Fluorescence at the high-energy end of the visible spectrum (blue and violet) is important because it potentially allows other colours with lower energies to be extracted, explains team leader Manish Chhowalla. By properly engineering a material, luminescence in different colours, or white, can be produced. This can be useful for lighting or colour displays.

Easily processed
The team, which includes scientists from Imperial College London in the UK and National Taiwan University, observed blue fluorescence from thin films of chemically derived graphene oxide. "Although there are other organic compounds that fluoresce at this wavelength, they are not easily processable in solution," Chhowalla told nanotechweb.org. "Graphene oxide, on the other hand, can be easily dissolved in water and deposited on a variety of substrates over large areas in the form of thin films."

Such films are transparent and their electrical properties can be tuned from semiconducting to metallic by controlling their thickness. These and other interesting optoelectronic properties could be exploited by fabricating devices on flexible platforms.

"Graphene oxide can be easily dissolved in water and deposited on a variety of substrates over large areas in the form of thin films" Manish Chhowalla, Rutgers University

The researchers obtained their results by depositing graphene oxide thin films on a substrate and then gradually reducing the films using chemicals while monitoring changes in the electrical and optical properties of the material. The photoluminescence changes as the graphene oxide reduces, which shows that it comes from recombining electron-hole pairs localized within small carbon clusters, according to the team.

The results suggest that even though a graphene oxide sheet is a giant molecule, its optical properties mimic those of smaller organic molecules. Combining these properties, and the fact that it is easy to process in solution, makes it attractive for printable optoelectronic devices.

The researchers now hope to engineer the chemical structure of graphene oxide to tune its fluorescence energy.

The work was reported on arXiv.