Berkeley Lab scientists use black phosphorus to discover possibilities for conductivity

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A team of researchers from the Lawrence Berkeley National Laboratory has experimentally discovered new possibilities for conductivity in a range of technological devices through the use of black phosphorus, a layered semiconductor.

Using black phosphorus nanoribbons — which have the width of a micron — and field effect transistors that act as electrical switches, researchers from the Berkeley Lab were able to confirm strong in-plane anisotropy in thermal conductivity in the material. This holds potential for future applications within devices such as computer chips, lasers and waste heat harvesters.

For corresponding author and campus associate professor of materials science and engineering Junqiao Wu, who also has an appointment in the lab’s division of materials sciences, this new property of black phosphorus could be important for electronic, optoelectronic and thermoelectric devices in the future. It could allow researchers to design electronic devices to avoid overheating and create higher-efficiency thermoelectric devices.

Unlike graphene, another semiconductor, the in-plane thermal conductivity of black phosphorus is anisotropic, meaning it has properties of being directionally dependent; graphene is isotropic, which implies identical properties in all directions.

The strong in-plane anisotropy is important because it is a new phenomenon in two-dimensional materials, according to co-lead author Fan Yang, a postdoctoral fellow at the lab’s Molecular Foundry.

On the application side, anisotropic materials in thermoelectrics improve their efficiency in energy conversion from heat to electricity. The materials’ presence can also limit overheating, Yang said.

In-plane anisotropy of thermal conductivity means the thermal conductivity in the two in-plane crystal directions are different: One is a zigzag direction, and the other is an armchair direction, according to Wu. The difference measured, he said, was a factor of two.

“You can imagine that in a football game, it is much more difficult for the players to run toward the goal than along the direction perpendicular to it,” Wu said in an email. “In such cases, the resistance the players face is anisotropic in the football field.”

Wu’s team measured the thermal conductivity of black phosphorus nanoribbons using suspended micropad devices in an artificial vacuum. Nanoribbons formed a bridge between the two suspended micropads in order to measure them. All components of the experiment were thermally isolated from the environment so that the tiny temperature gradient along a single nanoribbon could be accurately determined.

According to Wu, the in-plane crystal direction is comparable to a book, which has many pages or layers.

“We made a sample with only a few layers stacked together,” Wu said in an email. “The in-plane direction is any directions in the page of the book.”

Wu said that there were theoretical predictions in literature last year but that no experimental demonstration had been done until his team’s work.

Yang said the group is planning to use its experimental platform and black phosphorus to investigate and understand how thermal conduction is affected at the nanoscale in a variety of scenarios.

Robert Tooke covers research and ideas. Contact him at [email protected] and follow him on Twitter at @robertono_t.

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