Ralph Spolenak

Last Name

Spolenak

First Name

Ralph

ORCID

0000-0003-3413-0594

Organisational unit

03692 - Spolenak, Ralph / Spolenak, Ralph

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Publications 1 - 8 of 8

Electrohydrodynamic redox printing vs. physical vapour deposition: a comparative study of nanoporous Ag morphology and SERS performance
Porenta, Nikolaus; Spolenak, Ralph; Piazza , Loredana; et al. (2025)
Discover Nano
The increasing demand for miniaturised, high-performance sensing platforms necessitates materials that can be deposited with high spatial precision. Surface-enhanced Raman spectroscopy (SERS) has emerged as a powerful analytical technique, offering significant signal amplification. Nanoporous (np) metals, particularly np Ag, are promising candidates for SERS substrates due to their high surface area and tunable nanostructure. In this study, we compare np Ag fabricated via electrohydrodynamic redox printing (EHD-RP), an additive manufacturing technique with high spatial resolution, to conventionally produced counterparts using physical vapour deposition (PVD). EHD-RP-derived np Ag exhibits comparable SERS performance to PVD samples. Structural analysis reveals that the density of sub-25 nm pores and the degree of structural disorder strongly contribute to enhancement factors. Additionally, EHD-RP-derived np Ag demonstrates excellent stability under varying illumination conditions and effectively catalyses the plasmon-driven dimerisation of 4-nitrobenzenethiol. These results underscore the potential of EHD-RP for fabricating functional nanostructured materials for integrated sensing applications.
Droplet-Confined Electroplating for Nanoscale Additive Manufacturing: Current Control of the Initial Stages of Growth of Copper Nanowires
Nydegger, Mirco; Spolenak, Ralph (2025)
ACS Electrochemistry
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 and needs to be analyzed operando. Nucleation and growth dynamics are evaluated systematically. Our results indicate different deposition regimes and link the current to both morphology and volume of deposited copper. This allows for optimized electroplating strategies and calibration of the slicing algorithms necessary for a controlled deposition of 3D structures with different solvents. The potential of selecting appropriate solvents further readies this novel technique for the reliable deposition of functional structures with submicron resolution.
Hybrid Resonant Metasurfaces with Configurable Structural Colors
Wohlwend, Jelena; Hilti, Anna; Polinari, Claudiadele; et al. (2024)
Advanced Optical Materials
Metasurfaces are sub-wavelength nanostructures with the potential to overcome the limitations of traditional optics. Existing dielectric metasurfaces, however, are often limited to geometric primitives, which hamper their application in emergent hybrid metasurfaces. Here, a simple fabrication scheme for non-primitive metasurfaces that addresses this limitation is introduced. Due to their broken out-of-plane symmetry, these nanostructures provide a new dimension in the design space. Taking inspiration from bio-photonic systems in nature, using these elements complex hybrid metasurfaces are designed that encompass an ordered and a disordered phase. The capabilities of this hybrid optical systems are illustrated by generating configurable structural colors with extra ordinary resolution. Furthermore, the local control of light over a broad bandwidth is demonstrated and a maximum coupling efficiency of more than 97% is achieved.
Hierarchically Porous SnO2/Cu Composites via Freeze Casting and Selective Cu Reduction
Fandré, Jean Pascal; Pennell, Samuel; Ramesh, Sapna Lalitha; et al. (2025)
Advanced Engineering Materials
Lamellar SnO2/Cu foams are created by directional freeze casting of SnO2/CuO slurries followed by liquid phase sintering and subsequent hydrogen reduction to achieve a two-phase, interpenetrating SnO2-30 vol% Cu lamellar structure. The resulting SnO2/Cu foams exhibit both lamellar channels (millimeters in length) from the freeze-casting step and submicron porosity within the Cu phase from the reduction step. This hierarchical microstructure provides increased electronic conductivity relative to unmodified SnO2. When applied as a negative electrode material for lithium-ion batteries, the interaction between the mesoporous Cu phase and embedded SnO2 enables the conversion reaction of the SnO2 and Li to become reversible, improving the capacity of the electrode. However, the lamellar structure is ultimately unable to accommodate the expansion of the Sn during lithiation, resulting in a breakdown of the architecture during cycling.
Micromechanical Insights into Sinter-Based Additively Manufactured NiTi with Nb as a Sintering Aid
Abando, Nerea; Gallivan, Rebecca Anne; Toncich, Nensi; et al. (2025)
Advanced Engineering Materials
The combination of active materials with complex geometries, as enabled by additive manufacturing techniques, facilitates access to unprecedented mechanical responses. Among different printing methods, filament-based materials extrusion (f-MEX) grants extraordinary design freedom for a wide range of metals. Yet, the final properties are often corrupted by poor sintering behavior. Current filament compositions are tailored toward performance improvement via polymeric additives, but the effect of metallic additives remains to be fully explored. This work studies the microstructural and micromechanical impact of incorporating Nb as a liquid-phase sintering aid to NiTi shape memory alloys printed via f-MEX. Printed and sintered systems are compared to bare sintered powders via chemically correlated nanoindentation to investigate the effect of the filament binder on the final properties. Nanoindentation mapping highlights the intricate microstructure influenced by the addition of Nb. Inhomogeneities from the as-received powder still remain due to the short sintering times, which hinder complete diffusion of Nb into the NiTi matrix. This work demonstrates the microstructural and micromechanical integrity of NiTiNb parts produced via f-MEX, proving liquid sintering a reliable alternative for sintering these promising alloys and making them suitable for engineering applications at a lower processing energy expense.
CO₂ Conversion in Cu–Pd Based Disordered Network Metamaterials with Ultrasmall Mode Volumes
Wohlwend, Jelena; Wipf, Oliver; Kiwic, David; et al. (2025)
Nano Letters
Composition effects on Ni/Al reactive multilayers: A comprehensive study of mechanical properties, reaction dynamics, and phase evolution
Toncich, Nensi; Schwarz, Fabian; Gallivan, Rebecca Anne; et al. (2025)
Journal of Applied Physics
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 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.
Electrohydrodynamic redox printing vs magnetron sputtering: Enhancing hardness of nanoporous silver with twinning and structural order
Porenta, Nikolaus; Gallivan, Rebecca Anne; Galvis Melo, Joshua; et al. (2025)
Materials & Design
Nanoporous (np) metals are used for a variety of applications including actuators, catalysts, and sensors. 3D printing enabled multi-functional design offers a pathway for enhancing performance in these applications. Electrohydrodynamic redox printing (EHD-RP), is a high resolution additive manufacturing technique which can produce complex 3D metal alloys that allows for the fabrication of np metal structures. To determine the influence of this additive manufacturing process on the mechanical properties, we compare EHD-RP derived np-Ag to films made via magnetron sputtering. Samples fabricated by EHD-RP exhibit higher hardness than sputtered films for similar relative densities as demonstrated with nanoindentation. To explain these differences we used FIB-tomography to quantify structural features in the np networks including ligament width, relative density, connectivity, and topology. Materials factors like twin density and grain size were investigated using STEM analysis. Together, the higher twin density and more ordered network structure found in EHD-RP explain the enhanced mechanical behaviour of np-Ag and highlight a key opportunity for additive manufacturing to enhance both materials and structural design in np networks.

Publications 1 - 8 of 8

Ralph Spolenak

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