A. Arena

Electrical Characterization and Hydrogen Peroxide Sensing Properties of Gold/Nafion:Polypyrrole/MWCNTs Electrochemical Devices

Electrochemical devices using as substrates copier grade transparency sheets are developed by using ion conducting Nafion: polypyrrole mixtures, deposited between gold bottom electrodes and upper electrodes based on Multi Walled Carbon Nanotubes (MWCNTs). The electrical properties of the Nafion:polypyrrole blends and of the gold/Nafion:polypyrrole/MWCNTs devices are investigated under dry conditions and in deionized water by means of frequency dependent impedance measurements and time domain electrical characterization. According to current-voltage measurements carried out in deionized water, the steady state current forms cycles characterized by redox peaks, the intensity and position of which reversibly change in response to H2O2, with a lower detection limit in the micromolar range. The sensitivity that is obtained is comparable with that of other electrochemical sensors that however, unlike our devices, require supporting electrolytes.

Photovoltaic properties of multi-walled carbon nanotubes deposited on n-doped silicon

Multi-wall carbon nanotubes (MWCNTs), grown by catalytic chemical vapor deposition (CCVD) over Fe supported on alumina catalyst, using isobutane as feedstock, are dispersed in aqueous solutions of sodium dodecyl sulfate. Stable and highly photosensitive heterojunctions are developed by liquid-phase deposition of MWCNTs/surfactant mixtures on the top of n-doped monocrystalline silicon wafers. Electrical measurements performed in the dark and under broad-band visible light, show that the hybrid solar cells, despite their non-optimized design, have conversion energy efficiency of the order of 3%.

Capacitive humidity sensors based on MWCNTs/polyelectrolyte interfaces deposited on flexible substrates

Inexpensive humidity sensors operating at room temperature are developed by casting on glossy paper linearly shaped carbon nanotubes electrodes, spaced by few hundreds microns, and sensitive layers consisting of iron oxide nanopowder dispersed in a polyelectrolyte host. The electrical behaviour of the devices, investigated by using triangular voltage inputs, is found to be mainly capacitive. The humidity sensing functionality is demonstrated by a reversible change of the devices capacitance, from few tens pF to hundreds pF, as the relative humidity level changes from 35% to 60%. The average response and recovery time of the devices is found to be of the order of a few min.

Preparation, characterization, and micropatterning of laser-dye-doped sol-gel films

Planar thin films of dye-doped silica were prepared by sol-gel synthesis. The optical properties of the films were characterized by transmittance measurements in the visible-infrared range and by photoluminescence. Micropatterns whose feature size was on the order of 2.4 μm, were imprinted on the film surface by a simple soft lithography approach. An optical microscope interfaced to a charge-coupled device camera was used to image the imprinted patterns and to analyze the patterned films photoluminescence. A sensitive spectral narrowing of the emission, which occurred only in micropatterned films, was attributed to the lateral confinement of the luminescence due to the micrometer-scale modulation of refractive index produced by the embossing procedure.

Electrical characterization of solid-state heterojunctions between PEDOT: PSS and an anionic polyelectrolyte

Stable organic heterojunctions are developed by deposition of the anionic polyelectrolyte poly(4-lithium styrenesulfonic acid) (LiPSSA) on the top of poly(3,4-ethylendioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) films. The electrical response of the heterojunctions to triangular voltage pulses in the range between -5 and +5V is characterized by hysteresis phenomena, observed for hundreds of cycles with low distortions, at scan rates between 1 and 100mV/s. The hysteresis is ascribed to the occurrence of redox transitions, evidenced by the presence of well-defined peaks in the current-voltage characteristics. The electrical behaviour of the PEDOT:PSS/LiPSSA devices is found to be mainly capacitive, and show humidity-sensitive functionality, demonstrated by the gradual and reversible increase of the area subtended by the J-V cycles as the relative humidity increases.

Preparation and optical characterization of photosensitive multilayered structures based on polythiophenes and tetracyanoquinodimethane

Organic double layers obtained by electrodeposition of a substituted polythiophene followed by spin-coating of a Tetracyanoquinodimethane (TCNQ) electron acceptor layer, are characterized by means of spectrophotometric measurements carried out in the UV-VIS-IR range. According to the optical data, the electron acceptor molecules reduce to the singly ionized state as a consequence of a charge-transfer reaction that takes place at the interface with the electrodeposited polymer, which acts as an electron donor. Rectifying heterojunctions are obtained by depositing the double layers on the top of monocrystalline silicon wafers, and on the top of monocrystalline silicon wafers coated with electrochemically de-doped poly(3,4-ethylendioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS). Current versus voltage measurements performed in the dark and under halogen light irradiation show that the developed heterojunctions exhibit photosensitive charge transport features, promising in view of photovoltaic applications.

Sol-Gel Glass from Organic Modified Silicates for Optics Applications

A room temperature sol-gel process starting from tetraethyl orthosilicate and triethoxy-vinyl silane is used to obtain dye doped organic modified glass, whose composition and optical properties are investigated by means of transmittance measurements in the VIS-IR range. Experimental conditions as the precursor ratio, the water amount, and the thermal treatment, are optimized to avoid the guest molecules degradation and to obtain good optical quality thin films, able to be micropatterned by embossing lithography. The photoluminescence of the patterned films, imaged on a μm scale, is analysed by means of a CCD camera and compared with the emission of the unstructured films.

Optical Properties of Laser-dye-doped Polyvinylcarbazole Films Coated with Polycarbonate

Spectrophotometry in the ultraviolet-visible-infrared range was used to investigate the optical properties of polymer bilayers obtained by spin coating polycarbonate films about 1 μm thick on top of pyrromethene-doped polyvinylcarbazole (PVK) layers deposited on glass. Due to the light confinement in the PVK plane, the laser-dye photoluminescence collected along the polymer plane was linearly polarized and shifted from green to yellow according to the active film thickness. To laterally contain light, microstructures were imprinted on laser-dye-doped PVK films by soft lithography. The photoluminescence coming out of a microstructured film and the film topography were analyzed by scanning near-field optical microscopy.

Low frequency noise measurements as an early indicator of degradation for devices on plastic substrates subjected to mechanical stress

We report on experiments performed in order to verify the ability of low frequency noise measurements to serve as a sensitive tool for the characterization of the degradation of electron devices on flexible substrates as a consequence of mechanical stress. We designed and build a dedicated stress chamber for performing stress (repeated bending) in controlled environmental conditions. Test devices are copper resistance onto plastic substrates. We observe that, whenever the flicker noise produced by the resistances is clearly detectable, the level of flicker noise increases as the mechanical stress increases. In some cases noise increment of 100 % are observed when the change in the DC resistance of the sample is as low as 0.1%.

Nafion/Polypyrrole Blends for Non Enzymatic Detection of Hydrogen Peroxide

Mainly capacitive electrochemical devices have been developed by depositing ion conducting Nafion/Polypyrrole mixtures, onto the gap between gold and Multi Walled Carbon Nanotubes (MWCNTs) electrodes, applied on copier grade transparency sheets. The electrical properties of the MWCNTs/Nafion:Polypyrrole/Au devices are investigated by means of frequency dependent impedance measurements, and time domain electrical characterization. The results of current measurements performed in 0.1 M NaOH aqueous solutions, in response to triangular voltage inputs, show that the MWCNTs/Nafion:Polypyrrole/Au devices reversibly respond to the presence of H2O2, with a lower detection limit of about 5 μM.