Towards Perfect Absorption of Single Layer CVD Graphene in an Optical Resonant Cavity: Challenges and Experimental Achievements.

Graphene is emerging as a promising material for the integration in the most common Si platform, capable to convey some of its unique properties to fabricate novel photonic and optoelectronic devices. For many real functions and devices however, graphene absorption is too low and must be enhanced. Among strategies, the use of an optical resonant cavity was recently proposed, and graphene absorption enhancement was demonstrated, both, by theoretical and experimental studies. This paper summarizes our recent progress in graphene absorption enhancement by means of Si/SiO2-based Fabry-Perot filters fabricated by radiofrequency sputtering. Simulations and experimental achievements carried out during more than two years of investigations are reported here, detailing the technical expedients that were necessary to increase the single layer CVD graphene absorption first to 39% and then up to 84%. Graphene absorption increased when an asymmetric Fabry-Perot filter was applied rather than a symmetric one, and a further absorption increase was obtained when graphene was embedded in a reflective rather than a transmissive Fabry-Perot filter. Moreover, the effect of the incident angle of the electromagnetic radiation and of the polarization of the light was investigated in the case of the optimized reflective Fabry-Perot filter. Experimental challenges and precautions to avoid evaporation or sputtering induced damage on the graphene layers are described as well, disclosing some experimental procedures that may help other researchers to embed graphene inside PVD grown materials with minimal alterations.

The Critical Raw Materials Issue between Scarcity, Supply Risk, and Unique Properties.

This editorial reports on a thorough analysis of the abundance and scarcity distribution of chemical elements and the minerals they form in the Earth, Sun, and Universe in connection with their number of neutrons and binding energy per nucleon. On one hand, understanding the elements' formation and their specific properties related to their electronic and nucleonic structure may lead to understanding whether future solutions to replace certain elements or materials for specific technical applications are realistic. On the other hand, finding solutions to the critical availability of some of these elements is an urgent need. Even the analysis of the availability of scarce minerals from European Union sources leads to the suggestion that a wide-ranging approach is essential. These two fundamental assumptions represent also the logical approach that led the European Commission to ask for a multi-disciplinary effort from the scientific community to tackle the challenge of Critical Raw Materials. This editorial is also the story of one of the first fulcrum around which a wide network of material scientists gathered thanks to the support of the funding organization for research and innovation networks, COST (European Cooperation in Science and Technology).

Critical Raw Materials Saving by Protective Coatings under Extreme Conditions: A Review of Last Trends in Alloys and Coatings for Aerospace Engine Applications.

Several applications, where extreme conditions occur, require the use of alloys often containing many critical elements. Due to the ever increasing prices of critical raw materials (CRMs) linked to their high supply risk, and because of their fundamental and large utilization in high tech products and applications, it is extremely important to find viable solutions to save CRMs usage. Apart from increasing processes' efficiency, substitution, and recycling, one of the alternatives to preserve an alloy and increase its operating lifetime, thus saving the CRMs needed for its manufacturing, is to protect it by a suitable coating or a surface treatment. This review presents the most recent trends in coatings for application in high temperature alloys for aerospace engines. CRMs' current and future saving scenarios in the alloys and coatings for the aerospace engine are also discussed. The overarching aim of this paper is to raise awareness on the CRMs issue related to the alloys and coating for aerospace, suggesting some mitigation measures without having the ambition nor to give a complete overview of the topic nor a turnkey solution.

Powder Bed Fusion Additive Manufacturing Using Critical Raw Materials: A Review.

The term "critical raw materials" (CRMs) refers to various metals and nonmetals that are crucial to Europe's economic progress. Modern technologies enabling effective use and recyclability of CRMs are in critical demand for the EU industries. The use of CRMs, especially in the fields of biomedicine, aerospace, electric vehicles, and energy applications, is almost irreplaceable. Additive manufacturing (also referred to as 3D printing) is one of the key enabling technologies in the field of manufacturing which underpins the Fourth Industrial Revolution. 3D printing not only suppresses waste but also provides an efficient buy-to-fly ratio and possesses the potential to entirely change supply and distribution chains, significantly reducing costs and revolutionizing all logistics. This review provides comprehensive new insights into CRM-containing materials processed by modern additive manufacturing techniques and outlines the potential for increasing the efficiency of CRMs utilization and reducing the dependence on CRMs through wider industrial incorporation of AM and specifics of powder bed AM methods making them prime candidates for such developments.

Experimental demonstration of mid-IR absorption enhancement in single layer CVD graphene.

Mid-IR absorption of single layer graphene (SLG) was simulated and experimentally demonstrated by embedding a SLG grown by chemical vapor deposition (CVD) inside a Fabry-Perot (FP) filter made by alternating quarter wave Si and SiO2 layers fabricated by radiofrequency sputtering. The absorption from the graphene layer was modeled by using COMSOL Multiphysics in four different configurations, depending on its position inside the filter, an asymmetric FP made of two different dielectric mirrors separated by a cavity. In the first three configurations, graphene was inserted at the center of the optical cavity and inside the top or bottom dielectric mirror forming the FP. The fourth configuration involves two layers of graphene, each positioned inside one of the dielectric mirrors. The calculated electric field distribution inside the FP shows two symmetric maxima just above and below the cavity, i.e., inside the mirrors, while the electric field at the center of the cavity is negligible. For the experimental demonstration, the graphene geometry corresponding to the maximum electric field intensity was chosen, and, between two equivalent alternatives, the one with the easiest fabrication procedure was selected. Results demonstrate a maximum experimental absorption of 50% at 4342 nm for SLG when inserted in the top mirror of the FP, in excellent agreement with the simulated value of 53%.

The Critical Raw Materials in Cutting Tools for Machining Applications: A Review.

A variety of cutting tool materials are used for the contact mode mechanical machining of components under extreme conditions of stress, temperature and/or corrosion, including operations such as drilling, milling turning and so on. These demanding conditions impose a seriously high strain rate (an order of magnitude higher than forming), and this limits the useful life of cutting tools, especially single-point cutting tools. Tungsten carbide is the most popularly used cutting tool material, and unfortunately its main ingredients of W and Co are at high risk in terms of material supply and are listed among critical raw materials (CRMs) for EU, for which sustainable use should be addressed. This paper highlights the evolution and the trend of use of CRMs) in cutting tools for mechanical machining through a timely review. The focus of this review and its motivation was driven by the four following themes: (i) the discussion of newly emerging hybrid machining processes offering performance enhancements and longevity in terms of tool life (laser and cryogenic incorporation); (ii) the development and synthesis of new CRM substitutes to minimise the use of tungsten; (iii) the improvement of the recycling of worn tools; and (iv) the accelerated use of modelling and simulation to design long-lasting tools in the Industry-4.0 framework, circular economy and cyber secure manufacturing. It may be noted that the scope of this paper is not to represent a completely exhaustive document concerning cutting tools for mechanical processing, but to raise awareness and pave the way for innovative thinking on the use of critical materials in mechanical processing tools with the aim of developing smart, timely control strategies and mitigation measures to suppress the use of CRMs.

Experimental near infrared absorption enhancement of graphene layers in an optical resonant cavity.

Graphene has recently emerged as a promising candidate for a wide range of photonic and optoelectronic applications, with a high application potential in devices using infrared radiation. The optical absorption of 2D materials and graphene can be uniquely enhanced when they are embedded in optical resonant cavities, since optically-thin atomic-thickness absorbers do not perturb the cavity itself. Despite the many theoretical studies, experimental validation is still lagging behind. Here, large near infrared (NIR) absorption of unpatterned chemical vapor deposition graphene is experimentally demonstrated for the first time in a large area (1 inch) passive optical device by exploiting the enhancement of the electric field at the center of a Fabry-Perot cavity. Test devices were fabricated with single layer, double layer and five layers graphene, sandwiched between two almost symmetric Bragg mirrors deposited by radio frequency sputtering and consisting of alternate layers of Si and SiO2. A thin evaporated MgF2 overlayer was used to reduce sputtering induced damage on graphene layers. Measured absorption values, in the range of 37%-45%, were found in very good accordance with simulated ones. A maximum absorption of 45% was measured at 2345 nm for the double-layer graphene.

Measuring ultrathin metal coatings using SPR spectroscopic ellipsometry with a prism-dielectric-metal-liquid configuration.

A new surface plasmon resonance (SPR) configuration is proposed, which consists of a prism, a dielectric layer, a metal coating, and a matching liquid. The optical constants of each layer in the proposed prism-dielectric-metal-liquid (PDML) configuration have been optimized to match the SPR conditions and reach the strongest intensity. Combining the PDML configuration with spectroscopic ellipsometry, SPR spectroscopic ellipsometry (SPRSE) with a PDML configuration was developed. The SPR wavelength can be adjusted to the desired wavelength by varying the thickness of the dielectric layer. The amplitude and phase change, magnified by the SPR in the visible and near-infrared wavelengths, were obtained to determine the optical constants and thickness of ultrathin metal coatings. The extracted optical constants were found to be in good agreement with the results obtained using transmission electron microscopy (TEM) and X-ray reflectivity (XRR) techniques. These SPRSE measurements show great potential for characterizing the interface between a metal coating and a dielectric layer, and the surface uniformity of ultrathin metal coatings.

Solutions for Critical Raw Materials under Extreme Conditions: A Review.

In Europe, many technologies with high socio-economic benefits face materials requirements that are often affected by demand-supply disruption. This paper offers an overview of critical raw materials in high value alloys and metal-matrix composites used in critical applications, such as energy, transportation and machinery manufacturing associated with extreme working conditions in terms of temperature, loading, friction, wear and corrosion. The goal is to provide perspectives about the reduction and/or substitution of selected critical raw materials: Co, W, Cr, Nb and Mg.

Behavior of optical thin-film materials and coatings under proton and gamma irradiation.

Optical materials and coatings are exposed to the flux of energetic particles when used in either space applications or nuclear energy plants. The study of their behavior in such an environment is important to avoid failure of the optical components during their operation. The optical performance of several thin-film materials ((HfO2, Ta2O5, Nb2O5, TiO2, SiO2) and coatings, under irradiation with high-dose gamma rays (5.8 MGy) and exposure to low-energy (60 keV) protons, has been investigated. Some variations of optical properties have been detected in silicon oxide after irradiation, while the other materials are stable in such conditions.

Optical transmission filters for observation of lightning phenomena in the Earth atmosphere.

Very narrowband transmission filters, as parts of an instrument for the study of lightning phenomena, are described. Their performance must be maintained at an incidence angle of ±5.5° and this condition poses some limitations on the minimum bandwidth of the order of a few nanometers. The fabrication of such coatings on large area substrates is quite challenging because of the required thickness accuracy. Moreover, their performance should be not influenced by the environmental conditions in space.

Optical parameters of oxide films typically used in optical coating production.

Wavelength dependencies of refractive indices of thin film materials differ for various deposition conditions, and it is practically impossible to attribute a single refractive index wavelength dependence to any typical thin film material. Besides objective reasons, differences in the optical parameters of thin films may also be connected with nonadequate choices of models and algorithms used for the processing of measurement data. The main goal of this paper is to present reliable wavelength dependencies of refractive indices of the most widely used slightly absorbing oxide thin film materials. These dependencies can be used by other researchers for comparison and verification of their own characterization results.