Lead halide perovskite nanocrystals are promising candidates for applications in light-emitting devices due to their high exciton binding energy, exceptionally high quantum efficiency, and good stability. Depending on crystal symmetry and shape anisotropy, a distinct exciton fine structure controls their emission properties [1]. Here, we demonstrate the strength of polarization-resolved micro-photoluminescence (PL) spectroscopy on a single nanocrystal level for getting insight into exciton fine structure states and their relation to crystal symmetry and shape anisotropy.

In nearly cubic FAPbBr₃ nanocrystals, the degeneracy of the bright exciton triplet is lifted leading to three bright states with transition dipoles oriented along the orthorhombic crystal symmetry at cryogenic temperatures. Depending on the orientation of the nanocrystals with respect to the optical axis between one and three polarized emission lines are visible [2]. Magneto-PL and time-resolved-PL experiments on single nanocrystals demonstrate that the dark singlet exciton is energetically below the bright one, shifted by about 2.6 meV.

In highly anisotropic CsPbBr₃ nanoplatelets (NPLs) either one, two or three resolvable emission lines (case I, II and III, respectively) with significantly different polarization patterns are found. In case a single peak is observed, the emission is mostly unpolarized and linewidths generally exceed 1 meV. In contrast, the polarization of the two emission lines of case II NPLs are oriented orthogonally with respect to each other. In case III NPLs, the lowest and highest energy peaks are polarized collinearly, while the central emission line is polarized in a direction orthogonal to the former two. This quite characteristic polarization pattern can be explained by the occurrence of orthorhombic CsPbBr₃ NPLs with two different orientations of the crystal axes [3]. In case II NPLs, one of the orthorhombic crystal axes is aligned with the NPL thickness and the observation direction and the radiation emitted by the corresponding dipole cannot be detected. In case III NPLs, in contrast, all orthorhombic crystal axes have a finite projection perpendicular to the observation direction allowing the observation of all emission lines of the fine structure split triplet. The negligible fine structure splitting in case I NPLs is consistent with the coexistence of these different lattice polymorphs within a single NPL. Our findings not only allow the unambiguous identification of the crystal configuration of individual CsPbBr₃ nanoplatelets from pure optical measurements, but in addition facilitate the determination of a nanoplatelets’ absolute spatial orientation with respect to the lab coordinates via its characteristic polarization pattern.[4]