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The morphological, rheological and electrical properties of nanocomposites based on multi-walled carbon nanotubes (MWNTs) and high density polyethylene (HDPE), prepared by melt mixing, have been investigated. The effects of temperature, MWNT concentration and nanotube surface treatment have been analyzed. A good dispersion of oxidized nanotubes in the HDPE matrix was found using electron microscopy investigations, as has also been found in the case of related nonfunctionalized MWNTs. The rheological oscillatory shear tests in the melt state have been carried out with strain amplitude values within the linear viscoelastic region. The rheological measurements on the nonfunctionalized MWNT/HDPEs revealed that the percolation threshold was temperature dependent with lower values at higher temperatures. At low frequencies, the oxidized MWNTs/HDPE composites showed storage modulus and complex viscosity values lower than the corresponding values measured for nanocomposites prepared with nonfunctionalized MWNTs. The conductivity decrease is more than 2 orders of magnitude for the sample with 2.5 wt% oxidized MWNTs, compared to the un-oxidized comparator materials. A constant voltage, long-term stability test allowed to estimate the average number of electrically active percolation paths and showed that stepwise conductivity loss is preceded by a strong increase in the noise amplitude related to the electrical current. POLYM. ENG. SCI., 57:665–673, 2017. © 2017 Society of Plastics Engineers

Low-frequency noise spectroscopy has been used to monitor electronic properties of solar cells under temperature or radiation stress. For all the investigated polymer:fullerene, silicon and perovskite solar cells this technique evidences a clear correlation of the recombination and the transport processes with the device performances.

The wet anisotropic etching process is generally used in the field of micromachining (MEMS), particularly for commercial products such as accelerometers. Hard masks like oxide or nitride play a key role in the transfer of patterns to the substrate during the lithography process. This work reports on the use of polycrystalline graphenic carbon as an etch mask for wet chemical processing and outlines a simple method to create patterned structures on (100) silicon wafers. Graphenic carbon (GC) was deposited on the silicon substrate by chemical vapor deposition (CVD) using C2H4 as precursor. The desired pattern was written in the spin-coated negative photoresist using UV laser lithography. Different geometrical shapes were printed on the substrate with dimensions ranging from 10 to 50 micrometers. In the next stage, the O2 plasma etched away the carbon from the area not covered by the photoresist, acting as an additional mask for this and the subsequent processing steps. Finally, the sample was immersed in the KOH bath saturated with isopropanol and the etching rate was evaluated for each crystal plane. Compared to the use of a sacrificial oxide mask, this technique is simpler and produces more reliable results.

The hole mobility of a semiconducting film with a new type of small molecule has been determined by electrical characterization with two different device geometries. The small molecule, namely Zn(OC)2, consists of oxadiazole groups (O), that act as electron conductor and blue emitter, and carbazole groups (C), that are hole conductors, which are arranged around a central Zn-atom. This disordered organic material has in principal ambipolar conduction properties. In both the organic devices investigated, that is, a vertical diode and a bottom-gate organic field effect transistor, the transport is dominated by holes. In the diode structure, the charge carrier transport shows a dependence from the electric field with a space charge limited current characteristic. The hole mobility value is several orders of magnitude higher as compared to the one extracted from the characteristics of the organic transistor. This large difference in the charge carrier transport properties for the two different device configurations is due to the strong influence of the dielectric/semiconductor interface which degrades the hole transport in the organic field effect transistor.

In this work it was investigated the possibility to use a low-cost sustainable filler, obtained from municipal solid waste incinerator (MSWI) residues stabilized by COlloidal Silica Medium to Obtain Safe inert (COSMOS) technology, for the development of new eco-friendly PP-based composites. The eco-composites were prepared by melt compounding and tested to verify the effects of the filler addition on the thermal, mechanical and electrical properties of PP. Moreover, in order to check the possible filler capacity to act as adjuvant for conventional flame-retardants, ternary PP composite systems, containing also a commercial non-halogenated flame-retardant in the formulation, were produced and characterized. The obtained results demonstrate that the MSWI residues can be an effective reinforcement for engineering value-added eco-composite materials.

A new type of tissue, based on the combination of tobacco cells and multi-walled carbon nanotubes exhibited good mechanical stability even after more than 2 years of room temperature storage. The original intermediate value of electrical conductivity in coplanar configuration, decreased strongly after high electric field stress before the storage period. In the visible wavelength range optical transmission values between 0.1% and 1% have been measured for the more than 100 μm thick sample. The bio/nano-composite in the low-conductivity state revealed to be a sensitive photoreceiver in the visible wavelength range, when illuminated with white LED light. A notable decrease of the low-frequency current noise amplitude with LED illumination has been observed.

In this work the preparation of flexible polymeric films with controlled electrical conductivity, light transmission and surface wettability is reported. A drop casted graphene oxide thin film is photo-reduced at different levels by UV light or laser irradiation. Optical microscopy, IR spectroscopy, electrical characterization, Raman spectroscopy and static water contact angle measurements are used in order to characterize the effects of the various reduction methods. Correlations between the optical, electrical and structural properties are reported and compared to previous literature results. These correlations provide a useful tool for independently tuning the properties of these films for specific applications.

In this paper, we investigate the electrical hole transport properties of an organic/inorganic heterostructure consisting of a thin organic film, that combines hole and electron conducting molecules around a bridging Zn-atom, deposited on top of an n-type crystalline silicon substrate. Current-voltage characteristics and capacitance voltage measurements have been used for the determination of the organic layer dielectric and hole conduction parameters.

A Candida albicans/multiwalled carbon nanotube (Ca/MWCNTs) composite material has been produced. It can be used as a temperature-sensing element operative in a wide temperature range (up to 100 °C). The Ca/MWCNTs composite has excellent linear current-voltage characteristics when combined with coplanar gold electrodes. We used growing cells of C. albicans to structure the CNT-based composite. The fungus C. albicans combined with MWCNTs coprecipitated as an aggregate of cells and nanotubes that formed a viscous material. Microscopic analyses showed that Ca/MWCNTs formed a sort of artificial tissue. Slow temperature cycling was performed up to 12 days showing a stabilization of the temperature response of the material.

A theoretical model, combining trapping/detrapping and recombination mechanisms, is formulated to explain the origin of random current fluctuations in silicon-based solar cells. In this framework, the comparison between dark and photo-induced noise allows the determination of important electronic parameters of the defect states. A detailed analysis of the electric noise, at different temperatures and for different illumination levels, is reported for crystalline silicon-based solar cells, in the pristine form and after artificial degradation with high energy protons. The evolution of the dominating defect properties is studied through noise spectroscopy.

The radiation hardness of CH3 NH3 PbI3 -based solar cells is evaluated from in situ measurements during high-energy proton irradiation. These organic-inorganic perovskites exhibit radiation hardness and withstand proton doses that exceed the damage threshold of crystalline silicon by almost 3 orders of magnitude. Moreover, after termination of the proton irradiation, a self-healing process of the solar cells commences.

Rheological and electrical properties of nanocomposites based on multi-walled carbon nanotubes (MWNTs) and high density polyethylene (HDPE), prepared by melt mixing in a micro-twin screw extruder, have been investigated. The effect of MWNT concentration (0.5 and 2.5 wt %) and nanotube surface treatment (oxidative treatment in a tubular furnace at 500°C for 1 hr in a 95% nitrogen, 5% oxygen atmosphere) has been analyzed. It has been found that the sample conductivity with oxidation of the nanotubes decreases more than 2 orders of magnitude.Scanning electron microscopy showed good adhesion and dispersion of nanotubes in the matrix, independently of the surface treatment. Electrical and rheological measurements revealed that the oxidative treatment, causing some reduction of the MWNT quality, decreases the efficiency of the nanotube matrix interaction.

Carbon nanotubes addition to polymer and epoxy material allows to realize a large variety of new types of sensors and actuators. For the development of these devices, a deeper understanding of the basic charge carriers transport mechanisms is needed and low-frequency noise spectroscopy can effectively contribute to this task. The dc and electrical noise characteristics of different polymer/carbon nanotubes composites are analyzed at temperatures from 10 to 300 K. It has been found that a random tunnel junctions resistive networks model describes all the systems under test. A crossover from a two-level tunneling mechanism, at low temperatures, to resistance fluctuations induced by percolative paths, in the high-temperature region, has also been observed. This behavior of the 1/f noise is a general feature of the investigated highly conducting nanocomposites, independent of the specific polymer matrix and nanotube concentration.

The hybrid perovskite methylammonium lead iodide CH3NH3PbI3 recently revealed its potential for the manufacturing of low-cost and efficient photovoltaic cells. However, many questions remain unanswered regarding the physics of the charge carrier conduction. In this respect, it is known that two structural phase transitions, occurring at temperatures near 160 and 310 K, could profoundly change the electronic properties of the photovoltaic material, but, up to now, a clear experimental evidence has not been reported. In order to shed light on this topic, the low-temperature phase transition of perovskite solar cells has been thoroughly investigated by using electric noise spectroscopy. Here it is shown that the dynamics of fluctuations detect the existence of a metastable state in a crossover region between the room-temperature tetragonal and the low-temperature orthorhombic phases of the perovskite compound. Besides the presence of a noise peak at this transition, a saturation of the fluctuation amplitudes is observed induced by the external DC current or, equivalently, by light exposure. This noise saturation effect is independent on temperature, and may represent an important aspect to consider for a detailed explanation of the mechanisms of operation in perovskite solar cells.

Some commercially available solar panels with very high efficiencies for terrestrial photovoltaic applications are based on the amorphous silicon on crystalline silicon material system. This type of heterostructure has more than 40 years' old history. The early development of the technology and the results, obtained in the last years with this type of solar cell are reviewed. In particular it is demonstrated why the physical understanding of the interface properties and band-structure was important for the development of high efficiency solar cells.

Organic/inorganic heterojunctions have been fabricated by spin-coating of an oxadiazole based polyether on top of either n-type or p-type crystalline silicon substrates. The prepared heterodiodes showed good rectification. During current-voltage measurements sharp current switching effects have been observed in the reverse bias characteristics for both types of heterodiodes. In particular a very stable hysteresis in the reverse characteristics of the polyether/n-type crystalline silicon has been measured at applied voltages below 1V. The application of this heterodiode as electrical memory with an on/off-ratio of about 2 orders of magnitude has been demonstrated (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of the electrical field on the conductivity of linear low-density polyethylene/carbon nanotubes/ (LLDPE/CNT) composites during temperature cycling has been investigated. Under applied voltage a positive resistivity temperature coefficient was observed during heating already at low (2–3 wt%) CNT content, followed by a large negative temperature coefficient during cooling whose value depends on the applied voltage. The resistivity values after thermal cycling were markedly lower, while they slightly increased in the absence of an electrical field. The effects of thermal cycling on structural and physical properties of the composites have been evaluated by X-ray diffraction and differential scanning calorimetry.

Carbon nanotubes added to polymer and epoxy matrices are compounds of interest for applications in electronics and aerospace. The realization of high-performance devices based on these materials can profit from the investigation of their electric noise properties, as this gives a more detailed insight of the basic charge carriers transport mechanisms at work. The dc and electrical noise characteristics of different polymer/carbon nanotubes composites have been analyzed from 10 to 300 K. The results suggest that all these systems can be regarded as random resistive networks of tunnel junctions formed by adjacent carbon nanotubes. However, in the high-temperature regime, contributions deriving from other possible mechanisms cannot be separated using dc information alone. A transition from a fluctuation-induced tunneling process to a thermally activated regime is instead revealed by electric noise spectroscopy. In particular, a crossover is found from a two-level tunneling mechanism, operating at low temperatures, to resistance fluctuations of a percolative network, in the high-temperature region. The observed behavior of 1/f noise seems to be a general feature for highly conductive samples, independent on the type of polymer matrix and on the nanotube density.

A biodegradable low-cost energy storage material has been obtained by using a self-assembly nanocomposite gelatin and graphene flakes. The blends with various ratio of graphene and gelatin have been prepared and characterized by Raman spectroscopy, cyclevoltammetry measurements, chronopotentiometry measurements and impedance spectroscopy. The ability of the biopolymer to bind the water–glycerol molecules and the graphene flakes leads to an improvement of the dielectric properties with a value of the surface capacitance of about 3 orders higher (10 mF cm −2 ) than that measured for the device based on activated carbon. An electrical oxidation at the anode contact has been observed by applying to the device a higher bias voltage. The modified active material exhibits an enhancement of the cycle stability with a further increase of the surface capacitance (100 mF cm −2 ) and of the respective specific capacitance up to a value of about 380 F g −1 .

Insulating materials play a vital role in the design and performance of electrical systems for both steady and transient state conditions. Among the other properties, also in this field, polymer nanocomposites promise to offer exciting improvements. Many studies in the last decade has witnessed significant developments in the area of nanodielectric materials and significant effects of nano-scale fillers on electric, thermal and mechanical properties of polymeric materials have been observed. However, the developments of new and advanced materials to be used the miniaturization of electronic devices fabrication require extensive studies on electrical insulation characteristics of these materials before they can be used in commercial systems. In this work, Polystyrene (PS) composites were prepared by the blend solution method using MoS2@Oleylamine nanosheets as filler. The dielectric properties of the resulting comoposite have been investigated at 300K and in the frequency range between 1000 Hz and 1 MHz. The addition of the MoS2@Oleylamine nanosheets leads to a decreasing of the relative dielectric constant and of the electrical conductivity measured in the voltage range between +/- 500V. Thanks to a possibility to tune the electrical permittivity with the control of MoS2 concentration, these materials could be used as a low-dielectric material in the microelectronics applications.

In this paper, we discuss the conditions under which we can correctly extract a variety of module parameters of single mode bidirectional optoelectronic duplexer and diplexer modules during relatively slow thermal cycling between -40/spl deg/C and +85/spl deg/C. The obtained values of parameters such as tracking error, optical crosstalk, and the laser diode characteristic temperature have been compared for bidirectional modules from different manufacturers and the impact on these parameters of different interconnect techniques, like microoptic (Mo), silicon optical bench, and planar lightwave circuit technologies is discussed. It is shown that there is a conflict between small threshold currents and good temperature stability for InGaAsP-based Fabry-Perot laser diodes emitting at 1300 and 1550 nm. Then the results of spatially low coherence reflectometry for the different bidirectional module types are presented.

Two types of lignin, alkali lignin and lignosulfonic acid sodium salt, are blended into thermoplastic zein through melt mixing in order to develop biodegradable insulator materials for multifunctional applications in electronics. The effects of lignin type and content on the dielectric properties of the resulting bio-nanocomposites are investigated. The results indicate that, by modifying the structural arrangement of the zein with the use of lignin, it is possible to obtain bio-nanocomposites characterized by tunable dielectric properties. The bio-nanocomposites containing low amounts of lignin derivatives exhibit extensive protein structural changes together with a modification of the dielectric properties compared to the pristine thermoplastic zein. Changes in the dielectric properties of these systems are also observed to change over time, indicating a loss of plasticizer, as is evident by a decrease in the glass-transition temperature. At high frequencies, the resulting values of the dielectric permittivity and of the loss tangent demonstrate that the bio-nanocomposite can be used as biodegradable dielectric material for transient (temporary) electronics.

A HDPE/CNT composite has been fabricated based on a commercial polyethylene loaded with a high concentration of CVD grown multiwalled carbon nanotubes with a high aspect ratio. Exploiting the resulting high conductivity of this composite material, we tested the possibility to use it as absorber of electromagnetic waves at microwave frequencies up to 25 GHz. For this purpose the device under test was placed as substrate of a microstrip transmission line. Measurements of the scattering parameters, done with a vectorial network analyzer, have been used in order to obtain the power absorption at different frequencies. Special attention was paid to the optimization of the microstrip geometry. A comparison of the microwave power absorption spectrum with those of commercial cavity absorbers showed encouraging results, regarding the microwave absorption capability of the HDPE/CNT composite material.

Temperature is one of the most specific external parameters that can accelerate the degradation rate in polymer:fullerene solar cells. To detect modifications of the active layer materials, electric noise spectroscopy is a sensitive experimental technique. A detailed characterization of the dc electric transport and voltage–noise properties shows how thermal ageing is detrimental for the investigated bulk heterojunction photovoltaic system. In particular, an increase of the energy barrier height at the interface between the metal contact and the blend, and a simultaneous decrease of the charge carrier zero-field mobility, evaluated through the analysis of the flicker noise component, are observed as a consequence of a thermal treatment. These effects can be related to morphological changes of the solar cell active layer and interface, and are revealed by monitoring the noise level.

The sensitivity of vertical cavity surface-emitting lasers to electrostatic discharge (ESD) pulses has been investigated under human body model test conditions. Very similar degradation behavior has been found for vertical-cavity surface-emitting lasers (VCSELs) from two different manufacturers, both with proton-implantation for lateral current confinement. For all investigated devices we observed during forward bias stress that the optical degradation precedes the electrical degradation and the forward bias damage threshold pulse amplitudes were only slightly higher than the reverse bias values. At the initial stage of the VCSEL degradation, damage of the upper p-DBR mirror region has been observed without modification of the active layer. During the ESD tests we monitored the electrical and the optical parameters of the VCSELs and measured during forward bias stress additionally the optical emission transients. The optical transients during ESD pulsing enable a fast evaluation of the damage threshold and give also an indication of the time scale of the junction heating during ESD pulses.

Human Body Model ESD tests on two commercially available different types of wavelength division multiplexing bidirectional optoelectronic modules, intended for data transmission over multimode optical fibers at 820 and 1320 nm wavelengths, have been performed. These bidirectional modules incorporated pin diode receivers and light emitting diode in a single package. In particular, the 820 nm light emitting diodes had rather high damage threshold pulse amplitudes of about +10,000 V under forward bias and −5000 V under reverse bias conditions, and no active layer degradation has been observed. In-situ measurements of the optical transients emitted during forward bias ESD pulses enabled a damage pulse threshold detection of a LED without the need of further optoelectronic characterization. The receiver photodiodes, however, were very sensitive to reverse bias ESD pulses. The InGaAs photodiodes degraded during ESD pulses with amplitudes as low as −150 V and in the case of the Silicon photodiodes as low as −175 V.

Here we present new materials obtained using, either fungal or isolated tobacco cells in association with different percentages of carbon nanotubes (CNTs). As a proof of concept, we used eitherCandida albicansor a non-green Tobacco BY-2 cell line combined with multi-walled CNTs. The electrical, mechanical, and conductivity vs temperature properties for some of these materials have been determined.C. albicans-based tissues have high conductivity and are stable at elevated temperatures. By lowering the CNTs content, we obtained a stable, electrically conductive optical transparent film, though with a relatively high sheet resistance. Further, we produced, using tobacco cells, a material that exhibits good electrical as well as mechanical properties.

A detailed electric noise investigation of polymer:fullerene solar cells, at 300 K under dark conditions, is reported. The experimental noise results are interpreted in terms of a model taking into account the device capacitance and recombination resistance. Relevant parameters of the solar cells can be computed through fluctuation spectroscopy, and the results have been compared with those obtained by alternative techniques. After a thermal treatment at 340 K, a modification of the voltage-spectral traces has been observed and related to a strong cell resistance reduction.

In the present study we investigated a combination of quantum dots with multi-walled carbon nanotubes as a possible future additive to the active layer of polymer solar cells. In this case the quantum dots should serve to enhance the long wavelength response of the solar cell, while the nanotubes enhance the charge carrier collection efficiency by favoring charge carrier separation and enhancement of the lateral conduction of the films. In order to clarify the interplay of the nanoparticles only, we deposited them into a non-conducting and transparent polymethyl-methalacrylate (PMMA) matrix. InP/ZnS quantum dots with an emission peak wavelength of 660 nm have been chosen in this study, because their addition can enhance the long wavelength response of conventional poly(3-hexylthiophene) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) bulk heterostructure polymer solar cells. In our study we kept the quantum dot concentration constant and varied the concentration of the carbon nanotubes (CNTs) in the deposited films. The characterization of the film morphology by scanning electron microscopy (SEM) imaging and of the optical properties by photoluminescence and transmittance revealed a rather complex interplay between nanotubes and quantum dots. In particular we found a strong quenching of the photoluminescence and an inhomogeneous CNT distribution for carbon nanotube concentrations exceeding 1%. The decrease in optical transmittance of the films with increasing CNT concentration is less pronounced, when quantum dots (QDs) are added. The optical transmittance in a wavelength range between 380 nm and 800 nm of the composites could be expressed empirically as a simple second order polynomial function. © 2013 Elsevier B.V. All rights reserved.

An overview over typical defects and reliability aspects regarding the operation of InGaAs InP avalanche photodiodes for the use in fiber-optic communication systems is given. Measurements regarding the performance and stability of different types of top illuminated InGaAs InP Separate Absorption Grading Multiplication avalanche photodiodes have been compared with special emphasis on the homogeneity of the photoresponse. High resolution scanning optical microscopy measurements at different wavelengths allowed to identify specific technological problems. Guard ring misalignment resulting in increased responsivity of the junction perimeter have been found on devices from one manufacturer. The responsivity images of the active area of some of the investigated photodiodes showed sharp peaks due to microplasma formation. The impact of these microplasmas on dark current and photocurrent of the devices has been investigated in detail. A step-stress test performed at successively increasing temperatures resulted in the catastrophic failure of the tested devices from one family. An analysis of the dark current-voltage characteristics before and after degradation leads to the conclusion that a modification of the semiconductor-insulator interface properties caused a dramatic increase in the surface leakage currents.

The decay of optically generated excess charge carriers at the heterointerface between crystalline silicon and amorphous silicon has been monitored in situ during plasma deposition of the amorphous layer. A dramatic increase in the interface recombination rate is seen instantly when the silane plasma is ignited. In a later stage of amorphous layer deposition, the recombination rate decreases. This interface passivation depends strongly on the substrate temperature and is more efficient at higher temperature and in the presence of silane plasma. The effect is not due to a change in the band bending at the heterointerface.

The damage induced by hydrogen and helium plasmas at the surface of crystalline silicon has been monitored in situ by time‐resolved microwave conductivity and by spectroscopic ellipsometry measurements. Both plasma treatments increase the decay rate of the optically generated excess charge carriers and decrease the amplitude of the microwave reflection transients. While for the helium plasma a high density of electronic defects is created immediately after plasma ignition, a continuously increasing number of recombination centers is observed in the case of the hydrogen plasma exposure. In support of the transient microwave measurements, the analysis of the spectroscopic ellipsometry measurements reveals the creation of a damaged surface layer, which in the case of the helium plasma exposure has a high and in the case of the hydrogen plasma a low fraction of amorphous silicon. This can be explained by the different nature of the processes involved in the interaction of hydrogen (chemical) and helium (physi...

We report on the application of a self electro-optic effect device in waveguiding configuration for the generation of a wide variety of different nonlinear optical transfer functions. It makes use of the Wannier-Stark effect in an InGaAs/InP short period superlattice and operates at room temperature for TE-polarized light around 1.55 μm. In particular, optical bistability, optical signal-frequency multiplication and the operation as an optical power discriminator are demonstrated.

In this work we describe the influence of the growth parameters on the optical and crystallographic properties of InGaAs/InP short period superlattices (SPSLs). The optimized growth conditions applied for multiquantum wells are no longer appropriate when the period thickness is below 85 A and quasi-continuous growth interruption times are required. The assessment of new growth conditions enabled the realization of SPSL structures with ultimate period of 31 A presenting good optical and structural properties. The electron wave-function coherence length of seven SPSLs was evaluated. Application of these devices enabled the realization of Wannier-Stark modulators operating at 1545 nm with extinction ratio of 22 dB at 2.5 V reverse bias. The electron wave-function was evaluated, for the first time in the InGaAs/InP system, to be at last seven SPSL periods using both transmittance and photocurrent methods. The on state attenuation determined using Wood's method is below 2 dB in the range between 1540 and 1555 nm. SPSL structures are therefore well suited for the realization of guided wave electro-optic modulators based on the Wannier-Stark effect.

In recent years measurement of permittivity signal has been increasingly used for online biomass monitoring of cell cultures. Breweries use it as an established method for fermenter inoculation and bioprocess control for instance [1]. In the case of animal cell cultures the correlation between permittivity and viable cell densities determined offline varies along cultivation time. Hence, several authors have used the permittivity signal as an indirect method for measuring oxygen and glutamine consumption as well as intracellular nucleotide phosphate concentrations [2,3]. The latest generation of biomass monitoring devices allows parallel measurement of permittivity at a range of frequencies leading to an improvement in the correlation between biomass and permittivity and providing a tool to further explore other aspects related to the physiological state of the cells.

An epoxy/carbon nanotubes (CNTs) composite material with a low concentration of multiwalled CNTs (0.5 wt%) has been shown to be applicable in a wide temperature range (up to 160 degrees C) as heating and temperature-sensing element. It can be prepared in any type of geometry allowing a simple application to all kinds of surfaces that have to be sensed and heated. The composite material itself and the electric contacts have demonstrated excellent stability even under extreme ambient conditions. The electrical resistivity of the composite has shown a temperature dependence consistent with the fluctuation-induced tunneling model. This model assumes that the electrical resistance of the nanotube network is dominated by the interconnections between the individual nanotubes rather than by the nanotube resistance itself.

We report on electrical transport and noise processes of Epoxy/multi-wall carbon nanotube composites. The dc measurements suggest that the investigated system can be regarded as a random resistor network in which the resistors are located at the junctions between carbon nanotubes and the nodes are the conducting pathways connecting different junctions, namely the nanotubes themselves. The noise-spectral density measurements reveal the existence of a threshold voltage, above which a markedly nonlinear behavior is found and is tentatively related to the specific properties of the Epoxy-mediated contact barrier among the carbon nanotubes.

A new type of Epoxy/CNT composite has been fabricated based on bisphenol A-epoxy resin, loaded with 0.5wt% of commercial multiwall carbon nanotubes. Exploiting the resulting high conductivity of this composite material, we tested the possibility to use it as absorber of electromagnetic waves at microwave frequencies up to 25 GHz. For this purpose the sample, to be tested, was placed as substrate of a microstrip transmission line. Measurements of the scattering parameters, done with a vectorial network analyzer, have been used in order to obtain the power absorption (PA) spectra. Special attention was paid to the optimization of the microstrip geometry. A comparison of the PA spectrum with the absorption spectra of commercial cavity absorbers showed encouraging results, regarding the microwave absorption capability of the new nanocomposite material.

Electrical current transport and low-frequency noise spectra of multiwall carbon nanotubes in high-density polyethylene matrix have been measured in a temperature range between 10 and 300K. The dc electrical investigations suggest that these composites can be regarded as a random resistor network, where the resistors are formed by tunnel junctions between carbon nanotubes. A crossover of the conduction from a low-field to a high-field regime is found in current-voltage characteristics. In particular, the high-field regime has a strong dependence on carbon nanotube concentration. Noise measurements reveal a standard 1/f behavior due to resistance fluctuations. However, in samples with different concentration of nanotubes, an unusual temperature dependence of the noise is observed. The samples with higher percentage of nanotubes seem to be the most promising ones for devices application, since their noise level is lower in the whole investigated temperature range.

Multilayers of a-Si:N/a-SiN:H and a-Si:H/a-SiC:H were investigated by contactless transient photoconductivity measurements. A faster decay of the photoconductivity was observed in the a-Si:H/a-SiC:H multilayer whereas the a-Si:H/a-SiN:H multilayer was characterized by a slower decay relative to a pure a-Si:H film. These effects are ascribed to defects associated with interface states. The electron mobility in the thin a-Si:H layers of the multilayers does not appear to be appreciably different from that in pure a-Si:H films.

Transport and noise measurements of multiwall carbon nanotubes in high-density polyethylene matrix are reported. In these composites current transport occurs through a random tunnel junctions network, formed by adjacent carbon nanotubes. Low-frequency noise investigations reveal a 1/f behavior induced by resistance fluctuations. An unusual temperature dependence in samples with different nanotube concentration is found. This can be explained by a transition from a fluctuation-induced tunneling mechanism to a thermally activated regime, occurring at increasing nanotube concentration and resulting in a decrease in the overall noise.

An ultra-low cost temperature sensor has been developed that uses as active sensing material an epoxy layer, mixed with a small concentration of multi-walled carbon nanotubes. A stable NTC characteristics has been measured during temperature cycling. Under Joule heating conditions, a linear current-temperature characteristics has been obtained.

Irradiation of unsupported samples of hydrogenated amorphous silicon below 100 K with 360-MeV Xe ions results in macroscopically visible changes in sample dimensions. These changes have two different causes. Formation and growth of cavities lead to an isotropic increase of the specimen dimensions and to a drastic decrease of the mass density. Simultaneously plastic flow occurs, which produces additional but anisotropic changes of the specimen dimensions. The dimensions perpendicular to the beam grow whereas the dimension parallel to the ion beam shrinks. Neither effect saturates in the investigated fluence range (Φt< 1014Xe/cm2) and both are absent in crystalline silicon. The effects are most likely provoked by electronic excitations and/or ionizations in the wake of the fast ions. With respect to plastic flow,a–Si:H behaves like metallic and oxide glasses. Formation of cavities and their growth, however, seem to occur only in tetrahedrally coordinated solids.

Contactless measurements of the transient photoconductivity in {ital a}-Si:H and in {ital a}-Si{sub 1{minus}{ital x}}Ge{sub {ital x}}:H alloys are reported. A decrease of the effective electron mobility with increasing Ge content is observed. Because of a difference of dependence of the excitation density, two different processes are distinguished that lead to this decrease of the mobility. One is attributed to trapping and the other one leads to a decrease of the drift mobility. No increase of the recombination rate upon alloying of {ital a}-Si:H with Ge is observed as, on the contrary, the long-time decay in alloys is slower than in pure {ital a}-Si:H.

Hydrogen profiles for layered a-Si:H structures are presented. Substantial accumulation of hydrogen at the p-i interface is found. Additional incorporation of hydrogen in p-layers is also observed for intermediate boron doping levels compared to intrinsic layers. Hydrogen profiles in single pin and in pinpin amorphous silicon structures enable determination of individual layer thickness.

The optoelectronic properties of a-Si:H films grown at different substrate temperatures between 30°C and 250°C were evaluated in-situ by contactless Time Resolved Microwave Conductivity (TRMC) measurements. The transient photoconductivity induced by 532nm laser pulses during the growth of multilayer systems composed of a-Si:H layers deposited at different temperatures made a direct comparison of these films possible. The maximum height of the TRMC signal was observed to decrease strongly with decreasing deposition temperature. Additionally, the annealing of layers deposited at low temperatures was studied by TRMC, stationary photoconductivity, IR and ESR measurements. Whereas a considerable increase of the photoconductivity and the TRMC signal and a decrease in the spin density was observed during annealing at 250°C, the IR transmission spectra remained unchanged.

At the moment there are efforts to replace the classical transparent conducting coating, Indium Tin Oxide, (or other TCO's) by a new class of materials, which are easier to handle and cheaper in mass production. Possible choices are transparent conductive coatings, which consist of polymers. In this investigation PEDOT:PSS is used. Carbon nanotubes are added in order to render the film electrically conductive. A prerequisite for its use is long-term stability. Therefore the degradation of PEDOT: PSS/ carbon nanotubes as a function of time is determined.

The electrical properties of various lead-free electrically conductive adhesives are investigated. They are intended to replace the solder, which is normally used to connect the interconnector tapes to the busbar. Compared to solder joints conductive adhesives offer the advantage of lower contact formation temperature with reasonable low contact resistances.