Electrical switching can control the optical transparency of thin films of graphene oxide.
Bilkent University researchers characterizing the optical response of biased graphene oxide films. From left to right: Okan Oner Ekiz, Aykutlu Dana, Bulend Ortac, Mustafa Urel, And Hasan Guner.
Electrical switching can control the optical transparency of thin films of graphene oxide, according to researchers in Aykutlu Dana’s group at Bilkent University (O.O. Ekiz et al., ACS Nano, doi: 10.1021/nn1014215). This could possibly be used to create bistable displays for electronic paper or to tune ultrafast optoelectronic devices.
Graphene, the single-atom-thick material formed by a planar honeycomb of carbon atoms, shows promise for many electrical and optical devices, and methods of mechanically and chemically altering the material to obtain desirable properties are a hot topic for research. Graphene is an excellent conductor with a zero-energy bandgap. Graphene oxide, however, is an insulator with a large effective band gap.
The group grew multilayer graphene oxide films about 30-60 nm thick on Pd/Au contacts on a glass substrate. The contacts were separated by roughly half a millimeter. When they created a 2.5 V difference between the two contacts, the film near the positive contact oxidized and the negative contact reduced (deoxygenated). The oxidized film was semitransparent. (This Youtube video shows the film near the negative contact becoming opaque when a voltage is applied.)
The changes in oxygenation—and thus transparency—are reversible. When they were driven slowly, over seconds, the films remained intact, but when the changes occurred quickly, the films became damaged. If the researchers can improve the repeatability and controllability of the electrochemical changes, the material could be used for low-energy displays, memory or other optoelectronic devices.
The researchers believe that, when the film is reduced, graphene quantum dots and channels appear, creating a network of more-conductive areas, until these areas form a sort of nanoscale mesh. The mesh size depends on the degree of reduction. The amount of oxygen in the material can be monitored by measuring the electrical resistance in the film. This suggests that researchers should take into account the nanoscale chemical changes, and not merely electronic changes, when making optoelectronic devices with modified graphene.
The material could also be used for ultrafast photonic devices. In another recent paper by the group and researchers at Ankara University, they demonstrated that they could electrically and optically tune the optical response of the films (Ulas Kurum, et al., Appl. Phys. Lett., doi:10.1063/1.3573797). They found that the optical properties can be manipulated by the electrical bias, as well as chemical changes made by femtosecond pulses of light. Graphene exhibits wideband nonlinear saturable absorption, while a different mechanism occurs in electrically biased graphene oxide. They can change the type of absorption in the ultrafast optical response using either electrically driven chemical changes or by modifying the wavelength.
Yvonne Carts-Powell is a freelance science writer who specializes in optics and photonics.