Berkeley Lab researchers have experimentally confirmed strong in-plane anisotropy in thermal conductivity along the zigzag (ZZ) and armchair (AC) directions of single-crystal black phosphorous nanoribbons. Credit: Junqiao Wu, Berkeley Lab
Researchers have confirmed single-crystal black phosphorous nanoribbons display a strong in-plane anisotropy in thermal conductivity, up to a factor of 2, along the zigzag and armchair directions of single-crystal black phosphorus nanoribbons. An experimental revelation that should facilitate the future application of this highly promising material to electronic, optoelectronic and thermoelectric devices.
“Imagine the lattice of black phosphorus as a 2D network of balls connected with springs, in which the network is softer along one direction of the plane than another,” says Junqiao Wu. “Our study shows that in a similar manner heat flow in the black phosphorus nanoribbons can be very different along different directions in the plane. This thermal conductivity anisotropy has been predicted recently for 2D black phosphorus crystals by theorists but never before observed.”
Black phosphorous, named for its distinctive color, is a natural semiconductor with an energy bandgap that allows its electrical conductance to be switched “on and off.”
Black phosphorus, named for its distinctive color, is a natural semiconductor with an energy bandgap that allows its electrical conductance to be switched “on and off.” It has been theorized that in contrast to graphene, black phosphorus has opposite anisotropy in thermal and electrical conductivities – i.e., heat flows more easily along a direction in which electricity flows with more difficulty. Such anisotropy would be a boost for designing energy-efficient transistors and thermoelectric devices, but experimental confirmation proved challenging because of sample preparation and measurement requirements.
“We fabricated black phosphorus nanoribbons in a top-down approach using lithography, then utilized suspended micro-pad devices to thermally isolate the nanoribbons from the environment so that tiny temperature gradient and thermal conduction along a single nanoribbon could be accurately determined,” Wu says. “We also went the extra mile to engineer the interface between the nanoribbon and the contact electrodes to ensure negligible thermal and electrical contact resistances, which is essential for this type of experiment.”
The results at Molecular Foundry revealed high directional anisotropy in thermal conductivity at >100 K. This anisotropy was attributed mainly to phonon dispersion with some contribution from phonon-phonon scattering rate, both of which are orientation-dependent. At 300 K, thermal conductivity decreased as the thickness of the nanoribbon thickness shrank from ~300 to 50 nm. The anisotropy ratio remained at a factor of 2 within this thickness range.
“The anisotropy we discovered in the thermal conductivity of black phosphorus nanoribbons indicates that when these layered materials are patterned into different shapes for microelectronic and optoelectronic devices, the lattice orientation of the patterns should be considered,” Wu says. “This anisotropy can be especially advantageous if heat generation and dissipation play a role in the device operation. Eg these orientation-dependent thermal conductivities give us opportunities to design microelectronic devices with different lattice orientations for cooling and operating microchips. We could use efficient thermal management to reduce chip temperature and enhance chip performance.”
Wu’s team plan to investigate how thermal conductivity in black phosphorus nanoribbons is affected under different scenarios, such as hetero-interfaces, phase-transitions and domain boundaries and effects of stress and pressure. http://newscenter.lbl.gov/2015/10/16/is-black-phosphorous-the-next-big-thing-in-materials/
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