SrMnO3 Thermochromic Behavior Governed by Size-Dependent Structural Distortions.

The influence of particle size in both the structure and thermochromic behavior of 4H-SrMnO3 related perovskite is described. Microsized SrMnO3 suffers a structural transition from hexagonal (P63/mmc) to orthorhombic (C2221) symmetry at temperature close to 340 K. The orthorhombic distortion is due to the tilting of the corner-sharing Mn2O9 units building the 4H structural type. When temperature decreases, the distortion becomes sharper reaching its maximal degree at ∼125 K. These structural changes promote the modification of the electronic structure of orthorhombic SrMnO3 phase originating the observed color change. nano-SrMnO3 adopts the ideal 4H hexagonal structure at room temperature, the orthorhombic distortion being only detected at temperature below 170 K. A decrease in the orthorhombic distortion degree, compared to that observed in the microsample, may be the reason why a color change is not observed at low temperature (77 K).

Chemical analysis at atomic resolution of isolated extended defects in an oxygen-deficient, complex manganese perovskite.

A general approach to the structural and analytical characterization of complex bulk oxides that exploits the advantage of the atomic spatial resolution and the analytical capability of aberration-corrected microscopy is described. The combined use of imaging and spectroscopic techniques becomes necessary to the complete characterization of the oxygen-deficient colossal magnetoresistant La(0.56)Sr(0.44)MnO(2.5)-related perovskite. In this compound, the formation of isolated (La/Sr)O and MnO rock-salt-type planar defects are identified from atomically resolved High Angle Annular Dark Field (HAADF) images. The location of the oxygen atomic columns from Annular Bright Field (ABF) images indicates edge-sharing MnO6 octahedra in the MnO planes and the study performed by Electron Energy Loss Spectroscopy (EELS) reveals different Mn oxidation states derived from the corner- or edge-sharing MnO6 octahedra environment.

Mechanically interlocked single-wall carbon nanotubes.

Extensive research has been devoted to the chemical manipulation of carbon nanotubes. The attachment of molecular fragments through covalent-bond formation produces kinetically stable products, but implies the saturation of some of the C-C double bonds of the nanotubes. Supramolecular modification maintains the structure of the SWNTs but yields labile species. Herein, we present a strategy for the synthesis of mechanically interlocked derivatives of SWNTs (MINTs). In the key rotaxane-forming step, we employed macrocycle precursors equipped with two π-extended tetrathiafulvalene SWNT recognition units and terminated with bisalkenes that were closed around the nanotubes through ring-closing metathesis (RCM). The mechanically interlocked nature of the derivatives was probed by analytical, spectroscopic, and microscopic techniques, as well as by appropriate control experiments. Individual macrocycles were observed by HR STEM to circumscribe the nanotubes.

Characterization of Tunneled Wide Band Gap Mixed Conductors: The Na2O-Ga2O3-TiO2 System.

This article focuses on the Na2O-Ga2O3-TiO2 system, which is barely explored in the study of transparent conductive oxides (TCOs). NaxGa4+xTin-4-xO2n-2 (n = 5, 6, and 7 and x ≈ 0.7-0.8) materials were characterized using neutron powder diffraction and aberration-corrected scanning transmission electron microscopy. Activation energy, as a function of different structures depending on tunnel size, shows a significant improvement in Na+ ion conduction from hexagonal to octagonal tunnels. New insights into the relationship between the crystal structure and the transport properties of TCOs, which are crucial for the design and development of new optoelectronic devices, are provided.

Structural effects behind the low temperature nonconventional relaxor behavior of the Sr2NaNb5O15 bronze.

An exhaustive temperature dependent structural and dielectric study of the tetragonal tungsten bronze-type Sr(2)NaNb(5)O(15) (SNN) compound has been performed in the 300-100 K temperature range, by combining X-ray, neutron diffraction, and transmission electron microscopy with dielectric measurements, in order to clarify the structural effects responsible for the observed low temperature dielectric properties. Interestingly, a relevant second anomaly in the dielectric constant, in addition to the ferroelectric (FE) to paraelectric (PE) transition at T(C) = 518 K is found at T ≈ 240 K, revealing a relaxor-like behavior of the material at low temperature. This phenomenon has been previously observed in FE perovskite-type phases and referred to as the re-entrant phenomenon. However, FE polarization tends to vanish below this low temperature dielectric anomaly and this fact is not expected for a classical relaxor-ferroelectric phase. Although there is no structural transition from RT to 100 K, there is a change in the elastic properties of the material in the considered temperature range and the intense anomaly at ~240 K could be associated to a smeared-out phase transition to a frustrated FE/ferroelastic (FEL) low temperature state in correlation with subtle structural effects.

The Role of Transmission Electron Microscopy in the Early Development of Mesoporous Materials for Tissue Regeneration and Drug Delivery Applications.

For the last 20 years, silica-based mesoporous materials have provided a sound platform for the development of biomedical technology applied to tissue engineering and drug delivery. Their unique structural and textural characteristics, chiefly, the ordered distribution of homogeneous and tunable pores with high surface areas and large pore volume, and their excellent biocompatibility provide an excellent starting point for bone tissue regeneration on the mesoporous surface, and also to load species of interest inside the pores. Adequate control of the synthesis conditions and functionalization of the mesoporous surface are critical factors in the design of new systems that are suitable for use in specific medical applications. Simultaneously, the use of appropriate characterization techniques in the several stages of design and manufacture of mesoporous particles allows us to ascertain the textural, structural and compositional modifications induced during the synthesis, functionalization and post-in vitro assays processes. In this scenario, the present paper shows, through several examples, the role of transmission electron microscopy and associated spectroscopic techniques in the search for useful information in the early design stages of mesoporous systems, with application in the fields of tissue regeneration and drug delivery systems.

Evaluation of the Nanodomain Structure in In-Zn-O Transparent Conductors.

The optimization of novel transparent conductive oxides (TCOs) implies a better understanding of the role that the dopant plays on the optoelectronic properties of these materials. In this work, we perform a systematic study of the homologous series ZnkIn2Ok+3 (IZO) by characterizing the specific location of indium in the structure that leads to a nanodomain framework to release structural strain. Through a systematic study of different terms of the series, we have been able to observe the influence of the k value in the nano-structural features of this homologous series. The stabilization and visualization of the structural modulation as a function of k is discussed, even in the lowest term of the series (k = 3). The strain fields and atomic displacements in the wurtzite structure as a consequence of the introduction of In3+ are evaluated.

Transmission electron microscopy evidence of spontaneous B-cation layered distribution in NaNb(1-x)Ta(x)O3.

A transmission electron microscopy (TEM) study of the complex NaNb(1-x)Ta(x)O(3) (0.4 < or = x < or = 0.6) perovskites, combining high-resolution TEM and high-angle annular dark-field scanning TEM, has revealed the formation of extended areas on the crystals where niobium and tantalum order into layers in a 1:1 ratio. NaNb(1-x)Ta(x)O(3) oxides are stoichiometric, and there is neither charge difference nor significant ionic size discrepancy between Nb(V) and Ta(V) cations. As d(0) octahedrally coordinated cations, they show a propensity to second-order Jahn-Teller distortion. This distortion, however, manifests itself to different extents for the two cations and is considered the driving force for the layered ordered distribution observed. The niobium-tantalum segregation we have found can also be interpreted as a naturally occurring nanometer-scale phase separation. Albeit occurring in wide regions of the crystals and not in the entire grains, it shows a clear trend toward a long-range ordered disposition. This is reminiscent of the more general behavior of a recently documented class of perovskites that suffer spontaneous nanoscale phase separation to form a superlattice.

Direct Transformation of Crystalline MoO3 into Few-Layers MoS2.

We fabricated large-area atomically thin MoS2 layers through the direct transformation of crystalline molybdenum trioxide (MoO3) by sulfurization at relatively low temperatures. The obtained MoS2 sheets are polycrystalline (~10-20 nm single-crystal domain size) with areas of up to 300 × 300 µm2, 2-4 layers in thickness and show a marked p-type behavior. The synthesized films are characterized by a combination of complementary techniques: Raman spectroscopy, X-ray diffraction, transmission electron microscopy and electronic transport measurements.

Synthesis and characterization of Ag2S and Ag2S/Ag2(S,Se) NIR nanocrystals.

Syntheses of metal sulfide nanocrystals (NCs) by heat-up routes in the presence of thiols yield NC arrangements difficult to further functionalize and transfer to aqueous media. By means of different NMR techniques, and exemplified by Ag2S NCs, a metal-organic polymer formed during the synthesis acting as a ligand has been identified to be responsible for such aggregation. In this work, a new synthetic hot-injection strategy is presented to synthesize Ag2S NCs which are easily ligand exchangeable in water. Furthermore, the hot-injection route allows an extra NC treatment with Se to produce Ag2S/Ag2(S,Se) NCs with improved optical properties with respect to the Ag2S cores, and better resistance to oxidation, as demonstrated by X-ray absorption experiments.

Structure and electrochromism of two-dimensional octahedral molecular sieve h'-WO3.

Octahedral molecular sieves (OMS) are built of transition metal-oxygen octahedra that delimit sub-nanoscale cavities. Compared to other microporous solids, OMS exhibit larger versatility in properties, provided by various redox states and magnetic behaviors of transition metals. Hence, OMS offer opportunities in electrochemical energy harnessing devices, including batteries, electrochemical capacitors and electrochromic systems, provided two conditions are met: fast exchange of ions in the micropores and stability upon exchange. Here we unveil a novel OMS hexagonal polymorph of tungsten oxide called h'-WO3, built of (WO6)6 tunnel cavities. h'-WO3 is prepared by a one-step soft chemistry aqueous route leading to the hydrogen bronze h'-H0.07WO3. Gentle heating results in h'-WO3 with framework retention. The material exhibits an unusual combination of 1-dimensional crystal structure and 2-dimensional nanostructure that enhances and fastens proton (de)insertion for stable electrochromic devices. This discovery paves the way to a new family of mixed valence functional materials with tunable behaviors.

Amyloid Assembly Endows Gad m 1 with Biomineralization Properties.

Acid proteins capable of nucleating Ca2+ and displaying aggregation capacity play key roles in the formation of calcium carbonate biominerals. The helix-loop helix EF-hands are the most common Ca2+-binding motifs in proteins. Calcium is bound by the loop region. These motifs are found in many proteins that are regulated by calcium. Gad m 1, an Atlantic cod ß-parvalbumin isoform, is a monomeric EF-hand protein that acts as a Ca2+ buffer in fish muscle; the neutral and acid apo-forms of this protein can form amyloids. Since Ca2+-nucleating proteins have a propensity to form extended ß-strand structures, we wondered whether amyloid assemblies of an EF-hand protein were able to influence calcium carbonate crystallization in vitro. Here, we used the Gad m 1 chain as a model to generate monomeric and amyloid assemblies and to analyze their effect on calcite formation in vitro. We found that only amyloid assemblies alter calcite morphology.

Copper-containing mesoporous bioactive glass nanoparticles as multifunctional agent for bone regeneration.

The application of mesoporous bioactive glasses (MBGs) containing controllable amount of different ions, with the aim to impart antibacterial activity, as well as stimulation of osteogenesis and angiogenesis, is attracting an increasing interest. In this contribution, in order to endow nano-sized MBG with additional biological functions, the framework of a binary SiO2-CaO mesoporous glass was modified with different concentrations of copper ions (2 and 5%mol.), through a one-pot ultrasound-assisted sol-gel procedure. The Cu-containing MBG (2%mol.) showed high exposed surface area (550m2g-1), uniform mesoporous channels (2.6nm), remarkable in vitro bioactive behaviour and sustained release of Cu2+ ions. Cu-MBG nanoparticles and their ionic dissolution extracts exhibited antibacterial effect against three different bacteria strains, E. coli, S. aureus, S. epidermidis, and the ability to inhibit and disperse the biofilm produced by S. epidermidis. The obtained results suggest that the developed material, which combines in single multifunctional agent excellent bioactivity and antimicrobial ability, offers promising opportunities for the prevention of infectious diseases and the effective treatment of bone defects. STATEMENT OF SIGNIFICANCE: In order to endow mesoporous bioactive glass, characterized by excellent bioactive properties, with additional biological functions, Cu-doped mesoporous SiO2-CaO glass (Cu-MBG) in the form of nanoparticles was prepared by an ultra-sound assisted one pot synthesis. The analysis of the bacterial viability, using different bacterial strains, and the morphological observation of the biofilm produced by the Staphylococcus epidermidis, revealed the antimicrobial effectiveness of the Cu-MBG and the relative ionic extracts against both the bacterial growth and the biofilm formation/dispersion, providing a true alternative to traditional antibiotic systemic therapies. The proposed multifunctional agent represents a promising and versatile platform for bone and soft tissues regeneration.

Surface-Driven Magnetotransport in Perovskite Nanocrystals.

Unique insights into magnetotransport in 20 nm ligand-free La0.67 Sr0.33 MnO3 perovskite nanocrystals of nearly perfect crystalline quality reveal a chemically altered 0.8 nm thick surface layer that triggers exceptionally large magnetoresistance at low temperature, independently of the spin polarization of the ferromagnetic core. This discovery shows how the nanoscale impacts magnetotransport in a material widely spread as electrode in hybrid spintronic devices.

Cobalt phosphate-modified barium-doped tantalum nitride nanorod photoanode with 1.5% solar energy conversion efficiency.

Spurred by the decreased availability of fossil fuels and global warming, the idea of converting solar energy into clean fuels has been widely recognized. Hydrogen produced by photoelectrochemical water splitting using sunlight could provide a carbon dioxide lean fuel as an alternative to fossil fuels. A major challenge in photoelectrochemical water splitting is to develop an efficient photoanode that can stably oxidize water into oxygen. Here we report an efficient and stable photoanode that couples an active barium-doped tantalum nitride nanostructure with a stable cobalt phosphate co-catalyst. The effect of barium doping on the photoelectrochemical activity of the photoanode is investigated. The photoanode yields a maximum solar energy conversion efficiency of 1.5%, which is more than three times higher than that of state-of-the-art single-photon photoanodes. Further, stoichiometric oxygen and hydrogen are stably produced on the photoanode and the counter electrode with Faraday efficiency of almost unity for 100 min.