Οπτικές και ηλεκτρονικές ιδιότητεςμερικών φύλλων γερμάνιου-σελήνιου

Atomically thin two-dimensional (2D) materials including graphene have attracted significant research interest due to their extraordinary physical properties. However, graphene is a zero-band gap material which in some cases is an undesirable property for optoelectronic applications. Transition metal dichalcogenides (TMDs) and some IV-VI compounds combine the 2D layered structure with a finite gap and therefore are alternatives of graphene. Single layers of TMDs have a direct-gap at the K-point of the Brillouin zone with a range of bandgaps in the visible spectrum, making them suitable for optoelectronic applications from light emitting diodes to light harvesting and sensors. An indirect-to-direct-gap transition occurs at the monolayer limit resulting in strong excitonic photoluminescence (PL) emission. Remarkable electrical properties of these materials have also been demonstrated in field effect transistors and new phenomena have arisen in the family of TMDs. In this thesis, we present a study of GeSe (IV-VI), a layered compound semiconductor that theoretical calculations reveal interesting electronic and optical properties. The predictions suggest that the crystal undergoes a direct to indirect energy bandgap transition at the limit of bilayer and monolayer. At the same time, the anisotropic spin splitting of the energy bands that comes from the interplay of spin-orbit coupling and lack of inversion symmetry renders this crystal optimal for spin related applications like spintronics. In addition, a monolayer of GeSe is found to be a strong absorber of visible light. We applied several exfoliation techniques with initial purpose to create a 2D layer of this crystal. However, GeSe is unstable in most of the situations we put it in and tends to change its phase in one or two phases of the more stable GeSe2. This change of phase motivated us to find certain conditions under which we can cause intentionally this phase change