Abstract 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. A new approach based on sequential self-assembly (SSA) that enables the formation of a two-phase metamaterial (TPM) composed of a disordered network metamaterial with embedded nanoparticles (NPs) is proposed. Taking advantage of both the high-spatial and high-energy resolution of electron energy loss spectroscopy (EELS), inhomogeneous localization of light in the network is observed, concurrent with dipolar and higher-order localized surface plasmon modes in the nanoparticles. Moreover, it is demonstrated 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.

Abstract Magnetically assembled superparamagnetic colloids are exploited as fluid mixers, swimmers, and delivery systems in several microscale applications. The encapsulation of such colloids in droplets may open new opportunities to build magnetically controlled displays and optical components. Here, the assembly of superparamagnetic colloids inside droplets under rotating magnetic fields is studied, and this phenomenon is exploited to create functional optical devices. Colloids are encapsulated in monodisperse droplets produced by microfluidics and magnetically assembled into dynamic 2D clusters. Using an optical microscope equipped with a magnetic control setup, the effect of the magnetic field strength and rotational frequency on the size, stability, and dynamics of 2D colloidal clusters inside droplets is investigated. The results show that cluster size and stability depend on the magnetic forces acting on the structure under the externally imposed field. By rotating the cluster in specific orientations, it is possible to magnetically control the effective refractive index and the transmission of light through the colloid-laden droplets, thus demonstrating the potential of the encapsulated colloids in optical applications.

Abstract 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, a broadband scanning reflectance anisotropy microscope as a phase-sensitive multi-material optical platform for strain mapping is introduced. 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. Cutting edge strain sensitivity is demonstrated 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.

The reflection of light from distributed microplatelets is an effective approach to creating color and controlling the optical properties in paints, security features, and optical filters. However, predictive tools for the design and manufacturing of such composite materials are limited due to the complex light–matter interactions that determine their optical response. Here, the optical reflectance of individual reflective microplatelets and of polymer-based composites containing these engineered platelets as an aligned, dispersed phase are experimentally studied and analytically calculated. Transfer-matrix calculations are used to interpret the effect of the platelet architecture, the number of platelets, and their size distribution on the experimentally measured reflectance of composites prepared using a previously established magnetic alignment technique. It is demonstrated that the reflectance of the composites can be understood as the averaged response of an array of Fabry–Pérot resonators, in which the microplatelets act as semi-transparent flat reflectors and the polymer as cavity medium. By using an analytical model and computer simulations to describe the interaction of light with platelets embedded in a polymer matrix, this work provides useful tools for the design and fabrication of composites with tailored optical reflectance.

Electrohydrodynamic redox 3D printing (EHD-RP) is an additive manufacturing (AM) technique with submicron resolution and multi-metal capabilities, offering the possibility to switch chemistry during deposition “on-the-fly”. Despite the potential for synthesizing a large range of metals by electrochemical small-scale AM techniques, to date, only Cu and Ag have been reproducibly deposited by EHD-RP. Here, we extend the materials palette available to EHD-RP by using aqueous solvents instead of organic solvents, as used previously. We demonstrate deposition of Cu and Zn from sacrificial anodes immersed in acidic aqueous solvents. Mass spectrometry indicates that the choice of the solvent is important to the deposition of pure Zn. Additionally, we show that the deposited Zn structures, 250 nm in width, can be partially converted into semiconducting ZnO structures by oxidation at 325 °C in air.

Abstract 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, Ag/a-Si multilayers are examined as a model system for the thermal stability of hyperbolic metamaterials. Using a combination of nanotomography, finite element simulations, and optical spectroscopy, the thermal and optical instability of the metamaterials is mapped. Although the thermal instability initiates at 300 °C, the hyperbolic dispersion persists up to 500 °C. 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. These findings open new avenues to understand multilayer instability and pave the way to design hyperbolic metamaterials able to withstand high temperatures.

Structural color is frequently exploited by living organisms for biological functions and has also been translated into synthetic materials as a more durable and less hazardous alternative to conventional pigments. Additive manufacturing approaches were recently exploited for the fabrication of exquisite photonic objects, but the angle-dependence observed limits a broader application of structural color in synthetic systems. Here, we propose a manufacturing platform for the 3D printing of complex-shaped objects that display isotropic structural color generated from photonic colloidal glasses. Structurally colored objects are printed from aqueous colloidal inks containing monodisperse silica particles, carbon black, and a gel-forming copolymer. Rheology and Small-Angle-X-Ray-Scattering measurements are performed to identify the processing conditions leading to printed objects with tunable structural colors. Multimaterial printing is eventually used to create complex-shaped objects with multiple structural colors using silica and carbon as abundant and sustainable building blocks.

Mechanical metamaterials are periodic lattice structures with complex unit cell architectures that can achieve extraordinary mechanical properties beyond the capability of bulk materials. A 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. The tunable non-linear modulus increase can be exploited to design materials mimicking the complex mechanical response of biological tissue.

The design and fabrication of large-area metamaterials is an ongoing challenge. In the present work, we propose a scalable design route and low-footprint strategy for the production of large-area, frequency-selective Cu–Sn disordered network metamaterials with quasi-perfect absorption. The nanoscale networks combine the robustness of disordered systems with the broad-band optical response known from connected wire-mesh metamaterials. Using experiments and simulations, we show how frequency-selective absorption in the networks can be designed and controlled. We observe a linear dependence of the optical response as a function of Sn content ranging from the near-infrared to the visible region. The absorbing state exhibits strong sensitivity to both changes in the global network topology and the chemistry of the network. We probe the plasmonic response of these nanometric networks by electron energy loss spectroscopy (EELS), where we resolve extremely confined gap surface-plasmon (GSP) modes.

Intentional breaking of the lattice symmetry in solids is a key concept to alter the properties of materials by modifying their electronic band structure. However, the correlation of strain-induced effects and breaking of the lattice symmetry is often indirect, resorting to vibrational spectroscopic techniques, such as Raman scattering. Here, we demonstrate that reflectance anisotropy spectroscopy (RAS), which directly depends on the complex dielectric function, enables the direct observation of electronic band structure modulation. Studying the strain-induced symmetry breaking in copper, we show how uniaxial strain lifts the degeneracy of states in the proximity of the both L and X symmetry points, thus altering the matrix element for interband optical transitions, directly observable in RAS. We corroborate our experimental results by analyzing the strain-induced changes in the electronic structure based on ab initio density functional theory calculations. The versatility to study breaking of the lattice symmetry by simple reflectance measurements opens up the possibility to gain a direct insight on the band structure of other strain-engineered materials, such as graphene and two-dimensional transition metal dichalcogenides.

Above a critical temperature, high-performance fibers may lose their mechanical properties resulting in catastrophic events of damage when, e.g., used as load-carrying ropes. Here, a method to functionalize polymer fibers with thermochromic optical coatings that enable signaling of damaging thermal history is introduced. These smart coatings are comprised of an index-tunable anti-reflection coating based on chalcogenide phase change materials (PCM). It is demonstrated that the insulator−metal phase transition of these materials can be aligned with the critical deterioration temperature of both polyethylene terephthalate (PET) monofilaments and liquid-crystal polyester (LCP) yarns by composition tuning. The carefully designed optical system amplifies the change in optical properties of its constituents upon phase change. The thermal and mechanical degradation of these fibers can thus be monitored and displayed by eye.

Zero-index (ZI) materials are synthetic optical materials with a 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 system, we demonstrate that the localized crystallization of the semiconductor at the metal/semiconductor interface can be used as a 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 Ångstrom-sized crystalline shell of the semiconductor. Photonic doping can be used to enhance the transmission in these disordered metamaterials, as shown by simulations. Our results establish novel large-area zero-index materials for wafer-scale applications and beyond.

An extensive range of metals can be dissolved and re-deposited in liquid solvents using electrochemistry. We harness this concept for additive manufacturing, demonstrating the focused electrohydrodynamic ejection of metal ions dissolved from sacrificial anodes and their subsequent reduction to elemental metals on the substrate. This technique, termed electrohydrodynamic redox printing (EHD-RP), enables the direct, ink-free fabrication of polycrystalline multi-metal 3D structures without the need for post-print processing. On-the-fly switching and mixing of two metals printed from a single multichannel nozzle facilitates a chemical feature size of <400 nm with a spatial resolution of 250 nm at printing speeds of up to 10 voxels per second. As shown, the additive control of the chemical architecture of materials provided by EHD-RP unlocks the synthesis of 3D bi-metal structures with programmed local properties and opens new avenues for the direct fabrication of chemically architected materials and devices.

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 measured strength and resistivity 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.

Self‐healing behavior, the ability to autonomously counteract damage, is observed in some inorganic materials, and it has recently been extended to various artificial systems. In metals, healing usually requires thermal activation by a furnace treatment that stimulates damage repair. High temperature exposure, however, renders these routes incompatible with temperature‐sensitive systems such as on‐chip microelectronic components. In this work, designing Ni/Al multilayers as on‐chip heat sources, a concept for on‐demand healing of metal films that no longer relies on external annealing, is demonstrated. The process is based on harvesting a solitary self‐sustained heat wave that is produced by a solid‐state reaction in the heat source to weld cracks in different metal films. Healing is activated at room temperature with a remarkably small current input and in situ probing reveals a large conductance recovery up to 500 nm wide cracks within 1 ms, orders of magnitude faster than furnace‐based approaches. Intrinsic heat source healing represents a unique concept for rapid on‐chip healing of metal films that will provide new flexibility to prevent failure in inaccessible electronic systems: from implantable healthcare devices, to space probe instrumentation.

Abstract To ensure safe and reliable operation of materials with high thermal loads, e.g., in turbines or thermal solar collectors, it is key to detect material degradation. Here, a sensor concept providing a direct optical feedback of thermally induced hardness and resistivity changes in transition-metal-nitride functional coatings is presented. The sensor concept relies on a lossy Gires–Tournois interferometer configuration using thermally induced detuning of a highly absorbing state in the optical spectrum as feedback. It is demonstrated for the specific case of TiAlN coatings that such detuning is due to a symmetry-breaking structural phase transition, which is accompanied by the formation of saturated structural colors.

Abstract In this work, the onset strains of fragmentation, the fracture strength, and fracture toughness of amorphous and crystalline Ge2Sb2Te5 (GST) films are determined. Using in situ methods, such as resistance measurements, reflectance anisotropy spectroscopy (RAS), and light microscopy during uniaxial tensile loading, we demonstrate that onset strain of fragmentation and delamination depend on both, crystallographic state of the \GST\ film and the film thickness. We observe that amorphous \GST\ fractures at larger strains than crystalline GST. However, due to its small Young’s modulus the fracture toughness \KIC\ of amorphous \GST\ is lower than that of crystalline \GST\ (amorphous: \MPa\ m0.5; crystalline: \MPa\ m0.5). The results presented are critically discussed with respect to the potential application of \GST\ films in flexible displays.