Atomically thin transition metal dichalcogenides (TMDCs) present a promising platform for numerous photonic applications due to excitonic spectral features, possibility to tune their constants by external gating, doping, or light, and mechanical stability. Utilization of such materials for sensing or optical modulation purposes would require a clever optical design, as by itself the 2D materials can offer only a small optical phase delay – consequence of the atomic thickness. To address this issue, we combine films of 2D semiconductors which exhibit excitonic lines with the Fabry-Perot resonators of the standard commercial SiO2/Si substrate, in order to realize topological phase singularities in reflection. Around these singularities, reflection spectra demonstrate rapid phase changes while the structure behaves as a perfect absorber. Furthermore, we demonstrate that such topological phase singularities are ubiquitous for the entire class of atomically thin TMDCs and other high-refractive-index materials, making it a powerful tool for phase engineering in flat optics. As a practical demonstration, we employ PdSe2 topological phase singularities for a refractive index sensor and demonstrate its superior phase sensitivity compared to typical surface plasmon resonance sensors.

We studied the tribological properties of amorphous molybdenum sulfide (MoSx) thin-film coatings during sliding friction in an oxidizing environment at a low temperature (−100 °C). To obtain films with different sulfur contents (x ~ 2, 3, and 4), we used reactive pulsed laser deposition, where laser ablation of the Mo target was performed in H2S at various pressures. The lowest coefficient of friction (0.08) was observed during tribo-testing of the MoS3 coating. This coating had good ductility and low wear; the wear of a steel counterbody was minimal. The MoS2 coating had the best wear resistance, due to the tribo-film adhering well to the coating in the wear track. Tribo-modification of the MoS2 coating, however, caused a higher coefficient of friction (0.16) and the most intensive wear of the counterbody. The MoS4 coating had inferior tribological properties. This study explored the mechanisms of possible tribo-chemical changes and structural rearrangements in MoSx coatings upon contact with a counterbody when exposed to oxygen and water. The properties of the tribo-film and the efficiency of its transfer onto the coating and/or the counterbody largely depended on local atomic packing of the nanoclusters that formed the structure of the amorphous MoSx films.

The feasibility of growing atomically thin MoS2 films (down to two monolayers) on several tens of cm2 area was demonstrated by first depositing the MoO3 thin film by using an atomic layer deposition and subsequent sulfurization at temperatures ranging from 500 to 1000 °C. The effect of sulfurization temperature on properties of thin MoS2 films was investigated in details. It was found that the annealing of the MoO3 film under the elemental sulfur vapor condition allows effective sulfurization from 500 °C, at which the converted MoS2 film contained a rather high concentration of elemental sulfur which might reside at the boundaries between the relatively low-crystallized edge-on MoS2 grains. The increase in sulfurization temperature from 500 to 1000 °C results in a significant grain size growth from ∼10 up to >∼100 nm, with the change of the edge-on grains to the flat grains with their (0001) planes being parallel to the sapphire substrate. Raman spectroscopy investigations also indicated that the defect concentration decreased with the increasing sulfurization temperature. The films obtained by the sulfurization at lower temperatures (500–700 °C) may have high catalytic activity, whereas the highly (0001) aligned films obtained at higher temperatures (900–1000 °C) could be useful for high functionality electronic applications, which was successfully demonstrated by their combination with thin ferroelectric HfO2-based film. Thus, the noticeable remnant polarization value and a good switching endurance were obtained directly in contact with MoS2 film, allowing to conclude the possibility of the memory MoS2-based FeFET concept realization.