Giant change of MoS2 optical properties along amorphous-crystalline transition: broadband spectroscopic study including the NIR therapeutic window

Abstract

This work deals with an ellipsometric study of magnetron sputtered thin MoS2 films. The evolution of the UV-VIS-NIR optical properties of as-deposited and subsequently annealed films is thoughtfully investigated, covering amorphous, amorphous relaxed, partially crystallized, and polycrystallized MoS2 films. The transition from the mixed 1T '@2H local order in the amorphous phase toward the long-range 2H order in the polycrystalline phase is systematically correlated with film optical properties. The early stage of a few-layer 2H ordering toward the 2H bulk-like polycrystalline structure during annealing is evidenced through the energy shift of MoS2 prominent excitonic peaks. A considerable change in optical response between metallic (amorphous) and semiconducting (polycrystalline) MoS2 phases is reported and presented in terms of dielectric permittivity and normal reflectance NIR-VIS-UV spectra. Results of light-heat conversion in the NIR therapeutic window show so far uncovered potential of amorphous MoS2 as an agent for photothermal therapy. Spectroscopic ellipsometry provided sensitive characterization disclosing essential results complementary to other characterization tools. The benefit of these results is expected to be employed in fundamental and application-motivated research, for example, in the field of phase change materials, photothermal cancer therapy, and magneto-optical study of magnetic ordering in metal transition dichalcogenides, among others.