Phosphorene is a two-dimensional material consisting of phosphorus. It consists of a single layer of the artificially made layered black phosphorus, the most stable allotrope of phosphorus. The designation phosphorene has been introduced in analogy to the designation of graphene as a single layer of graphite. Among two-dimensional materials, phosphorene emerged as a strong competitor to graphene because, in contrast to graphene, phosphorene has a nonzero fundamental band gap that can be furthermore modulated by strain and the number of layers in a stack. Phosphorene was first isolated in 2014 by mechanical exfoliation. History In 1914 black phosphorus, a layered semiconducting allotrope of phosphorus, was synthesized. This allotrope has been shown to exhibit high carrier mobility. In 2014, several groups isolated single-layer phosphorene, a monolayer of black phosphorus. It attracted renewed attention because of its potential in optoelectronics and electronics due to its band gap, which can be tuned via modifying its thickness, anisotropic photoelectronic properties and high carrier mobility. Phosphorene was initially prepared using mechanical cleavage, a commonly used technique in graphene production that is difficult to scale up. Liquid exfoliation is a promising method for scalable phosphorene production. Synthesis Synthesis of phosphorene is a significant challenge. Currently, there are two main ways of phosphorene production: scotch-tape-based microcleavage and liquid exfoliation, while several other methods are being developed as well. Phosphorene production from plasma etching has also been reported. In scotch-tape-based microcleavage, phosphorene is mechanically exfoliated from a bulk of black phosphorus crystal using scotch-tape. Phosphorene is then transferred on a Si/SiO2 substrate, where it is then cleaned with acetone, isopropyl alcohol and methanol to remove any scotch tape residue. The sample is then heated to 180 °C to remove solvent residue. In the liquid exfoliation method, first reported by Brent et al. in 2014 and modified by others, bulk black phosphorus is first ground in a mortar and pestle and then sonicated in deoxygenated, anhydrous organic liquids such as NMP under an inert atmosphere using low-power bath sonication. Suspensions are then centrifuged for 30 minutes to filter out the unexfoliated black phosphorus. Resulting 2D monolayer and few-layer phosphorene unoxidized and crystalline structure, while exposure to air oxidizes the phosphorene and produces acid. Another variation of liquid exfoliation is "basic N-methyl-2-pyrrolidone (NMP) liquid exfoliation". Bulk black phosphorene is added to a saturated NaOH/NMP solution, which is further sonicated for 4 hours to conduct liquid exfoliation. The solution is then