Very recently, all-inorganic perovskite CsPbX3 (X = Cl, Br, I) nanostructures such as nanoparticles, nanoplates, and nanorods have been extensively explored. These CsPbX3 nanostructures exhibit excellent optical properties; however, the photophysics involved is not yet clear. Herein, the emission properties and luminescence mechanism of CsPbBr3 nanosheets (NSs) were investigated using steady-state and time-resolved photoluminescence (PL) spectroscopic techniques. Moreover, two kinds of excitonic emissions (Peak 1 and Peak 2) are observed at low temperatures (<80 K) under the conditions of low excitation level. They are revealed to stem from the radiative recombination of trapped and free excitons by examining their spectral features and emission intensity dependences on excitation power. Thermally induced exchange between the two kinds of excitons is found and modeled quantitatively; this has led to the determination of an activation energy of 13 meV. Thermal redistribution of trapped excitons and thermal expansion-induced blueshift of the bandgap are jointly responsible for the abnormal temperature dependence of the position of Peak 1, whereas the latter is predominant for the monotonic blueshift of the position of Peak 2 with an increase in temperature. These results and findings shed some light on the complicated luminescence mechanism of CsPbBr3 NSs.

The role of exciton–phonon coupling in light emission in cesium lead bromide (CsPbBr3) nanosheets is investigated with combined photoluminescence spectroscopy and the multimode Brownian oscillator model. A good agreement between theory and experiment in the low temperature range of 5–40 K enables us to determine several key parameters, including the dimensionless Huang–Rhys factor characterizing the exciton–longitudinal-optical 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 free 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 interested temperature range. These new findings may deepen the understanding of the exciton–phonon coupling in CsPbBr3 nanosheets and relevant solids.

For excited carriers or electron-hole coupling pairs (excitons) in disordered crystals, they may localize and broadly distribute within energy space first, and then experience radiative recombination and thermal transfer (i.e., non-radiative recombination via multi-phonon process) processes till they eventually return to their ground states. It has been known for a very long time that the time dynamics of these elementary excitations is energy dependent or dispersive. However, theoretical treatments to the problem are notoriously difficult. Here, we develop an analytical generalized model for temperature dependent time-resolved luminescence, which is capable of giving a quantitative description of dispersive carrier dynamics in a wide temperature range. The two effective luminescence and nonradiative recombination lifetimes of localized elementary excitations were mathematically derived as Carrier localization (CL) in real crystalline solids due to various disorders, e.g., defects, impurities, composition fluctuation, lattice distortion etc. is a ubiquitous phenomenon which was theoretically treated by Anderson for the first time 1 . To date, CL and related phenomena still remain as a subject of extensive interest primarily because of their scientific significance and profound impact on electrical, magnetic and optical properties of material systems [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] . With the rapid development of the InGaN alloy based blue-green light emitting diodes, recently, the CL effect induced by structural imperfections has been increasingly addressed [19][20][21] . For example, it has been well shown that localized carriers due to alloy disorder, especially indium content fluctuation, can produce efficient luminescence and unusual thermodynamic behaviors [19][20][21][22][23][24][25][26][27][28] . In order to interpret these unusual luminescence behaviors associated with the carrier localization, many attempts have been devoted. For example, Eliseev et al. proposed an empirical formula to interpret temperature-induced "blue" shift in peak position of luminescence 25 . This model agrees well with experimental data at high temperatures, but does not work at low temperatures. Wang applied the pseudopotential approach to study the CL mechanism in different InGaN systems 23 , which mainly focuses on the contribution of component fluctuation and quantum-dot formation to the carrier localization.