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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: 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 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Adsorption from binary liquid solutions into mesoporous silica: a capacitance isotherm on 5CB nematogen/cyclohexane mixtures(2021); ; We present a capacitance method to measure the adsorption of rod-like nematogens (4-cyano- (Formula presented.) -pentylbiphenyl, 5CB) from a binary liquid 5CB/cyclohexane solution into a monolithic mesoporous silica membrane traversed by tubular pores with radii of 5.4 nm at room temperature. The resulting adsorption isotherm is reminiscent of classical type II isotherms of gas adsorption in mesoporous media. Its analysis by a model for adsorption from binary solutions, as inspired by the Brunauer–Emmett–Teller (BET) approach for gas adsorption on solid surfaces, indicates that the first adsorbed monolayer consists of flat-lying (homogeneously anchored) 5CB molecules at the pore walls. An underestimation of the adsorbed 5CB amount by the adsorption model compared to the measured isotherm for high 5CB concentrations hint towards a capillary filling transition in the mesopores similar to capillary condensation, i.e. film-growth at the pore walls is replaced by filling of the pore centres by the liquid crystal. The experimental method and thermodynamic analysis presented here can easily be adapted to other binary liquid solutions and thus allows a controlled filling of mesoporous materials with non-volatile molecular systems.Publicationtype: Journal Article56