Graphene — a layer of graphite only one atom thick — has been hailed by some as the new silicon. It has some extraordinary electronic properties, most notably very low resistance to electron flow. But it also suffers from one of the drawbacks associated with silicon in that it is difficult to make graphene emit light. This hinders its integration into optoelectronic circuits. Recently, the related material graphene oxide has been investigated as a way to circumvent these problems, and it has so far been shown to generate both infrared and visible light.

Now, a collaborative team led by Chun-Wei Chen of the National Taiwan University and Manish Chhowalla, now at Imperial College London, has gone even further by demonstrating blue and near-ultraviolet luminescence from graphene oxide films.1

Semiconductors are widely used for fabricating optoelectronic devices, where electrons undergo optical transitions across an energy band gap. The difficulty in generating light from graphene stems from the fact that it is a semimetal, and thus does not have a band gap. However, a band gap can be opened by the addition of oxygen to the carbon, which results in the formation small clusters in which atoms are covalently bonded in a different way. The clusters effectively behave as luminescence centers, and the emission wavelength is determined by the cluster size and density. “We have recently succeeded in producing light emission from red to blue by controlling the size of the emission clusters,” says Chen.

Fig. 1: Blue-light emission from a suspension of graphene oxide.

The research team deposited thin films of graphene oxide from suspension (Fig. 1) onto a silicon substrate. Light was excited from the sample by exposure to ultraviolet radiation. The intensity of the emission could be increased by controlling the concentration of the clusters, which was achieved by exposing the thin films to a vapor of the inorganic chemical hydrazine.

While graphene oxide does not have the superb electronic properties of graphene, its advantage lies in the fact that it has an optical band gap. It is also soluble, which makes it amenable to large-area and inexpensive device manufacture. “These results provide us with the potential for fabricating printable devices, such as displays or light-emitting diodes, on flexible substrates.”