A two-dimensional CaSi monolayer with quasi-planar pentacoordinate silicon

Area of Science:

• Materials Science
• Solid-State Physics
• Computational Chemistry

Background:

• Predicting novel materials with unique topological properties is crucial for advancing technological applications.
• Planar pentacoordinate silicon (ppSi) represents a departure from conventional bonding, offering potential for new material functionalities.

Purpose of the Study:

• To explore the possibility of incorporating rule-breaking planar pentacoordinate silicon (ppSi) into a periodic structure.
• To computationally investigate the stability and properties of a novel two-dimensional (2D) CaSi monolayer.

Main Methods:

• Density functional theory (DFT) computations were employed to predict and analyze the material.
• A global minimum search was conducted to ascertain the lowest-energy structure.
• Phonon dispersion calculations were performed to assess thermodynamic stability.

Main Results:

• A stable 2D CaSi monolayer, built from a Ca4Si2 motif containing ppSi, was identified as the global minimum energy structure.
• The CaSi monolayer exhibits thermodynamic and kinetic stability, evidenced by moderate cohesive energy and absence of imaginary phonon modes.
• The material is a natural semiconductor with a 0.5 eV band gap, strong visible light absorption, and an unusual negative Poisson's ratio.

Conclusions:

• The ppSi-containing CaSi monolayer is a promising, experimentally realizable 2D material.
• Its unique electronic, optical, and mechanical properties suggest potential applications in electronics, optoelectronics, and advanced mechanics.