Synthesis, Surface Engineering, and Emerging Applications of Black Phosphorus Nanosheets

Black phosphorus nanosheets (BPNS) are emerging two-dimensional (2D) materials with exceptional optical and electronic properties, making them promising candidates for a wide range of nanotechnology applications. Their distinctive structural, electronic, and optical characteristics, particularly their thickness-dependent direct band gap, which varies from approximately 0.3 eV in bulk black phosphorus to 2.0 eV in the monolayer, have been extensively investigated using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations. This review focuses on the anisotropic optical responses of BPNS, including their polarization-dependent linear absorption and reflection, as well as nonlinear optical phenomena such as saturable absorption and the Kerr effect. In addition, the potential applications of BPNS in solar cells, sensors, and biomedical devices are discussed, with particular emphasis on their tunable band gap and enhanced sensitivity. Their reported biodegradability and generally favorable biocompatibility in preclinical studies further support their potential in nanomedicine, although these properties depend on formulation, dose, and surface modification. Despite these promising properties, challenges related to large-scale synthesis and environmental stability remain. Addressing these limitations through scalable synthesis approaches and improved stability will be essential for translating the unique capabilities of BPNS into practical applications.