Lead halide perovskite nanocrystals are promising candidates for applications in light-emitting devices due to their exceptionally high quantum efficiency. Single nanocrystals are even considered as efficient single photon sources. For fully exploiting this application potential, a detailed understanding of the recombination mechanism - in particular of the exchange-split excitonic fine structure - is essential. 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.

The specific band structure of lead halide perovskites – s-states forming the valence band and p-states that form the conduction band - leads to an exciton fine structure consisting of a singlet and a triplet state. These can be energetically split depending on crystal symmetry and shape anisotropy [1]. 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. The polarization of the two emission lines of case II NPLs are oriented orthogonally with respect to each other. In case III NPLs, in contrast, 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]. Interestingly, we found that the occupation of the fine structure split states cannot be explained by a simple Boltzmann distribution, indicating an inhibited transfer of excitons between the exchange-split states [4]. We will discuss the potential role of the distinctly different band structure of lead halide perovskites compared to II-VI nanocrystals on this finding.

References

[1] Tamarat et al. (2019): Nature Materials 18, 717. DOI: 10.1038/s41563-019-0364-x

[2] Pfingsten et al. (2018): Nano Letters 18, 4440. DOI: 10.1021/acs.nanolett.8b01523.

[3] Bertolotti et al. (2019): ACS Nano 13, 14294. DOI: 10.1021/acsnano.9b07626.

[4] Schmitz et al. (2021): Nano Letters 21, 9085. DOI: 10.1021/acs.nanolett.1c02775.