Synthesis and Optical Properties of Colloidal M3Bi2I9 (M = Cs, Rb) Perovskite Nanocrystals

Bulk Cs3Bi2I9 exhibits zero-dimensional (0-D) perovskite crystal structure at the molecular level, providing scopes for novel optical properties compared to three-dimensional perovskite structures. Here, 0-D refers to the crystal structure irrespective of the size of the crystal. We have prepared colloidal Cs3Bi2I9 nanocrystals and elucidated the unique optical properties arising from their 0-D crystal structure. Absorption spectrum at 10 K confirms that the electronic band gap of Cs3Bi2I9 nanocrystals is at 2.86 eV, along with a sharp excitonic peak at 2.56 eV, resulting in a very high excitonic binding energy, E-b(X) = 300 meV. Interestingly, we observe two peaks in the photoluminescence spectra at room temperature on both sides of the excitonic absorption energy. Because EbX (300 meV) >> effective phonon energy (36 meV), the phonon-mediated relaxation of carriers from conduction band minimum to the excitonic state is suppressed to an extent. Consequently, two photoluminescence peaks related to both the bulk band edge and the excitonic transitions are observed. Furthermore, Rb3Bi2I9 nanocrystals have also been synthesized, but they exhibit two-dimensional layered structure, unlike the 0-D structure of Cs3Bi2I9.