NPG Asia Materials featured highlight | doi:10.1038/asiamat.2010.45

Graphene quantum dots: true blue

Fig. 1: Transmission electron microscopy image of graphene quantum dots.

 

It has been shown that graphene has excellent electronic, thermal and mechanical properties, but it is unusual to think about it as a potential candidate for optical applications as well. Still, this one-atomic-layer-thick sheet of carbon never ceases to surprise. Now, Dengyu Pan, Minghong Wu and colleagues at Shanghai University in China¹ have used a chemical route to obtain graphene quantum dots that emit bright blue luminescence.

In its two-dimensional form, a graphene sheet does not have a semiconducting energy gap, which is essential for optical activity. It is known, however, that a gap can be induced by reducing the dimensions of the sheet, by cutting out ribbons or producing quantum dots for example. But obtaining optical emission in the visible range would require a quantum dot of less than 10 nm in size, which is quite difficult to achieve.

Dengyu Pan and his colleagues have now achieved this through a chemical route. The quantum dots were obtained from graphene sheets oxidized in sulfuric and nitric acids. As Pan explains, “the oxidation results in the formation of oxygen-containing functional groups, typically C=O and C–O–C, which tend to organize into lines.” Exposing the oxidized sheets to a hydrothermal process removes the C–O–C groups and effectively cuts out quantum dots with a size of about 10 nm (Fig. 1).

The quantum dots exhibit strong luminescence at a wavelength of around 400 nm, which is not observed in the original graphene sheets. Clearly such a short emission wavelength is the signature of a large bandgap. This is a consequence of the strong lateral confinement, but the team believes that this strong optical activity is also related to the particular structure of the quantum dot edges.

How do these blue emitters compare with other systems? The quantum yield — an indication of the optical efficiency — is quite low with respect to well-developed dyes and semiconductor quantum dots. But, notes Pan, “the graphene quantum dots show more stable photoluminescence, have lower cytotoxicity, and pose less of an environmental hazard, making it attractive for biological applications.” The team also believes that the quantum yield can be improved with further work, which would make these graphene quantum dots attractive for optoelectronic applications.