2D Single Crystal of High-Temperature Phase Cuprous Iodide under Ambient Conditions

Two-dimensional (2D) materials, with their structural uniqueness, exceptional properties, and wide-ranging applications, show unprecedented prospects in fundamental physics research and industrial applications. 2D \b{eta}-phase cuprous iodide (\b{eta}-CuI) is a promising candidate for overcoming the challenges of insufficient P-type transparent conductive materials, with multiple predicted unique properties. Previous experimental results show that \b{eta}-CuI only occurs at elevated temperatures between 645 and 675 K. Many efforts are made to stabilize \b{eta}-CuI at room temperature through surface/interface engineering. However, the resulting composites limit the performance and application of pure 2D \b{eta}-CuI. Here, we demonstrate experimentally that isolated 2D \b{eta}-CuI single crystals can exist stably under ambient conditions, a high-temperature phase CuI found at room temperature. We validate the simultaneous existence of {\gamma}-CuI and \b{eta}-CuI in the synthesized CuI. The previous neglect of \b{eta}-CuI crystals can be ascribed to factors including their low content, small dimensions, and lack of ingenious experimental characterization. Moreover, the theoretical calculation further confirms dynamically and thermally stable of the monolayer \b{eta}-CuI, which has an ultra-wide direct band-gap (3.66 eV). Our findings challenge the traditional understanding of \b{eta}-CuI as a high-temperature phase of CuI, instead providing a new definition that 2D \b{eta}-CuI exhibits remarkable stability under ambient conditions.