Lattice Dynamic and Instability in Pentasilicene: A Light Single-Element Ferroelectric Material with High Curie Temperature

The compatibility of Si semiconductor technology with current electronic devices has led to a continuing search for new Si-based materials with novel properties. The recent synthesis of penta-Si nanoribbons [Phys. Rev. Lett. 117, 276102 (2016); Nano Lett. 18, 2937 (2018); Nat. Commun. 7, 13076 (2016)] is a new addition to this family. However, pentasilicene, a two-dimensional (2D) sheet composed of only Si pentagons, was found to be unstable in pentagraphenelike configuration and could only be stabilized dynamically by surface decoration. Using first-principles calculations and a thorough analysis of its imaginary frequencies, we show that pentasilicene can indeed be made dynamically stable by tilting the Si dimers to reduce the Coulomb repulsion between them. The consequence of this tilting leads to a much more interesting discovery: the stabilized pentasilicene breaks the structural inversion symmetries, resulting in spontaneous electrical polarization and ferroelectricity with a high Curie temperature of 1190 K. This is the first report of a 2D ferroelectric system composed of a light single element, which can have potential applications in nonvolatile random access memory.