Methylammonium lead halide perovskite materials have received remarkable interest because of their extraordinary operating capacity in solar cell, light-emitting diode, laser, and water splitting devices, coupled with the cheap, flexible nature of the material synthesis. In many instances, perovskites are developed in thin-film, platelet, or nanocrystal form but for some applications more atypical 3D shapes would also be appropriate.
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Carbonate salts, including the familiar biomineral calcium carbonate, can be fashioned into a substantial diversity of composite 3D forms through the implementation of either biological or bio-inspired mineralization functions. Through the chemical substitution of these carbonate salts with perovskite, in a process that preserves shape, the protean optoelectronic characteristics of perovskites can be synthesized with the geometrical tunability of carbonate salts .
Using Cathodoluminescence Imaging for Analyzing Optical Properties of 3D Perovskites
Cathodoluminescence (CL) imaging and spectroscopy offers a robust method to examine the optical properties of such 3D perovskites structures on deep-subwavelength scales. Figure 1 shows CL intensity maps of complex shaped perovskite structures, obtained from synthetically produced carbonate material. CL maps indicate that there is (efficient) light emission throughout the structures, demonstrating that the transformation into a light emitting perovskite semiconductor was successful. CL images were collected on a range of shapes and perovskite compositions with varying halides in the crystal matrix (chlorine, bromine, and iodine) as shown in Figure 1.
Figure 1. CL intensity maps of synthetically grown 3D perovskite structures acquired with the SPARC CL system. Images show (a) spiral, (b) trumpet, and (c) spiked-coral shapes of CH3NH3PbBr3. (d) CH3NH3PbCl3 coral shape. (e) Field of CH3NH3PbI3 vase shapes. The colors chosen for the color scale represent the emission range of the measured cathodoluminescence (see Figure 2), with lighter colors corresponding to higher emission intensities. The CL maps were acquired at 5 kV acceleration voltage, a beam current of 45 pA, and a pixel dwell time of 100 μs per pixel. Credits: Noorduin Lab, AMOLF. See .
Figure 2 exhibits CL spectra obtained on microstructures with alternative perovskite compositions, comparable to the structures shown in Figure 1. As anticipated the primary emission peak position strongly relies on which halide is contained in the perovskite crystal (420, 550, and 800 nm for Cl, Br, and I respectively), denoting that CL can be productively utilized to (locally) ascertain the spectral emission properties of diverse perovskite compounds.
It is understood that methylammonium perovskite materials are less substantial compared to some of their comprehensively inorganic counterparts. Nonetheless, top-quality CL maps could be secured on the structures without consequential degradation, indicating that this imaging methodology is not overly injurious for perovskites.
Figure 2. Spatially averaged CL spectra of CH3NH3PbCl3 (blue curve), CH3NH3PbBr3 (green curve), and CH3NH3PbI3 (red curve) acquired on individual microstructures. The spectra were acquired using 5 kV acceleration voltage, a beam current of 45 pA, and a dwell time of 100 ms per scanning pixel Credits: Noorduin Lab, AMOLF. See 
The CL imaging approaches presented here can naturally be extended to other perovskite geometries/materials (e.g. thin-films) . As such the SPARC CL system can be used as an analytical tool to facilitate the continuous development and improvement of a wide range of perovskite-based (nano) materials and devices.
 T. Holtus, et al., Shape-preserving transformation of carbonate minerals into lead halide perovskite semiconductors based on ion exchange/insertion reactions, Nature Chemistry, https://doi.org/10.1038/s41557-018-0064-1 (2018)
 W. Li et al. Phase Segregation Enhanced Ion Movement in Efficient Inorganic CsPbIBr2 Solar Cells Adv. Energy Mater. 7, 1700946 (2017)
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