Ultrafast time-resolved investigations of excitons and biexcitons at room temperature in layered WS ₂

Abstract

Strong light-matter interactions in layered transition metal dichalcogenides (TMDs) open up vivid possibilities for novel excitonic quasiparticle-based devices. The optical properties of TMDs are dominated mostly by the tightly bound excitons and more complex quasiparticles, the biexcitons. Instead of physically exfoliated monolayers, the solvent-mediated chemical exfoliation of these 2D crystals is a cost-effective, large-scale production method suitable for substantial practical implications. Here, we explore the ultrafast excitonic phenomena in layered WS2 (mono-to-quad) dispersion using broadband (350–750 nm) femtosecond pump-probe spectroscopy at room temperature (300 K) which are inaccessible to the steady-state absorption or emission spectroscopy. The transient absorption spectra (TAS) suggest that the mono-to-quad layered dispersion of WS2 has similar spectral features as monolayer WS2 in terms of saturation absorptions (SA) and excited state absorptions (ESA). Similar to monolayer TMDs, we are able to identify excitons and biexcitons in multi-layered 2D stratum of WS2 as well as calculate the biexciton binding energies ( 69 meV and 66 meV), which are in excellent agreement with earlier theoretical predictions. Furthermore, using many-body physics, we demonstrate that the excitons in layered WS2 behave like Wannier–Mott excitons and explain their origins via first-principles calculations. Our detailed time-resolved investigation provides ultrafast radiative and non-radiative lifetimes of the excitons and biexcitons in layered WS2. Indeed, our results unravel the complex optical response of layered TMDs, which should lead to numerous technological applications for developing excitonic quasiparticle-based valleytronic devices and ultrafast biexciton lasers at room temperature.

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

“…In transient absorption measurements of 2D TMDs, one commonly observed feature is an ultrafast decay process that causes loss of about half of the signal in about 1 ps. This is remarkably consistent across different TMDs studied, including MoS 2 [11,25,53,54], WS 2 [27,28,55,56], MoSe 2 [24], and WSe 2 [22], with samples fabricated by different groups and with different techniques. This process shows no apparent dependence on the temperature [27] or the dielectric environment [24] of the samples.…”

Enhanced Trion Emission and Carrier Dynamics in Monolayer WS2 Coupled with Plasmonic Nanocavity

“…The fastest component (τ 1 ) of the recovery process could be assigned to Auger recombination or exciton–exciton annihilation (indicated by the red arrow labeled by Auger (τ 1 ) in Figure 3G), the faster component (τ 2 ) could be assigned to the nonradiative decay of trions (indicated by the blue arrow labeled by trap states (τ 2 ) in Figure 3G), and the slowest component (τ 3 ) could be assigned to the radiative recombination of trions (indicated by the orange arrow labeled by PL (τ rise ) in Figure 3G). [ 48 ] Triple exponential function was used for fitting trion dynamic, for the WS 2 off‐cavity, the obtained three components are τ 1 = 26.82 ± 0.88 ps, τ 2 = 267.74 ± 42.89 ps, and τ 3 = 3979.18 ± 252.93 ps, respectively; for the WS 2 in‐cavity, the related trion dynamic components are τ 1 = 18.11 ± 1.85 ps, τ 2 = 86.83 ± 38.49 ps, and τ 3 = 698.45 ± 243.45 ps. Compared with the trion dynamics of WS 2 off the plasmonic nanocavity, all three parameters, including τ 1 , τ 2 , and τ 3 get much shorter on the plasmonic nanocavity.…”

“…Figure 3A,B shows the differential reflection spectrum as a 2D color contour plot for the WS 2 off and on plasmonic nanocavity, excited with a 520 nm laser pulse. In the transient reflection spectrum, the photoinduced bleaching peaks at the 1.98 eV (negative band) can be attributed to the A-exciton, [48][49][50][51] which is the result of transient bandgap renormalization and state-filling effect. [52] During the A-exciton formation process, the hot carriers are generated in the conduction and valence bands once the sample is excited by the pump beam, then the hot electrons relax to the conduction band edge in the pulse width-limited time and hot holes relax to the lower valence band in the timescale of hundreds of femtoseconds.…”

Visualizing Hot‐Carrier Expansion and Cascaded Transport in WS2 by Ultrafast Transient Absorption Microscopy

“…It shows a biexponential decay dominated by the slow component, coincident with that of the PB feature in the whole measurement window (see Figure S4 , Supporting Information), implying that the low‐energy PA feature shares the same excited‐state source as the PB feature (i.e., band‐edge excitons). Except for the early factor of band renormalization, [ 42 ] such a PA feature may be caused by the formation of biexcitons [ 46 , 47 , 48 ] or trap state excitons [ 32 , 49 , 50 ] induced by pump‐injected band‐edge excitons.…”