Effects of Interlayer Coupling on Hot‐Carrier Dynamics in Graphene‐Derived van der Waals Heterostructures

Abstract

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...

Efficiency Limits of Solar Energy Harvesting via Internal Photoemission in Carbon Materials

“…Furthermore, unlike in noble metals, electron-phonon scattering is predicted to be the dominant scattering process for the non-equilibrium electrons, especially for those with higher energies. This effect was attributed to softer phonon modes and stronger electron-phonon coupling with lighter atoms in carbon materials than in metals [58]. The above observations hold high promise for the use of hot-carrier carbon IPE devices as photodetectors for both visible and infrared spectral ranges [12,59].…”

“…It is known from prior measurements and DFT modeling that the mean free path for hot electrons in noble metals such as Au range from a few nanometers for electrons with energies exceeding the Fermi level by 2-5 eV, to 50-100 nm for electrons with energies close to the Fermi level [9]. Recent DFT modeling predicts comparable mean free path values for higher-energy electrons in graphene and graphite, and significantly larger ones (from~100 nm to above 1000nm) for electrons with energies within 0.5eV above the Fermi level [58]. Furthermore, unlike in noble metals, electron-phonon scattering is predicted to be the dominant scattering process for the non-equilibrium electrons, especially for those with higher energies.…”

Quantum plasmons with optical-range frequencies in doped few-layer graphene

“…This takes into account the detailed electronic structure effects such as the response of electrons far from the Dirac point, as well as scattering against both acoustic and optical phonons, including umklapp and intervalley processes [35][36][37][38]. Doping, that is, a change in the position of the Fermi level E F changes the value of τ , and hence calculations were carried out for several different values of E F ranging from the neutral (undoped) value to 1.5 eV above it (see the Supplemental Material [34] for details of formulation and [35] for values of τ ). Interestingly, our results show that the extremely large τ ≈ 1 ps for freestanding undoped graphene drops to ≈ 29 fs in doped graphene.…”

“…To obtain reliable values of τ , we used DFT results for the energies and matrix elements of both electrons and phonons (see the Supplemental Material [34] and Ref. [35]). This takes into account the detailed electronic structure effects such as the response of electrons far from the Dirac point, as well as scattering against both acoustic and optical phonons, including umklapp and intervalley processes [35][36][37][38].…”

Optoelectronic response of the type-I Weyl semimetals TaAs and NbAs from first principles

“…Then the Fermisurface averaged momentum relaxation time we use in (5) is weighted by the Fermi occupation and square of the velocity: In addition to the above components of the Drude contribution to the frequency dependent dielectric function, we also explicitly calculate the imaginary components of the dielectric function due to direct and phonon-assisted electronic transitions, as in our prior works. 51,[56][57][58] The corresponding expressions are Equation 10 and 11 in the Computational Methods. We then also predict the real part of the dielectric function via the Kramers-Kronig relation.…”