We show that CdSe nanoplatelets are a model system to investigate the tunability of trions and excitons in laterally finite 2D semiconductors.

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.

Photoluminescence spectra, shows that monolayer Transition-metal dichalcogenides (ML-TMDCs), possess charged exciton binding energies, conspicuously similar to the energy of optical phonons. This enigmatic coincidence has offered opportunities to investigate many-body interactions between trion (𝑋 − ), exciton (𝑋) and phonon and led to efficient excitonic anti-Stokes processes with the potential for laser refrigeration and energy harvesting. In this study, we show that in WSe 2 materials, the trion binding energy matches two phonon modes, the out-of-plane 𝐴 1 ′ and the in-plane 𝐸 ′ mode. In this respect, using the Fermi golden rule together with the effective mass approximation, we investigate the rate of the population transfers between 𝑋 and 𝑋 − , mediated by a single phonon. We demonstrate that, while the absolute importance of the two phonon modes on the upconversion process strongly depend on the experimental conditions such as the temperature and the dielectric environment (substrate), both modes lead to an up-conversion process on time scales in the range of few picoseconds to sub-nanosecond, consistent with recents experimental findings. The conjugate process is also investigated in our study, as a function of temperature and electron density 𝑁 𝑒 . We prove that exciton to trion down-conversion process is very unlikely at low electron density 𝑁 𝑒 < 10 10 𝑐𝑚 −2 and high temperature 𝑇 > 50 𝐾 while it increases dramatically to reach few picoseconds time scale at low temperature and for electron density 𝑁 𝑒 > 10 10 𝑐𝑚 −2 . Finally, our results show that conversion process occurs more rapidly in exemplary monolayer molybdenum-based dichalcogenides (MoSe 2 and MoTe 2 ) than tungsten dichalcogenides . I. INTRODUCTION AND MOTIVATIONTransition-metal dichalcogenides (TMDCs) (MX2, M=Mo,S; X=Se,S,Te), as atomically thin two-dimensional materials have had a tremendous impact on semiconductor physics in the last years. Due to their direct bandgap located at the ±K points in the Brillouin zone, a large interband dipole moment, spin-valley coupled physics, atomically thin layers emit strong photoluminescence and represent a semiconducting supplement to the twodimensional and zero-gap material grapheme [1][2][3][4][5][6][7]. The attractive Coulomb interaction between the conduction band electron and the valence band hole in TMDCs can bound them into a hydrogen-like state, known as exciton 𝑋, which is an elementary excitation that plays a key role in optoelectronic phenomena [8][9][10][11][12][13][14][15]. In TMDCs materials, owing to the dielectric screening and spatial quantum confinement, their optical transitions and lightmatter interaction are governed by robust exciton feature with binding energies of several hundred meV [8][9][10][11][12][13][14].This strong binding energy, which is more than one order of magnitude larger than conventional semiconductors such as GaAs, leads to large optical transition dipoles of excitons [8-14, 16, 17]. Additionally, when the TMDCs sample is negatively doped, the bound electron-hole...