Thesis title: Van der Waals materials under high pressure: an optical spectroscopy study from bulk to monolayers
Van der Waals (vdW) materials are among the most studied systems in Condensed Matter Physics research as their layered, anisotropic crystal structure results in a wide variety of physical properties and offers the unique possibility to scale vdW crystals down to the two-dimensional (2D) limit by isolating monolayers (1L). Current research on vdW materials is moving towards the controlled tuning of their physical properties, as it would represent a significant achievement for practical applications. However, achieving this goal requires a deep understanding of the physics at play in these systems.
This thesis investigates the possibility of tuning the physical properties of bulk and monolayer vdW materials by applying pressure. Pressure is a clean and direct strategy to reduce the inter-atomic distances within crystals, resulting in variations of their vibrational, optical and electronic properties, that we examined by employing different optical spectroscopy techniques, specifically photoluminescence (PL), Raman and traditional and photothermal infrared spectroscopy. We focused our high-pressure investigation on two different vdW crystals: the semiconducting Transition Metal Dichalcogenides (TMDs) and the insulating hexagonal Boron Nitride (hBN), promising candidates for low-dimensional heterostructures and miniaturized electronics and optoelectronics.
Starting from an introduction to TMDs and hBN and to the state-of-the-art of high-pressure research on these crystals, this thesis presents our experimental results, offering novel insights into the high-pressure evolution of TMDs and hBN. Indeed, our high-pressure experiments on TMDs enabled us to observe the significant impact of the n-type doping in the pressure evolution of both bulk and 1L-TMDs at the GPa scale, leading to an early metallization of bulk crystals, signalled by our high-pressure far-infrared spectroscopy measurements, and to the presence of trion recombination in the high-pressure PL spectra of 1L-TMDs. Furthermore, by studying 1L-TMD domes under pressure by Raman and PL spectroscopies, we found that the interplay of pressure and strain leads to a unique nonlinear broadening of the monolayer bandgap, suggesting a hybridization of electronic levels predicted in planar 1L-TMDs at much higher pressure. We then focused on the pressure evolution of the vibrational properties of hBN, which was studied by optical photothermal infrared (OPTIR) spectroscopy. These measurements represent the first attempt at sub-diffraction limit resolved IR spectroscopy in the high-pressure regime, and our results demonstrate its feasibility by tracking the pressure-induced hardening of the E1u phonon of hBN and the hexagonal-to-wurtzite phase transition. Concurrently, we observed a peculiar pressure trend in the OPTIR intensity, evidencing the need for further experimental and theoretical investigations to gain a deeper understanding of the physics of photothermal spectroscopy.
These findings bring valuable contributions to the understanding of the pressure-induced evolution of TMDs and hBN, advancing the current knowledge in this area while providing novel spectroscopic signatures and experimental strategies, paving the way for further exploration of low-dimensional systems under pressure.