Aerogel-Based Single-Ion Magnets: A Case Study of a Cobalt(II) Complex Immobilized in Silica.

The chemical immobilization of cobalt(II) ions in a silica aerogel matrix enabled the synthesis of the first representative example of aerogel-based single-ion magnets. For the synthesis of the lyogels, methyl-trimethoxysilane and N-3-(trimethoxysilyl)propyl ethylenediamine were co-hydrolyzed, then the ethylenediamine groups that were immobilized on the silica matrix enabled the subsequent binding of cobalt(II) ions. Lyogels with various amounts of ethylenediamine moieties (0.1-15 mol %) were soaked in isopropanol solutions of cobalt(II) nitrate and further supercritically dried in carbon dioxide to obtain aerogels with a specific surface area of 210-596 m2·g-1, an apparent density of 0.403-0.740 cm3·g-1 and a porosity of 60-78%. The actual cobalt content in the aerogels was 0.01-1.50 mmol per 1 g of SiO2, which could easily be tuned by the concentration of ethylenediamine moieties in the silica matrix. The introduction of cobalt(II) ions into the ethylenediamine-modified silica aerogel promoted the stability of the diamine moieties at the supercritical drying stage. The molecular prototype of the immobilized cobalt(II) complex, bearing one ethylenediamine ligand [Co(en)(MeCN)(NO3)2], was synthesized and structurally characterized. Using magnetometry in the DC mode, it was shown that cobalt(II)-modified silica aerogels exhibited slow magnetic relaxation in a nonzero field. A decrease in cobalt(II) concentration in aerogels from 1.5 mmol to 0.14 mmol per 1 g of SiO2 resulted in a weakening of inter-ion interactions; the magnetization reversal energy barrier likewise increased from 4 to 18 K.

Stained Glass Effect in Anodic Aluminum Oxide Formed in Selenic Acid.

A combination of the unique porous structure and physical and chemical properties of anodic aluminum oxide (AAO) makes it widely used in cutting-edge areas of materials science and nanotechnology. Selenic acid electrolyte provides the ability to obtain AAO with low porosity and high optical transparency and thus is promising for the preparation of AAO photonic crystals (PhCs). Here, we show the influence of crystallographic orientation of Al on the electrochemical oxidation rate in 1 M H2SeO4 as well as on the growth rate, porosity, and the effective refractive index of AAO. The cyclic anodization regime is used to prepare AAO PhCs with photonic band gaps, their wavelength positions are used to measure the AAO growth rate. At an anodization voltage of 40-45 V, the growth rate varies by up to 22.6% with crystallographic orientation of Al grains, causing the stained glass effect, which can be seen with the naked eye.

Tunable sub-terahertz resonance absorption in high-coercivity magnetodielectric ceramics.

Recently, giant coercivities (20-42 kOe) and sub-terahertz natural ferromagnetic resonance (NFMR) at 100-300 GHz were observed for single-domain M-type hexaferrite particles with high aluminum substitution. Herein, we fabricated dense ceramics of Sr0.67Ca0.33Fe8Al4O19 and, for the first time, investigated their magnetostatic and magnetodynamic properties in the temperature range of 5-300 K. It was shown that dense ceramics maintain their high magnetic hardness (a coercivity of 10-20 kOe) and NFMR frequencies of 140-200 GHz durably in the entire temperature range. Magnetizing the initially non-magnetized ceramics leads to a considerable decrease in the resonance absorption and to almost complete vanishing of the resonance line at 5 kOe. At the same time, an efficient linear frequency tuning by the external magnetic field was observed for the remanent sample. These findings open new horizons for developing industrial terahertz electronics based on dielectric ferrimagnets.

Three-Dimensional Photonic Crystals Based on Porous Anodic Aluminum Oxide.

Photonic crystals (PCs) consisting of a periodic arrangement of holes in dielectric media have found success in light manipulation and sensing. Among them, three-dimensional (3D) PCs are in high demand due to their unique properties originating from multiple photonic band gaps (PBGs) and even full ones. Here, 3D PCs based on porous anodic aluminum oxide (AAO) were fabricated for the first time. Our approach involves prepatterning of the aluminum surface by a focused ion beam to form a hexagonal array of pore nuclei. Subsequent anodization in 1 M H3PO3 using a sine wave profile of voltage provides AAO with a defect-free in-plane porous structure and out-of-plane porosity modulation. The ability to tune the position, width, and depth of the PBGs is demonstrated. The combination of the flexibility of the proposed approach with the unique properties of AAO extends the range of practical applications of 3D PCs far beyond the current achievements.

Towards High-Temperature MEMS: Two-Step Annealing Suppressed Recrystallization in Thin Multilayer Pt-Rh/Zr Films.

Platinum-based thin films are widely used to create microelectronic devices operating at temperatures above 500 °C. One of the most effective ways to increase the high-temperature stability of platinum-based films involves incorporating refractory metal oxides (e.g., ZrO2, HfO2). In such structures, refractory oxide is located along the metal grain boundaries and hinders the mobility of Pt atoms. However, the effect of annealing conditions on the morphology and functional properties of such multiphase systems is rarely studied. Here, we show that the two-step annealing of 250-nm-thick Pt-Rh/Zr multilayer films instead of the widely used isothermal annealing leads to a more uniform film morphology without voids and hillocks. The composition and morphology of as-deposited and annealed films were investigated using X-ray diffraction and scanning electron microscopy, combined with energy-dispersive X-ray spectroscopy. At the first annealing step at 450 °C, zirconium oxidation was observed. The second high-temperature annealing at 800-1000 °C resulted in the recrystallization of the Pt-Rh alloy. In comparison to the one-step annealing of Pt-Rh and Pt-Rh/Zr films, after two-step annealing, the metal phase in the Pt-Rh/Zr films has a smaller grain size and a less pronounced texture in the <111> direction, manifesting enhanced high-temperature stability. After two-step annealing at 450/900 °C, the Pt-Rh/Zr thin film possessed a grain size of 60 ± 27 nm and a resistivity of 17 × 10-6 Ω·m. The proposed annealing protocol can be used to create thin-film MEMS devices for operation at elevated temperatures, e.g., microheater-based gas sensors.

One-Dimensional Photonic Crystals with Nonbranched Pores Prepared via Phosphorous Acid Anodizing of Aluminium.

One-dimensional photonic crystals (1D PhCs) obtained by aluminium anodizing under oscillating conditions are promising materials with structure-dependent optical properties. Electrolytes based on sulphuric, oxalic, and selenic acids have been utilized for the preparation of anodic aluminium oxide (AAO) 1D PhCs with sub-100-nm pore diameter. AAO films with larger pores can be obtained by anodizing in phosphorous acid at high voltages. Here, for the first time, anodizing in phosphorous acid is applied for the preparation of AAO 1D PhCs with nonbranched macropores. The sine wave profile of anodizing voltage in the 135-165 V range produces straight pores, whose diameter is above 100 nm and alternates periodically in size. The pore diameter modulation period linearly increases with the charge density by a factor of 599 ± 15 nm·cm2·C-1. The position of the photonic band gap is controlled precisely in the 0.63-1.96 µm range, and the effective refractive index of AAO 1D PhCs is 1.58 ± 0.05.

Long-Term Operational Stability of Ta/Pt Thin-Film Microheaters: Impact of the Ta Adhesion Layer.

Microheaters with long-term stability are crucial for the development of a variety of microelectronic devices operated at high temperatures. Structured Ta/Pt bilayers, in which the Ta sublayer ensures high adhesion of the Pt resistive layer, are widely used to create microheaters. Herein, a comprehensive study of the microstructure of Ta/Pt films using high-resolution transmission electron microscopy with local elemental analysis reveals the twofold nature of Ta after annealing. The main fraction of Ta persists in the form of tantalum oxide between the Pt resistive layer and the alumina substrate. Such a sublayer hampers Pt recrystallization and grain growth in bilayered Ta/Pt films in comparison with pure Pt films. Tantalum is also observed inside the Pt grains as individual Ta nanoparticles, but their volume fraction is only about 2%. Microheaters based on the 10 nm Ta/90 nm Pt bilayers after pre-annealing exhibit long-term stability with low resistance drift at 500 °C (less than 3%/month).

Mosaic of Anodic Alumina Inherited from Anodizing of Polycrystalline Substrate in Oxalic Acid.

The anodizing of aluminium under oscillating conditions is a versatile and reproducible method for the preparation of one-dimensional photonic crystals (PhCs). Many anodizing parameters have been optimised to improve the optical properties of anodic aluminium oxide (AAO) PhCs. However, the influence of the crystallographic orientation of an Al substrate on the characteristics of AAO PhCs has not been considered yet. Here, the effect of Al substrate crystallography on the properties of AAO PhCs is investigated. It is experimentally demonstrated that the cyclic anodizing of coarse-grained aluminium foils produces a mosaic of photonic crystals. The crystallographic orientation of Al grains affects the electrochemical oxidation rate of Al, the growth rate of AAO, and the wavelength position of the photonic band gap.

Pore Ordering in Anodic Aluminum Oxide: Interplay between the Pattern of Pore Nuclei and the Crystallographic Orientation of Aluminum.

Anodization of aluminum with a pre-patterned surface is a promising approach for preparing anodic aluminum oxide (AAO) films with defect-free pore arrangement. Although pronounced effects of crystallographic orientation of Al on the AAO structure have been demonstrated, all current studies on the anodization of pre-patterned aluminum consider the substrate as an isotropic medium and, thus, do not consider the azimuthal orientation of the pattern relative to the basis vectors of the Al unit cell. Here, we investigate the interplay between the azimuthal alignment of the pore nuclei array and the crystallographic orientation of aluminum. Al(100) and Al(111) single-crystal substrates were pre-patterned by a Ga focused ion beam and then anodized under self-ordering conditions. The thickness-dependent degree of pore ordering in AAO was quantified using statistical analysis of scanning electron microscopy images. The observed trends demonstrate that the preferred azimuthal orientation of pore nuclei rows coincides with the <110> directions in the Al unit cell, which is favorable for creating AAO with a high degree of pore ordering. In the case of an unspecified azimuthal orientation of the pore nuclei array, crystallography-affected disorder within the AAO structure occurs with increasing film thickness. Our findings have important implications for preparing defect-free porous films over 100 µm in thickness that are crucial for a variety of AAO applications, e.g., creating metamaterials and 2D/3D photonic crystals.

Correction: High-coercivity hexaferrite ceramics featuring sub-terahertz ferromagnetic resonance.

Correction for 'High-coercivity hexaferrite ceramics featuring sub-terahertz ferromagnetic resonance' by Evgeny A. Gorbachev et al., Mater. Horiz., 2022, 9, 1264-1272, DOI: https://doi.org/10.1039/D1MH01797G.

Ni/(R2O3,CaO) Nanocomposites Produced by the Exsolution of R1.5Ca0.5NiO4 Nickelates (R = Nd, Sm, Eu): Rare Earth Effect on the Catalytic Performance in the Dry Reforming and Partial Oxidation of Methane.

In order to clarify the role of R2O3 in the metal-oxide catalysts derived from complex oxide precursors, a series of R1.5Ca0.5NiO4 (R = Nd, Sm, Eu) complex oxides was obtained. A significant systematic increase in the orthorhombic distortion of the R1.5Ca0.5NiO4 structure (K2NiF4 type, Cmce) from Nd to Eu correlates with a corresponding decrease in their ionic radii. A reduction of R1.5Ca0.5NiO4 in the Ar/H2 gas mixture at 800 °C causes a formation of dense agglomerates of CaO and R2O3 coated with spherical 25-30 nm particles of Ni metal. The size of metal particles and oxide agglomerates is similar in all Ni/(R2O3,CaO) composites in the study. Their morphology is rather similar to the products of redox exsolution obtained by the partial reduction of complex oxides. All obtained composites demonstrated a significant catalytic activity in the dry reforming (DRM) and partial oxidation (POM) of methane at 700-800 °C. A systematic decrease in the DRM catalytic activity of composites from Nd to Eu could be attributed to the basicity reduction of R2O3 components of the composite catalysts. The maximum CH4 conversion in POM reaction was observed for Ni/(Sm2O3,CaO), while the maximum selectivity was demonstrated by Nd2O3-based composite. The possible reasons for the observed difference are discussed.

High-coercivity hexaferrite ceramics featuring sub-terahertz ferromagnetic resonance.

Herein, we demonstrate for the first time compact ferrite ceramics with giant coercivity. The materials are manufactured via sintering single-domain Sr0.67Ca0.33Fe8Al4O19 particles synthesized by a citrate-nitrate auto-combustion method. The obtained ceramics show coercivities up to 22.5 kOe and natural ferromagnetic resonance frequencies (NFMR) in a sub-THz range of 160-282 GHz. At a maximum density of 95%, the sample displays coercivity of 18.5 kOe, which is the highest value among dense ferrite materials reported so far. In addition, we report an unusual blueshift of the NFMR frequency from 160 to 200 GHz, which occurs during material sintering.

Uniform arrays of gold nanoelectrodes with tuneable recess depth.

Nanoelectrode arrays are much in demand in electroanalytical chemistry, electrocatalysis, and bioelectrochemistry. One of the promising approaches for the preparation of such systems is templated electrodeposition. In the present study, porous anodic alumina templates are used to prepare Au nanoelectrode arrays. Multistage electrodeposition is proposed for the formation of recessed electrodes with the ability to tune the distance between the surface of the porous template and the top surface of the nanoelectrodes. A set of complementary techniques, including chronoamperometry, coulometry, and scanning electron microscopy, are used to characterize the nanoelectrode arrays. The number of active nanoelectrodes is experimentally measured. The pathways to further improve the recessed nanoelectrode arrays based on anodic alumina templates are discussed.

Ca-Al double-substituted strontium hexaferrites with giant coercivity.

We demonstrate that the simultaneous substitution of calcium and aluminum for strontium and iron in strontium hexaferrite results in a significant increase of coercivity up to a record high of 21.3 kOe. We propose that the effect is originated from a crystal structure distortion causing an increase of the magnetocrystalline anisotropy.