Molecular Network Heat Engines

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Heat engines form one of the cornerstones of classical thermodynamics. By converting heat into mechanical work they powered the industrial revolution in the 19th century. Molecular heat engines have the potential to convert thermal energy to electrical power and vice versa with efficiency close to the thermodynamic limit. The topic of single-molecule thermoelectricity is therefore of fundamental importance for the development of on-chip cooling and heat-to-electricity energy harvesting technologies that could power the quantum revolution of the 21st century. The key challenge in harnessing the thermoelectric energy conversion capabilities of single molecules is gaining a better understanding of the quantum mechanical interactions between molecular electronic and vibrational degrees of freedom, which could prove transformative for experiments in the research area of open quantum systems. These experiments will deliver impact in two ways: by exploring new science and by laying the foundation for new technologies.New science: Molecular heat engines form an ideal platform for exploring the dialogue between quantum mechanics and thermodynamics. While some theoretical efforts have been undertaken towards this end, many predictions remain to be verified by experiments. New insights into thermodynamics on the molecular scale will also raise further questions: Does quantum coherence boost the thermoelectric efficiency of single-molecule heat engines? What happens if the Born-Oppenheimer approximation breaks down? Can molecular vibrational modes be electrically cooled to their ground state?New technologies: Thermoelectrics have a long history of providing simple, reliable power generation. Yet, the use of thermoelectric materials to recover waste heat has remained limited due to their scarcity and toxicity, and the unfortunate fact that the properties that determine their efficiency - the electrical conductance, the thermal conductance, and the Seebeck coefficient - are contra-indicated, meaning that an improvement to one will deteriorate another. Quantum effects in single-molecule heat engines lift the link between these contra-indicated properties, thereby opening up the possibility for highly efficient thermoelectric generators that could provide a low-cost, environmentally-friendly means of scavenging waste heat that would drastically decrease global energy consumption.This proposal seeks to develop the instrumentation and experimental methodology to investigate controlled thermoelectric heat-to-energy conversion in a single molecule, where the emphasis is on controlling the molecular interactions. This control will be achieved by using two-dimensional networks of nanoparticles linked via molecular junctions. Building on recent ground-breaking experiments, I will use electric-field control to tune the molecular energy level alignment with respect of the Fermi level of the substrate, while simultaneously controlling the tunnel coupling and applied bias voltage. A local heater will drive a thermally generated flow of electrons through a single molecule, which I will be able to optimize thanks to the unprecedented degree of tunability in the system. By probing the thermoelectric efficiency over a wide parameter space, I will establish the intrinsic thermodynamic limits to single-molecule energy conversion.