Unveiling Core-Shell Structure Formation in a Ni3Fe Nanoparticle with In Situ Multi-Bragg Coherent Diffraction Imaging

Solid-state reactions play a key role in materials science. The evolution of the structure of a single 350 nm Ni3Fe nanoparticle, i.e., its morphology (facets) as well as its deformation field, has been followed by applying multireflection Bragg coherent diffraction imaging. Through this approach, we unveiled a demixing process that occurs at high temperatures (600 °C) under an Ar atmosphere. This process leads to the gradual emergence of a highly strained core-shell structure, distinguished by two distinct lattice parameters with a difference of 0.4%. Concurrently, this transformation causes the facets to vanish, ultimately yielding a rounded core-shell nanoparticle. This final structure comprises a Ni3Fe core surrounded by a 40 nm Ni-rich outer shell due to preferential iron oxidation. Providing in situ 3D imaging of the lattice parameters at the nanometer scale while varying the temperature, this study─with the support of atomistic simulations─not only showcases the power of in situ multireflection BCDI but also provides valuable insights into the mechanisms at work during a solid-state reaction characterized by a core-shell transition.

Physical Separation of Enantiomeric Products by Compartmentalized Parallel Kinetic Resolution

Accessing each enantiomer of a chiral molecule starting from a racemic mixture remains a daunting challenge in chemistry. Indeed, until now, only a few solutions exist to separate enantiomers of an equimolar mixture of a chiral precursor. In this study, we establish a new strategy to prepare simultaneously and physically separate both enantioenriched enantiomers of a molecule starting from a racemic substrate. This process combines two enantiomeric catalytic systems, working in parallel, and separation by an achiral membrane with selective permeability. This unprecedented system was successfully applied to the simultaneous preparation of both enantiomers of chiral 1,2-diols starting from racemic epoxides using Jacobsen's hydrolytic kinetic resolution (HKR) in parallel.

Enhanced Refractive Index Sensitivity through Combining a Sol-Gel Adsorbate with a TiO2 Nanoimprinted Metasurface for Gas Sensing

We combine a gas-adsorbent microporous hybrid silica layer and a dense TiO2 Mie resonator array (metasurface), both obtained by sol-gel deposition and nanoimprint lithography, to form nanocomposite systems with high sensitivity for refractive index (RI) variations induced by gas adsorption. Using optical transduction based on direct specular reflection, we show spectral shifts of 4470 nm/RIU corresponding to 0.2 nm/ppm gas (air concentration) and reflection intensity changes of R* = 17 (R/RIU) and 0.55 × 10-3 R/ppm (air concentration). The metasurface is composed of hexagonally arranged TiO2 nanopillar arrays, whereas the surrounding sensitive material is a class II microporous hybrid silica, containing methyl and phenyl covalently bonded organic functions. This hybrid layer shows efficient adsorption capability of volatile organic molecules such as isopropanol, which is used to induce slight variations of RI around the TiO2 antennas. Specular reflectance variations at 45° incidence and refractive index measurements are performed using a spectroscopic ellipsometer. The presence of the titania metasurface enhances the signal by almost an order of magnitude with respect to the 2D counterpart (simulated as an effective medium approximation) and is attributed to the antenna effect, enhancing the interaction of the confined electromagnetic wave with the sensitive microporous medium. This sol-gel nanocomposite system presents many advantages such as high throughput and low-cost elaboration of elements and a high chemical, mechanical, and thermal resistance, ensuring high stability as a potential gas-sensitive nanocomposite layer for long periods. This work is a case study of improving the sensitivity of sol-gel gas-sensitive materials in optical transduction, which will be exploited in further works to develop artificial noses.

Twin boundary migration in an individual platinum nanocrystal during catalytic CO oxidation

At the nanoscale, elastic strain and crystal defects largely influence the properties and functionalities of materials. The ability to predict the structural evolution of catalytic nanocrystals during the reaction is of primary importance for catalyst design. However, to date, imaging and characterising the structure of defects inside a nanocrystal in three-dimensions and in situ during reaction has remained a challenge. We report here an unusual twin boundary migration process in a single platinum nanoparticle during CO oxidation using Bragg coherent diffraction imaging as the characterisation tool. Density functional theory calculations show that twin migration can be correlated with the relative change in the interfacial energies of the free surfaces exposed to CO. The x-ray technique also reveals particle reshaping during the reaction. In situ and non-invasive structural characterisation of defects during reaction opens new avenues for understanding defect behaviour in confined crystals and paves the way for strain and defect engineering.

Spontaneous shape transition of Mn x Ge1- x islands to long nanowires

We report experimental evidence for a spontaneous shape transition, from regular islands to elongated nanowires, upon high-temperature annealing of a thin Mn wetting layer evaporated on Ge(111). We demonstrate that 4.5 monolayers is the critical thickness of the Mn layer, governing the shape transition to wires. A small change around this value modulates the geometry of the nanostructures. The Mn-Ge alloy nanowires are single-crystalline structures with homogeneous composition and uniform width along their length. The shape evolution towards nanowires occurs for islands with a mean size of ≃170 nm. The wires, up to ≃1.1 μm long, asymptotically tend to ≃80 nm of width. We found that tuning the annealing process allows one to extend the wire length up to ≃1.5 μm with a minor rise of the lateral size to ≃100 nm. The elongation process of the nanostructures is in agreement with a strain-driven shape transition mechanism proposed in the literature for other heteroepitaxial systems. Our study gives experimental evidence for the spontaneous formation of spatially uniform and compositionally homogeneous Mn-rich GeMn nanowires on Ge(111). The reliable and simple synthesis approach allows one to exploit the room-temperature ferromagnetic properties of the Mn-Ge alloy to design and fabricate novel nanodevices.

New Strategies for Engineering Tensile Strained Si Layers for Novel n-Type MOSFET

We report a novel approach for engineering tensely strained Si layers on a relaxed silicon germanium on insulator (SGOI) film using a combination of condensation, annealing, and epitaxy in conditions specifically chosen from elastic simulations. The study shows the remarkable role of the SiO₂ buried oxide layer (BOX) on the elastic behavior of the system. We show that tensely strained Si can be engineered by using alternatively rigidity (at low temperature) and viscoelasticity (at high temperature) of the SiO₂ substrate. In these conditions, we get a Si strained layer perfectly flat and free of defects on top of relaxed Si_1-xGe_x. We found very specific annealing conditions to relax SGOI while keeping a homogeneous Ge concentration and an excellent thickness uniformity resulting from the viscoelasticity of SiO₂ at this temperature, which would allow layer-by-layer matter redistribution. Remarkably, the Si layer epitaxially grown on relaxed SGOI remains fully strained with -0.85% tensile strain. The absence of strain sharing (between Si_1-xGe_x and Si) is explained by the rigidity of the Si_1-xGe_x/BOX interface at low temperature. Elastic simulations of the real system show that, because of the very specific elastic characteristics of SiO₂, there are unique experimental conditions that both relax Si_1-xGe_x and keep Si strained. Various epitaxial processes could be revisited in light of these new results. The generic and simple process implemented here meets all the requirements of the microelectronics industry and should be rapidly integrated in the fabrication lines of large multifinger 2.5 V n-type MOSFET on SOI used for RF-switch applications and for many other applications.

Hyperuniform Monocrystalline Structures by Spinodal Solid-State Dewetting

Materials featuring anomalous suppression of density fluctuations over large length scales are emerging systems known as disordered hyperuniform. The underlying hidden order renders them appealing for several applications, such as light management and topologically protected electronic states. These applications require scalable fabrication, which is hard to achieve with available top-down approaches. Theoretically, it is known that spinodal decomposition can lead to disordered hyperuniform architectures. Spontaneous formation of stable patterns could thus be a viable path for the bottom-up fabrication of these materials. Here, we show that monocrystalline semiconductor-based structures, in particular Si_{1-x}Ge_{x} layers deposited on silicon-on-insulator substrates, can undergo spinodal solid-state dewetting featuring correlated disorder with an effective hyperuniform character. Nano- to micrometric sized structures targeting specific morphologies and hyperuniform character can be obtained, proving the generality of the approach and paving the way for technological applications of disordered hyperuniform metamaterials. Phase-field simulations explain the underlying nonlinear dynamics and the physical origin of the emerging patterns.

Raman microscopy and infrared optical properties of SiGe Mie resonators formed on SiO2 via Ge condensation and solid state dewetting

All-dielectric photonics is a rapidly developing field of optics and material science. The main interest at visible and near-infrared frequencies is light management using high-refractive-index Mie-resonant dielectric particles. Most work in this area of research focuses on exploiting Si-based particles. Here, we study monocrystalline Mie-resonant particles made of Ge-rich SiGe alloys with refractive index higher than that of Si. These islands are formed via solid state dewetting of SiGe flat layers by using two different processes: (i) dewetting of monocrystalline SiGe layers (60%-80% Ge content) obtained via Ge condensation of SiGe on silicon on insulator; and (ii) dewetting of a SiGe layer deposited via molecular beam epitaxy on silicon on insulator and ex situ Ge condensation, forming a Ge-rich shell surrounding a SiGe-core. Using high-spatial-resolution Raman microscopy we monitor Ge content x and strain ϵ of flat layers and SiGe-islands. We observe strain relaxation associated with formation of trading dislocations in the SiGe islands compared to the starting SiGe layers, as confirmed by TEM images. For initial high Ge concentration in the flat layers, the corresponding Ge content in the dewetted islands is lower, owing to diffusion of Si atoms from Si or SiO₂ into SiGe islands. The Ge content also varies from particle to particle on the same sample. Size and shape of the dewetted particles depend on the fabrication process: thicker initial SiGe layers lead to larger particles. Samples with narrow island size distribution display rather sharp Mie resonances in the 1000-2500 nm spectral range. Larger islands display Mie resonances at longer wavelength. Positions of the resonances are in agreement with the theoretical calculations in the discrete dipole approximation.

Templated dewetting of single-crystal sub-millimeter-long nanowires and on-chip silicon circuits

Large-scale, defect-free, micro- and nano-circuits with controlled inter-connections represent the nexus between electronic and photonic components. However, their fabrication over large scales often requires demanding procedures that are hardly scalable. Here we synthesize arrays of parallel ultra-long (up to 0.75 mm), monocrystalline, silicon-based nano-wires and complex, connected circuits exploiting low-resolution etching and annealing of thin silicon films on insulator. Phase field simulations reveal that crystal faceting and stabilization of the wires against breaking is due to surface energy anisotropy. Wires splitting, inter-connections and direction are independently managed by engineering the dewetting fronts and exploiting the spontaneous formation of kinks. Finally, we fabricate field-effect transistors with state-of-the-art trans-conductance and electron mobility. Beyond the first experimental evidence of controlled dewetting of patches featuring a record aspect ratio of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document}~1/60000 and self-assembled \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document}~mm long nano-wires, our method constitutes a distinct and promising approach for the deterministic implementation of atomically-smooth, mono-crystalline electronic and photonic circuits.

Fabricating defect-free micro- and nano-circuits over large scales with controlled interconnections remains a challenge. Here, Bollani et al. show a dewetting strategy for engineering arrays of parallel Si-based nanowires up to 0.75 mm and complex interconnected circuits of monocrystalline Si on a chip.

Capillary-driven elastic attraction between quantum dots

We present a novel self-assembly route to align SiGe quantum dots. By a combination of theoretical analyses and experimental investigation, we show that epitaxial SiGe quantum dots can cluster in ordered closely packed assemblies, revealing an attractive phenomenon. We compute nucleation energy barriers, accounting for elastic effects between quantum dots through both elastic energy and strain-dependent surface energy. If the former is mostly repulsive, we show that the decrease in the surface energy close to an existing island reduces the nucleation barrier. It subsequently increases the probability of nucleation close to an existing island, and turns out to be equivalent to an effective attraction between dots. We show by Monte-Carlo simulations that this effect describes well the experimental results, revealing a new mechanism ruling self-organisation of quantum dots. Such a generic process could be observed in various heterogeneous systems and could pave the way for a wide range of applications.

Conductive-bridge memory cells based on a nanoporous electrodeposited GeSbTe alloy

We report on the fabrication of memory devices based on a nanoporous GeSbTe layer electrodeposited inbetween TiN and Ag electrodes. It is shown that devices can operate along two distinct electrical modes consisting of a volatile or a non-volatile resistance switching mode upon appropriate preconditioning procedures. Based on electrical measurements conducted in both switching modes and physical analysis performed on a device after electrical stress, resistance switching is attributed to the formation/dissolution of a conductive filament from the Ag electrode into the GST layer whereas the volatile/non-volatile resistance switching is attributed to the presence of an interface layer between the GST and the Ag top electrode. Due to their simple, low-cost and low-temperature fabrication procedure, these devices could be advantageously exploited in flexible electronic applications or embedded into the back-end of line CMOS technology.

New strategies for producing defect free SiGe strained nanolayers

Strain engineering is seen as a cost-effective way to improve the properties of electronic devices. However, this technique is limited by the development of the Asarro Tiller Grinfeld growth instability and nucleation of dislocations. Two strain engineering processes have been developed, fabrication of stretchable nanomembranes by deposition of SiGe on a sacrificial compliant substrate and use of lateral stressors to strain SiGe on Silicon On Insulator. Here, we investigate the influence of substrate softness and pre-strain on growth instability and nucleation of dislocations. We show that while a soft pseudo-substrate could significantly enhance the growth rate of the instability in specific conditions, no effet is seen for SiGe heteroepitaxy, because of the normalized thickness of the layers. Such results were obtained for substrates up to 10 times softer than bulk silicon. The theoretical predictions are supported by experimental results obtained first on moderately soft Silicon On Insulator and second on highly soft porous silicon. On the contrary, the use of a tensily pre-strained substrate is far more efficient to inhibit both the development of the instability and the nucleation of misfit dislocations. Such inhibitions are nicely observed during the heteroepitaxy of SiGe on pre-strained porous silicon.

Tailoring Strain and Morphology of Core-Shell SiGe Nanowires by Low-Temperature Ge Condensation

Selective oxidation of the silicon element of silicon germanium (SiGe) alloys during thermal oxidation is a very important and technologically relevant mechanism used to fabricate a variety of microelectronic devices. We develop here a simple integrative approach involving vapor-liquid-solid (VLS) growth followed by selective oxidation steps to the construction of core-shell nanowires and higher-level ordered systems with scalable configurations. We examine the selective oxidation/condensation process under nonequilibrium conditions that gives rise to spontaneous formation of core-shell structures by germanium condensation. We contrast this strategy that uses reaction-diffusion-segregation mechanisms to produce coherently strained structures with highly configurable geometry and abrupt interfaces with growth-based processes which lead to low strained systems with nonuniform composition, three-dimensional morphology, and broad core-shell interface. We specially focus on SiGe core-shell nanowires and demonstrate that they can have up to 70% Ge-rich shell and 2% homogeneous strain with core diameter as small as 14 nm. Key elements of the building process associated with this approach are identified with regard to existing theoretical models. Moreover, starting from results of ab initio calculations, we discuss the electronic structure of these novel nanostructures as well as their wide potential for advanced device applications.

Complex dewetting scenarios of ultrathin silicon films for large-scale nanoarchitectures

Dewetting is a ubiquitous phenomenon in nature; many different thin films of organic and inorganic substances (such as liquids, polymers, metals, and semiconductors) share this shape instability driven by surface tension and mass transport. Via templated solid-state dewetting, we frame complex nanoarchitectures of monocrystalline silicon on insulator with unprecedented precision and reproducibility over large scales. Phase-field simulations reveal the dominant role of surface diffusion as a driving force for dewetting and provide a predictive tool to further engineer this hybrid top-down/bottom-up self-assembly method. Our results demonstrate that patches of thin monocrystalline films of metals and semiconductors share the same dewetting dynamics. We also prove the potential of our method by fabricating nanotransfer molding of metal oxide xerogels on silicon and glass substrates. This method allows the novel possibility of transferring these Si-based patterns on different materials, which do not usually undergo dewetting, offering great potential also for microfluidic or sensing applications.

Templated Solid-State Dewetting of Thin Silicon Films

Thin film dewetting can be efficiently exploited for the implementation of functionalized surfaces over very large scales. Although the formation of sub-micrometer sized crystals via solid-state dewetting represents a viable method for the fabrication of quantum dots and optical meta-surfaces, there are several limitations related to the intrinsic features of dewetting in a crystalline medium. Disordered spatial organization, size, and shape fluctuations are relevant issues not properly addressed so far. This study reports on the deterministic nucleation and precise positioning of Si- and SiGe-based nanocrystals by templated solid-state dewetting of thin silicon films. The dewetting dynamics is guided by pattern size and shape taking full control over number, size, shape, and relative position of the particles (islands dimensions and relative distances are in the hundreds nm range and fluctuate ≈11% for the volumes and ≈5% for the positioning).

Fabrication of core-shell nanostructures via silicon on insulator dewetting and germanium condensation: towards a strain tuning method for SiGe-based heterostructures in a three-dimensional geometry

We report on a novel method for the implementation of core-shell SiGe-based nanocrystals combining silicon on insulator dewetting in a molecular beam epitaxy reactor with an ex situ Ge condensation process. With an in situ two-step process (annealing and Ge deposition) we produce two families of islands on the same sample: Si-rich, formed during the first step and, all around them, Ge-rich formed after Ge deposition. By increasing the amount of Ge deposited on the annealed samples from 0 to 18 monolayers, the islands' shape in the Si-rich zones can be tuned from elongated and flat to more symmetric and with a larger vertical aspect ratio. At the same time, the spatial extension of the Ge-rich zones is progressively increased as well as the Ge content in the islands. Further processing by ex situ rapid thermal oxidation results in the formation of a core-shell composition profile in both Si and Ge-rich zones with atomically sharp heterointerfaces. The Ge condensation induces a Ge enrichment of the islands' shell of up to 50% while keeping a pure Si core in the Si-rich zones and a ∼25% SiGe alloy in the Ge-rich ones. The large lattice mismatch between core and shell, the absence of dislocations and the islands' monocrystalline nature render this novel class of nanostructures a promising device platform for strain-based band-gap engineering. Finally, this method can be used for the implementation of ultralarge scale meta-surfaces with dielectric Mie resonators for light manipulation at the nanoscale.

Correction: Fabrication of poly-crystalline Si-based Mie resonators via amorphous Si on SiO2 dewetting

Correction for 'Fabrication of poly-crystalline Si-based Mie resonators via amorphous Si on SiO2 dewetting' by Meher Naffouti, et al., Nanoscale, 2016, 8, 2844-2849.

Effect of Infant Formula Containing a Low Dose of the Probiotic Bifidobacterium lactis CNCM I-3446 on Immune and Gut Functions in C-Section Delivered Babies: A Pilot Study

BACKGROUND

In the absence of breast-feeding and its immunomodulatory factors, supplementation of starter infant formula (IF) with probiotics is currently used to support immune functions and gut development.

AIM

To assess whether immune-related beneficial effects of regular dose (10⁷ CFU/g of powder) of the probiotic Bifidobacterium lactis CNCM I-3446 (hereafter named B. lactis) in starter IF supplementation can be maintained with starter IF containing a low dose (10⁴ CFU/g of powder) of B. lactis.

METHOD

This trial was designed as a pilot, prospective, double-blind, randomized, single-center clinical trial of two parallel groups (n = 77 infants/group) of C-section delivered infants receiving a starter IF containing either low dose or regular dose of the probiotic B. lactis from birth to six months of age. In addition, a reference group of infants breast-fed for a minimum of four months (n = 44 infants), also born by C-section, were included. All groups were then provided follow-up formula without B. lactis up to 12 months of age. Occurrence of diarrhea, immune and gut maturation, responses to vaccinations, and growth were assessed from birth to 12 months. The effect of low-dose B. lactis formula was compared to regular-dose B. lactis formula, considered as reference for IF with probiotics, and both were further compared to breast-feeding as a physiological reference.

RESULTS

Data showed that feeding low-dose B. lactis IF provides similar effects as feeding regular-dose B. lactis IF or breast milk. No consistent statistical differences regarding early life protection against gastrointestinal infections, immune and gut maturation, microbiota establishment, and growth were observed between randomized formula-fed groups as well as with the breast-fed reference group.

CONCLUSION

This pilot study suggests that supplementing C-section born neonates with low-dose B. lactis-containing starter formula may impact immune as well as gut maturation similarly to regular-dose B. lactis, close to the breast-feeding reference.

Fabrication of poly-crystalline Si-based Mie resonators via amorphous Si on SiO2 dewetting

We report the fabrication of Si-based dielectric Mie resonators via a low cost process based on solid-state dewetting of ultra-thin amorphous Si on SiO2. We investigate the dewetting dynamics of a few nanometer sized layers annealed at high temperature to form submicrometric Si-particles. Morphological and structural characterization reveal the polycrystalline nature of the semiconductor matrix as well as rather irregular morphologies of the dewetted islands. Optical dark field imaging and spectroscopy measurements of the single islands reveal pronounced resonant scattering at visible frequencies. The linewidth of the low-order modes can be ∼20 nm in full width at half maximum, leading to a quality factor Q exceeding 25. These values reach the state-of-the-art ones obtained for monocrystalline Mie resonators. The simplicity of the dewetting process and its cost-effectiveness opens the route to exploiting it over large scales for applications in silicon-based photonics.

Ordered arrays of Au catalysts by FIB assisted heterogeneous dewetting

Synthesizing Au0.8Si0.2 nanocatalysts that are homogeneous in size and have controlled position is becoming a challenging and crucial prequisite for the fabrication of ordered semiconductor nanowires. In this study, Au0.8Si0.2 nanocatalysts are synthesized via dewetting of Au layers on Si(111) during thermal annealing in an ultra-high vacuum. In the first part of the paper, the mechanism of homogeneous dewetting is analyzed as a function of the Au-deposited thickness (h Au). We distinguish three different dewetting regimes: (I) for a low thickness ([Formula: see text]), a submonolyer coverage of Au is stabilized and there is no dewetting. (II) For an intermediate thickness ([Formula: see text]), there is both dewetting and Au0.8Si0.2 phase formation. The size and density of the Au0.8Si0.2 clusters are directly related to h Au. When cooling down to room temperature, the clusters decompose and reject the Si at the Au/Si substrate interface. (III) For a large thickness ([Formula: see text]), only dewetting takes place, without forming AuSi clusters. In this regime, the dewetting is kinetically controlled by the self-diffusion of Au (activation energy ∼0.43 eV) without evidence of an Si-alloying effect. As a practical consequence, when relying solely on the homogeneous dewetting of Au/Si(111) to form the Au0.8Si0.2 catalysts (without a supply of Si atoms from vapor), regime II should be used to obtain good size and density control. In the second part of the paper, a process for ordering the catalysts using focused ion beam-(FIB) assisted dewetting (heterogeneous dewetting) is developed. We show that no matter what the FIB milling conditions and the Au nominal thickness are, dewetting is promoted by ion beam irradiation and is accompanied by the formation of Au0.8Si0.2 droplets. The droplets preferentially form on the patterned areas, while in similar annealing conditions, they do not form on the unpatterned areas. This behavior is attributed to the larger Au-Si interdiffusion in the patterned areas, which results from the Si amorphization induced by the FIB. A systematic analysis of the position of the nanodroplets shows their preferential nucleation inside the patterns, while thicker platelets of almost pure Au are observed between the patterns. The evolutions of the size homogeneity and the occupancy rate of the patterns are quantified as a function of the FIB dose and annealing temperature. Nice arrays of perfectly ordered AuSi catalysts are obtained after optimizing the FIB and dewetting conditions.

[DXA imaging: the multifunction Swiss army knife?]

The significant progress on the quality and resolution of the images obtained by "Dual X-ray Absorptiometry" or DXA has permitted on one hand to improve some existing features and on the other to develop new ones, significantly refining the care of our patients in various pathologies. For example, by improving the prediction of fracture risk by indirect analysis of micro- and macro-architecture of the bone, by looking for markers of associated bone diseases (research vertebral fractures or atypical femoral fractures), or by assessing the metabolic status by the measurement of body composition. With the best performing DXA devices we will soon be able, in clinical routine, to determine bone age, to estimate cardiovascular risk (by measuring the calcification of the abdominal aorta) or to predict the progression of joint osteoarthritis and its evolution after surgical management.

Crystallization of hollow mesoporous silica nanoparticles

Amorphous structured silica can now be crystallized to pure α-quartz under relatively mild conditions, while preserving initial complex architectures.

Bright photoluminescence from ordered arrays of SiGe nanowires grown on Si(111)

We report on the optical properties of SiGe nanowires (NWs) grown by molecular beam epitaxy (MBE) in ordered arrays on SiO2/Si(111) substrates. The production method employs Au catalysts with self-limited sizes deposited in SiO2-free sites opened-up in the substrate by focused ion beam patterning for the preferential nucleation and growth of these well-organized NWs. The NWs thus produced have a diameter of 200 nm, a length of 200 nm, and a Ge concentration x = 0.15. Their photoluminescence (PL) spectra were measured at low temperatures (from 6 to 25 K) with excitation at 405 and 458 nm. There are four major features in the energy range of interest (980-1120 meV) at energies of 1040.7, 1082.8, 1092.5, and 1098.5 meV, which are assigned to the NW-transverse optic (TO) Si-Si mode, NW-transverse acoustic (TA), Si-substrate-TO and NW-no-phonon (NP) lines, respectively. From these results the NW TA and TO phonon energies are found to be 15.7 and 57.8 meV, respectively, which agree very well with the values expected for bulk Si1- x Ge x with x = 0.15, while the measured NW NP energy of 1099 meV would indicate a bulk-like Ge concentration of x = 0.14. Both of these concentrations values, as determined from PL, are in agreement with the target value. The NWs are too large in diameter for a quantum confinement induced energy shift in the band gap. Nevertheless, NW PL is readily observed, indicating that efficient carrier recombination is occurring within the NWs.

Wafer scale formation of monocrystalline silicon-based Mie resonators via silicon-on-insulator dewetting

Subwavelength-sized dielectric Mie resonators have recently emerged as a promising photonic platform, as they combine the advantages of dielectric microstructures and metallic nanoparticles supporting surface plasmon polaritons. Here, we report the capabilities of a dewetting-based process, independent of the sample size, to fabricate Si-based resonators over large scales starting from commercial silicon-on-insulator (SOI) substrates. Spontaneous dewetting is shown to allow the production of monocrystalline Mie-resonators that feature two resonant modes in the visible spectrum, as observed in confocal scattering spectroscopy. Homogeneous scattering responses and improved spatial ordering of the Si-based resonators are observed when dewetting is assisted by electron beam lithography. Finally, exploiting different thermal agglomeration regimes, we highlight the versatility of this technique, which, when assisted by focused ion beam nanopatterning, produces monocrystalline nanocrystals with ad hoc size, position, and organization in complex multimers.

Selective growth and ordering of SiGe nanowires for band gap engineering

Selective growth and self-organization of silicon-germanium (SiGe) nanowires (NWs) on focused ion beam (FIB) patterned Si(111) substrates is reported. In its first step, the process involves the selective synthesis of Au catalysts in SiO₂-free areas; its second step involves the preferential nucleation and growth of SiGe NWs on the catalysts. The selective synthesis process is based on a simple, room-temperature reduction of gold salts (Au³⁺Cl₄⁻) in aqueous solution, which provides well-organized Au catalysts. By optimizing the reduction process, we are able to generate a bidimensional regular array of Au catalysts with self-limited sizes positioned in SiO₂-free windows opened in a SiO₂/Si(111) substrate by FIB patterning. Such Au catalysts subsequently serve as preferential nucleation and growth sites of well-organized NWs. Furthermore, these NWs with tunable position and size exhibit the relevant features and bright luminescence that would find several applications in optoelectronic nanodevices.

Diffusion induced effects on geometry of Ge nanowires

We report diffusion induced germanium nanowire growth and its dependence on the Ge evaporation flux. The wires show a growth rate (dL/dt) in agreement with the previously reported models, but detection of anomalies in the grown wires may indicate the prevalence of the direct Ge impinging effect on large diameter wires. Additionally, we demonstrate that change in deposition flux could directly affect the diffusion length of the Ge adatoms on the wire sidewalls. This in turn modifies the geometry of the grown wires by introducing a lateral growth starting from the base of the wire. A detailed understanding of the deposition flux effect on the growth and geometry of wires will result in improved knowledge of physical properties of wires.

[Long-term implication of bariatric surgery: beyond deficiencies]

Bariatric surgery frequently leads to several nutritional deficiencies that are well reported in the literature. However, numerous other aspects of the patients' life are concerned by bariatric interventions, so that the follow-up of these patients after bariatric surgery does not only encompass a mere correction of the well-known nutritional deficiencies. Only a thorough knowledge of all possible consequences of bariatric interventions permits an adequate follow-up of these patients. Overtly addressing all these issues with the patients before performing a bariatric intervention is the best way for them to be prepared for the intervention itself and to understand the need of a life-long follow-up thereafter.

Ferromagnetic Mn-doped Si0.3Ge0.7 nanodots self-assembled on Si(100)

Densely packed epitaxial Mn-doped Si(0.3)Ge(0.7) nanodots self-assembled on Si(100) have been obtained. Their structural properties were studied using reflection high-energy electron diffraction, energy dispersive x-ray diffraction, atomic force microscopy, extended x-ray absorption fine structure measurements and high-resolution transmission electron microscopy. Mn(5)Ge(1)Si(2) crystallites embedded in Si(0.3)Ge(0.7) were found. They exhibit a ferromagnetic behaviour with a Curie temperature of about 225 K.

Photoluminescence efficiency and size distribution of self assembled ge dots on porous TiO2

For Ge nanodots approximately 20 nm in diameter grown by annealing a thin amorphous Ge layer deposited by molecular beam epitaxy on a mesoporous TiO2 layer on Si(001), photoluminescence (PL) was observed as a wide near-infrared band near 800 meV. Using a tight binding theoretical model, the energy-dependent PL spectrum was transformed into a dependence on dot size. The average dot size determined the peak energy of the PL band and its shape depended on the size distribution, including bandgap enlargement due to quantum confinement. Combining the dot sample PL with an established dependence of emission efficiency on dot diameter, it was possible to derive a dot size distribution and compare it with results obtained independently from atomic force microscopy.

Low dose endoluminal photodynamic therapy improves murine T cell-mediated colitis

BACKGROUND AND STUDY AIMS

Low dose photodynamic therapy (LDPDT) may modify the mucosal immune response and may thus provide a therapy for Crohn's disease. We evaluated the efficacy and safety of this technique in a murine T cell-mediated colitis model.

METHODS

The safety of LDPDT was first tested in BALB/c mice. Naïve T cells were used to induce colitis in mice with severe combined immunodeficiency, which were followed up endoscopically, and a murine endoscopic index of colitis (MEIC) was developed. The efficacy of LDPDT (10 J/cm (2); delta-aminolevulinic acid, 15 mg/kg bodyweight) was then tested on mice with moderate colitis, while a disease control group received no treatment. The MEIC, weight, length, and histology of the colon, cytokine expression indices, number of mucosal CD4 (+) T cells, percentage of apoptotic CD4 (+) T cells, body weight, and systemic side effects were evaluated.

RESULTS

LDPDT improved the MEIC ( P = 0.011) and the histological score ( P = 0.025), diminished the expression indices of the proinflammatory cytokines, interleukin-6 ( P = 0.042), interleukin-17 ( P = 0.029), and interferon-gamma ( P = 0.014), decreased the number of mucosal CD4 (+) T cells, and increased the percentage of apoptotic CD4 (+) T cells compared with the disease control group. No local or systemic side effects occurred.

CONCLUSION

LDPDT improves murine T cell-mediated colitis, decreases the proinflammatory cytokines interleukin-6, interleukin-17, and interferon-gamma, and decreases the number of CD4 (+) T cells. No adverse events were observed. Therefore, this technique is now being evaluated in patients with inflammatory bowel disease.

[Management of hyperinsulinemic hypoglycemia post-gastric bypass surgery]

Gastric bypass surgery is an effective treatment for morbid obesity, allowing a substantial weight loss together with an improvement of the cardiovascular and metabolic comorbidities, particularly the glucose control. However, after gastric bypassing, an imbalance between sensitivity and insulin secretion may be observed. This disorder gives rise to hyperinsulinemic hypoglycemia (late dumping syndrome) and is characterized by a dizziness that can be disabling. This problem must be distinguished from conditions provoking similar symptoms, as for instance the early dumping syndrome and the food restriction-related hypoglycemia. Since all these conditions need a particular management, their distinction is essential.

[Progress in bronchoscopy: ultrasound guided fine-needle aspiration (EBUS-TBNA)]

In case of lung cancer, evaluation of the extension disease is mandatory. Mediastinal nodes must also be evaluated: if a controlateral to the tumor node is metastatic (N3), surgery is no more indicated. As specificity of PET-CT scan is not satisfaying (78%), cytology or histology of a suspect node is necessary. Even if PT-CT scan is negative, node micrometastasis are not excluded. To investigate a mediastinal adenopathy, mediastinoscopy or mediastinotomy are gold standard. Cytoponction guided by ultrasonography (EBUS-TBNA) is a new method which has been proved to be efficient and safe, but necessating some expertise and regular practice. It could be a valuable alternative to mediastinoscopy.

[Pneumonology. Ultrasound-guided transbronchial needle aspiration]

After discovering a lung cancer without extrathoracic metastases, the preoperative mediastinal check-up is of great importance to determine the patients operability. The 78% specificity of activity measuring FDG in a PET examination is clearly insufficient to avoid a biopsy of the incriminated lymph nodes. The investigation of a suspected malignant adenopathy justifies a complete examination by mediastinoscopy or-tomy, which are considered gold standards. The transbronchial fine needle aspiration guided by ultrasound (EBUS-TBNA) constitutes the recent evolution of TBNA, method known for 20 years now but underused. Depending on the operators skill, this new procedure needs a learning curve before being able to replace mediastinoscopy.

Role of Fas ligand expression in promoting escape from immune rejection in a spontaneous tumor model

Tumors escape immune-mediated rejection by a variety of mechanisms during tumor progression. The elucidation of these mechanisms in vivo suffers from a lack of suitable models of spontaneous tumor formation escaping active specific immunotherapy (ASI). In a rat neu transgenic (rNeu-TG) mouse model of spontaneous breast tumor formation, we showed that rNeu-TG mice developed late escape tumors despite the presence of a persistent rNeu-specific immune response after ASI. Cell suspensions derived from these escape tumors grew in vaccinated tumor-free mice, whereas injected spontaneous tumor cells were rejected. Escape tumors retained rNeu or MHC class I expression but significantly upregulated Fas (CD95, Apo-1) ligand. We further demonstrated that Fas-L on escape tumor cells correlated with apoptosis of infiltrating T lymphocytes. Thus, our results provide evidence that spontaneous breast tumors upregulate Fas-L expression after vaccination that may promote tumor escape in vivo after ASI.

Targeting HER-2/neu for active-specific immunotherapy in a mouse model of spontaneous breast cancer

The identification of tumor-associated antigens has led to increased interest in vaccination strategies to treat and/or prevent cancer. This study examined the feasibility of active-specific immunotherapy against the breast-tumor antigen HER-2/neu using a HER-2/neu transgenic (rNeu-TG) mouse model. rNeu-TG mice develop spontaneous breast tumors after pregnancy, indicating that they fail to mount an effective immune response against rNeu. Allogeneic fibroblasts expressing HER-2/neu were used as a cell-based vaccine. Vaccination induced a rNeu-specific anti-tumor immune response that prevented tumor formation of transplanted breast-tumor cells, and also protected mice from spontaneous tumor formation. Both T-cell-mediated and humoral immune responses were detectable in vaccinated mice. Vaccination also protected tumor-bearing mice from a challenge with cell suspensions isolated from spontaneous tumors, indicating that rNeu-TG mice are not tolerant to rNeu, even after spontaneous tumor formation. However, established spontaneous tumors themselves were never affected. This observation correlated with T-cell infiltrations in the injected but not in the established spontaneous tumor. Thus, allogeneic fibroblasts are efficient vaccine vectors to prime a specific immune response against an over-expressed tumor antigen. Moreover, our results suggest striking differences in the immunological requirements for the rejection of an established vs. a transplanted tumor.

Female-biased dispersal in the monogamous mammal Crocidura russula: evidence from field data and microsatellite patterns

We investigated dispersal patterns in the monogamous Crocidura russula, based both on direct field observations (mark-recapture data) and on genetic analyses (microsatellite loci). Natal dispersal was found to be low. Most juveniles settled within their natal territory or one immediately adjacent. Migration rate was estimated to two individuals per year and per population. The correlation between genetic and geographical distances over a 16 km transect implies that migration occurs over short ranges. Natal dispersal was restricted to first-litter juveniles weaned in early May; this result suggests a direct dependence of dispersal on reproductive opportunities. Natal dispersal was highly female biased, a pattern unusual among mammals. Its association with monogamy provides support for the resource-competition model of dispersal. Our results demonstrate that a state-biased dispersal can be directly inferred from microsatellite genotype distributions, which opens new perspectives for empirical studies in this area.

Origin and evolution of homologous repeated sequences in the mitochondrial DNA control region of shrews

The complete mitochondrial DNA (mtDNA) control region was amplified and directly sequenced in two species of shrew, Crocidura russula and Sorex araneus (Insectivora, Mammalia). The general organization is similar to that found in other mammals: a central conserved region surrounded by two more variable domains. However, we have found in shrews the simultaneous presence of arrays of tandem repeats in potential locations where repeats tend to occur separately in other mammalian species. These locations correspond to regions which are associated with a possible interruption of the replication processes, either at the end of the three-stranded D-loop structure or toward the end of the heavy-strand replication. In the left domain the repeated sequences (R1 repeats) are 78 bp long, whereas in the right domain the repeats are 12 bp long in C. russula and 14 bp long in S. araneus (R2 repeats). Variation in the copy number of these repeated sequences results in mtDNA control region length differences. Southern blot analysis indicates that level of heteroplasmy (more than one mtDNA form within an individual) differs between species. A comparative study of the R2 repeats in 12 additional species representing three shrew subfamilies provides useful indications for the understanding of the origin and the evolution of these homologous tandemly repeated sequences. An asymmetry in the distribution of variants within the arrays, as well as the constant occurrence of shorter repeated sequences flanking only one side of the R2 arrays, could be related to asymmetry in the replication of each strand of the mtDNA molecule. The pattern of sequence and length variation within and between species, together with the capability of the arrays to form stable secondary structures, suggests that the dominant mechanism involved in the evolution of these arrays in unidirectional replication slippage.

Twenty-four hour blood pressure and heart rate profiles of diabetic patients with abnormal cardiovascular reflexes

Twenty-four hour ambulatory blood pressure and heart rate profiles of 24 patients with diabetes were monitored in order to assess the effect of autonomic neuropathy on 24-h haemodynamic profiles. Eighteen patients had abnormal cardiovascular reflexes. Mean arterial pressure rose at night in six of the patients with autonomic neuropathy and fell by less than or equal to 5 mmHg in seven. In the remaining five patients with autonomic neuropathy and in the six diabetic patients with normal cardiovascular reflexes, the fall in nocturnal mean arterial pressure was comparable to that of 11 non-diabetic patients with essential hypertension. Median 24-h mean arterial pressure was similar in all four groups of diabetic patients. Prevalence of autonomic symptoms was not related to the change in blood pressure in those with autonomic neuropathy. Twenty-seven months after monitoring, three fatal and five severe non-fatal cardiovascular or renal events had occurred in four of the six patients with a rise in nocturnal blood pressure, compared with one non-fatal event in those with a small fall and no severe events in those with a pronounced fall (p = 0.02). Blood pressure rises at night in certain diabetic patients with abnormal cardiovascular reflexes and the nocturnal rise appears to be associated with a poor prognosis.

Interaction of oxindanac and frusemide in man

Oxindanac, a moderately active cyclo-oxygenase inhibitor in vitro, is a new antiinflammatory agent under clinical investigation. Its effects on frusemide-induced natriuresis have now been studied. Eight male volunteers receiving frusemide 40 mg b.d. were also given either oxindanac 300 mg b.d. or placebo in two consecutive periods separated by a treatment-free period, according to a randomized cross-over study design. Urinary prostaglandin excretion (PGF2 alpha) fell by 75% after 3 days on oxindanac. Frusemide-induced renin activity reached 66% of the control value in the presence of oxindanac. However, the natriuresis induced by frusemide did not differ significantly whether oxindanac or placebo was administered, despite the inhibitory action of the former on prostaglandin synthesis in vivo.

Effect of calcium-channel blockade on the aldosterone response to sodium depletion and potassium loading in man

Angiotensin II (Ang II) and potassium (K+) increase aldosterone (Aldo) production in vitro via Ca2+-dependent mechanisms. To determine the effects of Ca2+ antagonism in vivo, we examined the influence of nifedipine on the Aldo response to Na+ depletion and K+ loading in 11 healthy subjects. On the fifth day of a low-Na+/high-K+ diet (10 mmol Na+/100 mmol K+) the subjects were randomly given either nifedipine 30 mg po or placebo, and on the sixth day they received the alternative drug. KCl in 5% glucose was infused on days 5 and 6 from 10:00 to 12:00 AM (0.6 mmol/kg over 2 hours). Dexamethasone was given to suppress adrenal corticotrophic hormone. Plasma renin activity (PRA) and plasma Aldo were determined every 20 minutes. Nifedipine induced a rise in heart rate at 60 minutes but did not change blood pressure. During KCl/glucose infusions, plasma glucose increased significantly, but plasma K+ remained stable. PRA, but not baseline plasma Aldo, was stimulated by nifedipine. KCl provoked a significant and similar Aldo rise (P less than .01) under placebo and nifedipine. Baseline Aldo/PRA ratio was reduced under nifedipine when compared to placebo (P less than .01), whereas during KCl infusions this ratio was similarly elevated under placebo and nifedipine. We conclude that acute inhibition of slow Ca2+ channels does not interfere with K+-induced Aldo secretion in man, suggesting that adaptive mechanisms operate in vivo.

[Nyctohemeral arterial pressure profile and heart rate in autonomic diabetic neuropathy]

Autonomic failure reduces the physiological nocturnal decline of blood pressure (BP) and heart rate (HR). To assess the effect on circadian hemodynamic rhythms of sympathetic (s) and parasympathetic (ps) impairment in diabetic autonomic neuropathy (DAN), we measured BP automatically every 15 min and HR continuously for 24 hour in 11 diabetic patients. They were divided into 3 groups according to the results of cardiovascular reflex tests: a) DAN s + ps, b) DAN ps, c) no DAN. Nine of the patients were hypertensive. Eleven non-diabetic hypertensives served as controls. The disturbance of the circadian BP profile was related to the severity of the DAN, nocturnal BP tending to rise in group a, to fall in group b, and falling markedly in group c and in the control group. FC fell to a similar extent in all the groups. We conclude that the circadian BP profile is more affected by DAN than the FC profile, and that study of the circadian BP profile could reveal the presence of predominantly nocturnal hypertension.

Influence of nifedipine and enalapril on osmoregulation of vasopressin

To determine whether calcium fluxes and angiotensin II influence osmoregulation of vasopressin (AVP) secretion, the effects of the calcium antagonist nifedipine and of the converting enzyme inhibitor enalapril on the AVP response to an osmotic load were compared to those of a placebo in seven normal female subjects. Plasma and urinary AVP were measured before and during a 3-h infusion of 2.5% hypertonic saline. Nifedipine (10 mg orally 2 h before and 10 mg at the start of the infusion) increased heart rate but did not change blood pressure. The changes in free water clearance and in urinary AVP induced by hypertonic saline under nifedipine were greater than in the control test, but the slope and the intercept of the regression line of plasma AVP upon plasma osmolality were not significantly different. Enalapril (10 mg 3 h before the infusion) did not change heart rate or blood pressure. Free water clearance and urinary AVP did not differ from the control test, but the slope of the regression line was less steep. These slight modifications of the response to an osmotic load suggest that calcium fluxes and angiotensin II only exert a limited influence on AVP osmoregulation in normal females.

Blood pressure regulation on low and high sodium diets in normotensive members of normotensive or hypertensive families

In members of hypertensive families a high sodium intake may have a pressor effect. The mechanism mediating the sodium sensitivity is unclear. Blood pressure, exchangeable sodium, plasma levels of norepinephrine, epinephrine, renin and aldosterone, the pressor response to infused norepinephrine or angiotensin II and the urinary excretion of prostaglandin (PG) E2 and F2 alpha were measured after 7 days of low sodium diet (urinary sodium 13 +/- 10 s.d. mmol/24 h) and 7 days of high sodium intake (urinary sodium 268 +/- 97 mmol/24 h) in 10 normotensive men without and 13 men with family history of essential hypertension. After the low sodium phase, blood pressure, heart rate, exchangeable sodium, plasma levels of norepinephrine, epinephrine, renin and aldosterone, pressor doses of norepinephrine or angiotensin II and the urinary excretion of PGE2 or PGF2 alpha did not differ between the two groups. After the high sodium phase, blood pressure increased only in subjects with positive (P less than 0.05) but not in those with negative family history. In the two groups exchangeable sodium increased (P less than 0.05) and plasma levels of NE (-33% versus -32%), renin (-31% versus -27%) or aldosterone (-74% versus -61%) and pressor doses of NE (-55% versus -54%) or ANG II (-72% versus -44%) decreased by a comparable extent. Urinary PGE2 or PGF2 alpha were unchanged. These findings suggest that a high dietary sodium intake exerts a pressor effect in subjects with familial predisposition to essential hypertension. This pressor effect is not explained by an abnormal adaptation of body sodium, sympathetic activity, renin-angiotensin-aldosterone axis, cardiovascular pressor responsiveness and renal prostaglandin excretion to a high sodium diet.