Synthesis and engineering of 2D materials and their heterostructures : probing nanomechanical and nanoelectrical phenomena

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Δημητρόπουλος, Μαρίνος

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In the previous couple decades, the surge of two-dimensional materials (2DM) has taken the world by storm and became a new research hotspot for exotic physical phenomena. Notable efforts have been made in the synthesis and engineering of these materials and their heterostructures with emphasis given in their industrial integration. However, in order to fully understand the extent of their unusual properties, characterization with nanometer precision became a necessity. Therefore, in order to incorporate 2DM in the application of interest, one must first understand the full range of their nanoscale attributes. Furthermore, thanks to their characteristic exceptional mechanical strength and flexibility, they can provide an ideal platform for wrinkle engineering, enabling tunable modulation and remarkable improvement of their properties. Purposely introducing and manipulating topological disorder is expected to yield significant degree of freedom in the design of devices. The present thesis showcases the importance of nanoscale manipulation of 2DM properties through wrinkling and entails synthesis and wrinkle engineering of 2DM and their heterostructures. Flexible control over wrinkling was achieved by either applying external stimuli or transferring the 2DM on top of pre-patterned substrates for guided localized strains. Nondestructive spectroscopic and microscopic characterization tools were harnessed to quantitatively determine strain-engineered alterations in these properties. The nanoelectrical and nanomechanical properties of the engineered materials were probed extensively by the means of Kelvin Probe Force Microscopy for nanoscale work function, Tunneling Atomic Force Microscopy for current distribution, Nanoindentation for fracture and Lateral Force Microscopy for friction measurements. In the first two chapters, the framework of the thesis is laid out by making an introduction on the world of 2DM and exploring the extent of nanomechanical and nanoelectrical properties for graphene as well as wrinkle engineering pathways, respectively. The third chapter involves the characterization tools necessary for the investigation of 2D properties, such as Atomic Force Microscopy (AFM), Raman spectroscopy and Photoluminescence. For AFM specifically, working principles and calibration protocols for the modes adopted in this thesis, are presented. In chapters four and five, the effect of graphene wrinkles is investigated in terms of their impact on nanoscale properties. It is found that the wrinkle path presents ultrahigh electrical conductivity on top of the corrugation by up to two orders of magnitude compared to flat areas and, depending on the substrate, nanoscale work function engineering can be achieved through wrinkle tuning. Furthermore, nanoindentation experiments on suspended wrinkled graphene appears to be suppressing crack propagation and induce a localization effect on the fracture of the membranes. In chapters six and seven, the prospect of wrinkle engineering in heterostructures is presented. By taking graphene/hBN as an example, the wrinkle density of the 2D sheets arising naturally from the Chemical Vapor Deposition (CVD) process can be exploited as a synthesis scaffold for direct graphene growth at wafer scale. Lastly, graphene wrinkles are tested as pre-patterned substrates for band gap tuning of MoS2, denoting a significant increase in strain engineering range with minimal external thermal treatment. Concluding in chapter eight, future research directions will be provided in the controlled patterning of 2DM with other means such as lasers, the ability of Atomic Force Microscopy as a tool for manipulating atomically thin flakes for twistronics will be highlighted and unidirectional graphene wrinkles from liquid metal CVD could be harnessed for guided electron flow.



Atomic Force Microscopy (AFM), 2D materials, Wrinkle engineering, Nanomechanical properties, Nanoelectrical properties