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John O | November 2018

Small tweaks to edges of graphene nanoribbons have significant impact on thermal conductivity


By Josh Perry, Editor
[email protected]

 

Through simulations, researchers at the Agency for Science, Technology, and Research (A*STAR) in Singapore discovered that changing the orientation of the edges of black phosphorene nanoribbons can have a significant impact on their thermal conductivity.

 


Tube-like atomic structures on the edges of phosphorous-based nanoribbons help keep this 2D material conductive during times of thermal or tensile stress.
(A*STAR Institute of High Performance Computing)

 

According to a report from A*STAR, black phosphorene differs from graphene, another two-dimensional compound, because the bonding pattern of the phosphorous atoms forms a distinct wrinkling shape.

 

“Investigations suggest that the zig-zag structure of this 2D film enables it to behave differently in different orientations: it can transport electrons slowly along one axis, for example, but rapidly in the perpendicular direction,” the report explained.

 

The researchers believe that by changing the orientation of the nanoribbons they could draw excess heat from nanoscale circuits. To affect the nanoribbons’ thermal conductivity, researchers focused on the atomic structures on the edges.

 

“To understand how different edge structures impact thermal conductivity, the A*STAR team used computer algorithms that simulate phonon transfer across a temperature gradient,” the report continued. “They modeled phosphorene films as narrow, rectangular nanoribbons and observed that heat conductivity was mostly uniform in pristine nanoribbons. The dimer and tube-terminated models, on the other hand, preferred to direct heat to central regions away from the edges.”

 

Simulations also showed that the tube-edged models produced a twisting movement from phonons, as well as breathing modes. Researchers believe this is what allows the material to scatter thermal vibrations and stay cool.

 

“Normally, 2D materials have reduced ability to diffuse heat when strained laterally,” the article added. “Tube-terminated nanoribbons, however, have nearly constant thermal conductivity under strain — a property that may be useful in future wearable technology.”

 

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

 

“By studying tube?terminated phosphorene nanoribbons (PNRs), it is found that unusual phonon and thermal properties can emerge from topologically new edges. The lattice dynamics calculations show that in tube?terminated PNRs, the breaking of rotation symmetry suppresses the degeneracy of phonon modes, causing the emergence of twisting mode.

 

“An anomalous change of an out?of?plane acoustic mode to breathing modes with nonzero energy at the center of Brillouin zone occurs when the phosphorene sheet is converted into a tube?terminated PNR. These unusual twisting and breathing modes provide a larger phase space for scattering phonons, thus explaining the low thermal conductivity of tube?terminated PNRs revealed by molecular dynamics calculations.

 

“Due to the change in the stress field distribution caused by the tube edge, a nearly strain?independent thermal conductivity in tube?terminated PNRs is observed, which is in contrast to the apparent enhancement of thermal conductivity in pristine and dimer?terminated PNRs under tensile strain. The work reveals intriguing phononic and thermal behaviors of tube?terminated 2D materials.”

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