Amit Soni

Bulk semiconductors possess an abundance of new scientific knowledge from the development of two-dimensional (2D) materials by modifying the quantum mechanical behaviour of the semiconductors. This study reports on the detailed first-principles computations of alkaline-earth tellurides with a rocksalt (NaCl type) structure, i.e. bulk (three-dimensional (3D)) structures CaTe, SrTe, and BaTe and their corresponding 2D monolayers. The density functional theory approach is used for these calculations, employing full-potential linearized augmented plane wave methods from the WIEN2k package. Structural relaxations are performed with the Perdew–Burke–Ernzerhof generalized gradient approximation functional, whereas band gaps are computed using the modified Becke–Johnson potential of Tran–Blaha for the bulk structures and the local modified Becke–Johnson potential for the 2D monolayers. It is observed that all three 2D monolayers are dynamically and mechanically stable due to the absence of imaginary modes and agreement with the Born–Huang stability criteria for the phonon dispersion. The bulk compounds have indirect-gaps ( E _g = 2.15–2.47 eV), whereas the monolayers exhibit direct gaps at Γ (2.46–2.85 eV). The direct comparison of all three varieties of tellurides in both bulk and 2D monolayer forms is shown. Although it involves a gradual transformation from indirect to direct type of materials due to dimensional reduction, the specific contribution here is the square P4/mmm monolayer polymorphs and its cross composition rather than the transition itself. This dimensionality reduction results in a significant modification in the optical properties also. Static dielectric constants, refractive indices, and reflectivity values decrease approximately 67%, 42%, and 87%, respectively, decreasing the static dielectric response of the monolayers relative to the bulk compounds with absorption onset in the visible (2.46–2.85 eV, ∼435–505 nm). The thermoelectric analysis shows a significant level of Seebeck coefficients. The 3D bulk tellurides and monolayers of CaTe/SrTe are the examples of materials with p-type Seebeck effect, while BaTe is characterized by n-type. These attributes together with the presence of wide band gaps and optical activity across the visible to ultraviolet spectrum mean that there are reasons to believe that alkaline earth tellurides with 2D structures can be further used in optoelectronics.