Metalorganic chemical vapor deposition of mesoporous SiTiOC nanostructured thin films

SiTiOC mesoporous thin films have been obtained by metalorganic chemical vapor deposition (MOCVD) using titanium iso-propoxide (TIP) and tetraethylorthosilicate (TEOS) as starting precursors. The influences of both carrier gas and deposition temperature on the properties of the produced films were extensively studied. The low-angle XRD analysis confirms that, all produced films under different conditions (gas type and temperature) have the mesoporous structure. However, the deposition temperature was found to be much effective in controlling both morphology and composition of the final films than the type of carrier gas. The morphology of the produced films was totally converted from spherical shape-like nanoparticles at 700 °C to lengthy at higher temperature of 1000 °C. The SEM-EDX investigations proved that the composition of the produced films was composed of SiTiOC structure system. The PL analysis has demonstrated along with FT-IR data that all the deposited films at various deposition parameters were composed mainly of SiO₂, SiOC, SiC, TiO₂ and TiOC bond structures and most probably nanocomposite SiTiOC system thin films.     

First principles study of oxygen-deficient centers in pure and Ge-doped silica

Using ab initio calculations on 108 atoms pure- and Ge-doped (2.8 mol%) silica-based supercells, we performed a statistical study on the electronic structure and energetic contribution of neutral oxygen vacancies, also named Oxygen Deficient Centers (ODCs). All the 72 oxygen sites in the amorphous silica (a-SiO₂) cell were considered as possible candidates for the formation of the vacancies leading to study 72 different Si-ODCs (SiSi bond) and 144 Ge-ODCs (GeSi bond). The distributions of structural parameters and formation energies of the ODCs were evaluated through Density Functional Theory calculations. The obtained parameters showed a wide distribution that can be mainly associated with the differences in the local environments surrounding the point defects. We show that the formation energies of Si and Ge-ODCs generated from the same oxygen site of our supercell are correlated. Moreover, the local asymmetry around the SiGe or GeSi bond can also affect their formation energies, providing a strong evidence for the influence of short-range environment on the ODC generation efficiency.     

Infrared attenuated total reflection spectroscopy studies of aprotic condensation of (EtO)3SiRSi(OEt)3 and RSi(OEt)3 systems with carboxylic acids

Aprotic hydrolysis and condensation reactions of bis end-capped trialkoxysilanes ((EtO)₃SiRSi(OEt)₃), linked via the organic chain R containing urea groups chemically bonded to a poly(propyleneglycol) (PPG) chain, in the presence of carboxylic acids, i.e. acetic-, chlorodifluoroacetic- and trifluoroacetic acids, were studied using infrared attenuated total reflection (IR ATR) spectroscopy. IR and ²⁹Si NMR spectral analysis revealed solvolysis reactions: the carboxylic acids interacted with ethoxysilane groups forming silyl esters leading to the formation of bridging SiOSi groups and carboxylic acid ester by-products. These results were compared with those obtained on simpler single capped methyltriethoxysilane (MeSi(OEt)₃, MTEOS) condensed with trifluoroacetic acid. Gelation of (EtO)₃SiRSi(OEt)₃ (catalyzed with acetic acid) encapsulated between a transparent conductive oxide (TCO) glass and a conductive and IR transparent silicon wafer was followed with the help of IR reflection–absorption spectroscopy. The results revealed that aprotic solvolysis of the hybrid precursor with acetic acid led to the formation of non-aqueous gels with low silanol content, confirming the advantages of aprotic solvolysis of organic–inorganic hybrids used as redox electrolytes in hybrid electrochromic (EC) and dye-sensitized photoelectrochemical (DSPEC) cells. Some comments regarding the accuracy of IR ATR spectral measurements compared to IR transmission spectra are also given.     

Density studies of liquid alloys SnAg and SnZn with near eutectic compositions

The density values for SnAg and SnZn alloys with near eutectic compositions were determined by the gamma-absorption method both in the liquid and solid states. Sn-rich melts of the AgSn system were found to be microheterogeneous, whereas SnZn melts demonstrate the behavior typical for true solutions.     

Transparent K2OTiO2P2O5SiO2 monolithic gels and inorganic amorphous solids through incomplete hydrolysis

Sol–gel derived transparent glasses are of technological interest because of its precisely controlled composition for multicomponent glasses at low temperature processing. The present work demonstrates a new and simple methodology for preparing transparent multicomponent oxide gels by incomplete hydrolysis of alkoxides. Through this processing, a small quantity of organic agencies resulted from incomplete hydrolysis of alkoxides self-disperses in inorganic oxide network, and thus control the formation of the monolith gel free of cracks. Specially, K₂OTiO₂P₂O₅SiO₂ gel monoliths have been synthesized through this route. The gels transformed into transparent K₂OTiO₂P₂O₅SiO₂ inorganic amorphous solids after heat treatment above 450 °C. This approach could be applied to many other multicomponent oxides.     

Structural characterization of lanthanum aluminosilicate glasses by 29Si solid-state NMR

We investigate the network structures of LaSiAlO glasses by ²⁹Si magic-angle-spinning (MAS) nuclear magnetic resonance (NMR). Their compositions span most of the glass-forming region of the ternary La₂O₃Al₂O₃SiO₂ system at 1600 °C. The ²⁹Si NMR resonances narrow and become progressively deshielded when Al substitutes for Si in the network, as well as for increasing La-content of the glass, which leads to network depolymerization. We compare experimental and calculated center of gravities of the ²⁹Si NMR peaks, the latter generated from different simplified models for the distributions of Al and Si as well as bridging oxygen (BO) and non-bridging oxygen (NBO) atoms over the networks. The data do not permit accurate quantifications and may only be interpreted in limiting scenarios. However, they indicate that both distributions are essentially randomized, implying a clear deviation of the Al/Si ordering from that according to a Loewenstein Al-avoidance, coupled with a nearly uniform partitioning of the NBO atoms between Al and Si tetrahedra.     

Correlation between the 2.7 eV and 4.3 eV photoluminescence bands in silica glass

Our previous studies have reported the excitation energy dependence of the 2.7 and 4.3 eV photoluminescence (PL) bands in oxygen deficient silica glass at low temperature (∼20 K). An oxygen vacancy (O₃SiSiO₃) was thought to be the origin of the two PL bands. In order to verify the origin of the 2.7 and 4.3 eV PL bands in silica glass, we measured the PL band of various thermally heat treated silica glasses. In the sample after heat treatment, we did not observe the 4.3 eV PL band, though we did observe the 2.7 eV PL band. These results suggest that these two PL bands do not have a common origin.     

Relationship between the short-range order and electrical resistivity in liquid indium-antimony

The composition dependencies of local structure and electronic structure, as well as the electric resistivity of liquid indium-antimony alloys have been investigated by the first-principles molecular dynamics simulations. It is shown that the variations of InIn, InSb and SbSb coordination tendency and the projected density of states of In and Sb in liquid In_xSb_1 ? x depend on the Sb concentration, and electric resistivity of liquid indium-antimony also reveals a regular change trends as a function of Sb concentration. Further analysis confirmed that there are explicit relationships between the short range structural parameters and electrical resistivities in liquid In_xSb_1 ? x.     

Interplay of ballistic and chemical effects in the formation of structural defects for Sn and Pb implanted silica

The electronic structures of Sn and Pb implanted SiO₂ are studied using soft X-ray absorption (XAS) and emission (XES) spectroscopy. We show, using reference compounds and ab initio calculations, that the presence of PbO and SnO interactions can be detected in the pre-edge region of the oxygen K-edge XAS. Via analysis of this interaction-sensitive pre-edge region, we find that Pb implantation results primarily in the clustering of Pb atoms. Conversely, with Sn implantation using identical conditions, strong SnO interactions are present, showing that Sn is coordinated with oxygen. The varying results between the two ion types are explained using both ballistic considerations and density functional theory calculations. We find that the substitution of Pb into Si sites in SiO₂ requires much more energy than substituting Sn in these same sites, primarily due to the larger size of the Pb ions. From these calculated formation energies it is evident that Pb requires far higher temperatures than Sn to be soluble in SiO₂. These results help explain the complex processes which take place upon implantation and determine the final products.     

Cobalt porphyrin covalently bonded to organo modified silica xerogels

Some of the fine physicochemical properties displayed by porphyrin solutions can be preserved when these species are trapped within inorganic oxide pore networks, such as silica. A successful outcome is related to inhibiting the interaction between the macrocycle and SiOH surface groups. Here, when porphyrins are chemically bonded to the walls of a silica network through molecular bridges arising from functionalized alkoxides alone or combined with monomer precursors (i.e. lactams, diamines, etc.), their spectroscopic properties can be kept similar to those shown in solution. This latest outcome consists in bonding porphyrinic species to the pore walls of an organo-modified silica networks. In the present work, a strategy has been designed to uphold these useful properties by covalently bonding cobalt porphyrin molecules inside a SiO₂ pore network where SiOH surface groups are exchanged by alkyl groups proceeding from organo-substituted alkoxides. In these hybrid systems, the electronic transitions are well preserved when SiOH surface groups are exchanged by groups such as methyl, ethyl, vinyl, etc. This separation action prevents flocculation among macrocyclic molecules (even if a change in polarity occurs inside the pores) due to the adsorption of bridging alkyl groups. A proper selection of both macrocyclic molecules and bridging molecules, in conjunction with a proper choice of the synthesis conditions, lead to the attainment of hybrid solid systems in which the spectroscopic properties are not only preserved but can even be adjusted by selecting suitable sizes and identities of the bridging alkyl groups.     

IR and ESR investigations on V2O5–P2O5–BaO glass system with opto-electronic potential

The changes observed in the IR and ESR spectra of the xV₂O₅(1 ? x)[0.8P₂O₅ ? 0.2BaO] glass system with 0 ≤ x ≤ 50 mol% show that vanadium oxide acts as a network modifier at low concentration (x ≤ 5 mol%) and as a network former for high content (x ≥ 10 mol%). Thus the IR bands belonging to the phosphate groups are strongly reduced except the specific bands of the short chain phosphate units due to the phosphate network depolymerization and the spectra are dominated by the vibrations characteristic for POP, POV and VOV linkages. At the same time the changes observed in the ESR spectra of these glasses are explained supposing the superposition of two signals, one with a well-resolved hyperfine structure typical for isolated V⁴⁺ ions and a broad line characteristic for clustered ions. The line width dependence versus V₂O₅ content shows that dipole–dipole interactions exist between vanadium ions until x = 5 mol% and the superexchange interactions prevail at high content (x > 10 mol%).     

Preparation,crystallization behavior and magnetic properties of nanoparticles magnetic glass–ceramics in the systems Fe2O3 CoO MnO2, Fe2O3 NiO MoO3 and Fe2O3 CoO V2O5

Ferrimagnetic glass–ceramics were prepared in the systems Fe₂O₃ CoO MnO₂ (S1), Fe₂O₃ NiO MoO₃ (S2) and Fe₂O₃ CoO V₂O₅ (S3). Small amount of H₃BO₄ was added to make the melting process easier. The samples were characterized using DTA, XRD, TEM and EDX. Sequence of crystallization was studied by applying heat-treatment at 800 and 1000 °C for 4 h. CoFe₂O₄ with crystallite sizes of ≈ 14–20 nm was successfully prepared beside FeCoOBO₃ and Co₃BO₅ in S1. NiMoO₄, (FeNi₂)O₂(BO₃) and NiO with crystallite size ≈ 56–79 nm were crystallized in S2. CoFe₂O₄, FeCoOBO₃ and Co₃BO₅ with crystallite size ≈ 6–8 nm were crystallized in S3. Magnetic hysteresis cycles were analyzed with a maximum applied field of 20 kOe at room temperature. From the obtained hysteresis loops Ms records higher values for S1 and S3 and lower value for S2, while coercivity reach maximum for S2. The variable, magnetic, data range gives a wide range for different applications.     

On the structure of tungstate–tellurite glasses

A continuous network model of xWO₃–(1 − x)TeO₂ glasses is developed, based on quantum-chemical calculation and Raman spectra analysis, in order to relate the structural and vibrational properties with glass composition. The tungstate–tellurite glass network is shown to be formed mainly by structural units of three types, TeO₄ trigonal bipyramids, OTeO₂ trigonal pyramids, and WO₆ octahedra with OW double bonds. Most of the W atoms are found to be incorporated into single OWO₅ octahedra, with no more than several percents of these atoms occurring in 2[OWO₅] paired tungstate centers. The structural and vibrational properties of tungstate–tellurite glasses of several compositions are analyzed by application of the model and a novel interpretation of the Raman spectra is suggested.     

Optical characterization of 316L stainless steel coated with sol–gel titania

In this work 316L stainless steel substrates were coated with sol–gel derived films by means of the dip-coating technique. Titanium isopropoxide and ethanol were used as chemical precursor and solvent, respectively. The dip-coating step was performed using withdrawal speeds of 6 mm/min, 30 mm/min, and 60 mm/min. Next, the samples were heat treated in air for 30 min at 100 °C, 300 °C, and 400 °C. The processed composites were examined by FTIR and UV–vis spectroscopies. We observed that the materials prepared in this work can exhibit a variety of colors depending on the heat treatment temperature, withdrawal speed, and precursor:solvent molar ratio used in their processing. It is an important finding since this behavior could lead to architectural application of these materials. We believe that the changes observed in the UV–vis spectra and the colors of these samples could be related to the variation of the coating thickness as the processing conditions were modified. FTIR tests revealed that the ratio between the intensities of features ascribed to hydroxyl groups and TiO bonds decreased as the heat treatment temperature was increased. On the other hand, the ratio between bands related to TiOTi and TiO bonds decreased when the heating temperature was raised from 100 °C to 300 °C.     

Radiation induced generation of non-bridging oxygen hole center in silica: Intrinsic and extrinsic processes

The generation of non-bridging oxygen hole center (Si–O) was investigated in a wide variety of natural (fused quartz) and synthetic silica samples exposed to different γ- and β-irradiation doses by looking at its optical bands. We distinguish two different generation processes: intrinsic associated with the cleavage of Si–O bond and characterized by a sublinear law and extrinsic due to the conversion of OH precursor characterized by a growth curve with a saturating tendency. The interplay between the two processes and the role of H are discussed.     

Insights into new calcium phosphosilicate xerogels using an advanced characterization methodology

Amorphous calcium phosphosilicate xerogels of high phosphate content were synthesized by a new sol–gel route. Their structural characterization was achieved through the combination of complementary analytical methods, including advanced solid state NMR and scattering techniques. Two representative compositions, with similar P contents but a different Ca:Si ratio, have been chosen for detailed study. Using ⁴³Ca solid state NMR and Ca K-edge XANES, the Ca local environment in the samples was characterized, revealing that it is similar for both compositions. It was found that POSi linkages are present in both compounds after calcination at 350 °C. However, for the sample with the lower Si content, a higher fraction of unusual 6-coordinated silicon was observed. Interestingly, calcium was also found to enhance the network connectivity and to enter the phosphosilicate network without the need for calcination at high temperature, which is advantageous in comparison with syntheses performed previously on similar compounds.     

Effect of thermal annealing and hydrogen-plasma treatment in boron-doped microcrystalline silicon

We have investigated the effects of temperature (during film growth and post-deposition thermal annealing) and H₂-plasma treatment on the electronic and structural properties of p-type microcrystalline silicon films (p-μc-Si:H) for solar cell applications. The highest dark conductivity is obtained in the thermally annealed p-μc-Si:H prepared at low substrate temperature of 50 °C. This dark conductivity is decreased by two orders of magnitude when the film is exposed to H₂-plasma, being completely restored after thermal annealing. Namely, reversible dual-conductivity cycle is observed between thermally annealed state and H₂-plasma-treated state in p-μc-Si:H. The dual-conductivity cycle is accompanied with the reversible change in the infrared-absorption spectrum at around 1845 cm^? 1 assigned as SiHB complex in p-μc-Si:H network structure. Taking into account of the reversible structural change by H₂-plasma-exposure and thermal-annealing cycles, necessary process-procedure condition has been proposed for obtaining high photovoltaic performance in thin-film-Si solar cells with high quality p-μc-Si:H.     

Electron paramagnetic resonance and thermal behavior of vanadyl-doped mixed alkali bismuth borate glasses

Glasses with composition 15 Li₂O15K₂O10Bi₂O₃(60 ? x) B₂O₃: xV₂O₅ where x = 1, 3, 5, 7, and 9 mol% are prepared by normal melt-quench technique. Characterization of the prepared samples is done using X-ray diffraction, density and differential scanning calorimetry. The density and molar volume are found to increase with increasing x whereas the glass transition temperature decreases. Electron paramagnetic resonance spectra of the prepared samples are recorded using EPR-spectrometer operating in the X-band frequency. The resonance spectra are well resolved for x ≥ 5 mol% and the intensity of the resonance peak is found to increase with increasing x. The values of spin-Hamiltonian parameters (SHP) and molecular orbital coefficients are evaluated. From the values of SHPs it is concluded that V⁴⁺ ions in the present glass system exist as vanadyl ions in octahedral co-ordination with tetragonal compression. It is observed that the SPHs depend slightly on the relative concentration of V₂O₅. Further, the theoretical optical basicity of the glasses has also been evaluated and it is observed that the changes in optical basicity values are in accordance with the changes in SHPs.     

The effects of ZnCl2 and ErCl3 on the vibrational spectra and structure of tellurite glasses

A series of zinc tellurite glasses, containing up to 40 mol% ZnCl₂ and doped with 1–10 mol% ErCl₃, was prepared by melting and casting and their structure was analyzed by polarized Raman and variable incidence infrared reflection spectroscopies. Kramers–Kronig analysis of the infrared reflectivity led to the identification of the vibrational mode components. The Raman spectra are dominated by an intense, depolarized boson peak at ∼45 cm⁻¹ and a high frequency, polarized peak at ∼767 cm⁻¹. The introduction of ZnCl₂ and ErCl₃ modifiers led to a blue shift of the high frequency peak, while the intensity of the boson peak was found to increase continuously with the Er³⁺ content. It is shown that the erbium and zinc compounds both break TeOTe bonds, introducing non-bridging chlorine species, connected mostly to the zinc atoms.     

Synthesis and characterization of the conductivity and polarization processes in supported ionic liquid-like phases (SILLPs)

Different crosslinked polystyrene resins containing ionic liquid fragments covalently attached (supported ionic liquid-like phases: SILLPs) have been prepared and their conductivity has been studied by means of dielectric impedance spectroscopy. The main structural parameters considered in this study have been the presence of C2H bonds in the imidazolium subunits and the nature (level of hydrophobicity) of the N-substituents (methyl vs. butyl). The results obtained show that the presence of C2H fragments can have an influence on intrinsic conductivity. In the same way the role of the hydrophobicity of the N-substituents seems to play only a minor role. The main structural component associated with conductivity is the nature of the counteranion. The bulkiest, less coordinating anions, in particular the NTf₂^? anion, seem to be associated with the highest conductivities, with the values obtained being two orders of magnitude higher than those calculated for the same systems containing the Cl^? anions for each of the temperatures under study. The results obtained rule out the participation of some given conduction mechanisms such as the involvement of carbene formation. Instead, the formation of ion-pairs of higher or lower association (strong/weak ion pairs) could be responsible for the observed behavior.