Papers by Walter mendes de azevedo
Semiconductor Science and Technology, 2015
There are many studies on the synthesis and structure of mesoporous silica, but few reports on me... more There are many studies on the synthesis and structure of mesoporous silica, but few reports on mesoporous silica-based electronic devices using planar technology. Fabrication of low-k insulator films from mesoporous silica has been investigated for years. Trapped water is a nuisance for those intending to use mesoporous silica films as low-k materials, but may be beneficial for other applications. In this work, we fabricated Si metal-insulator-semiconductor capacitors (MIS) with a hexagonal mesoporous silica (MCM-41) film as the dielectric and studied their electrical characteristics. We show that water confined within the dielectric is associated with high values of capacitance per unit area (approximately 1 μF cm −2 at 100 Hz) and frequency dispersion of the accumulation capacitance. These devices hold potential for the development of high value MIS capacitors, sensors and biosensors.
Materials Research Express, 2018
Silica-coated carbon nanotube has been synthesized through a variety of
experimental techniques. ... more Silica-coated carbon nanotube has been synthesized through a variety of
experimental techniques. In this work, we present results of a route of synthesis for multiwalled
carbon nanotubes (MWCNTs)/Silica composites and Silica. This technique involves a reaction
of carbon nanotubes dispersed using Gum Arabic with tetraethylorthosilicate (TEOS), Si
(OC2H5)4 in solution of ethanol, with NH4OH as catalyst. Characterization of X-ray diffraction,
infrared spectroscopy and scanning/transmission electronic microscopy confirmed the
MWCNTs/Silica composite synthesis. In this methodology, acids were not used for
functionalizing the surface of the carbon nanotubes. We also studied the composite thermal
stability. The results of this paper suggest that our methodology can be used to develop less
expensive electronic devices such as sensors, biosensors, capacitors, etc. Additionally, our
methodology is easy to operate
Radiation Measurements, 2014
Characterization of the gamma sensor radiation Tb,Tm-Cu-C 12 A 7 . The majority of the sample can... more Characterization of the gamma sensor radiation Tb,Tm-Cu-C 12 A 7 . The majority of the sample can be a candidate for dosimetric application. Thermoluminescent results with excellent reproducibility at 240 C and 320 C. Linear response between 5 Â 10 À3 Gy and 10 Gy. Tm-Cu-C12A7 showed the best response for 5 mGy remaining linear near to 100 Gy.
Applied Surface Science, 2008
In this work, we present an alternative route to prepare silver hexacyanoferrate(II)/polyaniline ... more In this work, we present an alternative route to prepare silver hexacyanoferrate(II)/polyaniline (PANI) composite thin films. Differently from the electrochemical method, used to synthesize the conducting polymer film on a electrode surface, this new chemical route makes use of dialysis membrane as a solid support to synthesize the silver hexacyanoferrate(III) compound, and subsequently uses this composite membrane as oxidizing agent
Synthetic Metals, 1999
A new semi-Interpenetrating Polymer Network (semi-IPN) based on polyvinyl alcohol–glutaraldehyde/... more A new semi-Interpenetrating Polymer Network (semi-IPN) based on polyvinyl alcohol–glutaraldehyde/polyaniline was synthesized and characterized by spectroscopic analysis in the UV–Visible–Infrared region and by scanning electron microscopy (SEM). The semi-IPNs exhibit good mechanical properties, characteristic of polyvinyl alcohol–glutaraldehyde polymer network, and excellent optical properties. The UV–Visible results show that the optical properties of the semi-IPNs are characteristic only of polyaniline polymer,
Analytical Letters, 1997
A simple conductimetric system to determine ammonia concentration using a sensor based on a condu... more A simple conductimetric system to determine ammonia concentration using a sensor based on a conductor polymer was developed. The sensitive element to ammonia is a thin polyaniline film deposited by chemical synthesis in an acrylic substrate prepared before hand with two graphite electrodes. The conductance of the polyaniline film decreases when exposed to the ammonia gas and this variation can
Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment, 2007
Thermoluminescent (TL) α-Al2O3:C dosimeters, produced in the form of single crystals, show a high... more Thermoluminescent (TL) α-Al2O3:C dosimeters, produced in the form of single crystals, show a high sensitivity to ionizing radiation (about 40–60 times higher than LiF:Mg,Ti). However, the crystal growth requires high temperatures (2050°C) and highly reducing atmospheres. This paper presents the TL response of thin nanoporous Al2O3:C membranes obtained by electrochemical anodizing of aluminum in organic acid solutions at room temperature.
physica status solidi (c), 2004
We investigated the morphology of layers formed by reaction-induced vapor phase stain etch. The l... more We investigated the morphology of layers formed by reaction-induced vapor phase stain etch. The layers consist of irregularly shaped, rough regions with sizes in the order of tens of microns bounded by cracks and separated by smoother regions. The AFM images revealed sub-micron cracks and hillocks with columnar aspect, which might correspond to luminescent Si columns a few nanometers wide.
Synthetic Metals, 2002
Conducting polymer image and diffraction grating pattern were recorded in dichromated poly(vinyl ... more Conducting polymer image and diffraction grating pattern were recorded in dichromated poly(vinyl alcohol) (DCPVA) ®lms by visible light exposure, in a process resembling photolithography. Aniline, used as a chemical developer, polymerized into the unexposed areas of the DCPVA matrix, yielding a composite material which allowed permanent optical information storage and exhibited optical properties characteristic of polyaniline. The photoreduction properties of the DCPVA ®lms and the optical properties of the composite were studied by UV±VIS spectroscopy. The characterization of the diffraction grating composite gave a line width of 12.15 mm and a diffraction ef®ciency of 32%. #
Materials Characterization, 2003
In this paper, we present a new and straightforward route to prepare polyaniline/poly acid acryli... more In this paper, we present a new and straightforward route to prepare polyaniline/poly acid acrylic (PAA) thin film composites in large areas and on almost any surface. This method was developed to improve the mechanical and adherence properties of polyaniline devices used as ionization radiation sensors. The route consists of the combination of the metal oxidant with polymer acid to form a highly homogeneous and viscous paste, which can be easily spread over any surface. In the second step, an aniline acid solution is brought in contact with the dried paste where polymerization occurs, yielding a high homogeneous and conducting polymer composite. The UV -visible absorption and infrared analysis confirm that a polyaniline/PAA complex is obtained. The four-point conductivity measurements show that the composite conductivity q is the order of 5 V À 1 cm À 1 . Preliminary gamma radiation interaction with the composite shows that the doped composite exhibits a linear response that can be used in the development of real-time radiation sensors for the dose range from 0 to 5000 Gy. D
Technical Digest. Summaries of Papers Presented at the Quantum Electronics and Laser Science Conference, 1999
The class of materials known as conjugated polymers (CP) has attracted great attention due to its... more The class of materials known as conjugated polymers (CP) has attracted great attention due to its optical and semiconductor properties. Among CPs, polyaniline (PANI) has been found to be a unique material that presents a reversibility character of its conductivity. For example, upon redox and protonation it is possible to change the polymer from an insulating (leucoemeraldine) form, to a
Aniline surface coated Fe3O4 nanoparticles have been successfully synthesized by UV irradiation v... more Aniline surface coated Fe3O4 nanoparticles have been successfully synthesized by UV irradiation varying the time and the acid media (HCl, HNO3, or H2SO4). The synthesized material represents a promising platform for application in nerve regeneration. XRD patterns are consistent with the crystalline structure of magnetite. Nevertheless, for UV irradiation times longer than 2 h, extra XRD lines reveal the presence of goethite. The mean crystallite size of uncoated particles is estimated to be 25.4 nm, meanwhile that size is reduced to 19.9 nm for the UV irradiated sample in HCl medium for 4 h. Mössbauer spectra of uncoated nanoparticles reveal the occurrence of thermal relaxation at room temperature, while the 77 K-Mössbauer spectrum suggests the occurrence of electron localization effects similar to that expected in bulk magnetite. The Mossbauer spectra of UV irradiated sample in HCl medium during 4 h, confirms the presence of the goethite phase. For this sample, the thermal relaxation is more evident, since the room temperature spectrum shows larger spectral area for the nonmagnetic component due to the smaller crystallite size. Meanwhile, the 77 K-Mössbauer spectrum suggests the absence of the electron localization effect above 77 K.
Optical Materials, 2005
A spectroscopic study performed on Eu(III) and Tb(III) complexes dispersed on polysiloxane sol-ge... more A spectroscopic study performed on Eu(III) and Tb(III) complexes dispersed on polysiloxane sol-gel glasses is reported. The complexes tris [(4,4,4-
Sensors and Actuators B: Chemical, 1996
Fl.antigen purified from gersinia pestis was covalently linked to polyvinyl alcohol (PVA) glutara... more Fl.antigen purified from gersinia pestis was covalently linked to polyvinyl alcohol (PVA) glutaraldehyde cross-linked discs syn. thesized by acid catalysis. This derivative was incubated wil h fluorescein labeled antibody against FI-antigen and excited at 4880 A by either an argon laser or a dye laser. The fluorescence was detected at 5200 A. The appearance of the transition at 5200 A was indicative of positive complex formation, since PVA-glutaraldehyde does not fluoresce at this wavelength. The observed fluorescence was compared with samples treated with patient sera. A positive response was indicated by a decrease in the fluorescence at 5200 A. An additional series of experiments was carried out by first treating the Fl-antigen-PVA-glutaraldehyde disc with patient sera, followed by fluorescein labeled goat anti-human IgG conjugate. A positive response was indicated by an increase in the observed fluorescence. Both approaches were equally capable of detecting antibodies in patient sera. However, the first technique was more rapid. When these data are compared against the standard passive hemagglutination assay presently used in Brazil, it has been found to have far greater sensitivity for the detection of plague.
Radiation Protection Dosimetry, 2006
In this work we present the synthesis, characterisation and the thermoluminescence (TL) response ... more In this work we present the synthesis, characterisation and the thermoluminescence (TL) response of nanoporous carbon doped aluminium oxide Al 2 O 3 :C produced by anodic oxidation of aluminium in organic and inorganic solvents. The X-ray and scanning electron microscopy (SEM) measurements reveal that the synthesised samples are amorphous and present highly ordered structures with uniform pore distribution with diameter of the order 50 nm. The photoluminescence and spectroscopic analysis in the visible and infrared regions show that the luminescence properties presented by the samples prepared in organic acid are due to carboxylate species, incorporated in anodic alumina films during the synthesis process. After an annealing treatment, part of the incorporated species decomposes and is incorporated into the structure of the aluminium oxide yielding a highly thermoluminescent detector (TL) . The results for X-ray irradiation in the range from 21 to 80 keV indicate a linear TL response with the dose in the range from 5 mGy to 1 Gy, suggesting that nanoporous aluminium oxide produced in the present route of synthesis is a suitable detector for radiation measurements.
Optics Letters, 1995
The nonlinear-optical properties of a semi-interpenetrating polymer network of poly(vinyl alcohol... more The nonlinear-optical properties of a semi-interpenetrating polymer network of poly(vinyl alcohol) glutaraldehyde -polyaniline were studied. Large ($10 212 cm 2 ͞W) and fast (,50 ps) refractive optical nonlinearites were observed. The potential of this novel material for photonic applications is evaluated.
Journal of the Optical Society of America B, 2001
The third-order nonlinear optical properties of polyaniline (PANI) solutions and films were inves... more The third-order nonlinear optical properties of polyaniline (PANI) solutions and films were investigated at 532 nm by use of Z-scan, power limiting, and optical Kerr gate techniques. The polymers studied were the undoped partially oxidized (emeraldine base) and fully reduced (leucoemeraldine base) forms of PANI. Our results demonstrate that the leucoemeraldine base is more suitable for use in devices such as all-optical switches and optical power limiters operating at 532 nm. The worse performance of the emeraldine base is due to the presence of defects inside the bandgap of the polymer.
Molecular Crystals and Liquid Crystals, 2002
The first hyperpolarizability β per monomeric residue of polyaniline in three oxidation states an... more The first hyperpolarizability β per monomeric residue of polyaniline in three oxidation states and in the doped emeraldine salt form was determined by hyper-Rayleigh scattering. Multiphoton induced fluorescence was observed and, together with light absorption at the fundamental (ω) and double (2ω) frequencies was taken into account in the calculation of β.
Molecular Crystals and Liquid Crystals, 2002
Diffraction gratings were recorded both optically and interferometrically in dichromated poly(vin... more Diffraction gratings were recorded both optically and interferometrically in dichromated poly(vinyl alcohol) (DCPVA) films by means of dichromate photoreduction. Aniline was used as a chemical developer, through in situ polymerization in the unreduced parts of the film. The resulting composite material shows the optical and chemical properties of polyaniline and the mechanical resistance of PVA. Characterization of the gratings gave
Microelectronics Journal, 2003
Thin polyaniline films in the emeraldine oxidation state, deposited by spin coating on glass subs... more Thin polyaniline films in the emeraldine oxidation state, deposited by spin coating on glass substrate when exposed to gamma-irradiation presents an interesting radiochromic effect, very similar to the protonic acid doping of polyaniline. The green color of the irradiated film and the two absorption bands developed at 420 and 832 nm is a strong evidence of the generation of polaron defect (radical cation) which is responsible for the high conductivity of the polymer. In this work we used the Atomic Force Microscopy (AFM) technique to investigate the morphology of polyaniline films irradiated with 60 Co gamma rays and compare with protonic acids doping effect. q
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Papers by Walter mendes de azevedo
experimental techniques. In this work, we present results of a route of synthesis for multiwalled
carbon nanotubes (MWCNTs)/Silica composites and Silica. This technique involves a reaction
of carbon nanotubes dispersed using Gum Arabic with tetraethylorthosilicate (TEOS), Si
(OC2H5)4 in solution of ethanol, with NH4OH as catalyst. Characterization of X-ray diffraction,
infrared spectroscopy and scanning/transmission electronic microscopy confirmed the
MWCNTs/Silica composite synthesis. In this methodology, acids were not used for
functionalizing the surface of the carbon nanotubes. We also studied the composite thermal
stability. The results of this paper suggest that our methodology can be used to develop less
expensive electronic devices such as sensors, biosensors, capacitors, etc. Additionally, our
methodology is easy to operate
experimental techniques. In this work, we present results of a route of synthesis for multiwalled
carbon nanotubes (MWCNTs)/Silica composites and Silica. This technique involves a reaction
of carbon nanotubes dispersed using Gum Arabic with tetraethylorthosilicate (TEOS), Si
(OC2H5)4 in solution of ethanol, with NH4OH as catalyst. Characterization of X-ray diffraction,
infrared spectroscopy and scanning/transmission electronic microscopy confirmed the
MWCNTs/Silica composite synthesis. In this methodology, acids were not used for
functionalizing the surface of the carbon nanotubes. We also studied the composite thermal
stability. The results of this paper suggest that our methodology can be used to develop less
expensive electronic devices such as sensors, biosensors, capacitors, etc. Additionally, our
methodology is easy to operate