University of California, Riverside (UCR) Professor of Electrical Engineering and Chair of Materials Science and Engineering Alexander Balandin is leading several projects to explore ways to use the unique capabilities of graphene “quilts” as heat conductors in high-power electronics.
Most of the current research on graphene has focused on its electronic properties and graphene’s potential for high-speed nano-circuits. Due to its unique structure, electrons travel at extremely high speeds throughout it.
Balandin is focusing on another of graphene’s remarkable properties: it’s extraordinarily high thermal conductivity, which can be used for heat removal in nanoscale and 3-D electronics. The higher speed, higher power densities and increased thermal residence in the state-of-the-art devices result in development of hot spots, performance degradation and thermal breakdown. Balandin’s proposed graphene-based approach for thermal management represents a radical departure from conventional methods and might lead to creation of a new technology for hot-spot spreading.
Because graphene is only one molecule thick, it didn’t lend itself to traditional methods of thermal conductivity measurement. Balandin led a team of researchers that first measured it using an original non-conventional technique in 2008. The procedure involved a non-contact approach on the basis of Raman spectroscopy utilizing the inelastic scattering of photons (light) by phonons (crystal vibrations). The power dissipated in graphene and corresponding temperature rise were detected by extremely small shifts in the wavelength of the light scattered from graphene. That was sufficient to extract the values of the thermal conductivity through an elaborate mathematical procedure.
Balandin’s research group discovered that the thermal conductivity of large suspended graphene sheets varies in the range from about 3000 to 5300 W/mK (watts per meter per degree Kelvin) near room temperature. These are very high values, which exceed those of carbon nanotubes (3,000-3,500 W/mK) and diamond (1,000-2,200 W/mK).
Measuring the thermal conductivity of something with a thickness of just one atomic layer is tricky. At the University of California, Riverside, we approached the problem this way: First, we prepared samples to measure, each consisting of a long graphene flake suspended across a trench in a silicon wafer and attached to heat sinks. We then heated the graphene flake with a laser. The heat wave propagated from the middle of the graphene to the heat sinks.
To measure the temperature at the center of the hot spot, we came up with an unconventional use for a micro Raman spectrometer. Ordinarily, Raman spectroscopy is an optical technique used to identify materials. The Raman spectrum of graphene has a clear peak, referred to as a G peak; the spectral position of the peak depends on the temperature of the sample. So by measuring the exact position of the G peak, we were able to use our spectrometer as a thermometer.
Only a small portion of the laser light ended up dissipating in the graphene. Most of the light passed through it and was reflected back. We determined the fraction of the light dissipated in the sample by comparing the Raman intensity of the graphene with that of bulk graphite. Knowing the temperature rise, the dissipated light power, and the geometry of the graphene flake, we then determined the graphene’s thermal conductivity. The measured values exceeded 3000 watts per meter per kelvin near room temperature and depended on the size of the graphene flake. We learned later that our results agreed with physicist Paul G. Klemens’s predictions years earlier