Science 331, 568–571 (2011)

Although graphene is the most famous two-dimensional crystal, it is not the only one. A variety of bulk layered materials can be exfoliated into single- and few-layer forms that exhibit distinct properties, including direct bandgaps and enhanced thermoelectric efficiency. However, exfoliation procedures have so far been relatively complex, or limited to small scales. Now, Jonathan Coleman and colleagues at Trinity College Dublin, Korea University, Texas A&M University, the University of Oxford and Imperial College London have demonstrated a simple and scalable method to exfoliate layered materials into individual two-dimensional flakes.

The researchers sonicated commercial powders of various layered materials in common solvents, and centrifuged the resulting dispersions. By choosing solvents with appropriate surface tensions, this procedure yielded two-dimensional nanoscale flakes tens to thousands of nanometres in lateral size. Transmission electron microscopy showed that the exfoliation left the crystal structure of the materials unaffected, in contrast to alternative approaches such as lithium intercalation.

Coleman and colleagues were also able to use these exfoliated materials to make free-standing films and composites. The composites consisted of a thermoplastic material filled with nanoscale flakes, and demonstrated enhanced mechanical properties comparable to the reinforcement achieved with graphene and nanoclay fillers. The new exfoliation technique is compatible with deposition onto substrates, and should be applicable to a wide variety of layered compounds.