Thermal rectification and phonon properties in partially perforated graphene

Abstract

In this work, a thermal rectification ratio $\eta$ of 18.5% was observed in partially perforated graphene with the use of Molecular Dynamics (MD) simulations. In all cases studied here, heat preferentially flows from the porous to the pristine region and both $\kappa$ and $\eta$ increase upon increasing the length of the pristine region and upon decreasing the size of the pores. To interpret the results, the macroscopic "R-Series Model" is applied, attributing rectification to the different temperature dependence of $\kappa$ of perforated and pristine graphene. According to the model, $\eta$ is maximized when the two regions composing the structure have matching thermal resistances and mismatching temperature-dependence of $\kappa$. The model agrees qualitatively with the MD results, indicating that the latter is the principal rectification mechanism, but it can significantly underestimate $\eta$. Phonon analysis further reveals the appearance of new 'defect' modes localized around and between pores, resulting in the emergence of a new prominent peak in the phonon Density of States at 520 $cm^{-1}$. The study considers key geometric factors such as the length of the pristine region, and the pore size, shape, alignment, and orientation. Pore shape and alignment exert minimal influence on $\eta$, although alignment greatly influences $\kappa$. Eventually, arranged pores are deemed more efficient than randomly distributed defects for increasing rectification.