Inorganic graphene

Molybdenum and tungsten sulfide nanosheets, analogous to graphene, have been made.

Credit: © 2010 Wiley

Inorganic analogues of carbon nanotubes and fullerenes made from molybdenum and tungsten sulfide have been known for some years. The bulk forms of these sulfides are similar to graphite in that they are layered and have a hexagonal structure. Although there have been some reports of exfoliated, single-sheet forms of these compounds, they have not been unambiguously characterized or comprehensively studied. These would mimic graphene, the single- or few-layered two-dimensional carbon material exciting much interest at present.

Now, C. N. R. Rao and colleagues from the Jawaharlal Nehru Centre for Advanced Scientific Research in Bangalore have used three chemical methods¹ to synthesize MoS₂ and WS₂ analogues of graphene. They have also used several characterization techniques to fully examine the materials. One of the synthetic methods involved using water to exfoliate lithium-intercalated bulk compounds, and the other two approaches involved building the sheets from the bottom up. All the methods resulted in single- or few-layered samples, confirmed by transmission electron microscopy and atomic force microscopy. A two- or three-layer-thick sample of WS₂ had an average thickness of 1.3 nm.

Rao and co-workers used density functional theory to examine the bonding and phonon structure in layered MoS₂. They found that in a triple-layered compound, molybdenum atoms are in a six-coordinate environment whereas the sulfur atoms are three-coordinate. Compared with the bulk compound, the bandgap in the layered system is larger, and the layer phonons have lower energy than those in the bulk.