Homogeneous Doping Research Articles

Structure and optical homogeneity of LiNbO3〈Mg〉 crystals grown from different charges

This paper reports a comparative study of the homogeneity of LiNbO3〈Mg〉 (≥5 mol % Mg) crystals grown from a charge synthesized using Nb2O5〈Mg〉 prepared through homogeneous magnesium doping of a reextract in the Nb2O5 extraction step, and LiNbO3〈Mg〉 crystals prepared through direct doping of a growth charge. The LiNbO3〈Mg〉 crystals were characterized by optical microscopy and Raman spectroscopy. The results demonstrate that the LiNbO3〈Mg〉 crystals prepared using homogeneous doping are more structurally uniform.

Tight-binding description of intrinsic superconducting correlations in multilayer graphene

Using highly efficient GPU-based simulations of the tight-binding Bogoliubov-de Gennes equations we solve self-consistently for the pair correlation in rhombohedral (ABC) and Bernal (ABA) multilayer graphene by considering a finite intrinsic s-wave pairing potential. We find that the two different stacking configurations have opposite bulk/surface behavior for the order parameter. Surface superconductivity is robust for ABC stacked multilayer graphene even at very low pairing potentials for which the bulk order parameter vanishes, in agreement with a recent analytical approach. In contrast, for Bernal stacked multilayer graphene, we find that the order parameter is always suppressed at the surface and that there exists a critical value for the pairing potential below which no superconducting order is achieved. We considered different doping scenarios and find that homogeneous doping strongly suppresses surface superconductivity while non-homogeneous field-induced doping has a much weaker effect on the superconducting order parameter. For multilayer structures with hybrid stacking (ABC and ABA) we find that when the thickness of each region is small (few layers), high-temperature surface superconductivity survives throughout the bulk due to the proximity effect between ABC/ABA interfaces where the order parameter is enhanced.

Synthesis and characterization of bulk cobalt-doped ZnO and their thin films

(ZnO)1−x(Co3O4)x≤0.05 thin-film series were deposited in vacuum Ar–O2 and N2 environment using the pulsed-laser deposition (PLD) technique. Bulk pellets were prepared by the solid-state reaction technique by mixing ZnO and Co3O4 powder and further utilized for deposition of thin films. XRD patterns of pellets exhibit homogeneous ZnCoO phase formation with no other impurity phases. Single-phase (002) oriented ZnCoO thin films were formed without any cobalt segregation. The cobalt core peaks confirmed the existence of +2 valence state of cobalt in thin-film samples, and the binding energy difference between the cores peaks showed the homogeneous doping of cobalt in ZnO matrix. The magnetic ordering was achieved in thin films samples which were deposited in vacuum and N2 ambience.

Homogeneous and stable p-type doping of graphene by MeV electron beam-stimulated hybridization with ZnO thin films

In this work, we demonstrate a unique and facile methodology for the homogenous and stable p-type doping of graphene by hybridization with ZnO thin films fabricated by MeV electron beam irradiation (MEBI) under ambient conditions. The formation of the ZnO/graphene hybrid nanostructure was attributed to MEBI-stimulated dissociation of zinc acetate dihydrate and a subsequent oxidation process. A ZnO thin film with an ultra-flat surface and uniform thickness was formed on graphene. We found that homogeneous and stable p-type doping was achieved by charge transfer from the graphene to the ZnO film.

Emitter Requirements for Nickel Contacts on Silicon Solar Cells-A Simulation Study

A unified model for the nickel-silicon theoretical contact resistivity was computed, using a 0.6eV Schottky barrier height, and compared to recent experimental data. This model was subsequently inserted in a classical co- optimization procedure. Numerical simulations were combined with analytical power losses equations to find out the effect of homogeneous emitter doping profile on an ideal lab-scale silicon solar cell. According to this work, an optimized emitter for plated contacts on a 10μm wide nickel seed layer should have a junction depth ranging from 0.5 to 4μm and a surface doping from 4 x 1018 to 2 x 1019 cm-3. This broad range allows getting more than 25.5% and up to 25.9% theoretical efficiency on a 2 x 2cm2 silicon solar cell metallized with 10μm thick fingers. Finally, contour plots were also simulated using a larger Schottky barrier height in order to figure out the effect of the nickel silicide contact interface on solar cell properties.

Ion conduction in nanoscale yttria-stabilized zirconia fabricated by atomic layer deposition with various doping rates

The ion conduction of yttria-stabilized zirconia (YSZ) was studied by varying the doping ratios during atomic layer deposition (ALD). The ALD cycle ratio for the yttria and zirconia depositions was varied from 1:1 to 1:6, which corresponded to the doping ratios from 28.8% to 4.3%. The in-plane conductivity of ALD YSZ was enhanced by up to 2 orders of magnitude; the optimal ALD doping ratio (10.4%) was found to differ from that of bulk YSZ (8%). This different relationship between the doping ratio and the ion conduction for ALD YSZ versus bulk YSZ is due to the inhomogeneous doping in the vertical direction of the ALD YSZ films, as opposed to the homogenous doping of bulk YSZ.

ChemInform Abstract: From Composites to Solid Solutions: Modeling of Ionic Conductivity in the CaF2—BaF2 System.

By using calcium fluorite and barium fluorite as test materials, we demonstrated that homovalent can greatly affect ionic conductivity through locally changing the defect density. Whilst this doping is a state-of-the-art effect in the case of dopants that replace native ions of different charge (heterovalent dopants), it is a rather surprising effect at a first glance for substitutional dopants of the same charge; here, the phenomenon is not electrostatic, but elastic in nature. As a consequence of size mismatch, the smaller Ca atoms in the BaF(2) lattice favored the formation of interstitial sites that were located close to the Ca atoms, whilst doping larger Ba species into the CaF(2) phase favored vacancy formation. In terms of conductivity, and in agreement with the different mobilities, the first doping effect was favorable, whilst the other decreased conductivity. The concentration effects were formalized by a heterogeneous Frenkel reaction that was distinguished from the mean Frenkel reaction by additional (elastic) trapping that became more pronounced the lower the temperature. It was very revealing to relate this phenomenon to CaF(2)-BaF(2) multilayers and composites. In very general terms, these effects in the solid solutions were understood as being the atomistic limit of the interfacial charge-transfer that occurred at the hetero-interface of the crystallites or films, and reflected the transition from heterogeneous doping (higher-dimensional doping) to homogeneous doping (zero-dimensional doping).

From Composites to Solid Solutions: Modeling of Ionic Conductivity in the CaF2–BaF2 System

By using calcium fluorite and barium fluorite as test materials, we demonstrated that homovalent "dopants" can greatly affect ionic conductivity through locally changing the defect density. Whilst this doping is a state-of-the-art effect in the case of dopants that replace native ions of different charge (heterovalent dopants), it is a rather surprising effect at a first glance for substitutional dopants of the same charge; here, the phenomenon is not electrostatic, but elastic in nature. As a consequence of size mismatch, the smaller Ca atoms in the BaF(2) lattice favored the formation of interstitial sites that were located close to the Ca atoms, whilst doping larger Ba species into the CaF(2) phase favored vacancy formation. In terms of conductivity, and in agreement with the different mobilities, the first doping effect was favorable, whilst the other decreased conductivity. The concentration effects were formalized by a heterogeneous Frenkel reaction that was distinguished from the mean Frenkel reaction by additional (elastic) trapping that became more pronounced the lower the temperature. It was very revealing to relate this phenomenon to CaF(2)-BaF(2) multilayers and composites. In very general terms, these effects in the solid solutions were understood as being the atomistic limit of the interfacial charge-transfer that occurred at the hetero-interface of the crystallites or films, and reflected the transition from heterogeneous doping (higher-dimensional doping) to homogeneous doping (zero-dimensional doping).

Strain-induced delamination of edge-grafted graphite

Edge-selectively grafted graphite (EGG) with poly(ether-ketone) was prepared by the Friedel-Crafts acylation in a mild polyphosphoric acid (PPA)-phosphorous pentoxide (P(2)O(5)) mixture. The homogeneous reaction dope was coagulated in air moisture at different temperatures. The morphology of expanded EGG was changed from balls, balls/rods and rods with respect to coagulation temperatures of 80, 60, 40 and 25 °C, respectively.

Electrical Conductivity Enhancement of PTFE (Teflon) Induced by Homogeneous Low Voltage Electron Beam Irradiation (HLEBI)

Electrical conductivity (·) enhancement, control, aging stability, and linear relation between · and reciprocal temperature (1/T) in PTFE (Teflon) had been examined by homogeneous low potential electron beam irradiation (HLEBI). Irradiations up to 0.432MGy were found to raise the · of PTFE more than two orders of magnitude. In addition, control of electron movement or Fermi level by HLEBI on polymers seems possible since at low HLEBI levels up to 0.216MGy, a strong dependence on · was observed. Aging stability investigation showed the saturated conductivities at 10 6 s( ·s) (2.5 © 10 ¹16 and 3.6 © 10 ¹15 Sm ¹1 ) obtained at EB doses of 0.0432 and 0.432MGy, respectively were approximately 4.9 and 70.6 times higher than · of untreated PTFE (5.1 © 10 ¹17 Sm ¹1 ). Although the aging initially reduced the · of irradiated samples, ·s were remarkably higher than the untreated. For example, the lower dose 0.0432MGy samples decayed significantly, but its ·s value (2.5 © 10 ¹16 Sm ¹1 ) was still about 490% above untreated condition at 5.1 © 10 ¹17 Sm ¹1 . ESR (electron spin resonance) measurements showed aging for 10 6 s only slightly reduced the peak intensity of irradiated PTFE. This indicated the charge carriers generated did not easily decay. Results agree with the literature where PTFE was irradiated with VUVand radicals such as CF3 + did not substantially decay with aging. Since the slopes of linear relationships between logarithmic electrical conductivity (ln·) and reciprocal temperature (1/T) of all PTFE samples irradiated were approximately equal to that before treatment from 303 to 375K, HLEBI didn’t convert the · system. The small irradiation dose of 0.0432MGy raised the electrical conductivity at room temperature. At higher temperatures above ³375K, the 0.0432 and 0.432MGy-HLEBI generated drops in conductivity ·. When unstable dangling bonds with isolated radicals at terminated sites are assumed to be formed in irradiated PTFE, the · recovery by heating to 424K as well as high · of irradiated PTFE could be explained. HLEBI probably generated dangling bonds in the form of radicals acting as acceptors to carry charge. Since a chemical dopant was not needed to act as a charge carrier, HLEBI could probably be an attractive method to attain homogeneous doping of organic films. [doi:10.2320/matertrans.M2011273]

Structural, optical, and magnetic properties of Co-doped ZnO nanorods fabricated by a facile solution route

In this study, a facile two-step solution route is demonstrated for the fabrication of Co-doped ZnO nanorods with diverse doping levels. The combination of XRD, EDS, and XPS measurements reveals that Co ions are successfully incorporated into the ZnO matrix and exist in the host lattice under the 2+ valence state. The substitution of Co 2+ for Zn 2+ does not change the wurtzite structure of ZnO nanocrystals. Co 2+ dopant inhibits the dissolution of the intermediate product, thus, allowing a relatively slow and uniform deposition of the effective ions on the growing tiny rods. As a consequence, the obtained ZnO nanorods become longer and thinner with the increase in dopant concentration. Homogeneous substitutional doping is further verified by UV–vis absorption and photoluminescence spectroscopy. An obvious redshift in the wavelength of the absorption edge is observed in the doped ZnO samples, which can be attributed to the sp–d exchange interactions between the electrons in the conduction band of ZnO and the localized d electrons of the Co 2+ cations. A remarkable quenching of yellow-green luminescence that results from doping is explained by an energy transfer mechanism. Furthermore, the doped ZnO exhibits room-temperature ferromagnetism, which is greatly suppressed and replaced by paramagnetism at higher doping levels.

Performance of microwave synthesized dual solvent dope solution and lithium bromide additives on poly(ethersulfone) membranes

AbstractBACKGROUND: Generally the fabrication of polymeric membranes is a complicated and expensive process since it involves several steps. The preliminary preparation steps involve polymer drying and dissolution and is very time consuming and expensive. Currently, conventional electrothermal heating (CEH) is used to dissolve polyethersulfone in an aprotic solvent for membrane fabrication. Usually CEH requires 6 to 8 h at temperatures of 80 to 95 °C. This paper reports the fabrication and characterization of polyethersulfone (PES) asymmetric ultrafiltration membrane made from microwave (MW) synthesis casting solution consisting of various compositions of double solvents and lithium bromide (LiBr) additive.RESULTS: Homogeneous dual solvent dope solutions prepared via microware irradiation took only 1 h instead of the 7 h when prepared using CEH. The results also revealed that the membrane permeation and rejection rates, pore size and porosity were dependent on the ratio of LiBr to acetone. Membranes with LiBr kept at 2 and 3 wt% exhibited both high rejection and permeation rates with minimum pore sizes of 1.067 and 1.214 nm respectively. The presence of LiBr and the occurrence of chain scission were elucidated using Fourier transform infrared (FTIR) spectroscopy while its hydrophilic property was confirmed by water absorption and contact angle measurements.CONCLUSIONS: It was concluded that the microwave technique is capable of producing 1 L membrane solutions in less than 1 h. The membranes prepared from the microwave solutions show good rejection and permeation rates. Copyright © 2011 Society of Chemical Industry

Simulation of the Incomplete Ionization of the <i>n</i>-Type Dopant Phosphorus in 4H-SiC, Including Screening by Free Carriers

The simulation of the incomplete ionization of substitutional dopants in Silicon Carbide (SiC) is often performed using Boltzmann statistics and ionization energy values that do not depend on free carrier concentrations. But in the case of heavy doping Fermi-Dirac statistics is needed, while the case of an inhomogeneous dopants distribution or that of an excess carrier injection requires local free carrier concentration-dependent impurity ionization energies. Here a model for describing partial ionization from diluted to high homogeneous doping densities in SiC and in thermal equilibrium is presented and compared with results on Phosphorus doped 4H-SiC.

Enhanced catalytic performance of assembled ceria necklace nanowires by Ni doping

Ceria and nickel-doped ceria necklace-like nanowires were synthesized by a facile, surfactant free method, through an unconventional assembly route. Homogeneous doping of lower-valent cations introduces elastic strain, resulting in extrinsic surface defects and higher catalytic performance in CO oxidization.

Bandgap narrowing of titanium oxide nanosheets: homogeneous doping of molecular iodine for improved photoreactivity

We developed a new strategy for homogeneous doping of iodine molecules (I2) to achieve bandgap narrowing of semiconducting metal oxide nanosheets. This was realized by electrostatic interactions between nonpolar I2 molecules and negatively charged exfoliated metal oxide nanosheets, mediated by the partial polarizability of the molecular iodine. Flocculated titania nanosheets doped with I2 molecules (I2-Ti0.91O2 sheets) were explored as a model system to demonstrate the effectiveness of this strategy. We observed an extension of the intrinsic absorption edge into the visible light region through a shifting of the valence band maximum at high doping levels of I2 molecules. Importantly, at the same time the geometric structure of the host retained its integrity. The experimental together with first-principles calculations reveal the molecular nature of adsorbed iodine atoms and imply that the mechanism of electronic structure modulation in the titania layers changes depending on the concentration of I2 molecules. This simple and efficient strategy may be suitable for other charged metal oxide nanosheets with large bandgaps to facilitate the design of new functional materials.

Synthesis, characterization and enhanced electrical properties of CTAB-directed polyaniline nanoparticles

Inverse microemulsion system of cetyltrimethylammonium bromide (CTAB) molecules is utilized for virtually monodispersed and controlled growth of HCl polyaniline (PANI) nanoparticles at room temperature (ca. 300 K). The templated electroconductive polymer reveals lamellar crystalline structure under X-ray diffraction signifying marked sub-chain alignment of the polymerized nanoparticles. The nanostructured polymer has spherically symmetric morphology in a size range of 2.0 nm to 6.0 nm under electron microscope examination. Gel permeation chromatography gives polydispersity index of 1.02 for nanostructured polymer in agreement with the size monodispersity transpired by electron microscopy. The d.c. electrical conductivity σ dc of PANI at room temperature is 10.11 S/cm whereas the variation of conductivity with temperature in the range 227–303 K reveals that the conducting mechanism can be considered as three-dimensional variable-range-hopping (3D-VRH). UV-Vis spectrum indicates two broad absorption bands due to polaron formation that contributes to enhanced electrical conductivity of the polymer. The fundamental absorption edge in the polymer is formed by direct allowed transitions to the extent that the optical band gap value was found to be 2.35 eV. The crystalline nanostructure and homogeneous doping attained in the cationic template of amphiphile are argued as contributing factors to the enhanced conductivity of the polymer.

First principles study of lithium insertion in bulk silicon

Si is an important anode material for the next generation of Li ion batteries. Here the energeticsand dynamics of Li atoms in bulk Si have been studied at different Li concentrations on thebasis of first principles calculations. It is found that Li prefers to occupy an interstitial site as ashallow donor rather than a substitutional site. The most stable position is the tetrahedral(Td) site. The diffusion of a Li atom in the Si lattice is through aTd–Hex–Td trajectory, where the Hex site is the hexagonal transition site with an energy barrier of0.58 eV. We have also systematically studied the local structural transition of aLixSi alloywith x varying from 0 to 0.25. At low doping concentration (x = 0–0.125),Li atoms prefer to be separated from each other, resulting in a homogeneous doping distribution. Startingfrom x = 0.125, Li atoms tend to form clusters induced by a lattice distortionwith frequent breaking and reforming of Si–Si bonds. Whenx ≥ 0.1875, Li atoms will break some Si–Si bonds permanently, which results in dangling bonds.These dangling bonds create negatively charged zones, which is the main driving force forLi atom clustering at high doping concentration.

Homogeneous doping of ceramics by infiltration–gelation

Advanced ceramics require small amounts of cation dopants to improve the sintering process or achieve certain properties. Dopant precursors are often dissolved in initial processing liquids, which may adversely affect ceramic colloidal stability, and dopant transport during drying. This would lead to chemical and microstructural inhomogeniety in the eventual ceramics. Here we present a method that circumvents these problems using the example of Al 3+-doped TiO 2 ceramics. Homogeneous TiO 2 compacts with 34% porosity are first prepared by colloidal casting and calcined at 700 °C. The obtained compacts are infiltrated with an aqueous solution of Al(NO 3) 3, citric acid and ethylene glycol. The solution composition is adjusted such that during drying, a gel forms that covers the internal pore surface. Subsequent decomposition of organics results in a homogeneous dopant distribution in the porous and then dense-sintered TiO 2 compacts. This is verified with SEM-EDX, and laser ablation coupled ICP-MS on dense compacts sintered at 1100 °C.

Electrochemical performance of Ti 4+-doped LiFePO 4 synthesized by co-precipitation and post-sintering method

Ti 4+-mixed FePO 4· xH 2O precursor was prepared by co-precipitation method, with which Ti 4+ cations were added in the process of preparing FePO 4· xH 2O to pursue an effective and homogenous doping way. Ti 4+-doped LiFePO 4 was prepared by an ambient-reduction and post-sintering method using the as-prepared precursor, Li 2CO 3 and oxalic acid as raw materials. The samples were characterized by scanning electron microscopy (SEM), X-ray diffractometry (XRD), electrochemical impedance spectroscopy (EIS), and electrochemical charge/discharge test. Effects of Ti 4+-doping and sintering temperature on the physical and electrochemical performance of LiFePO 4 powders were investigated. It is noted that Ti 4+-doping can improve the electrochemical performance of LiFePO 4 remarkably. The Ti 4+-doped sample sintered at 600 °C delivers an initial discharge capacity of 150, 130 and 125 mA·h/g with 0.1 C, 1 C and 2 C rates, respectively, without fading after 40 cycles.

High performance magnetically controllable microturbines

Reported in this paper is two-photon photopolymerization (TPP) fabrication of magnetic microturbines with high surface smoothness towards microfluids mixing. As the key component of the magnetic photoresist, Fe(3)O(4) nanoparticles were carefully screened for homogeneous doping. In this work, oleic acid stabilized Fe(3)O(4) nanoparticles synthesized via high-temperature induced organic phase decomposition of an iron precursor show evident advantages in particle morphology. After modification with propoxylated trimethylolpropane triacrylate (PO(3)-TMPTA, a kind of cross-linker), the magnetic nanoparticles were homogeneously doped in acrylate-based photoresist for TPP fabrication of microstructures. Finally, a magnetic microturbine was successfully fabricated as an active mixing device for remote control of microfluids blending. The development of high quality magnetic photoresists would lead to high performance magnetically controllable microdevices for lab-on-a-chip (LOC) applications.