Seminário Geral 08/07/2022: High-pressure studies of atomically-thin van der Waals materials

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 (HP) 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 talk, I will describe how I addressed some of these challenges and I will discuss three different HP studies involving those systems using diamond anvil cells. In the first study, I will report on the electronic-band tuning and multivalley scattering at high pressures in monolayer MoS₂ and WSe₂ revealed by double-resonance Raman. Our work establishes the double-resonance 2LA and LA Raman bands as sensitive probes of strain-induced modifications to the electronic structure of TMDs. In the second study, I will detail the pressure-tuning of minibands in MoS₂/WSe₂ heterostructures revealed by moiré phonons– finite-momentum phonons from the individual layers downfolded to zone center through the moiré potential. In this work, we reveal that moiré phonons are associated with vertical electronic transitions between regions of high-joint density of states in the minibands, establishing moiré phonons as sensitive probes of the mini-band electronic structure and their modifications under external tuning parameters. For the last work, I will 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.