Large range modification of exciton species in monolayer WS₂.

Unconventional emissions from excitons and trions in monolayer WS₂ are studied by photoexcitation. When excited by a 532 nm laser beam, the carrier species in the monolayer WS₂ are affected by the excess electrons escaping from photoionization of donor impurity, the concentration of which varies with different locations of the sample. Simply by increasing the excitation power at room temperature, the excess electrons and, thus, the intensity ratio of excited trions and excitons can be continuously tuned over a large range from 0.1 to 7.7. Furthermore, this intensity ratio can also be manipulated by varying temperature. However, in this way, the resonance energy of the excitons and trions shows redshifts with increasing temperature due to electron-phonon coupling. The binding energy of the trion is determined to be ∼26  meV and independent of temperature, indicating strong Coulomb interaction of carriers in such 2D materials.