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The schematic synthesis process for the MAPbBr3@PbBr(OH) is illustrated in Fig. 1. The pH value of N, N-dimethylformamide (DMF) was preadjusted to 9.0 through dropwise addition of ammonium hydroxide; a mixture of PbBr2 and MABr with a mole ratio of 1.05:1 was dissolved in DMF solvent under continuous stirring until a white precipitate was formed. Subsequently, the precipitate was placed into an oven at 70 ℃ to obtain dried MAPbBr3 perovskite, as shown in the middle of Fig. 1. The dried MAPbBr3 perovskite is named MA-d for convenience, and the yellow MA-d exhibits very weak fluorescence under UV illumination. After addition of water, bright green fluorescence appears. Detailed characterizations indicate that MA-d changes to rod-shaped PbBr(OH), and MAPbBr3 QDs are embedded in situ into the PbBr(OH) microrods to form MAPbBr3@PbBr(OH). MAPbBr3@PbBr(OH) is named MA-h for convenience. Based on the first-principles calculations, the bandgap of PbBr(OH) is ~3.1 eV, which is close to the bandgap of the material (3.44 eV). The valence band maximum originates from the Br and O orbitals, and the conduction band (CB) minimum is dominated by the Pb orbitals, as shown in Supplementary Fig. 1. The decomposition enthalpies of MAPbBr3 and PbBr(OH) are 0.38 and 16.15 eV, respectively, (Supplementary Fig. 2, Supplementary Table 1), indicating that PbBr(OH) has higher thermodynamic stability than MAPbBr3, which can prevent the decomposition of internal MAPbBr3 QDs. Different from that apt to degrade and lose fluorescence in the presence of moisture43, the MA-h synthesized in this work can maintain its bright fluorescence for a year in water.
Fig. 2
a PL spectra of MA-d (PL QY: 2.50%) and MA-h (PL QY: 71.54%), and the inset shows the corresponding optical images of the two samples under indoor lighting and UV illumination. b XRD patterns of MA-d and MA-h. c Magnification of regions marked in b. d SEM image of MA-d. e Elemental mapping of MA-d. f SEM image of MA-h, and the inset shows a laser confocal fluorescence microscopy image. g Elemental mapping of MA-h. h TEM image of the crushed MA-h. i HRTEM image of the QDs in MA-hStrategy Perovskite Emission peak (nm) FWHM (nm) PL QYs (%) Stability Ref. Compositional engineering CsPbBr3 QDs 513 20 95 90% (30 d, air) 31 FAPbBr3 NCs 530 22 85 38% (100 ℃) 51 CsPbBr3: Mn QDs 514–517 20 90 60% (120 d, air) 52 Surface engineering MAPbBr3–APTES NCs 505 42 55 70% (2.5 h, isopropanol) 53 CsPbBr3–TDPA QDs 522 22 68 80% (300 min, water) 54 CsPbBr3–CTAB QDs 515 39 71 63% (80 min, UV) 55 Matrix encapsulation CsPbBr3–Meso-SiO2 QDs 515 20 55 60% (100 ℃) 33 CsPbBr3–Ami-SiO2 powders 520 26 56 80% (108 h, UV) 47 CsPbBr3–PMMA powders 510 25 45 75% (3 d, air) 56 MAPbBr3 powders 518 50 11.7 82% (6 months, water) 42 CsPbBr3 powders 508 45 53.9 74% (6 months, water) 42 MAPbBr3 powders 514 28 71.5 90% (1 year, water, DMF), 80% (100 ℃, water), 50% (60 h, UV) Our work Table 1. Summary of PL properties and stability of perovskite QDs
The MA-d powders are yellow in color and show negligible green fluorescence under UV illumination. The MA-h powders are a greenish color and show bright green fluorescence under UV illumination, and the corresponding picture is shown in the inset of Fig. 2a. The fluorescence spectra of MA-d and MA-h are shown in Fig. 2a, and the emission intensity of MA-h increases 35-fold compared with that of MA-d. To compare the PL peaks of MA-d and MA-h, their PL intensities were normalized, as shown in Supplementary Fig. 3. The PL peak of MA-d is located at 523 nm, while that of MA-h is located at 514 nm, indicating that the size of MA-d decreases after adding water. In addition, the full-width at half-maximum (FWHM) of MA-d is 51 nm, and the FWHM of MA-h is only 26 nm, indicating the smaller and uniform size distribution of MA-h. The crystal structures of MA-d and MA-h were characterized by X-ray diffraction (XRD), as shown in Fig. 2b. All the diffraction peaks of MA-d are from the cubic perovskite structure of MAPbBr3, while the spectrum of MA-h contains many extra peaks in addition to those of MAPbBr3, and the extra peaks can be assigned to PbBr(OH) (JCPDS No. 89-2492), indicating the formation of PbBr(OH). The enlarged XRD pattern (Fig. 2c) reveals that the diffraction peaks slightly shift to higher angles by ~0.1° compared with the standard data of PbBr(OH), which might be a result of the lattice mismatch between PbBr(OH) and MAPbBr3. To further investigate the structural evolution of the samples before and after adding water, scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) images were taken. Before adding water, MA-d is a typical cubic structure (Fig. 2d), which matches well with its XRD results. To further explore the microscopic morphology of the samples, the MA-d powders were broken by an ultrasonic cell crusher in toluene. In the TEM image of the crushed MA-d (Supplementary Fig. 4), monodisperse and uniform QDs (10 nm in size) with cubic shapes can be observed, and the QDs tend to agglomerate on the TEM grids, as previously reported44. The inset of Supplementary Fig. 4 highlights that MA-d possesses a well-defined crystalline structure with a characteristic lattice distance of 0.58 nm, corresponding to the d-spacing of the (100) crystal planes of MAPbBr3. C, N, Pb, and Br can be observed from the elemental mapping of MA-d (Fig. 2e); the elements of C and N come from methylamine, while the elements of Pb and Br stem from the PbBr6 octahedron. MA-h exhibits uniform rod morphology with an average diameter of 1.5 μm and length of 4 μm, as shown in Fig. 2f and Supplementary Fig. 5. The inset of Fig. 2f shows the corresponding laser confocal fluorescence microscope image, and uniform green fluorescence can be observed along the rod structure. Elemental mapping of MA-h is shown in Fig. 2g, and the elements O, Pb, and Br are uniformly distributed along the rod structure, which indicates the formation of PbBr(OH). To explore the inner structure of the rod, the MA-h powders were broken in a cell crusher. Some spherical QDs with an outer shell can be observed (Fig. 2h), and the lattice spacing of the QDs is 0.29 nm, corresponding to the d-spacing of the (200) crystal planes of MAPbBr3 (Fig. 2i). The above results confirm that the MAPbBr3 QDs were coated by PbBr(OH). Thus, one can conclude that the bulk cubic shape of MAPbBr3 changes to a rod-like shape through the addition of water, and the MAPbBr3 QDs are embedded into PbBr(OH). In addition, X-ray photoelectron spectroscopy (XPS) spectra were collected to detect the surface chemistry of MA-d and MA-h, as shown in Supplementary Fig. 6. The Br/Pb atomic ratios calculated from the XPS spectra are summarized in Supplementary Table 2. From the table, one can see that the Br/Pb atomic ratios of MA-d and MA-h are 3.06 and 1.11, respectively, confirming that these materials are MAPbBr3 and PbBr(OH).
Fig. 3
a The PL intensity of the sample during cycling. b The corresponding PL spectra of MA-h, and the inset is the magnified spectra. c, d The PL spectra and images of the MA-h powders immersed in different solvents. e The PL intensity of the sample in the presence of water at different temperatures, and the inset is the schematic diagram of the reaction. f The PL intensity of the sample in the presence of water under UV irradiation for different hours, and the inset shows the schematic diagram of the testFig. 4
PL, PLE, and absorption spectra of a MA-d and b MA-h. c Temperature-dependent PL spectra. d PL decay curves of MA-d and MA-h, and the inset is the magnified spectra. e Time-resolved PL spectra of MA-h. f PL spectra of MA-h at different time delays. g Schematic illustration of the morphology evolution of the as-prepared MAPbBr3 perovskite. h Energy level diagram of PbBr(OH) and inner QDsFig. 5
PL spectra of a FA lead bromide perovskite and b all-inorganic halide perovskites, and the insets show digital images of the corresponding samples in ambient light and under UV light. c Digital images of the large-scale synthesized sample (upper images) and LED based on the sample under different driven currents (bottom images). d EL spectra of the LEDs under different driven currents from 10 mA to 200 mA. e Schematic diagram of potential fingerprint detection based on the as-prepared LED. f Photographs of fingerprint (upper images) and the corresponding pseudocolor map and gray values along the profile (bottom images)