Local Phonon Modes Concerned with the Self-Trapped Exciton State in CsPbBr3 Nanocrystals

The exciton-phonon coupling plays a significant role in the optical property of lead halide perovskite semiconductors, but the concerned phonon modes have not been well identified. Under the resonant excitation and low temperature conditions, strong and sharp Raman signals in CsPbBr3 nanocrystals (NCs) were found to superimpose on the fluorescence band of the free exciton, including both Stokes and anti-Stokes signals. Also, two basic phonon modes with energies of 3.7 and 6.3 meV were obtained, respectively. At the same time, the emission band from the self-trapped exciton (STE) recombination was clearly observed under the weak excitation conditions. The Raman signals and STE band show very high similarity in temperature dependence. Both of them decrease quickly with the rise of temperature from 10 to 50 K and then tend to vanish at higher temperatures. Therefore, the two phonon modes are proposed to be concerned with the STE state.