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Functional Nanostructured Materials (including low-D carbon)

Postsynthesis Phase Transformation for CsPbBr3/Rb4PbBr6 Core/shell Nanocrystals with Exceptional Photostability Bo Wang, Congyang Zhang, Shouqiang Huang, Zhichun Li, Long Kong, Ling Jin, Junhui Wang, Kaifeng Wu, and Liang Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04198 • Publication Date (Web): 15 Jun 2018 Downloaded from http://pubs.acs.org on June 15, 2018

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ACS Applied Materials & Interfaces

Postsynthesis Phase Transformation for CsPbBr3/Rb4PbBr6 Core/shell Nanocrystals with Exceptional Photostability Bo Wang, † Congyang Zhang, † Shouqiang Huang, † Zhichun Li, † Long Kong † Ling Jin, § Junhui Wang, ‡ Kaifeng Wu, ‡ and Liang Li*,†,# †

School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800

Dongchuan Road, Shanghai 200240, China §

Shanghai Starriver Billingual School, 2588 Jindu Road, Shanghai 310112, China

‡

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical

Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China #

Shanghai Institute of Pollution Control and Ecological Security, 1239 Siping Road, Shanghai

200092, China KEYWORDS: cation exchange, core/shell nanocrystals, perovskites, phase transformation, photostability

ABSTRACT

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Lead halide perovskite nanocrystals (NCs) as promising optoelectronic materials are intensively researched. However, the instability is one of the biggest challenges needed to overcome before fulfill their practical applications. To improve their stability, we present a postsynthetic controlled phase transformation of CsPbBr3 toward CsPbBr3/Rb4PbBr6 core/shell structure triggered by rubidium oleate treatment. The resulted core/shell NCs show exceptional photostability both in solution and on-chip. The solution of CsPbBr3/Rb4PbBr6 NCs can remain over 90% of the initial emission photoluminescence quantum yields (PLQY) after 42 h of intense light-emitting diodes illumination (450 nm, 175 mW/cm2), which is even better than the conventional CdSe/CdS quantum dots whose emission drop to 50% after 18 h under the same condition. We believe that the exceptional photostability should be resulted from the protection of the robust Rb4PbBr6 shell on CsPbBr3 NCs.

INTRODUCTION In recent years, cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals (NCs) have drawn large attention due to their outstanding optical properties, such as tunable emissions in the whole visible spectrum, high photoluminescence quantum yields (PLQY), and narrow emission line width,1,2 making them candidates for various optoelectronic applications, such as solar cells, lasers, light-emitting diodes (LED), and backlight display.3-5 Furthermore, due to the relative weak formation energy and high defect tolerance, they can be easily synthesized with simple procedures under a much low temperature. Benefiting from these excellent properties, CsPbX3 NCs show great potential to be an alternative of traditional metal chalcogenide-based quantum dots (QDs). However, the fatal drawback of perovskite NCs seems to be their inherent poor


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stability when exposure to oxygen, light, temperature, and moisture,6,7 which tremendously hinders their practical applications. To improve the stability of perovskite NCs, many efforts have been done, including inorganic oxide (silica, alumina or titanium dioxide) and polymer encapsulation,8-13 incorporation in mesoporous silica,14 embedding in the graphene oxide matrix,15 and surface anchoring with silica sphere.16 Unfortunately, the existing insulating barrier layer and relative large size of these integrated composite materials make them not well suited for many optoelectronic and bioimaging applications. In addition, many surface treatment strategies have been employed to post-treatment of perovskite NCs,17-21 and some of them are helpful to protect perovskite NCs and improve their PLQY, such as didodecyl dimethylammonium sulfide (S2−-DDA+) passivation,22 introducing high-affinity ligands and thiocyanate surface treatment.23,24 However, the ligands is still dynamically binding on NCs surface, easy to dissociate in certain conditions, such as post-treatment process or extreme dilution. Ligands surface treatment still cannot improve the intrinsic stability of perovskite NCs. Therefore, it is still a great challenge to simultaneously improve stability and PLQY of individual perovskite NCs. As we know, for traditional QDs, an important strategy to improve QDs’ surface passivation and stability is overgrowth with an epitaxial shell of a second semiconductor, resulting in a core/shell structure. According to this concept, core/shell structure seems an ideal approach to obtain luminescent perovskite NCs with excellent property (especially the stability). Hence, we envisage that if the perovskite NCs can be coated with an epitaxial shell, both the stability and PLQY could be dramatically enhanced. However, most efforts of conventional methods for shell coating failed due to intrinsic “soft nature” (high ion mobility), such as, intermixing of halide anions between the core and shell,2,25 resulting in mixed-halide perovskite NCs. To our best knowledge, mixed


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methylammonium–octylammonium lead bromide perovskite NCs and Mn2+:CsPbCl3/CsPbCl3 are the only two reported core/shell structured NCs.26,27 Lately, several papers reported another zero-dimensional (0D) cesium lead halide perovskite (Cs4PbBr6) with higher exciton binding energy and broader band gap than CsPbBr3,28,29 which shows better structural stability against surrounding environments. Further, according to Quan’s work,30 CsPbBr3 NCs embedded in Cs4PbBr6 matrix with high PLQY and stability can be synthesized due to good lattice matching of them and improved passivation. In addition, Grätzel et al.31 and Ko et al.32 reported that photovoltaic performance and stability of perovskite solar cells could be effectively improved by incorporation of small and oxidation-stable rubidium cation (Rb+), suggesting that Rb doping is also a robust strategy to stabilize perovskite structure. Inspiration by these findings, we came up a concept that the 0D perovskite (such as Cs4PbBr6 or Rb4PbBr6) may be a potential passivation shell material for constructing the core/shell structure of perovskite NCs. In this communication, we developed a novel approach to prepare highly stable and luminescent perovskite NCs through postsynthetic controlled phase transformation and cation exchange Cs+ with Rb+ simultaneously. To realize this concept, rubidium oleate (RbOA) was used to trigger the phase transformation and cation exchange process toward a hybrid CsPbBr3/Rb4PbBr6 NCs, which is similar to the surface treatment of PbSe by cadmium oleate for obtaining PbSe/CdSe core/shell QDs.33 Due to the excellent passivation of the CsPbBr3 NCs surface, the PLQY of CsPbBr3/Rb4PbBr6 NCs was enhanced significantly. Most impressively, the obtained NCs showed outstanding stability under blue LED illumination and ambient atmosphere, which was comparable to CdSe/CdS QDs. Through the optical, structure characterizations and the stability


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results, we preliminarily determined the core/shell structure of the hybrid perovskite NCs consists of a CsPbBr3-riched core and a Rb4PbBr6-riched shell. EXPERIMENTAL SECTION Chemicals. Oleic acid (OA, 90%, Aldrich). Cesium carbonate (Cs2CO3, 99.9%), rubidium carbonate (Rb2CO3, 99.8%), potassium oleate (KOA, 98%), sodium acetate trihydrate (99.5%), lead bromide (PbBr2, 99%), 1-octadecene (ODE, 90%), oleylamine (OAm, 90%) were purchased from Aladdin. Methyl acetate (≥97.5%), toluene (≥99.5%), acetone (≥99.5%) were purchased from Sinopharm Chemical Reagent. All the chemicals were used without further purification. Preparation of CsOA precursor. 6 mmol Cs2CO3 (1.955 g), 12 mL OA, and 28 mL ODE were added into a 100 mL 3-neck flask and evacuated for 1 h at 120 °C, then the temperature was raised to 150 °C under argon flow to maintain 0.5 h until the solution became optically clear. After that, the solution was allowed to cool to room temperature, and stored as stock solution (0.3 M CsOA). Preparation of RbOA precursor. 2.46 mmol Rb2CO3 (0.569 g), 2.5 mL OA, and 30 mL ODE were added into a 100 mL 3-neck flask and evacuated for 1 h at 120 °C, then the temperature was raised to 180 °C under argon flow to maintain 0.5 h until the solution became optically clear. After that, the solution was allowed to cool to room temperature, and stored as stock solution (0.15 M RbOA). Preparation of 0.15 M NaOA and KOA precursor took the same procedures, except for addition of sodium acetate trihydrate and KOA, respectively. Preparation of Cd(OA)2 precursors. 175 mmol CdO (22.472g), 175 ml OA and 175 ml ODE were added into a 500 mL 3-neck flask and evacuated for 1 h at 120 ºC, then the temperature was raised to 250 ºC under argon flow to maintain 0.5 h until the solution became optically clear.


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After that, the solution was allowed to cool to room temperature, and stored as stock solution (0.5 M Cd(OA)2). Synthesis of CsPbBr3 NCs. 1.44 mmol PbBr2 (528.4 mg), 20 mL ODE, 4 mL OAm, and 4 mL OA, were added into a 100 mL three-neck flask and evacuated for 30 min at 120 ºC to completely dissolve the PbBr2 salt. Then, the temperature raised to 180 °C under argon atmosphere, and 1.26 mL of CsOA precursor (0.3 M), which was pre-heated at 100 °C, was quickly injected into the prepared solution. After 15 s, the three-neck flask was placed in an icewater bath and cooled to room temperature. Equal methyl acetate was added into the crude solution and precipitated via centrifugation at 10000 rpm. Then, the precipitate was dispersed in 30 mL toluene solution. To purifying the final NCs oncemore centrifugation was required. Several batches of products mixed together as original solution for subsequently rubidium cation exchange, and the concentration of this solution was 2.5 µmol/mL determined by ICP. Synthesis of CdSe core QDs. 16 mmol CdO (2.054g), 40 mL ODE, and 16 mL OA were added into a 250 mL 3-neck flask and evacuated for 30 min at 120 ºC, then the temperature was raised to 270 ºC under argon flow until the solution became optically clear, following 8 mmol Se (dissolved in TOP) was quickly injected into the reaction solution. After that, the temperature was set at 240 ºC for the growth of the nanocrystals. The solution was cooled down to room temperature after 3 min growth, and then CdSe QDs were precipitated and purified with methanol and hexane, then stored in toluene as stock solution. Synthesis of CdSe/CdS core/shell QDs. 0.8 mmol of washed CdSe core (cadmium atomic concentration) was dispersed into 40mL ODE in a 250 mL 3-neck flask and evacuated for 30 min at 120 ºC to remove the toluene, the temperature was raised to 280 ºC under a argon


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atmosphere. Starting at 230 ºC, precursor solutions of Cd(OA)2 (0.5 M) and 1-dodecanethiol (0.6 M) diluted in ODE was injected from separate syringes pump by a rate of 3.2 mL/h, and the injected time set as 25 h for the growth of CdS shell. Postsynthesis of CsPbBr3/Rb4PbBr6 core/shell NCs. Quantitive RbOA solution (Rb/Cs=0.6, 0.9, 1.2, and 1.32) and 10 mL CsPbBr3 NCs solution loaded into 20 mL bottle, and mixed for cation exchange reaction. Aliquots of the sample were taken at different time intervals and injected into cuvette containing fixed volume toluene for recording their absorption spectra. Photoluminescence (PL) spectra of different time intervals were recorded in situ by ocean optical spectrometer. After 20 min of the cation exchange reaction, equal volume of acetone was added to precipitate and separate products from the mixed solution via centrifugation at 10000 rpm. The precipitates were collected for further characterization. The process for NaOA, KOA and CsOA reaction with CsPbBr3 NCs were similar to RbOA. LED package. 1 mL CsPbBr3 and RbOA treated NCs (A450=2.7) in toluene solution were mixed with 300 mg of PDMS A and 30 mg PDMS B (A:B = 10:1, wt. %), respectively. In order to remove the toluene and bubbles, the resulting mixtures were heated to 50 °C for 1 h. After that, the mixtures were dropped onto a blue chip and then thermally cured for 1 h at 80 °C and 120 °C in an oven, respectively. Characterization. X-ray power diﬀraction (XRD) patterns of the CsPbBr3 and treated NCs were detected by the BrukerD8 Advance X-ray Diffractometer at 40 kV and 30 mA using Cu Kα radiation (λ=1.5406 Å). The high-resolution transmission electron microscope (HR-TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were obtained from the FEI Talos F200X TEM instruments operated at an accelerating


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voltage of 200 kV. The surface composition measurement was performed with XPS (Kratos Axis UltraDLD), and all the spectra were calibrated to the C 1s peak at 284.8 eV. UV-vis absorption spectra were recorded on a Cary-60 UV-vis spectrophotometer. Photoluminescence (PL) spectra were taken using a F-380 fluorescence spectrometer (Tianjin Gangdong Sci. & Tech. Development Co., Ltd., China). In situ PL spectra were recorded by ocean optical spectrometer. The absolute photoluminescence quantum yields (PLQY) and fluorescence lifetimes were performed on a time-resolved fluorescence spectrofluorometer (QM/TM/IM, PTI, USA). The excitation wavelength of PLQY and fluorescence lifetimes was 450 nm, and the emission wavelengths of CsPbBr3, Rb/Cs=0.6, 0.9, and 1.2 samples were 509, 508, 508, and 505 nm, respectively. The Valance band (VB) spectra were measured with a monochromatic He I light source (21.2 eV) and a VG Scienta R4000 analyzer. Femtosecond pump-probe transient absorption (TA). The femtosecond pump-probe TA measurements were performed using a regenerative amplified Ti:sapphire laser system (Coherent; 800 nm, 70 fs, 6 mJ/pulse, and 1 kHz repetition rate) as the laser source and a femtoTA100 (Time-Tech Spectra) as the spectrometer. Briefly, the 800 nm output pulse from the regenerative amplifier was split in two parts with a 50% beam splitter. The transmitted part was used to pump a TOPAS Optical Parametric Amplifier (OPA) which generated a wavelengthtunable laser pulse from 250 nm to 2.5 µm as pump beam. The reflected 800 nm beam was split again into two parts. One part with less than 10% was attenuated with a neutral density filter and focused into a 2 mm thick calcium fluoride window to generate a white light continuum (WLC) used for probe beam. The probe beam was focused with an Al parabolic reflector onto the sample. After the sample, the probe beam was collimated and then focused into a fiber-coupled spectrometer with CMOS sensors and detected at a frequency of 1 KHz. The intensity of the


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pump pulse used in the experiment was controlled by a variable neutral-density filter wheel. The delay between the pump and probe pulses was controlled by a motorized delay stage. The pump pulses were chopped by a synchronized chopper at 500 Hz and the absorbance change was calculated with two adjacent probe pulses (pump-blocked and pump-unblocked). The samples were placed in 1 mm airtight cuvettes in a N2-filled glove box and measured under ambient conditions. Samples were vigorously stirred in all the measurements. RESULTS AND DISCUSSION The original CsPbBr3 NCs were pre-synthesized by a modified hot-injection synthesis strategy.1 To obtain the CsPbBr3/Rb4PbBr6 NCs, different amounts (15, 22.5, and 30 µmol) of RbOA were added to react with 10 mL CsPbBr3 NCs solution. For simplicity, these three samples were labeled as the nominal ratios of Rb/Cs calculated as 0.6, 0.9, and 1.2, respectively. After the reaction was evolved for 20 min, the products were precipitated with acetone and redissolved in toluene for further characterizations (more details in Experimental Section). The normalized absorption and PL spectra were shown in Figure 1a, b. At the initial stage, the original CsPbBr3 NCs presented an absorption band edge of 496 nm and PL emission of 507 nm. After Rb treatment, the absorption and PL peaks were gradually blue-shifted to shorter wavelength (Figure S1), and this blue-shift became more obvious as Rb increased (Figure 1a, b). However, when excess RbOA adding (Rb/Cs=1.32), the excitonic absorption and PL peaks disappeared eventually (Figure S1g, h). Meanwhile new absorption peaks appeared at 313 nm, and the intensities also enhanced as Rb increased (insert in Figure 1a). This emerging absorption, similar to the previous report of Cs4PbBr6 NCs,28 maybe originated from the absorption of 0D perovskite (such as Rb4PbBr6) in our hybrid NCs. As shown in Figure 1c, the absolute PLQY of hybrid perovskite NCs were gradually increased with the increase of the amount of Rb. Especially,


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when the Rb/Cs=1.2, the value reached as high as 85% compared to the 40% of the original sample. To detail understand the decay dynamics, the time-resolved PL decay spectra were obtained and fitted with a bi-exponential decay model (Figure 1d). Obviously, after Rb treatment, the average lifetime increased from 8.0 ns of CsPbBr3 NCs to 11.5 ns of Rb/Cs=1.2 sample (Table S1). This result indicates that the Rb treatment provide better passivation of CsPbBr3 NCs and suppress the non-radiative recombination, which is also consistent with the enhanced PLQY.

Figure 1. Normalized absorption spectra (a), PL (b), absolute PLQY (c), and time-resolved PL decays spectra and the fitted curves (d) of CsPbBr3 NCs with/without Rb treatment. The images of CsPbBr3 NCs solution with/without Rb treatment under ambient (left) and 365 nm lamp (right) inserted in (c), I=CsPbBr3, II=Rb/Cs=0.6, III=Rb/Cs=0.9, IV=Rb/Cs=1.2. X-ray diﬀraction (XRD) was carried out to further investigate the crystal structure after the Rb treatment. As show in Figure 2a, the XRD pattern of the pure CsPbBr3 NCs shows the dominant


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diffraction peaks at 15.1°, 21.5°, and 30.6°, which can be assigned to the (100), (110), and (200) planes of cubic perovskite structure (JCPDS 54-0752). After treatment, the main peaks of CsPbBr3 become weaker as the ratio of Rb/Cs increased from 0.6 to 1.2. Meanwhile, several new diffraction peaks appear and gradually increase, eventually become the major phase, which is resulted from the phase transformation process from CsPbBr3 to Rb4PbBr6 structure (JCPDS 250724). In order to confirm the cation exchange reaction and formation of Rb4PbBr6 rather than Cs4PbBr6, Cs-oleate (CsOA, 30 µmol) was used as the reaction agent for comparison. The XRD patterns of the obtained products are shown in Figure 2b. Compared to the Cs treated sample, the diffraction peaks of Rb treated perovskite NCs all shift toward higher angles due to the reduced lattice parameters with smaller alkali metal ion (Rb+).32

(c)

(d)

Figure 2. (a) XRD patterns of CsPbBr3 NCs with/without Rb treatment (black line: CsPbBr3 JCPDS 54-0752, red line: Rb4PbBr6 JCPDS 25-0724), (b) XRD patterns of Cs and Rb treated CsPbBr3 NCs. TEM and HR-TEM (insert) images of CsPbBr3 NCs (c) and Rb/Cs=1.2 sample (d).


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Figure 2c, d and Figure S2 show the transmission electron microscope (TEM) images of CsPbBr3 NCs before and after Rb treatment. From the Figure 2c, CsPbBr3 NCs exhibit cubic shape, and the average size is about 8.5 nm (Figure S2c). After Rb treatment (Rb/Cs=1.2), the hybrid perovskite NCs still uniformly distribute without any isolated impurities, but the average size increase to 10.2 nm (Figure S2d). In addition, the corners of Rb treated perovskite NCs become smooth compared to that of original CsPbBr3 NCs (Figure S2), which may be caused by the formation of Rb4PbBr6, since they are the rhombohedral phases.34 The element mappings of Rb treatment sample has been obtained and shown in Figure S3. It is obvious that the Rb and Cs clearly distribute in the NCs, which further demonstrates Rb+ incorporation into CsPbBr3 NCs and exchange with Cs+. From the above optical, structure and morphology results, we can confirm the phase transformation and cation exchange processes in our approach. In order to determine the ratio of phase transformation, the Rb treated sample (Rb/Cs=1.2) was analyzed by X-ray photo-electron spectroscopy (XPS). As shown in Figure S4, the new signal of Rb appears distinctly, and the atomic ratio of Cs/Rb is 1.00/5.71 (Table S2). Accordingly, we speculated that the molar percentage of the Rb4PbBr6 phase in the hybrid perovskite NCs is 58.8%. Further, from the energy dispersive X-ray spectroscopy (EDS) data (Table S3), the atomic ratio of Cs/Rb is 1.00/4.27 after Rb treatment with 51.6% of Rb4PbBr6 phase in the hybrid perovskite NCs, which is consistent with the result of XPS.


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Figure 3. Photostability of solution (a,d) under illumination with a 450 nm LED light (175 mW/cm2). PL peak shift (b) as illumination time prolonged. Photostability of powders sealed with PDMS on the LED chips (c). While, what surprise us in this work is the amazing photostability of the Rb treated perovskite NCs. The photostability tests of perovskite NCs solutions were performed on a blue LED module (peak at 450 nm, Philips Fortimo) with a power of 175 mW/cm2. As comparison, we chose CdSe/CdS QDs as a reference synthesized follow our previous report (more details in Experimental Section).35 The absorption and PL spectra, TEM of CdSe core and CdSe/CdS QDs (Figure S5, S6, S7) testified formation of CdSe/CdS core/shell QDs. The stability results (Figure 3a) showed that the relative PLQY of CsPbBr3 NCs decreased dramatically under continuous light illumination, which only retained 4.1% of its original value after 2 h. After Rb treatment, the photostability of perovskite NCs was significantly improved. Especially, the Rb/Cs=1.2 sample exhibited the best stability (the initial PLQY was still preserved over 90% after


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illumination 42 h), which was even much better than the CdSe/CdS QDs (the initial PLQY only remained 50% after illumination 18 h). Besides, the stability of Rb/Cs=0.9 sample was also enhanced (retained 50% after illumination 26 h) and comparable to the CdSe/CdS QDs (Figure 3a). As we all known, PL peak position shifts will affect the performance of QDs based optoelectronic devices. For further detail analysis, the emission peak shift was also investigated. As shown in Figure 3b, under continue illumination, the original CsPbBr3 NCs exhibit obvious red-shift. This can attribute to the dynamic nature of ligand and the crystal growth of CsPbBr3 NCs.6 However, after Rb treatment, this PL peak shift phenomenon is gradually suppressed. For the Rb/Cs=1.2 sample, PL peak position can be keep almost unchanged, which is also superior to CdSe/CdS QDs. To exclude the effects from the increasing OA ligands density with adding RbOA, equal molar of OA was substituted for RbOA. As shown in Figure S8, obviously, the parallel OA treatment won’t significantly improve their photostability. Meanwhile, the photostability of CsPbBr3/Rb4PbBr6 NCs film was also significantly improved, the relative PLQY of Rb/Cs=1.2 sample still remained 60% of its original value after illumination 10 h (Figure S9, 175 mW/cm2), however, the remnant PLQY of CsPbBr3 NCs was less than 30% of its original value after illumination 2 h. As contrast, the stability of CdSe/CdS QDs film was better than CsPbBr3/Rb4PbBr6 NCs film, kept over 86% of its original value after illumination 10 h. To further prove the superior stability in practical display application, we test the operational stability of CsPbBr3/Rb4PbBr6 NCs blending in polydimethylsiloxane (PDMS) on the blue LED chips (peak at 455 nm) operated at 20 mA and 3.0 V under ambient temperature. As shown in Figure 3c, the remnant PLQY of CsPbBr3 sample was lower than 18% of the initial value after operation 2 h, while, the Rb/Cs=1.2 sample still remained nearly 80% after operation 8 h, which is comparable to the stability of CdSe/CdS sample (the value of relative PLQY


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remained over 90% after operation 8 h). We should mention that, being different from the NCs solution, the stability of Rb treated perovskite NCs sample as film and on LED chips were lower than CdSe/CdS sample, maybe due to intrinsic “soft nature” of perovskite NCs. Nonetheless, all these experiments data both demonstrate that CsPbBr3/Rb4PbBr6 hybrid NCs possesses excellent stability than original CsPbBr3 NCs. Accordingly, from these aforementioned results, including the blue-shift of absorption and PL spectra, improved lifetime, enhanced PLQY, the phase transformation process, NCs with increased size and smooth corners, and the surprisingly photostability, we primarily conclude that Rb treatment with CsPbBr3 NCs partly transform CsPbBr3 into Rb4PbBr6 and generate a core/shell structure consists of a CsPbBr3-rich core and a Rb4PbBr6-rich shell. Compared to the original CsPbBr3 NCs, this Rb treatment may cause the size decrease of CsPbBr3 core, and thick Rb4PbBr6 shell is likely formed on the surface of CsPbBr3 NCs, with the significant enhancement of photostability and PLQY. However, similar to previous reports of perovskite core/shell NCs,26,27 we cannot identify the existence of core and the shell from HR-TEM owing to the low electron density contrast between them. In order to explore the band alignment in the Rb/Cs samples for explain their enhanced PL properties as compared to the CsPbBr3 sample, we performed transient absorption (TA) measurements on CsPbBr3 and Rb/Cs=1.2 samples. In these experiments, a 400 pump pulse (with an energy density of ~8.4 µJ/cm2) was used to excite the samples and the induced absorption changes as a function of both wavelength and time were recorded by a white light probe pulse. According to the absorption spectra in Figure 1a inset, the 400 nm light should only excite the CsPbBr3 domain in the Rb/Cs=1.2 sample because it has considerably lower energy (by ~0.34 eV) than the onset of the Rb4PbBr6 domain at ~360 nm. Figure 4a shows the TA


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spectra of the Rb/Cs=1.2 and pure CsPbBr3 samples. The spectral features of both samples are dominated by a bleach feature at ~500 nm due to the state filling effect in CsPbBr3. Neither sample shows obvious bleach feature in the

Postsynthesis Phase Transformation for CsPbBr3 ... - ACS Publications

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