Estudo das propriedades óptico-térmicas em heteroestruturas de van der Waals (gipsita@MoS2) por Espectroscopia Raman e Fotoluminescência

Abstract

Van der Waals (vdW) heterostructures have been a crucial platform for exploring two-dimensional materials, offering opportunities for innovations in electronics, optoelectronics, and photonics. Among the most investigated two-dimensional (2D) heterostructures, the graphene-MoS2 system stands out for its enhanced modulation of trions due to charge transfer, while the hBN-MoS2 heterostructure enhances the photoluminescence (PL) of MoS2 by reducing roughness and providing electrostatic shielding. On the other hand, gypsum (CaSO4 · 2H2O) is a 2D material that remains relatively unexplored and could serve as an intriguing substrate for MoS2, given its phase transitions near ambient temperatures. Therefore, this dissertation aims to fabricate samples of gypsum@MoS2 bilayer (2L) heterostructure through mechanical exfoliation and the pick-up method, and to investigate their optical-thermal properties using Raman and PL spectroscopy. Optical measurements were conducted at room temperature (300 K), under thermal annealing (343 K), and in-situ (123 to 423 K). From the PL results, we observed that bulk gypsum reduces the intensity of excitons A and B in MoS2-2L compared to SiO2, attributed to the induction of non-radiative states from gypsum’s hydrated structure. Furthermore, the interaction of gypsum@MoS2-2L induces compressive strain that increases the energies of excitons A and B. In contrast, few-layer gypsum, with lower water content and greater structural flexibility, applies less strain, resulting in a lower exciton A energy closer to that on SiO2. Raman analysis showed that gypsum exhibits slightly higher temperature coefficients of the E2g and A1g modes compared to SiO2, indicating greater thermal sensitivity in its vibrational modes. The blueshift in the A1g and E2g bands of MoS2 on gypsum suggests p-type doping likely due to the presence of sulfate ions, reinforced by the significantly higher A1g/E2g intensity ratio on gypsum, characteristic of materials with excess positive carriers. Our results highlight the notable thermal properties of gypsum as a substrate, opening promising prospects for the application of gypsum@MoS2 heterostructure in nanophotonic devices controlled via strain and doping.