Phase and dissipative behaviour of complex 2D materials

Abstract

Being a relatively new field, the vast chemical space spanned by two-dimensional materials has so far only seen limited exploitation to optimise properties. In the field of tribology, layered materials have been used for decades as solid lubricants, but the design of optimised surfaces for friction in a systematic way, which relies on a predictive model of friction that scales from the nanoscale up to micro/macro-scales, remained elusive. This thesis makes a step in exploring the chemical space and stacking possibilities of two-dimensional materials, informing experimental efforts to design tribological contacts. Moreover, it extends the fundamental understanding of lubrication mechanisms, on which the design of solid lubricants can be built.

The materials design aspects of this thesis focus on the phase behaviour of monolayers and the inter-layer phase stability of twisted heterostructures. A framework was developed to identify novel compounds obtained from substitutional alloys on transition metal sites in dichalcogenide crystals, yielding a road-map for experimental efforts. A generalised theory of epitaxy developed here rationalises the behaviour of heterostructures observed as a function of the twist angle introduced between the lattices.

To advance the fundamental understanding of solid lubrication, the dissipative behaviour of two-dimensional materials has been studied both in hetero- and homo-structures. Super- lubric behaviour in large-mismatched heterostructures has been rationalised in terms of edge effects and identified as a promising road to robust superlubricity. A novel framework, based on the Prandtl-Tomlinson model and stochastic thermodynamics, allows evaluation of a set of thermodynamic quantities in homostructures, providing a clear picture of the dissipation in the system and a straightforward way to test the model against experiments.

In conclusion, the phase behaviour analysis shows that alloyed transition metal dichalco- genides offer many opportunities for material science research, thanks to the densely populated chemical space. Flexural phonons play an important role in the stability of misaligned heterostructures, with implications for novel twistronic devices. The analysis of tribological contacts highlights the importance of potential energy surface topology, in addition to simple energy barriers, and of edge effects in determining the dissipative behaviour of the system.

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Identifiers

ORCID for Andrea Silva: orcid.org/0000-0001-6699-8115

ORCID for Tomas Polcar: orcid.org/0000-0002-0863-6287

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