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 (WS₂). 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.

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 X_A and X_B, 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 X_B exciton fill up before those near the X_A 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 X_A and X_B excitons may open new horizons for the incorporation of 2D-TMD materials in future optoelectronics.

Direct growth and patterning of atomically thin two-dimensional (2D) materials on various substrates are essential steps toward enabling their potential for use in the next generation of electronic and optoelectronic devices. The conventional gas-phase growth techniques, however, are not compatible with direct patterning processes. Similarly, the condensed-phase methods, based on metal oxide deposition and chalcogenization processes, require lengthy processing times and high temperatures. Here, a novel self-limiting laser crystallization process for direct crystallization and patterning of 2D materials is demonstrated. It takes advantage of significant differences between the optical properties of the amorphous and crystalline phases. Pulsed laser deposition is used to deposit a thin layer of stoichiometric amorphous molybdenum disulfide (MoS2) film (~3 nm) onto the fused silica substrates. A tunable nanosecond infrared (IR) laser (1064 nm) is then employed to couple a precise amount of power and number of pulses into the amorphous materials for controlled crystallization and direct writing processes. The IR laser interaction with the amorphous layer results in fast heating, crystallization, and/or evaporation of the materials within a narrow processing window. However, reduction of the midgap and defect states in the as crystallized layers decreases the laser coupling efficiency leading to higher tolerance to process parameters. The deliberate design of such laser 2D material interactions allows the self-limiting crystallization phenomena to occur with increased quality and a much broader processing window. This unique laser processing approach allows high-quality crystallization, direct writing, patterning, and the integration of various 2D materials into future functional devices.