July 20, 2011

Transforming C60 molecules into graphene quantum dots

Scientists from the A*STAR Institute of High Performance Computing and the National University of Singapore have now developed a technique that collapses spherical carbon nanostructures down into perfectly formed quantum dots—structures useful for electronics because of their ability to trap single electrons.

The carbon atoms in graphite are arranged into stacked sheets that are weakly bound to one another. A single layer, referred to as graphene, can be peeled from bulk graphite using adhesive tape. The properties of graphene differ radically from the bulk material because it is only one atom thick, and so scientists are keen to harness this potential in a form compatible with existing optical and electronic devices. Small, regular-shaped graphene nanostructures called quantum dots, for example, could open up new possibilities in such applications.

Quantum dots, at just a few nanometers in diameter, can influence the flow of current at the single-electron level. However, making lots of quantum dots all with the same dimensions, and therefore with the same electronic properties, has proved to be tricky. The researchers came up with a simple solution: they took a hollow spherical form of carbon known as a ‘buckyball’—which is composed of 60 carbon atoms and always has the same shape—and fragmented it to produce uniformly sized quantum dots of graphene

Nature Nanotechnology - Transforming C60 molecules into graphene quantum dots

The fragmentation of fullerenes using ions, surface collisions or thermal effects is a complex process that typically leads to the formation of small carbon clusters of variable size. Here, we show that geometrically well-defined graphene quantum dots can be synthesized on a ruthenium surface using C60 molecules as a precursor. Scanning tunnelling microscopy imaging, supported by density functional theory calculations, suggests that the structures are formed through the ruthenium-catalysed cage-opening of C60. In this process, the strong C60–Ru interaction induces the formation of surface vacancies in the Ru single crystal and a subsequent embedding of C60 molecules in the surface. The fragmentation of the embedded molecules at elevated temperatures then produces carbon clusters that undergo diffusion and aggregation to form graphene quantum dots. The equilibrium shape of the graphene can be tailored by optimizing the annealing temperature and the density of the carbon clusters.

13 pages of supplemental information
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