Mechanics and Thermal Transport in Low-Dimensional Heterostructures and Applications to Sensors

The immense growth of skin-like electronic devices based on two-dimensional (2D) heterostructures is considered to revolutionize technologies of communication, health, and fitness for grand applications in healthcare monitoring and human-machine interfaces. The large deformation exerted to the devices that are required to accommodate the complex human motions while maintaining a conformal attachment to human skin is a current bottleneck in thermal management. Understanding of fundamental thermal transport in response to large mechanical loading conditions is expected to lay the foundation for controllable thermal management in these devices.

This dissertation presents the fundamental understanding of thermal transport in 2D nanomaterials and heterostructures in response to an externally large mechanical loading and proposes a novel measuring principle for mechanical sensors based on the mechanics-thermal coupling.

Specifically, in Chapter 2 the thermal responses in 2D materials and their corresponding heterostructures subjected to loadings are systematically introduced. A small mechanical loading within the stretchability of materials can alter the geometric features, influence the heat transfer path and the effective conducting area of the material, leading to the change in thermal transport. A large external mechanical stimulation beyond the stretchability can intrigue lattice deformation and thus directly coordinate with the phonon properties, intriguing dramatic variation in thermal transport capabilities. Extensive calculations on mechanical, thermal transport and phonon properties support these findings.

To elucidate the competition mechanism of thermal transport under intrinsically small mismatch-induced strain and externally applied mechanical loads, several types of heterostructures with either covalent bonded or van der Waals (vdW) heterojunctions, are discussed in Chapter 3 and 4, supported by extensive analyses of stress, deformation, geometric morphology, phonon activities and atomic interactions. Generalized models that take into account structural features, mechanical deformation and thermal activities in heterostructures were proposed to quality thermal properties.

In Chapter 5 and 6, two types of mechanical sensors that both rely on thermal responses to mechanical loading were proposed and exemplified: a pressure sensor capable of sensing, locating and mapping pressure loadings; a multifunctional sensor capable of sensing and differentiating modes of mechanical stimuli, including tension, compression, bending and external pressure.

These results and new findings provide a firsthand fundamental understanding of thermal transport in deformed nanomaterials and immediate guidance to the development of stretchable devices whose thermal management could be mechanically tunable. Besides, they will help open a new route for the exploration of future devices by leveraging unique thermal properties of materials, thereby strategically extending design solutions of mechanical sensors from electrical resistance to thermal transport-based responses.