In addition, the nanosheets are produced directly in the liquid phase and are thus inherently processable and can be easily formed into composites, coatings or films 21, facilitating their use in a range of applications. LPE is a powerful technique to produce nanosheets in large quantities. If this could be achieved it would yield numerous advantages. We hypothesized that LPE of BP may be practical if the solvent is carefully chosen to minimize oxidation of the exfoliated nanosheets in the liquid phase due to the solvation shell acting as a barrier to prevent oxidative species reaching the nanosheet surface. It is known that BP can be protected from reacting with environmental species by encapsulation, suggesting a possible way forward 7, 24. For this method to be useful, ways must be found to stabilize liquid-exfoliated FL-BP nanosheets against oxidation.
While phosphorene nanosheets have very recently been produced by liquid exfoliation 29, 30, 31, this method remains problematic, largely because BP is known to be unstable 7, 8, 32, degrading via reactions with water and oxygen. This technique involves the sonication 22, 23 or shearing 24, 25 of layered crystals in appropriate liquids and has previously been applied to graphene and boron nitride, as well as a range of other 2D materials 21, 22, 23, 26, 27, 28. One way to prepare nanosheets in large quantities is by liquid phase exfoliation (LPE) 20, 21. For most applications, it will be necessary to produce FL-BP in much larger quantities than can be achieved by mechanical exfoliation. Furthermore, theory predicts that FL-BP shows potential for use in gas sensors 18 and thermoelectrics 19. Indeed, BP has already been fabricated into electrodes in lithium ion batteries 17. In addition, like other 2D materials, it is probable that BP has the potential to perform in a range of applications beyond (opto)electronics. As a result, BP is extremely attractive both for electronics and optoelectronics and has therefore been extensively studied in applications such as transistors 7, 12, 15, photodetectors 15, 16 and solar cells 9. This is in contrast to graphene 1 which has no bandgap and materials such as MoS 2, which display direct bandgaps only in the monolayer form 6.
This new material has a direct bandgap in mono-, few-layer and bulk forms, which varies with nanosheet thickness from ∼1.5 eV for monolayer phosphorene to ∼0.3 eV for bulk BP 8, 13, 14. Recently, it was shown that BP can be exfoliated by mechanical cleavage to form mono- and few-layer phosphorene, which we refer to as FL-BP 8, 9, 10, 11, 12. In BP, the monolayers stack together via van der Waals interactions to form layered crystals in much the same way as graphene stacks together to form graphite. Phosphorene consists of atomically thin, 2D nanosheets of black phosphorus (BP). However, in the past year a new 2D material has been generating considerable excitement in the research community 7. Over the last few years, the study of two-dimensional (2D) materials 1, 2, 3, 4, 5, 6 such as graphene, BN and MoS 2 have become one of the most exciting areas of nano-science.