Silver Ions Assisted Inversion Temperature Crystallization of 2D Cs₃Bi₂Br₉ Nanoflakes for Highly Sensitive X‐Ray Detection

Abstract 2D perovskites have attracted wide attention for optoelectronic applications because of their unique layer structure and tunable outstanding optical/electrical properties. In addition, 2D Cs 3 Bi 2 Br 9 nanoflakes possess large effective atomic number, high resistivity, high density as well as excellent stability, rendering it a promising material for X‐ray detection. Nevertheless, it is full of challenges to synthesize 2D Cs 3 Bi 2 Br 9 nanoflakes by conventional inversion temperature crystallization (ITC) strategy due to the existence of Br ‐ vacancies in the Cs 3 Bi 2 Br 9 crystal nucleus. Herein, an Ag + assisted ITC (SAITC) strategy to grow 2D Cs 3 Bi 2 Br 9 nanoflakes is proposed. The synthesis mechanism revealed by both experiments and theoretical calculations can be mainly ascribed to the passivated Br − vacancies and enhanced structure stability by adding Ag + which can effectively prevent the oxidation of 2D Cs 3 Bi 2 Br 9 nanoflakes from growth of hybrid crystals. The synthesized high‐crystallinity 2D Cs 3 Bi 2 Br 9 nanoflakes possess direct bandgap characteristic, and the mobility lifetime can reach 9.8 × 10 −4 cm 2 V −1 . Excitingly, the fabricated device based on 2D Cs 3 Bi 2 Br 9 nanoflakes demonstrates ultrahigh sensitivity of detecting X‐ray (1.9 CGy air −1 cm −2 ) at very low driven voltage (0.5 V) due to the photoconductive gain mechanism. The 2D Cs 3 Bi 2 Br 9 nanoflakes synthesized by SAITC method have great potential for developing highly sensitive optoelectronic devices.