Modulation of optoelectronic properties of layered two-dimensional materials by strain, twist and dielectric engineering

Abstract

Since Sir Andre Geim and Sir Kostya Novoselov successfully isolated graphene in Manchester in 2004, numerous layered two-dimensional (2D) materials have been uncovered. Among these, transition metal dichalcogenides (TMDs) (WS2, MoS2) and group-IV monochalcogenides (GeS) have garnered significant attention recently because of their distinctive electrical and optical characteristics. Several external elements, such as the surrounding dielectric medium, mechanical strain, and twisting, can greatly affect these properties. The dielectric medium in the vicinity can impact charge carrier screening and exciton creation, while mechanical strain can be employed to adjust the material’s band structure. Alternatively, a moiré pattern may develop when two layers of 2D materials are rotated relative to one another. This twist-induced superlattice can dramatically alter the electronic properties of the material. This thesis explores the utilization of these external perturbations to uncover novel optoelectronic properties in layered 2D materials. First, the tensile strain-induced brightening of momentum-forbidden ‘dark’ exciton in monolayer WS2 by placing them on nanotextured substrates will be discussed. This is shown by photoluminescence and Raman measurements, supported by theoretical calculations. Excitons and trions are quasiparticles that arise due to the interaction of electrons and holes in semiconductors. Further, the modulation of exciton and trion formation in monolayer WS2 by dielectric and substrate engineering will be discussed. We conducted a systematic investigation of surface defects by progressively increasing the distance between the WS2 monolayer and the substrate. This approach allowed us to accurately adjust the exciton and trion contributions in the photoluminescence (PL) spectra of WS2. Additionally, excitation power-dependent measurements on dielectric-engineered and patterned substrates provided insights into the mechanism behind PL modulation in monolayer WS2. Next, the emergence of ferroelectricity due to a twist in germanium sulfide nanowires will be discussed. GeS is a material with inversion symmetry and therefore ferroelectricity is forbidden. The production of twisted nanowires using low-pressure chemical vapour deposition (LPCVD) will be covered. In this regard, the details for the development of a custom-built LPCVD system will also be discussed. The electrical measurement, piezoelectric force microscopy (PFM), second-harmonic generation (SHG) spectroscopy, and theoretical results that confirm the presence of unconventional ferroelectricity in twisted GeS nanowires will also be discussed in the thesis. Furthermore, I will discuss our most recent findings on electronic transport in the graphene-MoS2 heterostructure and provide an account of an inexpensive, custom-built transfer stage system crucial for fabricating 2D van der Waals heterostructures. The thesis wraps up by highlighting the major experimental findings and suggesting some potential avenues for future research based on these results.