2-dimensional (2D) materials have stunned the scientific community and have triggered immense interest due to their unique optoelectronic properties. These properties strongly depend on the thickness of 2D layers and can be controlled by an externally applied voltage providing experimental “knobs” for property tuning. Possibilities of engineering properties by design expand when 2D materials are reassembled into heterostructures in a vertical stack. This thesis is devoted to novel optoelectronic phenomena in 2D materials and heterostructures. Here, we first solve obstacles with lattice mismatch and misalignment in van der Waals interfaces for efficient broad-spectrum optoelectronics. Then, we provide pathways to detect and identify different 2D layers when conventional techniques fail. Finally, we realize double ionic gated devices that enable the application of colossal electric fields to tune the band structure of atomically thin layers on the unprecedented eV scale.