Formation and properties of iodine- and acetonitrile-functionalized two-dimensional Si materials: a Density Functional Theory study

Topotactic transformations of suitable layered three-dimensional precursors are among the most robust methods to prepare two-dimensional (2D) materials based on silicon or germanium. Here we use Density Functional Theory calculations to probe the mechanisms underlying the formation of 2D-Si sheets functionalized with iodine atoms (SiI) or acetonitrile molecules [Si(MeCN)] starting from a layered CaSi2 precursor. We identify the sequence of exothermic surface reactions that enable the adsorption of, not only iodine atoms, but, surprisingly, also of solvent acetonitrile molecules on both sides of the top layer of a Si-terminated CaSi2 surface and its ensuing exfoliation as a standalone 2D sheet. In the acetonitrile case, the as-formed 2D material exhibits intriguing structural and electronic properties with an unusual quasi-one-dimensional substructure of silicon chains and a Dirac-like cone in the energy band diagram. The results elucidate the atomic-scale details of the established experimental technique of topotactic synthesis of functionalized silicene and identify new structural motifs for 2D materials.