Defect-Induced Supercollision Cooling of Photoexcited Carriers in Graphene

Abstract

Defects play a fundamental role in the energy relaxation of hot photoexcited carriers in graphene, thus a complete understanding of these processes are vital for improving the development of graphene devices. Recently, it has been theoretically predicted and experimentally demonstrated that defect-assisted acoustic phonon supercollision, the collision between a carrier and both an acoustic phonon and a defect, is an important energy relaxation process for carriers with excess energy below the optical phonon emission. Here, we studied samples with defects optically generated in a controlled manner to experimentally probe the supercollision model as a function of the defect density. We present pump and probe transient absorption measurements showing that the decay time decreases as the density of defect increases as predicted by the supercollision model.

Effect of defect-induced cooling on graphene hot-electron bolometers

“…We note that the pump probe measurements were performed about one month before the transport measurements and the sample properties might have slightly deteriorated for the unprotected sample SA, due to sample handling and its exposure to ambient conditions. Nevertheless, the pump probe measurements confirm that the cooling coefficient is higher for the sample treated with Pd sputtering, leading to a faster response time and consistent with a shorter mean free path and higher defect concentration [7,12]. Moreover, the values of obtained from the pump-probe measurements are well within the range of the values obtained from the linear fits of Te vs. P 1/3 from the transport measurements of the quantum dot bolometer samples in Figure 2(b).…”

“…Previous work on CVD grown graphene [10] and exfoliated graphene [11] measured this cubic power law at temperatures higher than TBG, where, in the presence of defects, the supercollisions are predicted to dominate over normal collisions [7]. Other work used pump-probe experiments on exfoliated graphene with the defect density systematically increased by exposure to near-infrared femtosecond pulses and showed that samples with higher defect density had faster cooling times [12]. For lower temperatures, Tx < T0, Te < TBG, the cooling occurs via normal collision, with the dependence P  (Te 4 -T0 4 ) [10] down to a crossover temperature Tx = [13,14].…”

Angle-tunable intersubband photoabsorption and enhanced photobleaching in twisted bilayer graphene

“…The extracted time constants are τ1 SLG = 120 ± 20 fs and τ2 SLG = 1,980 ± 60 fs. These values are characteristic of SLG samples with low defect density [43]. The transient transmissivity depends superlinearly on Te through the FD distribution function, so that the time evolution of Te and ΔT/T are not correlated in time.…”

“…The cooling of the thermalized electron-phonon bath is finally driven by the lattice and achieved in less than 10 picoseconds via anharmonic decay of the hot phonons. In defected SLG, hot carriers can release their excess energy also by defect-mediated emission of acoustic phonons (supercollision mechanism), which occurs on a picosecond time scale dependent on substrate and defect densities [31,33,[37][38][39][40][41][42][43]. The hot carrier distribution inhibits interband absorption over a broad energy range due to Pauli blocking, so that in the TA spectra of SLG a decreased absorption, or photo-bleaching (PB), signal is detected.…”

Suppression of supercollision carrier cooling in high mobility graphene on SiC( 0001¯ )

“…Thus SC becomes an effective cooling pathway. Indeed, SCs have been observed as the dominant cooling channel in a variety of graphene samples, such as suspended 27 and substrate-supported 11,[28][29][30] graphene. As a consequence of these complicated interplays between carriers, optical phonons, acoustic phonons and defects, photo-excited carrier dynamics in graphene has not as yet been clarified.…”