Phosphorene

Phosphorene is a two-dimensional material and allotrope of phosphorus. Phosphorene can be viewed as a single layer of black phosphorus, much in the same way that graphene is a single layer of graphite. Phosphorene is predicted to be a strong competitor to graphene because, in contrast to graphene, phosphorene has a band gap. Phosphorene was first isolated in 2014 by mechanical exfoliation.

Elemental phosphorus was discovered in 1669 by Hennig Brand. In the 1960s black phosphorus, a layered semiconducting allotrope of phosphorus, was synthesized, which exhibited high carrier mobility. In 2014, several groups introduced 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 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 is also 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 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.

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