An interdisciplinary global research team have shown the ability to control heat flow in ultrathin films, by building them one atomic layer at a time. This layer-by-layer assembly technique is commonly used in the two-dimensional materials community to build new van der Waals materials and devices for applications in energy efficient transistors, energy conversion and energy storage.
The research team, comprised of members from Boise State University, the University of Illinois at Urbana-Champaign, Sungkyunkwan University, the Korea Advanced Institute of Science and Technology and Stanford University, employed a method known as suspended bridge thermometry to measure heat flow along graphene-based van der Waals solids. These artificial materials are not available in nature, so the researchers, led by Assistant Professor David Estrada of Boise State and Associate Professor Eric Pop of Stanford University, created them through a layer-by-layer assembly of graphene films. Graphene is a single layer of carbon atom arranged in a hexagonal crystal, and the subject of the 2010 Nobel prize in physics.
To measure their heat flow, the researchers transferred the graphene films onto a thin insulating membrane that is about one hundred times thinner than the diameter of a human hair. The team then used standard semiconductor manufacturing techniques to fabricate their suspended thermal test platform, including patterning metal heaters and thermometers. Their study has been published in the Nature Partner Journal – npj 2D Materials and Applications.
“Such thermal measurement platforms are difficult to make, but are essential to understand heat flow in nanomaterials,” said Estrada. “The nanofabrication challenges pay off, because this measurement approach is more sensitive than other techniques, in terms of evaluating the thermal conductivity of nanostructures.”
The team employed a so-called electrical thermometry approach by using a metal’s intrinsic temperature coefficient of resistance to sense the heat flow across the sample. By applying a large heating power in one metal line (the “heater”), a second nearby metal line (the “sensor”) changes in resistance when heat flowing through the sample reaches it. Measuring this resistance change with one-billionth of a volt resolution allows for temperature sensing with one-thousandth of a degree sensitivity.
“These careful measurements have found that these films have excellent thermal properties, similar to those of bulk copper,” Pop said. “However, copper films are quite resistive in sub-10 nanometer thin films due to their surface roughness, whereas these graphene layers retain their excellent thermal properties.”
The team also measured heat flow across the layered graphene films, using an optical method known as time-domain thermoreflectance in collaboration with Professor David Cahill at the University of Illinois. These measurements revealed that heat flow across the graphene films is significantly impeded by the weak van der Waals coupling between them.
Ultimately, the researchers succeeded in creating an artificial material with large thermal anisotropy (having different heat flow properties along different directions) between the in-plane and cross-plane direction. Such materials could have applications as nanoscale heat spreaders, helping to sink heat from high-power circuitry, while simultaneously blocking heat from reaching other layers of the circuit, which could be sensitive to heating. Such challenges are pervasive in the semiconductor industry as keeping pace with Moore’s Law has increased the power density of computer processors to a point where the current materials have difficulty withstanding the heat.