Covalent 2D and 3D networks from 1D nanostructures: designing new materials

We show extensive theoretical studies related to the generation and characterization of 2D and 3D ordered networks using 1D units that are connected covalently. We experimentally created multi-terminal junctions containing 1D carbon blocks in order to study the most common morphologies and branched structures that could be used in the theoretical design of network models. We found that the mechanical and electronic characteristics of ordered networks based on carbon nanotubes (ON-CNTs) are dominated by their specific super-architecture (hexagonal, cubic, square, and diamond-type). We show that charges follow specific paths through the nodes of the multi-terminal systems, which could result in complex integrated nanoelectronic circuits. The 3D architectures reveal their ability to support extremely high unidirectional stress when their mechanical properties are studied. In addition, these networks are shown to perform better than standard carbon aerogels because of their low mass densities, continuous porosities, and high surface areas.     

Self-assembly hierarchical micro/nanostructures of leaf-like polyaniline with 1D nanorods on 2D foliage surface

A novel 2D leaf-like polyaniline with special hierarchical micro/nanostructures, a length of about 3 μm, width of about 2.3 μm and thickness of about 120 nm, has been successfully synthesized in the presence of poly (acrylic acid-co-maleic acid) sodium salt (PAA/MA-SS), which is self-assembled from 2D square nanoplate and 1D nanorods. Its surface consists of highly cross-linked nanorods of approximately 100 nm in length and 30 nm in diameter. In order to investigate the formation mechanism of such 2D leaf-like polyaniline, some micro/nanostructures of polyaniline are synthesized at different polymerization times and the results show that the polyaniline microleaves originate from square nanoplates, which then self-assemble into leaf-like micro/nanostructures with nanorods on the surface.     

The transition from 2D to 3D nanoclusters of silicon carbide on silicon

We have evaluated the critical average distance between the nanoclusters of silicon carbide grown by molecular beam epitaxy on silicon and estimated the time of the transition from two-to three-dimensional growth.     

Ultrasound-induced morphology transformation from 3D microspheres to 1D nanorods of luminescent coordination polymer

Hierarchically spherical architectures self-assembled by nanorods of coordination polymer La(1,3,5-BTC)(H2O)6:Dy3+ have been successfully prepared on a large scale via rapid static growth in solution phase at room temperature. Interestingly, these uniform and well-dispersed 3D microspheres are sensitive to ultrasound, which can be completely transformed into 1D nanorods through simple ultrasonic treatment. The photoluminescence properties of the nano/microstructured La(1,3,5-BTC)(H2O)6:Dy3+ are also investigated in detail, indicating that the obtained lanthanum 1,3,5-benzenetricarboxylate is a new promising host material for the luminescence of Dy3+ ions.     

Room temperature synthesis and ammonia sensing of monodispersed hierarchical 1 and 3-dimensional Co3O4 nanostructures: switching from p to n-type sensing

This study report on a sonochemical synthesis of 1- and 3-dimensional hierarchical nanostructured cobalt oxide systems (Co₃O₄) and their application in ammonia sensing at room temperature (i.e., 30 °C). The Co₃O₄ nanostructures were synthesized via a room temperature-assisted precipitation and subsequent thermal treatment of the oxalate precursor. The resulted nanostructures were characterized by SEM, XRD, TEM, FTIR spectroscopy, BET, and TGA/DTA. The synthesis mechanism was proposed on the basis of morphology analyzed at various stages of the particle growth. It was observed that the final hierarchical microspheres structure resulted from the self-aggregation of the initially formed nanorods. The microspheres and nanorods were used as efficient room temperature gas sensors for ammonia detection in the concentration range of 0.01–500 ppm. The nanorod-based sensor showed an unusual n-type sensing behavior to ammonia in a temperature range of 30–300 °C. This transition of p to n-type was correlated to the formation of successive layers of physisorbed water molecules at the surface of the synthesized Co₃O₄. However, in case of the microspheres, the n-type behavior and superior sensitivity were observed at 30 °C followed by a negligible response up to 200 °C, while the intrinsic p-type behavior was recorded at an elevated temperature (200–300 °C). The observed unusual sensing performance may be associated with the crystallographic nature and lattice strain in the material structures. Additionally, the large specific surface area and the change in crystalline structure with temperature made the as prepared novel hierarchical Co₃O₄ structures a distinctive material for sensing ammonia at 30 °C.

     

From sheets to fibers: A novel approach to gamma-AlOOH and gamma-A12O3 1D nanostructures

The gamma-AIOOH (boehmite) nanofiber bundles have been synthesized via a convenient quencher method. Most nanofibers contain even smaller nanowires with an average diameter of 5 nm. A series of contrast experiments reveal that the evolvement from nanosheets to nanofibers occurs in the quencher process. gamma-Al2O3 nanofiber bundles with mesoporous character can be obtained by calcining relevant gamma-AlOOH nanostructures at 500 degrees C.     

Tunable Magnetic Antiskyrmion Size and Helical Period from Nanometers to Micrometers in a D2d Heusler Compound

Skyrmions and antiskyrmions are magnetic nano-objects with distinct chiral, noncollinear spin textures that are found in various magnetic systems with crystal symmetries that give rise to specific Dzyaloshinskii–Moriya exchange vectors. These magnetic nano-objects are associated with closely related helical spin textures that can form in the same material. The skyrmion size and the period of the helix are generally considered as being determined, in large part, by the ratio of the magnitude of the Heisenberg to that of the Dzyaloshinskii–Moriya exchange interaction. In this work, it is shown by real-space magnetic imaging that the helix period λ and the size of the antiskyrmion d_aSk in the D_2d compound Mn_1.4PtSn can be systematically tuned by more than an order of magnitude from ≈100 nm to more than 1.1 µm by varying the thickness of the lamella in which they are observed. The chiral spin texture is verified to be preserved even up to micrometer-thick layers. This extreme size tunability is shown to arise from long-range magnetodipolar interactions, which typically play a much less important role for B20 skyrmions. This tunability in size makes antiskyrmions very attractive for technological applications.     

Synthesis of In2O3 nanostructures: from pyramidal monuments, nanotowers to nanopencils

We report on the synthesis of In2O3 nanostructures grown at three different growth temperatures by using the thermal evaporation method. The obtained nanostructural morphologies of In2O3 were characterized by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). In2O3 nanopencils and pyramidal monument-like structures were reported. The reaction temperature and the difference of temperature between the central heating zones and the product deposition zones, the reaction time, and the surface energies of different growth planes are all responsible for the final crystalline morphologies of the In2O3 nanostructures. A growth mechanism proposed to elucidate the kinetic factors leading the growth of In2O3 nanostructures. The obtained results may not only assist the investigation of new approaches of preparing various nanostructures for potential technical applications and how to modulate the desired morphology, but also give a deeper understanding of the fundamental physical and chemical processes of CVD methods.     

The influence of the reaction temperature on the morphology of sodium titanate 1D nanostructures and their thermal stability

We have synthesized large quantities of sodium-titanate-based nanotubes and nanoribbons with high yields under hydrothermal conditions from anatase powder in an aqueous NaOH solution. The reaction temperatures were from 95 to 195 degrees C, in steps of 20 degrees C. We observed that the morphology of the nanomaterials, which is reflected in their specific surface areas, depends strongly on the reaction temperature. For the materials synthesized in the range 95-135 degrees C and above 155 degrees C only a single morphology type was observed for the nanostructures, i.e., nanotubes and nanoribbons, respectively. In contrast, when the reaction was carried out at 155 degreesC, both nanotubes and nanoribbons were found in the product. SEM, TEM, and XRD techniques were used to determine the materials' morphological and structural properties, and the thermal stability of the materials was investigated with TGA and DSC. The largest weight loss, of approximately 25%, was observed in a temperature range from 25 up to 600 degrees C for the product obtained at 95 degrees C, probably due to the presence of unrolled titanate sheets.     

Crossover from 3D to 2D metallic conduction in Sr2RuO4

We have investigated the normal-state conduction of superconducting Sr₂RuO₄ (T_c 1K), which is isostructural to La_2–xSr_xCuO₄. The resistivity of single crystals shows a crossover at T_M130K from 3D metallic conduction at lower temperatures to 2D one at higher temperatures. Concerning the temperature dependence of the out-of-plane resistivity, we present a systematic interpretation based on competition between the life time of the quasiparticles and the time for the quasiparticles to travel between the adjacent RuO₂ planes.     

V2O5 nanorods on TiO2 nanofibers: a new class of hierarchical nanostructures enabled by electrospinning and calcination

Electrospinning provides a simple approach to fabricating nanofibers and assemblies with controllable hierarchical structures. In this communication, we demonstrate that electrospinning can be combined with calcination to further maneuver the morphology and phase structure of nanofibers. More specifically, single-crystal V2O5 nanorods could be grown on rutile nanofibers by carefully calcining composite nanofibers consisting of amorphous V2O5, amorphous TiO2, and poly(vinylpyrrolidone). The size of the resulting V2O5 nanorods could be conveniently controlled by varying the composition of the nanofibers and/or the calcination temperature. In addition to the nanorod-on-nanofiber hierarchical structure, we believe this approach can also be extended to fabricate other more complex architectures.     

Dynamics of the 1st order phase transition between U2D2 and high field phases of 3He

No Heading Dynamics of the 1st order phase transition between the U2D2 and the high field phases (HFP) of nuclear ordered solid ³He was studied by field-cycling method between the two phases. Single crystals of U2D2 ³He were grown at the bottom of the compressional cell in superfluid ³He-B at about 0.5 mK. The domain distribution in the U2D2 crystal was determined by ULT-MRI (Ultra Low Temperature – Magnetic Resonance Imaging)¹. We have measured evolution of the HFP after the static magnetic field was suddenly changed to B = B_C1 + B through the lower critical field B_C1. The HFP developed exponentially in time and showed two stage evolution with a fast stage rate and a slow stage rule. We observed that the fast stage rate increased with B. Similar behavior was observed in the reversed phase transition. MRI measurements indicate that this phase transition occurs, at least for small B, by the heterogeneous nucleation mechanism, where new seeds appear at the liquid-crystal interface.PACS numbers: 64.60.Qb, 67.80.–s, 67.80.Jd, 68.08.–p, 76.60.Pc.     

3D-to-2D phase transformation through highly ordered 1D crystals from transition-metal oxides to dichalcogenides

Although solid-state phase transformations through chemical reaction with the surrounding environment are important in the field of materials science, the atomic-level dynamics at reacting surfaces have been difficult to observe directly. Herein, we found highly ordered arrays of 1D intermediate crystals with a unique atomic configuration during the thermal sulfidation of 3D-structured MoO₂ to 2D layer-structured MoS₂. These arrays reveal a dimension-breaking reconstruction process (3D → 1D → 2D) as well as a unique electronic structure evolution. Theoretical calculations show that the 1D crystals have a cross-sectional structure of four transition-metal atoms arranged in a diamond shape; these are critical to the atomic layer-by-layer formation of 2D transition-metal dichalcogenides. Furthermore, electronic structure analyses reveal that the 1D intermediate crystals alter the MoO₂/MoS₂ contact structure from p- to n-type with increases in the number of formed MoS₂ layers.     

pH-sensitive morphological transition from nanowire to nanovesicle of a single amino acid-based water soluble molecule

Employment of single amino acid-based molecules for the construction of various nanostructures is a challenging issue. Moreover, it is fascinating to see the controlled fabrication of specific nanostructures from the self-assembly of molecular building blocks through the fine tuning of pH of the solution. The present study demonstrates pH-responsive nanostructural transformation of single amino acid-based nanostructures from nanowires to nanovesicles. The molecules have been developed by coupling the carboxyl group of natural amino acids with the amino group of unnatural m-aminobenzoic acid (m-ABA), such as NH₂–Xx–m-ABA–CO₂H, where Xx are amino acids Phe, Tyr, Gly, and Pro. The presence of m-aminobenzoic acid helps in self-assembly through insertion of conformational constraints in the peptide backbone and also through aromatic π–π interactions. The insertion of m-aminobenzoic acid also induces proteolytic stability in the nanostructures. The formation of nanowires has been observed at acidic pH (pH 4.2–6.0), while both nanowires and nanovesicles are formed simultaneously at nearly neutral pH (pH 6.4). With an increase in the pH of the solution, only one nanoscopic species, i.e., nanovesicles have been formed exclusively, and these nanovesicles are stable within the range of pH 7.0–9.1. A further increase in pH (pH 10) triggers the disruption of nanovesicular structures that starts at the peripheral wall of the vesicles and is completed after total disintegration at pH 11. Studies show that alkali metal salt KCl can also disrupt these nanovesicles efficiently. These peptide-based nanovesicles can encapsulate a potent drug curcumin, which can be effectively released through the disruption of the vesicles either in presence of KCl or alkaline pH. Single crystal X-ray diffraction analysis indicates the sheet-mediated self-assembly in the formation of different nanostructures. Studies also show that the tyrosine-based peptide nanovesicles are elegant hosts for in situ preparation and stabilization of silver nanoparticles.     

Effects of BaO on the glass formation of the PbO-B2O3-TiO2-BaO system in relation to transition temperatures

The variation of the BaO content on the quaternary PbO-B₂O₃-TiO₂-BaO system's glass formation tendency was investigated in relation to transition temperatures, such as melt temperature (T _f), liquidus temperature (T _l), crystallization temperature (T _c) and glass transition temperature (T _g). Compositions were melted between 800°C and 1300°C. In order to obtain bulk glass samples, glass formation was carried out using a preheated cylindrical brass mould without forcing the cooling rate to increase. Glass formation tendency increased with increasing the temperature ratios of T _g/T _l and T _c/T _l and with decreasing T _g/T _c. Eutectic compositions preferentially formed glasses from melts due to their low melting temperatures.     

On the extension of the grain loop concept from 2D to 3D granular assemblies

Granular Matter - We study the local structural changes along the jamming transitions in asymmetric bidisperse granular packings. The local structure of the packing is assessed by the contact...