Synthesis and integration of monolayer Transition Metal Dichalcogenides for piezoelectronics

Mechanical deformation and electrical potentials are coupled in piezoelectric materials, and these materials therefore can be used in energy harvesting and self-powered systems. However, the performance of three dimensional piezoelectric materials drops below a critical thickness due to cancellation of intrinsic piezoelectric effect by the effect of interfaces, limiting the suitablility of these materials for lightweight, ultrathin and highly flexible applications.
Non-centrosymmetric 2D materials with atomic thickness such as odd-layer transition metal dichalcogenides (TMDCs), hexagonal boron nitride and group IV monochalcogenides, etc. have been theoretically predicted to have piezoelectricity. While some have been confirmed experimentally, achieving a high energy conversion ratio, large-scale synthesis with high quality and non-destructive transfer are still challenges for application.
The thesis explores new methods for large-scale synthesis of monolayer TMDCs, optimization of transfer technology and documents the testing of piezoelectric effects in flexible TMDCs device. For exfoliation of large area samples, since PVA has a strong interaction with TMDCs, I show that PVA foil lamination can be used to isolate monolayer TMDCs with 200-1000 μm² and high room-temperature mobility and off/on ratios in air. Second, Ni was added as a catalyst to improve monolayer MoS₂ synthesis in interfacial limited growth thanks to edge modification of Ni-Mo-S. The grain size and coverage ratio of MoS₂ was increased and CrS₂ was synthesized successfully, and is predicted to have high piezoelectric coefficient and stiffness.
A new set of techniques adapted from the 2D materials tool kit was developed for handling the transfer of thin transition metal oxide membranes, enabling millimeter-scale few nm thick transition metal oxide membrane transfer onto arbitrary substrates, and twisting of stacked oxides with a controlled angle was demonstrated for the first time. In addition, new techniques for making metal contacts that can be transferred by wedging, support a simple route for fabricating flexible devices. Finally, a custom-made piezoelectric characterization system was built, where noise and spurious signals originating from the substrate, circuit and electrometer were analyzed and reduced or eliminated. The piezoelectric effect of flexible device based on Au-exfoliated MoS₂ was measured but no unambiguous piezoelectric response was observed. The reason might be roughness of the substrate, low quality of MoS₂ and contamination from the transfer process.