However, since the TF nanogenerators are designed using indium tin oxide (ITO) electrodes to create a transparent device, the ITO-based nanogenerators have limited flexibility due to the ceramic structure of the ITO, and defects can easily be introduced if the device is overflexed."

Professor Sang-Woo (right) Kim's new work in the School of Advanced Materials Science & Engineering at Sungkyunkwan University.together with Jae-Young Choi, a senior researcher at Samsung Advanced Institute of Technology (SAIT), and collaborators from Sungkyunkwan University, SAIT and Kumoh National Institute of Technology, have now demonstrated the first use of CVD -grown large-scale graphene sheets as the transparent electrodes for fully transparent and flexible nanogenerators.

The team published its findings in Advanced Materials ("Fully Rollable Transparent Nanogenerators Based on Graphene Electrodes"). In their technique, they prepare large graphene sheets using CVD on a nickel-coated silicon dioxide wafer (a technique based on the previous work.)

The graphene sheets are used as the platform for the growth of ZnO nanorods, synthesised at 95°C using the aqueous solution method, resulting in vertically well-aligned ZnO nanorods on graphene. The graphene-based nanogenerator can then be completed by integrating the 3D heterogeneous nanostructure with another graphene sheet as a top electrode.

The Korean team points out that graphene-based RT nanogenerators are suitable for futuristic device applications such as flexible self-powered touch sensors, wearable artificial skins, fully rollable display mobile devices, and battery supplements for wearable cellular phones.

Firstly, the transparent property of the power generator can make a big impact on the size and shape of various electronic gadgets since transparent elements can be embedded anywhere in the device.

So a transparent power generator could be integrated into the display of a cell phone, enabling a significant reduction in size of the cell phone. The main impact of this work, though, is for the area of flexible electronics. "Even though most elements in flexible electronics can provide the flexibility, if the power generator is not flexible then a fully flexible electronic device cannot be achieved" explains Kim.

"We had previously developed ITO-based nanogenerators but the mechanical durability of the device was a major weak point since ITO is brittle. With graphene-based nanogenerators we are able to overcome the durability issues of ITO-based devices."

The research team is still at work.
"In a nanogenerator, Schottky contact formation between the top electrode and the ZnO nanorods is very important to enhance the output performance of the device," says Kim. "Our current graphene electrode is only providing a weak Schottky contact. However, we can improve the output power level of our graphene-based nanogenerators via controlling the work function of the graphene electrode by a doping process."

Another problem can arise from the top electrode. Depending on the top electrode morphology, the output performance can vary significantly since the contact probability of ZnO nanorods to the top electrode is not uniform. 

Kim says, "We assume that only about 1/100 or 1/1000 of ZnO nanorods are involved in the electrical energy generation since the length of ZnO nanorods is not uniform at this present stage and many of them don't make contact with the electrode. We are working on this."