Despite advances in the miniturization of micro-electronic chips, an important technical challenge is producing good electrical contacts between the active devices—such as a carbon nanotube, ultra-thin film or molecule —and power sources. The problem is particularly tricky in devices based on organic films, such as field-effect transistors, where it often requires some fraction of a volt or higher to move charge carriers from the metal contact into the molecular layer. Even with enough energy to overcome the metal-organic potential barrier, the efficiency of charge carriers passing through the contact can be low, which results in poor device performance.

Scientists from the Institute of Chemistry at the Chinese Academy of Sciences in Beijing1 are taking a new approach to making electrode contacts in organic field-effect transistors. The team, including Gui Yu and Yunqi Liu, grew graphene based electrodes with arbitrary shapes and showed that they can form high quality contacts to pentacene—an organic material widely used for fabricating organic field-effect transistors.

Fig. 1: Graphene can be patterned into a variety of shapes for making electronic devices. In the device shown here, the graphene is grown on pre-patterned copper or silver (yellow) on top of SiO2(dark blue) on silicon (light blue).

To produce graphene electrodes with the desired shapes the group started with patterned silver or copper on top of an insulating SiO2 substrate and exposed the pattern to ethanol (a source of carbon) and H2/Ar at high temperature. The silver and copper act as catalysts for carbon growth, thus the graphene structures only grew on the patterned metal regions (Fig. 1).

The team then deposited pentacene on top of the graphene contacts. To assess the quality of their contacts, they measured the mobility of the charge carriers through the device and found it to be relatively high for the particular geometry used.

The group believes that these contacts were good because graphene has a structure that is similar to that of the pentacene molecules used in their devices. In addition to the desired physical characteristics, the used method for synthesizing the patterned graphene contacts was inexpensive. “The biggest contribution of this work is the demonstration of a novel approach to fabricate patterned graphene,” says Liu. “The simple technique together with a low-cost process suggests that patterned graphene could serve as excellent electrodes for organic field-effect transistors.”