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Item type:Publication, Access status: without files , Shaping the electric field: overcoming geometric inhomogeneities in the electrochemical fabrication of mesoporous materialsIntegrating porous systems such as mesoporous silicon (pSi) into reliable macroscopic devices for sensing, photonics and energy storage requires stringent control over their structural homogeneity. While radial variations in layer thickness and porosity are frequently attributed to edge sealing effects or pinned gas, the macroscopic impact of the electrochemical cell’s internal geometry is largely overlooked. Using pSi as a model system, we demonstrate that the insulating cell body acts as a significant field-shaping component. Combining 2D axisymmetric finite element simulations of the current distribution with spatially resolved near-infrared (NIR) spectrometry, we investigate distortions caused by standard cell features. We reveal that the internal container widening step, commonly used to accommodate counter electrodes, strongly compresses the electric field and generates localized, ring-shaped zones of accelerated etching. Optimizing this step height reduced the radial thickness deviation of a pSi layer 6-fold to just 2%. Furthermore, predictive simulations demonstrate that internal obstacles, such as stirring rods, can be deliberately positioned to either prevent unwanted current shadows or actively manipulate the current distribution for precise three-dimensional structuring. Ultimately, mindful hardware design that prioritizes spatial current distribution is a critical and efficient strategy to maximize the yield of highly homogeneous porous media, directly enhancing their scalability and performance in functional nanomaterial applications.Publicationtype: Journal Article11 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On the applicability of the Maxwell Garnett effective medium model to media with a high density of cylindrical pores(Optica Publishing Group, 2024-04-01); ; ; ; The optical properties of dielectric materials with subwavelength cylindrical pores are commonly described by effective medium models. We compare the Maxwell Garnett and the Bruggeman effective medium models for porous silicon with simulations and experiments for the case of polarization orthogonal to the pore axis. The Maxwell Garnett model matches the results of the simulations even up to very high porosities. An experimental study of the effective permittivity of macroporous and mesoporous silicon is conducted by analyzing the Fabry-Pérot oscillations in the long-wavelength limit. These experimental results are also consistent with the Maxwell Garnett model. We advocate using this model for media with cylindrical pores in the future.Publicationtype: Journal Article36 58 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Laser-excited elastic guided waves reveal the complex mechanics of nanoporous siliconPublicationtype: Book part15 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , How nanoporous silicon-polypyrrole hybrids flex their muscles in aqueous electrolytes: in operando high-resolution x-ray diffraction and electron tomography-based micromechanical computer simulations(American Physical Society (APS), 2022-11-28); ; ; ; Macroscopic strain experiments have revealed that silicon crystals traversed by parallel, channel-like nanopores functionalized with the artificial muscle polymer polypyrrole (PPy) exhibit large and reversible electrochemomechanical actuation in aqueous electrolytes. On a macroscopic scale these actuation properties are well understood. However, on the microscopical level this system still bears open questions, as to how the electrochemical expansion and contraction of PPy acts on to np-Si pore walls and how the collective motorics of the pore array emerges from the single-nanopore behavior. Here we present synchrotron-based, in operando x-ray diffraction on the evolving electrostrains in epilayers of this material grown on bulk silicon. An analysis of these experiments with micromechanical finite-element simulations, that are based on a full three-dimensional reconstruction of the nanoporous medium by transmission electron microscopy (TEM) tomography, shows that the in-plane mechanical response is dominantly isotropic despite the anisotropic elasticity of the single-crystalline host matrix. However, the structural anisotropy originating from the parallel alignment of the nanopores led to significant differences between the in- and out-of-plane electromechanical response. This response is not describable by a simple two-dimensional arrangement of parallel cylindrical channels. Rather, the simulations highlight that the dendritic shape of the silicon pore walls, including pore connections between the main channels, causes complex, highly inhomogeneous stress-strain fields in the crystalline host. Time-dependent x-ray scattering experiments on the dynamics of the actuator properties hint towards the importance of diffusion limitations, plastic deformation, and creep in the nanoconfined polymer upon (counter)ion adsorption and desorption, the very pore-scale processes causing the macroscopic electroactuation. From a more general perspective, our study demonstrates that the combination of TEM tomography-based micromechanical modeling with high-resolution x-ray scattering experiments provides a powerful approach for in operando analysis of nanoporous composites from the single nanopore up to the porous-medium scale.Publicationtype: Journal ArticleScopus© Citations 5 54 54 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Laser-excited elastic guided waves reveal the complex mechanics of nanoporous silicon(Nature Publishing Group, 2021-06-14); ; ; ; Nanoporosity in silicon leads to completely new functionalities of this mainstream semiconductor. A difficult to assess mechanics has however significantly limited its application in fields ranging from nanofluidics and biosensorics to drug delivery, energy storage and photonics. Here, we present a study on laser-excited elastic guided waves detected contactless and non-destructively in dry and liquid-infused single-crystalline porous silicon. These experiments reveal that the self-organised formation of 100 billions of parallel nanopores per square centimetre cross section results in a nearly isotropic elasticity perpendicular to the pore axes and an 80% effective stiffness reduction, altogether leading to significant deviations from the cubic anisotropy observed in bulk silicon. Our thorough assessment of the wafer-scale mechanics of nanoporous silicon provides the base for predictive applications in robust on-chip devices and evidences that recent breakthroughs in laser ultrasonics open up entirely new frontiers for in-situ, non-destructive mechanical characterisation of dry and liquid-functionalised porous materials.Publicationtype: Journal ArticleScopus© Citations 25 149 327