Optical Probing of Crystal Lattice Configurations in Single CsPbBr₃ Nanoplatelets

Abstract

Quantum-confined nanostructures of CsPbBr 3 with luminescence quantum efficiencies approaching unity have shown tremendous potential for lighting and quantum light applications. In contrast to CsPbBr 3 quantum dots, where the fine structure of the emissive exciton state has been intensely discussed, the relationship among lattice orientation, shape anisotropy, and exciton fine structure in lead halide nanoplatelets has not yet been established. In this work, we investigate the fine structure of the bright triplet exciton of individual CsPbBr 3 nanoplatelets by polarization-resolved micro-and magnetophotoluminescence spectroscopy at liquid helium temperature and find a large zero-field splitting of up to 2.5 meV. A unique relation between the crystal structure and the photoluminescence emission confirms the existence of two distinct crystal configurations in such nanoplatelets with different alignments of the crystal axes with respect to the nanoplatelet facets. Polarization-resolved experiments eventually allow us to determine the absolute orientation of an individual nanoplatelet on the substrate purely by optical means.

Exciton–Phonon and Trion–Phonon Couplings Revealed by Photoluminescence Spectroscopy of Single CsPbBr3 Perovskite Nanocrystals

“…The polarization behavior of a Type III NC is shown in Figure S4. These fine-structure splittings are induced by the crystal field. , The number of observable peaks is determined by the three transition dipoles (bright triplet excitons) and the orientation of the NCs. , Figure c summarizes the fine-structure splitting energy of bright excitons of the Type II NCs as a function of the exciton peak energy. The size dependence of the splitting energy is under discussion in perovskite NC. − , The splitting energy of ∼1 meV is in good agreement with findings from the previous studies on CsPbBr 3 NCs ,,,, and shows no clear size dependence under our experimental condition.…”

“…26,52 The number of observable peaks is determined by the three transition dipoles (bright triplet excitons) and the orientation of the NCs. 26,53 peak energy. The size dependence of the splitting energy is under discussion in perovskite NC.…”

Quantized Exciton Motion and Fine Energy-Level Structure of a Single Perovskite Nanowire

“…If we set the axial direction of a single CsPbBr 3 NWR along the z -axis, according to previous optical studies of single perovskite QDs, − the energy-level structure of band-edge excitons is composed of in-plane X and Y orthogonal states, out-of-plane Z state, and dark D state (see theoretical discussions in the Supporting Information). Although the X , Y , and Z states should be all emissive due to their bright-exciton feature, it was quite common to observe only doublet PL peaks from a single perovskite QD with orthogonally linear polarizations. − The missing bright-exciton state might have the highest energy to prevent its thermal population, a weak oscillator strength or an emission dipole parallel to the PL collection axis. − For the single NWRs studied here with a flat-lying configuration on the sample substrate (Figure a,c), the emission dipoles of X / Y and Z excitons should be distributed within the cross-sectional plane and along the axial direction, respectively. , Because of the strong anisotropy of depolarization field in a single NWR, luminescence polarized along the long axis ( Z -state) is strong whereas the electric fields of light within the cross-sectional plane ( X and Y states) are reduced by 2ε/(ε + ε NWR ) with ε and ε NWR being dielectric constants of the environment and the NWR, respectively.…”

“…The above findings have thus provided a comprehensive view on the intrinsic energy-level structures and recombination dynamics of excitons in 1D semiconductor nanostructures. These single NWRs can then serve as an ideal platform for further explorations of fascinating 1D photophysics, thus complementing their well-studied 2D ,, and 0D ,− ,, counterparts in terms of both fundamental studies and practical applications.…”

Thickness-Dependent Dark-Bright Exciton Splitting and Phonon Bottleneck in CsPbBr3-Based Nanoplatelets Revealed via Magneto-Optical Spectroscopy

“…The presence of a dark ground state is potentially detrimental for applications of lead halide perovskites as light emitters. The exciton fine structure of perovskite nanocrystals has been recently the object of extensive investigation. ,,,− The substantially larger confinement of charge carriers in nanoplatelets greatly enhances the electron–hole exchange interaction and is expected to increase the splitting between the bright and dark states significantly. , The fine structure of perovskite nanoplatelets has received considerably less attention, and experimental efforts have only recently focused on obtaining quantitative measurements. ,− …”