Black phosphorus is a crystalline allotrope of phosphorus that is, in contrast to the more familiar white phosphorus, non-toxic and stable in air. When exposed to light or heat, however, it becomes increasingly unstable and will eventually ignite at around 300°C. In its pure form, black phosphorus is a semimetal with properties similar to those of graphite. It is an excellent conductor of electricity and has a large band gap making it suitable for use in optoelectronics applications.
Atomic structure
The crystal structure of black phosphorus consists of alternating layers of atoms bonded together by covalent bonds. These layers are held together by van der Waals forces. The atomic arrangement within each layer is similar to that found in graphite, with each atom bonded to three others in a honeycomb-like lattice. This gives black phosphorus its characteristic metallic lustre.
Properties
Due to the nature of the bonding in black phosphorus, it exhibits many interesting physical properties. It is anisotropic, meaning that its physical properties vary depending on the direction in which they are measured. For example, it is significantly harder to compress along the basal planes than perpendicular to them. This anisotropy also results in interesting electrical properties; whileblack phosphorus is an insulator when measured along the basal planes, it conducts electricity readily perpendicular to them due largely to the presence of conducting channels formed by delocalised electrons between layers. Black phosphorus also has a high surface area-to-volume ratio due to its layered structure, making it attractive for use as a catalyst support or electrode material. Finally, because of its unique atomic structure, black phosphorous fluoresces brightly when exposed to ultraviolet light making it useful as a marker or tracer material