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Ternary transition metal nitrides (TMNs) promise to significantly expand the material design space by opening new functionality and enhancing existing properties. However, most systems have only been investigated computationally and limited understanding of their stabilizing mechanisms restricts translation to experimental synthesis. To better elucidate key factors in designing ternary TMNs, we experimentally fabricate and analyze the physical properties of the ternary Zr-Pt-N system. Structural analysis and DFT modeling demonstrate that Pt substitutes nitrogen on the non-metallic sublattice, which destabilizes the rock-salt structure and forms a complex cubic phase. We also show insolubility of Pt in the Zr-Pt-N at 45 at % with the formation of a secondary Pt-rich phase. The measured reduced plasma frequency, decrease in resistivity, and decrease in hardness reflect a dominance of metallic behavior in bonding. Contrary to previous computational predictions, Zr-Pt-N films are shown to be metastable systems where even low Pt concentrations (1%) facilitate a solid reaction with the Si-substrate, that is inaccessible in ZrN films., Comment: 6 pages, 3 figures

Metasurfaces play a key role in functionalizing light at the nanoscale. Existing dielectric metasurfaces, however, are often limited to geometric primitives and their usage in emergent hybrid metasurfaces is hampered as confinement of light occurs only in their interior. Taking inspiration from biophotonic systems in nature, we introduce a new class of hybrid metasurfaces, which combine ordered and disordered elements. While the ordered phase relies on non-reciprocal meta-atoms - whose breaking of the out-of-plane symmetry enables the confinement of visible light in air, the disordered phase exploits global plasmonic network modes and their ability to localize energy at nanometric scales. By generating configurable structural colors with extra-ordinary resolution, we demonstrate that coupling of these elements provides a new dimension in the design space. We showcase that control of the local light-matter interaction enables the creation of intricate, customizable optical patterns, which open new avenues for information encoding and high-security features.

Molybdenum disulfide, $MoS_2$, is a next-generation semiconductor and is frequently integrated into emergent optoelectronic technologies based on two-dimensional materials. Here, we present a method that provides direct optical feedback on the thickness and crystallinity of sputter-deposited $MoS_2$ down to the few-layer regime. This smart sensing enables tracking the material's functional properties, such as excitonic response, sheet resistance, and hardness across the amorphous-crystalline transition. To illustrate the potential of such feedback-controlled fabrication, we realized $MoS_2$-based hyperbolic metamaterials (HMM) with controllable optical topological transitions and hardness., Comment: 17 pages, 4 figures, 68 references. Supplementary available with 16 pages, 8 figures, and 10 references

Self-assembly processes provide the means to achieve scalable and versatile metamaterials by "bottom-up" fabrication. Despite their enormous potential, especially as a platform for energy materials, self-assembled metamaterials are often limited to single phase systems, and complex multi-phase metamaterials have scarcely been explored.We propose a new approach based on sequential self-assembly that enables the formation of a two-phase metamaterial composed of a disordered network metamaterial with embedded nanoparticles. Taking advantage of both the high-spatial and high-energy resolution of electron energy loss spectroscopy, we observe inhomogeneous localization of light in the network, concurrent with dipolar and higher-order localized surface plasmon modes in the nanoparticles. Moreover, we demonstrate that the coupling strength deviates from the interaction of two classical dipoles when entering the strong coupling regime. The observed energy exchange between two phases in this complex metamaterial, realized solely through self-assembly, implies the possibility to exploit these disordered systems for plasmon-enhanced catalysis., Comment: 14 pages, 5 figures

Strain-engineering of materials encompasses significant elastic deformation and leads to breaking of the lattice symmetry and as a consequence to the emergence of optical anisotropy. However, the capability to image and map local strain fields by optical microscopy is currently limited to specific materials. Here, we introduce a broadband scanning reflectance anisotropy microscope as a phase-sensitive multi-material optical platform for strain mapping. The microscope produces hyperspectral images with diffraction-limited sub-micron resolution of the near-normal incidence ellipsometric response of the sample, which is related to elastic strain by means of the elasto-optic effect. We demonstrate cutting edge strain sensitivity using a variety of materials, such as metasurfaces, semiconductors and metals. The versatility of the method to study the breaking of the lattice symmetry by simple reflectance measurements opens up the possibility to carry out non-destructive mechanical characterization of multi-material components, such as wearable electronics and optical semiconductor devices., Comment: 9 pages, 4 figures. Added missing department in the authors' affiliation

Hyperbolic metamaterials show exceptional optical properties, such as near-perfect broadband absorption, due to their geometrically-engineered optical anisotropy. Many of their proposed applications, such as thermophotovoltaics or radiative cooling, require high-temperature stability. In this work we examine Ag/a-Si multilayers as a model system for the thermal stability of hyperbolic metamaterials. Using a combination of nanotomography, finite element simulations and optical spectroscopy, we map the thermal and optical instability of the metamaterials. Although the thermal instability initiates at 300C, the hyperbolic dispersion persists up to 500C. Direct finite element simulations on tomographical data provide a route to decouple and evaluate interfacial and elastic strain energy contributions to the instability. Depending on stacking order the instability's driving force is either dominated by changes in anisotropic elastic strain energy due thermal expansion mismatch or by minimization of interfacial energy. Our findings open new avenues to understand multilayer instability and pave the way to design hyperbolic metamaterials able to withstand high temperatures., Comment: Number of figures: 4 figures comprising 32 separate panels in total Number of pages: 10 pages Number of citations: 61 citations Presented at MRS Spring 2022

Mechanical metamaterials are periodic lattice structures with complex unit cell architectures that can achieve extraordinary mechanical properties beyond the capability of bulk materials. A new class of metamaterials is proposed, whose mechanical properties rely on deformation-induced transitions in nodal-topology by formation of internal self-contact. The universal nature of the principle presented, is demonstrated for tension, compression, shear and torsion. In particular, it is shown that by frustration of soft deformation modes, large highly non-linear stiffening effects can be generated. Tunable non-linear elasticity can be exploited to design materials mimicking the complex mechanical response of biological tissue.

Zero-index (ZI) materials are synthetic optical materials with vanishing effective permittivity and/or permeability at a given design frequency. Recently, it has been shown that the permeability of a zero-index host material can be deterministically tuned by adding photonic dopants. Here, we apply metal-induced crystallization (MIC) in quasi-random metal-semiconductor composites to fabricate large-area zero-index materials. Using Ag-Si as a model systems, we demonstrate that the localized crystallization of the semiconductor at the metal/semiconductor interface can be used as design parameter to control light interaction in such a disordered system. The induced crystallization generates new zero-index states corresponding to a hybridized plasmonic mode emerging from selective coupling of light to the \r{a}ngstr\"om-sized crystalline shell of the semiconductor. Photonic doping can be used to enhance the transmission in these disordered metamaterials as is shown by simulation. Our results break ground for novel large-area zero-index materials for wafer scale applications and beyond., Comment: 23 pages, 4 figures

Controlling anisotropy in self-assembled structures enables engineering of materials with highly directional response. Here, we harness the anisotropic growth of ice walls in a thermal gradient to assemble an anisotropic refractory metal structure, which is then infiltrated with Cu to make a composite. Using experiments and simulations, we demonstrate on the specific example of tungsten-copper composites the effect of anisotropy on the electrical and mechanical properties. The results are compared to isotropic tungsten-copper composites fabricated by standard powder metallurgical methods. Our results have the potential to fuel the development of more efficient materials, used in electrical power grids and solar-thermal energy conversion systems. The method presented here can be used with a variety of refractory metals and ceramics, which fosters the opportunity to design and functionalize a vast class of new anisotropic load-bearing hybrid metal composites with highly directional properties., Comment: 11 pages, 4 figures

Structural colours have drawn wide attention for their potential as a future printing technology for various applications, ranging from biomimetic tissues to adaptive camouflage materials. However, an efficient approach to realise robust colours with a scalable fabrication technique is still lacking, hampering the realisation of practical applications with this platform. Here we develop a new approach based on large scale network metamaterials, which combine dealloyed subwavelength structures at the nanoscale with loss-less, ultra-thin dielectrics coatings. By using theory and experiments, we show how sub-wavelength dielectric coatings control a mechanism of resonant light coupling with epsilon-near-zero (ENZ) regions generated in the metallic network, manifesting the formation of highly saturated structural colours that cover a wide portion of the spectrum. Ellipsometry measurements report the efficient observation of these colours even at angles of $70$ degrees. The network-like architecture of these nanomaterials allows for high mechanical resistance, which is quantified in a series of nano-scratch tests. With such remarkable properties, these metastructures represent a robust design technology for real-world, large scale commercial applications., Comment: 23 pages, 6 figures

Pt-Al and Pt-Y-Al thin film electrodes on yttria-stabilised zirconia electrolytes were prepared by dealloying of co-sputtered Pt-Al or Pt-Y-Al films. The selective dissolution of Al from the Pt-alloy compound causes the formation of a highly porous nanowire network with a mean branch thickness below 25 nm and a pore intercept length below 35 nm. The oxygen reduction capability of the resulting electrodes was analysed in a micro-solid oxide fuel cell setup at elevated temperatures (598-873 K). Here, we demonstrate that these nanoporous thin films excel "state-of-the-art" fuel cell electrodes in terms of catalytic activity and thermal stability. The nanoporous Pt electrodes exhibit exchange current densities that are up to 13 times higher than conventional Pt electrodes, measured at 648 K. It is shown that the enhanced catalytic activity of these Pt electrodes is achieved through the engineering of the materials d-bands due to the addition of yttrium as ternary constituent., Comment: 6 pages, 4 figures, 11 panels

During the growth of metal thin films on dielectric substrates at a given deposition temperature T, the film's morphology is conditioned by the magnitude and asymmetry of up- and downhill diffusion. Any severe change of this mechanism leads to a growth instability, which induces an alteration of the thin film morphology. In order to study this mechanism, ultra-thin Pt films were deposited via pulsed laser deposition (PLD) onto yttria-stabilized-zirconia single crystals at different deposition temperatures. The morphological evolution of Pt thin films has been investigated by means of scanning electron microscopy (SEM), atomic force microscopy (AFM) and standard image analysis techniques. The experimentally obtained morphologies are compared to simulated thin film structures resulting from a two-dimensional kinetic Monte Carlo (KMC) approach. Two main observations have been made: i) Thin Pt films deposited onto zirconia undergo a growth transition from two-dimensional to three-dimensional growth at T > 573 K. The growth transition and related morphological changes are a function of the deposition temperature. ii) A critical cluster size of i\ast = 4 in combination with an asymmetric Ehrlich-Schwoebel (ES) barrier favoring the uphill diffusion of atoms allows for a computational reproduction of the experimentally obtained film morphologies., Comment: 7 pages, 6 figures, 1 table

The stability of a metal thin films on a dielectric substrate is conditioned by the magnitude of the interactive forces at the interface. In the case of a non-reactive interface and weak adhesion, the minimization of free surface energy gives rise to an instability of the thin film. In order to study these effects, Pt thin films with a thickness of 50 nm were deposited via ion-beam sputtering on yttria stabilized zirconia single crystals. All Pt films were subjected to heat treatments up to 973 K for 2 h. The morphological evolution of Pt thin films has been investigated by means of scanning electron microscopy (SEM), atomic force microscopy (AFM) and standard image analysis techniques. Three main observations have been made: i) the deposition method has a direct impact on the morphological evolution of the film during annealing. Instead of hole formation, that is typically observed as response to a thermal treatment, anisotropic pyramidal shaped hillocks are formed on top of the film. ii) It is shown by comparing the hillocks' aspect ratio with finite element method (FEM) simulations that the hillock formation can be assigned to a stress relaxation process inside the thin film. iii) By measuring the equilibrium shapes and the shape fluctuations of the formed Pt hillocks the anisotropy of the step free energy and its stiffness have been derived in addition to the anisotropic kink energy of the hillock's edges., Comment: 8 pages, 7 figures

Highly porous Pt/Al thin film electrodes on yttria stabilized zirconia electrolytes were prepared by dealloying of co-sputtered Pt/Al films. The oxygen reduction capability of the resulting electrodes was analyzed in a solid oxide fuel cell setup at elevated temperatures. During initial heating to 523 K exceptionally high performances compared to conventional Pt thin film electrodes were measured. This results from the high internal surface area and large three phase boundary length obtained by the dealloying process. Exposure to elevated temperatures of 673 K or 873 K gave rise to degradation of the electrode performance, which was primarily attributed to the oxidation of remaining Al in the thin films., Comment: 5 pages, 4 figures

Applying focused ion beam (FIB) nanotomography and Rutherford backscattering spectroscopy (RBS) to dealloyed platinum-aluminum thin films an in-depth analysis of the dominating physical mechanisms of porosity formation during the dealloying process is performed. The dynamical porosity formation due to the dissolution of the less noble aluminum in the alloy is treated as result of a reaction-diffusion system. The RBS analysis yields that the porosity formation is mainly caused by a linearly propagating diffusion front, i.e. the liquid/solid interface, with a uniform speed of 42(3) nm/s when using a 4M aqueous NaOH solution at room temperature. The experimentally observed front evolution is captured by the normal diffusive Fisher-Kolmogorov-Petrovskii-Piskounov (FKPP) equation and can be interpreted as a branching random walk phenomenon. The etching front produces a gradual porosity with an enhanced porosity in the surface-near regions of the thin film due to prolonged exposure of the alloy to the alkaline solution., Comment: 4 pages, 5 figures