It is relatively easy to p-dope graphene using adsorbates and oxygen groups on edges but for real-world applications, scientists need to be able to make n-doped material too. This is more difficult because special strategies are needed.

Now, Hongjie Dai of Stanford University in the US and colleagues have shown that graphene can be n-doped through high-power electrical joule heating in ammonia gas. The high power causes the material to heat up to hundreds of degrees and the graphene edges/defect sites (which are more reactive) start to react with the ammonia gas to form carbon–nitrogen groups. The researchers confirmed the formation of carbon–nitrogen species in the thermally annealed graphene using X-ray photoelectron spectroscopy and nanoscale secondary ion mass spectroscopy.

No loss of mobility
The process can dope graphene without degrading electrical properties such as mobility, something that occurred with previous n-doping methods that involved depositing electron-rich molecules/species on the graphene plane, explains Dai. Such techniques introduced scattering centres that lowered the carrier mobility. "Our doping method modifies mostly the edges of graphene without introducing large numbers of impurities in the plane," he told

The team, which includes researchers from the University of Florida and the Lawrence Livermore National Lab, then made an n-type graphene FET that operates at room temperature. The researchers have already demonstrated p-type transistors using as-made graphene nanoribbons and palladium in previous experiments. "With complementary n- and p-type transistors we can now envision the possibility of making more complicated logic functions such as AND and OR gates," said Dai.

Spurred on by its result, Dai says that his group will now study detailed doping chemistry in graphene and optimize transistor performance.

The work was reported in Science.