Synthesis of Millimeter-Size Single-Crystal 2D BiOI Sheets and Ribbons on Mica

Atomically thin two-dimensional (2D) materials have received widespread interest both scientifically and technologically. Recently, the 2D layered BiOI crystal has been revisited for its low-dimensional features and semiconducting properties, yet the production of large-size 2D BiOI single crystals remains challenging. Here, we report the synthesis of millimeter-size single-crystal 2D BiOI sheets and ribbons for the first time by using a sacrifice strategy to optimize the O₂ partial pressure in an atmospheric-pressure growth system. Two types of synthesis mechanism are proposed for these BiOI crystals: (1) 2D nucleation and growth for BiOI sheets (bottom-up); (2) layer-by-layer oxidation and exfoliation for BiOI sheets and ribbons (up-bottom). The top-gated 2D BiOI-based field-effect transistor shows n-type behavior and moderate performance with ∼10⁴ current modulation. Our study gives deep insights into the synthesis of large-size 2D BiOI single crystals and provides a new up-bottom method that could also be used for synthesizing many other 2D materials.