(284e) Controlling the Orientation of Photoluminescent Transition States in Anisotropic CsPbBr3 Perovskite Nanoparticles Using Alignment, Fusing, and Surface Interactions

Next generation optoelectronic devices require extremely bright emitters with tunable properties, such as color tunability and preferential light emission angle, that can be produced at large scales. Perovskite nanocrystals (PNCs) are an excellent candidate for this challenge as they can achieve extremely high quantum yields and a wide color gamut [1]. Recently, the alignment electric dipoles within perovskite nanocrystal films, which governs angular light emission and energy transfer rates, was shown to be variable based on the local environment [2], [3], [4], and this is unique to these materials. While most of these studies focus on isotropic nanoparticles, confinement and anisotropic particle shape also strongly influence the alignment of these dipoles and the degree to which the particles are influenced by their local environment [5].

This presentation will explore how surface effects and neighbor interactions are affected by assembles of anisotropic CsPbBr3 nanoparticles. We synthesized films of CsPbBr3 nanocrystals and used back focal plane microscopy to quantify the angle between the horizontal and vertical electronic dipole states, TDM (the effective transition dipole moment angle relative to the substrate) [6]. We synthesized CsPbBr3 nanoplates (~3-5 nm thick by ~30 nm in length and width, emission = 485 nm) and used two methods to achieve highly anisotropic photoluminescent properties. First, we used slow evaporation in solvents with varying polarity (hexane, heptane, and octane) to induce anisotropic fusing into long nanowires (100s nm in length) [7]. Back focal plane fluorescence microscopy showed that the unfused particles had a nearly isotropic dipole alignment (TDM = 33°). This higher vertical component is present despite the confinement because of the nanocrystal interactions with the glass substrate. After fusing into 100s nm long nanowires, the surface effects were reduced, inducing a more horizontally aligned dipole component (TDM = 22-27°) where the horizontal strength correlated directly to fused particle length. Next, we used liquid-liquid self-assembly to create large, low defect films of highly oriented particles. By varying the solvent, we can tune between kinetically trapped (face down) or thermodynamically favored (edge up) assemblies and achieve strongly horizontal or strongly vertical dipole alignments, respectively. The face down assemblies of the nanoplates also show an enhanced vertical component (actual TDM = 39°, expected TDM = 18°) as seen in prior studies. Interestingly, the edge-up plates show a dipole alignment (actual TDM = 44°) that is nearly unaffected by the surface effects seen in other perovskite nanocrystal films (expected TDM = 42°). Additionally, if more quantum confined particles are used (emission = 460 nm) and the nanoparticles are allowed to anisotropically fuse (lengths ~100 nm), the dipole alignment becomes almost completely horizontal, showing that the continuity of the films plays a significant role. Understanding and controlling the interplay of anisotropy and surface interactions could elucidate why these nanocrystals can achieve such unique photophysics and allow us to design materials for next generation technologies.

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[2] M. J. Jurow et al., “Tunable Anisotropic Photon Emission from Self-Organized CsPbBr3 Perovskite Nanocrystals,” Nano Letters, vol. 17, no. 7, pp. 4534–4540, 2017.

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[4] L. E. Parsons, B. Russ, and C. N. Eisler, “Tunable angular light emission of sparse lead halide perovskite nanocrystal thin films via solution-processed substrate treatment,” Nano Letters, Under Review, Submitted March 2025.

[5] M. J. Jurow et al., “Manipulating the Transition Dipole Moment of CsPbBr 3 Perovskite Nanocrystals for Superior Optical Properties,” Nano Letters, vol. 19, no. 4, pp. 2489–2496, 2019.

[6] B. Russ, T.-T. Lin, and C. N. Eisler, “Quantifying the Accuracy and Uncertainty in Back Focal Plane Imaging for Nanostructured Materials and Optoelectronics,” ACS Photonics, vol. Accepted, Mar. 2025.

[7] N. Soetan, W. R. Erwin, A. M. Tonigan, D. G. Walker, and R. Bardhan, “Solvent-Assisted Self-Assembly of CsPbBr₃ Perovskite Nanocrystals into One-Dimensional Superlattice,” J. Phys. Chem. C, vol. 121, no. 33, pp. 18186–18194, Aug. 2017.