Ballistic phonon transport analysis of twisted graphene nanoribbons

Abstract

Although an individual carbon nanotube (CNT) has superior thermal property, it is known that the property is deteriorated when forming CNT-based bulk materials such as film and fiber. To resolve the issue, encapsulation of materials into an inner space of CNT has been considered. Encapsulated material usually plays the role of scatterer for phonons and then suppresses heat conduction. However, the influence of encapsulating material on heat conduction is not fully understood. Thus, exploration of encapsulated materials for improving thermal conductivity is demanded. Graphene nanoribbon (GNR) has inherently high thermal conductivity and it can be used to enhance heat conduction of CNT-based bulk material. However, a previous experiment observed a twist of GNR encapsulated in CNT and this twist may negatively influence heat conduction. By using a combination of atomistic Green's function and mode-matching methods, we here investigate the impacts of a single twist and series of twists on phonon transport inside a GNR. As for the single twist, we found that the thermal conductance linearly decreases with increasing specific twist angle (twist angle per unit length). Furthermore, we found that phonons with long-wavelength longer than a length of twisted GNR are largely impeded and geometrical scattering is dominant. On the other hand for series of twists, although mutual interferences of phonons scattered by twists were expected to reduce overall heat conduction, it is found that a single twist mostly determines heat conduction, and thus non-local effect on phonon transport is negligible.