Synthesis of an open-framework allotrope of silicon

Abstract

Silicon is ubiquitous in contemporary technology. The most stable form of silicon at ambient conditions takes on the structure of diamond (cF8, d-Si) and is an indirect bandgap semiconductor, which prevents it from being considered as a next-generation platform for semiconductor technologies. Here, we report the formation of a new orthorhombic allotrope of silicon, Si24, using a novel two-step synthesis methodology. First, a Na4Si24 precursor was synthesized at high pressure; second, sodium was removed from the precursor by a thermal 'degassing' process. The Cmcm structure of Si24, which has 24 Si atoms per unit cell (oC24), contains open channels along the crystallographic a-axis that are formed from six- and eight-membered sp(3) silicon rings. This new allotrope possesses a quasidirect bandgap near 1.3 eV. Our combined experimental/theoretical study expands the known allotropy for element fourteen and the unique high-pressure precursor synthesis methodology demonstrates the potential for new materials with desirable properties.

Generalized evolutionary metadynamics for sampling the energy landscapes and its applications

“…Such irreversibility indicates a chance to synthesize new metastable forms by high-pressure modification. Quest for new forms of silicon is surged by photovoltaic application [31][32][33]. In order to achieve a better absorption in the visible light, the ideal material should have a direct band gap of 1-1.5 eV, and strong absorption within the solar spectrum.…”

Novel silicon phases and nanostructures for solar energy conversion

“…However, when such guest atoms occupy the cages, they donate electrons to the framework making the material metallic and thus not useful for photovoltaic applications. Recently, Kim et al 26 reported the synthesis of a new Cmcm Si 24 allotrope free of metals, with open channels along the aaxis, formed from six and eight membered sp 3 silicon rings, and with a quasi-direct band gap of 1.3 eV. Baranowski and coauthors 24 successfully synthesized a type-II Si-Ge clathrate alloy.…”

Formation of inclusion type silicon phases induced by inert gases

“…Electronic structure calculations 5,6 also suggested the band gap of Si clathrate is either direct or quasi-direct (namely, formally indirect, but the direct and indirect gaps are nearly degenerate). A recently synthesized new allotrope of silicon, Si 24 , was identified to possess an orthorhombic lattice and to contain eight-membered rings 7 . Both experimental measurements and theoretical calculations suggested Si 24 has a quasi-direct band gap of 1.3 eV.…”