Abstract 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.

中文翻译：

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超薄萤石晶体的层相关带隙和介电常数

摘要 通过 ab initio 计算研究了在二维 (2D) 场效应晶体管中具有广阔应用前景的超薄结晶氟化钙（萤石，CaF2）的结构、稳定性和厚度相关特性。形成焓和声子计算证明了超薄 CaF2 的相位和动力学稳定性。单层和少层 CaF2 的稳定性验证，可作为通过 CVD/MBE 或其他生长方法生长的 CaF2 超薄层的理想参考系统，表明厚度从单层到体极限变化的二维材料可以获得具有硬键层间相互作用的 CaF2。显示出少层 CaF2 中相邻层之间的强化学键，这与层状 MoS2 或石墨中的弱层间范德华 (vdW) 相互作用完全不同，由于与 vdW 层状晶体相比更强的层间恢复力，导致低光学声子模式的频率增加。通过电子结构和介电响应的计算，研究了超薄CaF2的层相关带隙和介电常数，可以通过调整层厚来设计材料以适应实际应用。DFT-HSE06 和线性响应计算证明了超薄 CaF2 的大且可调的带隙和优异的介电性能，表明超薄 CaF2 作为新兴的 2D 介电层的前景。由于与 vdW 层状晶体相比更强的层间恢复力，导致低光学声子模式的频率增加。通过电子结构和介电响应的计算，研究了超薄CaF2的层相关带隙和介电常数，可以通过调整层厚来设计材料以适应实际应用。DFT-HSE06 和线性响应计算证明了超薄 CaF2 的大且可调的带隙和优异的介电性能，表明超薄 CaF2 作为新兴的 2D 介电层的前景。由于与 vdW 层状晶体相比更强的层间恢复力，导致低光学声子模式的频率增加。通过电子结构和介电响应的计算，研究了超薄CaF2的层相关带隙和介电常数，可以通过调整层厚来设计材料以适应实际应用。DFT-HSE06 和线性响应计算证明了超薄 CaF2 的大且可调的带隙和优异的介电性能，表明超薄 CaF2 作为新兴的 2D 介电层的前景。研究了超薄 CaF2 的层相关带隙和介电常数，并且可以通过调整层厚度来设计材料以适应实际应用。DFT-HSE06 和线性响应计算证明了超薄 CaF2 的大且可调的带隙和优异的介电性能，表明超薄 CaF2 作为新兴的 2D 介电层的前景。研究了超薄 CaF2 的层相关带隙和介电常数，并且可以通过调整层厚度来设计材料以适应实际应用。DFT-HSE06 和线性响应计算证明了超薄 CaF2 的大且可调的带隙和优异的介电性能，表明超薄 CaF2 作为新兴的 2D 介电层的前景。