Photocarrier thermalization bottleneck in graphene

Abstract

We present an ab-initio study of photocarrier dynamics in graphene due to electron-phonon (EP) interactions. Using the Boltzmann relaxation-time approximation with parameters determined from density functional theory (DFT) and a complementary, explicitly solvable model we show that the photocarrier thermalization time changes by orders of magnitude, when the excitation energy is reduced from 1 eV to the 100 meV range. In detail, the ultrafast thermalization at low temperatures takes place on a femtosecond timescale via optical phonon emission, but slows down to picoseconds once excitation energies become comparable with these optical phonon energy quanta. In the latter regime, thermalization times exhibit a pronounced dependence on temperature. Our DFT model includes all the inter-and intraband transitions due to EP scattering. Thanks to the high melting point of graphene we extend our studies up to 2000 K and show that such high temperatures reduce the photocarrier thermalization time through phonon absorption. arXiv:1804.07467v1 [cond-mat.mtrl-sci]

Intertube Excitonic Coupling in Nanotube Van der Waals Heterostructures

“…Instead, the formation of indirect intertube excitons via charge transfer can account for the slow response under IR excitation. Indeed the characteristic timescale of 220 ps for its rise (Figure 4c) is consistent with our estimates of hole tunneling times from the CNTs to the MoS 2 : using a model of quantum tunneling through the BN barrier [ 30 ] we estimated hole tunneling times of 100 ps–1 ns (Figure S9, Supporting Information). While the indirect excitons thus created do not directly absorb, the transfer of quasiparticles from one layer to another modifies the absorption rate and thus creates a non‐zero ΔOD in the same way as discussed for the ultrafast intertube biexcitonic response.…”

Thermalization of photoexcited carriers in two-dimensional transition metal dichalcogenides and internal quantum efficiency of van der Waals heterostructures

“…In what follows, we use an approach already applied successfully to bulk and 2D materials [13,42,44,45]. For instance, we have demonstrated in our recent work that phonon emission by photocarriers in graphene is strongly suppressed at low excess energies of about 100 meV [13]. This phenomenon occurs because of high optical phonon frequencies due to strong carbon-carbon bonding and has been observed experimentally as a thermalization bottleneck [10].…”

“…The scheme has first been realized in a graphene-boronnitride-graphene vdW heterostructure [7], where the interlayer tunneling time ranges from 1 fs to 1 ps depending on bias voltage. Since the photocarrier thermalization time in graphene spans the interval between 10 fs and 10 ps depending on carrier concentration and excitation energy [8][9][10][11][12][13], there is a parameter range within which high-energy photocarrier extraction is feasible. Substituting graphene by a 2D transition metal dichalcogenide (TMDC) in a stack [14][15][16][17] opens new perspectives thanks to the possibility to create a diode configuration [18] and to realize stronger light-matter interactions [19].…”

Bypassing the Structural Bottleneck in the Ultrafast Melting of Electronic Order

“…relaxation process initially driven by the interaction with strongly coupled phonons. Remarkably, the fast relaxation component stops for both fluences at approximately the same value of k B ΔT e ≈ 40 meV, hinting at a closure of the fast relaxation channel at a characteristic threshold energy for phonon emission [48,49]. In contrast, the transient ΔE shift displays a slower rise to 65 meV with a sigmoidal time constant τ Δ ¼ 60 AE 10 fs, stationary behavior between 100 and 280 fs, and then exponential decay (with a time constant of 220 AE 60 fs).…”