Dielectric control of ultrafast carrier dynamics and transport in graphene

Abstract

Understanding the ultrafast dynamics of photoexcited charges in graphene is essential, as the microscopic mechanisms underlying these dynamics determine many of graphene's optical, optothermal, and optoelectronic properties. These are crucial properties for many functionalities and devices enabled by graphene, such as high-speed photodectors. Therefore, beyond scientific understanding, it is highly desirable to control ultrafast carrier dynamics for practical applications. Here, we establish this control by engineering the dielectric environment of graphene, thereby regulating both heating and cooling dynamics without altering the Fermi energy, optical power, or ambient temperature. By combining optical pump-terahertz probe experiments with theoretical calculations, we show that dielectric screening suppresses carrier-carrier interactions and slows the dynamics. In particular, reduced carrier-carrier scattering delays the formation of a quasi-equilibrium hot electron distribution, thus slowing carrier heating. It also slows carrier cooling because re-thermalization after optical-phonon emission depends on the same interactions. The enhanced screening further reduces the energy of electron-hole puddles, thereby increasing charge mobility and the Seebeck coefficient. This ability to externally control internal graphene dynamics and transport properties enables the optimization of device performance, such as the sensitivity of photodetectors for data communication and wireless communication applications.

Figures (3)

• Figure 1: Experimental observation of slower photoexcited carrier dynamics controlled by the dielectric environment.a, Illustration of the experimental approach, where graphene inside a modified silica flow cell is surrounded by different dielectric environments with static dielectric constant $\epsilon$ (in green), in particular nitrogen gas, toluene, 1-hexanol, acetophenone, and isopropanol (IPA), see chemical structures on the top left. Time-resolved optical pump -- THz probe measurements provide access to the ultrafast carrier dynamics, as shown on the right. b, The pump-induced change in THz transmission $\Delta T/T_0$ as a function of pump-probe delay time $\Delta t$ for nitrogen ($\epsilon$ = 1) and IPA ($\epsilon$ = 19.7) environments, showing a markedly slower rise and slower decay for the IPA case. c, The photoexcited carrier dynamics for the different environments, with the dashed line indicating the delay time where the peak signal occurs. The results show a gradual slowing of the photoexcited carrier dynamics -- both heating and cooling -- as the dielectric constant of the environment increases. d, Peak THz transmission as a function of pump fluence for the different dielectric environments, showing a lower signal and faster saturation for larger dielectric constants.
• Figure 2: Calculated dielectric tuning of photoexcited carrier dynamics by controlling carrier-carrier interactions.a-b, Calculated photoexcited carrier dynamics in an environment with $\epsilon$ = 1 (a) and $\epsilon$ = 20 (b), showing the dynamics of the density of photoexcited carriers $\Delta n$ and of the carrier temperature $T$. The latter is obtained by describing the corresponding carrier distributions with Fermi-Dirac statistics. c, Illustration of the carrier dynamics that occur after optical excitation creates an additional electron in the conduction band and an additional hole in the valence band, for $p$-doped graphene with $\epsilon$ = 1. The initially excited carriers relax through carrier-carrier scattering, which creates additional hot carriers, in this case, hot holes with an energy above the Fermi energy. d-e, Snapshots of calculated carrier distributions at several delay times after photoexcitation for $\epsilon$ = 1 (d) and $\epsilon$ = 20 (e). In the former case, all distributions after $\approx$10 fs are thermalized due to efficient carrier-carrier interactions, whereas in the latter case, they are non-thermal. f, Illustration of the carrier dynamics after photoexcitation for $\epsilon$ = 20. In this case, the relaxation of initially excited carriers by the emission of optical phonons plays an important role, since carrier-carrier interactions are screened.
• Figure 3: Tuning electronic transport in graphene by modulating the electron-hole puddle height via dielectric screening.a, Carrier mobility as a function of carrier density. b, Seebeck coefficient as a function of Fermi energy, for different puddle heights. Upper right inset: scaling of the mobility with puddle height at $n = 10^{12}$ cm$^{-2}$. The dashed line indicates the scaling $\mu \propto 1/W^2$. Lower left inset: scaling of the maximal value of $S$ with puddle height.