Abstract
The successful mechanical exfoliation of monolayer graphene and the characterization of its outstanding electronic properties not only brought the 2010 Nobel Prize in Physics, but also opened up new areas of research on two-dimensional (2D) materials. Numerous 2D materials have been exfoliated or synthesized, forming a family of 2D materials covering conductors (e.g. graphene), semiconductors (e.g. transition metal dichalcogenides (TMDs)), and insulators (e.g. h-BN). The unique atomically thin structure endows them excellent physical and chemical properties, and has broad application prospects in mechanical, electrical, optical and many other functional fields. The magic angle graphene superconductivity was reported in 2018 and selected as Physics World Breakthrough of The Year Award Winner, igniting the passion for investigating 2D moiré materials. Graphene, known as one of the strongest materials, has attracted worldwide attention for the characterization of its mechanical properties. Hexagonal boron nitride (h-BN), the most ideal insulator substrate for all other 2D materials, and MoS2, the most promising TMD, are also required to uncover their responses to mechanical deformation. However, as the most comprehensive mechanical characterization method, direct tensile test is still a big challenge for these monolayer 2D materials due to technique limit of manipulating atomically thick 2D materials.Besides, the elastic strain engineering (ESE) and deep elastic strain engineering (DESE) have been proved to be an effective strategy to modulate physical and chemical properties of crystal materials who possess great elasticity. The superior stretchability of 2D materials originated from atomical thickness and in plane covalent bonds makes them ideal candidates for ESE even DESE applications. What’s more, the unique structure of 2D van der Waals (vdW) materials provides various degrees to tailor their properties including stacking order, building blocks, thickness, and twist angle especially, which has opened up a new field so called “twistronics”. All these features can be coupled with ESE even DESE to tailor the properties of 2D materials further. However, introducing the large uniform strain into 2D materials for ESE and DESE is still challenging in experiments.
Thus, the thorough understanding of 2D materials’ mechanical properties and the success of introducing large uniform strain in relatively large area 2D materials are essential for their reliable device applications, modulating properties in wide range, as well as inducing exotic features in 2D materials by multi-strategies synergistic modulation. Here, the mechanical behaviors of monolayer graphene are comprehensively investigated based on our developed in situ quantitative micro-/nano-mechanical platform. Especially, its nonlinear elastic mechanical properties under large strain are firstly uncovered by direct tensile tests via synergistic optimal design of sample geometry and local strength. The extracted nonlinear parameter provides useful data for theoretical simulations of graphene under large strain. The defects’ impact to mechanical properties of h-BN is also explored by direct tensile tests and its defects tolerance is summarized by multiscale simulations based on our experimental data. Coupled with DFT simulation, the large strain achieved in monolayer h-BN provides large polarization in it for its piezoelectrical applications. Then, via modifying the transfer method, the elastic limit of MoS2 is approached, which is comparable to local maximum strain in AFM-nanoindentation. And the preliminary result for modulating bandgap of MoS2 is demonstrated. Finally, the twisted bilayer graphene is successfully fabricated and demonstrated with large elastic properties, which can be used to modulate the moiré pattern in it for synergistic modulation of ESE and twistronics.
In summary, the research work in this thesis not only provides comprehensive understanding of mechanical properties of monolayer 2D materials, including elastic mechanical properties, fracture behaviors, and defect tolerance, but also demonstrates great potential of monolayer 2D materials and twisted bilayer graphene in ESE, DESE and multi-strategies synergistic modulation. We aim to pave a pathway to realize the continuous, reversible, and wide range modulation of large area 2D material’s properties via multi-strategies synergistic modulation for functional applications in multi-fields.
| Date of Award | 29 Jul 2021 |
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| Original language | English |
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| Supervisor | Yang LU (Supervisor) |