We investigate the influence of grain boundaries (GBs), line defects (LDs), and chirality on thermal transport in graphene using non-equilibrium Green's functions. At room temperature the ballistic thermal conductance is ~4.2 GWm -2 K -1 , and single GBs or LDs yield transmission from 50-80% of this value. LDs with carbon atom octagon defects have lower thermal transmission than GBs with pentagon and heptagon defects. We apply our findings to study the thermal conductivity of polycrystalline graphene for practical applications, and find that the type and size of GBs play an important role when grain sizes are smaller than a few hundred nanometers.

Abstract-We measure top-gated graphene field effect transistors (GFETs) with nanosecond-range pulsed gate and drain voltages. Due to high-κ dielectric or graphene imperfections, the drain current decreases ~10% over time scales of ~10 μs, consistent with charge trapping mechanisms. Pulsed operation leads to hysteresis-free I-V characteristics, which are studied with pulses as short as 75 ns and 150 ns at the drain and gate, respectively. The pulsed operation enables reliable extraction of GFET intrinsic transconductance and mobility values independent of sweep direction, which are up to a factor of two higher than those obtained from simple DC characterization. We also observe drain-bias-induced charge trapping effects at lateral fields greater than 0.1 V/µm. In addition, using modeling and capacitancevoltage measurements we extract charge trap densities up to 10 12 cm -2 in the top gate dielectric (here Al2O3). Our study illustrates important time-and field-dependent imperfections of top-gated GFETs with high-κ dielectrics, which must be carefully considered for future developments of this technology.

We investigate transport in graphene supported on various dielectrics (SiO2, BN, Al2O3, HfO2) through a hydrodynamic model which includes self-heating and thermal coupling to the substrate, scattering with ionized impurities, graphene phonons and dynamically screened interfacial plasmonphonon (IPP) modes. We uncover that while low-field transport is largely determined by impurity scattering, high-field transport is defined by scattering with dielectric-induced IPP modes, and a smaller contribution of graphene intrinsic phonons. We also find that lattice heating can lead to negative differential drift velocity (with respect to the electric field), which can be controlled by changing the underlying dielectric thermal properties or thickness. Graphene on BN exhibits the largest highfield drift velocity, while graphene on HfO2 has the lowest one due to strong influence of IPP modes.

The interaction between graphene and various substrates plays an important and limiting role on the behavior of graphene films and devices. Here we uncover that dynamic screening of so-called remote substrate phonons (RPs) has a significant effect on the thermal coupling at the graphene-substrate interface. We calculate the thermal conductance h RP between graphene electrons and substrate, and its dependence on carrier density and temperature for SiO 2 , HfO 2 , h-BN, and Al 2 O 3 substrates. The dynamic screening of RPs leads to one order of magnitude or more decrease in h RP and a change in its dependence on carrier density. Dynamic screening predicts a decrease of ∼1 MW K −1 m −2 while static screening predicts a rise of ∼10 MW K −1 m −2 when the carrier density in Al 2 O 3-supported graphene is increased from 10 12 to 10 13 cm −2 .

New technologies are emerging which allow us to manipulate and assemble 2-dimensional (2D) building blocks, such as graphene, into synthetic van der Waals (vdW) solids. Assembly of such vdW solids has enabled novel electronic devices and could lead to control over anisotropic thermal properties through tuning of inter-layer coupling and phonon scattering. Here we report the systematic control of heat flow in graphene-based vdW solids assembled in a layer-by-layer (LBL) fashion. In-plane thermal measurements (between 100 K and 400 K) reveal substrate and grain boundary scattering limit thermal transport in vdW solids composed of one to four transferred layers of graphene grown by chemical vapor deposition (CVD). Such films have room temperature in-plane thermal conductivity of ~400 Wm−1 K−1. Cross-plane thermal conductance approaches 15 MWm−2 K−1 for graphene-based vdW solids composed of seven layers of graphene films grown by CVD, likely limited by rotational mismatch between layers and trapped particulates remnant from graphene transfer processes. Our results provide fundamental insight into the in-plane and cross-plane heat carrying properties of substrate-supported synthetic vdW solids, with important implications for emerging devices made from artificially stacked 2D materials.