Crystallographic, electronic and vibrational properties of 2D silicate monolayers

Phyllosilicates are promising materials for optoelectronic applications because of their interesting electronic and magnetic properties that can be modulated by specific ionic substitutions. They can be easily exfoliated down to a single layer, enabling their use in specific 2D applications, such as the creation of van der Waals heterostructures and other materials with tailored physical properties. The present work reports a theoretical investigation of the structural, electronic, Raman and infrared properties of the (001) monolayer of phlogopite [K(Mg,Fe)₃Si₃AlO₁₀(OH)₂, with Mg/Fe ratio ≥ 2] and how they change with different Fe²⁺/Mg²⁺ substitutions in the structure. Although other cations could occupy the octahedral sheet positions in phlogopite (and phyllosilicate in general), here the focus is only on Fe²⁺/Mg²⁺ substitution. To this aim, density functional theory simulations were performed using the B3LYP functional, including long-range interactions in the physical treatment. The structure of the single layer of phlogopite showed a decrease of the tetrahedral rotation angle near the interlayer cations in comparison with that of the bulk mineral, which led to a tetrahedral sheet with a hexagonal pattern close to ideality, and an electronic band structure with a decreased band gap energy, down to about 3 eV. All results were discussed against the few available experimental and theoretical data in the scientific literature, finding good agreement but also further extending the knowledge of this interesting natural 2D material.