Optical properties of aged WS2 monolayers grown by CVD method on Si/SiO2 substrates are studied using temperature dependent photoluminescence and reflectance contrast spectroscopy. Aged WS2 monolayers have a typical surface roughness about 0.5 nm and, in addition, a high density of nanoparticles (nanocaps) with the base diameter about 30 nm and average height of 7 nm. The A-exciton of aged monolayer has a peak position at 1.951 eV while in as-grown monolayer the peak is at about 24 meV higher energy at room temperature. This red-shift is explained using local tensile strain concept, where strain value of 2.1% was calculated for these nanocap regions. Strained nanocaps have lower band gap energy and excitons will funnel into these regions. At T=10K a double exciton and trion peaks were revealed. The separation between double peaks is about 20 meV and the origin of higher energy peaks is related to the optical band gap energy fluctuations caused by random distribution of local tensile strain due to increased surface roughness. In addition, a wide defect related exciton band XD was found at about 1.93 eV in all aged monolayers. It is shown that the theory of localized excitons describes well the temperature dependence of peak position and halfwidth of the A-exciton band. The possible origin of nanocaps is also discussed.

Postdeposition treatments (PDTs) are common technological approaches to achieve high-efficiency chalcogenide solar cells. For SnS, a promising solar cell material, most PDT strategies to control the SnS properties are overwhelmingly based on an annealing in sulfur-containing ambient atmosphere that is described by condensed-state reactions and vapor-phase transport. In this work, a systematic study of the impact of PDTs in a N2 atmosphere, ampules at temperatures between 400 and 600 °C, and a SnCl2 treatment at 250–500 °C on the properties of SnS films and SnS/CdS solar cells prepared by close-spaced sublimation is reported. The ampule and N2 annealing conditions do not affect the grain size of the SnS layers but significantly impact the concentration of intrinsic point defects, carrier density, and mobility. Annealing at 500–600 °C strongly enhances the hole concentration and decreases the carrier mobility, having detrimental impacts on the device performance. SnCl2 treatment promotes grain growth, sintering, and doping by mass transport through the melted phase; it adjusts the hole density and improves the carrier mobility in the SnS layers. SnS/CdS solar cells with an efficiency of 2.8% are achieved in the SnCl2 treatment step, opening new possibilities to further improve the performance of SnS-based devices.

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Photoluminescence spectroscopy has been used to investigate the details of B band emission from single‐layer MoS2 samples grown by chemical vapor deposition. The evolution of the peak shape upon variable excitation power is well described by a combination of exciton and trion contributions, from which B trion binding energy of 18 meV has been extracted. It has been suggested that the absence of accompanying A trion emission in our samples, as well as the enhanced intensity of the B band, can be ascribed to a fast non‐radiative recombination channel arising from the large defect density in our samples. It has been found that band‐selective recombination channels or formation of dark/bright trions are possible microscopic scenarios for our observations.

Temperature‐ and laser power‐dependent photoluminescence (PL) properties of the asymmetric defect‐bound exciton band X normalD in defective WS2 monolayers, grown by chemical vapor deposition, are studied. Based on PL mapping, a monolayer region with an intensive X normalD band emission at about 1.9 eV is chosen for further studies. The X normalD band is thermally quenched above 180 K, and the thermal activation energy is found to be E normala  = 33 ± 4 meV. At T = 15 K, the X normalD band intensity reveals a sublinear dependence with increasing excitation power and the peak position shows a blueshift of about 15 meV per decade of laser power. It is shown that the X normalD band is related to the deep defect states within the band gap of WS2.