  Despite the importance of high thermal conductance (i.e. low thermal
resistance) of metal contacts to thermal management of graphene devices, prior
reported thermal conductance of metal/graphene interfaces are all relatively
low, only 20-40 MW m$^{-2}$ K$^{-1}$. One possible route to improve the thermal
conductance ($G$) of metal/graphene interfaces is through additional heat
conduction by electrons, since graphene can be easily doped by metals. In this
paper, we evaluate the electronic heat conduction across metal/graphene
interfaces by measuring the thermal conductance of Pd/transferred graphene
(trG)/Pd interfaces, prepared by either thermal evaporation or radio-frequency
(rf) magnetron sputtering, over a wide temperature range of 80 to 500 K. We
find that for the samples prepared by thermal evaporation, the thermal
conductance of Pd/trG/Pd is 42 MW m$^{-2}$ K$^{-1}$. The thermal conductance
only weakly depends on temperature, which suggests that heat is predominantly
carried by phonons across the intrinsic Pd/graphene interface. However, for
Pd/trG/Pd samples with the top Pd films deposited by rf magnetron sputtering,
we observe a significant increment of thermal conductance from the intrinsic
value of 42 MW m$^{-2}$ K$^{-1}$ to 300 MW m$^{-2}$ K$^{-1}$, and $G$ is
roughly proportional to $T$. We attribute the enhancement of thermal
conductance to an additional channel of heat transport by electrons via
atomic-scale pinholes formed in the graphene during the sputtering process. We
thus conclude that electrons play a negligible role in heat conduction across
intrinsic interfaces of metal and pristine graphene, and the contribution of
electrons is only substantial if graphene is damaged.
