Jean Spiece

Field-Effect Control of Graphene–Fullerene Thermoelectric Nanodevices

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.

Improving accuracy of nanothermal measurements via spatially distributed scanning thermal microscope probes

Advances in material 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 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. Although some aspects of the response of this sensor have 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 selfheating 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, which allowed reducing 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 operations, providing a comprehensive pathway improving the precision of the nanothermal measurements in SThM.

Probing thermal transport and layering in disk media using scanning thermal microscopy

With the advent of heat-assisted magnetic recording (HAMR) [1] the thermal transport properties of magnetic recording media have become a key performance characteristic.

Thermal transport in epitaxial Si1-xGex alloy nanowires with varying composition and morphology

We report on structural, compositional, and thermal characterization of self-assembled in-plane epitaxial Si1-xGex alloy nanowires grown by molecular beam epitaxy on Si (001) substrates. The thermal properties were studied by means of scanning thermal microscopy, while the microstructural characteristics, the spatial distribution of the elemental composition of the alloy nanowires and the sample surface were investigated by transmission electron microscopy and energy dispersive x-ray microanalysis. We provide new insights regarding the morphology of the in-plane nanostructures, their size-dependent gradient chemical composition, and the formation of a 5 nm thick wetting layer on the Si substrate surface. In addition, we directly probe heat transfer between a heated scanning probe sensor and Si1-xGex alloy nanowires of different morphological characteristics and we quantify their thermal resistance variations. We correlate the variations of the thermal signal to the dependence of the heat spreading with the cross-sectional geometry of the nanowires using finite element method simula-tions. With this method we determine the thermal conductivity of the nanowires with values in the range of 2-3 Wm-1K-1. These results provide valuable information in growth processes and show the great capability of the scanning thermal mi-croscopy technique in ambient environment for nanoscale thermal studies, otherwise not possible using conventional tech-niques.We report on structural, compositional, and thermal characterization of self-assembled in-plane epitaxial Si1-x Ge x alloy nanowires grown by molecular beam epitaxy on Si (001) substrates. The thermal properties were studied by means of scanning thermal microscopy (SThM), while the microstructural characteristics, the spatial distribution of the elemental composition of the alloy nanowires and the sample surface were investigated by transmission electron microscopy and energy dispersive x-ray microanalysis. We provide new insights regarding the morphology of the in-plane nanostructures, their size-dependent gradient chemical composition, and the formation of a 5 nm thick wetting layer on the Si substrate surface. In addition, we directly probe heat transfer between a heated scanning probe sensor and Si1-x Ge x alloy nanowires of different morphological characteristics and we quantify their thermal resistance variations. We correlate the variations of the thermal signal to the dependence of the heat spreading with the cross-sectional geometry of the nanowires using finite element method simulations. With this method we determine the thermal conductivity of the nanowires with values in the range of 2-3 W m-1 K-1. These results provide valuable information in growth processes and show the great capability of the SThM technique in ambient environment for nanoscale thermal studies, otherwise not possible using conventional techniques.