Left: Members of the Osaka research team (Credit: NanoJapan) Right: Amplitude map of THz radiation on graphene, showing adsorbed oxygen (1) and bare InP substrate (2) (Credit: Rice and Osaka universities)
A novel method of identifying contaminants on the surface of graphene has been developed, using terahertz spectroscopy and an indium phosphide substrate.
The research, published in the Nature journal Scientific Reports, was conducted by Masayoshi Tonouchi of Osaka’s Institute of Laser Engineering and Junichiro Kono of Rice University.
Even one molecule of a foreign substance on graphene can affect its optical and electrical properties - a fact which has already been used to characterize the purity of graphene. The major advantage of Kono and Tonouchi's new method is that it is completely contact-free, unlike regular electrical measurements.
The change in the terahertz signal due to adsorption of molecules is remarkable - not just the intensity but also the waveform of emitted terahertz radiation totally and dynamically changes in response to molecular adsorption and desorption.
Junichiro Kono
Single-molecule sensitivity
Indium phosphide emits terahertz waves when it is excited by near-infrared radiation. The researchers found that graphene's absorption of this terahertz radiation changes drastically with even the smallest amount of contamination.
This is due to charge transfer between the graphene and the molecules on it's surface.
By transferring CVD graphene onto an indium phosphide substrate and monitoring the terahertz radiation given off under excitation by a femtosecond-pulsed infrared laser, Tonouchi and Kono found that it was possible to detect even the tiniest amount of contamination on the graphene - right down to a single oxygen molecule.
Purity is paramount in graphene devices
The researchers highlighted the fact that this new technique could be very interesting to electronics manufacturers, who are looking to use graphene in their devices in the future.
Kono commented:
"We have to take into account the influence of the surroundings - graphene in a vacuum or sandwiched between noncontaminating layers would probably be stable, but exposure to air would contaminate it."
It's not all bad news though - this type of method could be extremely useful in the manufacturing process, to ensure the purity of graphene circuits without physically disturbing them.
The researchers at both Rice and Osaka are continuing this research - their next task is to investigate the terahertz conductivity of graphene sheets on a range of different substrates.