Degrees Of Doping Research Articles

Fabrication of electrically-conducting nonwoven porous mats of polystyrene–polypyrrole core–shell nanofibers via electrospinning and vapor phase polymerization

This paper describes the growth of polypyrrole (PPy) layers over polystyrene (PS) nanofibers via a vapor phase polymerization process. PS nanofibers were produced through electrospinning of PS solutions containing chemical oxidants capable of polymerizing pyrrole monomers. The electrospun PS nanofibers provided a robust and stable template for growth of conducting polymer nonwoven porous mats. A mixture of tetrahydrofuran, acetone, and butanol was able to dissolve both hydrophobic PS and ionic salts (oxidants). Two chemical oxidants widely used in PPy synthesis – ferric chloride and ferric tosylate – were compared. Upon exposure to pyrrole vapor, both oxidants produced PPy conducting polymers over PS nanofibers. The ferric tosylate containing template fibers showed much faster PPy growth kinetics than the ferric chloride containing fibers. The PPy growth on ferric tosylate containing template fibers produced highly faceted fibers and gave a higher degree of crystallization of PPy than the PPy grown with ferric chloride containing templates. The intrinsic doping of PPy was slightly higher in the PPy layers grown with ferric tosylate. The combination of higher degrees of crystallization and doping resulted in a higher conductivity for PS–PPy nanofiber mats produced with ferric tosylate than those produced with ferric chloride.

Spin-polarization and electronic properties of half-metallic Heusler alloys calculated fromfirst principles

Half-metallic Heusler alloys are amongst the most promising materials for futuremagneto-electronic applications. We review some recent results on the electronicproperties of these compounds. The origin of the gap in these half-metallic alloysand its connection to the magnetic properties are well understood. Changing thelattice parameter slightly shifts the Fermi level. Spin–orbit coupling induces stateswithin the gap but the alloys keep a very high degree of spin polarization at theFermi level. Small degrees of doping and disorder as well as defects with lowformation energy have little effect on the properties of the gap, while temperatureeffects can lead to a quick loss of half-metallicity. Finally, we discuss two specialissues: the case of quaternary Heusler alloys and the half-metallic ferrimagnets.

Effect of doping and disorder on the half metallicity of full Heusler alloys

Heusler alloys containing Co and Mn are amongst the most heavily studied half metallic ferromagnets for future applications in spintronics. Using state-of-the-art electronic structure calculations, we investigate the effect of doping and disorder on their electronic and magnetic properties. Small degrees of doping by substituting Fe or Cr for Mn scarcely affect the half metallicity. A similar effect is also achieved by mixing the sublattices occupied by the Mn and sp atoms. Thus the half metallicity is a robust property of these alloys.

Elastic constant evaluation in piezoelectric ceramics by ultrasonic velocity measurement methods

2- . The ceramic system PZT consists in a binary system x PbZrO3 - (1-x) PbTiO3 (0 < x < 1) called lead zirconate-titanate. It have been noted that doping the system im- proves some of its properties. At the present time, studying materials having different degrees of doping, in the complete diagram of phases of such systems, is of great interest. At the same time, measurement methods such as the thermoelectric, the resonance-antiresonance and hysteresis loop methods, providing accurate determination of properties of these materials, have been developed. The ultrasonic velocity measurement method (UVM) (2) consists in measuring the time spent by a trav- elling pulse inside a material. By improving the measurement equipments, it is possible to apply this method to the investigation of the internal structure of a material. In this work, the results obtained by the application of the UVM to determinate longitudinal and tangential velocities of sound and elastic coeffi- cients during the polarization process of ceramic materials type Pb(Zr0.54Ti0.46)O3+1.0 wt% Nb2O5 (PZTN 1/54/46) are presented. 2 Theoretical developments

Effect of Nitrogen Doping on Sliding Wear Behaviour of Stainless Steel Coatings

Nitrogen-doped austenitic stainless steel coatings have been deposited onto a mild steel substrate by the magnetron sputtering technique, with the purpose to enhance the tribologica1 and chemical properties of the steel. The resultant coatings with different degrees of nitrogen doping have been characterised by X-ray diffraction, scanning electron microscopy and nanoindentation. Dry sliding friction and wear tests have been conducted to investigate the tribological behaviour of the coating-substrate systems. The results show that nitrogen doping stabilises and expands the austenite lattices, which leads to a significant hardening effect in the coating without formation of harmful nitride compound phases. With an appropriate amount of nitrogen doping, the coating exhibits much enhanced sliding wear resistance. However, excessive nitrogen doping leads to the deterioration in tribological performance due to coating embrittlement and reduced coating-substrate integrity.

On the nature of the anisotropy of the resistivity of Nd2−xCexCuO4+δ with different cerium and oxygen concentrations

The results of a study of the temperature dependences of the resistivity and its anisotropy coefficient in the normal phase of single-crystal Nd2−xCexCuO4+δ films with different degrees of cerium doping x and oxygen content δ are presented. Two types of films of identical composition are considered, grown with the orientation of the c axis of the crystal perpendicular to and parallel to the plane of the substrate. The parameters found from an analysis of the conductivity and Hall coefficient are presented for the 18 samples studied.

Electron Paramagnetic Resonance Observation of Critical Behaviors in a Disordered Conducting Polymer System

While most polymers are good insulators, some of them known as conducting polymers show remarkable electrical conductivities upon doping with electron donors or acceptors. The electrical conductivity is generally known to be governed by the one-dimensional diffusion of the chargecarrying solitons or polarons, and the main charge transfer mechanism in conjugated conducting polymers is known to be the intrachain diffusion and interchain hopping of polarons and/or bipolarons. Electron paramagnetic resonance (EPR) is a powerful tool for spin dynamics, and have been employed to study various conducting polymers as they can sensitively reflect the spin carrier motions and interactions. In conjugated conducting polymers like poly[2buthoxy-5-methoxy-1,4-phenylenevinylene] (PBMPV) where the charge carriers are polarons and bipolarons, the EPR intensity is proportional to the density of the spincarrying polarons (spin 1=2, charge e). A pair of polarons can form spinless singlet bipolarons (spin 0, charge 2e), which are energetically more stable. The spin-carrying polarons can give rise to EPR signals, whereas the spinless singlet bipolarons cannot make EPR transitions. Thus EPR can serve as a sensitive probe of spin carriers in the conducting polymers. PBMPV is a derivative of the conjugated PPV (polyphenylenevinylene) polymer. It can be easily doped to controllable degrees of doping and provides a unique opportunity to systematically study the doping as well as conductivity dependence of various properties including the dopant kinetics and spin/charge dynamics. Recently, we have reported a critical behavior associated with the delocalization of the charge-carrier wave function in a series of I2-doped PBMPV conducting polymers by means of nuclear magnetic resonance (NMR) measurements. In this work, we have investigated the charge/spin dynamics in the same series of samples by means of EPR measurements. While there have been some indirect indications of bipolaron formation in some electrochemically synthesized conducting polymers through the EPR and susceptibility measurements, this quantitative and systematic work is believed to represent a comprehensive observation of the collective critical behavior of bipolaron formation and spin–charge carrier delocalization in an iodine-doped conjugated polymer system. The PBMPV conducting polymer samples were prepared by thermal elimination of polyelectrolyte precursor polymer films and iodine-doping, and the iodine concentration was determined from the weight gain after doping. While the doped PBMPV samples are known to have inherent inhomogeneities, relatively thick good quality samples have been obtained according to visual inspection. The electrical conductivity was measured by the standard four-in-lineprobe method. The room temperature EPR measurements were made at 9.4 GHz using an X-band spectrometer, and the magnetic susceptibilities were obtained by doubleintegrating the differential lineshapes. The room temperature electrical conductivity was measured as a function of the degree of I2-doping, and the conductivity displayed a characteristic increase with the doping. A rapid increase of the electrical conductivity was noticed around the dopings of pc1 1⁄4 10 3 I3 /RU (repeating unit) and pc2 1⁄4 10 1 I3 /RU, which can be attributed to the dynamics governing the charge conduction mechanism. Figure 1 shows the doping degree dependence of the spinto-charge ratio obtained from the spin susceptibility, assuming that the charge density is the same as the dopant density per repeating unit of the polymer. Figure 1 displays an initial decrease of the spin-to-charge ratio until it reaches a minimum of less than 10 2 around pc2, which can readily be attributed to the polaron–bipolaron recombination. It is also interesting to note the spin-to-charge ratio minimum, followed by an increase of the ratio, which may be explained by the dissociation of some of the spinless bipolarons to polarons. Figure 2 shows the EPR lineshapes for samples with various degrees of doping. It is also worth noting the abrupt increase of the peak-to-peak EPR linewidth, in Fig. 3, at the same doping degrees where the spin-to-charge ratio shows a minimum, which indicates that they arise from the same spin dynamics. The abrupt increase in the EPR linewidth at pc2 appears to be a lifetime broadening, i.e., line broadening due to the lifetime shortening, arising from the delocalization of the spin/charge-carrier wavefunctions at the percolation threshold. Thus, EPR is shown to reflect sensitively the spin dynamics in our systems. Figure 4 shows the temperature dependencies of the EPR spin susceptibility and the linewidth for a heavily doped sample with p 1⁄4 3 10 . The spin susceptibility undergoes a decrease with decreasing temperature before showing a typical Curie behavior below a critical temperature around which the linewidth shows a characteristic peak. This

Anisotropy in Thermoelectric Properties of CsBi4Te6

ABSTRACTCsBi4Te6 (ZT ∼ 0.8 at 225 K) shows highly anisotropic features in its crystal morphology and structure as expressed by the parallel infinite [Bi4Te6] rods which are linked via Bi-Bi bonds. Band calculations also point to a significant anisotropy in the carrier effective masses, and for this reason we examined the anisotropic thermoelectric properties of CsBi4Te6. The electrical conductivity, thermopower and thermal conductivity were measured along the three different crystallographic directions of the monoclinic structure of CsBi4Te6. These measurements were performed on samples with different degrees of doping. The strong charge transport anisotropy of these samples was confirmed and also observed that the thermopower values along the c-axis direction (which is perpendicular to the layer of Cs atoms) was negative (-80 μV/K) while those along the needle direction (b-axis) and parallel to the [Bi4Te6] layers (a-axis) were p-type (50–100 μV/K at room temperature. Other anisotropic features in the crystal growth habit, electronic band structure, and electrical and thermal conductivities are also presented.

Spectroscopic study of polycrystalline TiO2 doped with vanadium

The structure of coordination sites (V4+ ions) and their spatial distribution in the polycrystalline titanium dioxide (rutile) lattice were studied by ESR. It was found that at low degrees of doping, at [V4+] 0.5 at.% a new microphase with the mixed composition {TiO2—VO2} is formed. The mixed microphase has a noticeably narrower band gap than the initial TiO2. Comparison of the photocurrent spectra and the plots of the integral photocurrent vs. vanadium content with the structural data obtained using ESR spectroscopy showed that the formation of the {TiO2—VO2} microphases deteriorates the photoelectrochemical properties of the modified photoelectrodes. Synthetic procedures interfering the formation of such microphases in the doped rutile are discussed.

Resonant Raman spectroscopy of ethylene-acetylene copolymers doped with iodine

The relative contents of short and long conjugated chains in ethylene-acetylene copolymers (EAC) are determined by varying the lasing wavelength, as was done earlier for pure poly(acetylene). The Raman spectra of the copolymer samples doped with iodine contain the bands attributed to the iodine polyions I3− and I5− Unlike poly(acetylene), the ethylene-acetylene copolymer is characterized by the Raman spectra in which the intensities of the bands assigned to the I5− polyions are higher than those of the I3− polyions even at low degrees of doping. The structural features responsible for this difference are discussed.

Self-absorption effects in a LEC with low Stokes shift

The electroluminescence spectra both in forward and reverse directions of a light-emitting electrochemical cell prepared from a methyl-substituted ladder-type poly(paraphenylene) (mLPPP), blended with a crown ether, Dicyclohexano18crown6 (DCH18C6), and a Li salt, lithium trifluoromethanesulfonate (litriflate, LiTf for short) are compared. Different from the forward direction, in reverse direction the spectrum is strongly influenced by self-absorption effects. We discuss this behavior by different degrees of doping of the p- and n-type doped zones. This effect can be applied for a new method to realize bias-dependent color emission from single-layer devices at low voltages that can be prepared by only one simple and easy spin-coating process.

Real structure of Cd2Nb2O7 pyrochlore single crystals

X-ray topography and diffractometry are used to study the crystal structure of cadmium pyroniobate Cd2Nb2O7 at room temperature. Structural quality parameters were determined for crystals grown with different degrees of impurity doping. The nature of the crystal lattice damage is analyzed as a function of the type and concentration of impurities.

Drift mobility of photogenerated current carriers in Bi12GeO20 crystals

The drift mobility of photogenerated electrons and holes in Bi12GeO20 crystals with different degrees of doping is investigated experimentally.

Raman intensity study of scaling properties of Na+, K+, Ag+, and Pb2+ doped NH4NO3 crystals during phase transitions

It is observed that, for molecular crystals, during phase transitions the Raman intensities of the vibrational modes obey the power law |T−Tc|β. The exponent β contains much critical information and is mode dependent. The scaling properties of the doped materials are imbedded in β which is found to be very sensitive to very small degrees of doping. One of the eminent phenomena is the so-called crossover point where β of the totally symmetric ν1 mode equals the nonsymmetric ν4 mode of the nitrate ion. The scaling is expressed through the scaled coordinate d/M1/2 where d is the averaged interdoped-cationic distance and M is the mass of the cation. β of the ν1 mode behaves as (d/M1/2)γ with exponent γ rather independent of the transitions and the kinds of doping. Resonant modes due to doped cations are employed to explain the origin of the scaling.

Electrical properties of polybithiophene—polystyrene composites

Conducting composites consisting of polybithiophene and based on porous crosslinked polystyrene as host polymer have been synthetized in the vicinity and above the percolation threshold by oxidative polymerization with FeCl3. Electrical conductivity and thermoelectric power measurements for different degrees of doping have been carried out in the temperature range 80–300 K. The electrical conductivity variations are weakly thermally activated while the thermoelectric power has metallic magnitude with positive sign and increases with temperature. Conduction mechanisms are interpreted on the basis of an hopping model involving bipolaronic clusters. © 1996 John Wiley & Sons, Inc.

Electronic structure of buried Si layers in GaAs(001) as studied by soft-x-ray emission.

This thesis addresses the electronic structure of molecules and solids using resonant X-ray emission and photoemission spectroscopy. The use of monochromatic synchrotron radiation and the improved performance of the instrumentation have opened up the possibility of detailed analyses of the response of the electronic systems under interaction with X-rays. The experimental studies are accompanied by numerical ab initio calculations in the formalism of resonant inelastic scattering. The energy selectivity has made it possible for the first time to study how the chemical bonds in a molecule break up during resonant inelastic X-ray scattering. In the conjugated polymer systems, the element selectivity of the X-ray emission process made it possible to probe the different atomic elements separately. The X-ray emission technique proved to be useful for extracting isomeric information, and for measuring the change in the valence levels at different degrees of doping. In this thesis, spectral satellite features in transition metals were thoroughly investigated for various excitation energies around a core-level threshold. By measuring the relative spectral intensity of the satellites it was possible to extract information on the partial core-level widths. Using the nickel metal system as an example, it was shown that it is possible to probe the different core-excited states close toshake-up thresholds by measuring the relative spectral intensity variation of the Auger emission.Resonant photoemission measurements showed unambiguous evidence of interference effects. Theseeffects were also thoroughly probed using angle-dependent measurements. The combination of X-rayemission and absorption were useful for studying buried layers and interfaces due to the appreciable penetration depth of soft X-rays. X-ray scattering was further found to be useful for studying low-energy excited states of rare earth metallic compounds and transition metal oxides.

Dimensional Instability of Doped Lanthanum Chromites in an Oxygen Pressure Gradient

The volume expansion of samples of La1−yCayCrO3−δ in reducing atmosphere was measured for various degrees of doping. Combining the experimental data with calculated profiles of oxygen partial pressure, pO2, through plates exposed to pO2 differences relevant for SOFC interconnectors, the overall expansion of the interconnector–plate, as well as the resulting stresses in the stack are estimated. The behaviour of Ca– and Sr–doped material is compared, and the optimal degree of doping, as judged from a mechanical point of view, is discussed.

Efficient luminescence from zeolites

Zeolites as potential luminescent materials is a subject that has received little attention previously. The optical properties–UV emission and reflectance/absorbance spectra–of zeolites X and Y prepared with various degrees of Ce doping are described and an interpretation of the results is proposed based on the occupation of the various zeolitic coordination sites (see Figure). magnified image

Effect of vacuum annealings on the electronic properties of clean Si(111) surfaces.

Clean cleaved Si(111) samples of various degrees of doping have been vacuum annealed at increasing temperatures, up to 1150 \ifmmode^\circ\else\textdegree\fi{}C and studied in situ by photoemission-yield spectroscopy, with low-energy electron-diffraction and Auger-electron-spectroscopy controls. The photoyield-spectrum changes upon annealing of 7\ifmmode\times\else\texttimes\fi{}7 reconstructed surfaces are interpreted as a sharp increase of the density of acceptor levels in the surface layer probed by photoemission. The effect starts above 700 \ifmmode^\circ\else\textdegree\fi{}C and saturates beyond 1050 \ifmmode^\circ\else\textdegree\fi{}C at (2\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}${10}^{19}$ acceptor states ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$; contamination decreases this p-type overdoping effect. It is attributed to the formation of the appropriate number of Si vacancies, which relaxes, and therefore measures, the residual surface stress of the perfect 7\ifmmode\times\else\texttimes\fi{}7 reconstruction.

Impurity scattering effects on electron transport in short GaAs channels

As high-speed requirements for field-effect transistors (FETs) force the active channel regions to become shorter, the doping in the channel is expected to be increased to maintain a high aspect ratio and gate voltage control in the device. In this paper we have examined electron transport in submicron GaAs FET channels with varying degrees of doping. We find that impurity scattering has a strong effect on electron transport, not only at low electric fields but also at high fields when the channel length is less than a micron. These results are different from those in long channels where at high fields (≥4 kV/cm) impurity scattering is relatively unimportant. We find, for example, that for a 0.25 μm channel, the peak velocity decreases from 4.9×107 cm sec−1 for undoped channel to 3.7×107 cm sec−1 for a doping of 8×1017 cm−3. Results are presented for 0.25- and 0.5-μm channels and compared to those for a 2-μm channel.