S. Fratini

Tunable Fröhlich polarons in organic single-crystal transistors

In organic field-effect transistors (FETs), charges move near the surface of an organic semiconductor, at the interface with a dielectric. In the past, the nature of the microscopic motion of charge carriers—which determines the device performance—has been related to the quality of the organic semiconductor. Recently, it was discovered that the nearby dielectric also has an unexpectedly strong influence. The mechanisms responsible for this influence are not understood. To investigate these mechanisms, we have studied transport through organic single-crystal FETs with different gate insulators. We find that the temperature dependence of the mobility evolves from metallic-like to insulating-like with increasing dielectric constant of the insulator. The phenomenon is accounted for by a two-dimensional Fröhlich polaron model that quantitatively describes our observations and shows that increasing the dielectric polarizability results in a crossover from the weak to the strong polaronic coupling regime. This represents a considerable step forward in our understanding of transport through organic transistors, and identifies a microscopic physical process with a large influence on device performance.

Substrate-limited electron dynamics in graphene

We study the effects of polarizable substrates such as

Electrostatic interactions between graphene layers and their environment

\mathrm{Si}{\mathrm{O}}_{2}

DYNAMICAL MEAN-FIELD THEORY OF THE SMALL POLARON

and SiC on the carrier dynamics in graphene. We find that the quasiparticle spectrum acquires a finite broadening due to the long-range interaction with the polar modes at the interface between graphene and the substrate. This mechanism results in a density dependent electrical resistivity, which exhibits a sharp increase around room temperature, where it can become the dominant limiting factor of electron transport. The effects are weaker in doped bilayer graphene due to the more conventional parabolic band dispersion. Amorphous substrates, such as polymethyl methacrylate, can induce a room temperature resistivity of comparable magnitude, although with a weaker temperature dependence.

The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials

We analyze the electrostatic interactions between a single graphene layer and a

Transient localization in crystalline organic semiconductors

{\text{SiO}}_{2}

Tailoring the molecular structure to suppress extrinsic disorder in organic transistors

substrate, and other materials which may exist in its environment. We obtain that the leading effects arise from the polar modes at the

Molecular Fingerprints in the Electronic Properties of Crystalline Organic Semiconductors: From Experiment to Theory

{\text{SiO}}_{2}

Band dispersion and electronic lifetimes in crystalline organic semiconductors.

surface, and water molecules, which may form layers between the graphene sheet and the substrate. The strength of the interactions implies that graphene is pinned to the substrate at distances greater than a few lattice spacings. The implications for graphene nanoelectromechanical systems, and for the interaction between graphene and a scanning tunneling microscopy tip, are also considered.

Current saturation and Coulomb interactions in organic single-crystal transistors

A dynamical mean-field theory of the small polaron problem is presented, which becomes exact in the limit of infinite dimensions. The ground-state properties and the one-electron spectral function are obtained for a single electron interacting with Einstein phonons by a mapping of the lattice problem onto a polaronic impurity model. The one-electron propagator of the impurity model is calculated through a continued fraction expansion, at both zero and finite temperature, for any electron-phonon coupling and phonon energy. In contrast to the ground-state properties, such as the effective polaron mass, which show a continuous behavior as the coupling is increased, spectral properties exhibit a sharp qualitative change at low enough phonon frequency: beyond a critical coupling, one energy gap and then more open in the density of states at low energy, while the high-energy part of the spectrum is broad and can be qualitatively explained by a strong coupling adiabatic approximation. As a consequence, narrow and coherent low-energy subbands coexist with an incoherent featureless structure at high energy. The subbands denote the formation of quasiparticle polaron states. Also, divergencies of the self-energy may occur in the gaps. At finite temperature such an effect triggers an important damping and broadening of the polaron subbands. On the other hand, in the large phonon frequency regime such a separation of energy scales does not exist and the spectrum always has a multipeaked structure.