Layer-dependent band gaps and dielectric constants of ultrathin fluorite crystals

Structure, stability, and thickness-dependent properties of ultrathin crystalline calcium fluoride (fluorite, CaF2) with promising application in two-dimensional (2D) field-effect transistors have been investigated by ab initio calculation. The enthalpy of formation and phonon calculations demonstrate the phase and dynamical stabilities of ultrathin CaF2. The verification of the stability for mono- and few-layer CaF2, which can be referred as idealized reference systems for ultrathin layers of CaF2 grown by CVD/MBE or other growth methods, indicates that 2D materials with thickness varying from monolayer to bulk limit can be obtained for CaF2 with hard-bond interlayer interaction. The strong chemical bonds between the adjacent layers in few-layer CaF2 are displayed, which is completely different from the weak interlayer van der Waals (vdW) interaction in layered MoS2 or graphite, resulting in the increased frequencies of low optical phonon modes due to the stronger interlayer restoring force compared to vdW layered crystals. Through the calculations of electronic structure and dielectric response, the layer-dependent band gaps and dielectric constants of ultrathin CaF2 are investigated, and the material can be designed by adjusting the layer thickness to fit practical applications. The large and tunable band gaps and superior dielectric properties of ultrathin CaF2, demonstrated from DFT-HSE06 and linear response calculations, signify the prospect of ultrathin CaF2 as an emerging 2D dielectric layer.

Automated summary

The research investigated the structure, stability, and thickness-dependent properties of ultrathin crystalline calcium fluoride, and demonstrated the potential of adjusting layer thickness for practical applications. The material's large and tunable band gaps and superior dielectric properties suggest its prospect as an emerging 2D dielectric layer.