Quantum-Enhanced Molecular Piezoresistivity

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Piezoresistivity, the change in electrical resistivity (conductivity) of a material when a mechanical strain is applied, is an important effect for the development of modern sensors. Devices with piezoresistive behaviour, that can be used to detect strain, pressure, acceleration and force, are used as sensors in many applications. Many of us carry a set of them in our pockets, as the accelerometers in our smartphones that detect orientation and movement are in fact based on the piezoresistive (or piezocapacitive) effect in silicon semiconducting microstructures. Other applications include vehicle technology (e.g. force sensors responsible for deploying the airbag), construction (e.g. to monitor the performances of pre-stressed concrete in bridges), robotics (e.g. tactile perception of hands and pincers in second-generation robots), hydraulics (e.g. pressure sensors to control release valves), toys (a notable example are the Nintendo consoles that include in their controllers sets of accelerometers and dynamometers) and health technology (e.g. sensory feedback in remote surgery, "smart" wearable health monitoring and drug delivery devices). This list is by no means exhaustive, as force sensors are among the most common type of sensors. Despite the fact that the piezoresistive effect was discovered more than 150 years ago, the development of such devices remains active and topical, especially with the current, constant need for miniaturisation and reduced power consumption.I propose here to develop a new kind of piezoresistive sensors, based on molecules as active components. Some molecules undergo a conformational change (a change in the relative position of the atoms in their structure) as they are compressed or stretched, and their electrical properties (conductance / resistance) change accordingly. This behaviour arises from effect unique to the nanoscale realm, where charge flows by quantum tunnelling, and results in extremely enhanced sensitivity to very small forces. Piezoresistive phenomena will be initially investigated at the single-molecule level, by fabricating single-molecule junctions (electrical devices made of 1 molecule only) employing nanomanipulation techniques in a scanning tunnelling microscope to identify the most promising structures. Few-molecules measurements will follow using an atomic force microscope, to extract force parameters and verify their suitability to be used in functional electronic sensors. Finally, prototype devices will be prepared by sandwiching a self-assembled monolayer (a 1-molecule thick layer) of flexible molecules between two metallic films, and their electrical properties under mechanical load/stress will be assessed. From a technological point of view, the final aim of the project is to develop ultra-thin (<100 nm, 1/1000 the thickness of a human hair), precise and sensitive force sensors that could be used in the applications mentioned earlier, where reduced size and enhanced performances are required. From a scientific point of view, the project will yield unprecedentedly detailed data about the mechanical behaviour of molecules, that will have impact in fields such as catalysis and polymer degradation.