Molecular Network Heat Engines

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.

Scientific impact: Single-molecule junctions have been studied extensively using STM-based and mechanically controlled break junctions. Despite the tremendous success of these approaches, a major drawback is the lack of electrostatic gate control. As a result the transmission - and the corresponding electrical and thermal conductance and Seebeck coefficient - can only be probed at a fixed energy. I have demonstrated that the electrical power generated by a thermal gradient across a single molecule contacted by graphene nanoelectrodes can be tuned over orders of magnitude by a gate electric field. While graphene nanoelectrodes form a robust platform for studying single-molecule charge12 and heat transport, challenges remain regarding the atomistic details of the graphene-molecule anchoring. Small variations in the binding geometry strongly influence the overall transport properties of the molecular junction. By using electric-field controlled two-dimensional molecule-nanoparticles arrays I will be able to overcome both the challenges. MolNet will build on recent experiments on ordered nanoparticle networks and go beyond them by studying thermal and optically driven charge transport through arrays of nanoparticle-molecule-nanoparticle junctions.

Technological impact: Results from STM-based experiments are notoriously difficult to translate into scalable applications. This is in part due to the inherent non-scalable nature of the STM geometry, and in part due to the relatively underwhelming performance characteristics of single-molecule devices - the cooling power of an individual molecule for instance is only on the order of an attowatt. Despite these challenges, single-molecule applications are now being transferred from STM-based experiments to more scalable platforms owing to ever-refined nanofabrication technologies. Moreover, bottom-up and self-assembly methods have the potential to form structured arrays of ten-to-the-fourteen molecules. Importantly, the physics driving heat and charge transport through an individual molecule are the same as those converting heat to electricity in a self-assembled monolayer and molecular network. The knowledge gained from previous single-molecule experiments will thus inform the design rules for real-world molecular thermoelectric and photovoltaic materials and devices that will be tested in this project. To benchmark the thermoelectric performance of molecular network heat engines with other technologically relevant bulk and nanostructured materials, such as PbTe and Nano-SiGe, MolNet will seek to determine the thermodynamic efficiency and the related dimensionless figure-of-merit ZT. Key to this comparison will be the disentanglement of the electrical and thermal properties that intrinsic to the molecule under investigation from those that are determined by the interaction with its environment.