Defect-Enhanced Exciton–Exciton Annihilation in Monolayer Transition Metal Dichalcogenides at High Exciton Densities

Abstract

The exciton–exciton annihilation (EEA) process easily occurs in monolayer transition metal dichalcogenides (TMDs) because of the strong Coulomb interaction and quantum confinement effect, which enhance the many-body interaction of excitons. This process can affect the performance of the optoelectronic devices. It is crucial to examine the effect of defect states on the EEA process and determine whether it is comparable to that at the low excitation intensities, particularly when applied to laser devices at a high exciton density. In this study, femtosecond transient absorption spectroscopy was used to explore the EEA process of four types of CVD-grown monolayer TMDs (i.e., WS2, WSe2, MoS2, and MoSe2). We demonstrated that the defect-assisted EEA process of local excitons is enhanced and plays a key role in the exciton relaxation process at high exciton densities of approximately 1012 cm–2 below a Mott density of approximately 1013 cm–2. The measured EEA rates for WS2, WSe2, MoSe2, and MoS2 were 0.016, 0.026, 0.049, and 0.102 cm2/s, respectively, implying that EEA is enhanced as defect states increase in monolayer TMDs. Our results provide a profound insight into the effect of defect states on the EEA process in monolayer TMDs at high exciton densities.

Ultrastrong Coupling Leads to Slowed Cooling of Hot Excitons in Few-Layer Transition-Metal Dichalcogenides

“…One possible explanation for the discrepancy is that the nonradiative scattering events that lead to poor photoluminescence efficiency are more effectively screened by polariton coupling than τ 2 decay mechanisms. Recent work suggests that defect states enhance exciton−exciton annihilation (EEA), 59 so if EEA is the dominant decay mechanism at the A exciton, then polariton screening of defect states would also suppress EEA and therefore enhance radiative decay.…”

Regulating the Auger Recombination Process in Two-Dimensional Sn-Based Halide Perovskites

“…Evolution-associated spectra with three different lifetimes were obtained to study the characteristics of the relaxation process of photogenerated carriers in (PEA) 2 SnI 4 and (BA) 2 SnI 4 perovskites using the global fitting method, as shown in Figure 3. 19 The density of the photogenerated excitons is calculated from eq 2 39,52…”

“…And owing to this large exciton binding energy, the exciton state contains a large component of momentum corresponding to the electron and hole wave functions, which relaxes the requirement of momentum conservation and enhances the probability of the AR process. 16,19 Therefore, further studies are necessary to regulate the AR rate and improve the efficiency of 2D perovskite LEDs.…”

MXene‐Based Broadband Ultrafast Nonlinear Activator for Optical Computing

“…To further investigate the carrier relaxation processes, we globally fitted and analyzed the TAS with three relaxation decay components with different lifetimes (Figure 7b). [ 39 ] The relaxation kinetic process of 2D Ti 3 C 2 T x can be expressed as: [ 40 ] $$\[ \begin{array}{*{20}{c}}{g(t) = \mathop \sum \limits_{i = 1}^{n = 3} {A_i} \cdot {\rm{erfc}}\;\left( {\frac{\sigma }{{\sqrt 2 {\tau _i}}} - \frac{t}{{\sqrt 2 \sigma }}} \right)\;\exp \;\left( { - \frac{t}{{{\tau _i}}}} \right)}\end{array} \]$$where A i and τ i are the magnitude and lifetime of each component, respectively, “erfc” represents the integral error function, and σ is the laser pulse duration. The lifetime of the first fast process was 0.3 ps, the middle process was 5.22 ps, and the last process was 46.1 ps.…”

“…To further investigate the carrier relaxation processes, we globally fitted and analyzed the TAS with three relaxation decay components with different lifetimes (Figure 7b). [39] The relaxation kinetic process of 2D Ti 3 C 2 T x can be expressed as: [40] ( ) erfc 2 2 exp…”