Graphene – a 2D sheet of carbon just one atom thick – could be ideal for use in future electronics applications thanks to the fact that electrons whiz through the material at extremely high speeds. The material also conducts heat extremely well and can be transferred onto virtually any substrate. The most promising applications for graphene will be those that exploit its superior current-carrying capacity rather than those that require an energy bandgap because the material does not have one in its pure state.

Graphene field-effect transistors and interconnects built on conventional SiO2/Si substrates already exist and these typically have a breakdown current density of around 1 µA/nm2. Now, Alexander Balandin and Anirudha Sumant working together with electrical engineering graduate students in Balandin's lab at UCR have shown that the current-carrying capacity of graphene can be increased to as high as around 20 µA/nm2 by replacing the SiO2 with synthetic diamond or inexpensive ultrananocrystalline diamond. Diamond conducts heat better than silicon or silicon dioxide and can thus remove more heat away from graphene, which in turn means that it can sustain much higher current densities, explains Balandin.

The UCR-ANL team looked at two types of diamond – ultrananocrystalline (UNCD), which is a special, now commercially available, form of diamond originally developed at Argonne, and single-crystal diamond (SCD). Unlike conventional nanocrystalline diamond films produced using a hydrogen and methane gas mixture that require substrate temperatures of as high as 800 °C for growth, UNCD can be deposited at much lower temperatures of 400 °C. The researchers have also already shown that UNCD is compatible with conventional silicon complementary metal-oxide semiconductor (CMOS) technology used in electronics today.

Advanced polishing

The only snag is that the diamond needs to be smooth on the nanometre scale, but the team has developed an advanced polishing process that greatly reduces the material's roughness. “The resulting diamond surfaces had very small roughness, which allowed for fabrication of the high-mobility graphene transistors,” explains Sumant who grew UNCD at Argonne.

“We have shown that a combination of two carbon materials – graphene and diamond – can lead to the creation of a new technology,” Balandin told “The hybrid material might be used to make interconnects that would replace those made from copper on SiO2 and transistors for high-frequency, high-power density applications to name but two.”

The work performed thus far focused on graphene devices that were tens of microns in size. The researchers would now like to look at the current-carrying properties of nanometre-scale graphene-on-diamond devices.

The work was reported in Nano Letters.