Atomically Thin Layers of Graphene and Hexagonal Boron Nitride Made by Solvent Exfoliation of Their Phosphoric Acid Intercalation Compounds

The development of scalable and reliable techniques for the production of the atomically thin layers of graphene and hexagonal boron nitride (h-BN) in bulk quantities could make these materials a powerful platform for devices and composites that impact a wide variety of technologies (Nature 2012, 490, 192-200). To date a number of practical exfoliation methods have been reported that are based on sonicating or stirring powdered graphite or h-BN in common solvents. However, the products of these experiments consist mainly of few-layer sheets and contain only a small fraction of monolayers. A possible reason for this is that splitting the crystals into monolayers starts from solvent intercalation, which must overcome the substantial interlayer cohesive energy (120-720 mJ/m²) of the van der Waals solids. Here we show that the yield of the atomically thin layers can be increased to near unity when stage-1 intercalation compounds of phosphoric acid are used as starting materials. The exfoliation to predominantly monolayers was achieved by stirring them in medium polarity organic solvents that can form hydrogen bonds. The exfoliation process does not disrupt the sp² π-system of graphene and is gentle enough to allow the preparation of graphene and h-BN monolayers that are tens of microns in their lateral dimensions.