Although it was demonstrated that discrete molecular levels determine the sign and magnitude 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 microheaters serve as a testbed for studying single-molecule thermoelectricity. Reduced screening of the gate electric field compared to conventional metal electrodes allows control of 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 suggest that the power factor of an 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.

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 Vth and heating/cooling response ΔTPeltier, 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.

Managing thermal transport in nanostructures became a major challenge in development of active microelectronic, optoelectronic and thermoelectric devices, stalling the famous Moore’s law of clock speed increase of microprocessors for...

2D materials with high in-plane thermal conductivity such as graphene, which is also highly electrically conductive, or hexagonal boron nitride (hBN), which is electrically insulating, have been proposed for heat management applications, [2] whereas MoS 2 has been used as an active channel on electronic devices due to its comparable bandgap to silicon. [3] 2D materials can also be excellent thermal insulators for cross-plane thermal transport, with WSe 2 known to possess one of the lowest thermal conductivity for a continuous solid-state material. [4] Furthermore, vertical, [5] lateral, [6] or composite [7] heterostructures of 2D materials are shown to be potential candidates for electronic applications. These unique properties open new possibilities for the development of highperformance thermoelectric and phase change memory structures.For thermoelectric applications, 2D materials are also highly attractive due to their high Seebeck coefficient values. For example, the Seebeck coefficient of graphene has been reported to be between 10 and 180 µV K −1 [8] while that of MoS 2 can be orders of magnitude higher (3 × 10 5 µV K −1 ). [9] Nevertheless, their implementation in such devices needs clever use of thermal anisotropy of the in-plane [10] and cross-plane [11] thermal conductivities. The efficiency of a thermoelectric device, which converts the waste heat to energy is determined by the dimensionless figure of merit ZT = S 2 σT/k, where S is the Seebeck coefficient, σ is the electrical conductivity, and k = k ph + k el is the thermal conductivity due to electrons (el) and phonons (ph). Therefore, a highly efficient thermoelectric device requires materials with high S and σ and low k in the direction of the electron flow.Recently, it has been proposed that stacking of 2D materials to create heterostructures is an efficient way of enhancing the efficiency of thermoelectric devices. [12] For graphene/MoS 2 stacks, high figure of merit values (up to 2.8) are predicted, due to the reduction of k and heat transport through the sharp edges of MoS 2 nanoribbons. [13] Additionally, the thermal conductivity of rippled graphene was predicted to notably drop compared to flat one [14] leading to more efficient thermoelectrics. It has also been predicted that reduced thermal conductivity can be achieved by engineering the thermal transport in graphene periodic phononic structures. [15] Furthermore, it has been reported that lateral confinement alone allows to change Nanoscale scanning thermal microscopy (SThM) transport measurements from cryogenic to room temperature on 2D structures with sub 30 nm resolution are reported. This novel cryogenic operation of SThM, extending the temperature range of the sample down to 150 K, yields a clear insight into the nanothermal properties of the 2D nanostructures and supports the model of ballistic transport contribution at the edge of the detached areas of exfoliated graphene which leads to a size-dependent thermal resistance of the detached material. The thermal resistance of graph...

Advances in materials design and device miniaturization lead to physical properties that may significantly differ from the bulk ones. In particular, thermal transport is strongly affected when the device dimensions approach the mean free path of heat carriers. Scanning Thermal Microscopy (SThM) is arguably the best approach for probing nanoscale thermal properties with few tens of nm lateral resolution. Typical SThM probes based on a microfabricated Pd resistive probes (PdRP) using a spatially distributed heater and a nanoscale tip in contact with the sample, provide high sensitivity and operation in ambient, vacuum and liquid environments. Whereas some aspects of the response of this sensor has been studied, both for static and dynamic measurements, here we build an analytical model of the PdRP sensor taking into account finite dimensions of the heater that improves the precision and stability of the quantitative measurements. In particular we analyse the probe response for heat flowing through a tip to the sample and due to probe self-heating and theoretically and experimentally demonstrate that they can differ by more than 50%, hence introducing significant correction in the SThM measurements. Furthermore, we analyzed the effect of environmental parameters such as sample and microscope stage temperatures, and laser illumination, allowed to reduce the experimental scatter by a factor of 10. Finally, varying these parameters, we measured absolute values of heat resistances and compared these to the model for both ambient and vacuum SThM operation, providing a comprehensive pathway improving the precision of the nanothermal measurements in SThM.