Transport studies in graphene-based materials and structures

Graphene, a single atomic layer of graphite, has emerged as one of the most attractive materials in recent years for its many unique and excellent properties, inviting a broad area of fundamental studies and applications. In this thesis, we present some theoretical/experimental studies about the thermal, electronic and thermoelectric transport properties in graphene-based systems. We employ the molecular dynamic simulations to study the thermal transport in graphene nanoribbons (GNRs) exhibiting various properties, including chirality dependent thermal conductivity, thermal rectification in asymmetric GNRs, defects and isotopic engineering of the thermal conductivity and negative differential thermal conductance (NDTC) at large temperature biases. We prove a theorem on the existence of NDTC in general one-dimensional diffusive thermal transport. We synthesis graphene composites and characterize their electric and thermal properties. Their electrical conductivity follows the percolation theory. We use 3-w method to measure the thermal conductivity and find that their thermal conductivities can be tuned by the graphene filler concentration. We build a micro-manipulator to accurately align and transfer two-dimensional materials, e.g., graphene and boron nitride (BN). We then fabricate the stacked BN/grapnene/BN/graphene/BN systems with isolated metal contacts to each graphene layer, to study the counterflow thermoelectric transport and Coulomb drag. In the last we present our theoretical considerations about the transport properties of multilayer systems with interlayer Coulomb interactions, using phenomenological arguments, Drude-like models and Boltzmann transport formalism, and discussed the potential for the interlayer interaction to enhance the thermoelectric figure of merit

Existence of negative differential thermal conductance in one-dimensional diffusive thermal transport

We show that in a finite one-dimensional (1D) system with diffusive thermal transport described by the Fourier's law, negative differential thermal conductance (NDTC) cannot occur when the temperature at one end is fixed. We demonstrate that NDTC in this case requires the presence of junction(s) with temperature dependent thermal contact resistance (TCR). We derive a necessary and sufficient condition for the existence of NDTC in terms of the properties of the TCR for systems with a single junction. We show that under certain circumstances we even could have infinite (negative or positive) differential thermal conductance in the presence of the TCR. Our predictions provide theoretical basis for constructing NDTC-based devices, such as thermal amplifiers, oscillators and logic devices

Nonlinear thermal transport and negative differential thermal conductance in graphene nanoribbons

We employ classical molecular dynamics to study the nonlinear thermal transport in graphene nanoribbons (GNRs). For GNRs under large temperature biases beyond linear response regime, we have observed the onset of negative differential thermal conductance (NDTC). NDTC is tunable by varying the manner of applying the temperature biases. NDTC is reduced and eventually disappears when the length of the GNR increases. We have also observed NDTC in triangular GNRs, where NDTC exists only when the heat current is from the narrower to the wider end. These effects may be useful in nanoscale thermal managements and thermal signal processing utilizing GNRs.Comment: 5 pages, 4 figure

Tuning the thermal conductivity of graphene nanoribbons by edge passivation and isotope engineering: a molecular dynamics study

Using classical molecular dynamics simulation, we have studied the effect of edge-passivation by hydrogen (H-passivation) and isotope mixture (with random or supperlattice distributions) on the thermal conductivity of rectangular graphene nanoribbons (GNRs) (of several nanometers in size). We found that the thermal conductivity is considerably reduced by the edge H-passivation. We also find that the isotope mixing can reduce the thermal conductivities, with the supperlattice distribution giving rise to more reduction than the random distribution. These results can be useful in nanoscale engineering of thermal transport and heat management using GNRs.Comment: 4 pages, 4 figure

Observation of Quantized Hall Effect and Shubnikov-de Hass Oscillations in Highly Doped Bi2Se3: Evidence for Layered Transport of Bulk Carriers

Bi2Se3 is an important semiconductor thermoelectric material and a prototype topological insulator. Here we report observation of Shubnikov-de Hass (SdH) oscillations accompanied by quantized Hall resistances (Rxy) in highly-doped n-type Bi2Se3 with bulk carrier concentrations of few 10^19 cm^-3. Measurements under tilted magnetic fields show that the magnetotransport is 2D-like, where only the c-axis component of the magnetic field controls the Landau level formation. The quantized step size in 1/Rxy is found to scale with the sample thickness, and average ~e2/h per quintuple layer (QL). We show that the observed magnetotransport features do not come from the sample surface, but arise from the bulk of the sample acting as many parallel 2D electron systems to give a multilayered quantum Hall effect. Besides revealing a new electronic property of Bi2Se3, our finding also has important implications for electronic transport studies of topological insulator materials.Comment: accepted by Physical Review Letters (2012