We will discuss our recent work on nanoscale engineering of semiconductors to enhance device performance and functionality. GaN interband tunnel junctions could improve the performance of GaN-based optoelectronic devices for lighting and laser applications, and are the critical component for many promising high-efficiency emitters and photovoltaic devices. However, achieving efficient tunneling in wide band gap III-nitrides has been a long-standing challenge, and we will discuss two approaches we have used to overcome it. Our first approach uses polarization engineering of III-nitride semiconductors to create large potential gradients at nanometer scales enabling the most efficient III-nitride tunnel junctions today. Our second approach exploits the small valence band offset in GdN/GaN interfaces to create direct hole injection into p-type GaN. We will then discuss our recent work on CVD growth of 2D layered metal dichalcogenide semiconductors such as MoS2 and WS2, where we have achieved large area single crystal thin films. We will discuss the structural and electronic properties of these films, as well as progress towards 2D/2D and 2D/3D semiconductor heterostructures.