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Two-dimensional transition metal dichalcogenides (2D-TMDs) hold a great potential to platform future flexible optoelectronics. The beating hearts of these materials are their excitons known as XA and XB, which arise from transitions between spin-orbit split (SOS) levels in the conduction and valence bands at the K-point. The functionality of 2D-TMD-based devices is determined by the dynamics of these excitons. One of the most consequential channels of exciton decay on the device functionality is the defect-assisted recombination (DAR). Here, we employ steady-state absorption and emission spectroscopies, and pump density-dependent femtosecond transient absorption spectroscopy to report on the effect of DAR on the lifetime of excitons in monolayers of tungsten disulfide (2D-WS2) and diselenide (2D-WSe2). These pump-probe measurements suggested that while exciton decay dynamics in both monolayers are driven by DAR, in 2D-WS2, defect states near the XB exciton fill up before those near the XA exciton. However, in the 2D-WSe2 monolayer, the defect states fill up similarly. Understanding the contribution of DAR on the lifetime of excitons and the partition of this decay channel between XA and XB excitons may open new horizons for the incorporation of 2D-TMD materials in future optoelectronics. 

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 indispensible processes in photovoltaic and light-emitting devices, respectively. These two processes depend drastically on the photogeneration efficiency and lifetime of excitons. Here, we incorporate femtosecond pump–probe spectroscopy to investigate the ultrafast dynamics of exciton formation and decay in a single crystal of monolayer 2D tungsten disulfide (WS 2 ). Investigation of the formation dynamics of the lowest exciton (X A ) indicated that the formation time linearly increases from ∼150 fs upon resonant excitation, to ∼500 fs following excitation that is ∼1.1 eV above the band-gap. This dependence is attributed to the time it takes highly excited electrons in the conduction band (CB) to relax to the CB minimum (CBM) and contribute to the formation of X A . This is confirmed by infrared measurements of electron intraband relaxation dynamics. Furthermore, pump–probe experiments suggested that the X A ground state depletion recovery dynamics depend on the excitation energy as well. The average recovery time increased from ∼10 ps in the case of resonant excitation to ∼50 ps following excitation well above the band-gap. Having the ability to control whether generating short-lived or long-lived electron–hole pairs in 2D-TMD monolayers opens a new horizon for the application of these materials. For instance, long-lived electron–hole pairs are appropriate for photovoltaic devices, but short-lived excitons are more beneficial for lasers with ultrashort pulses. 

Organic–inorganic hybrid perovskites have emerged as promising optoelectronic materials for applications in photovoltaic and optoelectronic devices. Particularly, 2D layer‐structured hybrid perovskites are of great interest due to their remarkable optical and electrical properties, which can be easily tuned by selecting suitable organic and inorganic moieties during the material synthesis. Here, the solution‐phase growth of a large square‐shaped single‐crystalline 2D hybrid perovskite, phenethylammonium lead bromide (C₆H₅C₂H₄NH₃)₂PbBr₄(PEPB), with thickness as few as 3 unit cell layers is demonstrated. Compared to bulk crystals, the 2D PEPB nanocrystals show a major blueshifted photoluminescence (PL) peak at 409 nm indicating an increase in bandgap of 40 meV. Besides the major peak, two new PL peaks located at 480 and 525 nm are observed from the hybrid perovskite nanocrystals. PEPB nanocrystals with different thicknesses show different colors, which can be used to estimate the thickness of the nanocrystals. Time‐resolved reflectance spectroscopy is used to investigate the exciton dynamics, which exhibits a biexponential decay with an amplitude‐weighted lifetime of 16.7 ps. The high‐quality 2D (C₆H₅C₂H₄NH₃)₂PbBr₄nanocrystals are expected to have high PL quantum efficiency and potential applications for light‐emitting devices.