Showing papers in "European Physical Journal-applied Physics in 2006"

Out-door testing and long-term stability of plastic solar cells

Abstract: In spite of the high potential of polymer photovoltaic (PV) cells, considerable improvement of their stability under operational conditions needs to be achieved. The few published data on the stability of such cells are devoted to accelerated indoor testing at elevated temperatures. The acceleration factor is undoubtedly dependent on the PV materials, the cell architecture and may vary with the degradation. We report preliminary results of long-term evaluation of PV performance of polymer cells under real sun operational conditions. The studied devices include three types of encapsulated polymer/fullerene cells which differed by the configuration and content of the photoactive layer: (1) bulk heterojunction of MEH-PPV:PCBM; (2) bulk heterojunction of P3HT:PCBM; (3) bilayer heterojunction P3CT-C 60 . The MEHPPV-PCBM cell exhibited the fastest degradation. The degradation of PV performance of the P3HT-PCBM cell was much slower while the P3CT-C 60 device was found to be the most stable. Effect of restoration of I sc and V oc was found when the P3HT-PCBM and P3CT-C 60 cells were kept in the dark overnight. The first I sc and V oc measurements every morning yielded the highest values compared to those during the rest of a day. While I sc recovered only partly and exhibited significant degradation during a month, V oc values recovered completely every night and showed almost no reduction on a long-term time scale.

Carbon nanotube/PEDOT:PSS electrodes for organic photovoltaics

Abstract: High conductive and transparent thin films based on carbon nanotube – poly(3,4-ethylene-dioxythiophene)-poly(styrene sulfonate), PEDOT-PSS blends have been used to replace the conventional indium tin oxide (ITO) as the hole collecting electrode in organic photovoltaic cells. Using PEDOT:PSS as the host material, excellent dispersion of functionalized single wall carbon nanotubes can be achieved enhancing the polymer's conductivity, while maintaining its excellent optical transparency. Photovoltaic cells with Poly(3-hexylthiophene), P3HT and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM as the electron donor and acceptor on respectively on polymer-nanotube substrates have been fabricated and characterized. A power conversion efficiency of 1.3%, with a fill factor of 0.4, an open-circuit voltage of 0.6 V and a short-circuit current of 5.6 mA/cm2 under 100 mW/cm2 white light illumination are reported. These values are close with the reference cells made on ITO glass substrates with the same device structure and fabrication process. The only drawback is on the fill factor which is considerably smaller due to the high resistance of the polymer-nanotube film. Nevertheless, the results indicate that the spin casted polymer-nanotubes thin films are a low cost alternative to ITO for organic electronics.

Rare earth doped III-nitrides for optoelectronics

Abstract: Rare-earth (RE) doped III-nitrides, prepared by in-situ doping during growth or by ion im- plantation and annealing, are promising materials for visible light emitting displays. In addition, they are extremely challenging theoretically, on account of the complexity of the sharp inter-4f optical transitions, which are allowed only through the mixing by non-centrosymmetric crystal fields of the inner 4f orbitals with higher-lying states of opposite parity. We review recent experimental and theoretical work on Er-, Eu- and Tm-doped III-nitride compounds and alloys which has been carried out with a view to establish- ing the lattice location of RE in these materials and the probable nanostructure of the centres which are responsible for their luminescence. The isolated site REIII is found to be both optically and electrically inactive, but in association with neighbouring intrinsic defects (most probably nitrogen vacancies) REIII can generate a small family of similar optically active sites. Such a family is held to be responsible for the site multiplicity that is a common feature of the spectroscopy of RE-doped III-nitrides.

Dual crystallization behaviour of polythiophene/fullerene blends

Abstract: The nanoscale morphology of the active layer in bulk-heterojunction solar cells consisting of regioregular poly(3-hexylthiophene-2, 5-diyl) (P3HT) and methanofullerene([6-6]-phenyl C 61 butyric acid methyl ester) (PCBM) was extensively studied using Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) and optical microscopy. Different weight ratios of P3HT:PCBM were investigated as a function of annealing temperature and time revealing the occurrence of crystallization of both components. Firstly, the as-prepared films can be described as a semi-crystalline blend. Secondly, it has been demonstrated that for a short annealing time (5 min) at lower annealing temperatures (75–115 °C) an increased crystallization of P3HT occurs. Thirdly, it has been observed that a prolonged annealing at the given temperature range or a short annealing at higher temperatures ( ≥ 120 °C) leads to the formation of a new ordered crystalline structure of PCBM. These new ordered structures, a few μ m up to 100 μ m in length, form a network of needle-like and even fan-shaped crystals. Key-parameters to “tune” this new ordered structure of PCBM are blend ratio and annealing conditions. The growth mechanism of these new PCBM-structures is described by means of diffusion.

Controlled growth of Au nanoparticles in co-evaporated metal/polymer composite films and their optical and electrical properties

Abstract: Nanocomposite films containing Au nanoparticles embedded in a polymer matrix were prepared by vapour phase co-deposition of Au and polymers (Teflon AF and Poly( -methylstyrene)) in high vacuum. The microstructure of the composite materials as well as metal content strongly depend on the condensation coefficient of the Au atoms, the deposition rates of the components, the substrate temperature, and the type of polymer matrix. The condensation coefficient, which varies between 0.03 and 1, was determined from energy dispersive X-ray spectrometer (EDX) and surface profilometry. It is shown that the microstructure of nanocomposites (size, size distribution, and interparticle separation of metal clusters), which was determined by transmission electron microscopy, can be controlled by the deposition parameters and the choice of polymer matrix. The optical absorption in the visible region due to the particle plasmon resonance has a strong dependence on the metal filling factor. The correlation between the microstructure of nanocomposites and optical properties, studied using UV-Vis spectroscopy, was also established. Further more, the electrical properties of the composites were studied as a function of the metal volume fraction. It was observed that the nanocomposite films exhibit a percolation threshold at a metal volume fraction of 0.43 and 0.20 for gold nanoclusters in Teflon AF and Poly(α-methylstyrene), respectively.

Solid polymer fuel cell synthesis by low pressure plasmas: a short review

Abstract: In this review, we report on the use of low pressure plasmas for elaborating materials at the heart of solid polymer fuel cells (SPFC), especially electrodes and the membrane electrolyte Electrodes are formed using plasma sputtering techniques while the ion conducting membranes are built up using plasma polymerization Fuel cell performance will be improved by these approaches The electrode catalyst profile is optimized while membrane working temperature is increased and methanol crossover is lowered compared to conventional PEM fuel cells

Plasma sterilization of Geobacillus Stearothermophilus by O2:N2 RF inductively coupled plasma

Abstract: The aim of this work is to identify the main process responsible for sterilization of Geobacillus Stearothermophilus spores in O2 :N2 RF inductively coupled plasma. In order to meet this objective the sterilization efficiencies of discharges in mixtures differing in the initial O2 /N2 ratios are compared with plasma properties and with scanning electron microscopy images of treated spores. According to the obtained results it can be concluded that under our experimental conditions the time needed to reach complete sterilization is more related to O atom density than UV radiation intensity, i.e. complete sterilization is not related only to DNA damage as in UV sterilization but more likely to the etching of the spore.

Hybrid solar cells based on thin-film silicon and P3HT : A first step towards nano-structured devices

Abstract: Hybrid concepts based on a nanoscale combination of organic and inorganic semiconductors are a promising way to enhance the cost efficiency of solar cells through a better use of the solar spectrum, a higher aspect ratio of the interface, and the good processability of polymers. A new type of solar cells has been investigated. It is based on a heterojunction between regio-regular poly(3-hexylthiophene) as an organic electron donor and silicon as an inorganic electron acceptor. In a first step towards nano-structured devices, cells made of flat thin films of these materials have been studied as a model case of the heterojunction. The materials were characterized through ellipsometry and absorption spectroscopy. The devices were studied by means of their spectral response and their I - V characteristics. By combining these results, the contribution of each layer and the mechanisms of photocurrent generation are explained. The best cells to-date show a power conversion efficiency of 1.6% under AM 1.5 illumination, with a V oc of 0.704 V and a J sc of 4.22 mA/cm 2 .

Hybrid organic-inorganic nanomaterials: ligand effects

Abstract: CdSe nanocrystals were modified with small ligand molecules in order to render them compatible with a conducting polymer matrix. Modified NCs were characterized by NMR, optical spectroscopy, electrochemistry. Hybrid materials based on such modified nanocrystals and a conducting polymer (MDMO-PPV, P3HT or P3OT) were prepared, characterized, and tested in photovoltaic devices. Modified NCs quench the polymer PL in solutions as well as in films. Photovoltaic devices display modest power conversion efficiencies mainly due to phase segregation.

Physical and technological limitations of NanoCMOS devices to the end of the roadmap and beyond

Abstract: Since the end of the last millenium, the microelectronics industry has been facing new issues as far as CMOS devices scaling is concerned. Linear scaling will be possible in the future if new materials are introduced in CMOS device structures or if new device architectures are implemented. Innovations in the electronics history have been possible because of the strong association between devices and materials research. The demand for low voltage, low power and high performance are the great challenges for the engineering of sub 50 nm gate length CMOS devices. Functional CMOS devices in the range of 5 nm channel length have been demonstrated. The alternative architectures allowing to increase devices drivability and reduce power consumption are reviewed. The issues in the field of gate stack, channel, substrate, as well as source and drain engineering are addressed. HiK gate dielectric and metal gate are among the most strategic options to consider for power consumption and low supply voltage management. By introducing new materials (Ge, diamond/graphite carbon, HiK, ...), Si based CMOS will be scaled beyond the ITRS as the future System-on-Chip Platform integrating also new disruptive devices. For example, the association of C-diamond with HiK, as a combination for new functionalized Buried Insulators, will bring new ways of improving short channel effects and suppress self-heating. Because of the low parasitics required to obtain high performance circuits, alternative devices will hardly compete against logic CMOS.

Intersubband optical absorption in double quantum well under intense laser field

Abstract: The intersubband absorption in symmetric double quantum wells under the laser field is theoretically calculated within the framework of the effective mass approximation. Results obtained show that intersubbband optical transition and energy levels in double quantum well can significantly be modified and controlled by intense laser field. This statement gives a new degree of freedom in various device applications based on the intersubband transition of electrons.

Donor-acceptor heterojunction solar cells based on perylene dimide and perylene bisbenzimidazole

Abstract: We have fabricated heterojunction solar cells comprising active layers of perylene diimide (PDI) or perylene bisbenzimidazole (CONPER, conjugated perylene dye) as electron acceptor and ZnPC as donor. Bilayer solar cells were produced by successive evaporation of zinc phthalocyanine (ZnPC) and perylene diimide (PDI) or perylene bisbenzimidazole (CONPER) on glass substrates coated with indium doped tin oxide. Active layers with different thickness were evaporated. The bilayer cells were characterized under simulated AM 1.5 illumination (100 mW/cm 2 ). The best results were obtained for the device structure of ITO/PEDOT/ZnPC (40 nm)/perylene bisbenzimidazole (60 nm)/Al (70 nm).

Application of manganese dioxide to electromagnetic wave absorber: effective permittivity and absorbing property

Abstract: This paper presents the application of manganese dioxide to electromagnetic wave absorber. The effective permittivity was calculated by modified Rayleigh mixing formula, and the electromagnetic wave absorbing properties of composites with different volume fractions of manganese dioxide were investigated by the comparison of calculated and experimental values in 2–12 GHz frequencies. It is found that the pure manganese dioxide holds a high dielectric constant, which is a function of frequency. The manganese dioxide composites show high absorbing performance in low frequencies, and the calculated electromagnetic properties fit the experimental values for large volume fractions used in this study.

ITO films realized at room-temperature by Ion Beam Sputtering for high-performance Flexible Organic Light-Emitting Diodes

Abstract: Indium-tin oxide (ITO) thin layers are obtained by an IBS (Ion Beam Sputtering) deposition process. We elaborated ITO films on flexible substrates of polyethylene terephthalate (PET), under soft conditions of low temperatures and fulfilling the requirements of fabrication processes of the organic optoelectronic components. With a non thermally activated (20 °C) ITO deposition assisted by an oxygen flow (1 cm 3 /min), we got an optical transmittance of 90% in the visible range, a resistivity around 10 −3 Ω.cm and a surface roughness lower than 1.5 nm. Thus we realized flexible organic light-emitting diodes (FOLEDs) with good performances: a maximum luminance of 12000 cd/m 2 at a voltage of 19 V and a maximum luminous power efficiency around 1 lm/W at a voltage of 10 V (or a maximum current efficiency of 4 cd/A at 14 V) for the (PET(50 μ m)/ITO(200 nm)/TPD(40 nm)/Alq3(60 nm)/Ca/Al) structure.

Synthesis, characterization and application of donor-acceptor block copolymers in nanostructured bulk heterojunction solar cells

Abstract: The synthesis and properties of two semiconducting donor-acceptor (D-A) block copolymers and their application in nanostructured bulk heterojunction solar cells are reported. The donor segments were obtained via nitroxide mediated polymerization of either bis(4-methoxyphenyl)-4'-vinylphenylamine or N,N'-bis(4-methoxyphenyl)-N-phenyl-N'-4-vinylphenyl-[1,1'biphenyl]-4,4'-diamine. Narrow-distributed macroinitiators, poly{bis(4-methoxyphenyl)-4'-vinylphenylamine} (PvDMTPA) and poly{N,N'-bis(4-methoxyphenyl)-N-phenyl-N'-4-vinylphenyl-[1,1'biphenyl]-4,4'-diamine} (PvDMTPD) were used to polymerize the acceptor monomer perylene diimide acrylate (PerAcr) to yield block copolymers with well-defined molecular weights. Different diblock copolymers, PvDMTPA-block-PPerAcr and PvDMTPD-block-PPerAcr, with high perylene diimide weight fractions were prepared. The block copolymers exhibited efficient fluorescence quenching. Transmission electron microscopy revealed wire-like and worm-like nanostructures throughout bulk samples. Thin film photovoltaic devices showed short circuit currents of up to 1.21 mA/cm2 and power conversion efficiencies η of 0.32% under AM 1.5 illumination conditions.

Structural, AC conductivity and dielectric properties of Sr-La hexaferrite

Abstract: A series of M-type hexaferrite samples with composition Sr 1− x La x Fe 12 O 19 ( x = 0.00, 0.05, 0.15 and 0.25) were prepared by standard ceramic technique. AC electrical conductivity measurements were carried out at different frequencies (20 Hz–1 MHz) and at different temperatures. The dielectric constant and dielectric loss tangent were measured in the same range of frequencies. The experimental results indicate that AC electrical conductivity increases on increasing the frequency as well as the temperature, indicating magnetic semiconductor behavior of the samples. The increase in AC electrical conductivity with frequency and temperature has been explained on the basis of Koops Model whereas dielectric constant and dielectric loss tangent has been explained with the Maxwell–Wagner type interfacial polarization in agreement with the Koops phenomenological theory.

Finite element approach to photon propagation modeling in semi-infinite homogeneous and multilayered tissue structures

Abstract: A computerized model of steady–state photon diffusion within turbid media such as biological tissues, solved by means of the Finite Element Method (FEM) is presented in this paper. Assuming that the different media are illuminated by a flat collimated laser beam source, we develop the basic theory including the suitable boundary conditions along the meshed domain. Model simulations depict firstly photon-flux density patterns in the $r-z$ plane associated with axis fluence rate profiles, plotted as functions of media properties and beam sizes. A second objective was to display both the rate of re-emitted optical power integrated by an optical fibre radially displaced away from the source and the power integrated by an optical fibre axially moved inside the tissue. Simulation studies are further extended to multilayered media such as skin and aorta, in order to describe the light propagation through these tissue structures more realistically.

Characterization of reactively evaporated TiO2 thin films as high and medium index layers for optical applications

Abstract: Reactively evaporated TiO 2 thin films were deposited on the glass substrate by electron beam heating technique. In the first stage evaporation rates are varied keeping substrate temperature fixed at 200 °C and oxygen flow rate at 9.0 sccm for all the samples. The deposition rates were varied from 0.12–0.20 nm/s with a step of 0.02 to optimize the optical constants for the film in the wavelength band of 380–850 nm as a high index material for multilayer optical coatings and filters. The films prepared at higher deposition rates have shown low refractive index values and comparatively higher extinction coefficients for the given substrate temperature in the desired spectral range. The best optical constants were observed for the sample prepared at 0.12 nm/s (at λ = 550 nm: n : 2.39 and k : of the order of 10 −9 ). However, the variation in refractive index for all the samples was found to be in the range of 2.39–2.21 at 550 nm. In the second stage, films were prepared at a fixed deposition rate of 0.12 nm/s at different substrate temperature (50 °C, 100 °C, 150 °C and 200 °C) to study the optical properties of the films as a function of substrate temperature in order to investigate the potential of TiO 2 film application as medium index layer. It was observed that refractive index decreases with decreasing substrate temperature. However, it was not found true for absorption. Instead, at Ts = 50 °C extremely low absorption (of the order of 10 −5 ) has been observed with refractive index of 1.83, which increased sharply for next higher substrate temperature. Variation in absorption characteristics is found with varying substrate temperatures. The result shows that TiO 2 films can be used as high and medium index layer in optical coating by varying the deposition parameters during growth. Effectively, one can achieve performance of two materials with only one. This effect could be utilized to produce variable index layers and coatings. Two samples with best high and medium optical constant values, were further investigated using Atomic force microscopy (AFM), X-ray diffraction (XRD) and scanning electron microscopy (SEM) for surface and structure analysis. The results show that the particle size for the film is very small (10–20 nm), and the surface appears to be very compact, smooth and free of pinholes with roughness in angstrom range, extremely suitable for optical applications in single and multilayer structures.

Characterization of Ta2O5 thin films prepared by reactive evaporation

Abstract: Reactively evaporated thin films of tantalum oxide are prepared on glass substrate, using electron beam heating, for optical applications Firstly, the deposition was carried out at 020 nm/s, with substrate temperature of 200 °C, and oxygen flow rate was varied from 00 to 300 seem to study the effect of flow rate on optical constants The optical constants evaluated by using transmission data of the samples, with curve fitting, show a strong dependence on oxygen flow rate Oxygen flow rate of 100 seem has been found to give reasonably high index (n:211 at λ = 500 nm) and low absorption of the order of 10 -3 in most part of the desired spectrum (380-850 nm) However for oxygen flow rates below 100 seem and above 200 seem the films have exhibited low index and comparatively high absorption In the next step, deposition rates were varied from 010-030 nm/s with steps of 001 keeping oxygen flow rate and substrate temperature constant at 100 seem and 200 °C respectively, to optimize the film properties A variation in refractive index and extinction coefficient values is observed with varying deposition rates An increase in refractive index (n:2125 at λ = 500 nm) with reduced absorption ("k" of the order of 10 -4 ) is achieved over the entire spectrum for the film deposited at 010 nm/s The film was found to be highly adherent to the substrate as revealed by qualitative adhesive tape peel test Keeping in view the application of the work, calculation of optical constants was extended up to 1100 nm for the sample deposited at 010 nm/s The results have shown nearly constant optical constant values over the extended range making the film useful over a broad spectral region APM studies show that the surface is extremely smooth and compact, giving average and rms roughness values of 551 and 7174 A respectively, for the studied area of 25 μ x 25 μ XRD and SEM studies carried out for structural analysis show that the film is generally amorphous with a sign that crystalline structure could be achieved at little higher substrate temperatures The surface of the film is very smooth, free of voids, with the particle size in the range of 17-35 nm, calculated from SEM studies The film has exhibited compact and dense structure The work reported in this paper will be useful in tailoring the optical properties of Ta 2 O 5 as per requirement

Effects of N2 post-discharge plasma treatments on wool fabrics

Abstract: Direct low temperature plasma (LTP) has been studied for some years now as an alternative eco-friendly technology for achieving shrink-resistant wool fabrics, its action being attributed to the varied active species existing in the plasma. In this paper, for the first time, the efficiency of a post-discharge plasma treatment on the shrink-resistance of wool fabrics has been evaluated. The density of active species present in the nitrogen plasma post-discharge (PD) has been determined by optical spectroscopy and their effects on the increase of hydrophilicity at different treatment times have been recorded through drop test and contact angle techniques. Analysis of the surface by X-ray Photoelectron Spectroscopy (XPS) has shown the increase in N and O – containing functionalities. The post-discharge has shown to be effective in reducing the area shrinkage of the fabrics and its effects have been compared to those obtained by direct LTP.

A novel simplified two-dimensional magneto-optical trap as an intense source of slow cesium atoms

Abstract: We describe the design and performance of a slow atom source based on a 2D magneto-optical trap (MOT) that uses an innovative simple optical configuration. Metal-coated retro-reflecting prisms replace mirrors and quarter-wave plates so the optical power of the cooling laser beam is recycled. This source has been characterised for three different configurations: with and without transverse magnetic field gradient, and with a pusher beam to obtain a 2D + -MOT. The longitudinal velocity is of the order of 25 m·s −1 , with a transverse velocity spread ≤1 m·s −1 , while the typical atomic flux density obtained is up to 1.3×10 14 at ·s −1 ·m −2 for a cesium vapour pressure of ~4×10 −8 mbar in the source. We use this slow atom beam, instead of cesium vapour, to load a 3D moving optical molasses that feeds a continuous cold atom fountain. We obtain a gain of a factor ~20 in the atomic flux launched by the fountain.

Finite-element analysis of positive and negative corona discharge in wire-to-plane system

Abstract: This paper deals with positive and negative corona discharge in atmospheric air using a wire-to-plane system. The Poisson's equation is solved numerically using the finite-element method and the current continuity equation. The space charge density is updated using the simplified method of characteristics, where the ion diffusion is neglected. The numerical model takes into account the avalanche length of the corona discharge and the potential corresponding to the minimum ionisation field is directly introduced in the method on the border of the ionisation region. The end geometry of the linear biased probe without wraparound bias plates has been adapted to a wire-to-plane system for use in DC corona discharge. The current density and the electric field are measured; their spatial distributions are given and compared with the computed values. The agreement between the calculated values and those obtained experimentally is satisfactory. The per unit electric field and current density are also represented by a unique function.

Influence of aluminium ions on physical properties of PbO-P2O5-As2O3 glasses

Abstract: PbO-P 2 O 5 -As 2 O 3 glasses containing different concentrations of Al 2 O 3 ranging from 0 to 5 mol% were prepared. Dielectric properties (viz., dielectric constant e', loss tan δ and a.c. conductivity σ ac over a moderately wide range of frequency and temperature and dielectric breakdown strength in air medium), optical properties (optical absorption, IR spectra and thermoluminescence) and differential thermal analysis have been studied. The interesting changes observed in these studies have been explained in the light of different structural groups of aluminium ions.

Transparent conducting ZnO-CdO thin films deposited by e-beam evaporation technique

Abstract: Thin films of Zn Cd x O with x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5 at.% were deposited by electron-beam evaporation technique. It has been found that, for as-deposited films, both the transmittance and electrical resistivity decreased with increasing the Cd content. To improve the optical and electrical properties of these films, the effect of annealing temperature and time were taken into consideration for Zn Cd x O film with x = 0.2. It was found that, the optical transmittance and the electrical conductivity were improved significantly with increasing the time of annealing. At fixed temperature of 300 °C, the transmittance increased with increasing the time of annealing and reached its maximum values of 81% in the visible region and 94% in the NIR region at annealing time of 120 min. The low electrical resistivity of 3.6 × 10 −3 Ω cm was achieved at the same conditions. Other parameters named free carrier concentrations, refractive index, extinction coefficient, plasma frequency, and relaxation time were studied as a function of annealing temperature and time for 20% Cd content.

Electrical model of the atmospheric pressure glow discharge (APGD) in helium

Abstract: This work is a contribution to the understanding of mechanisms controlling the Atmospheric Pressure Glow Discharge (APGD). The approach consists in developing an electrical model of the discharge based on electrical circuit compounds. This model takes into account the main phenomena of the discharge including the memory effect and the creation of the cathode fall. It allows to have a general view of the process and can be easily associated to the power supply with short computational duration; it is also an interesting tool for the optimization of the whole process.

Role of hydrogen diffusion on the growth of polymorphous and microcrystalline silicon thin films

Abstract: Hydrogen diffusion is monitored during plasma conditions corresponding to the growth of poly- morphous and microcrystalline silicon thin films in the temperature range from 50 to 300 ◦ C. Structural changes of the films were monitored in situ by Spectroscopic Ellipsometry measurements and interpreted with the Bruggemann Effective Medium Approximation along with the Tetrahedron Model. For both ma- terials, diffusion of hydrogen leading to the formation of a hydrogen-rich subsurface layer during the first minute of exposure to the hydrogen plasma is observed. This initial phase was followed by a steady-sate regime during which the thickness of the subsurface layer stayed relatively constant and the total film thickness decreased as a function of time. This steady-state is explained as the result of the equilibrium between hydrogen diffusion and etching. The hydrogen diffusion coefficient and etching rate were calcu- lated from these measurements and found to be higher in amorphous than in polymorphous silicon. This result is consistent with the growth dynamics of polymorphous silicon. It also provides information con- cerning the nucleation of micro-crystallites in amorphous silicon under very large atomic hydrogen flow conditions. Moreover, it also explains why polymorphous silicon films do not undergo the phase transition to microcrystalline, even if deposited under relatively similar large atomic hydrogen flow conditions. PACS. 61.43.Dq Amorphous semiconductors, metals, and alloys - 66.30.-h Diffusion in solids - 68.55.Ac Nucleation and growth: microscopic aspects

Light-harvesting fullerenes for organic solar cells

Abstract: Novel dyads containing [60]fullerene-perylenediimide units were developed as light-harvesting acceptors for the preparation of efficient solar cells The antenna was grafted onto C-60 with the aim to improve the absorption spectrum of materials used in bulk-heterojunction devices Electrochemical and photophysical studies of these dyads in solution have revealed that there was no significant ground-state electronic interaction between the covalently bonded PDI and fullerene moieties Steady-state fluorescence experiments evidenced an effective photoinduced energy transfer from the PDI moiety to C-60 The potential use of these light-harvesting fullerenes in organic solar cells was estimated with their incorporation in bulk-heterojunctions using poly(3-hexylthiophene) as the conjugated pi-donor polymer

Algal biosensors for aquatic ecosystems monitoring

Abstract: The harmful effect of toxic chemicals on natural ecosystems has led to an increasing demand for early-warning systems to detect those toxicants at very low concentrations levels. Whole-cell biosensors based either on chlorophyll fluorescence or enzyme (phosphatase and esterase) inhibition are constructed for real-time detection and on-line monitoring. Results show that these devices are sensitive to heavy metals and pesticides. The system allows the cells to operate in their natural environment which favours long term stability and reflects the toxic action mechanism providing therefore an ecological interest.

Inactivation of escherichia coli in the flowing afterglow of an N2 discharge at reduce pressure : study of the destruction mechanisms of bacteria and hydrodynamics of the afterglow flow

Abstract: We present the optimization of a sterilization process with a N 2 flowing afterglow post-discharge First we study the inactivation of bacteria (E coli) submitted to the post-discharge, and discuss the mechanisms of bacteria inactivation Then we evaluate the volume repartition of the active species (N atoms produced by plasma discharge) in the chamber of treatment

Microstructure, dielectric and ferroelectric characterization of Ba doped PLZT ceramics

Abstract: Ba doped PLZT ceramic compositions with formula Pb 0.982− z La 0.012 Δ 0.006 Ba z (Zr 0.55 Ti 0.45 )O 3 for z = 0 to 6 mole% are synthesized by mixed-oxide method. Powder XRD studies indicate the coexistence of rhombohedral and tetragonal phase in 5 mole% Ba doped PLZT composition. SEM pictures show grain size control as Ba concentration increases in PLZT system. and tan δ are increased up to 4 mole% and 5 mole% Ba doped PLZT composition respectively. Dielectric maximum () is decreased to 4 mole% Ba doped PLZT and Curie temperature ( T C ) is decreased with increase of Ba concentration in PLZT system. Coercive field ( E c ) has shown gradual enhancement over series. Remanent ( P r ) and spontaneous ( P s ) polarizations are decreased to 5 mole% Ba doped PLZT composition.