on surface by a five-layer slab of CsPbCl3 lattice (Figure 7 (a)), with a slab surface with dimensions of a = 20.92, b = 17.78 Å, and c = 23.59 Å, separated by a vacuum space of 14 Å, exposing the (100) facet, shown in Figure 7 (b). The model of CuCl adsorbed on CsPbCl3 slab is provided in the supporting information. The adsorbed CuCl2 and CuCl molecules in the top three-layer are allowed to fluctuate by a given perturbation. The adsorption energy (Eads) of CuCl and CuCl2 adsorbed on the CsPbCl3(100) and defect CsPbCl3(100) systems was calculated, respectively as shown in Table 1. The most favorable adsorption site for the CuCl2 and CuCl on CsPbCl3(100) system with the molecular axis oriented perpendicular to Cs site and Pb site, respectively. The CuCl2 molecule finally adsorb in on-top site on CsPbCl3(100) system, a tilted configuration is shown in Figure 7 (c). From DFT calculations, the adsorption energy of CuCl2 are estimated to be -0.53 eV on Cs site of CsPbCl3 (100) and -0.4 eV on Pb site of CsPbCl3(100); the adsorption energy of CuCl is -0.22 eV on Cs site of CsPbCl3(100) and -0.38 eV on Pb site of CsPbCl3(100), respectively. The adsorption energy of CuCl2 is -3.60 eV on defect site of CsPbCl3(100). By contrast, CuCl is not able to be adsorbed on the defect sites. In summary, the interaction between CuCl2 and CsPbCl3(100) slab surfaces is much stronger than that between CuCl and CsPbCl3(100) slab surfaces. Moreover, the defect


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site of CsPbCl3 (100) surface systems can be stabilized by CuCl2 due to higher adsorption energy to form CuCl2-CsPbCl3(100). This finding can be comprehended by in terms of hard/soft acid/base interactions.32 A hard acid metal ion prefers to bind to hard halide ion. In the order of hard and soft acid metal ions, Cu2+ ion is a borderline hard acid and Cu+ ion is a soft acid. Therefore, a stronger interaction between Cu2+ ions and hard anion halide ions exposed on the surface of CsPbCl3 NCs or CsPbClxBr3-x NCs could be formed to prevent surface halide ion from reacting with organic cations in the solution, such as oleylamine. With Cu2+ ions absorbing on the surface of perovskite NCs, the stability of inorganic perovskite NCs crystal structure can certainly be improved. Conclusions It was found that the PLQY of CsPbCl3 and CsPbBr3-xClx NCs were strongly enhanced by CuX2 treatment via anion exchange reaction. The perovskite NCs with adsorption of CuX2 went through a recrystallization process, which resulted in the formation of better crystal structure with few surface defects and higher optical stability. DFT results indicated that CuX2 played an important role in stabilizing the surface of perovskite NCs with high adsorption energy. Especially, the re-growth rate of CsPbCl3 was found to be much slower than that of CsPbBr3. Therefore, the detailed studies of anion exchanging kinetics for the perovskite NCs will be further investigated. The understandings of growth rate for different perovskite CsPbX3 NCs could open a route for designing new perovskite structures for PVs, LEDs, and other applications.

Experimental section. Materials and chemicals. Cesium carbonate (Cs2CO3, 99.9%, 60-80 mesh, AK Scientific), oleic acid (OA, reagent grade, Fisher), 1-octadecene (ODE, 90%, Acros), oleylamine (OLA, 80-90%, Acros), lead chloride (PbCl2, 99%, Alfa), lead bromide (PbBr2, 98+%, Acros), n-trioctylphosphine (TOP, tech. 90%, Acros),copper( Ⅰ ) bromide (CuBr, 98%, Acros), copper( Ⅱ ) bromide (CuBr2, 99+%, Acros), copper( Ⅰ ) chloride (CuCl, 99%, Acros), copper( Ⅱ ) chloride (CuCl2, 99%, Acros), toluene (HPLC grade, Fisher), tert-butyl alcohol (tBuOH, 99.50%, Acros), methyl acetate (anhydrous, 99.5%, Aldrich), hexane (99%, Duksan) were all used without further purification. Preparation of Cs-oleate precursor 0.814 g Cs2CO3, 2.5 mL OA and 40 mL ODE were mixed in a 100 mL 3-neck flask and dried for 1h under N2 atmosphere at 120℃. The resulting mixture was heated to 150℃ under N2 until all Cs2CO3 reacted with OA to form Cs-oleate complex. The resulting complex was kept



at 150 °C to prevent precipitate of Cs-oleate from the solution at room-temperature. Synthesis and purification of CsPbX3 (X=Cl, Br) NCs For preparation of CsPbBr3 NCs, 15mL ODE and 0.207g PbBr2 were mixed in a 50 mL 3-neck flask and degassed under vacuum condition at 120 °C for 1h. Then, the solution of 1.5mL OA and 1.5mL OLA were injected into the flask at 120 °C after the resulting mixture was placed under N2 at 120 °C. After all PbBr2 salts were dissolved, the temperature was raised to 180°C. 1.5 mL pre-heated Cs-oleate solution was injected and finished in 10 seconds. Then the resulting solution was quenched by ice-water bath for 10 minutes to obtain the solution of CsPbBr3 NCs. For CsPbCl3 NCs synthesis, 15mL ODE, 0.156g PbCl2 and 3mL TOP were mixed in a 50 mL 3-neck flask and degassed at 120°C for 1h. The solution of 1.5mL OA and 1.5mL OLA were injected in to the flask under N2 at 120 °C. Until the PbCl2 salt was completely dissolved, the temperature was raised to 180°C and 1.5 mL pre-heated Cs-oleate solution was quickly injected and reacted for 5s. Then the resulting solution was quenched by ice-water bath for 10 minutes to obtain the solution of CsPbCl3 NCs. CsPbCl3 NCs treated with CuX and CuX2 during anion exchange reaction. First, 2 mL CsPbCl3 NCs solution was used and diluted with 4 mL toluene to form the precursor of perovskite NCs. Second, the copper halide solutions were prepared by dissolving CuXn (X=Cl, Br ; n=1 or 2) salt (1mmol for CuX2 and 2mmol for CuX, separately) in the solution consisting of 10mL toluene and 1mL OLA. To proceed the anion exchanging reaction, the resulting CuXn solution (2 mL) was added into the precursor of CsPbCl3 NCs solution under vigorous stirring and reacted for about 16 hours at room temperature. The treated CsPbX3 NCs were formed. Purification of obtained NCs 1mL of the treated NCs was added with 500μL methyl acetate and centrifuged at 13,000rpm for 30min. After the supernatant was discarded, the precipitate was re-dispersed in 300μL hexane. Then, 300μL methyl acetate was added and centrifuged at 13,000rpm for 30min. After purification process, the precipitate was re-dispersed in hexane for further HR-TEM and XRD analysis.

Characterizations The morphology and structure of CsPbX3 perovskites NCs were revealed by transmission electron microscope (TEM) and high-resolution TEM, respectively. The images of NCs morphology were received in a 200 kV transmission electron microscope (JEOL, 2100F) and


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120 kV transmission electron microscope (HITACHI, HT7700). The crystal structures and qualities of CsPbX3 perovskites NCs were determined by powder X-ray diffractometer (Rigaku Miniflex 600). UV/Vis spectra and Photoluminescence spectra were obtained from UV/VIS/NIR Spectrophotometer (Jasco V-770) and fluorescence spectrophotometer (HITACHI F-4500). PLQY was measured from Jasco spectrophotometer (FP-8500) with an integrating sphere (Jasco ILF-835). The excitation-wavelength of each sample is determined by the maximum intensity in the excitation spectrum.

Synchrotron Radiation measurements The in-situ XRD measurement was taken with the samples placed in a 1 mm inner diameter capillary tube. In-situ XRD analysis was performed at the beamline of BL01C at Taiwan Light Source (TLS) and the beamline of 09A at the Taiwan Photon Source (TPS) in National Synchrotron Radiation Research Center. The Cu K- and Pb L3-edge XANES/EXAFS spectra were obtained at beamline 44A of TPS in NSRRC. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detailed description of DFT calculation method of Cu+ and Cu2+ ion adsorbed on CsPbCl3 NCs. The additional in-situ XRD spectra, PL spectra, XPS spectra, TRPL spectra and HRTEM images of the perovskite NCs after treated with copper halide.

Acknowledgements This work has been financially supported by the Ministry of Science and Technology of Taiwan (MOST 106-2113-M-029-006-MY2 and MOST 106-2632-M-029-001) and Tunghai University. We thank Dr. U-Ser Jeng for assistance at beamline BL23A at Taiwan Light Source (TLS) and Dr. Hwo-Shuenn Sheu, Dr. Yu-Chun Chuang for assistance at beamline 09A at the Taiwan Photon Source (TPS).

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Figure Caption

Figure 1. The PL spectra of pristine CsPbCl3 and CsPbCl3 treated with (a) CuCl-OLA and CuCl2-OLA, (b) CuBr-OLA and CuBr2-OLA complex. The insets in (a) and (b) are photographs of fluorescent solution in vials and tables of PLQY values of pristine and treated samples.



Figure 2. TEM images of (a) pristine CsPbCl3, and CsPbCl3 treated with (b) CuCl2, (c) CuCl, (d) CuBr2 and (e) CuBr. (f) HRTEM image of the representative CuCl2 treated CsPbCl3 NCs. The inset is the image of Fast Fourier transforms of the CuCl2 treated CsPbCl3.


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Figure 3. XRD patterns of pristine CsPbCl3 NCs and CsPbBr3 and CsPbCl3 NCs treated with CuCl2, CuCl, CuBr2 and CuBr.



Figure 4. In-situ PL measurements of CsPbCl3 NCs treated with (a) CuBr2 and (b) CuCl2. (c) In-situ XRD spectra of CsPbCl3 NCs treated with CuCl2. (d) The schematic illustration of the recrystallization process of CsPbX3 perovskite NCs during anion exchanging process.


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Figure 5. (a) The emission stability test of pristine CsPbCl3 and the several treated CsPbCl3 NCs. (b) The tracking PL spectra of CsPbCl3 NCs treated with CuCl2 (upper) and CuBr2 (lower) for 1, 5 10, 20 and 30 weeks.

Figure 6. Local structure characterizations of pristine and treated CsPbCl3 NCs. (a) Pb L3-edge XANES spectra of pristine CsPbCl3, CsPbCl3 treated with CuCl2 and CuBr2 and (b) their k2-weighted Pb L3-edge EXAFS. (c) Cu K-edge XANES spectrum of CsPbCl3 treated with CuBr2 and (d) its k2-weighted Cu K-edge EXAFS spectrum.



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Figure 7. The CuCl2 molecule adsorbs on defect CsPbCl3(100) surface.(a) along y-axis direction, (b) top-view and (c) side-view.

Table 1. Adsorption energy, CuCl and CuCl2 adsorb on CsPbCl3(100) surface and defect CsPbCl3(100) surface, respectively. CuCl2

CuCl

Eads (eV)

Eads (eV)

Cs site-CsPbCl3(100)

-0.53

-0.22

Pb site-CsPbCl3(100)

-0.40

-0.38

defect-CsPbCl3(100)

-3.60

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TOC graphic


Enhanced Luminescence and Stability of Cesium Lead Halide

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