Maximum Doping Level Research Articles

X-ray photoelectron spectroscopy analysis for the chemical impact of solvent addition rate on electromagnetic shielding effectiveness of HCl-doped polyaniline nanopowders

An in-depth analysis of the chemical functionality in HCl-doped polyaniline (PANI) nanopowders is discussed through interpretations of x-ray photoelectron spectra. The distinctions between three PANI sample types, produced under varied synthesis conditions, are compared on the basis correlations between newly collected electron spectra for chemical analysis (or also x-ray photoelectron spectroscopy) and electromagnetic (EM) shielding effectiveness (SE) within two frequency bands (100–1500 MHz and ∼2–14 GHz). The findings are discussed with reference to previous data analysis of electrical conductivities and Raman and UV-vis spectra analyzed from replicates of the same PANI nanopowders, where only the 8–12 GHz range for SE was tested. They further corroborate previous results for limited-solvent conditions that enhance EM shielding. The three nanopowder types show distinctive differences in polaron, bipolaron, and polar lattice contributions. The collective findings describe the chemical connections between controlling and, most importantly, limiting the available solvent for polymerization with simultaneously doping and how it is that the newly developed solvent-limited approach for HCl-PANI nanopowders provides better shielding than traditionally solvent-rich methods by having more extended and perhaps even faster polaron delocalization than other PANI-based products. The maximum oxidation (50%) and doping (49%) levels obtained in the solvent-free nanopowders also produced the highest SE values of 37.3 ± 3.7 dB (MHz band) and 68.6 ± 4.6 dB (GHz band).

Efficient nitrogen doping of graphene by plasma treatment

Doping of pristine materials can change their chemical and electrical properties. Namely nitrogen doping of graphene results in modulation of electronic properties of graphene. In this work we present experimental results on nitrogen doped graphene fabricated in two steps. At first, the graphene samples were synthesized by a chemical vapor deposition method on copper foils. Then they were treated with ammonia radio frequency discharge plasma. The prepared samples were investigated by atomic-force microscopy (AFM), scanning electron microscopy (SEM), Raman spectroscopy, optical absorption spectroscopy including Fourier transform infrared spectroscopy (FTIR), X-ray and ultraviolet photoelectron spectroscopy. In doped graphene a dependence of N-atom concentration on the treatment parameters has been revealed. A maximum doping level of 3 atomic % has been obtained and the shift of valence band maximum of 0.2 eV was observed at this concentration of nitrogen.

Fuel Cell Application of High Temperature Polymer Electrolyte Membranes Obtained by Graft Copolymerization of Acrylic Acid and 2-Hydroxyethylmethacrylate on ETFE Backbone Material

Proton exchange membranes for high temperature fuel cell applications were obtained via graft copolymerizations on commercial poly(ethylene-alt-tetrafluoroethylene) (ETFE) films. ETFE was activated by electron beam treatment, and the hydrophilic monomers acrylic acid and hydroxyethyl methacrylate were grafted to ETFE. The maximum grafting level was 620%. The grafted membranes were doped with the proton carrier phosphoric acid leading to a maximum doping level of 450%. Stress–strain measurements indicate good mechanical stability of the composite membranes. The polymer–acid membranes were tested in a H2/O2 fuel cell. At a current density of 200 mA · cm−2 a stable power density of 113 mW · cm−2 was determined at 120 °C. The results show that high phosphoric acid contents associated with high proton conductivity may be obtained with a copolymer containing acrylic acid.

Novel concept of polymer electrolyte membranes for high-temperature fuel cells based on ETFE grafted with neutral acrylic monomers

High temperature proton exchange membranes were synthesized via electron beam treatment of commercial poly(ethylene-alt-tetraflouroethylene) (ETFE) films and subsequent graft radical copolymerizations with 2-hydroxyethyl methacrylate (HEMA) and 2-hydroxyethyl acrylate (HEA). The maximum degree of grafting achieved was 193%. Both grafted monomers are hydrophilic and lead to high membrane affinity to phosphoric acid. Doping with phosphoric acid resulted in a maximum doping level of 310%. The grafted membranes combine stable ETFE backbone polymer with hydrophilic side chains. As indicated by stress–strain curves the graft copolymer membranes show good mechanical stability. In addition, the polymer–acid-composites are thermally stable up to around 210°C. The polymer–acid-composite materials were tested in H2/O2 fuel cells at 120°C. Power densities of up to 108mWcm‒2 were obtained at a current density of 200mAcm‒2. It is shown that the alternate concept of polymer–acid composites without any basic units is also suitable for the application in high temperature fuel cells.

Co-co-doping Effect on Superconducting Properties of 112-Type Ca0.8La0.2FeAs2 Single Crystals

We systematically investigated the Co-co-doping effects in Ca0.8La0.2FeAs2 superconductors. The superconducting transition temperature (Tc) decreases almost linearly with increasing Co content. Simultaneously, it is found that the (Ca,La)112 phase is so sensitive to the Co doping level that chemical phase separation becomes more and more apparent as a result of formation of the (Ca,La)122 phase. The maximum Co doping level for 112 phase seems very low, indicating a quite cruel growth condition for 112 compared with other IBSs.

(Invited) AlGaN Higher Power Devices

The maximum power that GaN and SiC devices can routinely handle with their present designs is ~105 watts. This is determined by the fact that for a given breakdown voltage, VB, the maximum doping level, nmax (for electrons) and therefore the current density, is limited primarily by the critical filed, ξC, and access to the carrier as defined by the donor, ED, or acceptor, EA, depth. To a lesser extent, it is also limited by the carrier – usually the electron – mobility, μn. GaN, and the III-V semiconductors in general, have an advantage over group IV semiconductors in that they usually form continuous alloys so that ξC can be increased incrementally by increasing the fraction of the larger band gap material in the ternary; for the group IV it is Si, SiC, or diamond, and nothing else in between. To a first approximation the energy gap, EG, increases linearly from 3.39 for GaN to 6.2 eV for AlN and ξC from 3.5 to 11.7 V/cm. They actually increase sublinearly with the concentration, and the effects of this will be briefly explored. The carrier of interest is the electron because the depth of the Mg acceptor, which is deep to begin with, increases quite rapidly with the Al concentration. The dopant of interest is the Si donor, which behaves like a hydrogenic atom up to ~80% Al; above this it is thought to form a DX center. ED increases in depth from 32 to ~75 meV up to this Al%, and although this is ~3kT at room temperature, the large density of states in the conduction band enables most of the donors to be ionized. μn does decrease with the Al%, and at 80% Al it is about half the value it is for GaN. However, this deficit is more than made up for by the increased ξC, which has more than doubled. The conductivity also has a higher order dependence on ξC than it does on μn; in the ideal vertical diode it is proportional to ξC 3 and it is only linearly proportional to μn. Utilizing AlGaN creates other issues, especially for the vertical device, which will have to be the structure used because very high power lateral devices would occupy a larger surface area and be more prone to surface breakdown. The higher source and drain contact resistance for AlGaN will be a problem for both lateral and vertical devices, as will be alloy scattering in the 2DEG. It will be difficult to create a vertical structure that can be easily turned on and off be it with a current aperture or a JFET structure. The AlGaN drift region will also contain mismatch dislocations due to its lattice mismatch substrate, and it will be more difficult to confine the majority of dislocations to the interface, as it is done in SiGe devices, because Si and Ge are cubic and have active primary slip systems exposed to a substantial amount of shear. AlN and GaN have the hexagonal wurtzite structure, and neither the primary basal slip system nor the secondary prismatic slip systems have any shear operating on the slip planes so slip, if it occurs, occurs on pyramidal planes which have large critical shear stresses.

Evolution of fluorine doping following the REPUSIL process for the adjustment of optical properties of silica materials

We report on the fluorine doping and codoping of powder-based silica bulk glasses fabricated using REPUSIL (Reactive Powder Sintering of Silica) technology. The maximum doping level of 1.5 mol% of SiF4 is associated with a refractive index decrease by −8 x 10−3 compared to undoped silica and an essential decrease in the transition temperature by about 200 K. Fluorine codoping is eminently suitable for the direct refractive index adjustment of actively-doped silica glass materials (e.g., Al/Yb or Al/Tm). The fluorine codoping of Al/Yb silica glass significantly reduces parasitic photodarkening processes.

Development of polymer–dopant interactions during electropolymerization, a key factor in determining the redox behaviour of conducting polymers

Investigation of ionic motion in connection with the redox transformation of conjugated polymers (CP) has always been at the leading edge of research. Motivated by recent proofs for the chemical bond formation between chloride ion and α-positioned carbon in poly (3,4–ethylenedioxythiophene) (PEDOT), comprehensive studies have been extended to another strongly electronegative halide (F−) and to another CP, polypyrrole (PPy). As the electrochemical quartz crystal nanobalance (EQCN) results proved, the movement of the bulky Bu4N+ cations has been exclusively experienced during the redox processes of both systems. Moreover, the decisive role of the anions being present in the polymerization solution in determining the redox capacity and, consequently, the maximum doping level of the films was evidenced. On the grounds of the systematic experiments, the strong and permanent chemical interaction of highly electronegative anions and the polymer has been demonstrated as a general phenomenon. Importantly, this observation requires the necessary reconsideration of specific polymer–dopant interactions and calls attention to the necessity of careful design of the polymerization procedure.

Phase diagram and weak-link behavior in Nd-doped CaFe2As2

The transport properties, phase diagram, and dopant distribution are investigated in systematically Nd-doped CaFe2As2 single crystals. Coexistence of two superconducting (SC) phases with different critical transition temperature (Tc) is observed. The low-Tc phase emerges as x ≥ 0.031, and the Tc value increases to its maximum value of about 20 K at x = 0.083, the maximum doping level in our study. As x ≥ 0.060, the high-Tc phase with a Tc value of about 40 K is observed. The structure transition (STr) from tetragonal to orthorhombic phase vanishes suddenly around x = 0.060, where a new STr from tetragonal to collapsed tetragonal phase begins to turn up. Compared to the low-Tc phase, the end point of SC transition of the high-Tc phase is more sensitive to the magnetic field, showing a characteristic of Josephson weak-link behavior. Possible scenarios about this system are discussed based on our observations. We also find that the non-uniform SC properties cannot be attributed to the heterogeneous Nd distribution on the micro scale, as revealed by the detailed energy dispersive x-ray spectroscopy measurements.

Nitrogen Enriched Porous Carbon Spheres: Attractive Materials for Supercapacitor Electrodes and CO2 Adsorption

A series of nitrogen-containing polymer and carbon spheres were obtained by the sol–gel method. In particular, the nitrogen-rich carbon spheres were prepared by one-pot hydrothermal synthesis in the presence of resorcinol/formaldehyde as carbon precursors and ethylenediamine (EDA) as both a base catalyst and nitrogen precursor, followed by carbonization in nitrogen and activation with CO2. The introduction of EDA to the sol–gel system resulted in structurally bonded nitrogen-containing carbon spheres. The nitrogen doping level and the particle size can be tuned by varying the EDA amount in the reaction mixture. The maximum nitrogen doping level of 7.2 wt % in carbon spheres could be achieved without sacrificing the spherical morphology. The diameter of these carbon spheres (CS) can be tuned in the rage of 50–1200 nm by varying the EDA amount. N2 adsorption analysis showed that the aforementioned activated carbon spheres exhibited high surface area reaching up to1224 m2/g. Ultra high CO2 adsorption capacit...

Optimization and kinetics of phosphoric acid doping of poly(1-vinylimidazole)-graft-poly(ethylene-co-tetrafluorethylene) proton conducting membrane precursors

Optimization of the reaction parameters affecting phosphoric acid (PA) doping behavior of poly(1-vinylimidazole), P(VIm), grafted poly(ethylene-co-tetrafluoroethylene) (ETFE) proton conducting membrane precursors obtained by radiation induced grafting was studied using the Taguchi method. The reaction parameters such as degree of grafting (G%) in the precursors, PA concentration, temperature and doping time were selected as independent parameters while doping level was the response. The optimum parameters for achieving the maximum doping level (7.45mmolrepeat polymer unit−1) were: G% of 54%, PA concentration of 60%, temperature of 100°C and reaction time of 5 days. The kinetics of the acid doping reaction was also studied and the doping rate was found to be a function of reaction parameters and followed a first order reaction. The PA doping was verified by Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis of the membranes. The proton conductivity and thermal stability of the membranes were also evaluated. It can be concluded that the Taguchi method provides an effective tool for prediction of acid doping level and optimization of reaction parameters. The kinetics of acid doping is also suggested to be a diffusion-driven reaction following a multi-layers adsorption model.

Luminescent latex particles loaded with anionic lanthanide complexes: a versatile platform for multicolour optical coding

Luminescent polymer latexes loaded with lanthanide complexes have been synthesized by miniemulsion polymerization. A self-assembled europium complex was embedded into poly(methyl methacrylate) nanoparticles without covalent linking and compared to a commercially-available neutral chelate. The nature of the surfactant was found to have a great impact on the incorporation process for the europium complex and only the latexes stabilized by a cationic surfactant exhibited a luminescence signal. A maximum doping level of about 2% in weight in the final monodispersed particles was obtained. The resulting polymeric luminescent nanoparticles showed good stability over leakage. The described synthetic method was used to incorporate multiple lanthanide complexes into latex nanoparticles affording multicolour nanolabels. Two series of polymeric latexes bearing codes are presented in this work.

MgB2 inclusions in Bi-2223 matrix: The evaluation of microstructural, mechanical and superconducting properties of new system, Bi-2223 + MgB2

This study reports the effect of MgB2 inclusions on the microstructural, electrical, mechanical and superconducting properties of Bi1.8Pb0.4Sr2(MgB2)xCa2.2Cu3.0Oy ceramics with x=0, 0.01, 0.03, 0.05, 0.1, 0.3, 0.5 and 1.0 by use of bulk density, dc resistivity, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), transport critical current density (Jc) and Vickers microhardness (Hv) measurements. The samples studied in this work are prepared by conventional solid-state reaction method at the annealing temperature of 840°C for 48h. For the potential technological and industrial applications, the important characteristics such as the normal state resistivity, density, porosity, critical (onset and offset) transition temperature, variation of critical temperature, self-field critical current density, crystallinity, phase purity, lattice parameter, texturing, surface morphology, Vickers microhardness and elastic modulus values are obtained for the pure and MgB2-doped samples and compared with each other. It is found that all the properties given above are sensitively dependent upon the MgB2 concentration in Bi-2223 matrix. The critical transition temperature and critical current density values of the samples are observed to improve significantly for the optimum doping level of x=0.05. The maximum Tconset of 121.3K, Tcoffset of 114.1K and Jc of 463A/cm2 for the sample doped with x=0.05 whereas the minimum values are found to be 118.3K, 101.3K and 235A/cm2 for the material doped with x=1.0 as against 118.6K, 109.4K and 306A/cm2, respectively, for the undoped sample. This may be attributed to the fact that the Bi-2223 system changes from the optimally doped to an underdoped position with the excess of the MgB2 nanoparticles. Furthermore, XRD measurements illustrate that both pure and MgB2 doped superconductors (new system) contain only Bi-2223 and Bi-2212 phases (no different phase of MgB2 or any other cation) and exhibit the polycrystalline superconducting phases with the variable intensity of diffraction lines, confirming that the MgB2 inclusions are incorporated into the Bi-2223 crystal structure in the form of nanoparticles. Besides, the smallest (largest) lattice parameter a (c) is noticed to belong to the sample doped with x=0.05 while the largest (smallest) cell parameter a (c) is observed for the maximum doping level of MgB2 (x=1.0). As for SEM images, similar to the XRD investigations, the former sample has the best crystallinity, texturing, grain connectivity, lowest porosity and largest grain size while the worst surface morphology is observed for the latter sample. Additionally, the bulk porosity analyses for the samples demonstrate that the bulk porosity decreases monotonously with the MgB2 content in the Bi-2223 system up to x=0.05 after which the porosity starts to increase regularly towards to maximum value, leading to the degradation of the grain connectivity. At the same time, the Vickers microhardness values indicate that the samples doped with x=0.01, 0.03 and 0.05 (towards optimally doped state) exhibit reverse indentation size effect (RISE) behavior; on the other hand, the others (towards underdoped state) present indentation size effect (ISE) feature. To sum up, the MgB2 addition in the Bi-2223 superconducting system promotes the velocity of the Bi-2223 phase formation up to the concentration level of x=0.05 beyond which the microstructural, electrical, mechanical and superconducting properties of the Bi-2223 materials studied are found to degrade systematically and in fact reach to their local minimum points for the doping level of x=1.0.

In situ doping of catalyst-free InAs nanowires

We report on in situ doping of InAs nanowires grown by metal–organic vapor-phase epitaxy without any catalyst particles. The effects of various dopant precursors (Si2H6, H2S, DETe, CBr4) on the nanowire morphology and the axial and radial growth rates are investigated to select dopants that enable control of the conductivity in a broad range and that concomitantly lead to favorable nanowire growth. In addition, the resistivity of individual wires was measured for different gas-phase concentrations of the dopants selected, and the doping density and mobility were extracted. We find that by using Si2H6 axially and radially uniform doping densities up to 7 × 1019 cm−3 can be obtained without affecting the morphology or growth rates. For sulfur-doped InAs nanowires, we find that the distribution coefficient depends on the growth conditions, making S doping more difficult to control than Si doping. Moreover, above a critical sulfur gas-phase concentration, compensation takes place, limiting the maximum doping level to 2 × 1019 cm−3. Finally, we extract the specific contact resistivity as a function of doping concentration for Ti and Ni contacts.

Polypyrrole/copper(II) acetylacetonate composites prepared by in situ chemical oxidative polymerisation

This study reports on structural, thermal and dielectric properties of Polypyrrole (PPy) and Polypyrrole/copper(II) acetylacetonate composites. Interaction mechanisms between polymer and metal ions have been determined by Fourier transform infrared (FT-IR), nuclear magnetic resonance (NMR) and atomic absorption spectroscopy (AAS). Differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) have shown that whilst doping process does not influence PPy cold crystallisation temperature substantially, it does have a relatively higher influence on glass transition and degradation temperatures. Different microstructures on the surface morphology have been observed depending on the doping level through the use of scanning electron microscopy (SEM). Temperature dependent dielectric measurements have shown that doping process causes a relatively monotonic increase on the real dielectric constant of PPy at high frequencies when compared to low frequencies and such increase was found to be up to 105% for maximum doping level at 1MHz frequency. Two different frequency regions in the frequency dependent conductivity have been observed under the power law for PPy and its composites. The charge transfer mechanism can be explained via correlated barrier hoping (CBH) model.

Polystyrene nanoparticles doped with a luminescent europium complex

Polystyrene nanoparticles doped with a luminescent europium complex, Eu(tta)3phen, are prepared by miniemulsion polymerization. The influence of the complex on the miniemulsion polymerization is investigated by the systematic variation of the initial concentration of Eu(tta)3phen from 2 to 7wt% relatively to styrene. A maximum doping level of about 2% by weight in the final particles can be achieved. At higher doping levels, destabilization of the miniemulsion leads to a loss of reproducibility with respect to both the degree of conversion and the final Eu content of the particles. Doped nanoparticles of varying diameter, ranging from 19 to 94nm, are successfully prepared. Steady-state and time-resolved luminescence measurements indicate that the luminescence properties of Eu(tta)3phen in the doped latexes are unchanged from those found in THF solution. Aqueous dispersions of the doped particles exhibit characteristic red emission under UV light irradiation. The luminescence intensity increases linearly with Eu(tta)3phen content, indicating the absence of self-quenching despite the relatively high local concentrations within the particles.

Polymer blends based on copolymers bearing both side and main chain pyridine units as proton exchange membranes for high temperature fuel cells

New blend systems consisting of copolymers based on aromatic polyethers bearing both main and side chain pyridine units have been prepared in order to be evaluated as proton exchange membranes. The blend membranes showed high glass transition temperatures up to 265°C and high thermal (Td's up to 450°C) as well as oxidative stability. The acid doping ability was controlled based on the blend constituents and composition while the maximum doping levels were up to 480wt%. Comparison of different blend membranes with the same doping level showed that different conductivity values were obtained (up to 1.1×10−1Scm−1), denoting the important role of the polymer matrix itself on proton conduction. Morphological investigation by means of SEM revealed nanophase separated structures with spherical clusters of average sizes between 50 and 70nm. MEAs based on the newly developed blends were prepared and preliminary fuel cell tests at temperatures up to 180°C using dry gases were conducted. The performance obtained was satisfactory (723mV at 0.2Acm−2 at 180°C using H2/O2 gases) and close to the state of the art PBI MEAs.

Controllable graphene N-doping with ammonia plasma

Here we show that gas-phase doping by means of NH3 plasma exposure is a highly flexible and manufacturable process for graphene electronics. The nitrogen-containing radicals can readily form covalent bonds with the carbon lattice and keep stable in the postannealing for damage restoration. The amount of charge transfer can be fine tuned by controlling the exposure time and monitored by the systematic shift in the Raman G mode and the Gds−Vg curves in transport measurements. The maximum doping level can reach 1.5×1013 cm−2.

Electrochemical and Raman spectroelectrochemical investigation of single-wall carbon nanotubes–polythiophene hybrid materials

Two types of polythiophene—single-wall carbon nanotubes (SWCNTs) composites are comparatively studied. Composites of the first type consisted of nonmodified SWCNTs and nonmodified regioregular poly(3-hexylthiophene) (P3HT), poly(3-octylthiophene) (P3OT) or poly(dioctylterthiophenes) (PDOTT). Composites of the second type (molecular composites) were obtained by grafting carboxylic acid functionalized poly(alkylthiophene)s on amine functionalized SWCNTs. In AFM images of the composites of nonfunctionalized components, individual carbon nanotubes can be easily distinguished, which are well dispersed in the polymer matrix showing fibrilar morphology. Cyclic voltammetry and Raman spectroelectrochemical experiments unequivocally show that all characteristic redox and spectroscopic features of both SWCTs and the polymer are retained in the composite material. Moreover the percolating network of SWCTs facilitates the electrochemical doping of the polymeric component. These new, solution processible materials are good candidates for printable contact electrodes in organic electronics. Molecular composites obtained by grafting show a different morphology. It is characterized by a network of fibers of diameters ranging from few dozens to few hundreds nm. Individual composite components cannot be distinguished which is an obvious consequence of the grafting reaction. Combined voltammetric and Raman spectroelectrochemical studies show that the grafting reaction lowers the electrochemical activity of the polymer component and the resulting maximum doping level is, in this case, lower than that found in composites of nonmodified components.

Synthesis and characterization of poly(arylene ether sulfone)-b-polybenzimidazole copolymers for high temperature low humidity proton exchange membrane fuel cells

Multiblock copolymers based on poly(arylene ether sulfone) and polybenzimidazole (PBI) with different block lengths were synthesized by coupling carboxyl functional aromatic poly(arylene ethers) with ortho diamino functional PBI oligomers in NMP, selectively doped with phosphoric acid, and evaluated as a high temperature proton exchange membrane (PEM). Transparent and ductile membranes were produced by solvent casting from DMAc. From dynamic mechanical analysis (DMA), the neat copolymer membranes showed two distinct glass transition temperatures which implies the existence of a nanostructured morphology in the membranes. These two nanophases became more distinct with increasing block length. The membranes were immersed in various concentrations of phosphoric acid solution to produce the proton conductivity. The doping level increased with increasing concentration of the acid solution and a maximum doping level of 12 was achieved when 14.6M phosphoric acid solution was used. The acid doped membranes showed significantly reduced swelling behavior compared to a control conventional phosphoric acid doped PBI homopolymer system which appears to be related to the selective sorption into the PBI phase. The ionic conductivity of the doped samples at 200°C afforded up to 47mS/cm without external humidification. The protonic conductivity was found to increase with block length at a given doping level, reflecting the sharpness of the nanophase separation and the effect was even more prominent at a low doping level of 6–7. It is suggested that the phosphoric acid doped multiblock copolymer system would be a strong candidate for high temperature and low relative humidity PEM applications such as those required for stationary power.