Researchers at Rice and Osaka universities have developed a method for detecting impurities in graphene using terahertz waves. The joint research team believes that the method could provide a relatively simple way for anyone trying to use graphene for electronic applications to identify problems with the material early in the manufacturing process.
The new technique can identify a single foreign molecule on graphene. This is critically important in electronic applications where a single foreign molecule in graphene can corrupt its electrical properties. Before this work, in order to test those electrical properties you needed to attach contacts to the material, which itself could damage the graphene.
"Traditionally, in order to measure conductivity in a material, one has to attach contacts and then do electrical measurements," said Junichiro Kono of Rice University, in a news release. "But our method is contactless."
The sensing method, which is described in Nature’s Scientific Reports, involves the use of indium phosphide, which emits a terahertz signal when it becomes excited. To exploit that, the researchers layered graphene onto a substrate of indium phosphide (InP) using chemical vapor deposition (CVD).
They hit the combined material with femtosecond laser pulses that excited the indium phosphide and triggered it to emit terahertz signals back through the graphene. A spectrometer is used to detect the terahertz signal and even one stray molecule can be detected and located.
"The change in the terahertz signal due to adsorption of molecules is remarkable," Kono said. "Not just the intensity but also the waveform of emitted terahertz radiation totally and dynamically changes in response to molecular adsorption and desorption. The next step is to explore the ultimate sensitivity of this unique technique for gas sensing."
A byproduct of striking the graphene with the femtosecond laser is that the oxygen molecules are removed and the sensing technique can detect those changes as they occur.
What’s not clear from the research is what happens if you want to remove the graphene from the InP. Graphene producers have told their tales of woe in trying to remove graphene that was grown on a copper substrate, and a fair amount of research has been devoted to overcoming that problem. Presumably, with this technique you will need to make your device with the combination of InP and graphene.
While the researchers have developed a novel sensing technique that could benefit anyone trying to make electronic devices using graphene, they also acknowledge that it serves as a caution as much as a solution.
"For any future device designs using graphene, we have to take into account the influence of the surroundings," said Kono. He added that graphene in a vacuum or sandwiched between noncontaminating layers would probably be stable, but exposure to air would contaminate it.
Maybe this is one of the reasons why IBM seems to have limited their interest in graphene for electronic applications to just RF electronic applications.