(333e) Tunable Angular Emission of Inorganic Lead Halide Perovskite Nanoplates Via Controlled Assembly on a Liquid Sublayer

Inorganic lead halide perovskites (CsPbX₃(X=halide)) have exhibited extraordinary optical behavior, including remarkably high quantum yields and long exciton diffusion lengths much larger than typical inorganic semiconductors [1], [2]. Further, the transition dipole moment of perovskite nanocrystals (which directly governs their directional emission) is highly controllable based on the local environment [3]. This unique property of perovskites is not observed in other semiconductor nanocrystals (i.e. CdSe quantum dots) whose transition dipole moment can only be changed by moving towards more confined, anisotropic shapes. Such properties make them an interesting candidate for new wave optoelectronic, quantum, and solar devices. However, low defect assembly and large-scale ordering are prerequisites for their commercial use [4]. To date, this has been difficult to achieve. While some have been able to achieve local ordering through solvent-based evaporation [5], [6], long range ordering through conventional liquid-sublayer assembly methods [7], [8] has not yet been realized because of the perovskites’ instability and incompatibility with many anti-solvent and polar environments (e.g. diethylene glycol, dimethylformamide).

Here we demonstrate large, low defect, assembled thin films of perovskite (CsPbBr₃) nanoplates via liquid-liquid self-assembly for the first time. We show that by instead employing a fluorinated solvent sublayer, we can preserve our perovskite nanocrystals and provide the necessary environment to create a monolayer of highly ordered anisotropic nanocrystals. The figure demonstrates how long-range face-down ordering of CsPbBr₃ nanoplates is possible using this assembly method. Further, performing back focal plane (Fourier) imaging allows us to extract the average transition dipole moment (TDM) angle, or the average orientation of electronic transitions, which is important for determining the ultimate efficiency of an optoelectronic device [9]. Previous studies on face-down CsPbBr₃ nanocrystals showed increased TDM angles (14-29°) due to local surface charging [3]. Interestingly, our face-down assemblies of much larger crystals and of different synthetic technique (adapted from Pan et al. [10]) show extremely small transition angles (0°), highlighting that the transition angle of these materials is not only sensitive to environment, but also to synthesis and preparation. Understanding and control of these interactions could unlock these materials for novel optoelectronics applications.

References

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[10] J. Pan et al., ‘Efficient and Stable Self-Assembly Blue-Emitting CsPbBr3Nanoplatelets with Self-Repaired Surface Defects’, ACS Appl Nano Mater, vol. 5, no. 10, pp. 15062–15069, Oct. 2022, doi: 10.1021/acsanm.2c03231.