In 2004, A. Geim and K. Novoselov realized the first isolation of a one-atom-thick material, graphene. The discovery of graphene, which was immediately followed by that of many other 2D materials with a large variety of physical properties, has paved the way to study physical properties in condensed matter systems down to the atomic limit. In this thesis, we investigate the properties of layered semiconducting compounds known as transition metal dichalcogenides. We investigate a peculiar effect known as the valley Hall effect, only present in the monolayers of these compounds, and gain an understanding on the detailed microscopic processes involved. We also report on the development of new gating techniques for atomically thin crystals using ionic gates that allow to apply extremely large electric fields perpendicular to the surface of the material. We demonstrate the closing of a 1.5eV semiconducting gap in atomically thin crystals thanks to this applied perpendicular electric field.