Luminescent Silicon Nanosheet Paracrystals from Elemental- and Hydride-Based Syntheses of CaSi₂ Precursors: Implications for Photonic and Optoelectronic Applications

Layered silicon nanosheets (SiNSs)have attracted considerableattention owing to their unique combination of chemical and physicalproperties, which makes them an exciting candidate for next-generationon-chip light sources and lasers. Despite over 150 years of researchon SiNSs, the effects of the CaSi2 precursor quality onSiNSs have not been studied. Here, we report a comparison of CaSi2 (and SiNSs derived therefrom) synthesized from two reactionpathways: (1) melting Ca and Si (elemental melting, or EM-CaSi2) and (2) the less-explored reaction between CaH2 and Si (hydride synthesis, or HS-CaSi2). We demonstratethat both reaction pathways lead to CaSi2, but the HS-CaSi2 pathway requires only a single step without the need to meltthe CaSi2 product and at a temperature below the peritecticdecomposition of CaSi2. We find that the EM-CaSi2 exhibits grains that lay flat against the substrate, whereas theHS-CaSi2 has little preferred orientation. We deintercalatedboth EM- and HS-CaSi2 with HCl at -35 degrees C toyield hydrogen-terminated SiNSs. We characterized the SiNSs and foundthat the HS-SiNSs and EM-SiNSs exhibit properties that are nearlyidentical, with the exception that the morphology of the precursoris imparted to the SiNSs. These results provide the community witha one-step method to synthesize CaSi2 and demonstrate thatthe morphology of CaSi2 and SiNSs can be controlled withdifferent synthetic techniques.