  Graphene exhibits promise as a plasmonic material with high mode confinement
that could enable efficient hot carrier extraction. We investigate the
lifetimes and mean free paths of energetic carriers in free-standing graphene,
graphite and a heterostructure consisting of alternating graphene and hexagonal
boron nitride layers using ab initio calculations of electron-electron and
electron-phonon scattering in these materials. We find that the extremely high
lifetimes (3 ps) of low-energy carriers near the Dirac point in graphene, which
are a hundred times larger than that in noble metals, are reduced by an order
of magnitude due to inter-layer coupling in graphite, but enhanced in the
heterostructure due to phonon mode clamping. However, these lifetimes drop
precipitously with increasing carrier energy, and are smaller than those in
noble metals at energies exceeding 0.5 eV. By analysing the contribution of
different scattering mechanisms and inter-layer interactions, we identify
desirable spacer layer characteristics - high dielectric constant and heavy
atoms - that could pave the way for plasmonic heterostructures with improved
hot carrier transport.
