One-dimensional silicon and germanium nanostructures with no carbon analogues

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Photoinduced hole-transfer in semiconducting polymer/low-bandgap cyanine dye blends: evidence for unit charge separation quantum yield

Power-conversion efficiencies of organic heterojunction solar cells can be increased by using semiconducting donor-acceptor materials with complementary absorption spectra extending to the near-infrared region. Here, we used continuous wave fluorescence and absorption, as well as nanosecond transient absorption spectroscopy to study the initial charge transfer step for blends of a donor poly(p-phenylenevinylene) derivative and low-band gap cyanine dyes serving as electron acceptors. Electron transfer is the dominant relaxation process after photoexcitation of the donor. Hole transfer after cyanine photoexcitation occurs with an efficiency close to unity up to dye concentrations of similar to 30 wt%. Cyanines present an efficient self-quenching mechanism of their fluorescence, and for higher dye loadings in the blend, or pure cyanine films, this process effectively reduces the hole transfer. Comparison between dye emission in an inert polystyrene matrix and the donor matrix allowed us to separate the influence of self-quenching and charge transfer mechanisms. Favorable photovoltaic bilayer performance, including high open-circuit voltages of similar to 1 V confirmed the results from optical experiments. The characteristics of solar cells using different dyes also highlighted the need for balanced adjustment of the energy levels and their offsets at the heterojunction when using low-bandgap materials, and accentuated important effects of interface interactions and solid-state packing on charge generation and transport.

A polaronic stacking fault defect model for CaCu3Ti4O12 material: an approach for the origin of the huge dielectric constant and semiconducting coexistent features

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Grain boundary electric characterization of Zn7Sb2O12 semiconducting ceramic: A negative temperature coefficient thermistor

The electrical properties of the grain boundary region of electroceramic sensor temperature based on inverse spinel Zn7Sb2O12 were investigated at high temperature. The zinc antimoniate was synthesized by a chemical route based on the modified Pechini method. The electric properties of Zn7Sb2O12 were investigated by impedance spectroscopy in the frequency range from 5 Hz to 13 MHz and from 250 up to 600 degreesC. The grain boundary conductivity follows the Arrhenius law, with two linear branches of different slopes. These branches exhibit activation energies with very similar values; the low-temperature (less than or equal to350 degreesC) and high-temperature (greater than or equal to400 degreesC) regions are equal to 1.15 and 1.16 eV, respectively. Dissimilar behavior is observed on the relaxation time (tau) curve as a function of temperature, where a single slope is identified. The negative temperature coefficient parameters and nature of the polarization phenomenon of the grain boundary are discussed. (C) 2003 American Institute of Physics.

Femtosecond nonlinear optical properties of lead-germanium oxide amorphous films

The nonlinear (NL) response of lead-germanium oxide amorphous films was investigated using a Ti:saphire laser delivering pulses of approximate to 150 fs at 800 nm. The Kerr shutter technique was employed to reveal the time response of the nonlinearity that is smaller than 150 fs. The sign and magnitude of the nonlinearity were obtained using a novel technique called thermally managed eclipse Z scan which allows the simultaneous characterization of cumulative and noncumulative NL effects. The NL refractive index of electronic origin, n(2)approximate to 2x10(-17) m(2)/W, and the NL absorption coefficient, alpha(2)approximate to 3x10(3) cm/GW, were determined. (c) 2007 American Institute of Physics.

Structural and electrical characterization of semiconducting barium-lead-titanate ceramics

BaTiO3 is usually doped to achieve the temperature stability required by device applications, as well as to obtain a large positive temperature coefficient anomaly of resistivity (PTCR). Uniform distribution of dopants among the submicron dielectric particles is the key for optimal control of grain size and microstructure to maintain a high reliability. The system Ba0.84Pb0.16TiO3 was synthesized from high purity BaCO3, TiO2, PbO oxide powders as raw materials. Sb2O3, MnSO4 and ZnO were used as dopants and Al2O3, TiO2 and SiO2 as grain growth controllers. Phase composition was analyzed by using XRD and the microstructure was investigated by SEM. EDS attached to SEM was used to analyze phase composition specially related to abnormal grain growth. Electrical resistivities were measured as a function of temperature and the PTCR effect characterized by an abrupt increase on resistivity.

Local order around of germanium atoms in Ga10Ge25S65 glass by EXAFS

Irreversible photoexpansion effect has been observed in amorphous Ga10Ge2S65 glasses when its surface was exposed to light with energy greater than the band gap, 3.52 eV. A volume change of about 5% was reached in bulk samples by controlling illumination time and the laser power density. To understand the atomic scale processes of the photoexpansion effect, extended X-ray absorption fine structure (EXAFS) spectroscopy has been used as a local probe of the germanium environment in the glass samples before and after illumination. Modifications are observed in the average coordination shell around Ge atoms in the illuminated sample compared to the non-illuminated one. For the non-illuminated sample, the Ge coordination shell is described by a distorted tetrahedron of sulfur atoms at around 2.20 Angstrom. After illumination, the EXAFS signal can be explained by introducing an additional contribution to this average environment. Based on an analysis of the EXAFS data we proposed a two-shell model of 0.5 oxygen atoms at 2.01 Angstrom and 3.6 sulfur atoms at a 2.20 Angstrom. The existence of Ge-O bonds in the glass after illumination was confirmed by infrared measurements. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Interface morphology snapshots of vertically segregated thin films of semiconducting polymer/polystyrene blends

We present atomic force microscopic images of the interphase morphology of vertically segregated thin films spin coated from two-component mixtures of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) and polystyrene (PS). We investigate the mechanism leading to the formation of wetting layers and lateral structures during spin coating using different PS molecular weights, solvents and blend compositions. Spinodal decomposition competes with the formation of surface enrichment layers. The spinodal wavelength as a function of PS molecular weight follows a power-law similar to bulk-like spinodal decomposition. Our experimental results indicate that length scales of interface topographical features can be adjusted from the nanometer to micrometer range. The importance of controlled arrangement of semiconducting polymers in thin film geometries for organic optoelectronic device applications is discussed. (c) 2007 Elsevier Ltd. All rights reserved.

Hardness and elastic modulus of carbon-germanium alloys

This work reports on the mechanical properties of germanium-rich amorphous carbon-germanium alloys prepared by RF sputtering of a germanium/graphite target under an argon/hydrogen atmosphere. Nano-hardness, elastic modulus and stress were investigated as a function of the carbon content. The stress, which is reduced by the incorporation of carbon, was related to the film structure and to the difference in the Ge-Ge and Ge-C bond length. Contrary to what was expected, the hardness and elastic modulus of the alloys are lower than the corresponding values for pure amorphous hydrogenated germanium film, which in turn has both properties also smaller than those of crystalline germanium. These properties are analyzed in terms of the structural properties of the films. (C) 2001 Elsevier B.V. B.V All rights reserved.

Nonlinear optical properties of antimony-germanium-sulfur glasses at 1560 nm

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

P-type semiconducting gas sensing behavior of nanoporous rf sputtered CaCu3Ti4O12 thin films

The fabrication of nanoporous sputtered CaCu3Ti4O12 thin films with high gas sensitivity is reported in this work. The porous microstructure and the nanocrystalline nature of the material promoted the diffusion of the atmosphere into the film, shortening the response time of the samples. Behaving as p-type semiconductor, the material presents enhanced sensitivity even at low working temperatures. Impedance spectroscopy measurements were performed in order to investigate the mechanisms responsible for the performance of the devices. (C) 2008 American Institute of Physics.

Tunable plasmon resonance modes on gold nanoparticles in Er 3 +-doped germanium-tellurite glass

Relative to the Er3 +:gold-nanoparticle (Er3 +:Au-NP) axis, the polarization of the gold nanoparticle can be longitudinal (electric dipole parallel to the Er3 +:Au-NP axis) or transverse (electric dipole perpendicular to the Er3 +:Au-NP axis). For longitudinal polarization, the plasmon resonance modes of gold nanoparticles embedded in Er3 +-doped germanium-tellurite glass are activated using laser lines at 808 and 488 nm in resonance with radiative transitions of Er3 + ions. The gold nanoparticles were grown within the host glass by thermal annealing over various lengths of time, achieving diameters lower than 1.6 nm. The resonance wavelengths, determined theoretically and experimentally, are 770 and 800 nm. The absorption wavelength of nanoparticles was determined by using the Frohlich condition. Gold nanoparticles provide tunable emission resulting in a large enhancement for the 2H11/2 → 4I13/2 (emission at 805 nm) and 4S 3/2 → 4I13/2 (emission at 840 nm) electronic transitions of Er3 + ions; this is associated with the quantum yield of the energy transfer process. The excitation pathways, up-conversion and luminescence spectra of Er3 + ions are described through simplified energy level diagrams. We observed that up-conversion is favored by the excited-state absorption due to the presence of the gold nanoparticles coupled with the Er3 + ions within the glass matrix. © 2013 Elsevier B.V.

New flexible and transparent solution-based germanium-sulfide polymeric materials

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Thermal inkjet printing of copper tetrasulfonated phthalocyanine (CuTsPc) as a semiconducting layer on flexible MIS capacitors

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Exploiting the versatility of taste sensors based on impedance spectroscopy

The versatility of sensor arrays made from nanostructured Langmuir-Blodgett (LB) and layer-by-layer (LBL) films is demonstrated in two ways. First, different combinations of sensing units are employed to distinguish the basic tastes, viz. sweet, sour, bitter, and salty tastes, produced, respectively, by small concentrations (down to 0.01 g/mol) of sucrose, HCl, quinine, and NaCl solutions. The sensing units are comprised of LB and/or LBL films from semiconducting polymers, a ruthenium complex, and sulfonated lignin. Then, sensor arrays were used to identify wines from different sources, with the high distinguishing ability being demonstrated in principal component analysis (PCA) plots. Particularly important was the fact that the sensing ability does not depend on specific interactions between analytes and the film materials, but a judicious choice of materials is, nevertheless, required for the materials to respond differently to a given sample. It is also shown that the interaction with the analyte may affect the morphology of the nanostructured films, as indicated with scanning electron microscopy. For instance, in wine analysis these changes are not irreversible and the original film morphology is retrieved if the sensing unit is washed with copious amounts of water, thus allowing the sensor unit to be reused.