Apr 30, 2010
Quantum Hall effect seen in CVD graphene
Graphene made by bottom-up chemical vapour deposition (CVD) techniques has the same properties as material obtained by the top-down approach, which involves exfoliating – or shaving off – layers of graphene from graphite using sticky tape. So say researchers in the US who have fabricated microdevices from CVD-produced graphene and measured their physical and electronic properties. Among the things observed were field-effect transistors with on/off ratios of around 5 and carrier mobilities of up to 3000 cm2/V s, and the "half-integer" quantum Hall effect. This is a hallmark of monolayer graphene and was first observed in graphene made by the sticky-tape route. The result confirms that graphene could be fabricated in much larger quantities using CVD, something that will bring real-world applications a step closer.
Yong Chen of Purdue University and Qingkai Yu from the University of Houston and colleagues grew graphene by CVD of carbon atoms, obtained by decomposing methane at 1000 °C on a copper surface. The advantage of CVD-grown graphene is that it can be made over areas as large as inches across, as opposed to sticky-tape-produced samples that are typically just a few to tens of microns in size. And importantly, it can easily be transferred onto any substrate, which will greatly facilitate many applications, says Chen.
Quantum Hall effect
The scientists observed the quantum Hall effect in the material using standard low-temperature magneto-transport measurements. Most of the data was obtained at the National High Magnetic Field Lab in Tallahassee in a helium-3 fridge at temperatures of 0.3 K and magnetic fields close to 18 Tesla applied perpendicular to the graphene sample.
"Observing the quantum Hall effect in CVD-synthesized graphene shows that this material possesses the intrinsic electronic properties of graphene," Chen told nanotechweb.org. "The high-quality material could thus be used not only for device applications but also for many fundamental studies."
The quantum Hall effect (which is the relativistic analogue of the conventional integer quantum Hall effect seen for free electrons in semiconducting systems) is important in metrology because it is used to define the resistance standard, he adds. Indeed, Chen is now collaborating with the National Institute of Science and Technology (NIST) in Gaithersburg to investigate whether graphene might be used as a new material for quantum Hall metrology. "Large-size graphene films that can be transferred to arbitrary substrates could be very valuable for exploring a large number of device configurations to find what is optimal for resistance metrology," he said.
New physics? And novel devices
Such large-sized graphene sheets have already proved useful as transparent conductive thin films that are only one atom thick, for use as electrodes in solar cells, for example, and as flexible, bendable and stretchable platforms for electronic devices or electronic paper. "Compared with sheets fabricated or grown on either SiO2 or SiC substrates, CVD graphene can easily be transferred onto any type of substrate, which means that it could easily be interfaced with other interesting electronic materials or substrates with unique functions", explained Chen. "This might even to lead to new physics, and definitely novel devices."
The researchers would now like to further improve the quality of their CVD-grown graphene, because, although the material is single layer over a large area, it may not be single crystalline. Understanding how differently oriented graphene crystals domains form and how to control these domains will be important here. These domain boundaries may also affect the electronic properties of graphene.
"At the moment, we are busy making devices from our large graphene samples and are doing experiments that are difficult to perform on exfoliated graphene," revealed Chen. "We are also interested in integrating transparent graphene sheets in solar cells and other energy-harvesting devices."
The work was published in Appl. Phys. Lett. .
About the author
Belle Dumé is contributing editor at nanotechweb.org