Transient absorption of transition metal dichalcogenide monolayers studied by a photodope-pump-probe technique

We report three-pulse photodope-pump-probe measurements on photocarrier dynamics in semiconducting transition metal dichalcogenide monolayers of ${\mathrm{MoS}}_2$, ${\mathrm{WS}}_2$, ${\mathrm{MoSe}}_2$, and ${\mathrm{WSe}}_2$. The samples are fabricated by metal-organic chemical vapor deposition and mechanical exfoliation techniques and characterized by photoluminescence spectroscopy. In the time-resolved measurement, the samples are first photodoped by a prepulse, which injects background photocarriers of various densities. A pump pulse then injects photocarriers, whose dynamics is monitored by measuring a differential reflection of a time-delayed probe pulse. We found that the ultrafast decay component of the differential reflection signal, which has been widely reported before, shows minimal dependence on the background exciton density. This observation shows that a previously suggested carrier-trapping model cannot account for this component. The results thus further support an exciton-formation model that was previously proposed based on spectroscopic evidence.

Figure 1

Schematics of the three-pulse photodope-pump-probe setup.

Figure 2

Photoluminescence spectra of monolayers of ${\mathrm{MoS}}_2$ (a), ${\mathrm{WS}}_2$ (b), ${\mathrm{MoSe}}_2$ (c), and ${\mathrm{WSe}}_2$ (d) under the continuous-wave excitation of 3.06 eV.

Figure 3

Schematics of the photocarrier injection and dynamics. (a) A prepulse excites electrons (solid circles with $-$) and holes ($+$) at $-4$ ps. (b) The electron-hole pairs excited by the prepulse thermalize and form excitons. Some of the carriers might be captures by the trap states (dashed circles). (c) At $t=0$ ps, a pump pulse injects electron-hole pairs again to the photodoped sample. (d) The dynamics of the pump-injected photocarriers is monitored by a probe pulse tuned to the exciton resonance.

Figure 4

Differential reflection signal of ${\mathrm{WS}}_2$ monolayer under the excitation of the pump (red circles) or the prepulse (blue squares) alone, with the other blocked. The photon energy of the prepulse and pulse is 3.12 eV. The probe photon energy is 2.01 eV.

Figure 5

Differential reflection signal measured with the photodope-pump-probe techniques from the TMD monolayers of ${\mathrm{MoS}}_2$ (a), ${\mathrm{WS}}_2$ (b), ${\mathrm{MoSe}}_2$ (c), and ${\mathrm{WSe}}_2$ (d), respectively. Different symbols represent results with different background exciton densities injected by the prepulse. Other experimental conditions are labeled in each panel. The sample is at room temperature. Parts (e)–(h) are the same as (a)–(d) but are normalized to shown their similarity.

Figure 6

Parameters describing the ultrafast decay component of the differential reflection signal shown in Fig. 5, from monolayers of ${\mathrm{MoS}}_2$ (a), ${\mathrm{WS}}_2$ (b), ${\mathrm{MoSe}}_2$ (c), and ${\mathrm{WSe}}_2$ (d), respectively. Top panels are the peak differential reflection signal. Middle panels show the ratio of the differential reflection signal at 3 ps to the peak signal, that is, the amount of the signal drop due to the ultrafast decay component. The bottom panels are the decay time constant as a function of the background exciton density.