Which chromium-sulfur compounds exist as 2D material?

Two-dimensional (2D) chromium-sulfides are synthesized by molecular beam epitaxy using graphene as a substrate. Structure characterization by employing scanning tunneling microscopy and low energy electron diffraction indicates that there are two 2D phases, Cr${}_2$S${}_3$-2D and Cr${}_{2\frac{2}{3}}$S${}_4$-2D, which have not been reported before. Cr${}_{2\frac{2}{3}}$S${}_4$-2D is related to bulk Cr${}_5$S${}_6$, but thinner than a bulk unit cell. For Cr${}_2$S${}_3$-2D, an even thinner material, no bulk counterpart exists. Both 2D materials are found to be structurally stable under ambient conditions and exhibit interesting electronic properties. Extensive first-principles calculations provide further insight into the electronic structure of these systems and indicate that they should be magnetic. Although single layers of CrS${}_2$ were predicted to be stable by density functional theory calculations and reported in previous experimental studies, we were unable to synthesize CrS${}_2$ under our range of experimental conditions.