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Structure Evolution of CH3NH3PbBr3 Single Crystal Grown in N, N-dimethylformamide Solution Feng Chen, Chunxiang Xu, Qingyu Xu, Yizhi Zhu, Zhu Zhu, Wei Liu, Xiuxiu Dong, Feifei Qin, and Zengliang Shi Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018
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Crystal Growth & Design
Structure Evolution of CH3NH3PbBr3 Single Crystal Grown in N, N-dimethylformamide Solution Feng Chen, Chunxiang Xu,* Qingyu Xu,* Yizhi Zhu, Zhu Zhu, Wei Liu，Xiuxiu Dong, Feifei Qin, Zengliang Shi State Key Laboratory of Bioelectronics, the National Demonstration Center for Experimental Biomedical Engineering Education (Southeast University), School of Physics, Southeast University, Nanjing 210096, PR China. Email:
[email protected],
[email protected] KEYWORDS. CH3NH3PbBr3, Single crystal, Morphology evolution, Growth mechanism
ABSTRACT. CH3NH3PbBr3 single crystal, one of organometal halide perovskites, as novel optoelectronic and photovoltaic materials could be easily precipitated using an anti-solvent method. Understanding of the growth mechanism will significantly improve reproducibility, quality and controllability of the single crystals. However, it is difficult to capture the growth process due to the super solubility of CH3NH3PbBr3 crystals in N, N-dimethylformamide solution. Here, filter paper was used to separate the intermediate states of CH3NH3PbBr3 crystals out of N, N-dimethylformamide solution in the growth process. The developed fabrication approach can monitor the morphology evolution of CH3NH3PbBr3 single crystals in whole. Meanwhile, the precursor solution’s properties and degradation of CH3NH3PbBr3 single crystals were also investigated systematically. It assigns that CH3NH3PbBr3 single crystals growth process is similar to sodium chloride crystals grown from solution in the presence of nitrilotriacetamide. The results help us to deeply discern the growth mechanism of CH3NH3PbBr3 single crystal in N, N-dimethylformamide solution and design controllable optoelectronic devices.
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■ INTRODUCTION Organic-inorganic halide perovskite, the prototype CH3NH3PbX3 (X= Cl, Br or I), have attracted a great deal of attentions for their remarkable performance in photovoltaic and optoelectronic devices.1-2 Crystal organometal halide perovskites could be easily precipitated through solution processed methods, vapor evaporation methods and templated synthesis methods.3-7 For a crystalline material, single crystals are often regarded as the ideal platform for discerning their intrinsic properties.8-10 Various methods have been used to prepare CH3NH3PbX3 single crystals, such as inverse temperature crystallization (ITC),11,12 top-seed solution growth (TSSG), 13 and anti-solvent
vapor-assisted
crystallization.14,15
In
anti-solvent
vapor-assisted
crystallization, an appropriate anti-solvent is slowly diffused into substance precursors solution for high quality single crystal formation.10,16, 17 Anti-solvent crystallization acts as the advantageous method where the substance crystallization is a weak function of temperature. It would requires less energy than a solvent evaporation process.18 The growth process can generally be carried out at room temperature and the obtained crystals possess a high purity and yield.18 For CH3NH3PbX3 growth, dichloromethane (CH2Cl2) vapor was used to diffuse into perovskite precursor N, N-dimethylformamide (DMF) solution resulting in supersaturation and subsequent crystallization. Shi et al. obtained crack-free CH3NH3PbX3 single crystals with volumes exceeding 100 cubic millimeters with exceptionally low trap-state densities by employing this crystallization method.14 In addition, material lattice mismatch and incompatible growth temperatures between these perovskite and any substrates are negligible. Using a modified solvent/antisolvent vapor-assisted crystallization approach, Liao et al. prepared single-crystalline square microdisks of CH3NH3PbBr3 on SiO2 wafer at room temperature. Meantime, the four side-faces of square CH3NH3PbBr3 microdisk constitute a built-in laser microresonator with high cavity quality factor.15 Recently, studies on the nucleation and growth mechanisms for ITC have been applied some in situ on-line characterization techniques, such as grazing incidence
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Crystal Growth & Design
X-ray diffraction (GIXD), in situ Fourier transform infrared spectroscopy (FTIR). These means can reveal the crystalline formation and chemical composition reactions of perovskite materials over relevant time and temperature scales.19 Otherwise, in situ changing the solvent composition (acidity of the solution) also can initiate and explore the mechanism of the CH3NH3PbX3 crystallization.11 In anti-solvent crystallization, CH2Cl2
is
a
poor
solvent
for
CH3NH3PbBr3
and
miscible
with
N,
N-dimethylformamide (DMF). However, since CH2Cl2 is volatile, CH3NH3PbBr3 crystals would be redissolved in DMF solution after dichloromethane volatilized.20 Therefore, it is difficult to monitor the nucleation and growth process of the CH3NH3PbBr3 in situ. Here, a developed approach was used to capture the CH3NH3PbBr3 morphology evolution. By using a filter paper insert between substrate and precursor solution droplet, the intermediate states of CH3NH3PbBr3 crystals can be separated from DMF solution effectively. It can reveal the growth details of CH3NH3PbBr3 and understand the crystallization mechanisms. Based on this mechanism, the morphology of CH3NH3PbBr3 single crystal can be controlled.
■MATERIALS AND METHODS 2.1. Materials. N, N-dimethylformamide and CH2Cl2 were used with high purity more than 99.5%(AR). CH3NH3Br was purchased from Xi’an Polymer Light Technology Corp. And PbBr2 was purchased from Shanghai Aladdin biochemical Polytron Technologies Inc. without any purification, respectively. 2.2. Experimental Setup. The single crystal microdisks of CH3NH3PbBr3 were synthesized by a modified self-assembly method reported by Liao, Fu and coworkers.12 Briefly, solutions of CH3NH3Br (0.2 M) and PbBr2 (0.2 M) in DMF were mixed to form 0.1 M precursor solution. 15 µL of CH3NH3PbBr3 solution was drop-coated on a filter paper stacked Si wafer. Two vials were used as the crystal growth device. As shown in scheme 1, the small vial acts as the stage to support the filter paper stacked Si wafer. To ensure the small vial can be held in the big vial, it is full filled with CH2Cl2.The big vial contained moderate CH2Cl2 leveled below the small vial. Both of the two vials were sealed and put in oven heating at 60 ℃. After
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predetermined period, the filter paper was extracted and dried at 60 ℃. A certain amount of the CH3NH3PbBr3 suspension precipitated on the filter paper, and the obtained samples were represented the CH3NH3PbBr3 crystal formed at the corresponding time. 2.3. Characterization. The scanning electron microscopy (SEM) images of CH3NH3PbBr3 micro-structures were obtained by a Zeiss field emission SEM operated at 1.5 kV. The optical absorption spectra were measured using a commercial UV-Vis spectrophotometer (Shimadzu UV-2600). The phase structure was investigated a powder X-ray diffractometer with a goniometer radius of 285 mm (Ultima IV with Cu Kalpha radiation, Rigaku Corporation). The photoluminescence (PL) were excited by a femtosecond laser at 325 nm with pulse duration of 150 fs and repetition rate of 1000 Hz, and the spectra were collected by a spectrometer (SpectraPro-2500i, Acton Research Corporation).
Scheme 1. Experimental setup and morphology evolution of the CH3NH3PbBr3 single crystal with anti-solvent crystallization.
■ RESULTS AND DISCUSSION It has been demonstrated that a high quality CH3NH3PbBr3 single crystal can be obtained following with Liao’s self-assembly method.15 The similar results were verified in our previous report10 and this experiment as shown in Fig.1a. Meanwhile, some defective crystals also can be found in the as-prepared samples (Fig. 1b). All of the defective CH3NH3Pb3 crystals have the external appearance of well-developed cubes with sharp edges, but each cube is partly hollowed (Fig. 1c). The morphology distribution is similar to sodium chloride crystals grown from solution in the presence of nitrilotriacetamide.21 The formation of agglomerate crystals in the early
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Crystal Growth & Design
crystallization stages resulted in the particles having hollow surfaces. The branches of a dendritic sodium chloride crystal were highly aligned and exhibited a roughly cubic morphology with (100) type faces.21 For these series of CH3NH3PbBr3 crystals formation, it may be attributed to the following reasons: (1) Perovskite precursors in solution are charged species, including an octahedral [PbBr6]4 centre and other cooperative ions which similar with intermediated [PbI6]4 cage nanoparticles.19 (2) Organic molecules hydrogen/ion interdigitate between both sides of inorganic perovskite sheets by van der Waals interaction.22 (3) The intrinsic property of the crystallines phase of CH3NH3PbBr3 is cubic or tetragonal.23, 24
Figure 1. (a) SEM images of as-prepared MAPbBr3 microcuboids, (b) partly hollowed MAPbBr3 microcuboids and (c) several enlarged individuals.
To confirm the growth details of CH3NH3PbBr3 single crystal, a filter paper was put forward to insert between Si wafer and precursor solution. The filter paper acts as the critical role in the entire preparation process. It can separate CH3NH3PbBr3 crystals from the residual precursor DMF solution and monitor the morphology evolution of CH3NH3PbBr3 perovskite crystallization. Generally, the whole growth process of CH3NH3PbBr3 crystal needs more than 24 h and the filter paper surface is uneven. Here, a frosted glass substrate and heating were carried to study the effects of rough surface and temperature on the crystallization. It was found that higher temperature was favorable for the formation of well-structured perovskite crystals on the roughened surface (see Supporting Information, Fig. SP1). As temperature
