V. Lavayen

Synthesis and characterization of cyclotriphosphazenes containing silicon as single solid-state precursors for the formation of silicon/phosphorus nanostructured materials.

The synthesis and characterization of new organosilicon derivatives of N(3)P(3)Cl(6), N(3)P(3)[NH(CH(2))(3)Si(OEt)(3)](6) (1), N(3)P(3)[NH(CH(2))(3)Si(OEt)(3)](3)[NCH(3)(CH(2))(3)CN](3) (2), and N(3)P(3)[NH(CH(2))(3)Si(OEt)(3)](3)[HOC(6)H(4)(CH(2))CN](3) (3) are reported. Pyrolysis of 1, 2, and 3 in air and at several temperatures results in nanostructured materials whose composition and morphology depend on the temperature of pyrolysis and the substituents of the phosphazenes ring. The products stem from the reaction of SiO(2) with P(2)O(5), leading to either crystalline Si(5)(PO(4))(6)O, SiP(2)O(7) or an amorphous phase as the glass Si(5)(PO(4))(6)O/3SiO(2).2P(2)O(5), depending on the temperature and nature of the trimer precursors. From 1 at 800 degrees C, core-shell microspheres of SiO(2) coated with Si(5)(PO(4))(6)O are obtained, while in other cases, mesoporous or dense structures are observed. Atomic force microscopy examination after deposition of the materials on monocrystalline silicon wafers evidences morphology strongly dependent on the precursors. Isolated islands of size approximately 9 nm are observed from 1, whereas dense nanostructures with a mean height of 13 nm are formed from 3. Brunauer-Emmett-Teller measurements show mesoporous materials with low surface areas. The proposed growth mechanism involves the formation of cross-linking structures and of vacancies by carbonization of the organic matter, where the silicon compounds nucleate. Thus, for the first time, unique silicon nanostructured materials are obtained from cyclic phosphazenes containing silicon.

Synthesis, functionalization, and properties of intercalation compounds

Abstract Layered compounds are nanostructured, intrinsically anisotropic materials which often undergo intercalation reactions producing host–guest complexes. In this work examples from the molybdenum disulfide chemistry are used for discussing how the properties of the products may be regulated by appropriate selection of the guests species used for functionalizing the pristine sulfide. Special attention is given to new intercalates based in the intercalation of surfactants, which under special conditions may act as template promoting the conversion of the layered products into micro and nanotubes. The form how this kind of surfactants may be used for obtaining laminar derivatives of cadmium disulfide with the sulfide in a confined state is also described.

Low‐Dimensional, Hinged Bar‐code Metal Oxide Layers and Free‐Standing, Ordered Organic Nanostructures from Turbostratic Vanadium Oxide

Both low-dimensional bar-coded metal oxide layers, which exhibit molecular hinging, and free-standing organic nanostructures can be obtained from unique nanofibers of vanadium oxide (VO(x)). The nanofibers are successfully synthesized by a simple chemical route using an ethanolic solution of vanadium pentoxide xerogel and dodecanethiol resulting in a double bilayered laminar turbostratic structure. The formation of vanadium oxide nanofibers is observed after hydrothermal treatment of the thiol-intercalated xerogel, resulting in typical lengths in the range 2-6 microm and widths of about 50-500 nm. We observe concomitant hinging of the flexible nanofiber lamina at periodic hinge points in the final product on both the nanoscale and molecular level. Bar-coded nanofibers comprise alternating segments of organic-inorganic (thiols-VO(x)) material and are amenable to segmented, localized metal nanoparticle docking. Under certain conditions free-standing bilayered organic nanostructures are realized.

Metallophosphazene precursor routes to the solid-state deposition of metallic and dielectric microstructures and nanostructures on Si and SiO2.

We present a method for the preparation and deposition of metallic microstructures and nanostructures deposited on silicon and silica surfaces by pyrolysis in air at 800 degrees C of the corresponding metallophosphazene (cyclic or polymer). Atomic force microscopy studies reveal that the morphology is dependent on the polymeric or oligomeric nature of the phosphazene precursor, on the preparation method used, and on the silicon substrate surface (crystalline or amorphous) and its prior inductively couple plasma etching treatment. Microscale and nanoscale structures and high-surface-area thin films of gold, palladium, silver, and tin were successfully deposited from their respective newly synthesized precursors. The characteristic morphology of the deposited nanostructures resulted in varied roughness and increased surface area and was observed to be dependent on the precursor and the metal center. In contrast to island formation from noble metal precursors, we also report a coral of SnP(2)O(7) growth on Si and SiO(2) surfaces from the respective Sn polymer precursor, leaving a self-affine fractal structure with a well-defined roughness exponent that appears to be independent (within experimental error) of the average size of the islands. The nature of the precursor will be shown to influence the degree of surface features, and the mechanism of their formation is presented. The method reported here constitutes a new route to the deposition of single-crystal metallic, oxidic, and phosphate nanostructures and thin films on technologically relevant substrates.

Large directional conductivity change in chemically stable layered thin films of vanadium oxide and a 1D metal complex

Electroactive hybrid and layered oxides and related materials where the inorganic phase is the host, offering the conductivity characteristics of semiconductors, have been used in thin film transistors and related electronic devices where the host–guest interaction offered conductivity with improved processability. We describe the synthesis and characterization of a nanocomposite that shows large conductivity anisotropy when deposited as a thin film. We prepared the material by inserting quasi 1-dimensional potassium tetracyanoplatinate metal complexes with insulating electrical properties in between stacked nanosheets of vanadium oxide xerogels. Detailed structural and compositional analysis using transmission electron microscopy and X-ray photoelectron spectroscopy confirms that the hybrid material forms from a topotactic reaction and the framework of the layered host oxide structure is maintained. The hybrid film demonstrates a ∼1000-fold conductivity change between transport parallel and perpendicular to the film at room temperature. Temperature dependent transport measurements confirm Ohmic conduction perpendicular to the stack and small polaron hopping conduction parallel to the layering direction of the film. The conductivity anisotropy and simple synthesis demonstrate that nanostructured layered hybrids can provide alternative materials for thin film complementary logic and resistive memory.

Layered graphitic carbon host formation during liquid-free solid state growth of metal pyrophosphates

We report a successful ligand- and liquid-free solid state route to form metal pyrophosphates within a layered graphitic carbon matrix through a single step approach involving pyrolysis of previously synthesized organometallic derivatives of a cyclotriphosphazene. In this case, we show how single crystal Mn(2)P(2)O(7) can be formed on either the micro- or the nanoscale in the complete absence of solvents or solutions by an efficient combustion process using rationally designed macromolecular trimer precursors, and present evidence and a mechanism for layered graphite host formation. Using in situ Raman spectroscopy, infrared spectroscopy, X-ray diffraction, high resolution electron microscopy, thermogravimetric and differential scanning calorimetric analysis, and near-edge X-ray absorption fine structure examination, we monitor the formation process of a layered, graphitic carbon in the matrix. The identification of thermally and electrically conductive graphitic carbon host formation is important for the further development of this general ligand-free synthetic approach for inorganic nanocrystal growth in the solid state, and can be extended to form a range of transition metals pyrophosphates. For important energy storage applications, the method gives the ability to form oxide and (pyro)phosphates within a conductive, intercalation possible, graphitic carbon as host-guest composites directly on substrates for high rate Li-ion battery and emerging alternative positive electrode materials.

Deconvolution of the EPR spectra of vanadium oxide nanotubes

In this work we report results of continuous wave (CW) electron paramagnetic resonance (EPR) spectroscopy of vanadium oxide nanotubes. The observed EPR spectra are composed of a weak well-resolved spectrum of isolated V(4+) ions on top of an intense and broad structure-less line shape, attributed to spin-spin exchanged V(4+) clusters. With the purpose to deconvolute the structured weak spectrum from the composed broad line, a new approach based on the Krylov basis diagonalization method (KBDM) is introduced. It is based on the discrimination between broad and sharp components with respect to a selectable threshold and can be executed with few adjustable parameters, without the need of a priori information on the shape and structure of the lines. This makes the method advantageous with respect to other procedures and suitable for fast and routine spectral analysis, which, in conjunction with simulation techniques based on the spin Hamiltonian parameters, can provide a full characterization of the EPR spectrum. Results demonstrate and characterize the coexistence of two V(4+) species in the nanotubes and show good progress toward the goal of obtaining high fidelity deconvoluted spectra from complex signals with overlapping broader line shapes.

Pressure Induced Anisotropic Electrical Conductivity in Vanadium (V) Oxide-Based Tubular Structures

The hydrothermal treatment of vanadium pentoxide xerogel previously intercalated with long-chain primary amines leads to micro and nano-tubular products whose walls are conformed by amine bilayers sandwiched between V2O5lamellae. The microcrystalline powder product may be partially ordered at a macroscopic scale by pressure. Effects of the pressure are observed in the morphology, the intensity of the reflections in the X-ray diffractograms as well as in the electrical conductivity. The anisotropy degree estimated as the ratio of the electrical conductivities perpendicular and parallel to the direction of the applied pressure is about 102at room temperature.

INCLUSION COMPOUN DS OF α AND γ -CYCLODEXTRINS WITH n-ALKYL AMINE (n= 12, 18)

We report the synthesis of - and -ciclodextrin inclusion compounds with dodecylamine (DDA), and octadecylamine (ODA). Elemental analysis, 13C CP- MAS NMR spectroscopy, scanning electron microscopy (SEM) and powder X-ray diffraction analysis confirm the inclusion process. The basic host structure of the products is similar to that of typical cyclodextrin inclusion compounds. Depending on the guests molecular length they could present two possible conformations. The extended linear (zig-zag) conformation occurs when the space periodicity along the prism axis, calculated from spacing of these crystallographic layer lines, is comparable to the predicted length of the guest (DDA guest). When the predicted lengths of the guest (ODA guest) are longer than the periodicity matrix layer lines, a helicoidal guest conformation is predicted

CONDUCTIVITY PROPERTIES OF THIOUREA- AND UREA-HALOGEN INCLUSION COMPOUNDS WITH DIQUINUCLIDINIUM CATION AS GUEST

Thiourea and urea can modify their typical host properties to form new ternary polymolecular anionic halogen hosts in which the diquinuclidinium cation is included. A comparative study of the proton conductivity properties of this kind of inclusion compounds is presented. The hexagonal binary inclusion compound [quinuclidine] 3 thiourea 1 was taken as reference. The study shows the conductivity properties of [quinuclidine 2 H]+[thiourea 2 Cl]- 2, [quinuclidine 2 H]+[thiourea 2 Br]3, [quinuclidine 2 H] 2 2+[thiourea 2 I 2 ]2- 4, [quinuclidine 2 H]+[urea 5 Cl]- 5, [quinuclidine 2 H]+[urea 2 Br]- 6, and [quinuclidine 2 H]+[urea 2 I]- 7. Ionic conductivities of all the compounds in pellets, and of 2 and 3 in large single crystals were measured by electrochemical impedance spectroscopy (EIS). Anisotropic conductivity behaviour in crystals of adequate dimensions of 2 and 3 was detected. The conductivity values of 2 and 3 in the crystals were 2.19x10-4 and 6.03x10-6 (S/cm), respectively, in the assumed channel direction, and 2.42x10-6 and 8.27x10-9 (S/cm), respectively, in the perpendicular direction to the former, at 298 K. Conductivities at room temperature of the thiourea-halide derivatives in pellets show a changing behaviour from insulator (10-11 S/cm) for 2, 3 and 4 measured in vacuum, to semiconductor (10-7-10-8S/cm) for the thiourea-halide derivative measured at atmospheric pressure. For the urea-halide system the highest conductivity value corresponds to derivative 7 (8.66x10-5 S/cm) at atmospheric pressure, and the lowest to derivative 5 (5.48x10-7 S/cm) measured in vacuum. Comparisons considering structural aspects are also discussed.