Dielectric Function of Undoped and Doped Poly2-methoxy-5-(3 `,7 `-dimethyloctyloxy)-1,4-phenylene-vinylene] by Ellipsometry in a Wide Spectral Range

Ellipsometric measurements in a wide spectral range (from 0.05 to 6.5 eV) have been carried out on the organic semiconducting polymer, poly2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene-vinylene] (MDMO-PPV), in both undoped and doped states. The real and imaginary parts of the dielectric function and the refractive index are determined accurately, provided that the layer thickness is measured independently. After doping, the optical properties show the presence of new peaks, which could be well-resolved by spectroscopic ellipsometry. Also for the doped material, the complex refractive index, with respect to the dielectric function, has been determined. The broadening of the optical transitions is due to the delocalization of polarons at higher doping level. The detailed information about the dielectric function as well as refractive index function obtained by spectroscopic ellipsometry allows not only qualitative but also quantitative description of the optical properties of the undoped/doped polymer. For the direct characterization of the optical properties of MDMO-PPV, ellipsometry turns out to be advantageous compared to conventional reflection and transmission measurements.

Correlating defect induced ferromagnetism and gas sensing properties of undoped tin oxide sensors

A correlation between gas sensing properties and defect induced Room Temperature Ferromagnetism (RTFM) is demonstrated in non-stoichiometric SnO2 prepared by solution combustion method. The presence of oxygen vacancies (V-O), confirmed by RTFM is identified as the primary factor for enhanced gas sensing effect. The as-prepared SnO2 shows high saturation magnetization of similar to 0.018 emu/g as compared to similar to 0.002 and similar to 0.0005 emu/g in annealed samples and SnO2 prepared by precipitation respectively. The SnO2 prepared by precipitation which is an equilibrium method of synthesis shows lesser defects compared to the combustion product and hence exhibits lesser sensitivity in spite of smaller crystallite size. The study utilizes RTFM as a potential tool to characterize metal oxide gas sensors and recognizes the significance of oxygen vacancies in sensing mechanism over the microstructure. (C) 2014 AIP Publishing LLC.

Comparison of the photocatalytic degradation of trypan blue by undoped and silver-doped zinc oxide nanoparticles

Zinc oxide (ZnO) and silver doped zinc oxide (ZnO:Ag) nanoparticles were prepared using nitrates of zinc and silver as oxidizers and ethylene diaminetetraacetic acid (EDTA) as a fuel via low-temperature combustion synthesis (LCS) at 500 degrees C. X-ray diffraction (XRD) pattern indicates the presence of silver in the hexagonal wurtzite structure of ZnO. Fourier transform infrared (FTIR) spectrum indicates the presence of Ag-Zn-O stretching vibration at 510 cm(-1). Transmission electron microscopy (TEM) images shows that the average particle size of ZnO and ZnO:Ag nanoparticles were found to be 58 nm and 52 nm, respectively. X-ray photoelectron spectroscopy (XPS) data clearly indicates the presence of Ag in ZnO crystal lattice. The above characterization techniques indicate that the incorporation of silver affects the structural and optical properties of ZnO nanoparticles. ZnO:Ag nanoparticles exhibited 3% higher photocatalytic efficiency than pure ZnO nanoparticles. ZnO:Ag nanoparticles show better photocatalytic activity for the degradation of trypan blue (TrB) compared to undoped ZnO nanoparticles. (C) 2014 Elsevier Ltd. All rights reserved.

Defect induced optical bandgap narrowing in undoped SnO2 nanocrystals

Unusual optical bandgap narrowing is observed in undoped SnO2 nanoparticles synthesized by the solution combustion method. The estimated crystallite size is nearly 7 nm. Though the quantum confinement effect predicts a larger optical bandgap for materials with small crystallite size than the bulk, the optical bandgap in the as synthesized materials is found to be 2.9 eV compared to the reported value of 3.6 eV for bulk SnO2 particles. The yellow-green photoluminescence emissions and the observed narrowing of the bandgap can be attributed to the deep donor levels of oxygen vacancies, owing to the high exothermicity of the combustion reaction and the faster cooling rates involved in the process. (C) 2013 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Ferroelectric origin in one-dimensional undoped ZnO towards high electromechanical response

Ferroelectricity in ZnO is an unlikely physical phenomenon. Here, we show ferroelectricity in undoped 001] ZnO nanorods due to zinc vacancies. Generation of ferroelectricity in a ZnO nanorod effectively increases its piezoelectricity and turns the ZnO nanorod into an ultrahigh-piezoelectric material. Here using piezoelectric force microscopy (PFM), it is observed that increasing the frequency of the AC excitation electric field decreases the effective d(33). Subsequently, the existence of a reversible permanent electric dipole is also found from the P-E hysteresis loop of the ZnO nanorods. Under a high resolution transmission electron microscope (HRTEM), we observe a zinc blende stacking in the wurtzite stacking of a single nanorod along the growth axis. The zinc blende nature of this defect is also supported by the X-ray diffraction (XRD) and Raman spectra. The presence of zinc vacancies in this basal stacking fault modulates p-d hybridization of the ZnO nanorod and produces a magnetic moment through the adjacent oxygen ions. This in turn induces a reversible electric dipole in the non-centrosymmetric nanostructure and is responsible for the ultrahigh-piezoelectric response in these undoped ZnO nanorods. We reveal that this defect engineered ZnO can be considered to be in the competitive class of ultrahigh-piezoelectric nanomaterials for energy harvesting and electromechanical device fabrication.

Modeling of Channel Potential and Subthreshold Slope of Symmetric Double-Gate Transistor

A new physically based classical continuous potential distribution model, particularly considering the channel center, is proposed for a short-channel undoped body symmetrical double-gate transistor. It involves a novel technique for solving the 2-D nonlinear Poisson's equation in a rectangular coordinate system, which makes the model valid from weak to strong inversion regimes and from the channel center to the surface. We demonstrated, using the proposed model, that the channel potential versus gate voltage characteristics for the devices having equal channel lengths but different thicknesses pass through a single common point (termed ``crossover point''). Based on the potential model, a new compact model for the subthreshold swing is formulated. It is shown that for the devices having very high short-channel effects (SCE), the effective subthreshold slope factor is mainly dictated by the potential close to the channel center rather than the surface. SCEs and drain-induced barrier lowering are also assessed using the proposed model and validated against a professional numerical device simulator.

Phase conversions in calcium manganites with changing Ca/Mn ratios and their influence on the electrical transport properties

The phase-interconversions between the spinel-, brownmillerite-, defect rocksalt and perovskite-type structures have been investigated by way of (i) introducing deficiency in A-sites in CaxMn2-xO3 (0.05 <= x <= 1) i.e., by varying Ca/Mn ratio from 0.025 to 1 and (ii) nonstoichiometric CaMnO3-delta (CMO) with 0.02 <= delta <= 1. The temperature dependence of resistivity (rho-T) have been investigated on nonstoichiometric CaMnO3-delta (undoped) as well as the CMO substituted with donor impurities such as La3+, Y3+, Bi3+ or acceptor such as Na1+ ion at the Ca-site. The rho-T characteristics of nonstoichiometric CaMnO3-delta is strongly influenced by oxygen deficiency, which controls the concentration of Mn3+ ions and, in turn, affects the resistivity, rho. The results indicated that the substitution of aliovalent impurities at Ca-site in CaMnO3 has similar effects as of CaMnO3-delta ( undoped) annealed in atmospheres of varying partial pressures whereby electron or hole concentration can be altered, yet the doped samples can be processed in air or atmospheres of higher P-O2. The charge transport mechanisms of nonstoichiometric CaMnO3-delta as against the donor or acceptor doped CaMnO3 (sintered in air, P-O2 similar to 0.2 atm) have been predicted. The rho (T) curves of both donor doped CaMnO3 as well as non-stoichiometric CaMnO3-delta, is predictable by the small polaron hopping (SPH) model, which changes to the variable range hopping (VRH) at low temperatures whereas the acceptor doped CaMnO3 exhibited an activated semiconducting hopping ( ASH) throughout the measured range of temperature (10-500 K).

Einstein Relation in n-Channel Inversion Layers of Nonlinear Optical, Optoelectronic and Related Materials: Simplified Theory, Relative Comparison and Suggestion for an Experimental Determination

In this paper, we study the Einstein relation for the diffusivity to mobility ratio (DMR) in n-channel inversion layers of non-linear optical materials on the basis of a newly formulated electron dispersion relation by considering their special properties within the frame work of k.p formalism. The results for the n-channel inversion layers of III-V, ternary and quaternary materials form a special case of our generalized analysis. The DMR for n-channel inversion layers of II-VI, IV-VI and stressed materials has been investigated by formulating the respective 2D electron dispersion laws. It has been found, taking n-channel inversion layers of CdGeAs2, Cd(3)AS(2), InAs, InSb, Hg1-xCdxTe, In1-xGaxAsyP1-y lattice matched to InP, CdS, PbTe, PbSnTe, Pb1-xSnxSe and stressed InSb as examples, that the DMR increases with the increasing surface electric field with different numerical values and the nature of the variations are totally band structure dependent. The well-known expression of the DMR for wide gap materials has been obtained as a special case under certain limiting conditions and this compatibility is an indirect test for our generalized formalism. Besides, an experimental method of determining the 2D DMR for n-channel inversion layers having arbitrary dispersion laws has been suggested.

Influence of cobalt doping on superconducting transition in as-grown YBCO single crystals

The role of a charge buffer layer in the superconductivity of high-T-c materials is best studied by cationic substitutions. In this work, the chain copper in YBCO single crystals is substituted by Co3+ ion and consequent effect on superconducting transition temperature (T-c) studied. The T-c is measured using non-resonant Microwave Absorption technique, which is a highly sensitive and contactless method. It is seen that T-c of as-grown crystals is considerably enhanced by cobalt doping in low concentration regime. In contrast, higher T-c is achieved in undoped crystals only after extended oxygen anneal. When dopant concentration increases beyond an optimal value, T-c decreases and the system does not show superconductivity when cobalt content is high (x > 0.5 in YBa2Cu3-xCOxO7+/-delta). This behaviour consequent to cobalt substitution is discussed with reference to the apical oxygen model. Optimal cobalt doping can be thought of as an alternative to extended oxygen anneal in as-grown crystals of YBCO.

Electrical transport in layered crystalline semiconductors GeS doped with Ag, P impurities at high pressure

A study of the transport properties of layered crystalline semiconductors GeS (undoped and doped with Ag, P impurity) under quasihydrostatic pressure using Bridgman anvil system is made for the first time. Pressure-induced effects in undoped crystals reveal initial rise in resistivity followed by two broad peaks at higher pressures. Silver doping induces only minor changes in the behaviour except removing the second peak. Phosphorous impurity is found to have drastic effect on the transport properties. Temperature dependence of the resistivity exhibits two activation energies having opposite pressure coefficients. Results are discussed in the light of intrinsic features of the layered semiconductors.

Pressure induced effects in bulk amorphous n-type semiconductors(GeSe3.5)100âxBix

The effect of pressure on the electrical resistivity of amorphous n-type (GeSe3.5)100�xBix been studied in a Bridgeman anvil system up to a pressure of 90 kbar down to liquid nitrogen temperature. A continuous amorphous semiconductor to metal-like solid transition in the undoped GeSe3.5 is observed at room temperature. Incorporation of Bi in the GeSe3.5 network is found to significantly disturb the behaviour of the resistivity with pressure. With increasing Bi concentration a much broader variation in resistivity with pressure is observed. The temperature dependence of the resistivity and activation energy at different pressures is also measured and they are found to be composition dependent. Results are discussed in the light of the Phillips Model of ordered clusters in chalcogenide semiconductors.

Einstein relation in n-i-p-i and microstructures of nonlinear optical, optoelectronic and related materials: Simplified theory, relative comparison and suggestions for an experimental determination

We investigate the Einstein relation for the diffusivity-mobility ratio (DMR) for n-i-p-i and the microstructures of nonlinear optical compounds on the basis of a newly formulated electron dispersion law. The corresponding results for III-V, ternary and quaternary materials form a special case of our generalized analysis. The respective DMRs for II-VI, IV-VI and stressed materials have been studied. It has been found that taking CdGeAs2, Cd3As2, InAs, InSb, Hg1−xCdxTe, In1−xGaxAsyP1−y lattices matched to InP, CdS, PbTe, PbSnTe and Pb1−xSnxSe and stressed InSb as examples that the DMR increases with increasing electron concentration in various manners with different numerical magnitudes which reflect the different signatures of the n-i-p-i systems and the corresponding microstructures. We have suggested an experimental method of determining the DMR in this case and the present simplified analysis is in agreement with the suggested relationship. In addition, our results find three applications in the field of quantum effect devices.

Gelatin hydrogel electrolytes and their application to electrochemical supercapacitors

Gelatin hydrogel electrolytes (GHEs) with varying NaCl concentrations have been prepared by cross-linking an aqueous solution of gelatin with aqueous glutaraldehyde and characterized by scanning electron microscopy, differential scanning calorimetry, cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic chronopotentiometry. Glass transition temperatures for GHEs range between 339.6 and 376.9 K depending on the dopant concentration. Ionic conductivity behavior of GHEs was studied with varying concentrations of gelatin, glutaraldehyde, and NaCl, and found to vary between 10(-3) and 10(-1) S cm(-1). GHEs have a potential window of about 1 V. Undoped and 0.25 N NaCl-doped GHEs follow Arrhenius equations with activation energy values of 1.94 and 1.88 x 10(-4) eV, respectively. Electrochemical supercapacitors (ESs) employing these GHEs in conjunction with Black Pearl Carbon electrodes are assembled and studied. Optimal values for capacitance, phase angle, and relaxation time constant of 81 F g(-1), 75 degrees, and 0.03 s are obtained for 3 N NaCl-doped GHE, respectively. ES with pristine GHE exhibits a cycle life of 4.3 h vs 4.7 h for the ES with 3 N NaCl-doped GHE. (c) 2007 The Electrochemical Society.

Influence of quantum confinement on the photoemission from superlattices of optoelectronic materials

We study the photoemission from quantum wire and quantum dot superlattices with graded interfaces of optoelectronic materials on the basis of newly formulated electron dispersion relations in the presence of external photo-excitation. Besides, the influence of a magnetic field on the photoemission from the aforementioned superlattices together with quantum well superlattices in the presence of a quantizing magnetic field has also been studied in this context. It has been observed taking into account HgTe/Hg1-xCdxTe and InxGa1-xAs/InP that the photoemission from these nanostructures increases with increasing photon energy in quantized steps and exhibits oscillatory dependences with the increase in carrier concentration. Besides, the photoemission decreases with increasing light intensity and wavelength, together with the fact that said emission decreases with increasing thickness exhibiting oscillatory spikes. The strong dependences of the photoemission on the light intensity reflects the direct signature of light waves on the carrier energy spectra. The content of this paper finds six applications in the fields of low dimensional systems in general. (C) 2010 Elsevier Ltd. All rights reserved.

ZnO nanocrystalline thin films: a correlation of microstructural, optoelectronic properties

The compositional, structural, microstructural, dc electrical conductivity and optical properties of undoped zinc oxide films prepared by the sol-gel process using a spin-coating technique were investigated. The ZnO films were obtained by 5 cycle spin-coated and dried zinc oxide films followed by annealing in air at 600 A degrees C. The films deposited on the platinum coated silicon substrate were crystallized in a hexagonal wurtzite form. The energy-dispersive X-ray (EDX) spectrometry shows Zn and O elements in the products with an approximate molar ratio. TEM image of ZnO thin film shows that a grain of about 60-80 nm in size is really an aggregate of many small crystallites of around 10-20 nm. Electron diffraction pattern shows that the ZnO films exhibited hexagonal structure. The SEM micrograph showed that the films consist in nanocrystalline grains randomly distributed with voids in different regions. The dc conductivity found in the range of 10(-5)-10(-6) (Omega cm)(-1). The optical study showed that the spectra for all samples give the transparency in the visible range.