  Unconventional emissions from exciton and trion in monolayer WS2 are studied
by photoexcitation. Excited by 532nm 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 increasing the excitation power at
room temperature, the excess electron and thus the intensity ratio of excited
trion and exciton 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 exciton and trion
show red-shifts with increasing temperature due to electron-phonon coupling.
The binding energy of the trion is determined to be ~23meV and independent to
temperature, indicating strong Coulomb interaction of carriers in such 2D
materials.
