Xerogel Films Research Articles

Contributions to the static dielectric constant of low- k xerogel films derived from ellipsometry and IR spectroscopy

Silica xerogel films with low dielectric constant were prepared by means of a sol–gel spin-coating method using different aging and hydrophobisation conditions. Non-destructive variable angle spectroscopic ellipsometry (VASE) studies allow a complete characterization of the xerogel films, in terms of thickness, optical constants and void fraction. The electronic and ionic contributions to the static dielectric constant of the xerogel films were calculated from the refractive index in the visible range and from infrared transmission spectra, respectively. The origin of the differences between the contributions to the static dielectric constant of the xerogel films produced with different preparation conditions is discussed.

Luminescence from Sol-Gel-Derived Europium-Doped Films Confined in Mesoporous Anodic Alumina

Photoluminescence (PL) properties of the structure porous anodic alumina (PAA)/europium-doped xerogel film have been investigated with respect to the xerogel content its transmittance and PL excitation wavelength. Three mechanisms of lanthanide optical excitation in the structure are proposed: (i) excitation through the PAA matrix; excitation through the xerogel matrix that depends on its transparency and, in turn, on xerogel content and thickness within the porous morphology; and direct excitation of a lanthanide ion due to the relative transparency of both the PAA and xerogel films. It is speculated that exploiting PAA allows significant change in the excitation conditions of lanthanides, probably due to multiple scattering of the exciting light, which is the main reason for the reported PL enhancement of lanthanides from the structure compared with similar PAA/lanthanide-doped xerogel films fabricated on flat substrates. © 2003 The Electrochemical Society. All rights reserved.

Influences of Treatment Temperature and Water Content on Capacity and Rechargeability of V 2 O 5 Xerogel Films

xerogel films prepared with spray deposition have been investigated for their lithium battery applications. The influences of spray deposition and postannealing temperatures on the water content, structure, and electrochemical performance of the films have been studied. Water contents of the films depend on the deposition and postannealing temperatures. One film sprayed at 150°C and annealed at 190°C exhibited a large discharge capacity and the longest cycling life. By contrast, the films sprayed or annealed at high temperatures (>250°C) showed poor electrochemical performance. The film deposited at a substrate temperature of 420°C demonstrated structural features and electrochemical behavior of orthorhombic X-ray diffraction patterns of the films treated at different temperatures reveal that removal of too much water at high temperatures results in crystallization or a small interlayer spacing, which may restrict the diffusion of lithium ions inside films. solid-state nuclear magnetic resonance spectra show that the films annealed at low temperatures (i.e., 120°C or room temperature) have broad peaks and significant downfield isotropic chemical shifts, suggesting that the interlayered water produces a distorted square-pyramidal ligand field that may favor lithium intercalation and electron transfer within the matrix of the films. © 2003 The Electrochemical Society. All rights reserved.

Photoluminescence investigation of europium-doped alumina, titania and indium sol–gel-derived films in porous anodic alumina

Photoluminescence (PL) properties of Eu-doped xerogel films fabricated on porous anodic alumina (PAA) and flat substrates are analysed with respect to the xerogel content (TiO 2, Al 2O 3, In 2O 3) and excitation wavelength. A strong enhancement of Eu-related emission in the broad UV range of the excitation has been found for the three xerogel systems. However, the enhancement factor is not simple to interpret, it possesses rather a complex nature. We have proposed that the increase of PL intensity is composed of two components. The first component is associated with an improvement of the energy transfer from the host xerogel matrix to lanthanide ions. The second one is evoked by multiple scattering of exciting light in the pore channels. This additional path of the excitation and energy transfer improves essentially the PL of lanthanide ions. Moreover, in periodic matrix of PAA the emission of lanthanide is redistributed by angle contributing into PL enhancement.

Nanocomposite materials based on organopolysiloxane/macrocyle systems for electrochemical sensors

Hybrid inorganic–organic nanocomposite materials are prepared via sol–gel from γ-metacryloxypropyltrimetoxysilane (MAPTMS) and tetramethoxysilane (TMOS) precursors of a organopolysiloxane matrix in which ionophore ligands (crown-ethers: 12-crown-4, 15-crown-5 and 18-crown-6) are incorporated under controlled experimental conditions. Chemical analysis, FTIR and solid-state NMR ( 13 C and 29 Si ) spectroscopies, TG and DTA thermal analyses and specific surface area determinations indicate that the ionophore remains homogeneously distributed in the organopolysiloxane network. Ion-selective electrodes are prepared by coating metal substrates with gels to form homogeneous and continuous thin xerogel films. Impedance and potentiometric techniques are applied for electrochemical characterisation of the sensors. The resulting electrodes are very sensitive towards alkaline metal ions giving linear potentiometric responses with near-Nernstian slopes in wide cation concentration ranges. Additional advantages of this new type of sensors are better adhesion, sensibility, reproducibility, and detection limit compared to “conventional” (i.e., with internal solutions or PVC) electrodes.

Influence of porous anodic alumina matrix upon europium luminescence from sol–gel-derived films

Photoluminescence (PL) and PL excitation (PLE) spectra of Eu-doped TiO 2, SnO 2, and In 2O 3 xerogel films fabricated on porous anodic alumina (PAA) and flat substrates of monocrystalline silicon are analyzed. PAA matrix changes significantly the excitation conditions of Eu in TiO 2 xerogel film, resulting in broad intense band at 280 nm in PLE spectra that is not presented for the same xerogel film fabricated on Si substrate. Moreover, PAA changes the PL behavior of xerogel subjected to thermal treatment resulting in non-monotonous dependence PL intensity on annealing temperature. It is revealed that increase of annealing temperature from 200 to 500 °C leads to decrease of intensity of predominant spectral band at 611 nm, whereas further temperature increase up to 900 °C increases the PL intensity over the value obtained at initial annealing temperature.

Preparation, characterization and properties of PVC/V 2O 5 hybrid organic–inorganic material

In this work, we describe the synthesis of a new organic–inorganic hybrid material, which is composed by polyvinylchloride (PVC) and vanadium pentoxide xerogel. The presence of the organic polymer resulted in a stabilization of the electrochemical response of vanadium pentoxide xerogel in aqueous medium as well as a reduction of its solubility. In addition, the hybrid material has electrochromic properties and is mechanically flexible in contrast to pure V 2O 5 xerogel film, which is fragile and breakable.

Novel preparation and XPS analysis of V2O5 xerogel

The V2O5 sol was fabricated by ultra-fast quenching. The vanadium with low valence (V4+) was found in V2O5 xerogel films by XPS analysis. The technology of oxygen top-blown was applied to analyze the XPS spectrum difference of V2O5 xerogel when the powder of V2O5 was melting in air or in oxygen atmosphere. The results show that the different melting atmosphere has certain influences on the chemical valence of V2O5 xerogel.

Spectroelectrochemical Characterization of Nanostructured, Mesoporous Manganese Oxide in Aqueous Electrolytes

Manganese oxide (MnOx) xerogel and ambigel films were investigated as ultracapacitor electrodes in unbuffered aqueous electrolytes. These nanostructured, mesoporous gels, derived from the sol-gel reaction of and fumaric acid, were cast as continuous thin films on indium-tin oxide glass. Further processing of the wet gels produces birnessite nanoarchitectures in either a densified xerogel or a highly porous ambigel form. The pseudocapacitances of these mesoporous MnOx films in unbuffered aqueous electrolytes are characterized by spectroelectrochemical methods. Monitoring the changes in the optical absorbance during electrochemical cycling provides an in situ measure of the electronic state of the MnOx as the electrode is charged and discharged. Manganese oxide films prepared as highly porous ambigels exhibit a more capacitive-like character than xerogels, and also undergo less capacity loss with cycling. © 2003 The Electrochemical Society. All rights reserved.

Electrochemical study of vanadia–silica xerogel film

In this report, the sol–gel synthesis and electrochemical properties of a homogeneous multicomponent material composed of vanadium pentoxide xerogel dispersed in a silica matrix are described. The cyclic voltammetric and electrochemical impedance spectroscopy studies demonstrated that the silica matrix provides an improvement of the electrochemical properties mainly in relation to the kinetics of the lithium electroinsertion into the oxide matrix with little decrease of total charge during successive redox cycles. This attractive behavior can be explained by the structural stability produced by the silica matrix that favors the kinetic of V IV/V V solid state redox transitions.

Electroactive Multilayer Films of Polyaniline and Vanadium Pentoxide

Nanoscale composites of polyaniline (PANI) and vanadium oxide (V2O5) were assembled via the electrostatic layer-by-layer (LBL) technique, with a thickness per bilayer of 2.5 nm. Interactions between PANI and V2O5 are maximized in comparison to the usual xerogel films due to the nanostructured nature of the LBL films, in which V2O5 causes PANI to be oxidized. This has been demonstrated in Raman spectroscopy measurements and is consistent with electrochemical data. These strong interactions make the LBL film to display a color that is different from the colors of the individual materials. Furthermore, they promote a cooperative effect that enhances the charge storage capability of the films, with a total charge of 2.25 mC cm-2 for the PANI/V2O5 LBL films, in comparison with 1.86 mC cm-2 for the sum of the isolated contributions from PANI and V2O5.

Sol-gel-derived sensor materials that yield linear calibration plots, high sensitivity, and long-term stability.

Novel O2-sensing materials based on spin-coated n-octyltriethoxysilane (Octyl-triEOS)/tetraethylorthosilane (TEOS) composite xerogel films have been synthesized and investigated. These sensors are based on the O2 quenching of tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) ([Ru(dpp)3]2+) sequestered within the xerogels. Scanning electron microscopy and luminescence measurements (steady state and time resolved) have been used to investigate the structure of these films and their analytical figures of merit and determine the underlying reasons for their observed performance. The results show that certain [Ru(dpp)3]2+-doped Octyl-triEOS/TEOS composites form uniform, crack-free xerogel films that can be used to construct high-sensitivity O2 sensors that have linear calibration curves and excellent long-term stability. For example, an 11-month-old sensor based on 50 mol % Octyl-triEOS exhibits more than 4-fold greater sensitivity in comparison to an equivalent sensor based on pure TEOS. Over an 11-month time period, the sensitivity of a pure TEOS-based sensor drops by more than 400% whereas a sensor based on 50 mol % Octyl-triEOS remains stable (RSD = 4%).

High‐efficiency luminescent sources fabricated in mesoporous anodic alumina by sol‐gel synthesis

Abstract— This paper summarizes our recent results on the synthesis and investigation of photoluminescence (PL) from lanthanide‐doped microporous xerogel solids mesoscopically confined in porous anodic alumina (PAA). It was demonstrated, for Tb‐doped samples, that the PL intensity is strongly enhanced in comparison to thin xerogel films processed onto flat surfaces and increased with the thickness of the PAA layer. It was revealed for both Tb‐ and Eu‐doped PAA‐based structures that maximum emission is achieved at a excitation wavelength near 285 nm for the employed TiO2 and Al2O3 xerogels. Strong Eu‐ and Tb‐related PL visible to the naked eye was demonstrated, and a method for the fabrication of luminescent images based on anodizing, photolithography, and sol‐gel processes is proposed.

Effect of modification by poly(ethylene-oxide) on the reversibility of Li insertion/extraction in MoO 3 nanocomposite films

MoO 3 xerogel films modified by poly(ethylene-oxide) (PEO) were obtained by combining ion-exchange method with sol–gel technique. Investigations were conducted by X-ray diffractometry (XRD), Fourier transformation infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry. The results show that when MoO 3 xerogel is modified by the intercalation of PEO, the H atoms in PEO are H-bonded with the O atoms in the MoO bonds of MoO 3 xerogel, which effectively shields the electrostatic interaction between MoO 3 interlayer and Li + ions. The reversibility of the insertion/extraction of Li + ions is greatly improved by the modification with PEO of MoO 3 nanocomposite films.

Influence of Filler on the Pore Structure of Films of Ultra-High-Molecular-Weight Polyethylene Xerogels

The pore structure of filled films of xerogels of ultra-high-molecular-weight polyethylene prepared by gel-casting technique was studied by Fourier IR spectroscopy, attenuated total internal reflection spectroscopy, and optical microscopy. The influence of the filler nature, particle shape, and concentration on the pore structure formation of filled xerogels of ultra-high-molecular-weight polyethylene was examined.

Studies on the spin coating process of silica films

Silica xerogel films with good uniformity and various thicknesses were successfully prepared by a two-step acid-base catalyst procedure and spin coating. After the addition of ammonia catalyst, the sol changes gradually from a Newtonian fluid to a shearthinning one due to the progress of polymerization reactions. An empirical equation using the relative ratio of reaction time to gel time can give successful prediction of sol viscosity. Our systematic study of the coating process indicates the existence of a process window in terms of t/tgel and spin speed can be defined for obtaining uniform and defect-free films. It is difficult to obtain films with good quality when the viscosity of the sol is either too high or too low. Another parameter that also exhibits strong influence on film thickness is the concentration of TEOS in the starting solution. Finally, the film thickness was found to correlate well with sol viscosity and spin speed as h=A � 0.45 $ � 0.62 . # 2002 Elsevier Science Ltd and Techna S.r.l. All rights reserved.

Luminescence of Eu 3+ and Tb 3+ Ions in the Structure Microporous Xerogel/Mesoporous Anodic Aluminum Oxide

We have investigated the luminescence, luminescence excitation, and transmittance of europium- and terbium-doped xerogel films formed on smooth, nanotextured surfaces and in the pores of anodic aluminum oxide. Some factors responsible for enhancement of luminescence in the structure lanthanide-doped xerogel/mesoporous anodic aluminum oxide have been analyzed. It is assumed that the optical excitation of lanthanide ions can be realized directly, through a xerogel matrix, and due to the multiple scattering of exciting radiation by the matrix of mesoporous anodic aluminum oxide.

Optical and electrical properties of vanadium pentoxide xerogel films: modification in electric field and the role of ion transport

Reversible modification of the optical and electrical properties of V2O5×nH2O gel films under the action of an electric field is studied. It is shown that under cathodic polarization (I = 10-6-10-5 A, t~10min, film thickness ~10 µm), the films turn from brownish-yellow to red. This internal electrochromic (EC)effect is caused by the redistribution of hydrogen ions inside the film, instead of insertion from the outside (from an electrolyte). A local increase in the hydrogen concentration occurs near the cathode, and this in turn results in modification of the optical properties. The rise of transmittance in the long-wavelength region of the spectrum, as well as the shift of the absorption edge near hν~2.5eV towards longer wavelengths, is observed. In addition, according to the infrared data, some increase in water content also contributes to the process of colouration. The change in the optical properties is accompanied by a change in the electrical properties, namely, an increase in ionic conductivity from ~4×10-5 to 10-4Ω-1 cm-1. At higher currents, electroforming resulting from the transport of oxygen ions occurs. This process leads to the formation of a channel consisting of vanadium dioxide due to reduction of V2O5 to VO2. Electrical switching with the S-type negative resistance, associated with an electrothermally driven metal-insulator transition in the channel, is observed in the sandwich M/V2O5×nH2O/M devices (unlike the planar devices described in the literature). Finally, applied potentialities of these phenomena for micro- and opto-electronics (EC devices, sensors) are discussed.

Interactions Between Nanoporous Silica and Copper

Porous materials have been proposed to replace as the interlayer dielectric in future microelectronic devices. The interaction of porous materials with contiguous layers is very important for the long-term stability of devices. To investigate the interaction of silica xerogels with Cu, thin Cu layers were deposited on xerogel films and subsequently annealed. When Cu films were directly exposed to an oxidizing environment, they underwent morphological changes, and these changes may lead to the erroneous interpretation of the Rutherford backscattering spectra as depicting metal diffusion. When the samples are capped with an layer, which precludes oxidation, Cu does not show diffusion into the xerogel when annealed up to 650°C. Bias-temperature stressing (BTS) was conducted to assess Cu drift through the xerogel in the presence of an electric field. Contrary to what is normally observed with other dielectrics, the leakage current and capacitance-voltage (C-V) curve shifts were larger with an Al electrode than with a Cu electrode. When the organic capping groups are removed from the xerogel by high temperature annealing before metal deposition, C-V tests indicate Cu ion diffusion occurs even as low as 100°C. Surface modification of the xerogel by trimethylsilyl groups may contribute to the smaller charge injection from the Cu/xerogel interface or to the inhibition of Cu diffusion, thus offering the possibility of designing future monolayer diffusion barriers for porous materials. A model of diffusion through porous solids has been developed. Two possible paths of mass transfer in porous solids are identified: bulk and surface. Three driving forces are also considered: concentration gradient, electric field, and curvature gradient. The upper limit of Cu diffusivity in xerogel was estimated, and the model simulated the current observed during BTS testing reasonably well. © 2002 The Electrochemical Society. All rights reserved.

Ellipsometric study of the change in the porosity of silica xerogels after chemical modification of the surface with hexamethyldisilazane.

Variable angle spectroscopic ellipsometry (VASE) and ellipsometric porosimetry (EP) have been used to study the effect of treatment with hexamethyldisilazane (HMDS) on the porosity of silica xerogel films. Chemical modification of the surface with HMDS was found to reduce the porosity by approximately 15%. This reduction was connected with changes which occur in the silica network, with further condensation or the reaction between neighbouring trimethylsilyl (TMS) surface groups being possible causes.