Large range modification of exciton species in monolayer WS_2

Abstract

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.

Probing exciton species in atomically thin WS2–graphene heterostructures

“…With few deviations, the energy difference between the exciton and trion for WS 2 /2LG remains constant, suggesting that the trion dissociation in monolayer WS 2 /graphene is independent of temperature, consistent with observations in previous studies of WS 2 on Si [5,32,35].…”

“…The temperature-dependence of the PL intensity for WS 2 /1LG and WS 2 /2LG is presented in figure 6. An initial increase in PL intensity is observed for excitonic peaks of WS 2 /1LG and WS 2 /2LG, in the 183-293 K temperature range, followed by a gradual decrease of PL intensity as the temperature decreases further to 83 K. A decrease of the neutral exciton intensity with the decrease in temperature was previously observed for WS 2 and WSe 2 and attributed to a low-lying dark state that quenches light emission with decreasing temperature [32,36,37]. Typical separation value between dark and bright excitonic states observed for WSe 2 and WS 2 is ∼50 meV [38], whereas the separation value observed in this study is 8 meV for the decrease in intensity from 183 to 153 K. However, the separation values observed in the experiment may not accurately reflect the separation between the dark and bright states due to competing non-radiative decay channels which exhibit strong temperature-dependence or non-equilibrium population for the two states [36].…”

“…The coupling constant S appears similar for all three cases (see table 1), however the extracted values are smaller than previously reported for WS 2 : e.g. S=2.4 [32], perhaps due to the interaction with graphene. The average phonon energy for WS 2 /1LG and WS 2 /2LG exciton compares well with previously reported values, e.g.…”

“…The sublinear trend is slightly more pronounced for WS 2 /2LG, suggesting that, in addition to trion generation due to doping by the graphene substrate, donor/acceptor states within the WS 2 band gap can also contribute to trion formation through photoionisation and subsequent coupling to free excitons. Photoionisation effects were previously shown to lead to trion formation in WS 2 [32,41]. However, the slightly sublinear trend observed here suggests that photoionisation effects are not the dominant factor in trion formation for WS 2 /2LG.…”

Controlled growth of transition metal dichalcogenide monolayers using Knudsen-type effusion cells for the precursors

“…Similarly, a PL spectrum obtained on a WS 2 crystal, figure 6(g), displays the excitonic peak at 643 nm [9]. A smaller peak at 665 nm is seen as a shoulder of the A-exciton peak is due to trion excitation [48]. The PL maps obtained from both MoS 2 and WS 2 crystals are presented in figures 6(f) and (h), respectively.…”