  In the last decade atomically thin 2D materials have emerged as a perfect
platform for studying and tuning light-matter interaction and electronic
properties in nanostructures. The optoelectronic properties in layered
materials such as transition-metal-dichalcogenides (TMDs) are governed by
excitons, Coulomb bound electron-hole pairs, even at room temperature. The
energy, wave function extension, spin and valley properties of optically
excited conduction electrons and valence holes are controllable via multiple
experimentally accessible knobs, such as lattice strain, varying atomic
registries, dielectric engineering as well as electric and magnetic fields.
This results in a multitude of fascinating physical phenomena in optics and
transport linked to excitons with very specific properties, such as bright and
dark excitons, interlayer and charge transfer excitons as well as hybrid and
moir\'e excitons. In this book chapter we introduce general optoelectronic
properties of 2D materials and energy landscapes in TMD monolayers as well as
their vertical and lateral heterostructures, including twisted TMD hetero- and
homobilayer bilayers with moir\'e excitons and lattice recombination effects.
We review the recently gained insights and open questions on exciton diffusion,
strain- and field-induced exciton drift. We discuss intriguing non-linear
many-particle effects, such as exciton halo formation, negative and anomalous
diffusion, the surprising anti-funneling of dark excitons.
