Fig. 1: Crystal structure of van der Waals GeS2 and fingerprints of its anisotropy.
a The monoclinic crystal structure of the GeS2 monolayer along the с-axis (top), and the bulk structure along the a-axis (bottom). A black dashed frame reveals the unit cell. Colored dashed lines encircle selected groups of atoms that define vibrational interaction: red – corner-sharing patterns, blue – edge-sharing patterns, green – bridge sulfur atoms b Optical image of GeS2 bulk crystal. c AFM scan of GeS2 flake, revealing the presence of GeS2 monolayer. d GeS2 in the framework of the calculated out-of-plane static dielectric constants of novel van der Waals materials vs their experimental optical bandgaps. The electronic bandstructure cuts of GeS2 along a-axis (e), b-axis (f), and c-axis (g). Orange and blue curves present conduction and valence bands, respectively
