Frontiers in Synthetic Moiré Quantum Matter Proceedings of a Workshop (2022) / Chapter Skim
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6 Slicing and Alloying Halide Perovskites
6 Slicing and Alloying Halide Perovskites
Pages 61-71
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From page 61... ...
She began by talking about the optical, electronic, and structural diversity of the halide perovskites and then offered two case studies that show the potential of these materials in the field of moiré lattice systems. CREATING HALIDE PEROVSKITE STRUCTURES Noting that Ramesh had offered an introduction to oxide perovskites, Karunadasa explained that replacing the oxide with a halide would produce halide perovskites.
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From page 62... ...
For her, Karunadasa said, the most compelling aspect of these materials is how they are formed: "We take a pot of water, and we throw all the precursors into the solution. You can imagine, in solution there is this crazy free for all with organic molecules and inorganic ions tumbling around in solution, and our job as synthetic chemists is to figure out the conditions under which this mess, on its own accord, chooses to form these elaborate structures." Through a great deal of experimentation with different materials, she said, one learns that there are certain organic molecules that allow for templating of specific inorganic architectures and, likewise, there are certain inorganic architectures that allow for templating of specific organic molecules.
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From page 63... ...
In another example, monomers were placed between the perovskite sheets and hit with gamma radiation which created one-dimensional polymers between the perovskite layers In short, she said, it is possible to do all sorts of tricks with the organic layers, but in terms of the electronic properties, the inorganic components are much more important. So she offered some examples of the sorts of electronic structures that can be formed depending on how the three-dimensional perovskite is conceptu ally sliced to form two-dimensional perovskites.
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From page 64... ...
On the other hand, some halide perovskites will spontaneously lose halogen gas, such as bromine gas, leading to electron doping and increased conductivity; in the case of three-dimensional Cs2AgTlBr6, once the bromine gas has escaped, it can be replaced by putting the material in a bromine atmosphere for a week or so. "We are quite interested in trying to control this halo gen degassing reaction," Karunadasa said, "because we think it is a powerful way of doping these materials to the point where we can get decent transport properties." The Uses of Impurities in Double Perovskites Homovalent and heterovalent impurities can produce dramatic changes in the optical and electronic properties of two-dimensional and three-dimensional halide perovskites, Karunadasa said, and she offered an example of each.
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From page 65... ...
In particular, with less than 1 percent thallium impurity, the indirect bandgap drops from nearly 2 eV to about 1.4 eV, she said, "which is exactly where you want to be for photovoltaics." The thallium atoms seem to be replacing bismuth atoms in the lattice, and replacing all of the bismuth with thallium to create Cs2AgTlBr6 results in "the lowest bandgap I have seen in a halide perovskite," she said, of about 0.95 eV.
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From page 66... ...
couple in a similar square lattice, she said. The Future of Two-Dimensional Perovskites One can also carry out dimensional reduction on various double perovskites, Karunadasa said, and this reduction has various interesting consequences.
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From page 67... ...
Smaha, J Li, et al., 2021, "Alloying a Single and a Double Perovskite: A Cu+/2+ Mixed-Valence Layered Halide Perovskite with Strong Optical Absorption," Chemical Science 12:8689, permission conveyed through Copyright Clearance Center, Inc.
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From page 68... ...
The perovskite homo bilayer -- that is, two sheets of MA2PbI4 held together with van der Waal forces -- is analogous in several ways to the MoS2–WS2 hetero bilayer structure, such as the interlayer dis tances between the various atoms in the two structures. The researchers calculated the band structure of the Pb–I bilayer and found that the rotation of the CH3NH3 groups can considerably change the charge distribution.
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From page 69... ...
Summing up, she said that the theoretical paper motivates the search for bilayer homo- and heterostructures made out of perovskites, while her group's synthesis work offers hope that the self-assembly of halide perovskite heterostructures as single crystals may offer a path for realizing the exotic properties of hetero bilayer interfaces in bulk materials that form in water. QUESTION-AND-ANSWER SESSION In a brief period following her presentation, which was moderated by Aha ron Kapitulnik, Karunadasa answered several questions about her work.
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From page 70... ...
Kapitulnik asked whether these halide perovskite materials with organic lay ers interspersed between the perovskite layers might have enough flexibility to be compressed or twisted as a way of modifying their electronic properties. That is definitely worth looking at, Karunadasa replied.
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From page 71... ...
2020. "Expanded Analogs of Three-Dimensional Lead-Halide Hybrid Perovskites." Angewandte Che mie International Edition 59(43)
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