Water-resistant, monodispersed and stably luminescent CsPbBr₃/CsPb₂Br₅ core–shell-like structure lead halide perovskite nanocrystals

Inorganic metal halide perovskite (CsPbX₃, where X is a halogen element such as F, Br or I) nanocrystals have attracted considerable attention as a versatile material platform for the realization of a new generation of solution-processible optoelectric devices, including light emitting diodes (LEDs) [1], lasers [2], photodetectors [3] and solar cells [4]. Perovskite nanocrystals have excellent optoelectronic properties, such as large bandgap tunability, efficient narrow band emission, low cost and facile synthesis process [5, 6]. However, all lead halide perovskite materials are normally very sensitive to moisture [7] due to the fact that they are easily hydrolyzed [8]. The light emission wavelength also varies during their hydrolysis and crystal-growing at ambient atmosphere. Hence, the instability of the emerging perovskite materials in a humid environment is a bottleneck for their potential applications [9].

'Coating' is an effective method for enhancing the stability of perovskite nanocrystals, such as coating with silica [10], silica spheres [11], mesoporous silica matrixes [12] and polyhedral oligomeric silsesquioxane [13]. The nanocrystals can also be dispersed in phosphate glasses [14], PMMA [5] or PMA [15] to enhance their stability. All these coating processes make nanocrystals clusters instead of being monodispersed. Surface engineering has been used, such as ligand exchange processes, to improve the luminescence efficiency but not to be stably enhanced [16]. Nevertheless, these coating structure are usually insulating, resulting in clusters or introducing a lot of defects that hinder them from use in optoelectronic applications. Until now, thin layer coated or monodispersed core–shell structures are of great interest and an unsolved issue. In particular, the wide bandgap coating layer and the narrow bandgap core may form a quantum well structure and enhance the light emission of the core. Researchers are looking for the proper materials and synthesis methods for such core–shell structures.

Recently, CsPb₂Br₅ has been reported with indirect bandgap and being photoluminescence (PL) inactive with a bandgap of 2.979 eV, while CsPbBr₃, with a bandgap of about 2.48 eV [17–20]. Tetragonal CsPb₂Br₅ is a 2D material that has an obviously different crystal structure from CsPbBr₃. A wide bandgap coating layer and narrow bandgap core may form a quantum well structure and enhance light emission. Dual-phase CsPbBr₃–CsPb₂Br₅ structures usually exist during the synthesis process of CsPbBr₃ nanocrystals [20]. The shape and phase evolution from CsPbBr₃ to CsPb₂Br₅ demonstrated that the reaction could take place at or over 396.5 K in the solvent and the ultrathin CsPb₂Br₅ nanosheets were also synthesized by adjusting the chain length of the ligand [17]. Some electrical and optical properties of CsPb₂Br₅ are still rarely investigated and many blanks need to be filled. The synthesis parameters are extremely strict for the formation of inorganic metal halide perovskite nanocrystals [21]. This also makes it possible to synthesize and control the crystal growth of CsPbBr₃–CsPb₂Br₅ system materials. Nevertheless, how to control the enlargement of the CsPb₂Br₅ 2D layer to form a coating layer is still an issue.

In view of the aforementioned facts, CsPb₂Br₅ coating CsPbBr₃ could be an option to improve the stability of perovskite nanocrystals. The crystal growth of perovskite materials at different crystal facets could be controlled via the synthesis parameters, such as the surfactant, ratio of reactant, temperature, and reaction time, even though the experiment parameters are extremely strict [22–28]. In this work, CsPbBr₃ nanocrystals coated with CsPb₂Br₅ were synthesized using a modified non-stoichiometric solution-phase method by controlling the density of Cs₂CO₃ and PbBr₂ in the precursor solution under specific synthesis parameters.

2.1. Chemicals

2.2. Preparation of the Cs–oleate solution

2.3. Synthesis of CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals and CsPbBr₃ nanocrystals

2.4. Characterization

The crystal structure of CsPbBr₃ nanocrystals and CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals were investigated by XRD, as shown in figures 1(a) and (b), respectively. It is worth noting that the diffraction peaks between the CsPbBr₃ nanocrystals and the standard XRD spectra of the CsPbBr₃-cubic are all very similar, both in location and relative strength, as shown in figure 1(a), which confirms that the CsPbBr₃ nanocrystals have relatively high crystallinity with cubic structure. The lattice parameters of CsPbBr₃ are a = b = c = 0.583 nm and $\alpha =\beta =\gamma =90^\circ .$ By contrast, for the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals, the diffraction peaks were derived from both the CsPbBr₃-cubic structure and the CsPb₂Br₅-tetragonal structure, as shown in figure 1(b). The lattice parameters of CsPb₂Br₅ are a = b = 0.848 nm, c = 1.525 nm and $\alpha =\beta =\gamma =90^\circ .$ The CsPbBr₃ contributes most of the percentage of materials, while the CsPb₂Br₅ contributes much less. This infers that the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals contain these two crystal structures. The crystallite size (${{D}}_{{\rm{c}}}$) of the two materials could both be estimated at approximately 10 nm according to the Debye–Scherrer formula

$$\begin{eqnarray}&&{{D}}_{{\rm{c}}}=0.89\lambda /({B}\,\cos \,\theta )\end{eqnarray} \tag{ 1 }$$

where B is the half-peak width and $\lambda$ in the XRD measurement is 0.154 nm under K$\alpha$ radiation of the Ni-filtered Cu target. The $\theta$ ranges from 10° to 80°.

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To further study the shape and crystal structure of the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals, the two samples were investigated by HRTEM as shown in figures 1(c), (d). There is no obvious difference between the shape of the two kinds of nanocrystals, all of which are nanocubes at about 10 nm. The crystal size is the same as that calculated with the Debye–Scherrer formula. The CsPb₂Br₅ layer appears lattice spaced at 0.3 nm (facet (213)) and that of CsPbBr₃ at 0.58 nm (facet (100)), as shown in figures 1(c2) and (d2). The characterizations demonstrate that the coating processes just result in several layers of CsPb₂Br₅ on the surface instead of changing or destroying the original crystal structure.

The x-ray photoelectron spectroscopy (XPS) characterizations of CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals are plotted in figure 2. There are three elements 'Cs', 'Pb' and 'Br' in the materials. The atomic ratio between 'Pb' and 'Br' is close to 2:5. The surface of the nanocrystals is made of layers of Pb–Br structures, while there is less Cs, which is all inside the nanocrystals. The structure of the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals will be demonstrated in the following. It must be stated that there are still some precursor remains in the materials that cannot be avoided by purification during the XPS test, which may affect the results.

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The PL decay spectra of the CsPbBr₃ nanocrystals and the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals were plotted, as shown in figures 3(a) and (b), respectively. The PL decay curve can be well fitted with a double-exponential function

$$\begin{eqnarray}&&{A}({t})={{A}}_{1}\exp \left(-\tfrac{{t}}{{{\rm{\tau }}}_{1}}\right)+{{A}}_{2}\exp \left(-\tfrac{{t}}{{{\rm{\tau }}}_{2}}\right)\end{eqnarray} \tag{ 2 }$$

where A, A₁, A₂ are constants, t is time, and ${\tau }_{1}$ and ${\tau }_{2}$ represent the decay lifetimes corresponding to the intrinsic exciton relaxation and interaction between excitons and defects, respectively. The average lifetime (${\tau }_{{\rm{ave}}}$) can be calculated as

$$\begin{eqnarray}&&{\tau }_{{\rm{ave}}}=\displaystyle \frac{{{A}}_{1}{\tau }_{1}^{2}+{{A}}_{2}{\tau }_{2}^{2}}{{{A}}_{1}{\tau }_{1}+{{A}}_{2}{\tau }_{1}}.\end{eqnarray} \tag{ 3 }$$

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The CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals exhibit two lifetimes: τ₁ of 7.85 ns accounting for 90.29% and τ₂ of 17.0 ns accounting for 9.71%, respectively, which reveals the extremely high ratio of the intrinsic exciton relaxation. It infers that the two samples have fewer defect or impurity states. The CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals exhibit lifetime almost the same with the CsPbBr₃ nanocrystals except only 0.3 ns difference in $\tau 2.$ This slightly longer $\tau 2$ of the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals means that defects are reduced, but not obviously after coating, which maybe a result of the blue shift of the PL peak of the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals compared with that of the CsPbBr₃ nanocrystals. Anyway, the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals still show the intrinsic luminescence properties of CsPbBr₃ nanocrystals. It infers that the coating processes did not introduce any impurities or defects in the materials.

The luminescence stability in the water and ethanol atmosphere of the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals and CsPbBr₃ nanocrystals were both measured. The prepared nanocrystals appear as bright green emission as shown in figure 4(a). Firstly, we added ethanol with three times volume of nanocrystals solution. The ethanol and the solvent of nanocrystals' solution can be dissolved with each other in any ratio. After 5 s, the CsPbBr₃ nanocrystals had been decomposed and its solution became nonluminous transparent solution, while the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals could not be decomposed but could be dissolved in the mixed solution and remained luminescent, as shown in figure 4(b). Then two samples were tested with water. Water with a three times volume of nanocrystals solution was added into the solution. Water was on the bottom and nanocrystals solution was on the top, as shown in figure 4(c), because water and the solvent cannot be dissolved. Then the two samples were ultrasonicated for 2 h and left to stand for 5 min. The CsPbBr₃ nanocrystal solution became nonluminous while the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals solution remained bright green, as shown in figure 4(d). The absorption and PL spectra before and after the test were investigated, as in figures 4(e) and (f), respectively. The absorption and PL spectra of the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals changed little after being added to ethanol, while that of the CsPbBr₃ nanocrystals were not detected in the visible range from 400 nm to 700 nm. The absorption spectra of CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals shifted to blue by about 5 nm compared to that of CsPbBr₃. The PL peak at 515 nm of the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals shifted to blue by about 5 nm and was enhanced compared to that of CsPbBr₃ nanocrystals at about 520 nm. In halide perovskite materials, the halide elements such as 'Cl', 'Br' and 'I' could determine the bandgap and PL spectra of the materials. In addition, the crystallite size could also affect the bandgap and PL spectra, but as a secondary factor. The blue shift is due to the interface modification of nanocrystals in CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals and the smaller core size of CsPbBr₃. It must be stated that the light emission of CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals in water and ethanol also decreased, but very slowly in the cuvette in the next few days.

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Furthermore, the two nanocrystals solutions were also deposited on ITO glasses without any purification and passivation treatment, which were later left in water atmosphere (filled with water vapor for humidity of 100%) for a few days. The result was similar to the former test that CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals films were much more stably luminescent than nanocrystals films, as shown in figure 5. The ethanol and water test in solution and solid films verifies that the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals are anti-water and luminescent stably in water and ethanol environments.

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The mechanisms of the formation of the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals will be demonstrated in the following statements. In the reaction process, CsPbBr₃ nanocrystals formed with an octahedral structure of lead and bromine elements on the surface of nanocrystals during crystal growth. Under the reaction at 463.15 K, the Pb²⁺ and Br^- ions were still superfluous, which would embed into the Pb–Br octahedral structures on the nanocrystals surface and change it to a Pb–Br capped polyhedron composed of a triangular prism and two rectangular pyramids, as shown in figure 6, which was unstable in the solution and easy to return back to CsPbBr₃ structures as:

$$\begin{eqnarray}&&{{\rm{Pb}}}^{2+}+2{{\rm{Br}}}^{-}+{{\rm{CsPbBr}}}_{3}\rightleftharpoons {{\rm{CsPb}}}_{2}{{\rm{Br}}}_{5}.\end{eqnarray} \tag{ 4 }$$

The structure of CsPbBr₃ at different temperatures was simulated, as shown in figure 6(c). CsPbBr₃ tends to be of ${{\rm{O}}}_{{\rm{h}}}^{1}-{\rm{p}}{\rm{m}}3{\rm{m}}$ space group structure at relative high temperature, which corresponds to the XRD and HRTEM characterizations of CsPbBr₃ nanocrystals. The crystal structure of CsPb₂Br₅ and CsPbBr₃ has also been reported in previous works [17, 31]. CsPb₂Br₅ material appears to be a 2D structure, as shown in figure 6(b), which prefers growing horizontally at two crystal facets (100) and (110) and forms 2D layer structures with Pb and Br elements while Cs is between the two layers. As the nanocrystals grew, the evolution between CsPbBr₃ and CsPb₂Br₅ persisted. However, CsPb₂Br₅ always changed back to CsPbBr₃ due to the existence of sufficient Cs⁺ at a relatively high temperature of 463.15 K as a function as:

$$\begin{eqnarray}&&{{\rm{CsPb}}}_{2}{{\rm{Br}}}_{5}+{{\rm{Br}}}^{-}+{{\rm{Cs}}}^{+}\rightleftharpoons 2{{\rm{CsPbBr}}}_{3}\end{eqnarray} \tag{ 5 }$$

at a relatively high temperature of 463.15 K. As the reaction was in progress, the amount of Cs⁺ decreased much faster than that of Pb²⁺ due to the superfluous Pb²⁺ during the reaction. This led to the fact that the core of the nanocrystals was mainly CsPbBr₃ structure. Li et al realized the evolution from CsPbBr₃ nanocubes to tetragonal CsPb₂Br₅ nanosheets above the temperature of 396.5 K (when the temperature is above 396.5 K the change in Gibbs free energy of the reaction is ΔG < 0) by varying the ligands successfully [17]. The temperature is another key factor for the synthetic processes except for the excess Pb²⁺ and Br⁻. More and more CsPB₂Br₅ formed and extended to layers on the crystal surface as the density of Cs⁺ decreased a lot at the end of the reaction. At the end of the synthetic process, the nanocrystal solution was put in ice water to cool it down rapidly when the reaction finished. The temperature decreases to zero promptly. Then, the formed CsPb₂Br₅ structure could not return back to CsPbBr₃ structure due to ΔG > 0 as:

$$\begin{eqnarray}&&{{\rm{CsPbBr}}}_{3}\nleftrightarrow {{\rm{CsPb}}}_{2}{{\rm{Br}}}_{5}\end{eqnarray} \tag{ 6 }$$

and remained as several layers on the surface of CsPbBr₃ nanocrystals. The CsPb₂Br₅ layer could be stable on the surface of the nanocrystals and formed a coating structure after the reaction finished. Hence, 2D CsPb₂Br₅ grows horizontally and forms several layers on the surface of CsPbBr₃ nanocrystals.

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In addition, the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals could form a quantum well structure due to the wider bandgap of CsPb₂Br₅ than CsPbBr₃, which infers that light emission could be enhanced. The CsPb₂Br₅ has an indirect bandgap, and is PL-inactive and anti-water, which makes it a good option for coating materials. CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals can be a candidate material to solve the bottleneck problem of instability for future water-stable perovskite-based applications.

In summary, water-resistant, monodispersed and stably luminescent CsPbBr₃/CsPb₂Br₅ core–shell like structure nanocrystals were obtained and reported for the first time using a modified non-stoichiometric solution-phase method. The temperature and excess Pb²⁺ are two key factors for the formation of CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals. During the synthesis processes, the evolution between CsPb₂Br₅ and CsPbBr₃ are reversible in the solution of superfluous Pb²⁺ and Br⁻. At the beginning, the formed CsPb₂Br₅ transformed mostly to CsPbBr₃ due to the existence of Cs–oleate. However, at the end of the reaction, the solution was put in ice water to cool down. The CsPb₂Br₅ formed several layers on the surface of CsPbBr₃ nanocrystals and did not transform back to CsPbBr₃ due to ΔG > 0. The CsPb₂Br₅ coating with indirect bandgap and without light emission makes the CsPbBr₃/CsPb₂Br₅ core–shell nanocrystals have a quantum well structure and be water-resistant, which greatly improves the PL properties. The water-stable enhanced nanocrystals make it possible for long-term stable optoelectronic applications in the atmosphere. Future works will focus on their electrical and optical properties, and their application to optoelectronics and biological applications.

We would like to thank the Fundamental Research Funds for the Central Universities (FRFCU) under Grant No. 2016JBM066, the National Natural Science Foundation of China (NSFC) under Grant No. 61704007 and No. 11474018 and the National Key Research and Development Program of China under Grant No. 2016YFB0401302 for financial support.