The ultrathin transition metal dichalcogenides (TMDs) have emerged as promising materials for various applications using two dimensional semiconductors. They have attracted increasing attention due to their unique optical properties originate from neutral and charged excitons. In this paper, we study the strong localization of exciton center-of-mass motion within random potential fluctuations caused by the monolayer defects. Here, we report negatively charged exciton formation in monolayer TMDs, notably tungsten disulfide WS2. Our theory is based on an effective mass model of neutral and charged excitons, parameterized by ab-initio calculations. Taking into the account the strong correlation between the monolayer WS2 and the surrounding dielectric environment, our theoretical results are in good agreement with one-photon photoluminescence (PL) and reflectivity measurements. We also show that the exciton state with p-symmetry, experimentally observed by two-photon PL emission, is energetically below the 2s-state. We use the equilibrium mass action law, to quantify the relative weight of exciton and trion PL. We show that exciton and trion emission can be tuned and controlled by external parameters like temperature, pumping, and injection electrons. Finally, in comparison with experimental measurements, we show that exciton emission in monolayer tungsten dichalcogenides is substantially reduced. This feature suggests that free exciton can be trapped in disordered potential wells to form a localized exciton and therefore offers a route toward novel optical properties.

Disorder derived from defects or strain in monolayer TMDs can lead to a dramatic change in the physical behavior of the interband excitations, producing inhomogeneous spectral broadening and localization; leading to radiative lifetime increase. In this study, we have modeled the disorder in the surface of the sample through a randomized potential in monolayer WSe2. We show that this model allows us to simulate the spectra of localized exciton states as well as their radiative lifetime. In this context, we give an in depth study of the influence of the disorder potential parameters on the optical properties of these defects through energies, density of states, oscillator strengths, photoluminescence (PL) spectroscopy and radiative lifetime at low temperature (4K).We demonstrate that localized excitons have a longer emission time than free excitons, in the range of tens of picoseconds or more, and we show that it depends strongly on the disorder parameter and dielectric environment. Finally, in order to prove the validity of our model we compare it to available experimental results of the literature.

Excitonic effects play an important role on the optoelectronic behavior of atomically thin two-dimensional (2D) crystals of the WS transition metal dichalcogenide. In this paper, neutral and charged exciton behaviors in monolayer WS are handled within effective-mass approximation for which the critical parameters are ensured from our ab initio calculations. Firstly, we reveal an exciton series with a novel energy dependence on the orbital angular momentum. Considerable control of the dielectric environment on neutral and charged excitons binding energies is elucidated. We demonstrate that for accepted values of effective masses, the negative and positive trion binding energies should be identical. Secondly, localization of neutral exciton center of mass motion by random potential arising from monolayer defects is also studied. The results obtained are in agreement with available experimental work.

We studied the effects of thermal annealing (in air and in ultra-high vacuum from room temperature up to 300 °C) of mechanically exfoliated mono-layer and few layer MoS2 onto 270 nm SiO2/Si(1 0 0). The experiments were performed with optical microscopy, atomic force microscopy, non resonant Raman spectroscopy, and photoluminescence (PL) spectroscopy on the mono-layer flakes. We demonstrate the presence of a nano-confined water layer at the interface with the silicon substrate. The thickness of this water layer can be increased by immersing the exfoliated samples in water for one hour, or decreased by post exfoliation annealing. Then, we directly demonstrate the sublimation with annealing of the bottom layer at the interface with SiO2. PL experiments performed on the mono-layers in the 250 °C–300 °C annealing range, together with previous x-ray photoemission experiments, demonstrate the direct correlation of the PL integrated spectral intensity with the concentration of sulfur vacancies that are passivated by oxygen after exposure to air. Namely, the vacancy formation is triggered in the 250 °C–300 °C range independently of the annealing environment, and correspondingly in the same temperature range the PL spectral intensity proportionally increases. With respect to pristine exfoliated MoS2 we estimate via PL an increase by a factor three of the vacancy concentration upon annealing to 300 °C. This, considering the typical vacancy concentration values of mechanical exfoliated MoS2, leads to sulfur vacancy concentration values close to 1014 cm−2. Considering such high value, the sublimation of the bottom layer is then explained via oxidation of the sulfur vacancies upon dissociation of water preferentially supplied by the film present at the interface with the SiO2 substrate.

We theoretically study the role of the Berry curvature on neutral and charged excitons in twodimensional transition-metal dichalcogenides. The Berry curvature arises due to a strong coupling between the conduction and valence bands in these materials that can to great extent be described within the model of massive Dirac fermions. The Berry curvature lifts the degeneracy of exciton states with opposite angular momentum. Using an electronic interaction that accounts for non-local screening effects, we find a Berry-curvature induced splitting of ∼ 17 meV between the 2p− and 2p+ exciton states in WS2, consistent with experimental findings. Furthermore, we calculate the trion binding energies in WS2 and WSe2 for a large variety of screening lenghts and different dielectric constants for the environment. Our approach indicates the prominent role played by the Berry curvature along with non-local electronic interactions in the understanding of the energy spectra of neutral and charged excitons in transition-metal dichalcogenides and in the the interpretation of their optical properties.