  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.
