By Josh Perry, Editor
[email protected]

Scientists from the Agency for Science, Technology and Research (A*STAR) Institute of Materials Research and Engineering in Singapore discovered why heat moves twice as fast in the wrinkle of a black phosphorous nanoribbon as it does across the wrinkle.

Researchers discovered how heat moved through black phosphorous nanoribbons. (Wikimedia Commons)

According to a report from A*STAR, scientists assumed that thermal conductivity being direction-dependent in black phosphorous was due to the dispersion of phonons across the material’s lattice structure, but no one had defined the reason behind this heat transfer mechanism.

“The team started with the premise that the travelling velocity of phonons is equivalent to the speed of sound in a material, which in turn has a well-defined relationship to the material’s stiffness,” the report explained. “They used their expertise in high-precision material measurements to set up an experiment that allowed them to measure both heat transport and stiffness in the same system, using black phosphorus nanoribbons with either a zigzag or armchair orientation.”

Researchers used electron-beam nanolithography to build the black phosphorous on thin films and a custom micro-electro-thermal system and an atomic force microscope enabled them to determine that there was a physical link between heat transport and Young’s modulus.

They discovered that the ratio of thermal conductivity between the wrinkle and across the wrinkle of the nanoribbons was identical to the ratio of Young’s modulus values.

The research was published in Advanced Materials. The abstract stated:

“Black phosphorus (BP) has emerged as a promising candidate for next?generation electronics and optoelectronics among the 2D family materials due to its extraordinary electrical/optical/optoelectronic properties. Interestingly, BP shows strong anisotropic transport behavior because of its puckered honeycomb structure.

“Previous studies have demonstrated the thermal transport anisotropy of BP and theoretically attribute this to the anisotropy in both the phonon dispersion relation and the phonon relaxation time. However, the exact origin of such strong anisotropy lacks clarity and has yet to be proven experimentally.

“Here, the thermal transport anisotropy of BP nanoribbons is probed by an electron beam technique. Direct evidence is provided that the origin of this anisotropy is dominated by the anisotropic phonon group velocity, verified by Young's modulus measurements along different directions. It turns out that the ratio of the thermal conductivity between zigzag (ZZ) and armchair (AC) ribbons is almost same as that of the corresponding Young modulus values.

“The results from first?principles calculation are consistent with this experimental observation, where the anisotropic phonon group velocity between ZZ and AC is shown. These results provide fundamental insight into the anisotropic thermal transport in low?symmetry crystals.