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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 56 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Morphology of nanoporous glass : stochastic 3D modeling, stereology and the influence of pore width(American Physical Society (APS), 2024-04-01); ; ; ; Excursion sets of Gaussian random fields are used to model the three-dimensional (3D) morphology of differently manufactured porous glasses (PGs), which vary with respect to their mean pore widths measured by mercury intrusion porosimetry. The stochastic 3D model is calibrated by means of volume fractions and two-point coverage probability functions estimated from tomographic image data. Model validation is performed by comparing model realizations and image data in terms of morphological descriptors which are not used for model fitting. For this purpose, we consider mean geodesic tortuosity and constrictivity of the pore space, quantifying the length of the shortest transportation paths and the strength of bottleneck effects, respectively. Additionally, a stereological approach for parameter estimation is presented, i.e., the 3D model is calibrated using merely two-dimensional (2D) cross-sections of the 3D image data. Doing so, on average, a comparable goodness of fit is achieved as well. The variance of the calibrated model parameters is discussed, which is estimated on the basis of randomly chosen, individual 2D cross-sections. Moreover, interpolating between the model parameters calibrated to differently manufactured glasses enables the predictive simulation of virtual but realistic PGs with mean pore widths that have not yet been manufactured. The predictive power is demonstrated by means of cross-validation. Using the presented approach, relationships between parameters of the manufacturing process and descriptors of the resulting morphology of PGs are quantified, which opens possibilities for an efficient optimization of the underlying manufacturing process.Publicationtype: Journal Article12 71 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Polymeric liquids in mesoporous photonic structures: from precursor film spreading to imbibition dynamics at the nanoscale(American Institute of Physics (AIP), 2024-02-14); ; ; ; Polymers are known to wet nanopores with high surface energy through an atomically thin precursor film followed by slower capillary filling. We present here light interference spectroscopy using a mesoporous membrane-based chip that allows us to observe the dynamics of these phenomena in situ down to the sub-nanometer scale at milli- to microsecond temporal resolution. The device consists of a mesoporous silicon film (average pore size 6 nm) with an integrated photonic crystal, which permits to simultaneously measure the phase shift of thin film interference and the resonance of the photonic crystal upon imbibition. For a styrene dimer, we find a flat fluid front without a precursor film, while the pentamer forms an expanding molecular thin film moving in front of the menisci of the capillary filling. These different behaviors are attributed to a significantly faster pore-surface diffusion compared to the imbibition dynamics for the pentamer and vice versa for the dimer. In addition, both oligomers exhibit anomalously slow imbibition dynamics, which could be explained by apparent viscosities of six and eleven times the bulk value, respectively. However, a more consistent description of the dynamics is achieved by a constriction model that emphasizes the increasing importance of local undulations in the pore radius with the molecular size and includes a sub-nanometer hydrodynamic dead, immobile zone at the pore wall but otherwise uses bulk fluid parameters. Overall, our study illustrates that interferometric, opto-fluidic experiments with mesoporous media allow for a remarkably detailed exploration of the nano-rheology of polymeric liquids.Publicationtype: Journal Article22 11 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Wafer-scale fabrication of hierarchically porous silicon and silica by active nanoparticle-assisted chemical etching and pseudomorphic thermal oxidation(Wiley-VCH, 2023-06-01); ; ; ; Many biological materials exhibit a multiscale porosity with small, mostly nanoscale pores as well as large, macroscopic capillaries to simultaneously achieve optimized mass transport capabilities and lightweight structures with large inner surfaces. Realizing such a hierarchical porosity in artificial materials necessitates often sophisticated and expensive top-down processing that limits scalability. Here, an approach that combines self-organized porosity based on metal-assisted chemical etching (MACE) with photolithographically induced macroporosity for the synthesis of single-crystalline silicon with a bimodal pore-size distribution is presented, i.e., hexagonally arranged cylindrical macropores with 1 µm diameter separated by walls that are traversed by pores 60 nm across. The MACE process is mainly guided by a metal-catalyzed reduction–oxidation reaction, where silver nanoparticles (AgNPs) serve as the catalyst. In this process, the AgNPs act as self-propelled particles that are constantly removing silicon along their trajectories. High-resolution X-ray imaging and electron tomography reveal a resulting large open porosity and inner surface for potential applications in high-performance energy storage, harvesting and conversion or for on-chip sensorics and actuorics. Finally, the hierarchically porous silicon membranes can be transformed structure-conserving by thermal oxidation into hierarchically porous amorphous silica, a material that could be of particular interest for opto-fluidic and (bio-)photonic applications due to its multiscale artificial vascularization.Publicationtype: Journal ArticleScopus© Citations 1 70 82 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Diffusionlike Drying of a Nanoporous Solid as Revealed by Magnetic Resonance Imaging(2022-11); ; ; Drying plays a central role in various fabrication processes and applications of functional nanoporous materials, most prominently in relation to energy storage and conversion. During such processes, liquid coexists with air inside the sample, leading to transport as a result of concentration gradients of vapor and/or liquid. Experimentally, it is extremely challenging to unravel this transport phenomenology inside the hidden geometry of porous media. Here, we observe the drying of a model nanoporous material (monolithic mesoporous silica glass, Vycor) with magnetic resonance imaging. We show that, for various boundary conditions (air-flux intensities), no dry region develops, but the sample desaturates in depth. This desaturation is almost homogeneous throughout the sample for weak air flux, while saturation gradients can be observed for sufficiently strong air flux. We demonstrate that the transport of water is mainly ensured by liquid flow towards the free surface, resulting from a gradient of vapor pressure, associated with local saturation (via the desorption curve), leading to a gradient of liquid pressure (via the Kelvin law). Assuming otherwise standard hydrodynamic characteristics of the nanoconfined liquid, this results in a diffusionlike model, which appears to represent experimental data very well in terms of the spatial distribution of water over time inside the sample for various boundary conditions (air-flux intensities). Finally, we propose a predictive model of the detailed drying characteristics of a nanomaterial from knowledge of its pore size, permeability, and desorption curve. This provides an insight into the rational design of drying-based processes employing functional nanoporous materials and allows for a mechanistic understanding of drying phenomenologies in natural nanoporous media.Publicationtype: Journal Article20 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , On the issue of textured crystallization of Ba(NO₃)₂ in mesoporous SiO₂: Raman spectroscopy and lattice dynamics analysis(Elsevier, 2022-07-05); ; ; ; The lattice dynamics of preferentially aligned nanocrystals formed upon drying of aqueous Ba(NO3)2 solutions in a mesoporous silica glass traversed by tubular pores of approximately 12 nm are explored by Raman scattering. To interpret the experiments on the confined nanocrystals polarized Raman spectra of bulk single crystals and X-ray diffraction experiments are also performed. Since a cubic symmetry is inherent to Ba(NO3)2, a special Raman scattering geometry was utilized to separate the phonon modes of Ag and Eg species. Combining group-theory analysis and ab initio lattice dynamics calculations a full interpretation of all Raman lines of the bulk single crystal is achieved. Apart from a small confinement-induced line broadening, the peak positions and normalized peak intensities of the Raman spectra of the nanoconfined and macroscopic crystals are identical. Interestingly, the Raman scattering experiment indicates the existence of comparatively large, ∼10–20 μm, single-crystalline regions of Ba(NO3)2 embedded in the porous host, near three orders of magnitude larger than the average size of single nanopores. This is contrast to the initial assumption of non-interconnected pores. It rather indicates an inter-pore propagation of the crystallization front, presumably via microporosity in the pore walls.Publicationtype: Journal Article37 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Multiple glassy dynamics of a homologous series of triphenylene-based columnar liquid crystals - a study by broadband dielectric spectroscopy and advanced calorimetryHexakis(n-alkyloxy)triphenylene) (HATn) consisting of an aromatic triphenylene core and alkyl side chains are model discotic liquid crystal (DLC) systems forming a columnar mesophase. In the mesophase, the molecules of HATn self-assemble in columns, which has one-dimensional high charge carrier mobility along the columns. Here, a homologous series of HATn with different length of the alkyl chain (n=5,6,8,10,12) is investigated using differential scanning calorimetry (DSC), broadband dielectric spectroscopy (BDS) and advanced calorimetric techniques including fast scanning calorimetry (FSC) and specific heat spectroscopy (SHS). The investigation of the phase behavior was done utilizing DSC experiments and the influence of the alkyl chain length on the phase behavior was revealed. By the dielectric investigations probing the molecular mobility, a γ-relaxation due to localized fluctuations as well as two glassy dynamics the α core and α alkyl relaxation were observed in the temperature range of the plastic crystalline phase. Moreover, the observed glassy dynamics were further studied employing advanced calorimetry. All observed relaxation processes are attributed to the possible specific molecular fluctuations and discussed in detail. From the results a transition at around n=8 from a rigid constrained (n=5,6) to a softer system (n=10,12) was revealed with increasing alkyl chain length. A counterbalance of two competing effects of a polyethylene like behavior of the alkyl chains in the intercolumnar domains and self-organized confinement is discussed in the context of a hindered glass transition.Publicationtype: Journal Article33 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Dynamic Kerr and Pockels electro-optics of liquid crystals in nanopores for active photonic metamaterials(Royal Society of Chemistry (RSC), 2021-11-05); ; ; ; Photonic metamaterials with properties unattainable in base materials are already beginning to revolutionize optical component design. However, their exceptional characteristics are often static, as artificially engineered into the material during the fabrication process. This limits their application for in-operando adjustable optical devices and active optics in general. Here, for a hybrid material consisting of a liquid crystal-infused nanoporous solid, we demonstrate active and dynamic control of its meta-optics by applying alternating electric fields parallel to the long axes of its cylindrical pores. First-harmonic Pockels and second-harmonic Kerr birefringence responses, strongly depending on the excitation frequency and temperature, are observed in a frequency range from 50 Hz to 50 kHz. This peculiar behavior is quantitatively traced by a Landau-De Gennes free energy analysis to an order-disorder orientational transition of the rod-like mesogens and intimately related changes in the molecular mobilities and polar anchoring at the solid walls on the single-pore, meta-atomic scale. Thus, our study provides evidence that liquid crystal-infused nanopores exhibit integrated multi-physical couplings and reversible phase changes that make them particularly promising for the design of photonic metamaterials with thermo-electrically tunable birefringence in the emerging field of space-time metamaterials aiming at full spatio-temporal control of light.Publicationtype: Journal Article70 392 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Paraelectric KH₂PO₄ nanocrystals in monolithic mesoporous silica: Structure and lattice dynamicsCombining dielectric crystals with mesoporous solids allows a versatile design of functional nanomaterials, where the porous host provides a mechanical rigid scaffold structure and the molecular filling adds the functionalization. Here, we report a study of the complex lattice dynamics of a SiO₂:KH₂PO₄ nanocomposite consisting of a monolithic, mesoporous silica glass host with KH₂PO₄ nanocrystals embedded in its tubular channels ~12 nm across. A micro-Raman investigation performed in the spectral range of 70–1600 cm−1 reveals the complex lattice dynamics of the confined crystals. Their Raman spectrum resembles the one taken from bulk KH₂PO₄ crystals and thus, along with X-ray diffraction experiments, corroborates the successful solution-based synthesis of KH₂PO₄ nanocrystals with a structure analogous to the bulk material. We succeeded in observing not only the high-frequency internal modes (~900–1200 cm−1), typical of internal vibrations of the PO₄ tetrahedra, but, more importantly, also the lowest frequency modes typical of bulk KH₂PO₄ crystals. The experimental Raman spectrum was interpreted with a group theory analysis and first-principle lattice dynamics calculations. The calculated phonon spectrum reveals a lattice instability against two optical modes probably related to an intrinsic anharmonicity of the vibrational dynamics of KH₂PO₄ crystals originating from the uncertainty of proton localization within the network of hydrogen bonds. The analysis of calculated eigen-vectors indicates the involvement of hydrogen atoms in most phonon modes corroborating the substantial significance of the hydrogen subsystem in the lattice dynamics of paraelectric bulk and of KH₂PO₄ crystals in extreme spatial confinement. A marginal redistribution of relative Raman intensities of the confined compared to unconfined crystals presumably originates in slightly changed crystal fields and interatomic interactions, in particular for the parts of the nanocrystals in close proximity to the silica pore surfaces.Publicationtype: Journal Article60 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Anisotropic confinement of chromophores Induces second-order nonlinear optics in a nanoporous photonic metamaterial(Soc., 2021-02-15); ; ; ; Second-order nonlinear optics is the base for a large variety of devices aimed at the active manipulation of light. However, physical principles restrict its occurrence to non-centrosymmetric, anisotropic matter. This significantly limits the number of base materials exhibiting nonlinear optics. Here, we show that embedding chromophores in an array of conical channels 13 nm across in monolithic silica results in mesoscopic anisotropic matter and thus in a hybrid material showing second-harmonic generation (SHG). This non-linear optics is compared to the one achieved in corona-poled polymer films containing the identical chromophores. It originates in confinement-induced orientational order of the elongated guest molecules in the nanochannels. This leads to a non-centrosymmetric dipolar order and hence to a non-linear light-matter interaction on the sub-wavelength, single-pore scale. Our study demonstrates that the advent of large-scale, self-organised nanoporosity in monolithic solids along with confinement-controllable orientational order of chromophores at the single-pore scale provides a reliable and accessible tool to design materials with a nonlinear meta-optics.Publicationtype: Journal Article68 5