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

Dr. Luiz Gustavo Pimenta Martins (Universidade de Harvard)

Resumo:

Two-dimensional (2D) materials and Moiré superlattices formed by certain stacking configurations of 2D crystals, represent a new frontier for quantum matter research due to 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 different HP studies involving those systems using diamond anvil cells. In the first study [Nature Nanotechnology (2023) Accepted], I will detail the pressure-tuning of minibands in MoS₂/WSe₂ heterostructures revealed by moiré phonons: Raman-inactive phonons from the individual layers that are activated by the moiré potential. Moiré phonons manifest as satellite Raman peaks appearing exclusively in the heterostructure region, increasing in intensity and frequency under applied pressure. Our theoretical analysis reveals that their scattering rate is directly connected to the moiré potential strength. By comparing the experimental and calculated pressure-induced enhancement, we obtain numerical estimates for the moiré potential amplitude and its pressure dependence. This work establishes moiré phonons as a sensitive probe of the moiré potential and of the electronic structure of moiré systems. In the second study [ACS nano 16.5 (2022): 8064-8075], I will report on the electronic-band tuning and multivalley scattering at high pressures in monolayer MoS₂ and WSe₂ revealed by double-resonance Raman. This work establishes the double-resonance 2LA and LA Raman bands as sensitive probes of strain-induced modifications to the electronic structure of monolayer TMDs. Time allowing, I will present experiments detailing the first spectroscopic evidence for the pressure-induced formation of 2D diamond from the compression of few-layer graphene [Nature communications 8.1 (2017): 96, Carbon 173 (2021): 744-757].