Ultrafast dynamics of exciton formation and decay in two-dimensional tungsten disulfide (2D-WS₂) monolayers

Abstract

Excitons in two-dimensional transition metal dichalcogenide monolayers (2D-TMDs) are of essential importance due to their key involvement in 2D-TMD-based applications. For instance, exciton dissociation and exciton radiative recombination are indisponsable...

Ultrafast nonequilibrium dynamic process of separate electrons and holes during exciton formation in few-layer tungsten disulfide

“…S1(d) in the ESM). Different from few-layer MoS 2 , 41 the transition bands of X A and X B are separated very well at the K-valley, 38 as shown in Fig. S1(c) in the ESM.…”

“…The detailed methods for ultrashort pulse generation and carrier density estimation were described in our previous work. 41 Compared to previous reports on exciton formation, [37][38][39] in which the carrier density is in a low regime (~1×10 12 cm -2 ), the carrier density in this work is in a high density regime, so as to increase the initial quasi-free carrier density of the electron-hole plasma and to enhance the many-body effects in carrier dynamics. All measurements are performed at 300 K. Excitation pump fluencedependence measurement was performed to verify that photoexcitation of the sample occurs in the one-photon regime (Fig.…”

“…The positive ΔA features are induced absorptions, which can be attributed to carrier-induced peak broadening or transitions to higher energy levels. 38 The dotted curves and the solid lines superimposed on the 2D plots in Figs. 2(a) and (b) are contour lines and first moment traces of the two GSBs, which will be used later to discuss the many-body effects on excitonic resonances.…”

“…1(a)) at the K-valley in 2D-TMDs, and have further verified that the formation time linearly increases with excitation photon energy. [37][38][39] A recent experiment evidenced that not all of the photogenerated charge carriers form excitons on a sub-ps time scale, indicative of a nonequilibrium state of the photoexcited electron-hole system. 39 Despite a few studies above, the dynamic picture of exciton formation is still unclear.…”

“…Thus the unbound quasi-free electrons and holes should play an important role during the formation process. However, the experiments above were all focused on exciton dynamics or combined dynamics of electrons and holes, [37][38][39] rather than individual dynamics of single type of carriers. Since the effective mass of electron and hole in semiconductors are usually different, [43][44][45] there should be obvious differences in the scattering probability and relaxation rate between them.…”