Photophysics of cesium lead bromide perovskite nanosheets

Abstract

All-inorganic perovskite CsPbX3 (X=Cl, Br, I) nanostructures such as nanocrystals, nanosheets and nanowires have been increasingly explored in recent years. These CsPbX3 nanostructures have been demonstrated to exhibit excellent optical properties, but their photophysics has not yet clearly understood. In this thesis study, the luminescence mechanisms of CsPbBr3 nanosheets (NSs) are investigated by using various spectroscopic techniques. Some new insights into the luminescence processes in this emerging light-emitting material are obtained for the first time, to the best of our knowledge. The main results and findings of this study are summarized as below. Two kinds of excitonic emissions are observed below 80 K under the conditions of low excitation level. It is revealed that they stem from the radiative recombination of trapped (TX) and free excitons (FX), respectively, in terms of their photoluminescence (PL) spectral features and integrated intensity dependence on the excitation power. Thermally induced exchange between the two kinds of excitons is found and modeled quantitatively, which leads to the determination of an activation energy of 13 meV. Both thermal redistribution of the trapped excitons and bandgap blueshift induced by thermal expansion are jointly responsible for the abnormal temperature dependence of the peak position of the trapped excitons. For the free excitons, the latter mechanism is predominantly for the monotonic blueshift of their PL peak position with the rise of temperature. The role of exciton−phonon coupling in the luminescence of CsPbBr3 NSs is investigated with a combined PL spectroscopy and the multimode Brownian oscillator (MBO) model. Good agreement between theory and experiment in a low temperature range of 5-40 K enables us to determine several key parameters, such as the dimensionless Huang-Rhys factor characterizing the exciton-phonon coupling strength and the damping constant accounting for the phonon bath (quasi-continuous acoustic phonons) dissipation. It is found that the Huang-Rhys factor of the FX excitons peculiarly tends to diminish upon increasing the temperature in the interested low temperature range. However, the damping constant shows a linear increase with temperature in the same temperature range. To have a deeper insight into the complex luminescence mechanisms of CsPbBr3 NSs, PL behaviors in a broad temperature range from 5 to 500 K are further investigated. The bandgap blueshift induced by thermal lattice expansion is found to be gradually compensated by the bandgap redshift caused by electron-phonon coupling when the temperature was varied from 5 to 360 K. As the temperature is further increased beyond 360 K, the nearly completely compensated PL peak position turns to exhibit a rapid blueshift again. Such turning behavior is consistent with an orthorhombic–tetragonal (γ–β) phase transition at this critical temperature. For the PL linewidth, it shows a continuous broadening for temperatures higher than 40 K, suggesting the dominant role of phonon scattering, especially at high temperatures. The impact of excitation energy on the luminescence of FX and TX excitons in CsPbBr3 perovskite NSs is examined both experimentally and theoretically. At 10 K, the relative PL intensity between the trapped excitons and the free excitons shows a peculiar reduction tendency with increasing the optical excitation energy from 2.431 eV (510 nm) to 3.758 eV (330 nm). To interpret such tendency, a quantitative model is developed on the basis of the biological logistic population growth theory. A good agreement between experiment and theory is obtained. Lower capture coefficient of the TX level to the excited excitons via multiphonon emission relative to the FX level is argued to be the major cause of the observed phenomenon.