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International Edition: DOI: 10.1002/anie.201610619 German Edition: DOI: 10.1002/ange.201610619
Quantum Dots
Colloidal Synthesis of CH3NH3PbBr3 Nanoplatelets with Polarized Emission through Self-Organization Lige Liu+, Sheng Huang+, Longfei Pan, Li-Jie Shi, Bingsuo Zou, Luogen Deng, and Haizheng Zhong* Abstract: We report a combined experimental and theoretical study of the synthesis of CH3NH3PbBr3 nanoplatelets through self-organization. Shape transformation from spherical nanodots to square or rectangular nanoplatelets can be achieved by keeping the preformed colloidal nanocrystals at a high concentration (3.5 mg mL@1) for 3 days, or combining the synthesis of nanodots with self-organization. The average thickness of the resulting CH3NH3PbBr3 nanoplatelets is similar to the size of the original nanoparticles, and we also noticed several nanoplatelets with circular or square holes, suggesting that the shape transformation experienced a self-organization process through dipole–dipole interactions along with a realignment of dipolar vectors. Additionally, the CH3NH3PbBr3 nanoplatelets exhibit excellent polarized emissions for stretched CH3NH3PbBr3 nanoplatelets embedded in a polymer composite film, showing advantageous photoluminescence properties for display backlights.
Colloidal nanocrystals (NCs) of halide perovskites have attracted a great of attention owing to their superior luminescence properties as well as promising applications in many optoelectronics.[1–5] Spectroscopic studies and application exploration strongly rely on high-quality materials, and great efforts have been devoted to refine the synthesis of such materials.[6–12] In particular, controlling the size and shape of NCs is greatly desired in order to understand the fundamental dimensional-dependent properties and optimize device performance.[13–16] Unlike the well-developed colloidal chemistry for inorganic NCs, halide perovskite NCs can be synthesized at room temperature by adopting the re-precipitation synthesis for organic nanoparticles through simple solvent mixing.[3, 17] By varying the ligands and solvents, a few reports have realized the fabrication of halide perovskite NCs with controllable size and shapes.[18–25] However, an understanding of their formation mechanism greatly lags behind that of the [*] S. Huang,[+] Prof. H. Z. Zhong Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science & Engineering, Beijing Institute of Technology 5 Zhongguancun South Street, Haidian District Beijing, 100081 (China) E-mail:
[email protected] L. G. Liu,[+] L. F. Pan, Dr. L. J. Shi, Prof. B. S. Zou, Prof. L. G. Deng School of Physics, Beijing Institute of Technology 5 Zhongguancun South Street, Haidian District, Beijing, 100081 (China) [+] These authors contributed equally to this work. Supporting information for this article can be found under: http://dx.doi.org/10.1002/anie.201610619.
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colloidal routes, and only a limited number of reports have studied the dimensional-dependent optical properties. Very recently, we developed the ligand-assisted re-precipitation (LARP) synthesis and emulsion synthesis to generate size-tunable CH3NH3PbBr3 NCs with bright photoluminescence (PL) emission.[3, 18] Owing to the high mobility of halide ions[11, 12] and low formation energy of CH3NH3PbBr3 (DH = @0.25 eV),[26] the crystallization process of halide perovskites during solvent mixing is very complicated. Because of the lack of precise control, shape tuning has been a great challenge. As demonstrated by Kotov and Tang et al., self-organization of the pre-formed nanoparticles in solution is an alternative way to achieve anisotropic NCs.[27, 28] Importantly, the inherent linearly polarized PL emissions from these anisotropic shapes are very attractive for liquid crystal display (LCD) backlights.[29–31] Herein, we report a self-organization process to synthesize CH3NH3PbBr3 nanoplatelets (NPLs) from pre-formed colloidal CH3NH3PbBr3 nanodots (NDs), and demonstrate their linearly polarized PL emissions. The original spherical CH3NH3PbBr3 NCs were synthesized following our reported procedure,[18] and the resulting NCs can be precipitated and then isolated from colloidal solution to remove excess ligands and unreacted reagents (see the Supporting Information). The purified NCs are spherical NDs with tunable diameter from 3 to 6 nm. A typical sample was dissolved in nonpolar solvent at a concentration of 3.5 mg mL@1 and aged in darkness at room temperature for several days. The morphology evolution was studied by applying transmission electron microscopy (TEM; Figure S1). In the TEM images, several two- and four-dot alignments appeared after incubating for 2 h (Figure S2). With longer incubations, NPLs began appearing along with the original spherical NDs, and the ratio of NPLs gradually increased. After three days, most of the NDs had transformed into NPLs. Figure 1 shows the TEM images of the original CH3NH3PbBr3 NDs and their corresponding NPLs. The enlarged side view of NPLs is shown as the inset of Figure 1. We also varied the average size of the original NDs from 3 to 6 nm (Figure S3). In comparison to the original NDs, the average thickness of the obtained NPLs exhibited a slight size reduction from 3.3 nm to 2.8 nm, 4.8 nm to 3.7 nm, and 5.5 nm to 4.4 nm (Figure S4). The thickness was also confirmed by applying atomic force microscopy (AFM; Figure S5). We further analyzed the transformation process of CH3NH3PbBr3 NCs, and the results are summarized in Figure S6. The detailed analysis demonstrated that the thickness of the NPLs is similar to the size of the original NDs, and the side length is about 4-times the original NDs. We also conducted structural
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Figure 1. Illustration of the transformation of CH3NH3PbBr3 NCs from NDs to NPLs, and the corresponding TEM images of the original NDs (left) and the resultant NPLs (right). The inset is the enlarged side view of the NPLs.
characterization by applying X-ray diffraction (XRD) measurements. As shown in Figure S7, the resultant NPLs had obvious diffraction patterns of cubic CH3NH3PbBr3 structure, with featured peaks of (100), (110), (200), (210), (220), and (300) crystal planes. The XRD results are summarized in Table S1. All of the above results suggest that the formation of CH3NH3PbBr3 NPLs experienced a self-organization process. It is noted that the CH3NH3PbBr3 NPLs can also be obtained through one-step synthesis by combining the synthesis of NDs with the self-organization process (the detailed procedure is described in the Supporting Information and Figure S8). This implies that the NDs are dynamic products and the formation of CH3NH3PbBr3 NPLs is thermodynamically preferable. The CH3NH3PbBr3 NPLs were further characterized by applying high-resolution TEM (HRTEM) measurements. As shown in Figure 2 a and b, the NPLs are highly crystalline, with obvious lattice distances of 5.98 c and 4.20 c, corresponding to the lattice distance of (100) and (110) planes for cubic CH3NH3PbBr3 structure. It is also noteworthy that a few NPLs had circular or square holes (Figure 2 c). Theses NPLs with holes were also crystalline, with clear (100) crystal planes (Figures 2 d and S9). Detailed analysis revealed that all of the NPLs, with or without holes, had a preferred direction along the [100] plane. The TEM observations suggest that the NPLs may be formed by the fusion of original NDs along the [100] direction at the final stage. This assumption was supported by calculations that (100) planes have the lowest surface energy of 0.089 J m@2, in comparison with (110) of 0.155 J m@2 and (111) of 0.259 J m@2 (Table S2). To understand the self-organization process of CH3NH3PbBr3 NDs into NPLs, we performed theoretical simulations (Supporting Information). According to classical Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, electrostatic and van der Waals interactions between particles are the driving forces for the self-organization process.[32, 33] These interactions between two particles are pronounced when their distance is less than 20 nm (Figure S10). The initial stage of self-organization was dominated by dipole–dipole attractions owing to the longer interacting distance of dipolar Angew. Chem. Int. Ed. 2017, 56, 1780 –1783
Figure 2. High-resolution TEM images of CH3NH3PbBr3 NPLs without (a, b) and with (c, d) holes. The blue, pink, and brown spheres represent the CH3NH3, Pb, and Br ions, respectively.
attraction forces ( & 1/r3) than that of van der Waals interaction forces ( & 1/r6).[34] The parallel alignment of dipolar vectors in dipole–dipole interactions have been demonstrated to be main driving force for the self-organization of CdTe, CdSe, PbSe, PbS, Fe3O4, and Au NDs.[35–38] Owing to the molecular rotations and ion/hydrogen migrations in CH3NH3PbBr3 NCs,[39, 40] the dipolar vectors are direction variable, and the realignment of the dipolar vectors cannot be ignored during self-organization. Considering the variation of dipolar vectors, two and four coalescent NDs have the lowest energy and could be stable during the self-organization process. Although a balance between electrostatic and van der Waals forces is needed, the directional changes in the dipolar vector play an essential role in determining the final shape, described as follows. Figure 3 a shows the alignment of dipolar vectors for the two coalescent NDs with a fluctuating angle of a = 6088. Figures 3 b and c show the distribution of dipolar and total potential field of the two-dot alignment. The distribution of the dipolar potential field for two coalescent NDs is asymmetrical owing to the rotation of the dipolar vectors during their combination. The total potential field in Figure 3 c shows that the neighboring two NDs can be attracted from the top and bottom to form a four-dot alignment. In this case, the dipolar vectors reorient to redistribute the dipolar potential field. The dipolar vectors of the four aligned NDs are reoriented with a fluctuating angle of about b = 9088 with each other to further form a square shape (Figure 3 d–f). Note that our simulations also reveal that the direction of the dipolar vector is changed all of the time when the NDs are close to each other under dipole– dipole attraction. This is a new point that is different from the common dipole–dipole interactions. The four-dot alignment could undergo a secondary process to form larger NPLs, whose axis size is four-times that of the original dots. As reported by Kotov and Tang et al., the side–side orientation
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Figure 3. a–c) Illustration of the dipolar alignment (a), the distribution of the dipolar potential field (b), and total potential field (c) in a twodot alignment of CH3NH3PbBr3 NCs. d–f) Illustration of the dipolar alignment (d), the distribution of the dipolar potential field (e), and the total potential field (f) in a four-dot alignment. Note: The brown spheres represent NDs and the black arrow represents the dipolar vectors in CH3NH3PbBr3 NCs. The color bar represents the intensity of the potential field around the NDs alignment, attractive force (blue), and repulsive force (red).
gives the lowest energy state. Therefore, the self-organization process preferred the side–side orientation with the coordination of van der Waals and electrostatic forces.[27] Based on the crystallization thermodynamics,[41] the concentration of nanoparticles greatly decreased during the self-organization process to achieve an energy equilibrium state. The above discussions are in good accordance with the experimental observations that the axes of resulting NPLs are about 4-times that of the original NDs, and the intermediates seen in the TEM images also support the proposed dipolar realignments. In comparison with the NPLs reported previously, the resulting CH3NH3PbBr3 NPLs exhibit good colloidal dispersibility and excellent optical properties (Table S3). As shown in Figure 4 a, the UV/Vis absorption and PL spectra of the CH3NH3PbBr3 NDs and NPLs have similar band absorption edges at 512 nm and PL peaks of 520 nm. The absolute PL quantum yield of NPLs is & 70 %, which is comparable with that of the original NDs (Figure S11). The PL decays were studied using time-resolved PL measurements, and the results are present in Figure S12. Both of the CH3NH3PbBr3 NDs and
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NPLs show similar biexponential PL decays, with short-lived PL lifetimes (t1) of 4.0–4.7 ns, and long-lived PL lifetimes (t2) of 25.2–26.8 ns. Although single anisotropic NCs exhibit inherently polarized emissions, the polarization effects can be reduced when they are randomly distributed.[29–31, 42] The unidirectional alignment of NCs in polymer films through stretching can efficiently enhance polarization.[31] To illustrate the polarized PL emission from CH3NH3PbBr3 NPLs, we incorporated them into a stretchable polymeric matrix (4-methyl-1-pentene, TPX) for the polarization determination. An optical setup with a 405 nm laser was applied for the measurement (Figure S13). As shown in Figure 4 b, the PL emission of stretched NPL-embedded TPX film showed the expected cos/ sin intensity dependence on the polarizer angles, while the stretched ND-embedded TPX film does not. We define the degree of linear polarization as P = (Imax@Imin)/(Imax + Imin), and a polarization degree of 0.11 was measured for 2- to 3times stretched NPL-embedded TPX film. In addition, no polarization was observed for both of the unstretched NPLand ND-embedded films (Figure S14). These results illustrated that the observed polarization emission mainly arises from the alignment of NPLs, but not from the ordered dielectric environment.[43] In summary, we report the synthesis of uniform and luminescent CH3NH3PbBr3 NPLs through self-organization of pre-formed colloidal CH3NH3PbBr3 NDs. Theoretical simulations and analysis reveal that the dipole–dipole interactions with a realignment of dipolar vectors are the main factors driving the self-organization process, which is a new mechanism for the assembly of NCs. This conclusion is supported by the observed intermediates in TEM observations and the fact that the size and thickness of the resulting NPLs are correlated with the size of the original NDs. Additionally, we demonstrated the linearly polarized PL emission from stretched CH3NH3PbBr3 NPL-embedded polymer films, and a linear polarization degree of 0.11 was obtained. This work opens up an alternative way to realize shape control of halide perovskite NCs and provides potential polarized emissive materials.
Acknowledgements This work was supported by National Natural Science Foundation of China (No. 21573018 and 11474021) and Beijing Nova program (No. xx2014B040). The authors would like to thank Prof. Kun Liu for reading the manuscript, Prof. Yunchao Li and Xiaoyi Gao for their help on the AFM measurements.
Conflict of interest The authors declare no conflict of interest. Figure 4. a) UV/Vis absorption and PL spectra of the original CH3NH3PbBr3 NDs (black line) and corresponding NPLs (red line). b) PL intensity as a function of polarizer angle (blue spheres), and can be fitted to sinusoidal function (red line).
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Keywords: halide perovskites · nanodots · nanoplatelets · photoluminescence · polarization
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Manuscript received: October 30, 2016 Revised: December 17, 2016 Final Article published: January 11, 2017
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