How to Characterize Thermal Transport Capability of 2D Materials Fairly? - Sheet Thermal Conductance and the Choice of Thickness

Ever since the discovery of the record-high thermal conductivity of single layer graphene, thermal transport capability of monolayer 2D materials has been under constant spotlight. Since thermal conductivity is an intensive property for 3D materials and the thickness of 2D materials is not well defined, different definitions of thickness in literature have led to ambiguity towards predicting thermal conductivity values and thus in understanding the heat transfer capability of different monolayer 2D materials. We argue that if conventional definition of thermal conductivity should be used as the quantity to compare the heat transfer capability of various monolayer 2D materials, then the same thickness should be used. Alternatively, to circumvent the problem of ambiguous thickness completely, we also suggest that a "sheet thermal conductance" to be defined as an intensive 2D material property when characterizing the heat transfer capability of 2D materials. When converting literature thermal conductivity values of monolayer materials to this new property, some new features that were not displayed when using different thicknesses show up.