Progress in the synthesis of 2D black phosphorus beyond exfoliation

A considerable number of recent research have focused on two-dimensional (2D) black phosphorus (BP) since it was successfully prepared through mechanical exfoliation in 2014. After scaling down, BP with atomistic thickness shows fascinating semiconducting features with layer-dependent direct bandgap and high carrier mobility. The synthesis of high-quality few-layer BP thin films is critical to investigate their distinctive crystal structure, fundamental characteristics, as well as the potential applications in electronics, biomedicine, energy storage, photonics, and optoelectronics. Therefore, this review provides an overview of mono- and few-layer BP topic in the synthesis methods beyond exfoliation, including thinning treatments accompanied to exfoliation, conversion from red phosphorus to BP, and direct growth techniques. We summarize various attempts to control the BP sample's thickness and lateral dimensions during the synthesis. Furthermore, we discuss the current challenges and perspectives of large-scale growth of ultrathin BP which has been a bottleneck hindering wafer-scale device's development in this field. We hope to provide an insight into exploring some potential approaches practicable to synthesize high quality BP thin films utilized for developing high-performance nano-electronics and photonics, which may accelerate the progress of 2D BP toward real applications.