High-pressure studies of atomically-thin van der Waals materials

Abstract

Two-dimensional (2D) materials and Moiré superlattices formed by certain stacking configurations of 2D crystals, represent a new frontier for quantum matter research due the emergent properties associated to their reduced dimensionality and tunability. To glean insight into the physics of these atomically-thin van der Waals materials, their properties have been extensively studied by tuning of external parameters such as temperature, electrostatic doping, magnetic field and strain. However, there is an external tuning parameter that has not been used systematically in studies of these systems – pressure. The relative scarcity of high-pressure studies involving atomically-thin materials is due to experimental challenges, e.g., loading of micron-sized samples into the also micron-sized pressure chamber. In this thesis, I address those issues and I investigate 2D materials and Moiré heterostructures via high-pressure optical-spectroscopic experiments using diamond anvil cells (DACs), with two main goals: (i) investigating the synthesis of novel 2D materials; and (ii), tuning and probing the electronic properties of 2D materials and Moiré heterostructures.

To address the first point, I present experiments detailing the first evidence for the formation of a hard, transparent, sp³-containing 2D phase by compression of few-layer graphene, providing robust corroboration for the existence of 2D diamond. For the second point, I present two studies. In the first study, I report on the electronic-band tuning and multivalley scattering at high pressures in monolayer MoS₂ and WS₂ revealed by double-resonance Raman. The ability to probe the modifications in the band structure and multivalley scattering as a function of strain shall advance our understanding of different multivalley phenomena in transition metal dichalcogenides such as superconductivity, valley coherence, and valley transport. In the second study, I detail the pressure-tuning of minibands in MoS₂/WSe₂ heterostructures revealed by moiré phonons– Raman silent q ̸= 0 phonons from the individual layers activated by the moiré potential. In this work, we establish Moiré phonons as a sensitive probe of the mini-band electronic structure and their modifications under hydrostatic strain in this system, which is poised to be essential in understanding the emergent phenomena observed in similar Moiré systems.