Frontiers in Synthetic Moiré Quantum Matter Proceedings of a Workshop (2022) / Chapter Skim
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Pages 62-68
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From page 62... ...
The resulting material can be in the form of thin films, large single crystals, or polycrystalline powders. In these constructs, Karunadasa noted, there are two pieces that can be manipu lated -- the organic layers and the inorganic layers -- and she offered a number of FIGURE 6-1 Layered hybrid perovskites.
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From page 63... ...
She discussed three types of these bandgaps in particular: ligand-to-metal charge transfer, metal-to-metal charge transfer, and inorganic-to-organic charge transfer. Ligand-to-metal charge transfer can be found in two-dimensional copper-halide perovskites such as R2CuCl4 or R2CuBr4, where R is a 1+ organic ion, or three-dimensional double perovskites with vacancies such as Cs2SnI6.
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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... ...
"This holds true for all the double perovskites we have looked at," she said. This allows them to predict, to some extent, whether a bandgap will be large, medium, or small for a particular pairing of metals.
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From page 66... ...
For instance, the alloyed material is black, while the single perovskite is bright yel low and the double perovskite is colorless. The reason the alloyed material is black is that it has strong low-energy absorption produced by intervalence charge transfer.
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From page 67... ...
The exfoliation can be done with single perovskites, double perovskites, and many others. "There is a huge phase space to explore." Some recent papers have described exfoliating such materials all the way down to a monolayer, she said (Leng et al.
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From page 68... ...
First, because organic groups are used to direct the self-assembly, modifying the organic molecules changes the heterostruc ture that will crystallize, she said, "and we think this method is infinitely tunable to form various different intergrowths between the perovskites." Furthermore, these materials form as single crystals, so single-crystal X-ray diffraction can be used to determine the structure of the crystals very precisely. To date, she said, her group has synthesized at least nine of these heterostruc tures.
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