Black phosphorous. Photo: Berkeley Lab.
Black phosphorous. Photo: Berkeley Lab.

A new experimental discovery about black phosphorous nanoribbons should facilitate the future use of this highly promising material in electronic, optoelectronic and thermoelectric devices. A team of researchers at the US Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) has experimentally confirmed that single-crystal black phosphorous nanoribbons have a strong in-plane anisotropy in thermal conductivity, meaning the conductivity differs in different directions.

"Imagine the lattice of black phosphorous as a two-dimensional (2D) network of balls connected with springs, in which the network is softer along one direction of the plane than another," explains Junqiao Wu, a physicist who holds joint appointments with Berkeley Lab's Materials Sciences Division and the University of California Berkeley's Department of Materials Science and Engineering. "Our study shows that in a similar manner heat flow in the black phosphorous nanoribbons can be very different along different directions in the plane. This thermal conductivity anisotropy has been predicted recently for 2D black phosphorous crystals by theorists but never before observed."

Wu is the corresponding author of a paper describing this research in Nature Communications. The lead authors are Sangwook Lee and Fan Yang.

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’. It has been theorized that, in contrast to graphene, black phosphorous has opposite anisotropy in thermal and electrical conductivities, i.e. heat flows more easily along a direction in which electricity flows with more difficultly. Such anisotropy would be a boost for designing energy-efficient transistors and thermoelectric devices, but experimental confirmation has proved challenging because of difficulties with sample preparation and measurement.

"We fabricated black phosphorous 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 of the study, which was carried out at the Molecular Foundry, a DOE Office Science User Facility hosted by Berkeley Lab, revealed high directional anisotropy in thermal conductivity at temperatures greater than 100K. This anisotropy was attributed mainly to the dispersion of phonons, which are quasi-particles that represent the collective excitation of atoms or molecules in solid matter, with some contribution from the phonon-phonon scattering rate, both of which are orientation-dependent. Detailed analysis revealed that at 300K, thermal conductivity decreased as the thickness of the nanoribbon shrank from approximately 300nm to approximately 50nm. The anisotropy ratio remained at a factor of two within this thickness range.

"The anisotropy we discovered in the thermal conductivity of black phosphorous 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. For example, 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 and his colleagues now plan to use their experimental platform to investigate how thermal conductivity in black phosphorous nanoribbons is affected under different scenarios, such as hetero-interfaces, phase transitions and domain boundaries. They also want to explore the effects of various physical conditions, including stress and pressure.

This story is adapted from material from Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.