arXiv subjects

Ralph Spolenak

Publications and source records attributed to Ralph Spolenak.

18 recordsLinked to original sources

arXiv2026

Ru Alloying in Ni/Al Reactive Multilayers: Experimental Observations and Molecular Dynamics Simulations

Reactive multilayer thin films, a class of energetic materials, are increasingly recognized for their potential in joining applications, utilizing the chemical energy released as heat during exothermic reactions. These materials hold also promise for additional diverse technological applications, which require precise control over heat release rates and reaction propagation velocities. The microstructural properties of reactive multilayers play a critical role in determining their chemical reaction behavior. Among these, Ni/Al reactive multilayers have been extensively studied and used due to their favorable characteristics. In this study, we explore the incorporation of ruthenium (Ru) as a co-alloying element with nickel (Ni) in the Ni/Al system to investigate its impact on the materials properties, with a particular focus on reaction velocity and temperature. Ru enhances the reaction rates, but also causes a composition dependent phase transition in the as-deposited state from fcc to hcp. Additionally, molecular dynamics simulations are employed to examine the effects of Ru co-alloying with Ni, providing deeper insights into the underlying mechanisms. This work aims to advance the understanding of Ru's role in influencing the performance of Al/Ni-based reactive multilayers for advanced applications.

cond-mat.mtrl-sci↗

arXiv2026

Nanodroplet-Confined Electroplating Enables Submicron Printing of Metals and Oxide Ceramics

The fabrication of functional micro- and nano-electronic devices requires the deposition of high-quality materials of different electronic material classes, such as conductors, semiconductors and insulators. To establish ultra-high-resolution additive manufacturing as a viable addition to existing fabrication methods requires the combinatorial additive deposition of different electronic material classes. However, current techniques do not provide such a capability. Here, we demonstrate that droplet confined electroplating, an ultra-high-resolution AM technique initially developed for metals as electrohydrodynamic redox printing (EHD-RP), allows not only the direct deposition of many metals, but also of metal-oxides. Particularly, we demonstrate that applying fundamental electrochemical principles in combination with on-the-fly switching of the deposited material allows for the direct co-deposition of metals, metal-hydroxides and -oxides. Our results exemplify the feasibility of leveraging simple water-based electrochemical concepts to produce intricate and multi-material structures at the nanoscale.

cond-mat.mtrl-sci↗

arXiv2025

Composition Effects on Ni/Al Reactive Multilayers: A Comprehensive Study of Mechanical Properties, Reaction Dynamics and Phase Evolution

Ni/Al reactive multilayers are promising materials for applications requiring controlled local energy release and superior mechanical performance. This study systematically investigates the impact of compositional variations, ranging from 30 to 70 at.% Ni, and bilayer thicknesses (30 nm and 50 nm) on the mechanical properties and reaction dynamics of Ni/Al multilayers. Multilayers with varying Ni-to-Al ratios were fabricated and subjected to instrumented nanoindentation testing to evaluate hardness and elastic modulus. Combustion experiments, conducted on dogbone-shaped multilayers deposited onto silicon wafers with thermal barrier coatings, characterized the reaction front's speed, temperature, and the resulting phases. The findings revealed that composition variations within this range enable precise tuning of reaction speed and temperature without significant changes in mechanical properties, while deviations in modulus and hardness at higher nickel concentrations suggest microstructural influences. Notably, phase formation in Al-rich samples deviated from equilibrium predictions, highlighting the role of kinetic factors, such as diffusion and rapid quenching, in driving non-adiabatic processes during phase evolution. Molecular dynamics simulations provided complementary atomistic insights into mechanical responses and reaction kinetics, bridging experimental observations with theoretical predictions. This integrated approach advances the understanding of Ni/Al multilayers, offering a framework for optimizing their composition and structural design to achieve tailored performance for application-specific requirements.

cond-mat.mtrl-sci↗

arXiv2024

CO$_2$ Conversion in Cu-Pd based Disordered Network Metamaterials with Ultra-Small Mode Volume

Plasmons can drive chemical reactions by directly exciting intramolecular transitions. However, strong coupling of plasmons to single molecules remains a challenge as ultra-small mode volumes are required. In the presented work, we propose Cu-Pd plasmonic network metamaterials as a scalable platform for plasmon-assisted catalysis. Due to the absence of translational symmetry, these networks provide a unique plasmonic environment featuring a large local density of optical states and an unparalleled density of hotspots that effectively localizes light in mode volumes $V<8\cdot10^{-24}$ m$^3$. Catalytic performance tests during CO$_2$ conversion reveal production rates of up to 4.3$\cdot$10$^2$ mmol g$^{-1}$h$^{-1}$ and altered reaction selectivity under light illumination. Importantly, we show that the selectivity of the catalytic process can be tuned by modifying the network's chemical composition, offering a versatile approach to optimize reaction pathways.

physics.optics↗

arXiv2024

Droplet-confined electroplating for nanoscale additive manufacturing: current control of the initial stages of the growth of copper nanowires

Droplet-confined electrodeposition enables a precise deposition of three dimensional, nanoscopic and high purity metal structures. It aspires to fabricate intricate microelectronic devices, metamaterials, plasmonic structures and functionalized surfaces. Yet, a major handicap of droplet-confined electrodeposition is the current lack of control over the process, which is owed to its dynamic nature and the nanoscopic size of the involved droplets. The deposition current offers itself as an obvious and real-time window into the deposition. Therefore, the current during droplet-confined deposition is analysed. Nucleation and growth dynamics are evaluated systematically. Our results indicate different deposition regimes and link current to both volume and morphology of deposited copper. This allows for optimized electroplating strategies and to calibrate the slicing algorithms necessary for a controlled deposition of 3D structures. The potential of selecting appropriate solvents further readies this novel technique for the reliable deposition of functional structures with submicron resolution.

cond-mat.mtrl-sci↗

arXiv2024

Hybrid resonant metasurfaces with configurable structural colors

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.

physics.optics↗

arXiv2023

Smart sensing of the multifunctional properties of magnetron sputtered $MoS_2$ across the amorphous-crystalline transition

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.

cond-mat.mtrl-sci↗

arXiv2023

Direct in- and out-of-plane writing of metals on insulators by electron-beam-enabled, confined electrodeposition with submicrometer feature size

Additive microfabrication processes based on localized electroplating enable the one-step deposition of micro-scale metal structures with outstanding performance, e.g. high electrical conductivity and mechanical strength. They are therefore evaluated as an exciting and enabling addition to the existing repertoire of microfabrication technologies. Yet, electrochemical processes are generally restricted to conductive or semiconductive substrates, precluding their application in the manufacturing of functional electric devices where direct deposition onto insulators is often required. Here, we demonstrate the direct, localized electrodeposition of copper on a variety of insulating substrates, namely Al2O3, glass and flexible polyethylene, enabled by electron-beam-induced reduction in a highly confined liquid electrolyte reservoir. The nanometer-size of the electrolyte reservoir, fed by electrohydrodynamic ejection, enables a minimal feature size on the order of 200 nm. The fact that the transient reservoir is established and stabilized by electrohydrodynamic ejection rather than specialized liquid cells could offer greater flexibility towards deposition on arbitrary substrate geometries and materials. Installed in a low-vacuum scanning electron microscope, the setup further allows for operando, nanoscale observation and analysis of the manufacturing process.

physics.app-ph↗

arXiv2023

Nanodroplet Flight Control in Electrohydrodynamic Redox 3D Printing

Electrohydrodynamic 3D printing is an additive manufacturing technique with enormous potential in plasmonics, microelectronics, and sensing applications, thanks to its broad materials palette, high voxel deposition rate, and compatibility with various substrates. However, the electric field used to deposit material is concentrated at the depositing structure resulting in the focusing of the charged droplets and geometry-dependent landing positions, which complicates the fabrication of complex 3D shapes. The low level of concordance between design and printout seriously impedes the development of electrohydrodynamic 3D printing and rationalizes the simplicity of the designs reported so far. In this work, we break the electric field centrosymmetry to study the resulting deviation in the flight trajectory of the droplets. Comparison of experimental outcomes with predictions of an FEM model provides new insights into the droplet characteristics and unveils how the product of droplet size and charge uniquely governs its kinematics. From these insights, we develop reliable predictions of the jet trajectory and allow the computation of optimized printing paths counterbalancing the electric field distortion, thereby enabling the fabrication of geometries with unprecedented complexity.

physics.flu-dyn↗

arXiv2023

Scanning Reflectance Anisotropy Microscopy for Multi-Material Strain Mapping

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.

physics.optics↗

arXiv2022

Strain-Driven Thermal and Optical Instability in Silver/Amorphous-Silicon Hyperbolic Metamaterials

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.

cond-mat.mtrl-sci↗

arXiv2022

Deformation-induced topological transitions in mechanical metamaterials and their application to tunable non-linear stiffening

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.

physics.app-ph↗

arXiv2019

Metals by micro-scale additive manufacturing: comparison of microstructure and mechanical properties

Many emerging applications in microscale engineering rely on the fabrication of three-dimensional architectures in inorganic materials. Small-scale additive manufacturing (AM) aspires to provide flexible and facile access to these geometries. Yet, the synthesis of device-grade inorganic materials is still a key challenge towards the implementation of AM in microfabrication. Here, we present a comprehensive overview of the microstructural and mechanical properties of metals fabricated by most state-of-the-art AM methods that offer a spatial resolution $\leq$10$μ$m. Standardized sets of samples were studied by cross-sectional electron microscopy, nanoindentation and microcompression. We show that current microscale AM techniques synthesize metals with a wide range of microstructures and elastic and plastic properties, including materials of dense and crystalline microstructure with excellent mechanical properties that compare well to those of thin-film nanocrystalline materials. The large variation in materials performance can be related to the individual microstructure, which in turn is coupled to the various physico-chemical principles exploited by the different printing methods. The study provides practical guidelines for users of small-scale additive methods and establishes a baseline for the future optimization of the properties of printed metallic objects $-$ a significant step towards the potential establishment of AM techniques in microfabrication.

cond-mat.mtrl-sci↗

arXiv2019

Disordered Zero-Index Metamaterials Based On Metal Induced Crystallization

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 ångström-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.

physics.app-ph↗

arXiv2017

Ice-Templated W-Cu Composites with High Anisotropy

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.

cond-mat.mtrl-sci↗

arXiv2016

Scalable, ultra-resistant structural colors based on network metamaterials

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.

physics.optics↗

arXiv2011

Dealloying of Platinum-Aluminum Thin Films Part II. Electrode Performance

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.

cond-mat.mtrl-sci↗

arXiv2011

Dealloying of Platinum-Aluminum Thin Films Part I. Dynamics of Pattern Formation

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.

cond-mat.mtrl-sci↗