Optical Signatures of Dirac Electrodynamics for hBN-Passivated Silicene on Au(111).

The allotropic affinity for bulk silicon and unique electronic and optical properties make silicene a promising candidate for future high-performance devices compatible with mature complementary metal-oxide-semiconductor technology. However, silicene's outstanding properties are not preserved on its most prominent growth templates, due to strong substrate interactions and hybridization effects. In this letter, we report the optical properties of silicene epitaxially grown on Au(111). A novel in situ passivation methodology with few-layer hexagonal boron nitride enables detailed ex situ characterization at ambient conditions via µ-Raman spectroscopy and reflectance measurements. The optical properties of silicene on Au(111) appeared to be in accordance with the characteristics predicted theoretically for freestanding silicene, allowing the conclusion that its prominent electronic properties are preserved. The absorption features are, however, modified by many-body effects induced by the Au substrate due to an increased screening of electron-hole interactions.

Epitaxial Growth of Crystalline CaF2 on Silicene.

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.

Highly Biaxially Strained Silicene on Au(111).

Many of graphene's remarkable properties arise from its linear dispersion of the electronic states, forming a Dirac cone at the K points of the Brillouin zone. Silicene, the 2D allotrope of silicon, is also predicted to show a similar electronic band structure, with the addition of a tunable bandgap, induced by spin-orbit coupling. Because of these outstanding electronic properties, silicene is considered as a promising building block for next-generation electronic devices. Recently, it has been shown that silicene grown on Au(111) still possesses a Dirac cone, despite the interaction with the substrate. Here, to fully characterize the structure of this 2D material, we investigate the vibrational spectrum of a monolayer silicene grown on Au(111) by polarized Raman spectroscopy. To enable a detailed ex situ investigation, we passivated the silicene on Au(111) by encapsulating it under few layers hBN or graphene flakes. The observed spectrum is characterized by vibrational modes that are strongly red-shifted with respect to the ones expected for freestanding silicene. By comparing low-energy electron diffraction (LEED) patterns and Raman results with first-principles calculations, we show that the vibrational modes indicate a highly (>7%) biaxially strained silicene phase.

Silicene Passivation by Few-Layer Graphene.

The stabilization of silicene at ambient conditions is essential for its characterization, future processing, and device integration. Here, we demonstrate in situ encapsulation of silicene on Ag(111) by exfoliated few-layer graphene (FLG) flakes, allowing subsequent Raman analysis under ambient conditions. Raman spectroscopy measurements proved that FLG capping serves as an effective passivation, preventing degradation of silicene for up to 48 h. The acquired data are consistent with former in situ Raman measurements, showing two characteristic peaks, located at 216 and 515 cm-1. Polarization-dependent measurements allowed to identify the two modes as A and E, demonstrating that the symmetry properties of silicene are unaltered by the capping process.

Hyperspectral imaging with deformable gratings fabricated with metal-elastomer nanocomposites.

We report the fabrication and characterization of a simple and compact hyperspectral imaging setup based on a stretchable diffraction grating made with a metal-polymer nanocomposite. The nanocomposite is produced by implanting Ag clusters in a poly(dimethylsiloxane) film by supersonic cluster beam implantation. The deformable grating has curved grooves and is imposed on a concave cylindrical surface, thus obtaining optical power in two orthogonal directions. Both diffractive and optical powers are obtained by reflection, thus realizing a diffractive-catoptric optical device. This makes it easier to minimize aberrations. We prove that, despite the extended spectral range and the simplified optical scheme, it is actually possible to work with a traditional CCD sensor and achieve a good spectral and spatial resolution.