Resumen de Conductance, thermopower and thermal conductance measurements in single-molecule junctions and atomic contacts
Molecular electronics is a very active research field whose ultimate goal is the use of single molecules as functional building blocks in electronic devices. Single-molecule thermoelectricity, in particular, holds the promise to pave the way for the development of more efficient organic-based thermoelectric devices and has potential applications in waste heat recovery or on-chip cooling.
More importantly, single molecules connected between two metallic electrodes, the so-called single-molecule junctions, are of great fundamental interest and have proved to be exceptional platforms to test quantum transport theories. Measurements of thermopower in molecular junctions offer complementary information to the traditionally performed conductance characterization and are becoming essential for the understanding of transport processes at the nanoscale.
Additionally, investigating heat transfer at the nanoscale results also essential to bring the proposed applications to fruition and to shed light on the limits of classical theories. This is particularly challenging from the experimental point of view mainly due to the non-equilibrium effects targeted, the scale of the thermal currents to be measured and the intrinsic nature of heat flow, fundamentally different to charge transport.
This PhD Thesis aims at the experimental characterization of charge and heat transport properties of single-molecule junctions and atomic contacts to get an insight into their fundamental mechanisms and to test strategies for thermopower enhancement. To achieve this goal, we have employed a home-built scanning tunnelling microscope (STM), operating in ambient conditions and at room temperature, and a powerful experimental technique previously initiated in our group that has allowed us to perform simultaneous measurements of conductance and thermopower of single-molecule junctions.
The technique has been exploited to characterize the transport properties of endohedral metallofullerenes formed by planar metallic clusters inside a C80 fullerene cage. Sc3N@C80, Sc3C2@C80 and Er3N@C80 molecules connected between gold electrodes have been investigated and are demonstrated to show a bi-thermoelectric behaviour, giving positive or negative thermopower depending on the molecule orientation, without chemical modification. The possibility to tune the value and sign of the thermopower by compression of the molecular junctions is also explored, as well as the thermoelectric response of dimer junctions, i.e. two endohedral metallofullerenes connected in series.
Different strategies for thermopower enhancement have been also addressed using the STM-Break Junction technique. Conductance and thermopower measurements with three different groups of molecules (fluorene derivatives, oligoyne wires and porphyrin oligomers) have been performed to investigate the tunability of thermopower by chemical substitution of side groups, by increasing the molecular length and by varying the coupling of the units conforming the molecular backbone. Enhancement of the thermopower is found to occur to more or less extent with all these strategies, demonstrating the suitability of molecular junctions as a versatile test bed to explore different approaches to tune charge transport at the nanoscale.
Finally, a new experimental technique to perform thermal conductance measurements in model nanoscale junctions, such as atomic contacts or molecular junctions, has been developed during this Thesis. It consists of a Pt hot wire used as local heater and thermometer as well as STM tip to form the nanoscale junctions and to perform simultaneous thermal and electrical conductance measurements. The technique has been tested with Au-Au atomic contacts, employing ac lock-in detection combined with a numerical theoretical model, and enough thermal resolution of the hot wire sensors even in ambient conditions is demonstrated. The possibility to scan the metallic surface with the hot wire via tunnelling current is also explored.
