Observation of the Dirac fluid and the breakdown of the Wiedemann-Franz law in graphene

Interactions between particles in quantum many-body systems can lead to collective behavior described by hydrodynamics. One such system is the electron-hole plasma in graphene near the charge neutrality point which can form a strongly coupled Dirac fluid. This charge neutral plasma of quasi-relativistic fermions is expected to exhibit a substantial enhancement of the thermal conductivity, due to decoupling of charge and heat currents within hydrodynamics. Employing high sensitivity Johnson noise thermometry, we report the breakdown of the Wiedemann-Franz law in graphene, with a thermal conductivity an order of magnitude larger than the value predicted by Fermi liquid theory. This result is a signature of the Dirac fluid, and constitutes direct evidence of collective motion in a quantum electronic fluid.Physic

Field-effect control of graphene–fullerene thermoelectric nanodevices

Although it was demonstrated that discrete molecular levels determine the sign and mag nitude of the thermoelectric effect in single-molecule junctions, full electrostatic control of these levels has not been achieved to date. Here, we show that graphene nanogaps combined with gold micro-heaters serve as a testbed for studying single-molecule their moelectricity. Reduced screening of the gate electric field compared to conventional metal electrodes allows controlling the position of the dominant transport orbital by hundreds of meV. We find that the power factor of graphene-fullerene junctions can be tuned over several orders of magnitude to a value close to the theoretical limit of an isolated Breit-Wigner resonance. Furthermore our data suggests that the power factor of isolated level is only given by the tunnel coupling to the leads and temperature. These results open up new avenues for exploring thermoelectricity and charge transport in individual molecules, and highlight the importance of level-alignment and coupling to the electrodes for optimum energy-conversion in organic thermoelectric materials

Thermoelectric effects in carbon nanostructures: From quantum to mesoscopic influences

Heat from electrical devices, car engines, industrial processes and even our own body heat are an abundant, albeit difficult to harvest, energy source in today's society. As the extent of the climate crisis becomes clearer, recovering such waste heat can be an important step to design improved devices and reduce greenhouse gas emissions as well as the use of fossil fuels. Thermoelectric materials and devices are uniquely positioned to recover this waste heat and transform it to electricity due to their conversion characteristics and potentially small size. In addition, ever-increasing demands on computing power as well as further downscaling in chip sizes necessitates on-chip spot cooling and temperature sensing, which can be provided by thermoelectric devices. \\ So far, whenever research in thermoelectrics seemed to be at a dead end, a new approach, such as for example most recently nanostructuring and reduced dimensionality, have reinvigorated the field. While this has lead to unprecedented advances in the figure of merit, now regularly reported above unity, thermoelectric devices are still lacking the required conversion efficiency to compete with conventional heat engines, leaving them an option mostly for niche applications. Particularly in low-dimensional devices and structures, theoretically predicted high and competitive efficiency values are hard to obtain due to environmental factors and inherent limits in device design. In this dissertation, influences on the thermoelectric properties of carbon nanostructures due to coupling configurations, defects, geometrical considerations and other local effects are studied. A first experiment investigates the power factor in C$_{60}$ molecules, contacted via electroburned graphene nanoconstrictions. The results suggest a threefold way to increase the achievable power factor in zero-dimensional structures: positioning the molecular energy levels close to the Fermi energy levels of the leads, ensuring the tunnel coupling is optimized for the desired operating temperature and, in order to achieve a maximum power factor, the tunnel couplings to the source and drain need to be equal. Building on these conclusions, a temperature dependent study of the power factor in graphene quantum dots was performed, suggesting that a quantum dot can work as a quantum heat valve especially when contacted by non-ideal heat conducting leads. Apart from highlighting the importance of leads that conduct heat well to achieve a high performance, the findings also again emphasize the potentially detrimental effect tunnel coupling has on the power factor. \\ In order to analyse more localised effects, a Scanning Thermal Microscopy (SThM) approach was used to record thermoelectric maps (of both the Seebeck and Peltier effect) of graphene bow ties, revealing a geometrically dependent change in the Seebeck coefficient due to the impact of electron scattering at the graphene edges. This effect is exploited later to fabricate single-material graphene thermocouples that achieve an order of magnitude higher sensitivity than previously reported single-material metal thin film thermocouples. Lastly, Scanning Thermal Gate Microscopy (STGM) is developed, improving on the previously used SThM technique to achieve high resolution in thermovoltage maps at a fast scanning rate. STGM is then applied to graphene single-layer/bilayer junctions, demonstrating the influence of metallic contacts, layer thickness changes and local strain on the spatially distributed Seebeck coefficient in graphene. \\ The effects studied in this dissertation are aimed at helping to further the understanding of local thermoelectric properties and opening the path to improve on current device design and performances as well as facilitating the elimination of parasitic noise caused by undesirable thermoelectric effects

Thermoelectric effects in carbon nanostructures

Heat from electrical devices, car engines, industrial processes and even our own body heat are an abundant, albeit difficult to harvest, energy source in today's society. As the extent of the climate crisis becomes clearer, recovering such waste heat can be an important step to design improved devices and reduce greenhouse gas emissions as well as the use of fossil fuels. Thermoelectric materials and devices are uniquely positioned to recover this waste heat and transform it to electricity due to their conversion characteristics and potentially small size. In addition, ever-increasing demands on computing power as well as further downscaling in chip sizes necessitates on-chip spot cooling and temperature sensing, which can be provided by thermoelectric devices. \\ So far, whenever research in thermoelectrics seemed to be at a dead end, a new approach, such as for example most recently nanostructuring and reduced dimensionality, have reinvigorated the field. While this has lead to unprecedented advances in the figure of merit, now regularly reported above unity, thermoelectric devices are still lacking the required conversion efficiency to compete with conventional heat engines, leaving them an option mostly for niche applications. Particularly in low-dimensional devices and structures, theoretically predicted high and competitive efficiency values are hard to obtain due to environmental factors and inherent limits in device design. In this dissertation, influences on the thermoelectric properties of carbon nanostructures due to coupling configurations, defects, geometrical considerations and other local effects are studied. A first experiment investigates the power factor in C$_{60}$ molecules, contacted via electroburned graphene nanoconstrictions. The results suggest a threefold way to increase the achievable power factor in zero-dimensional structures: positioning the molecular energy levels close to the Fermi energy levels of the leads, ensuring the tunnel coupling is optimized for the desired operating temperature and, in order to achieve a maximum power factor, the tunnel couplings to the source and drain need to be equal. Building on these conclusions, a temperature dependent study of the power factor in graphene quantum dots was performed, suggesting that a quantum dot can work as a quantum heat valve especially when contacted by non-ideal heat conducting leads. Apart from highlighting the importance of leads that conduct heat well to achieve a high performance, the findings also again emphasize the potentially detrimental effect tunnel coupling has on the power factor. \\ In order to analyse more localised effects, a Scanning Thermal Microscopy (SThM) approach was used to record thermoelectric maps (of both the Seebeck and Peltier effect) of graphene bow ties, revealing a geometrically dependent change in the Seebeck coefficient due to the impact of electron scattering at the graphene edges. This effect is exploited later to fabricate single-material graphene thermocouples that achieve an order of magnitude higher sensitivity than previously reported single-material metal thin film thermocouples. Lastly, Scanning Thermal Gate Microscopy (STGM) is developed, improving on the previously used SThM technique to achieve high resolution in thermovoltage maps at a fast scanning rate. STGM is then applied to graphene single-layer/bilayer junctions, demonstrating the influence of metallic contacts, layer thickness changes and local strain on the spatially distributed Seebeck coefficient in graphene. \\ The effects studied in this dissertation are aimed at helping to further the understanding of local thermoelectric properties and opening the path to improve on current device design and performances as well as facilitating the elimination of parasitic noise caused by undesirable thermoelectric effects

Direct mapping of local Seebeck coefficient in 2D material nanostructures via scanning thermal gate microscopy

Studying local variations in the Seebeck coefficient of materials is important for understanding and optimizing their thermoelectric properties, yet most thermoelectric measurements are global over a whole device or material, thus overlooking spatial divergences in the signal and the role of local variation and internal structure. Such variations can be caused by local defects, metallic contacts or interfaces that often substantially influence thermoelectric properties, especially in two dimensional materials. Here, we demonstrate scanning thermal gate microscopy, a non-destructive method to obtain high resolution 2-dimensional maps of the thermovoltage, to study graphene samples. We demonstrate the efficiency of this newly developed method by measuring local Seebeck coefficient in a graphene ribbon and in a junction between single-layer and bilayer graphene

Role of metallic leads and electronic degeneracies in thermoelectric power generation in quantum dots

The power factor of a thermoelectric device is a measure of the heat-to-energy conversion efficiency in nanoscopic devices. Yet, even as interest in low-dimensional thermoelectric materials has increased, experimental research on what influences the power factor in these systems is scarce. Here, we present a detailed thermoelectric study of graphene quantum dot devices. We show that spin degeneracy of the quantum dot states has a significant impact on the zero-bias conductance of the device and leads to an increase of the power factor. Conversely, we demonstrate that nonideal heat exchange within the leads can suppress the power factor near the charge degeneracy point and nontrivially influences its temperature dependence.QN/van der Zant La

Geometrically Enhanced Thermoelectric Effects in Graphene Nanoconstrictions

The influence of nanostructuring and quantum confinement on the thermoelectric properties of materials has been extensively studied. While this has made possible multiple breakthroughs in the achievable figure of merit, classical confinement, and its effect on the local Seebeck coefficient has mostly been neglected, as has the Peltier effect in general due to the complexity of measuring small temperature gradients locally. Here we report that reducing the width of a graphene channel to 100 nm changes the Seebeck coefficient by orders of magnitude. Using a scanning thermal microscope allows us to probe the local temperature of electrically contacted graphene two-terminal devices or to locally heat the sample. We show that constrictions in mono- and bilayer graphene facilitate a spatially correlated gradient in the Seebeck and Peltier coefficient, as evidenced by the pronounced thermovoltage $V_{th}$ and heating/cooling response $\Delta T_{Peltier}$, respectively. This geometry dependent effect, which has not been reported previously in 2D materials, has important implications for measurements of patterned nanostructures in graphene and points to novel solutions for effective thermal management in electronic graphene devices or concepts for single material thermocouples

Thermoelectric Limitations of Graphene Nanodevices at Ultrahigh Current Densities

Graphene is atomically thin, possesses excellent thermal conductivity, and is able to withstand high current densities, making it attractive for many nanoscale applications such as field-effect transistors, interconnects, and thermal management layers. Enabling integration of graphene into such devices requires nanostructuring, which can have a drastic impact on the self-heating properties, in particular at high current densities. Here, we use a combination of scanning thermal microscopy, finite element thermal analysis, and operando scanning transmission electron microscopy techniques to observe prototype graphene devices in operation and gain a deeper understanding of the role of geometry and interfaces during high current density operation. We find that Peltier effects significantly influence the operational limit due to local electrical and thermal interfacial effects, causing asymmetric temperature distribution in the device. Thus, our results indicate that a proper understanding and design of graphene devices must include consideration of the surrounding materials, interfaces, and geometry. Leveraging these aspects provides opportunities for engineered extreme operation devices