CommuniCation(1 of 5) 1600914 2D materials exhibit a diverse array of optical and electronic properties, ranging from insulating hexagonal boron nitride and semiconducting transition metal dichalcogenides to semimetallic graphene. [1][2][3][4][5] Stacked 2D materials, or van der Waals (vdW) heterostructures, [6][7][8] have generated considerable recent interest as designer plasmonic, photonic, and optoelectronic materials. Combining 2D layers in different arrangements makes it possible to realize a variety of new optical phenomena and nanophotonic devices, covering spectral ranges from the microwave to the ultraviolet. [9][10][11] Simultaneously, the field of utilizing the energetic 'hot' carriers generated by surface plasmon decay for photodetection Graphene exhibits promise as a plasmonic material with high mode confinement that could enable efficient hot carrier extraction. The lifetimes and mean free paths of energetic carriers have been investigated 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. It is found that the extremely high lifetimes (3 ps) of low-energy carriers near the Dirac point in graphene, which are a 100 times larger than that in noble metals, are reduced by an order of magnitude due to interlayer 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 analyzing the contribution of different scattering mechanisms and interlayer interactions, desirable spacer layer characteristics-high dielectric constant and heavy atoms-that could pave the way for plasmonic heterostructures with improved hot carrier transport have been identified. and solar energy conversion has grown rapidly. [12][13][14] Hot carrier extraction has also been demonstrated in graphene, [15,16] with experimental techniques such as pump-probe spectroscopy [17] and 4D electron microscopy [18,19] used to explore the energy relaxation dynamics. [20,21] However, these techniques conventionally provide indirect signatures of the response of a large number of thermalizing carriers, [22] and extensive theoretical modeling is necessary to extract information about the sub-picosecond nonequilibrium carrier dynamics of interest. [23] With an ab initio framework for calculating optical response and electronphonon interactions, we previously evaluated mechanisms of hot carrier generation and relaxation in plasmonic metals, [24,25] and identified their signatures in ultrafast pump-probe measurements. [26,27] In particular, the small mean free paths of higher energy carriers helped elucidate the efficiency limits in plasmonic energy conversion devices and potential strategies to overcome them. [13] Here, we investigate the dynamics of hot carriers in graphene and in graphene-derived v...

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