Aqueous Solution Growth of Millimeter-Sized Nongreen-Luminescent

†School of Materials, ‡School of Physics, §School of Materials and Engineering, ¶State Key Laboratory of Optoelectronic Materials and Technologi...
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Aqueous Solution Growth of Millimeter-Sized Nongreen-Luminescent Wide Bandgap Cs4PbBr6 Bulk Crystal Zhaojun Zhang,†,¶ Yanming Zhu,†,¶ Weiliang Wang,‡,¶ Wei Zheng,†,¶ Richeng Lin,‡,¶ Xubiao Li,§,¶ Hao Zhang,⊥ Dingyong Zhong,‡,¶ and Feng Huang*,†,¶ †

School of Materials, ‡School of Physics, §School of Materials and Engineering, ¶State Key Laboratory of Optoelectronic Materials and Technologies, and ⊥Instrumental Analysis & Research Center, Sun Yat-Sen University, Xingang Xi Road No. 135, Guangzhou 510275, P. R. China

Crystal Growth & Design 2018.18:6393-6398. Downloaded from pubs.acs.org by STOCKHOLM UNIV on 04/24/19. For personal use only.

S Supporting Information *

ABSTRACT: Millimeter-sized Cs4PbBr6 bulk single crystal that has no green luminescence was successfully grown from concentrated CsBr aqueous solution for the first time. Absorption spectrum indicates an optical absorption starting from 3.6 eV and a localized absorption in the range of 3.8−4.2 eV. Photoluminescence spectrum clearly shows that the crystal has no green emission. Along with the calculated electronic band structure, it is concluded that Cs4PbBr6 has a wide bandgap of 3.6 eV. Through vacuum annealing treatment, the green luminescence of the original nongreen-luminescent Cs4PbBr6 crystal was successfully activated, which is possibly due to the formation of CsPbBr3. This work gives the method to obtain millimeter-sized Cs4PbBr6 crystals from aqueous solution and suggests a route to activate its green luminescence. The results are significant for the deep understanding of the intrinsic properties and exploring the potential applications of wide bandgap Cs4PbBr6.

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wide bandgap semiconducor materials, which have particular application in the fields of UV detection, etc.12−14 Therefore, in this work we hope to obtain intrinsic Cs4PbBr6 bulk crystal. In the choice of growth method, we first avoided using melting growth method due to the known difficulty in holding back the formation of CsPbBr3 phase.15 We design solution method and create a near-equilibrium process that is benefical for growing single phase and high-quality crystal.16,17 Aqueous-solution growth will also be a unique advantange of Cs4PbBr6, since most wide bandgap semiconductor crystals are generally obtained using complex high-temperature melting or vapor method.12,13 Apparently, Cs4PbBr6 belongs to ionic crystal. For ionic crystals, we know, the ion radius of anion is usually much larger than that of cations. Thus, the crystal lattice is mainly composed of closely packed anions, with cations occupying fixed interstitial positions.18 Obviously, the absence of anions will cause large local distortion of crystal lattice, while the effect of cations’ absence is much smaller. Therefore, to obtain well-ordered crystal lattice, the primary thermodynamic growth condition should ensure the perfection of anions’ lattice at priority.19 This philosophy of “ensuring perfection of anions’ lattice” has been successfully applied in our early studies on high-mobility ZnO-based materials.20−22 Specifically, in Cs4PbBr6 single

s4PbBr6 is expected to be a promising candidate in the field of electroluminescence (EL), due to its green luminescence (GL) (520 nm) with maximum quantum yield of photoluminescence up to 54%.1−3 At an earlier time, the synthesized Cs4PbBr6 samples, whether powders1,4 and nanocrystals2,5 or bulk crystals,3 all showed strong green luminescence, and thus Cs4PbBr6 was once believed to have a 2.4 eV band gap, and its GL was ascribed to band-to-band transition, while several further researches obtained nongreen-luminescent Cs4PbBr6 nanocrystals and indicated that Cs4PbBr6 has a wide bandgap larger than 3.0 eV.6−9 For example, Akkerman et al.6 reported the nongreen-luminescent Cs4PbBr6 and ascribed the obsereved green emission in previous studies to CsPbBr3 impurity. Recently, the point that intrinsic Cs4PbBr6 has a wide bandgap is more and more accepted. Although there are several reports on growing Cs4PbBr6 bulk crystals,10,11 these samples still have green luminescence. Chen et al.11 reported centimeter-sized Cs4PbBr6 crystals with CsPbBr3 inclusions emitting green luminescence. And De Bastiani et al.10 also obtained Cs4PbBr6 bulk crystal with having Br vacancies playing the role of green emission. However, the growth of nongreen-luminescent intrinsic high-crystalline-quality Cs4PbBr6 bulk crystals has not been reported yet. Growing intrinsic high-crystalline-quality nongreen-luminescent Cs4PbBr6 bulk crystal is significant for the deep understanding of its detailed intrinsic optical and electrical properties.3 More important, due to its large bandgap, the crystal growth of Cs4PbBr6 is also a significant issue for the fields of © 2018 American Chemical Society

Received: May 30, 2018 Revised: August 28, 2018 Published: October 8, 2018 6393

DOI: 10.1021/acs.cgd.8b00817 Cryst. Growth Des. 2018, 18, 6393−6398

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Figure 1. (a) Schematic diagram of growing Cs4PbBr6 bulk SC from concentrated CsBr aqueous solution. (b) Photo of the millimeter-sized rhombohedral Cs4PbBr6 SC. (c) Schematic diagram of the crystal structure of Cs4PbBr6 SC.

solution (nearly saturated at 20 °C). Next, dissolving PbBr2 in hydrobromic acid. Then, dropwise adding the PbBr2 solution into CsBr solution at 90 °C under vigorous stirring. (As local Pb2+ enrichment in the solution possibly causes formation of CsPbBr3, vigorous stirring is required.) Afterward, clear and transparent solution was obtained. Finally, as the above clear solution is slowly cooled from 90 to 20 °C, Cs4PbBr6 single crystals will be obtained. The crystals during the solution cooling process are given in Figure S1 in Supporting Information. The slow cooling rate was aimed to provide a near-equilibrium dissolution− crystallization process for growing high-quality crystals. The obtained crystals were washed by toluene, vacuum-dried, and then stored under atmospheric condition with humidity lower than 25%. For comparison, we also use unconcentrated CsBr aqueous solution, saturated anhydrous CsBr ethanol solution for crystal growth, and found that CsPbBr3 rather than Cs4PbBr6 was obtained, as shown in Figure S2. This indicates that only concentrated CsBr aqueous solution can guarantee the successful growth of pure phase Cs4PbBr6 crystal. The following considerations are the reasons why we choose concentrated CsBr aqueous solution. As above-mentioned, the perfection of Br lattices is a cornerstone of obtaining highquality Cs4PbBr6 single crystal. On the one hand, the excess Br in CsBr solution will exactly ensure the perfection of Br lattice of the crystal. On the other hand, the excess Cs in the solution stabilizes Cs4PbBr6 and prevents its transition to CsPbBr3. Previously, Wu et al. pointed out that H2O molecules could trigger transformation from nonluminescent nanophase Cs4PbBr6 to highly luminescent CsPbBr3 through a CsX-stripping mechanism.8 However, when using concentrated CsBr water solution, the solubility of CsBr in water is nearly saturated; then the water molecules will not trigger the stripping of CsBr from Cs4PbBr6, and thus Cs4PbBr6 stably exists. Our results clearly indicate that Cs4PbBr6 crystals can stably existed in

crystal, local absence of Br ions will lead to large lattice distortion, resulting in large amount of Br vacancy (VBr) and even screw dislocation. Therefore, Br-excess growth condition is essential for the lattice perfection of Cs4PbBr6 crystal. In our previous work, such kind of strategy has guided us to obtain high-quality CsPb2Br5 single crystal.16 Additionally, as for Cs4PbBr6, which has a large atom ratio of Cs, the growth condition should also guarantee excess Cs naturally. Therefore, in this work, we design a new scheme using concentrated CsBr aqueous solution for growing high-quality nongreen-luminescent Cs4PbBr6 SC. We successfully obtained high-crystalline-quality Cs4PbBr6 bulk single crystal from concentrated CsBr aqueous solution. Absorption and luminescence spectra indicate that the obtained Cs4PbBr6 crystal has a wide bandgap of 3.6 eV. More importantly, the obtained crystal has no green luminescence, indicating the successful prevention of the formation of CsPbBr3 impurities/inclusions. This breaks the restriction that water can trigger the transformation of Cs4PbBr6 to CsPbBr3 in a previous report8 and indicates that the simultaneous excess of Cs and Br ions stabilizes Cs4PbBr6, prevents the formation of CsPbBr3, and guarantees the perfection of crystal lattice. These results are not only significant for the growth and deep understanding of the properties of intrinsic Cs4PbBr6 bulk crystal but also beneficial for exploring broader potential application possibility of Cs4PbBr6, such as high-energy radiation detection, UV detection, and UV luminescence. The schematic diagram of growing Cs4PbBr6 single crystal is shown in Figure 1a. Although there has been several studies on the growth of Cs4PbBr6 bulk SC up to now (such as antisolvent vapor-assisted crystallization3,10 or cooling solution from mixture of organic and water phase11), our work is the first to use only aqueous solution for growing millimeter-sized Cs4PbBr6 crystals. First, preparing concentrated CsBr aqueous 6394

DOI: 10.1021/acs.cgd.8b00817 Cryst. Growth Des. 2018, 18, 6393−6398

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water environment in the premise of having excess Cs ions. The chemical equation can be expressed as follows: 4Cs+ + Pb2 + + 6Br − F Cs4PbBr6(nCs ≫ nPb , nBr ≫ nPb)

The crystal was grown based on the above dissolution− crystallization process. Such kind of dynamic equilibrium process is beneficial for obtaining high-quality products. For example, recently, Sun et al. reported the synthesis of Cs4PbBr6/ CsPbBr3 composites with near-unity luminescence quantum yield based on a dissolution−crystallization process in the mixed solvent of water and dimethyl sulfoxide (DMSO).23 In our work, the concentrations of Cs ions and Br ions in the n n solution are much larger than that of Pb ( nCs > 180, nBr > 180) Pb

Pb

ions, and this condition guarantees that the Cs-rich Cs4PbBr6 phase is the most stable one. In our previous report, we found that, when the concentration of Pb is much larger than that of Cs and Br, Pb-rich phase CsPb2Br5 is most stable.16 Additionally, when the concentration of Cs and Pb are almost in the same level, CsPbBr3 phase will be most stable one. These results clearly indicate that every single phase of Cs−Pb−Br family can stably exist in aqueous solution if they are under appropriate thermodynamic conditions. For Cs4PbBr6, the condition that it can exist stably is under sufficiently high concentration of Cs ions. That is why we choose concentrated CsBr aqueous solution for growing pure-phase intrinsic Cs4PbBr6 crystal. Through modulating the relative concentration of different ions (Cs+, Pb2+, Br−) in the external environment, it is also possible to realize the transformation between the three different phases, and these works will be our future pursuit. The as-grown millimeter-sized Cs4PbBr6 single crystals are shown in Figure 1b. The structure of the obtained single crystal was analyzed. Single-crystal structure information (Table S1) and the cif file are given in the Supporting Information. Our Cs4PbBr6 crystal belongs to space group R3̅c with lattice constants of a = b = 13.6443(13) Å, c = 17.2561(16) Å, α = β = 90°, γ = 120°. As Figure 1c shows, the structure of our crystal is identical to that of the previous results.10,24 The powder X-ray diffraction (XRD) of the as-grown Cs4PbBr6 crystal is shown in Figure 2a, which is also consistent with previous studies,3,10,11 indicating that we have definitely obtained Cs4PbBr6. Raman spectrum of the Cs4PbBr6 crystal was given in Figure 2b. There are four Raman-active peaks located at 55.3, 67.9, 81.1, and 122.4 cm−1, respectively. These Raman-active peaks are asigned to the vibrational mode of PbBr6 octahedron.3,25 The high symmetry and narrow lineshapes of the observed Raman peaks indicate the high-crystalline-quality of the obtained Cs4PbBr6 crytal.26−28 Besides, we also made transmission electron microscopy (TEM) test, and results are shown in Figure S3. The sample was prepared by grinding the bulk crystal and then ultrasonically dispersing with toluene. As can be seen, the highresolution TEM image shows clear regular lattice stripes, indicating the single pure phase and high-crystalline quality of the as-grown Cs4PbBr6 crystal.11 The observed lattice distance of 0.40 nm is corresponding to the crystal plane of (300).5 Note that, being similar to the phenomenon observed in previous studies,2 Cs4PbBr6 is easily damaged under irradiation of focused electron beam, which is possibly due to its low lattice energy. The absorption spectrum shown in Figure 2c was obtained from measuing the UV−vis diffuse reflectance spectrum of the grinding powder using the obtained Cs4PbBr6 single crystal.

Figure 2. (a) PXRD patterns, (b) Raman scattering spectrum, (c) absorption spectrum, and (d) PL spectrum of the as-grown white Cs4PbBr6 crystal. (inset) Photo during PL measurement, clearly showing that there is no green luminescence.

Apparently, there is a clear absorption edge at ∼3.6 eV, clearly demonstrating that Cs4PbBr6 has a wide bandgap of 3.6 eV. The absorption band between 3.6 and 4.1 eV can be attributed to the localized absorption from the isolated PbBr6 octahedra in the lattice of Cs4PbBr6.15 The room-temperature photoluminescence (PL) spectrum with using 325 nm laser as excitation is shown in Figure 2d. Obviously, our Cs4PbBr6 single crystal has no green luminescence. The observed broadband UV (full width at half-maximum (fwhm) ≈ 20 nm) luminescence peak at ∼370 nm was ascribed to the radiative decay of a Frenkel exciton at Pb2+ sites.29 Our work is the first one obtaining millimeter-sized bulk Cs4PbBr6 crystal that has no green luminescence, as previously reported nongreen-luminescent Cs4PbBr6 crystals are generally nanocrystals.6,8 Moreover, different with previous works, which generally used organic solvent,6,8,10,11,30 we used a novel growth method only employing aqueous solution, which can provide enough high concentration of Cs ions to stabilize the presence of pure-phase Cs4PbBr6. Our method will be a significant reference for the crystal growth studies of the similar series of materials. The energy-dispersive spectroscopy (EDS; Table S2 and Figure S4) measurement results show that the element ratio of the as-grown Cs4PbBr6 crystal is 4.3:1: 6.9 (Cs/Pb/Br). The excess of Cs and Br than the stoichiometric ratio is possibly due to the residual CsBr when growing crystal from CsBr solution. XPS characterization results were given in Figure S5. The binding energies of Cs 3d 5/2, Pb 4f 7/2, and Br 3d 5/2 core levels were located at 724.25, 138.56, and 67.73 eV, respectively, which are in good agreement with previously reported values of Cs4PbBr6 bulk crystal.11 And their profiles with peaks of only one valence state also indicates the singlephase composition of the obtained Cs4PbBr6 crystal. XPS valence band spectrum (Figure S5d) shows that the energy difference between the Fermi level and the valence band maximum (VBM) is ∼1.77 eV. Considering the band gap of Cs4PbBr6 is ∼3.6 eV, we conclude that Fermi level locates slightly closer to the VBM than to the conduction band minimum (CBM). Thus, it can be speculated that Cs4PbBr6 will possibly be intrinsic or weak p-type semiconductor. This means that 6395

DOI: 10.1021/acs.cgd.8b00817 Cryst. Growth Des. 2018, 18, 6393−6398

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results in localized absorption in the range from 3.5 to 4.2 eV (Figure 2b). As seen from Figure 3b, the localized DOS in the range from 3.5 to 4.2 eV is mainly composed of Br-4p and Pb-6s orbitals; thus, we suggest that such localization possibly results from the isolated PbBr6 octahedron.15 The above elaboration demonstrates that we successfully obtained intrinsic high-crystalline quality Cs4PbBr6 bulk crystals with no green luminescence ability. Next, in view of application in visible light luminescence, we hope to find a method to activate the green luminescence from our GL-active Cs4PbBr6 crystal. We conducted vacuum annealing treatment on the asgrown Cs4PbBr6 bulk crystals and found that the green luminescence was successfully activated after annealing at 150 °C for 10 h under vacuum condition as shown in Figure 4.

Cs4PbBr6 may have intrinsic high electrical resistivity, which will be beneficial for achieving low dark current for application in photon detection.12,31 The electronic band structure and density of states (DOS) of Cs4PbBr6 are calculated using density functional theory (DFT), and the results are shown in Figure 3. As seen from

Figure 3. (a) The calculated band structure of Cs4PbBr6 relying on ideal perfect crystal structure. CBM is located at Γ point, and VBM is at A point. The VB energy difference between A point and Γ point is only 5 meV.

Figure 3a, the conduction band (CB) bottom is at Γ point in the Brillouin zone. Valence band (VB) top is in A point; it seems that there is a minimum indirect bandgap. However, given that the difference of the VB energy between A point and Γ point is only 5 meV, we can speculate that Cs4PbBr6 tends to be a direct band gap semiconductor. Besides, we can find that the band dispersion in the calculated VB and CB are both extremely small, indicating that the PbBr6 octahedron are nearly uncoupled.6 And such small band dispersion also means a large effective mass for electron and holes in Cs4PbBr6. The calculated band gap of 3.78 eV agrees well with experimental value. Such a wide bandgap of Cs4PbBr6 implies its possible application in the field of UV detection, which has more important practical significance than general visible light detection.12 Additionally, considering that Cs4PbBr6 is composed of heavy elements (ZCs = 55, ZPb = 82, ZBr = 35), bulk Cs4PbBr6 crystal may also have possible excellent performance in radiation detection such as X-ray detection.31And studies in these two aspects will be our pursuit in the future. As seen from Figure 3b, it could be concluded that the CB and VB of Cs4PbBr6 are mainly composed of Pb-6p orbital and some Br-4p orbital, respectively. Cs orbitals almost have no contribution to the DOS near the VB and CB, and this characteristic is consistent with that of CsPbBr3 and CsPb2Br516 and previously reported DOS calculation of Cs4PbBr6.6 Additionally, we found that the DOS in the energy range from 3.5 to 4.2 eV is nearly localized (Figure 3b), and this can also be reflected from the band structure. We suggest that such localized DOS

Figure 4. (a) The photoluminescence spectrum and absorption spectrum of one Cs4PbBr6 crystal after vacuum annealing. (inset) The photo of a pale yellow Cs4PbBr6 crystal and its strong green luminescence when irradiated under 325 nm laser.

During our experiment, we found that the originally colorless Cs4PbBr6 single crystal becomes pale yellow, and the photos are compared in Figure S6. As shown in Figure S6, we found that vacuum exposure at room temperature will not change the original white crystal. But heating at 150 °C under vacuum exposure will significantly change the crystal. Photoluminescence spectrum of the pale yellow Cs4PbBr6 crystals was measured as shown in Figure 4a, which shows strong green luminescence. The absorption spectrum of the pale yellow crystal is shown in Figure 4b; compared to the result in Figure 2c, we found that the absorption in the region of 2.4−3.5 eV appeared, which may be attributed to generated CsPbBr3 phase after vacuum annealing.15,32 We measured the powder XRD patterns of the pale yellow Cs4PbBr6 crystal (Figure S7) and found that it remains nearly unchanged. This is possibly because the amount of the generated CsPbBr3 is so small that it cannot be detected by XRD within the measurement error. To identify the existence of CsPbBr3 phase in the crystal after annealing, we made high-resolution TEM test on this pale 6396

DOI: 10.1021/acs.cgd.8b00817 Cryst. Growth Des. 2018, 18, 6393−6398

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Guangzhou, China (Grant No. 201607020036). Additionally, we greatly thank Prof. Y. Li in Sun Yat-Sen Univ. for the singlecrystal measurement and structure analysis. We are also very grateful to Miss L. Yang for improving the English writing. We greatly appreciate Prof. J. Wang in Graduate School at Shenzhen, Tsinghua Univ. for XRD measurement.

yellow Cs4PbBr6 crystal, and the results are shown in Figure S8. Compared with Figure S3, we can find that, after vacuum annealing, there are a lot of small particles that appeared inside the Cs4PbBr6 crystal. The high-resolution (HR) TEM images shows the particles have a smaller lattice distance (red dotted circle) than that of Cs4PbBr6 (blue dotted line); therefore, we concluded that CsPbBr3 phase is generated, and it caused the appearance of strong green luminescence. Previously, De Bastiani et al.10 suggested that the generation of CsPbBr3 is caused by Br vacancies in the Cs4PbBr6 crystal. In our experiment, we suggest that vacuum annealing treatment probably caused Br vacancies, as the chemical potential of gaseous bromine molecules (Br2) is relatively small, and it decreases under vacuum condition. Then Br vacancies further led to formation of CsPbBr3 phase inside the Cs4PbBr6 crystal. The above results indicate that we found a suitable thermodynamic treatment condition to make the original GL-inactive Cs4PbBr6 transform to be GL-active. Additionally, this phenomenon also reminds us that the storage of intrinsic nongreenluminescent Cs4PbBr6 crystal should be far from excessive high temperature owing to the possible generation of CsPbBr3. In summary, we have successfully grown intrinsic wide band gap (3.6 eV) high-quality Cs4PbBr6 single crystal from concentrated CsBr aqueous solution. Benefiting from our designed growth system, the obtained Cs4PbBr6 SC does not have CsPbBr3 inclusion or possible Br vacancies, which may lead to green luminescence, indicating the high-crystalline-quality of our crystal. Compared to the previously reported green-luminescent Cs4PbBr6 crystal, such intrinsic wide band gap nongreenluminescent Cs4PbBr6 crystal has unique potential applications in UV detection, X-ray detection, etc. Additionally, after vacuum annealing treatment, we successfully activated the green luminescence from original nongreen-luminescent Cs4PbBr6, which is possibly due to the formation of CsPbBr3. This provides the method for the possible modulation strategy of the green luminescence of wide band gap Cs4PbBr6 crystal.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b00817.

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REFERENCES

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Discussion of growth of Cs4PbBr6 single crystal, characterization, computational details, photos of the crystals during the growth process, tabulated crystal data and elemental composition, TEM images, EDS data, binding energy data, photos and XRD patterns of crystals before and after annealing treatment (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wei Zheng: 0000-0003-4329-0469 Feng Huang: 0000-0002-4623-2216 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate the support from the National Natural Science Foundation of China (Nos. 61427901, 61604178, 91333207, and U1505252) and Science and Technology Program of 6397

DOI: 10.1021/acs.cgd.8b00817 Cryst. Growth Des. 2018, 18, 6393−6398

Crystal Growth & Design

Communication

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DOI: 10.1021/acs.cgd.8b00817 Cryst. Growth Des. 2018, 18, 6393−6398