Optical properties of two-dimensional tin nanosheets epitaxially grown on graphene.

Heterostacks formed by combining two-dimensional materials show novel properties which are of great interest for new applications in electronics, photonics and even twistronics, the new emerging field born after the outstanding discoveries on twisted graphene. Here, we report the direct growth of tin nanosheets at the two-dimensional limit via molecular beam epitaxy on chemical vapor deposited graphene on Al2O3(0001). The mutual interaction between the tin nanosheets and graphene is evidenced by structural and chemical investigations. On the one hand, Raman spectroscopy indicates that graphene undergoes compressive strain after the tin growth, while no charge transfer is observed. On the other hand, chemical analysis shows that tin nanosheets interaction with sapphire is mediated by graphene avoiding the tin oxidation occurring in the direct growth on this substrate. Remarkably, optical measurements show that the absorption of tin nanosheets exhibits a graphene-like behavior with a strong absorption in the ultraviolet photon energy range, therein resulting in a different optical response compared to tin nanosheets on bare sapphire. The optical properties of ultra-thin tin films therefore represent an open and flexible playground for the absorption of light in a broad range of the electromagnetic spectrum and technologically relevant applications for photon harvesting and sensors.

All-around encapsulation of silicene.

Silicene or the two-dimensional (2D) graphene-like silicon allotrope has recently emerged as a promising candidate for various applications in nanotechnology. However, concerns on the silicene stability still persist to date and need to be addressed aiming at the fabrication of competing and durable silicene-based devices. Here, we present an all-around encapsulation methodology beyond the current state-of-the-art silicene configuration, namely silicene sandwiched in between a capping layer (e.g., Al2O3) and the supporting substrate (e.g., Ag). In this framework, the insertion of one or two sacrificial 2D Sn layers enables the realization of different atomically thin encapsulation schemes, preserving the pristine properties of silicene while decoupling it from the growth template. On one hand, the epitaxy of a 2D Sn layer before silicene allows for the removal of the Ag substrate with no effect on silicene which in turn can be easily gated, for example, with an oxide layer on its top face. On the other hand, a full 2D encapsulation scheme, where top and bottom faces of silicene are protected by 2D Sn layers, gives rise to an atomically thin and cm2-scaled membrane preventing degradation of silicene for months. Both schemes thus constitute an advancement for the silicene stability and encapsulation in ambient conditions, paving the way to further exploitation in flexible electronics and photonics.

Crystal phase engineering of silicene by Sn-modified Ag(111).

The synthesis of silicene by direct growth on silver is characterized by the formation of multiple phases and domains, posing severe constraints on the spatial charge conduction towards a technological transfer of silicene to electronic transport devices. Here we engineer the silicene/silver interface by two schemes, namely, either through decoration by Sn atoms, forming an Ag2Sn surface alloy, or by buffering the interface with a stanene layer. Whereas in both cases Raman spectra confirm the typical features as expected from silicene, by electron diffraction we observe that a very well-ordered single-phase 4 × 4 monolayer silicene is stabilized by the decorated surface, while the buffered interface exhibits a sharp phase at all silicon coverages. Both interfaces also stabilize the ordered growth of a phase in the multilayer range, featuring a single rotational domain. Theoretical ab initio models are used to investigate low-buckled silicene phases (4 × 4 and a competing one) and various structures, supporting the experimental findings. This study provides new and promising technology routes to manipulate the silicene structure by controlled phase selection and single-crystal silicene growth on a wafer-scale.

Bendable Silicene Membranes.

Due to their superior mechanical properties, 2D materials have gained interest as active layers in flexible devices co-integrating electronic, photonic, and straintronic functions altogether. To this end, 2D bendable membranes compatible with the technological process standards and endowed with large-scale uniformity are highly desired. Here, it is reported on the realization of bendable membranes based on silicene layers (the 2D form of silicon) by means of a process in which the layers are fully detached from the native substrate and transferred onto arbitrary flexible substrates. The application of macroscopic mechanical deformations induces a strain-responsive behavior in the Raman spectrum of silicene. It is also shown that the membranes under elastic tension relaxation are prone to form microscale wrinkles displaying a local generation of strain in the silicene layer consistent with that observed under macroscopic mechanical deformation. Optothermal Raman spectroscopy measurements reveal a curvature-dependent heat dispersion in silicene wrinkles. Finally, as compelling evidence of the technological potential of the silicene membranes, it is demonstrated that they can be readily introduced into a lithographic process flow resulting in the definition of flexible device-ready architectures, a piezoresistor, and thus paving the way to a viable advance in a fully silicon-compatible technology framework.

Solid phase crystallization of amorphous silicon at the two-dimensional limit.

The epitaxy of silicene-on-Ag(111) renewed considerable interest in silicon (Si) when scaled down to the two-dimensional (2D) limit. This remains one of the most explored growth cases in the emerging 2D material fashion beyond graphene. However, out of a strict silicene framework, other allotropic forms of Si still deserve attention owing to technological purposes. Here, we present 2D Solid Phase Crystallization (SPC) of Si starting from an amorphous-Si on Ag(111) in atomic coverage to gain a crystalline-Si layer by post-growth annealing below 450 °C, namely Complementary Metal Oxide Semiconductor (CMOS) Back-End-of-Line (BEOL) thermal budget limit. Moreover, considering the benefit of the 2D-SPC scheme, we managed to write crystalline-Si pixels on the amorphous-Si matrix. Our in situ and ex situ analyses show that an in-plane interface or a lateral heterojunction between amorphous and crystalline-Si is formed. This amorphous-to-crystalline phase transformation suggests that 2D silicon may stem from an epitaxially grown layer and thermal self-organization/assembling.

Optical and thermal responses of silicene in Xene heterostructures.

Stabilization of silicene and preservation of its structural and electronic properties are essential for its processing and future integration into devices. The stacking of silicene on stanene, creating a Xene-based heterostructure, proves to be a viable new route in this respect. Here we demonstrate the effectiveness of a stanene layer in breaking the strong interaction between silicene and the Ag(111) substrate. The role of stanene as a 'buffer' layer is investigated by analyzing the optical response of epitaxial silicene through both power-dependent Raman spectroscopy and reflectivity measurements in the near infrared (NIR)-ultraviolet (UV) spectral range. Finally, we point out a Xene-induced shift of the silver plasma edge that paves the way for the development of a new approach to engineering the metal plasmonic response.

The Rise of the Xenes: From the Synthesis to the Integration Processes for Electronics and Photonics.

The recent outcomes related to the Xenes, the two-dimensional (2D) monoelemental graphene-like materials, in three interdisciplinary fields such as electronics, photonics and processing are here reviewed by focusing on peculiar growth and device integration aspects. In contrast with forerunner 2D materials such as graphene and transition metal dichalcogenides, the Xenes pose new and intriguing challenges for their synthesis and exploitation because of their artificial nature and stabilization issues. This effort is however rewarded by a fascinating and versatile scenario where the manipulation of the matter properties at the atomic scale paves the way to potential applications never reported to date. The current state-of-the-art about electronic integration of the Xenes, their optical and photonics properties, and the developed processing methodologies are summarized, whereas future challenges and critical aspects are tentatively outlined.

Probing the Laser Ablation of Black Phosphorus by Raman Spectroscopy.

Laser ablation in conjunction with Raman spectroscopy can be used to attain a controllable reduction of the thickness of exfoliated black phosphorus flakes and simultaneous measurement of the local temperature. However, this approach can be affected by several parameters, such as the thickness-dependent heat dissipation. Optical, thermal, and mechanical effects in the flakes and the substrate can influence the laser ablation and may become a source of artifacts on the measurement of the local temperature. In this work, we carry out a systematic investigation of the laser thinning of black phosphorus flakes on SiO2/Si substrates. The counterintuitive results from Raman thermometry are analyzed and elucidated with the help of numerical solutions of the problem, laying the groundwork for a controlled thinning process of this material.

Hydrophilic Character of Single-Layer MoS2 Grown on Ag(111).

The study of MoS2/metal interfaces is crucial for engineering efficient semiconductor-metal contacts in 2D MoS2-based devices. Here we investigate a MoS2/Ag heterostructure fabricated by growing a single MoS2 layer on Ag(111) by pulsed laser deposition under ultrahigh vacuum (UHV) conditions. The surface structure is observed in situ by scanning tunneling microscopy, revealing the hexagonal moiré pattern characteristic of the clean MoS2/Ag(111) interface. Ex situ Raman spectroscopy reveals an anomalous behavior of vibrational modes, induced by the strong MoS2-Ag interaction. After few-hours exposure to ambient conditions the Raman response significantly changes and the formation of molybdenum oxysulfides is revealed by X-ray photoelectron spectroscopy. These effects are due to the interplay with water vapor and can be reversed by a moderate UHV annealing. A polymeric (PMMA) capping is demonstrated to hinder water-induced modifications, preserving the original interface quality for months.

Optical Properties of Stanene-like Nanosheets on Al2O3(0001): Implications for Xene Photonics.

Stanene is one of the most intriguing two-dimensional (2D) materials because of its nontrivial topological properties. Here, the optical properties from THz to UV of molecular beam deposited tin nanosheets on Al2O3(0001) are reported. The experimental absorption coefficient cannot be described in terms of metallic tin or tin oxides. Nonetheless, a similar optical behavior was predicted by theory for freestanding stanene, thus suggesting the formation of the 2D tin nanosheets with stanene-like properties. These findings show that 2D tin bears appealing optical properties in a broad range of the electromagnetic spectrum, thus paving the way to Xenes-based nanophotonics.

Stability and universal encapsulation of epitaxial Xenes.

In the realm of two-dimensional material frameworks, single-element graphene-like lattices, known as Xenes, pose several issues concerning their environmental stability, with implications for their use in technology transfer to a device layout. In this Discussion, we scrutinize the chemical reactivity of epitaxial silicene, taken as a case in point, in oxygen-rich environments. The oxidation of silicene is detailed by means of a photoemission spectroscopy study upon carefully dosing molecular oxygen under vacuum and subsequent exposure to ambient conditions, showing different chemical reactivity. We therefore propose a sequential Al2O3 encapsulation of silicene as a solution to face degradation, proving its effectiveness by virtue of the interaction between silicene and a silver substrate. Based on this method, we generalize our encapsulation scheme to a large number of metal-supported Xenes by taking into account the case of epitaxial phosphorene-on-gold.

Thickness determination of anisotropic van der Waals crystals by raman spectroscopy: the case of black phosphorus.

The large foreseeable use two-dimensional materials in nanotechnology consequently demands precise methods for their thickness measurements. Usually, having a quick and easy methodology is a key requisite for the inspection of the large number of flakes produced by exfoliation methods. An effective option in this respect relies on the measurement of the intensity of Raman spectra, which can be used even when the flakes are encapsulated by a transparent protective layer. However, when using this methodology, special attention should be paid to the crystalline anisotropy of the examined material. Specifically, for the case of black phosphorus flakes, the absolute experimental determination of the thickness is rather difficult because the material is characterized by a low symmetry and also because the Raman tensors are complex quantities. In this work, we exploited Raman spectroscopy to measure the thickness of black phosphorous flakes using silicon as reference material for intensity calibrations. We found out that we can determine the thickness of a flake above 5 nm with an accuracy of about 20%. We tested the reproducibility of the method on two different setups, finding similar results. The method can be applied also to other van der Waals materials with a Raman band characterized by the same Raman tensor.

Embedding epitaxial (blue) phosphorene in between device-compatible functional layers.

The newly predicted allotropic phase of phosphorus termed blue phosphorus has been recently synthesized in its two-dimensional (2D) single layer fashion via epitaxial growth on a Au(111) substrate. The large scale epitaxy and the semiconductive character with a reported bandgap of ∼1.1 eV suggest that epitaxial phosphorene might be a suitable candidate to overcome the lack of a sizeable bandgap in semimetal X-enes. In close similarity to other X-enes, like silicene, the epitaxial phosphorene shows technological issues towards possible integration into devices, such as the metallic supporting template at the bottom and oxidation under ambient conditions on the top interface of its 2D lattice. Here, we report on a detailed structural and chemical analysis of epitaxial phosphorene and a newly developed methodology to allow for easy transfer of the chemically protected epitaxial phosphorene in between amorphous Al2O3 and thin Au(111) films grown on mica. The large scale epitaxy achieved on a portable Au(111)/mica template and the low reactivity with molecular oxygen of phosphorene pave the way for easy encapsulation of epitaxial phosphorene fostering its exploitability in devices through a versatile transfer methodology, as in the case of epitaxial silicene.

Engineering Epitaxial Silicene on Functional Substrates for Nanotechnology.

Two-dimensional materials are today a solid reality in condensed matter physics due to the disruptive discoveries about graphene. The class of the X-enes, namely, graphene-like single element artificial crystals, is quickly emerging driven by the high-momentum generated by silicene. Silicene, in addition to the graphene properties, shows up incidentally at the end of Moore's law debate in the electronic era. Indeed, silicene occurs as the crafted shrunk version of silicon long yearned by device manufacturers to improve the performances of their chips. Despite the periodic table kinship with graphene, silicene and the X-enes must deal with the twofold problem of their metastable nature, i.e., the stabilization on a substrate and out of vacuum environment. Synthesis on different substrates and deep characterization through electronic and optical techniques of silicene in the early days have been now following by the tentative steps towards reliable integration of silicene into devices. Here, we review three paradigmatic cases of silicene grown by molecular beam epitaxy showing three different possible applications, aiming at extending the exploitation of silicene out of the nanoelectronics field and thus keeping silicon a key player in nanotechnology, just in a thinner fashion.

Optical Conductivity of Two-Dimensional Silicon: Evidence of Dirac Electrodynamics.

The exotic electrodynamics properties of graphene come from the linearly dispersive electronic bands that host massless Dirac electrons. A similar behavior was predicted to manifest in freestanding silicene, the silicon counterpart of graphene, thereby envisaging a new route for silicon photonics. However, the access to silicene exploitation in photonics was hindered so far by the use of optically inappropriate substrates in experimentally realized silicene. Here we report on the optical conductivity of silicon nanosheets epitaxially grown on optically transparent Al2O3(0001) from a thickness of a few tens of nanometers down to the extreme two-dimensional (2D) limit. When a 2D regime is approached, a Dirac-like electrodynamics can be deduced from the observation of a low-energy optical conductivity feature owing to a silicene-based interfacing to the substrate.

Silicene, silicene derivatives, and their device applications.

Silicene, the ultimate scaling of a silicon atomic sheet in a buckled honeycomb lattice, represents a monoelemental class of two-dimensional (2D) materials similar to graphene but with unique potential for a host of exotic electronic properties. Nonetheless, there is a lack of experimental studies largely due to the interplay between material degradation and process portability issues. This review highlights the state-of-the-art experimental progress and future opportunities in the synthesis, characterization, stabilization, processing and experimental device examples of monolayer silicene and its derivatives. The electrostatic characteristics of the Ag-removal silicene field-effect transistor exhibit ambipolar charge transport, corroborating with theoretical predictions on Dirac fermions and Dirac cone in the band structure. The electronic structure of silicene is expected to be sensitive to substrate interaction, surface chemistry, and spin-orbit coupling, holding great promise for a variety of novel applications, such as topological bits, quantum sensing, and energy devices. Moreover, the unique allotropic affinity of silicene with single-crystalline bulk silicon suggests a more direct path for the integration with or revolution to ubiquitous semiconductor technology. Both the materials and process aspects of silicene research also provide transferable knowledge to other Xenes like stanene, germanene, phosphorene, and so forth.

Silicon Nanosheets: Crossover between Multilayer Silicene and Diamond-like Growth Regime.

The structural and electronic properties of nanoscale Si epitaxially grown on Ag(111) can be tuned from a multilayer silicene phase, where the constitutive layers incorporate a mixed sp2/sp3 bonding, to other ordinary Si phases, such as amorphous and diamond-like Si. Based on comparative scanning tunneling microscopy and Raman spectroscopy investigations, a key role in determining the nanoscale Si phase is played by the growth temperature of the epitaxial deposition on Ag(111) substrate and the presence or absence of a single-layer silicene as a seed for the successive growth. Furthermore, when integrated into a field-effect transistor device, multilayer silicene exhibits a characteristic ambipolar charge carrier transport behavior that makes it strikingly different from other conventional Si channels and suggestive of a Dirac-like character of the electronic bands of the crystal. These findings spotlight the interest in multilayer silicene as a different nanoscale Si phase for advanced nanotechnology applications such as ultrascaled nanoelectronics and nanomembranes, as well as for fundamental exploration of quantum properties.

Silicene field-effect transistors operating at room temperature.

Free-standing silicene, a silicon analogue of graphene, has a buckled honeycomb lattice and, because of its Dirac bandstructure combined with its sensitive surface, offers the potential for a widely tunable two-dimensional monolayer, where external fields and interface interactions can be exploited to influence fundamental properties such as bandgap and band character for future nanoelectronic devices. The quantum spin Hall effect, chiral superconductivity, giant magnetoresistance and various exotic field-dependent states have been predicted in monolayer silicene. Despite recent progress regarding the epitaxial synthesis of silicene and investigation of its electronic properties, to date there has been no report of experimental silicene devices because of its air stability issue. Here, we report a silicene field-effect transistor, corroborating theoretical expectations regarding its ambipolar Dirac charge transport, with a measured room-temperature mobility of ∼100 cm(2) V(-1) s(-1) attributed to acoustic phonon-limited transport and grain boundary scattering. These results are enabled by a growth-transfer-fabrication process that we have devised--silicene encapsulated delamination with native electrodes. This approach addresses a major challenge for material preservation of silicene during transfer and device fabrication and is applicable to other air-sensitive two-dimensional materials such as germanene and phosphorene. Silicene's allotropic affinity with bulk silicon and its low-temperature synthesis compared with graphene or alternative two-dimensional semiconductors suggest a more direct integration with ubiquitous semiconductor technology.

Two-dimensional Si nanosheets with local hexagonal structure on a MoS(2) surface.

The structural and electronic properties of a Si nanosheet (NS) grown onto a MoS2 substrate by means of molecular beam epitaxy are assessed. Epitaxially grown Si is shown to adapt to the trigonal prismatic surface lattice of MoS2 by forming two-dimensional nanodomains. The Si layer structure is distinguished from the underlying MoS2 surface structure. The local electronic properties of the Si nanosheet are dictated by the atomistic arrangement of the layer and unlike the MoS2 hosting substrate they are qualified by a gap-less density of states.