Phaedon Avouris and colleagues of the TJ Watson Research Center in Yorktown Heights performed optical measurements on long graphene strips measuring 55 × 2.5 µm. The researchers were able to resolve features measuring about 3 µm across using a 20× near-infrared objective lens with a long working distance. The infrared radiation emitted from the graphene was detected using a liquid nitrogen cooled HgCdTe detector array. Cooled, short-pass filters in the range of 1.8 to 2.5 µm were also use to reduce the number of "dark counts" in the detector.

Next, the team used a transmission grating fabricated on top of a prism to disperse the infrared radiation perpendicular to the direction of the graphene strip, and then measured emissions in the energy range of 0.5 to1 eV.

By analysing these measurements, Avouris and colleagues obtained the temperature distribution, carrier (electron and hole) densities and the position of the Dirac point in the graphene channel. The Dirac (or charge neutrality) point is the point in graphene's bandstructure where the valence and conduction bands touch, and is the point at which the sample becomes hottest. The Fermi level of undoped (or intrinsic) graphene coincides with the Dirac point, and position of this point is crucial for defining graphene's properties, explains Avouris.

The team also found that the graphene transistor gate electric field determines the position of the bandstructure in graphene relative to the Fermi level of the metal contacts used to make the device, and therefore the nature and number of charge carriers in graphene. "The current flowing through the graphene, the amount of energy dissipated and the spatial distribution of the dissipated energy are all determined by the gate field," Avouris told nanotechweb.org.

The new work shows that infrared emission measurements are a powerful, non-invasive way to analyse graphene devices. "Thermal management in these devices is essential for technological applications," said Avouris, "and hopefully IR emission will be very valuable in achieving this goal."

The results were published in Nature Nanotechnology.