Fine structure of excitons and electron–hole exchange energy in polymorphic CsPbBr₃single nanocrystals

Abstract

All inorganic CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) belong to the novel class of confined metal-halide perovskites which are currently arousing enthusiasm and stimulating huge activity across several fields of optoelectronics due to outstanding properties. A deep knowledge of the band-edge excitonic properties of these materials is thus crucial to further optimize their performances. Here, high-resolution photoluminescence (PL) spectroscopy of single bromide-based NCs reveals the exciton fine structure in the form of sharp peaks that are linearly polarized and grouped in doublets or triplets, which directly mirror the adopted crystalline structure, tetragonal (D4h symmetry) or orthorhombic (D2h symmetry). Intelligible equations are found that show how the fundamental parameters (spin-orbit coupling, ΔSO, crystal field term, T, and electron-hole exchange energy, J) rule the energy spacings in doublets and triplets. From experimental data, fine estimations of each parameter are obtained. The analysis of the absorption spectra of an ensemble of NCs with a "quasi-bulk" behavior leads to ΔSO = 1.20 ± 0.06 eV and T = -0.34 ± 0.05 eV in CsPbBr3. The study of individual luminescence responses of NCs having sizes comparable to the exciton Bohr diameter, 7 nm, allows us to estimate the value of J to be around ≈3 meV in both tetragonal and orthorhombic phases. This value is already enhanced by confinement.

Photoluminescence Mechanisms of All‐Inorganic Cesium Lead Bromide Perovskites Revealed by Single Particle Spectroscopy

“…By investigating micro‐luminescence of single CsPbBr 3 NCs at low temperature (5 K), Ramade et al. have confirmed the existence of three bright exciton states with well‐defined polarization properties: doublets for tetragonal and triplets for orthorhombic crystal structures (Figure b–e) . Lounis et al.…”

“…Emission of CsPbBr 3 NCs could involve a highly emissive triplet state due to their unique structure and composition. The existence of this bright triplet exciton has been proposed by different groups using different theoretical models . The contribution of heavy metal (Pb) to the conduction band induces strong spin‐orbit coupling, which causes the splitting of the conduction band.…”

“…The existence of this bright triplet exciton has been proposed by different groups using different theoretical models. [19,[23][24] The contribution of heavy metal (Pb) to the conduction band induces strong spin-orbit coupling, which causes the splitting of the conduction band. Electron-hole exchange interactions result in the formation of a lower-lying dark singlet state and a higher-lying bright triplet state in the cubic structure (Figure 3a).…”

“…(b and c) Single-dot PL spectra of two individual CsPbBr 3 NCs at 5 K. (d) Typical triplet and (e) doublet recorded. [24] Copyright 2018, Royal Society of Chemistry. (f) The proposed fine structure of the band-edge exciton under orthorhombic symmetry.…”

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 triplets for orthorhombic crystal structures (Figure 3b-e). [24] Lounis et al employed low-temperature magneto-optical spectroscopy to reveal the exciton fine structure of CsPbBr 3 NCs and confirm the triplet nature of bright exciton. [27] Becker et al have also found that PL spectrum of individual CsPbBr 2 Cl NC at cryogenic temperatures exhibits one, two or three peaks, all with near-linear polarization (Figure 3f-g).…”