Terahertz and Infrared Plasmon Polaritons in PtTe2 Type-II Dirac Topological Semimetal.

Surface plasmon polaritons (SPPs) are electromagnetic excitations existing at the interface between a metal and a dielectric. SPPs provide a promising path in nanophotonic devices for light manipulation at the micro and nanoscale with applications in optoelectronics, biomedicine, and energy harvesting. Recently, SPPs are extended to unconventional materials like graphene, transparent oxides, superconductors, and topological systems characterized by linearly dispersive electronic bands. In this respect, 3D Dirac and Weyl semimetals offer a promising frontier for infrared (IR) and terahertz (THz) radiation tuning by topologically-protected SPPs. In this work, the THz-IR optical response of platinum ditelluride (PtTe2) type-II Dirac topological semimetal films grown on Si substrates is investigated. SPPs generated on microscale ribbon arrays of PtTe2 are detected in the far-field limit, finding an excellent agreement among measurements, theoretical models, and electromagnetic simulation data. The far-field measurements are further supported by near-field IR data which indicate a strong electric field enhancement due to the SPP excitation near the ribbon edges. The present findings indicate that the PtTe2 ribbon array appears an ideal active layout for geometrically tunable SPPs thus inspiring a new fashion of optically tunable materials in the technologically demanding THz and IR spectrum.

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.

Extreme Bendability of Atomically Thin MoS2 Grown by Chemical Vapor Deposition Assisted by Perylene-Based Promoter.

Shaping two-dimensional (2D) materials in arbitrarily complex geometries is a key to designing their unique physical properties in a controlled fashion. This is an elegant solution, taking benefit from the extreme flexibility of the 2D layers but requiring the ability to force their spatial arrangement from flat to curved geometries in a delicate balance among free-energy contributions from strain, slip-and-shear mechanisms, and adhesion to the substrate. Here, we report on a chemical vapor deposition approach, which takes advantage of the surfactant effects of organic molecules, namely the tetrapotassium salt of perylene-3,4,9,10-tetracarboxylic acid (PTAS), to conformally grow atomically thin layers of molybdenum disulphide (MoS2) on arbitrarily nanopatterned substrates. Using atomically resolved transmission electron microscope images and density functional theory calculations, we show that the most energetically favorable condition for the MoS2 layers consists of its adaptation to the local curvature of the patterned substrate through a shear-and-slip mechanism rather than strain accumulation. This conclusion also reveals that the perylene-based molecules have a role in promoting the adhesion of the layers onto the substrate, no matter the local-scale geometry.

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.

Ambient Pressure Chemical Vapor Deposition of Flat and Vertically Aligned MoS2 Nanosheets.

Molybdenum disulfide (MoS2) got tremendous attention due to its atomically thin body, rich physics, and high carrier mobility. The controlled synthesis of large area and high crystalline monolayer MoS2 nanosheets on diverse substrates remains a challenge for potential practical applications. Synthesizing different structured MoS2 nanosheets with horizontal and vertical orientations with respect to the substrate surface would bring a configurational versatility with benefit for numerous applications, including nanoelectronics, optoelectronics, and energy technologies. Among the proposed methods, ambient pressure chemical vapor deposition (AP-CVD) is a promising way for developing large-scale MoS2 nanosheets because of its high flexibility and facile approach. Here, we show an effective way for synthesizing large-scale horizontally and vertically aligned MoS2 on different substrates such as flat SiO2/Si, pre-patterned SiO2 and conductive substrates (TaN) benefit various direct TMDs production. In particular, we show precise control of CVD optimization for yielding high-quality MoS2 layers by changing growth zone configuration and the process steps. We demonstrated that the influence of configuration variability by local changes of the S to MoO3 precursor positions in the growth zones inside the CVD reactor is a key factor that results in differently oriented MoS2 formation. Finally, we show the layer quality and physical properties of as-grown MoS2 by means of different characterizations: Raman spectroscopy, scanning electron microscopy (SEM), photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS). These experimental findings provide a strong pathway for conformally recasting AP-CVD grown MoS2 in many different configurations (i.e., substrate variability) or motifs (i.e., vertical or planar alignment) with potential for flexible electronics, optoelectronics, memories to energy storage devices.

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.

Hybrid MoS2/PEDOT:PSS transporting layers for interface engineering of nanoplatelet-based light-emitting diodes.

Colloidal semiconductor nanoplatelets (NPLs) are a subgroup of quantum confined materials that have recently emerged as promising active materials for solution processed light-emitting diodes (LEDs) thanks to their peculiar structural and electronic properties as well as their reduced dimensionality. Nowadays, the conventional structure for NPL-based LEDs makes use of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as a hole transporting layer (HTL). This is a well-known conjugated conductive polymer because it leads to high LED efficiency, though it has limited stability in air due to its intrinsic acidity and hygroscopicity. Here, we develop a nanocomposite aqueous ink, obtained by blending commercial PEDOT:PSS with water-based, stable and highly concentrated molybdenum disulfide (MoS2) nanosheets, obtained via liquid phase exfoliation (LPE), which is suitable as a HTL for solution processed NPL-based LEDs. We demonstrate that the MoS2 additive effectively works as a performance booster in unpackaged devices, thereby prolonging the lifetime up to 1000 hours under ambient conditions. Moreover, the addition of MoS2 induces a modification of the anode interface properties, including a change in the work function and a significant enhancement of the permittivity of the HTL.

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.

Broadband and Tunable Light Harvesting in Nanorippled MoS2 Ultrathin Films.

Nanofabrication of flat optic silica gratings conformally layered with two-dimensional (2D) MoS2 is demonstrated over large area (cm2), achieving a strong amplification of the photon absorption in the active 2D layer. The anisotropic subwavelength silica gratings induce a highly ordered periodic modulation of the MoS2 layer, promoting the excitation of Guided Mode Anomalies (GMA) at the interfaces of the 2D layer. We show the capability to achieve a broadband tuning of these lattice modes from the visible (VIS) to the near-infrared (NIR) by simply tailoring the illumination conditions and/or the period of the lattice. Remarkably, we demonstrate the possibility to strongly confine resonant and nonresonant light into the 2D MoS2 layers via GMA excitation, leading to a strong absorption enhancement as high as 240% relative to a flat continuous MoS2 film. Due to their broadband and tunable photon harvesting capabilities, these large area 2D MoS2 metastructures represent an ideal scalable platform for new generation devices in nanophotonics, photo- detection and -conversion, and quantum technologies.

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.

Ultra-broadband photon harvesting in large-area few-layer MoS2 nanostripe gratings.

Flat optics nanoarrays based on few-layer MoS2 are homogeneously fabricated over large-area (cm2) transparent templates, demonstrating effective tailoring of the photon absorption in two-dimensional (2D) transition-metal dichalcogenide (TMD) layers. The subwavelength subtractive re-shaping of the few-layer MoS2 film into a one-dimensional (1D) nanostripe array results in a pronounced photonic anomaly, tunable in a broadband spectral range by simply changing the illumination conditions (or the lattice periodicity). This scheme promotes efficient coupling of light to the 2D TMD layers via resonant interaction between the MoS2 excitons and the photonic lattice, with subsequent enhancement of absorption exceeding 400% relative to the flat layer. In parallel, an ultra-broadband absorption amplification in the whole visible spectrum is achieved, thanks to the non-resonant excitation of substrate guided modes promoted by MoS2 nanoarrays. These results highlight the potential of nanoscale re-shaped 2D TMD layers for large-area photon harvesting in layered nanophotonics, quantum technologies and new-generation photovoltaics.

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.

High-Density Sb2 Te3 Nanopillars Arrays by Templated, Bottom-Up MOCVD Growth.

Sb2 Te3 exhibits several technologically relevant properties, such as high thermoelectric efficiency, topological insulator character, and phase change memory behavior. Improved performances are observed and novel effects are predicted for this and other chalcogenide alloys when synthetized in the form of high-aspect-ratio nanostructures. The ability to grow chalcogenide nanowires and nanopillars (NPs) with high crystal quality in a controlled fashion, in terms of their size and position, can boost the realization of novel thermoelectric, spintronic, and memory devices. Here, it is shown that highly dense arrays of ultrascaled Sb2 Te3 NPs can be grown by metal organic chemical vapor deposition (MOCVD) on patterned substrates. In particular, crystalline Sb2 Te3 NPs with a diameter of 20 nm and a height of 200 nm are obtained in Au-functionalized, anodized aluminum oxide (AAO) templates with a pore density of ≈5 × 1010 cm-2 . Also, MOCVD growth of Sb2 Te3 can be followed either by mechanical polishing and chemical etching to produce Sb2 Te3 NPs arrays with planar surfaces or by chemical dissolution of the AAO templates to obtain freestanding Sb2 Te3 NPs forests. The illustrated growth method can be further scaled to smaller pore sizes and employed for other MOCVD-grown chalcogenide alloys and patterned substrates.