Phonon Interaction and Phase Transition in Single Formamidinium Lead Bromide Quantum Dots

Abstract

Formamidinium lead bromide (FAPbBr) quantum dots (QDs) are promising materials for light emitting applications in the visible spectral region because of their high photoluminescence (PL) quantum yield (QY) and the enhanced chemical stability as compared to, for instance, methylammonium based analogues. Toward practical harnessing of their compelling optical characteristics, the exciton recombination process, and in particular the exciton-phonon interaction and the impact of crystal phase transition, has to be understood in detail. This is addressed in this contribution by PL studies on single colloidal FAPbBr QDs. Polarization-resolved PL measurements reveal a fine structure splitting of excitonic transitions due to the Rashba effect. Distinct phonon replica have been observed within energetic distances of 4.3 ± 0.5, 8.6 ± 0.9, and 13.2 ± 1.1 meV from the zero phonon line, which we attribute to vibrational modes of the lead bromide lattice. Additional vibrational modes of 18.6 ± 0.3 and 38.8 ± 1.1 meV are found and related to liberation modes of the formamidinium (FA) cation. Temperature-dependent PL spectra reveal a line broadening of the emission caused by exciton phonon interaction as well an unusual energy shift which is attributed to a crystal phase transition within the single QD.

Properties of Excitons and Photogenerated Charge Carriers in Metal Halide Perovskites

“…Wide excitonic linewidth broadening was also found in perovskite nanocrystals . Saran et al attributed the PL linewidth broadening in CsPbX 3 (X = I, Br, and Cl) nanocrystals to the strong exciton–phonon coupling which also induced a giant bandgap renormalization in the materials with redshift of the bandgap by decreasing temperature .…”

“…Of particular interesting, instead of focusing on the emissive excitonic states, some researchers turn to investigating the dynamics of dark excitons in perovskite nanocrystals, since the presence of dark excitonic states may notably influence the performance of perovskite nanocrystal devices . Recently, the excitonic levels in perovskite nanocrystals have been found with fine structures due to the combined effects of electron–hole exchange interaction, strong spin–orbit coupling, and symmetry distortion . According to these studies, the lowest excitonic level is bright or dark strongly depending on the degree of asymmetry (structure distortion) of the sample.…”

“…Whereas, the vibrational analysis in perovskite nanocrystals is relatively rare. Very recently, Pfingsten et al found phonon replicas appeared as satellite emission peaks in FAPbBr 3 quantum dots, with the phonon replica at 4.3 meV assigned to the TO 1 mode, the one at 8.6 meV to the TO 2 mode, and the peak at 13.2 meV to either the TO 3 or the LO 1 mode or a combination of both. Additional peaks at around 18.6 and 38.8 meV were attributed to the FA liberation modes but could be also possible to the PbBr 6 cage octahedral mode.…”

“…Exciton–phonon coupling has been studied in both hybrid and all‐inorganic perovskites, with most investigations focusing on the understanding of the temperature dependence of PL linewidth broadening by such coupling . For example, Phuong et al investigated the exciton–phonon coupling in MAPbI 3 single crystals at low temperatures (15–80 K) using both temperature‐dependent photocurrent (PC) and PL spectroscopy ( Figure a) .…”

Long Exciton Dephasing Time and Coherent Phonon Coupling in CsPbBr2Cl Perovskite Nanocrystals

“…NCs. 32 The three-pulse photon-echo signal ( 12 , 13 ) in the direction = − 1 + 2 + 3 of an inhomogeneously broadened two-level system coupled to a single harmonic mode of angular frequency can be calculated as: 29 28,29 In Figure 4c, we show the modelled electric fields of the three-pulse photon echo with ( 12 , 23 )~√ ( 12 , 23 ) according to equation (1). The model uses a Huang-Rhys parameter of 1 2 Δ 2 = 0.038, a phonon mode at 3.45 meV (as measured in single NC PL measurements, see SI) and the above measured decay rates Γ 1 and 1,2 with their corresponding amplitudes.…”