E.A. de Vasconcelos

Growth of sub-micron fibres of pure polyaniline using the electrospinning technique

In this work we report on the formation of pure polyaniline fibres by the electrospinning method. The fibres present diameters ranging from hundreds of nanometres to a few micrometres and have lengths of hundreds of micrometres. The fibres were collected on SiO2/Si and Si wafer pieces in the form of isolated fibres. Morphological characterization by scanning electron microscopy shows smooth fibre surfaces and lack of failure effects such as necking and fibrillation. The current?voltage characteristics are linear and the conductivity values vary in the range usually observed for partially doped polyaniline.

An improved description of the dielectric breakdown in oxides based on a generalized Weibull distribution

In this work, we address modal parameter fluctuations in statistical distributions describing charge-to-breakdown (QBD) and/or time-to-breakdown (tBD) during the dielectric breakdown regime of ultra-thin oxides, which are of high interest for the advancement of electronic technology. We reobtain a generalized Weibull distribution (q-Weibull), which properly describes (tBD) data when oxide thickness fluctuations are present, in order to improve reliability assessment of ultra-thin oxides by time-to-breakdown (tBD) extrapolation and area scaling. The incorporation of fluctuations allows a physical interpretation of the q-Weibull distribution in connection with the Tsallis statistics. In support to our results, we analyze tBD data of SiO2-based MOS devices obtained experimentally and theoretically through a percolation model, demonstrating an advantageous description of the dielectric breakdown by the q-Weibull distribution.

Polyaniline nanofilms as a monitoring label and dosimetric device for gamma radiation

Abstract In this work, an optical dosimeter for gamma radiation ( 60 Co) based on the color change of polyaniline nanofilms in the emeraldine oxidation state was developed by spin coating of the polymer films on glass substrates. After irradiation, the nanofilms were characterized by UV-visible (UV-Vis) absorption spectroscopy as a function of the 60 Co radiation dose. The deep blue color, characteristic of undoped polyaniline films before irradiation subsequently becomes green as the film is irradiated, and the film absorption coefficient exhibits a linear dependence with logarithm of the irradiation dose from 1 to 10 kGy. These results strongly suggest that polyaniline nanofilms can be used as a qualitative and quantitative dosimeter for gamma radiation in the analyzed dose range.

A silicon-polymer heterostructure for sensor applications

We report the development and characterization of high quality polyaniline-silicon heterojunction diodes appropriated for use as gas and/or ionizing radiation sensors. Polyaniline thin films 40 nm thick are an active part of the junction structure, that presents excellent electrical characteristics, with rectifying ratio of 50,000 at ± 1.0 Volt bias. The devices are very sensitive to g-radiation up to 6,000 Gy and to gas moistures such ammonia, nitric acid and trichloroethylene. The sensitivity of the diodes is observed through shifts of the current-voltage (I-V) curves which can be easily monitored to provide a calibration curve of the sensor either as a radiation dosimeter or as a gas sensor for use in applications in environments demanding gas monitoring or radiation dosimetry.

Thermal-lens and photo-acoustic methods for the determination of SiC thermal properties

In this paper we report the use of photothermal techniques such as Thermal lens (TL) spectrometry, Photoacoustic and heat capacity, rcp, to determine the thermo-optical parameters, such as thermal conductivity (K), thermal diffusivity (D), specific heat (cp) and the optical path dependence with temperature (ds/dT), of an undoped polycrystalline 3C-SiC. To our knowledge, this is the first time that Thermal lens technique is used for wide band-gap systems. Results obtained for the polycrystalline sample with TL technique indicates that ds/dT is negative at room temperature. Moreover, the obtained values of thermal diffusivity and thermal conductivity are in good agreement with that found in the literature, indicating that the phototermal techniques can be used to obtain the referred parameters in circumstances where other techniques cannot be used, for example, in harsh environments. q 2005 Elsevier Ltd. All rights reserved.

Time evolution of SiO 2 /Si interface defects and dopant passivation in MOS capacitors

Abstract We study the dopant passivation in p- and n-type Metal-Oxide-Semiconductor capacitors after exposure to high doses of X-ray radiation. Passivation characteristics are analyzed as a function of device size, total dose and time evolution. The results suggest that passivation is influenced by interface charge generated during and after exposure to the radiation. In particular, the interfacial defect generation dynamics suggests that mechanisms associated with tunneling of carriers and/or hydrogen-related species at the SiO 2 /Si interface may be involved.

Fabrication and electrical characterization of polyaniline/silicon carbide heterojunctions

We report on the fabrication of silicon carbide/polyaniline heterojunctions produced by spin coating of polyaniline films onto n-type 6H-SiC and 4H-SiC substrates. Atomic force microscopy was used to estimate the surface roughness, and their electrical characteristics were investigated by means of current, capacitance and conductance measurements as a function of frequency and voltages. Reproducible characteristics and rectification ratios as high as 2 × 10 6 at ±2V for the 6H-SiC based heterojunctions were obtained. The devices were modelled as Schottky diodes with series resistance and an oxide interfacial oxide layer to account for interface traps. By analysing the forward bias I‐V characteristics, we found that the interface trap density for 4H-SiC/PANI heterojunctions is approximately one order of magnitude higher than for 6H-SiC/PANI heterojunctions, which is consistent with previous studies on SiC/SiO2 interface traps. The average value of interface trap densities for 6H-SiC devices was 8.4 × 10 11 eV −1 cm −2 and for 4H-SiC it was 2.7 × 10 13 eV −1 cm −2 . These values are in the range of previous reports on Schottky diodes with polymer layers. (Some figures in this article are in colour only in the electronic version)

AFM studies of polyaniline nanofilms irradiated with gamma rays

Abstract 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.

Statistical analysis of topographic images of nanoporous silicon and model surfaces

Abstract We investigated the quantitative description of nanoporous silicon morphology and its correlation with electrical and optical properties. We performed first-order as well as second-order statistical analysis of AFM images of nanoporous silicon fabricated by two different methods: reaction-induced-vapor-phase-stain-etch and electrochemical. We also simulated AFM images by generation of various model surfaces. From the height–height correlation plots, we were able to observe the effects of the growth method on RMS roughness parameter and verify that photoluminescence can be observed despite very small changes in silicon surface morphology. The analysis of the model surfaces showed that it is possible to reproduce the self-affine character of the porous surfaces by means of a linear combination of sine surfaces.

Polyaniline nanofilms as a sensing device for ionizing radiation

Abstract In this work an optical dosimeter for gamma radiation ( 60 Co ) based on the color change of polyaniline nanofilms (thickness in the range 30– 50 nm ) in the emeraldine oxidation state was developed by spin coating of the polymer on glass substrates. The irradiated polyaniline nanofilms were characterized by UV-visible absorption spectroscopy as a function of the 60 Co radiation dose. The deep blue color, characteristic of undoped polyaniline film before irradiation, becomes subsequently green as the film is being irradiated in the dose range from 0 to 10 kGy .