Black and blue phosphorus are both hexagonal allotropes of phosphorus made of stacked two-dimensional layers. But a single layer of blue phosphorus bears a closer resemblance to graphene – the layers have ‘zig-zag’ ridges rather than the deeper ‘armchair’ ridges of black phosphorus, making them slightly flatter. Calculations performed by Zhen Zhu and David Tomanek at Michigan State University showed that both forms are equally stable, but blue phosphorus may have superior electrical properties, such as a wider bandgap and higher electron mobility, which would make it better suited to applications in two-dimensional electronics. Like black phosphorus, it should be possible to exfoliate blue phosphorus to produce two-dimensional layers.
Blue phosphorus has not been synthesised yet, but the team suggest it could be made by chemical vapour deposition onto a substrate with hexagonal symmetry, or by depositing black phosphorus onto a stepped substrate.