Oct 11, 2017

A team of Researchers from Japan and Taiwan have created a new CVD approach to grow graphene at temperatures as low as 50 °C using a dilute methane vapor source and a molten gallium catalyst. Reducing the temperature in graphene CVD synthesis methods can be extremely beneficial integration of graphene in various applications, like the direct integration of CVD-grown graphene into electronic devices.

The team explains that in silicon-based electronics, the upper temperature threshold that the components can withstand upon graphene integration is around 400 °C. The threshold is even lower for plastic semiconducting devices, which can only withstand up to 100 °C during the graphene growing process. Under traditional conditions, graphene growth occurs at around 1000 °C and has not been suitable for the direct integration into such electronic devices.

This new method could change that situation though, as the team managed to grow CVD-graphene onto sapphire and polycarbonate substrates with the help of a molten gallium catalyst and dilute methane atmosphere. The gallium catalyst was chosen as it was a proven catalyst in recent graphene growth methods and can be easily removed by a gas jet after the graphene has been synthesized. The methane was diluted to 5% by mixing the atmospheric gas with a nitrogen and argon mixture.

The Researchers inspected the quality of the grown graphene using Raman spectroscopy (RAMANplus, Nanophoton Corporation), scanning electron microscopy (SEM, S-4800, Hitachi High-Technologies Corporation) and high-resolution transmission electron microscopy (HR-TEM, JEM-ARM200F, JEOL Ltd).

The new CVD process was able to create high quality graphene at near room temperature (relatively speaking), with graphene being grown onto sapphire substrates at 50 °C and at 100 °C on polycarbonate substrates.

The low-temperature synthesis was made possible through carbon attachment to island edges of pre-grown graphene nuclei islands and resulted in no damage to the substrate or surrounding components. The pre-existing nuclei island themselves were produced through conventional CVD processes or by a special nuclei transfer technique using a mixture 12C and 13C at low temperatures.

The presence of the molten gallium catalyst enhanced the methane absorption at lower temperatures and ultimately led to a low reaction barrier of 0.16 eV below 300 °C and 0.58 eV above 500 °C. This was confirmed through Arrhenius plots. The molten state of gallium was also found to be fluidic enough to facilitate an enhanced transport and growth of carbon atoms.

The fast growth kinetics associated with the low reaction barrier and low-temperature nuclei transfer process were found to facilitate the growth of graphene down to as low as 50 °C and is a result of competing pathways, i.e. methane decomposition at the gallium surface; and methane adsorption in bulk liquid gallium, followed by the subsequent deposition in gallium.

The two pathways were found to be favored at high and low temperatures, respectively and explains the reasons why a weak temperature dependence and low reaction barrier are present during the process. The methane absorption pathway is also thought to be unique for molten gallium as the process was found to be ineffective when other metals were used, including common graphene catalysts such as copper and nickel.

Although in this instance the growth was confined to the gallium droplet-substrate interface, the fluidic nature of the gallium catalyst is thought to be applicable to conformal graphene on 3D objects.

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