Spectroscopic characterization of intrinsic losses in an organic-inorganic hybrid waveguide synthesized by the sol-gel process

A promising way of fabricating integrated optics components is based on the sol-gel synthesis and photocuring of hybrid materials. However, the presence of OH groups in these materials is a major factor in optical amplification inhibition. In particular, high losses at 1550 nm are mainly due to non-condensed OH groups originating from the sol-gel process at low temperature. Thus, improvement of the final properties of these materials is correlated with the inhibition of OH group concentration. In this study, we used ²⁹Si NMR and near infrared spectroscopy to demonstrate the catalytic effect of zirconium (IV) n-propoxide on the condensation reactions of silanol groups. ²⁹Si NMR showed the absence of silanol species at the end of the synthesis. This result is attributed to the zirconate hydrophilic effect which consumes OH groups by catalysing the polycondensation of Si-OH bonds. In parallel, near-infrared experiments showed the presence of a high proportion of OH species at the end of the synthesis showing that the remaining OH groups are only present in the zirconium species.     

Structure of organic-inorganic hybrid low-melting glasses from Si NMR and ab initio molecular orbital calculations

The structure of organic-inorganic hybrid glass precursor Me₂Si(OPO(OH)₂)₂ and low-melting glasses SnO-Me₂SiO-P₂O₅ has been examined by ²⁹Si static and MAS NMR, respectively. The ²⁹Si MAS NMR spectra of SnO-Me₂SiO-P₂O₅ were deconvoluted into three Gaussian peaks, whose chemical shifts were located at −3.1, −10.2 and −18.1 ppm. Ab initio molecular orbital calculations have also been carried out for the clusters modeling the structure of precursor and glasses. From the calculations of ²⁹Si NMR shielding constants, the experimental chemical shifts at −10.2, −3.1 and −18.1 ppm were assigned to ²⁹Si atoms in P-O-Si(Me)₂-O-P network linkage, in P-O-Si(Me)₂-OH terminating structure and in P-O-Si(Me)₂-O-Si network linkage, respectively. Based on the area ratio of the assigned three peaks, the network structures around Si constructing SnO-Me₂SiO-P₂O₅ low-melting glasses were discussed in detail.     

Effect of Y2O3 addition on the phase transition and growth of YSZ nanocrystallites prepared by a sol-gel process

The effect of Y₂O₃ addition on the phase transition and growth of yttria-stabilized zirconia (YSZ) nanocrystallites prepared by a sol-gel process with various mixtures of ZrOCl₂ · 8H₂O and Y(NO₃)₃ · 6H₂O ethanol-water solutions at low temperatures has been studied. DTA/TGA, XRD, SEM, TEM and ED have been utilized to characterize the YSZ nanocrystallites. The crystallization temperature of 3YSZ, in which Y₂O₃/(Y₂O₃ + ZrO₂) = 0.03, gel powders estimated by DTA/TG is about 427 °C. When 3YSZ and 5YSZ gels are calcined at 500-700 °C, their crystal structures as composed of coexisting tetragonal and monoclinic ZrO₂, and tetragonal phase decreases with calcination temperature increasing from 500 to 700 °C. Pure cubic ZrO₂ is obtained when added Y₂O₃ is greater than 8 mol%. A nanocrystallite size distribution between 10 and 20 nm is obtained in TEM observations.     

Preparation and optical properties of titania-doped hybrid polymer via anhydrous sol-gel process

Incorporation of metal alkoxides (Ti, Zr, etc.) for tuning the optical properties of silica glasses by the sol-gel process is of significant interest for optical applications. In this paper, we report an anhydrous sol-gel process for preparation of photosensitive titania-doped hybrid glassy polymer with good homogeneity and high doping concentration (TiO₂ up to 40 mol%). The process consists of two steps: in the first step methacryloxypropyltrimethoxysilane (MPS) is hydrolyzed by boric acid through ligand exchange reaction (OH↔OR) under anhydrous conditions; and in the second step dimethyldimethoxysilane (DMDMS), diphenyldimethoxysilane (DPhDMS) and titanium ethoxide (TET) were added to condense with the silanols formed in the first step. The optical properties of the synthesized hybrid polymer were studied, and results showed that the hybrid material has low OH absorption, low optical losses (0.45 dB/cm at 1550 nm and 0.16 dB/cm at 1310 nm respectively), and good thermo-optical linearity with tuneable refractive index. The effect of TiO₂ doping in reducing the OH concentration of the hybrid material was observed, and the mechanism for this effect is discussed.     

A comparative FTIR study of thermal and photo-polymerization processes in hybrid sol-gel films

Controlling the organic polymerization in organic-inorganic hybrids is a key point in the development of new materials with high homogeneity of the nanostructure. The main difficulty is related with the achievement of a simultaneous control of the organic and inorganic network formation. Thermal and photocuring represent the main routes to form the organic chains when polymerizable organic groups are present in the hybrid materials. In the present work hybrid organic-inorganic films were synthesized from 3-methacryloxypropyltrimethoxysilane (MPTMS) cohydrolyzed with tetraethylorthosilicate (TEOS) and N-[(3-trimethoxysilyl)propyl]ethylenediamine (TMESPE) or 3-(triethoxysilyl)-propylamine (TESPA). This an example of basic catalyzed hybrid material with a polymerizable methacrylate functionality whose micro-structure is modified by the amine groups. FTIR spectroscopy was used to compare the effects of thermal or photo-induced polymerization on the materials. TESPA and TMESPE showed a different catalytic effect on the condensation of the inorganic network, with TMESPE the more efficient one. The presence of a more extended silica backbone reduced the curing efficiency in TMESPE derived samples. UV curing was also very effective in catalyzing the inorganic condensation of un-reacted species still present in the film after the deposition. A photo-induced polymerization of the inorganic side was observed in the hybrid films. Thermal polymerization in TMESPE films induces the reaction between the secondary amine and CO bonds in MPTMS, this reaction is, instead, not observed in films cured by UV radiation.     

Electrical characterization of gate oxides by scanning probe microscopies

Ballistic electron emission microscopy (BEEM) and non-contact atomic force microscopy (NC-AFM) are used to characterize SiO₂ and Al₂O₃ layers grown on Si(1 0 0). The effective conduction band mass and its energy dispersion in SiO₂ and an offset between Al₂O₃ and Si conduction bands of 2.78 eV were obtained with BEEM. NC-AFM was used to image electrons, and in some instances holes, trapped in the oxide layers near the surface and in the bulk of the oxide. Modeling of the tip-surface interaction supports the interpretation of image features arising from a single electron occupying a trap. The polarity of the trapped charge was deduced from Kelvin (potential difference) images that were simultaneously recorded with the topographic images.     

Silicon-29 cross-polarization/magic-angle-spinning NMR study of inorganic-organic hybrids: homogeneity of sol-gel derived hybrid gels

The structures of sol-gel derived hybrid gels prepared by co-hydrolysis of tetraethoxysilane (TEOS)-organotrialkoxysilane (RTES, RSi(OC₂H₅)₃, R = CH₃, C₅H₁₁, C₈H₁₇ and C₆H₅) mixtures (TEOS : RTES : CH₃CH₂OH : H₂O : HNO₃ = 0.5 : 0.5 : 10 : x : 0.3; x = 2, 5, 10 and 20) were characterized based on signal intensities of ²⁹Si CP/MAS NMR as a function of the contact time. The contact time dependencies of the signals for Q⁴ (Si(OSi)₄) units were successfully simulated by assuming distributed cross relaxation times (T_SiH) and an intrinsic ¹H spin-lattice relaxation time in the rotating frame (); the latter was different from the apparent value estimated by assuming single T_SiH. The distribution ranges of T_SiH for the TEOS-RTES gels broadened as the water content increased, suggesting that the Q⁴ units tended to be separated from the T units and that the local concentration of ¹H spins around the Q⁴ unit tended to decrease.     

Effects of organic content and H2O/TEOS molar ratio on the porosity and pore size distribution of hybrid naphthaleneaminepropylsilica xerogel

The hybrid naphthaleneaminepropylsilica material was obtained by a sol-gel route, varying the organic loading and the water/TEOS molar ratio. Infrared spectroscopy was used to identify the organic and inorganic phases. The morphology of the hybrid material was studied by using scanning electron microscopy and N₂ adsorption-desorption isotherms. It was observed that the increase in the organic content produces a decrease in the size and volume of the pores as well as in the surface area of the xerogel. The best porosity was obtained for water/TEOS molar ratio between 4 and 6.     

Calibration of cyclohexane solid-solid phase transition thermoporosimetry and application to the study of crosslinking of elastomers upon aging

Photo-aging of elastomers results in two competitive processes: chain- scissions and crosslinking. The solid-to-solid cyclohexane phase transition was used to investigate the crosslinking extent in photo-oxidised ethylene propylene diene monomer (EPDM) samples by thermoporosimetry technique. Before apply this technique to EPDM, texturally well-known silica gel samples were used to calibrate the method. By contrast to the case of liquid-to-solid heptane transition, no linear relationship was obtained between the solid-to-solid point depression (ΔT) and the pore radius (R_p). The following correlation was obtained:

     

Silica gel with continuous macropores prepared from water glass in the presence of poly(acrylic acid)

Monolithic silica gel with macropores was prepared in the solution of water glass and polyacrylic acid (HPAA) by freezing transitional structures of phase separation. In the system, phase separation proceeds between silica polymers and HPAA, so that the porous morphology varies from closed macropores to particle aggregates through bicontinuous morphology, where both macropores and silica skeleton are three-dimensionally interconnected, with increasing HPAA/silica ratio. In addition, we can control the macropore size in bicontinuous morphology by varying the concentration of both silica and HPAA, or by changing the molecular weight of HPAA. The pore size distribution is quite sharp indicating the presence of pores with the same size all through the monolithic samples.     

High resolution patterning of silica aerogels

Three-dimensional metallic structures are fabricated with high spatial resolution in silica aerogels. In our method, silica hydrogels are prepared with a standard base-catalyzed route, and exchanged with an aqueous solution typically containing Ag⁺ ions (1 M) and 2-propanol (0.2 M). The metal ions are reduced photolytically with a table-top ultraviolet lamp, or radiolytically, with a focused X-ray beam. We fabricated dots and lines as small as 30 × 70 μm, protruding for several mm into the bulk of the materials. The hydrogels are eventually supercritically dried to yield aerogels, without any measurable change in the shape and spatial resolution of the lithographed structures. Transmission electron microscopy shows that illuminated regions are composed by Ag clusters with a size of several μm, separated by thin layers of silica.     

Nanosized gold clusters formation in selected areas of soda-lime silicate glass

Nanosized gold clusters were developed in a gold-doped soda-lime silicate glass after X-ray irradiation followed by annealing, which provided an alternative way of forming metallic nanoclusters in glass to ion implantation. The formation of gold nanoclusters can be controlled by irradiation time or annealing time.     

Crystallization kinetics and optical band gap studies of Se96In4 glass before and after slow neutron irradiation

Results of differential scanning calorimetry (DSC) under non-isothermal condition on Se₉₆In₄ semiconducting chalcogenide glass before and after slow neutron irradiation, for different exposure times, have been reported and discussed. Some of Sn atoms have been injected into the glass by nuclear transmutation processes and the binary glass is converted into a ternary. This is accompanied by an increase in the activation energy of crystallization, E_c, and in the glass transition temperature, T_g and a decrease in the glass transition activation energy, E_t, in the onset crystallization temperature, T_c and in the peak temperature of crystallization T_p. Optical band gap measurements have also been carried out, before and after irradiation, on identical thin pellets of Se₉₆In₄ glass. The energy band gap, E_g, is found to decrease upon irradiation. These effects have been attributed to a structural change upon doping and to irradiation induced defects.     

A characterization study of xerogel silicapropylaniline powders

Silicapropylaniline nanometric materials with varying organic content were obtained using a sol-gel synthesis. By increasing the organic load, the scanning electron microscopy technique shows a slight increase in the average size of aggregated particles. N₂ isotherms and positron annihilation lifetime spectroscopy measurements show that the average pore size decreases accompanied by a surface area reduction. FTIR thermal analysis was used to estimate the thermal stability of the organic phase and also to detect the presence of trapped organic groups in closed pores. From the organic coverage and surface area measurements the surface density of the immobilized organic molecules as well as the average intermolecular distance between them could be estimated.     

The structure of zinc polyphosphate glass studied by diffraction methods and P NMR

Zinc polyphosphate glasses with molar ratios, y=n(ZnO)/n(P₂O₅), ranging from 1.0 to 2.5 have been examined by using X-ray and neutron diffraction and ³¹P magic angle spinning nuclear magnetic resonance spectroscopy to extract information on their short-range, intermediate-range and submicroscopic structure. The depolymerization of the PO₄ chains with decreasing P₂O₅ mol% content is quantitatively described by the Qⁿ distributions determined by NMR. The shortening of the real-space distances, r_m, extracted from the diffraction experiments, indicates a structural densification with progressive depolymerization. This effect correlates with the continuous increase in packing density of the ions, ρ_p. The Zn-O coordination number, N_ZnO, was found to be four across the entire polyphosphate concentration range. The presence of two differently-sized heterogeneity regions of electron density is indicated by the measured small angle X-ray scattering.     

The structural transformation and properties of spin-on poly(silsesquioxane) films by thermal curing

In this study, the structural transformation and properties of five commercially available poly(silsesquioxanes) by thermal curing were investigated, including poly(hydrogen silsesquioxanes) (HSQ and T12), and poly(methylsilsesquioxanes) (MSQ, T7 and T9). These materials with a different cage/network ratio and side groups (Si-H and Si-CH₃). The FTIR spectra show that the poly(silsesquioxane) films have different contents of the Si-O-Si cage and network structures, which significantly affects the refractive index and dielectric constant. The shifting of the Si-O-Si network band in the FTIR spectra can be correlated with their molecular structures. The refractive indices and dielectric constants of the studied poly(silsesquioxane) films increase with increasing the Si-O-Si network content. The retention of the Si-H or Si-CH₃ side group suggests the existence of the cage structures in the poly(silsesquioxane) films. The Si-O-Si cage structure results in a larger free volume than the Si-O-Si network structure in the poly(silsesquioxane) films and thus reduces the refractive index and dielectric constant. It is supported by the porosity result. The order of the refractive index in the studied poly(silsesquioxanes) films is T12>HSQ for the Si-H side group and T7>T9>MSQ with the Si-CH₃ side group, which can be correlated with the Si-O-Si network content. The poly(silsesquioxane) film with the Si-CH₃ side group has a lower refractive index than the Si-H side group at the same Si-O-Si network content, which is probably due to the steric hindrance effect of the CH₃ group.     

Application of mercury porosimetry to the study of xerogels used as stone consolidants

Alkoxysilanes, low-viscosity monomers that polymerize into the porous network of stone by a sol-gel process, are widely used in the restoration of stone buildings. We have used the mercury porosimetry technique to characterize changes in microstructure of three granites following their consolidation with two popular commercial products (Wacker OH and Tegovakon V). The suitability of this technique is questioned because a surprising increase of stone porosity is observed. In order to investigate the feasibility of porosimetry, we analyze the behavior of xerogels prepared from the two commercial products, under mercury pressure. Gels are basically compacted and not intruded by mercury. Thus, the increase of stone porosity after consolidation can actually be associated with gel shrinkage. Mercury porosimetry, therefore, has been found unsuitable for characterizing the microstructure of consolidated rocks. However, it can be employed usefully to evaluate shrinkage of gels under mercury pressure, which permits the behavior of a consolidant during the process of drying in stone to be predicted. It is a key factor because many problems of consolidants are related to their drying process within the stone. Gels under study exhibit a high rigidity and an elastic behavior, as consequence of their microporous structure. Finally, the reduction in the porous volume of gels after the porosimetry test demonstrates that the shrinkage mechanism is based on pore collapse.     

Characterization of the elasticity and anelasticity of bulk glasses by dynamical subresonant and resonant techniques

The determination of the elastic and anelastic characteristics by means of original non-destructive techniques has been applied to glasses and glass composites in order to link together the macroscopic data with structural aspects. The dynamical Young’s modulus determined by a free resonance technique allows a good accuracy measurement. Some examples concerning oxides, Ge(As)Se or metallic glasses are presented: the abrupt drop of the modulus in the range of the glass-transition temperature T_g is a general observation, which leads us to an attempt at normalization of the curves E versus T on master curves E/E(T_g) versus T/T_g. To study the viscoelastic properties, a low frequency torsional spectrometer is preferentially used to measure the damping due to viscous movements at a microscopic scale. A study of MgSiAlON glasses allows us to show that the intrinsic activation energy is much smaller than the one measured by creep or relaxation tests and that the glassy transition is characterized by a smooth change from vitreous solid (highly correlated) to quasi-liquid behavior; this has been confirmed on a metallic glass.     

Particle size control of PbTe quantum dots incorporated in the germano-silicate glass optical fiber by heat treatment

The effect of heat treatment of the germano-silicate glass optical fiber incorporated with PbTe quantum dots on the absorption characteristics was investigated. Incorporation of the PbTe QDs in the fiber core was confirmed by the absorption peaks that appeared at 687 nm and 1055 nm and their shift upon heat treatment. The absorption peak was found to shift linearly toward a longer wavelength after heat treatment at 700 °C, 1000 °C, and 1100 °C for 1 h. The red shift of the excitonic absorption peak was attributed to the increase in the average size of the PbTe QDs in the fiber core.