Epitaxial Growth of Crystalline CaF₂ on Silicene

Abstract

Silicene is one of the most promising two-dimensional (2D) materials for the realization of next-generation electronic devices, owing to its high carrier mobility and band gap tunability. To fully control its electronic properties, an external electric field needs to be applied perpendicularly to the 2D lattice, thus requiring the deposition of an insulating layer that directly interfaces silicene, without perturbing its bidimensional nature. A promising material candidate is CaF2, which is known to form a quasi van der Waals interface with 2D materials as well as to maintain its insulating properties even at ultrathin scales. Here we investigate the epitaxial growth of thin CaF2 layers on different silicene phases by means of molecular beam epitaxy. Through electron diffraction images, we clearly show that CaF2 can be grown epitaxially on silicene even at low temperatures, with its domains fully aligned to the lattice of the underlying 2D structure. Moreover, in situ X-ray photoelectron spectroscopy data evidence that, upon CaF2 deposition, no changes in the chemical state of the silicon atoms can be detected, proving that no Si–Ca or Si–F bonds are formed. This clearly shows that the 2D layer is pristinely preserved underneath the insulating layer. Polarized Raman experiments show that silicene undergoes a structural change upon interaction with CaF2; however, it retains its two-dimensional character without transitioning to a sp3-hybridized silicon. For the first time, we have shown that CaF2 and silicene can be successfully interfaced, paving the way for the integration of silicon-based 2D materials in functional devices.

Silicene Applications in Nanotechnology: From Transistors to Bendable Membranes

“…Indeed, the mixed sp 2 /sp 3 hybridization, resulting from the substrate-induced stabilization, makes silicene prone to chemical reactivity [24]. To prevent oxidation, silicene is encapsulated with a protective layer like Al 2 O 3 [25], or graphene [26], or hBN [27], [28], or CaF 2 [29]. The use of graphene or hBN flakes has the main drawback to protect limited area of the Fig.…”

Silicene Applications in Nanotechnology: From Transistors to Bendable Membranes

“…Indeed, the mixed sp 2 /sp 3 hybridization, resulting from the substrate-induced stabilization, makes silicene prone to chemical reactivity [18]. To prevent oxidation, silicene is encapsulated with a protective layer like Al2O3 [19], CaF2 [20], or graphene [21]. Intriguingly, 5 nm-thick Al2O3 film grown by molecular beam deposition turns out to be effective in protecting silicene as confirmed by X-ray photoelectron spectroscopy (Figure 1b) [19] and Raman spectroscopy (Figure 1c) [17], [18].…”

Silicene Applications in Nanotechnology: From Transistors to Bendable Membranes

“…Indeed, the mixed sp 2 /sp 3 hybridization, resulting from the substrate-induced stabilization, makes silicene prone to chemical reactivity [18]. To prevent oxidation, silicene is encapsulated with a protective layer like Al2O3 [19], CaF2 [20], or graphene [21]. Intriguingly, 5 nm-thick Al2O3 film grown by molecular beam deposition turns out to be effective in protecting silicene as confirmed by X-ray photoelectron spectroscopy (Figure 1b) [19] and Raman spectroscopy (Figure 1c) [17], [18].…”