Ultrathin and Flat Layer Black Phosphorus Fabricated by Reactive Oxygen and Water Rinse

Hyuksang Kwon, Sung Won Seo, Tae Gun Kim, Eun Seong Lee, Phung Thi Lanh, Sena Yang, Sunmin Ryu*, and Jeong Won Kim*

Ultrathin black phosphorus (BP) is one of the promising two-dimensional (2D) materials for future optoelectronic devices. Its chemical instability in ambient conditions and lack of a bottom-up approach for its synthesis necessitate efficient etching methods that generate BP films of designed thickness with stable and high-quality surfaces. Herein, reporting a photochemical etching method, we demonstrate a controlled layer-by-layer thinning of thick BP films down to a few layers or a single layer and confirm their Raman and photoluminescence characteristics. Ozone molecules generated by O₂ photolysis oxidize BP, forming P₂O₅-like oxides. When the resulting phosphorus oxides are removed by water, the surface of BP with preset thickness is highly flat and self-protective by surface oxygen functional groups. This method provides a fabrication strategy of BP and possibly other 2D semiconductors with band gaps tuned by their thickness.