983 resultados para surface electronic phenomena


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The vibrational spectra of palladium phthalocyanine (PdPc) evaporated thin solid films are reported, including the resonance Raman scattering, surface-enhanced resonance Raman scattering (SERRS) and SERRS mapping of the film surface using micro-Raman spectroscopy with 633 nm laser radiation. SERRS of PdPc was obtained by evaporating an overlayer of Ag nanoparticles on to the PdPc film on glass. The SERRS enhancement factor is estimated as similar to10(4) with reference to PdPc evaporated films on glass. The molecular organization of the PdPc evaporated films was probed using transmission and reflection-absorption infrared spectra. It was established that a random molecular distribution found in PdPc evaporated films is independent of temperature. No evidence of thermal degradation due to thermal annealing was found in the films. Electronic absorption and emission spectra are also discussed. Copyright (C) 2002 John Wiley Sons, Ltd.

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Nanostructured films of lignin (macromolecule extracted from sugar cane bagasse), polypyrrole (conducting polymer) and bis butylimido perylene (organic dye) were used in the detection of trace levels of fluorine (from H2SiF6), chlorine (from NaCIO), Pb+2, Cu+2, and Cd+2 in aqueous solutions. Langmuir monolayers on ultrapure water were characterised by surface pressure-mean molecular area (II-A) isotherms. Langmuir-Blodgett (LB) films were transferred onto gold interdigitated electrodes and used as individual sensing units of an electronic tongue system. Impedance spectroscopy measurements were taken with the sensor immersed into aqueous solutions containing the ions described above in different molar concentrations. Fourier transform infrared absorption (FTIR) was employed to identify possible interactions between the LB films and the analytes in solution, and no significant changes could be observed in the FTIR spectra of BuPTCD and Ppy. Therefore, the results for lignin point to an interaction involving the electronic cloud of the phenyl groups with the metallic ions.

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We have used the periodic quantum-mechanical method with density functional theory at the B3LYP level in order to study TiO2/Sn doped (1 1 0) surfaces and have investigated the structural, electronic and energy band properties of these oxides. Our calculated relaxation directions for TiO2 is the experimental one and is also in agreement with other theoretical results. We also observe for the doped systems relaxation of lattice positions of the atoms. Modification of Sri, O and Ti charges depend on the planes and positions of the substituted atoms. Doping can modify the Fermi levels, energy gaps as well as the localization and composition of both valence and conduction band main components. Doping can also modify the chemical, electronic and optical properties of these oxides surfaces increasing their suitability for use as gas sensors and optoelectronic devices. (c) 2005 Elsevier B.V. All rights reserved.

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The surface modifications induced on Teflon FEP and Mylar C polymer films by a low energy electron beam are probed using Raman and FTIR spectroscopy. The electron beam, which does not affect the Mylar C, surface, may break the copolymer chain into its monomers degrading the Teflon FEP surface. For Mylar C the electron beam decreases the roughness of the polymer surface. This difference in behavior may explain recent results in which the surface modifications investigated by measuring the second crossover energy shift in the electronic emission curve differed for the two polymers (Chinaglia et al [1]). In addition, the Raman data showed no evidence of carbon formation for either polymer samples, which is explained by the fact that only a low energy electron beam is used.

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A perylene derivative, n-(n-butyl)-n'-(4-aminobutyl) perylene-3,4,9,10-tetracarboxylic acid diimide (simplified as nBu-PTCD-(CH2)(4)-NH2) has been chosen as the target molecule for studies involving single molecule detection (SMD) using Raman scattering. The enhancement of the Raman signal is the result of the multiplicative effects of two phenomena, resonance Raman scattering (RRS) and surface-enhanced Raman scattering (SERS), which leads to the resulting surface-enhanced resonance Raman scattering (SERRS) process. The SERRS spectra from a single molecule have been collected using both silver and gold colloids. The SMD detection of the fundamental vibrational frequencies characteristic of nBu-PTCD-(CH2)(4)-NH2 is complemented with the detection of some overtones and combinations from ring stretching modes at the single molecule level. The background characterization of the ensemble vibrational spectroscopy of the target perylene and its SERRS is also presented, which includes the UV-vis absorption, experimental and calculated Raman scattering and infrared absorption, and molecular organization using reflection-absorption infrared spectroscopy (RAIRS).

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The nanoscale interactions between adjacent layers of layer-by-layer (LBL) films from poly(allylamine hydrochloride) (PAH) and azodye Brilliant Yellow (BY) have been investigated, with the films employed for optical storage and the formation of surface-relief gratings. Using Fourier transform infrared spectroscopy, we identified interactions involving SO3- groups from BY and NH3+ groups from PAH. These electrostatic interactions were responsible for the slow kinetics of writing in the optical storage experiments, due to a tendency to hinder photoisomerization and the subsequent reorientation of the azochromophores. The photoinduced birefringence did not saturate after one hour of exposure to the writing laser, whereas in azopolymer films, saturation is normally reached within a few minutes. on the other hand, the presence of such interactions prevented thermal relaxation of the chromophores after the writing laser was switched off, leading to a very stable written pattern. Moreover, the nanoscale interactions promoted mass transport for photoinscription of surface-relief gratings on PAH/BY LBL films, with the azochromophores being able to drag the inert PAH chains when undergoing the trans-cis-trans photoisomerization cycles. A low level of chromophore degradation was involved in the SRG photoinscription, which was confirmed with micro-Raman and fluorescence spectroscopies.

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The surface corrosion process associated with the hydrolysis of fluorozirconate glass, Z-BLAN (53ZrF(4), 20BaF(2), 20NaF, 4LaF(2), 3AlF(3)), and the corrosion protection efficiency of a nanocrystalline transparent SnO2 layer were investigated by X-ray photoelectron spectroscopy. The tin oxide film was deposited by the sol-gel dip-coating process in the presence of Tiron(R) as particle surface modifier agent. The chemical bonding structure and composition of the surface region of coated and non-coated ZBLAN were studied before water contact and after different immersion periods (5-30 min). In contrast to the effects occurring for non-coated glass, where the surface undergoes a rapid selective dissolution of the most soluble species inducing the formation of a new surface phase consisting of stable zirconium oxyfluoride, barium fluoride and lanthanum fluoride species, the results for the SnO2-coated glass showed that the hydrolytic attack induces a filling of the film nanopores by dissolved glass material and the formation of tin oxylluoride and zirconium oxyfluoride species. This process results in a modified film, which acts as a hermetic diffusion barrier protecting efficiently the glass surface. (C) 2006 Elsevier B.V. All rights reserved.

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The present study is concerned with the structural and electronic properties of the TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 composite systems. Periodic quantum mechanical method with density functional theory at the B3LYP level has been carried out. Relaxed surface energies, structural characteristics and electronic properties of the (I 10), (0 10), (10 1) and (00) low-index rutile surfaces for TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 models are studied. For, comparison purposes, the bare rutile TiO2 and SnO2 structures are also analyzed and compared with previous theoretical and experimental data. The calculated surface energy for both rutile TiO2 and SnO2 surfaces follows the sequence (110) < (010) < (101) < (001) and the energy increases as (010) < (101) < (110) < (001) and (010) approximate to (110) < (101) < (001) for SnO2/TiO2/SnO2 and TiO2/SnO2/TiO2 composite systems, respectively. SnO2/TiO2/SnO2 presents larger values of surface energy than the individual SnO2 and TiO2 metal oxides and the TiO2/SnO2/TiO2 system renders surface energy values of the same order that the TiO2 and lower than the SnO2. An analysis of the electronic structure of the TiO2/SnO2/TiO2 and SnO2/TiO2/SnO2 systems shows that the main characteristics of the upper part of the valence bands for all the studied surfaces are dominated by the external layers, i.e., by the TiO2 and the SnO2, respectively, and the topology of the lower part of the conduction bands looks like the core layers. There is an energy stabilization of both valence band top and conduction band bottom for (110) and (010) surfaces of the SnO2/TiO2/SnO2 composite system in relation to their core TiO2, whereas an opposite trend is found for the same surfaces of the TiO2/SnO2/TiO2 composite system in relation to the bare SnO2. The present theoretical results may explain the growth of TiO2@SnO2 bimorph composite nanotape.

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The influence of the equatorial ligand on the electrochemical oxidation of the compounds [H3CCo(chel)B], where chel is bis (dimethylglyoximato), (DH)2; bis(salicylaldehyde)ethylenediimine, salen; bis(salicylaldehyde) o-phenylenediimine, salophen; bis(salicylaldehyde)cyclohexylenediimine, salcn; bis(acetylacetone) ethylenediimine, bae; and where B is pyridine when chel is (DH2), and dimethylformamide (DMF) when chel represents a Schiff base (salen, salcn, salophen and bae), was studied by means of cyclic voltammetry in DMF, 0.2 M in tetraethylammonium perchlorate, between 25 and -25°C, with a platinum disk working electrode. Absorption spectra in the visible and near ultraviolet regions for these compounds in DMF at 25°C were obtained. The complexes exhibit a reversible one-electron oxidation, at -20°C with scan rates >0.5 V s-; chemical reactions following electron transfer are not detected under these conditions. At slower potential or higher temperatures, the oxidized product decomposes chemically in a solvent-assisted (or nucleophile-assisted) reaction, yielding products which are electroactive in the applied potential range. The behavior of the [H3CCo (DH2)py] derivative is better described as a quasi-reversible charge transfer followed by an irreversible chemical reaction. Experimental evidence suggests that in the case of the [H3CCo(bae)] derivative at -20°C, the reactive -species is pentacoordinated and weakly adsorbed at the electrode surface. The value of E 1 2 and the energies of the first two absorption bands in the visible spectra reveal the ability of the studied complexes to donate and to delocalize electronic charge. © 1982.

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The nucleation and growth model, which is usually applied to switching phenomena, is adapted for explaining surface potential measurements on the P(VDF-TrFE) (polyvinylidene fluoride-trifluoroethylene) copolymer obtained in a constant current corona triode. It is shown that the growth is one-dimensional and that the nucleation rate is unimportant, probably because surface potential measurements take much longer than the switching ones. The surface potential data can therefore be accounted for by a growth model in which the velocity of growth varies exponentially with the electric field. Since hysteresis loops can be obtained from surface potential measurements, it is suggested that similar mechanisms can be used when treating switching and hysteresis phenomena, provided that account is taken of the difference in the time scale of the measurements.

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By using the long-wave approximation, a system of coupled evolutions equations for the bulk velocity and the surface perturbations of a Bénard-Marangoni system is obtained. It includes nonlinearity, dispersion and dissipation, and it is interpreted as a dissipative generalization of the usual Boussinesq system of equations. Then, by considering that the Marangoni number is near the critical value M = -12, we show that the modulation of the Boussinesq waves is described by a perturbed Nonlinear Schrödinger Equation, and we study the conditions under which a Benjamin-Feir instability could eventually set in. The results give sufficient conditions for stability, but are inconclusive about the existence or not of a Benjamin-Feir instability in the long-wave limit. © 1995.

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The isotherms of adsorption of CuX2 (X=Cl-, Br-, ClO- 4) by silica gel chemically modified with 2-aminothiazole were studied in acetone and EtOH solutions, at 25°C. The 2-aminothiazole molecule, covalently bond to the silica gel surface, adsorbs CuX2 from solvent by forming a surface complex. At low loading, the electronic and E.S.R. spectral parameters indicate that the Cu2+ complexes have a distorted tetragonal symmetry. The d-d eletronic transition spectra show that for ClO- 4 complex, the peak of absorption do not change for any degree of metal loading whilst for Cl- and Br- complexes, the peak maxima shift to higher energy with lower metal loading. © Elsevier Science Ltd.

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The objective of this study was to evaluate, through scanning electronic microscopy, the effect of sharpening with different sharpening stones on the cutting angle of periodontal curettes (Gracey 5-6), and the influence on root surfaces after debridement and planing. The experimental model consisted of two different phases. In the first, the cutting angles of fifteen stainless steel Gracey 5-6 curettes were analyzed under a scanning electronic microscope after being sharpened with different types of stones. In the second phase, the root surfaces of 25 newly extracted teeth were evaluated with a scanning electronic microscope after being debrided with curettes sharpened with different stones. Analysis of the results showed that the synthetic stones (aluminum oxide and carborundum) are more abrasive and produce more irregular cutting angles, whereas Arkansas stones are less abrasive and produce smoother and more defined cutting angles. There was no significant statistical differences among the five groups tested with regard to the degree of irregularity of the root surfaces after instrumentation.

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About 99% of mechanical failures are consequence of the phenomena of fatigue, which consists on the progressive weakening of the resistant section of a mechanical component due to the growing of cracks caused by fluctuating loadings. A broad diversity of factors influences the fatigue life of a mechanical component, like the surface finishing, scale factors, among others, but none is as significantly as the presence of geometric severities. Stress concentrators are places where fatigue cracks have a greater probability to occur, and so on, the intuit of this work is to develop a consistent and trustfully methodology to determine the theoretical stress concentration factor of mechanical components. Copyright © 2007 SAE International.

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To properly describe the interactions between the ocean and atmosphere, it is necessary to assess a variety of time and spatial scales phenomena. Here, high resolution oceanographic and meteorological data collected during an observational campaign carried out aboard a ship in the tropical Atlantic Ocean, on May 15-24, 2002, is used to describe the radiation balance at the ocean interface. Data collected by two PIRATA buoys, along the equator at 23°W and 35°W and satellite and climate data are compared with the data obtained during the observational campaign. Comparison indicates remarkable similarity for daily and hourly values of radiation fluxes components as consequence of the temporal and spatial consistence presented by the air and water temperatures measured in situ and estimated from large scale information. The discrepancy, mainly in the Sao Pedro and Sao Paulo Archipelago area, seems to be associated to the local upwelling of cold water, which is not detected in all other estimates investigated here. More in situ data are necessary to clarify whether this upwelling flow has a larger scale effect and what are the meteorological and oceanographic implications of the local upwelling area on the tropical waters at the Brazilian coast.