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J. Appl. Cryst.の発刊に際して

Materials research plays a vital role in transforming breakthrough scientific ideas into next-generation technology. Similar to the way silicon revolutionized the microelectronics industry, the proper materials can greatly impact the field of plasmonics and metamaterials. Currently, research in plasmonics and metamaterials lacks good material building blocks in order to realize useful devices. Such devices suffer from many drawbacks arising from the undesirable properties of their material building blocks, especially metals. There are many materials, other than conventional metallic components such as gold and silver, that exhibit metallic properties and provide advantages in device performance, design flexibility, fabrication, integration, and tunability. This review explores different material classes for plasmonic and metamaterial applications, such as conventional semiconductors, transparent conducting oxides, perovskite oxides, metal nitrides, silicides, germanides, and 2D materials such as graphene. This review provides a summary of the recent developments in the search for better plasmonic materials and an outlook of further research directions.

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Alternative Plasmonic Materials: Beyond Gold and Silver

Alloying has long been used to confer desirable properties to materials. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. The multi-dimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here, we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behaviour has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking. High-entropy alloys have greatly expanded the compositional space for alloy design. In this Review, the authors discuss model high-entropy alloys with interesting properties, the physical mechanisms responsible for their behaviour and fruitful ways to probe and discover new materials in the vast compositional space that remains to be explored.

High-entropy alloys

Numerical Initial Value Problems in Ordinary Differential Equations

The thin film formation of magnetron sputtered polycrystalline coatings was monitored by in situ X-ray reflectivity measurements. The measured intensity was analyzed using the Parratt algorithm for time-dependent thin film systems. Guidelines for the on-line interpretation of the data were developed. For thick coatings, the experimental resolution needs to be included in the data evaluation in order to avoid misinterpretations. Based on a simple layer model, the time-dependent mean electron density, roughness and growth velocity were extracted from the data. As an example, the method was applied to the hard coating material vanadium carbide. Both instantaneous and slowly varying changes of the coating could be detected. It was shown that the growth velocity is proportional to the DC power. Significant changes of the microstructure induced by the working gas pressure are mainly driven by the chemical composition.

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Monitoring the thin film formation during sputter deposition of vanadium carbide

Mn + 1AXn (MAX; n = 1–3) phases are ternary layered nitride and carbide compounds featuring a combination of metallic and ceramic properties. Highly basal-plane textured and polycrystalline Cr2AlC, Ti2AlC, and Ti3AlC2 single-phase coatings have been synthesized on both amorphous and polycrystalline substrates via controlled thermal annealing of magnetron-sputtered nanoscale multilayers built by individual transition metal, carbon, and aluminum layers. Formation of substitutional solid solution carbide phases was triggered via solid-state diffusion reactions during annealing. Lower ordered Ti2AlC initially crystallized at an intermediate temperature range and was recognized as an intermediate reactant in the case of synthesizing the Ti3AlC2 312 MAX phase via annealing corresponding stoichiometric multilayers. The crystallization onset temperatures identified via in-situ high-temperature x-ray diffraction measurements were approximately 480, 660, and 820 °C for Cr2AlC, Ti2AlC, and Ti3AlC2, respectively. Contrary to the usually observed columnar structure representative of magnetron-sputtered coatings, the coatings synthesized via the current approach are composed of plateletlike, elongated crystallites. The nanoscale multilayered design stimulates the textured growth of MAX structures during thermal annealing. More specifically, the preferred crystallographic orientation relationships among the as-deposited transition metal layers, the intermediate solid solution phases, and the end-product MAX phases facilitate the growth of textured MAX phase films.

Textured growth of polycrystalline MAX phase carbide coatings via thermal annealing of M/C/Al multilayers

Boron nitride thin films were deposited by unbalanced radio frequency magnetron sputtering of a hot-pressed hexagonal boron nitride target in a mixed argon/nitrogen atmosphere of 0.2 Pa. The intensity of ion bombardment during deposition was varied by a direct current substrate bias between 0 and −500 V. Thin films containing more than 85% of the sp 3 -bonded cubic phase (c-BN), as confirmed by infrared spectroscopy, were produced in a wide voltage range from −200 to −400 V. The high c-BN content can be maintained even at an appreciably reduced bias of −80 V when a nucleation layer is formed in advance. In that case a decrease of compressive stress as well as an alteration of surface structure of the deposited c-BN films was observed. An analysis in light of electron energy loss spectroscopy indicates moreover the existence of an sp 2 -coordinated surface layer with an ion-energy-dependent thickness. The results are discussed within the frame of the subplantation model.

Sven Ulrich

Papers

Monitoring the thin film formation during sputter deposition of vanadium carbide

Textured growth of polycrystalline MAX phase carbide coatings via thermal annealing of M/C/Al multilayers

The constitution and properties of cubic boron nitride thin films: a comparative study on the influence of bombarding ion energy

Characterisation of silicon carbide and silicon nitride thin films and Si3N4/SiC multilayers

Influence of the energy of sputtered carbon atoms on the constitution of diamond-like carbon thin films