Synthesis and characterization of CaSi2 films for hydrogenated 2D Si nanosheets

Abstract

Hydrogenated two-dimensional (2D) Si nanosheet is a layered semiconductor synthesized by topotactic conversion of CaSi2 in HCl solution at low temperatures. The unavailability of scalable thin film deposition techniques has hindered the experimental analysis of physical properties. This study has investigated the synthesis of CaSi2 films by close-spaced evaporation and their topotactic conversion to 2D Si nanosheet films. A single-phase CaSi2 (6R) formation window was identified in the growth temperature range of 800–950 °C. Secondary phases (CaSi and Si polycrystals) formed outside the optimal window have also been identified. The x-ray pole figure of the representative CaSi2 film has shown that the film grows epitaxially with two epitaxial variants on the Si(111) surface with (0001) orientation. The CaSi2 films with and without the Si polycrystals have been subjected to low-temperature HCl solution treatment. Mostly hydrogenated 2D Si nanosheet films were synthesized without the Si polycrystals, while CaSi2 remained with the Si polycrystals, highlighting the importance of single-phase CaSi2 films.