Transition-metal dichalcogenide (TMD) monolayers are atomically thin two-dimensional (2D) semiconductors with the direct bandgap providing ideal platforms to study the interaction between light and confined excitons. Due to the 2D nature and strong Coulomb interaction resulting in large binding energies, these TMD monolayers host various exciton complexes of not only neutral excitons also charged excitons (trions) and biexcitons, routinely observable in room temperature. Using nanoscale optical imaging and spectroscopy, we will show that these exciton complexes coexist competing for the light emission, which largely depend on the surface conditions such as local defects or excess charge populations and the enhanced optical properties can be engineered by heterostructures of TMD monolayers.