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Nanomechanics in van der Waals Heterostructures
[Thesis]. Manchester, UK: The University of Manchester; 2018.
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Abstract
Due to the small lattice mismatch, graphene crystallographically aligned to hexagonal boron nitride (hBN) forms an hexagonal moire superlattice whose periodicity depends on the relative crystal orientation. Near perfect alignment, the periodicity of the superlattice is ~13.9nm with central regions characterised by the graphene lattice stretching to fit the potential of the underlying hBN. Small out-of-plane deformations occurring at the edges of the superlattice accumulate this built up strain. These two van der Waals materials are the subject of study in this thesis due to their large intrinsic strengths (70.5 - 130GPa) and Young's moduli (0.865 - 1TPa). Frictional forces between hBN and graphene flakes are increased as atoms are pinned near the potential trough. In the language of the Frenkel-Kontorova model, graphene is said to be in the commensurate state separated by out-of-plane kinks/- solitons. Potential barriers to kink motion are much lower than those seen by individual commensurate atoms. By introducing new kinks or destabilizing those present, flake motion may be possible using minimal forces. It is these kink excitations that are the topic of study in this thesis. New fabrication procedures for producing complex suspended van der Waals heterostructures are an important additional aim of this project. They will help push the boundaries of nano-fabrication in the realm of two dimensional materials. Aligned graphene/hBN devices are fabricated incorporating a suspended region. An electrostatic force is applied to the suspended region and used to impart a lateral force to the graphene/hBN stack. In-situ longitudinal resistance measurements of a 2 um2 device identify the need for reduced sample sizes or alternative higher resolution characterisation methods. High resolution atomic force microscopy measurements are used to track the changes of the superlattice period under increasing loads in a 300nm scan window. However, thermal drift becomes an acute adversary that prevents conclusive verdicts on the nature of kink motion. Scanning tunnelling microscopy measurements afford the most promise and although their are important challenges to overcome, namely accurately positioning the tip, offers the best opportunity of studying graphene/hBN kinks.