Conversion of Snse to SnSe2 Via Vapor Transport in Ferecrystals

Ferecrystals, which are rotationally-disordered misfit layered compounds that self assemble from elemental precursors, are useful for probing fundamental questions in solid state chemistry due to their turbrostratic (“rotational”) disorder along the Z axis. They can be used to answer questions about band gaps that are relevant to the semiconductor field. Previous studies have shown that it may be possible for a conversion of layered SnSe to SnSe₂ in misfit layered compounds via vapor transport in a sealed vessel that was outgassed and pumped down to 10^-6 Torr. Ferecrystals with SnSe₂ are hard to crystallize because the annealing process sublimates selenium, which leaves the SnSe phase and many defects. In this study, we take the ferecrystal (SnSe)_1.16NbSe₂ and perform selenium vapor annealing from 200 to 400 degrees °C to determine the best condition for a successful conversion to (SnSe₂)_1+δNbSe₂. In order to confirm a successful conversion, we used X-ray reflectivity and diffraction to characterize the structure of the film. High angle annular dark field imaging (HAADF) was also used to confirm findings from X-ray characterization. Our results have shown that the samples had all or some of the SnSe convert to SnSe₂, however the superlattice of these compounds were lost due to phase segregation. The HAADF images have shown that the conversion of SnSe to SnSe₂ in the ferecrystal can cause large grain boundaries in the SnSe₂ layers and confirmed that phase segregation was occurring more often in the higher temperatures of the reaction. This study has shown that a clean conversion from (SnSe)_1.16NbSe₂ to (SnSe₂)_1+δNbSe₂ is not possible with these conditions. However, this study did show us that we have taken right steps for figuring out the conditions that are required for a clean conversion to (SnSe₂)_1+δNbSe₂. The temperature of 250 °C was able to prevent massive phase segregation and still allowed for conversion of SnSe to SnSe₂. In the future, this project could lead to other topotactic reactions such as anion exchange with sulfur to post-synthetically convert misfit layered films.