Embedded Two-Dimensional Perovskite Nanoplatelets with Air-Stable

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Embedded two dimensional perovskite nanoplatelets with air-stable luminescence Xiaoli Xu, Haiping He, Jing Li, Zhishan Fang, Lu Gan, Lingxiang Chen, and Zhizhen Ye ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21396 • Publication Date (Web): 31 Jan 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Applied Materials & Interfaces

Embedded two dimensional perovskite nanoplatelets with air-stable luminescence Xiaoli Xu¶,ϯ, Haiping He¶,*, Jing Li¶, Zhishan Fang¶, Lu Gan¶, Lingxiang Chen,§ Zhizhen Ye¶,* ¶ State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Ϯ Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China § College of Biomedical Engineering & Instrument Science, Key Laboratory for Biomedical Engineering of Ministry of Education, Zhejiang University, Hangzhou 310027, China ABSTRACT: Two-dimensional (2D) perovskites represent a class of promising nanostructures for optoelectronic applications owing to their giant oscillator strength transition of excitons and high luminescence. However, major challenges lie in the surface ligand engineering and ambient stability. Here we show that air-stable quasi-2D CsPbBr3 nanoplatelets (NPLs) can be formed in the matrix of Cs4PbBr6 nanosheets by reducing the thickness of Cs4PbBr6 to ~7.6 nm, the scale comparable to the exciton Bohr radius of CsPbBr3. The 2D behavior of excitons is evidenced by the linear increase of radiative lifetime with increasing temperature. Moreover, the wide bandgap Cs4PbBr6 plays roles of surface passivation and protection, which leads to good photoluminescence properties without photobleaching effect and with ambient stability for over one month. Our work demonstrates a unique quasi2D heterostructure of perovskite nanomaterials which may either serve as a workbench for studying the exciton recombination dynamics or find application in high performance optoelectronic devices. Keywords: lead halide perovskite, two dimensional exciton, heterostructure, luminescence

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INTRODUCTION All-inorganic lead halide perovskites have been regarded as promising light-emitters since the successful synthesis of CsPbX3 nanocrystals (NCs) with high photoluminescence quantum yield (PLQY) exceeding 90%, narrow emission linewidth, and tunable wavelength covering the blue to red spectral range.1-6 Most importantly, the emissions from CsPbX3 NCs are excitonic in nature, which reveal much faster recombination than free charges in perovskite films.7,8 Among these nanostructures, quasi-two dimensional (2D) nanoplatelets (NPLs) are of particular interests because the ground state of excitons in such nanostructures could have a giant oscillator strength transition,9 which not only enhances the absorption cross-section but also further shortens the radiative lifetime of exciton recombination.9-12 It has been reported that the radiative lifetime of 2D excitons in CsPbBr3 NPLs reaches only about 1 ns at low temperature.1,11 Such a feature is very promising because a short radiative lifetime is critical to achieving perovskite light-emitting diodes (PeLEDs) with high brightness and low turn-on voltage.13,14 Despite the above advantages, the performances of PeLEDs based on colloidal nanostructures may still be limited by several drawbacks, including the charge/energy transfer resulting from particle agglomeration when forming films and the unsatisfied ambient stability.15-19 Recently, Sargent group developed an effective strategy to address these issues by embedding CsPbBr3 NCs into lattice matching Cs4PbBr6 crystalline matrix.20 Such a design brings about improved surface passivation, enhanced radiative recombination, and suppressed agglomeration, which eventually lead to a high PLQY of perovskites in solid state. The strategy can in principle be extended to perovskite NPLs to realize efficient 2D exciton recombination. However, a challenge lies in the growth mechanism and shape control of perovskite nanostructures in the 3D Cs4PbBr6 matrix, which is little understood and rarely studied so far. Alternatively, it is possible to obtain perovskite NPLs in the Cs4PbBr6 matrix via reducing the dimension of Cs4PbBr6 into 2D-like nanosheets, which provides restriction of the CsPbBr3 growth in the thickness dimension.


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In this work, we report for the first time a quasi-2D heterostructure of CsPbBr3/Cs4PbBr6 perovskites. We demonstrate that NPL-like CsPbBr3 can be formed in the matrix of 7.6 nm-thick Cs4PbBr6 nanosheets. Such a thickness is comparable to the exciton Bohr radius of CsPbBr3, which enables the formation of 2D exciton states in the embedded CsPbBr3. The 2D nature of CsPbBr3 pervoskite is evidenced by both transmission electron microscopic (TEM) observations and the linear temperature dependence of the radiative lifetime of excitons. The excitonic emission shows short lifetime and good ambient-stability. EXPERIMENTAL SECTION Materials Lead bromide (PbBr2, >98%, Sigma-Aldrich), cesium bromide (CsBr, 99.999%, SigmaAldrich), and cetyltrimethylammonium bromide (CTAB, 98%, Aladdin) were of analytical grade and be used as received without further purification. N,N dimethyl formamide (DMF, 99.9%) was purchased from Alfa Aesar. Toluene (AR, QS) was purchased from Shanghai Hushi Co. Ltd. and be used as received. Synthesis of Cs4PbBr6/CsPbBr3 perovskite crystals The perovskite crystals were synthesized by one-step antisolvent precipitation synthetic strategy. First, 15 mM CsBr, 15 mM PbBr2 and 7.5 mM CTAB were fully dissolved in 3 mL DMF under vigorous magnetron stirring. Second, 0.75 mL toluene was sprayed into the precursor solution, accompanied by continuous vigorous magnetron stirring. The toluene acts as an antisolvent and induces the formation of micelle (nucleation), from which process the subsequent crystalline growth can be obtained. The final precipitation was washed with toluene for 4 times, and be preserved at open atmosphere. The whole synthesizing process was accomplished in ambient condition and without intentionally heating. Instruments and Characterization X-ray diffraction (XRD) characterization was conducted using an X’pert PRO diffractometer (PANalytical) operated at 40 KeV and 40 mA equipped with a Cu Ka radiation source (λ = 1.5406 Å). Scanning Electron Microscopy (FESEM) images were carried out using a Hitachi-S4800 microscope. ACS Paragon Plus Environment


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Transmission Electron Microscopy (TEM) and HR-TEM images were obtained using FEI Technai G2 F20 transmission electron microscope at an accelerating voltage of 200 kV. Atomic force microscope (AFM, Bruker Vecco and Oxford Cypher S) was used to measure the thickness of perovskite nanosheets. X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCALAB 250 spectrometer using Al Kα radiation as the excitation source (1486.6 eV, 150 W). The PL, PL excitation spectra and PL Kinetic scan measurements were carried out on an FLS920 fluorescence spectrometer equipped with a Xenon lamp and a 405 nm laser (Edinburgh Instruments Ltd.). The PLQY was measured using an cary-4000 integration sphere incorporated into a ZCEC-150172F fluorescence spectrometer at room-temperature. The sample for PLQY measurement was prepared by spin-coating on glass slide.

Scheme 1. Schematic illustration of (a) ambient condition synthesis processes for the CsPbBr3 /Cs4PbBr6 composite, and (b) quasi-2D CsPbBr3 nanoplatelets embedded in Cs4PbBr6 nanosheet matrix. CsPbBr3/Cs4PbBr6 nanosheets were synthesized by one-step antisolvent precipitation strategy,21 by adding CTAB in the precursor solution to restrict the growth along certain dimensions.22 A schematic growth process is illustrated in Scheme 1a. We carried out a series of experiments to investigate the effect of CTAB on perovskite nanosheets, and the content of CTAB is found critical to the morphology of products (Figure S1). To obtain Cs4PbBr6 nanosheets, generally the mole ratio of CTAB to CsBr is fixed at 1:2. CsPbBr3 nanostructures are formed in the Cs4PbBr6 matrix during the precipitation process, as reported by Seth et al.22 Wang and Xuan, et al also presented a similar embedded CsPbBr3 nanocrystals (NCs) in the microscale Cs4PbBr6 matrix (CPB113/CPB416).23 As will be confirmed later,


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our growth process leads to CsPbBr3 /Cs4PbBr6 nanosheets in which the CsPbBr3 is NPL-like. A schematic microstructure of the products is shown in Scheme 1b.

Figure 1. Fundamental characterization of the CsPbBr3 /Cs4PbBr6 composites. (a) SEM image shows the thin circular nanosheets. Scale bar: 1 μm. (b) AFM image of a representative nanosheet. The height profile of the nanosheet shows a thickness of ~6 nm. (c) XRD pattern of the composites, showing the co-existence of Cs4PbBr6 and CsPbBr3. (d) TEM image of the composite nanosheets showing the dark ACS Paragon Plus Environment


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areas with lateral size up to tens of nanometers. Scale bar: 50 nm. (e) HRTEM image of the area labeled by yellow dash in (d). It shows the lattice of CsPbBr3 and the presence of Morie fringe pattern. Scale bar: 5 nm. The SEM image of the synthesized material is shown in Figure 1a. The product is thin circular nanosheets with diameter ranging from submicron to several microns. Atomic force microscopy (AFM) measurements (Figure 1b, Figure S2) reveal that the nanosheet is very thin with an average thickness of ~7.6 nm. The structure of the product was characterized by powder XRD, as presented in Figure 1c. The diffraction peaks can be indexed to a majority of hexagonal Cs4PbBr6 and a minor cubic-phase CsPbBr3, similar to the results recently reported by Sargent group.20 The results indicate that the synthesized material is CsPbBr3 /Cs4PbBr6 composite nanostructures. We note that the concentration of CTAB in the precursor solution was elaborately tuned and all the samples show the coexistence of CsPbBr3 and Cs4PbBr6, as illustrated by XRD patterns (Figure S3). The coexistence of CsPbBr3 and Cs4PbBr6 is also supported by quantitative analysis of XPS measurements (Figure S4). The elemental mole ratio of Cs:Pb:Br is estimated to be 3.28:1:4.29, which deviates from the stoichiometric ratio of both CsPbBr3 and Cs4PbBr6 obviously, indicating a CsPbBr3 content of ~7.5 at% in the composite. Figure 1d presents the TEM image of the composite nanosheets. Interestingly, some nanostructures with lateral size ranging from several nanometers to tens of nanometers can be clearly observed as the dark areas in the much larger matrix. High resolution images of the dark region (Figure 1e and Figure S5) show lattice spacing of 0.595 nm and 0.295 nm corresponding to the (100) and (200) planes of CsPbBr3, confirming the presence of NPL-like CsPbBr3. Electron diffraction pattern of the entire nanosheet (Figure S6) shows diffraction spots with lattice spacing of 0.70 nm, which corresponds to the (102) plane of Cs4PbBr6 thus confirming the Cs4PbBr6 nature of the matrix. Figure 1e also shows a high-contrast area with a spacing of ~1.235 nm, which agrees well with the Moire fringe pattern reported in CsPbBr3 /Cs4PbBr6 composite.24 According to the analysis of Sargent group, such a pattern is a result of the beating of two overlapping lattice fringes, thus is a solid evidence of CsPbBr3 embedded in Cs4PbBr6 matrix.20 Unfortunately, it is rather difficult to determine the thickness of ACS Paragon Plus Environment

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CsPbBr3 directly. Nevertheless, it is reasonable to identify the large CsPbBr3 insertions as NPLs considering the small thickness of Cs4PbBr6 matrix. Below we will provide spectroscopy evidences for the 2D nature of CsPbBr3.

Figure 2. Photoluminescence properties of the CsPbBr3 /Cs4PbBr6 composite. (a) Room temperature PL and PLE spectra. Inset shows a digital photograph of a drop-casted film under UV irradiation. (b) Logarithm plot of the integrated PL intensity versus excitation density of the emission. The slope is 1.186. (c) PL decay traces at various temperatures, showing monotonous increase of lifetime with increasing temperature. (d) The calculated radiative lifetime as a function of temperature, showing clear T1 dependence till 260 K. The CsPbBr3/Cs4PbBr6 nanosheets show intense green emission, as shown in Figure 2a. The PL spectrum shows a peak located around 520 nm (2.38 eV) with a full width at half-maximum (FWHM) ACS Paragon Plus Environment


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of ~18 nm. The PL excitation (PLE) spectrum shows an absorption edge in the range of 510-520 nm, indicating an inter-band transition nature of the emission. The PL emission agrees well with that of CsPbBr3, with a slight blueshift compared with the value (535 nm) of bulk crystal,25 due to quantum confinement effect (QCE) induced by the small thickness comparable to the Bohr radius of excitons (~ 7 nm) in CsPbBr3.1 The QCE is not strong, probably due to 1) the actual thickness of the NPLs, which cannot be determined precisely at this moment, is close to exciton Bohr radius, and 2) the PL is redshifted by the dielectric environment of surrounding Cs4PbBr6.26 Some researchers argued that the emission comes from the Cs4PbBr6 matrix.22,25,27-29 However, in our experiments this seems unlikely because of the following two facts. First, Manna group30 reported that pure-phase Cs4PbBr6 nanocrystals do not show any absorption around 520 nm. Second, the bandgap of Cs4PbBr6 is reported to be ~ 4 eV (310 nm).30,31 However, the PLE spectrum of our sample shows a sharp dip around 310 nm (Figure 2a), which indicates that the interband absorption of Cs4PbBr6 does not contribute to the green emission at all. Such a feature has been observed in CsPbBr3 /Cs4PbBr6 composite by several groups, who also attributed the emission to CsPbBr3.30-34 The relative content of CsPbBr3 in the composite nanosheets can be tuned by changing the PbBr2/CsBr ratio in the precursor solution according to the ternary phase diagram.20 Richness (deficiency) of PbBr2 leads to more (less) CsPbBr3 in the composite. Reaction with excess PbBr2 can even transform pure Cs4PbBr6 to pure CsPbBr3.30 As shown in Figure S7, reducing the PbBr2/CsBr ratio to 0.87 results in dramatic weakening of the green emission. On the contrary, increasing the ratio to 1.13 leads to increased PL intensity. These results are consistent with above conclusion that the green emission comes from the embedded CsPbBr3 nanostructure. In order to investigate the recombination nature of the green emission, the PL spectra under various excitation intensity were measured (Figure S8). The logarithm plot of the integrated PL intensity versus excitation density is shown in Figure 2b. It shows a power-law dependence I PL ~ I exk with a k value of 1.186. This value is close to 1, which indicates the excitonic nature of the green emission.35-37 Interestingly, the PL lifetime decreases with decreasing temperature (Figure 2c) and reaching ~3 ns at ACS Paragon Plus Environment

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20 K. Such a feature is a signature of giant oscillator strength effect in 1D and 2D semiconductors.9,11 To reveal the dimensional characteristic of the exciton recombination, we calculated the radiative 1   r-1   nr-1 , where τnr is the nonradiative lifetime (see lifetime (τr) from the PL lifetime (τPL) via  PL

Supporting Note 1 and Table S1 for calculation details).38,39 It has been well established in semiconductors that the radiative lifetime of excitons with different dimensionalities show different temperature (T) dependencies. For 0D, 1D, 2D and 3D excitons, the radiative lifetime is proportional to T0, T1/2, T1, and T3/2, respectively.40 Figure 2d plots the temperature-dependent radiative lifetime of the CsPbBr3 /Cs4PbBr6 nanosheets. The lifetime shows good linear dependence on temperature till ~260 K, which is an evidence for the 2D-like recombination of the excitons. The fast increase of radiative lifetime above 260 K is accompanied by fast decrease of the PL intensity (Figure S9), which is likely induced by thermalization of 2D excitons into 3D states41 followed by exciton dissociation at high temperature.

Figure 3. (a) Arrhenius plot of integrated PL intensity as a function of temperature. Solid line represent the fit result using eq. (1). (b) PL decay spectra measured at various emission energies accrossing the emission peak. Inset shows the derived lifetime as a function of photon energy.



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Figure 3a shows the Arrhenius plot of the integrated PL intensity as a function of temperature. As is well known, the PL suffers thermal quenching with increasing temperature. The data can be described by a well-established expression11 I (T )  I 0 /[1  CT exp(

Ea )] k BT

(1),

where Ea is activation energy, I0 is the PL intensity at low temperature limit, kB is the Boltzmann constant and C is a constant. It should be noted that in most work adopting eq. (1) the radiative lifetime is usually assumed to be temperature independent. However, we suggest to take into account the linear dependence of the radiative lifetime on temperature based on the observation of 2D exciton recombination. Therefore, the exponential term in eq. (1) should be multiplied by T to describe the data in a better manner. The best fit gives rise to Ea = 6.4 ± 2.6 meV. In quantum well-like structures, the PL quenching is usually attributed to thermal emission of excitons in the potential minima or dissociation of excitons.42 In the present case, the activation energy is much smaller than either the band offset between CsPbBr3 and Cs4PbBr6 or the exciton binding energy of quasi-2D CsPbBr3,11 indicating that there should be additional weak localization effect existing in our samples. A likely candidate is the localization of excitons in potential minima due to roughness at the well/barrier interface, which has been frequently observed in epitaxial quantum wells42 with localization energy of about 10 meV. The localization of excitons is also evidenced by the spectral distribution of PL lifetime, as shown in Figure 3b. We found that the PL lifetime depends on the exciton recombination energy. With increasing photon energy, the lifetime first increases and then keeps constant when the emission energy exceeds the peak energy. Such a feature has been frequently observed in inorganic semiconductor quantum wells, and has been regarded as a signature of exciton localization.43


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Figure 4. Emission stability of the CsPbBr3 /Cs4PbBr6 composite in ambient air. (a) PL intensity evolution with continuous illumination time of the excitation light. (b) Normalized PL spectra of the CsPbBr3 /Cs4PbBr6 composites after storage in air. (c) PLQY of the CsPbBr3 /Cs4PbBr6 composites after storage in air. The PL results, together with the observed microstructure, thus strongly suggest the formation of quasi-2D CsPbBr3 NPLs in Cs4PbBr6 nanosheets. While for Cs4PbBr6 nanosheets themselves, currently we are unable to say whether it is quasi-2D or not in nature. Because the excitonic emission cannot be identified in our sample and the fundamental parameters such as exciton Bohr radius are not available in the literature. It is generally accepted44 that organic ligands are necessary in achieving highly luminescent colloidal CsPbBr3 nanocrystals. However, the plenty of ligands on the surface induces difficulties in efficient carrier transport.19 In the present case, the Cs4PbBr6 may passivate the surface of CsPbBr3 NPLs. This is evidenced by the PL intensity evolution with continuous illumination. As shown in Figure 4a, with increasing illumination time the PL intensity does not show any photo-bleaching or trap filling effect, indicating low trap density in the NPLs.44-47 Moreover, the sample show reasonable optical stability when illuminated for hours. On the other hand, the air-stable Cs4PbBr6 matrix will improve the ambient stability of CsPbBr3 NPLs. As shown in Figure 4b, during 5-week storage in air with a humidity of 50-60%, the PL lineshapes of the sample are almost identical with a tiny peak shift less than 1 nm. Meanwhile, the PLQY (Figure 4c) shows only slow degradation after maintaining the ACS Paragon Plus Environment


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value for about 5 weeks, revealing reasonable stability during the storage. It seems that the PLQY even has a slight increase during the storage, which might result from the decomposition of Cs4PbBr6 to CsPbBr3 and CsBr as reported in the literature.48 We monitored the XRD patterns of our sample over 15 and 30 days (shown in Fig. S10) and indeed found the signature of such transformation. Over time, the peak of Cs4PbBr6 located at 28.70° shows decrease to some extent. At the same time, the peak intensity of CsPbBr3 located at about 31° increases. However, such decomposition should be not prominent because CsBr was not detected. CONCLUSIONS In summary, quasi-2D CsPbBr3 NPLs embedded in Cs4PbBr6 nanosheets were synthesized by onestep anti-solvent precipitation strategy. Adding certain amount of CTAB in the precursor solution results in the formation of Cs4PbBr6 nanosheets with an average thickness of 7.6 nm and a lateral size of several microns. The formation of CsPbBr3 NPLs in the Cs4PbBr6 matrix is supported by the presence of diffraction peaks in XRD and Morie fringe pattern in high resolution TEM. The radiative lifetime increases with increasing temperature and shows a T1 dependence, which is a signature of 2D exciton recombination. The surrounding Cs4PbBr6 matrix provides both surface passivation and protection for the CsPbBr3 NPLs, which improves the PL properties by eliminating the trap filling and enhancing the air stability. The air-stable, highly luminescent, and unique quasi-2D CsPbBr3 nanoplatelets /Cs4PbBr6 nanosheets heterostructure may either serve as a workbench for studying the exciton recombination dynamics or find application in various optoelectronic devices. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications web-site. SEM, XRD, AFM and HRTEM images, XPS spectra, PL spectra, note for the lifetime calculation, numerical results of radiative lifetime. AUTHOR INFORMATION Corresponding Authors ACS Paragon Plus Environment

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*E-mail: [email protected] (H. He). *E-mail: [email protected] (Z. Ye). ORCID Haiping He: 0000-0001-8246-0286 Zhizhen Ye: 0000-0002-0886-0115 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (no.51772271), the Natural Science Foundation of Zhejiang Province (no. LY17A040008), China Postdoctoral Science Foundation (2018M642426), and the Scientific Research Project of Gansu Province (no. 17JR5RA072).

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