Microplasma illumination enhancement of vertically aligned conducting ultrananocrystalline diamond nanorods

Vertically aligned conducting ultrananocrystalline diamond (UNCD) nanorods are fabricated using the reactive ion etching method incorporated with nanodiamond particles as mask. High electrical conductivity of 275 Ω·cm⁻¹ is obtained for UNCD nanorods. The microplasma cavities using UNCD nanorods as cathode show enhanced plasma illumination characteristics of low threshold field of 0.21 V/μm with plasma current density of 7.06 mA/cm² at an applied field of 0.35 V/μm. Such superior electrical properties of UNCD nanorods with high aspect ratio potentially make a significant impact on the diamond-based microplasma display technology.     

Effects of grain size and temperature on mechanical and failure properties of ultrananocrystalline diamond

Molecular dynamics simulations of ultrananocrystalline diamond (UNCD), with random distribution of grain sizes and grain boundaries (GBs), are performed to investigate the effect of grain size and temperature on the mechanical properties and failure mechanisms under tensile loading. Results show that when the grain size of UNCDs decreases from 4.1 nm to 2.26 nm, the Young's modulus decreases from 891 GPa to 840 GPa, while the obtained intrinsic fracture strength, 113 GPa, is insensitive to the grain size. Elastic softening is attributed to the increased volume fraction of amorphous-like atoms. Our analysis reveals that at room temperature, UNCD fails via sliding along a grain boundary with a large shear stress. Such sliding triggers crack initiation at an adjacent triple junction and subsequent propagation along an adjacent grain boundary with a large normal stress. With increasing temperature, a crossover from grain sliding to a direct intergranular fracture is observed. The crossover is caused by a different dependence of GB shear and tensile strength on temperature. The present work provides information that may be useful to the design and optimization of the mechanical properties and failure behavior of UNCDs.     

EPR study of hydrogen-related defects in boron-doped p-type CVD homoepitaxial diamond films

Boron-doped p-type single crystalline chemical vapor deposition (CVD) homoepitaxial diamond films were investigated by electron paramagnetic resonance (EPR). Carbon dangling bond defects, which were accompanied by a nearby hydrogen atom, were observed in boron-doped p-type CVD diamond films on a IIa substrate similar to those observed in undoped diamond. This result suggested that the energy level position of the defects is located below the Fermi energy of boron-doped diamond, at around 0.3 eV above the valence-band top. The reason why the Fermi energy could be changed by the incorporation of boron atoms at low density (10¹⁶–10¹⁷/cm³) in the film in spite of the existence of the large defect density of EPR centers (10¹⁸/cm³) is thought to be that the singly occupied electron states of defects are located near the band edge. As for the thermal annealing effect of the defects, it was revealed that the concentration of the defects and the mobility of the p-type film did not change after annealing up to 1200 °C which is much higher than the temperature of boron–hydrogen pair dissociation.     

Electron Paramagnetic Resonance, Optical, and Electronic Structure Studies of Photorefractive Barium Titanate

Barium titanate (BaTiO₃) is an important electrooptic ferroelectric oxide material which exhibits the unusual optical nonlinearity of photorefractivity. There is evidence that the photorefractive effect, which refers to the change in the index of refraction of the crystal by the action of light, is related to the presence of defects/impurities such as transition metal ions and/or cation or anion vacancies. This paper will review our electron paramagnetic resonance, optical, and theoretical eletronic structure studies aimed at probing the nature of the defects/impurities in BaTiO₃ and their role in the photorefractive effect.     

Identification of the deep-level defects in AlN single crystals: EPR and TL studies

X-band electron paramagnetic resonance studies were performed on three different types of AlN single crystals. These studies revealed the presence of two different types of shallow donors, oxygen located in the N site of the AlN lattice and carbon or silicon located in the Al site. These findings are supported by the observation of the light-induced EPR signals with almost similar slightly anisotropic g factors typical for shallow donor in AlN, but strongly different anisotropic EPR line-width ΔB. Distribution of the wave-function density of the unpaired donor electron for both shallow donors is discussed. Moreover, findings related to the deep-donor of the V_N centers are presented. The V_N center EPR spectra intensity increased drastically after the x-ray irradiation of the sample. The annealing results in recombination thermoluminescence and the deep donor (V_N) energy level was estimated to be about 0.75 eV.     

Three-Dimensional Electron Paramagnetic Resonance Imaging Technique for Mapping Porosity in Ceramics

A three-dimensional (3D) electron paramagnetic resonance imaging (EPRI) method was developed to probe the structure and size of pores in ceramic materials. The imaging device that was added to the EPR instrument consisted of a computer-controlled current source and magnetic field gradient. This add-on facility was tested using a well-defined diphenylpicrylhydrazyl phantom sample. Pumice was then used to demonstrate the potential of the technique. This stone was immersed in a 0.5 m M ¹⁵N-substituted perdeutereted tempone water solution to fill the pores with spin labels. Images were reconstructed using a filtered backprojection technique. A two-dimensional (2D) imaging plane was constructed by collecting 33 projection planes over 180°. A 3D image was derived from 22 planes each constructed by 22 projections. At present, the facility allows a resolution of 69 and 46μm for 2D and 3D imaging, respectively. Advancements of the imaging apparatus, software, and line width of the spin labels will be needed to enhance the resolution of this technique.     

Direct electron paramagnetic resonance monitoring of the peptide synthesis coupling reaction in polymeric support

This work demonstrates, for the first time, a time-resolved electron paramagnetic resonance (EPR) monitoring of a chemical reaction occurring in a polymeric structure. The progress of the coupling of a N^α-tert-butyloxycarbonyl-2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (Boc-TOAC) spin probe to a model peptide-resin was followed through EPR spectra. Progressive line broadening of EPR peaks was observed, indicative of an increased population of immobilized spin probe molecules attached to the solid support. The time for spectral stabilization of this process coincided with that determined in a previous coupling study, thereby validating this in situ quantitative monitoring of the reaction. In addition, the influence of polymer swelling degree and solvent viscosity, as well as of the steric hindrance within beads, on the rate of coupling reaction was also addressed. A deeper evaluation of the latter effect was possible by determining unusual polymer parameters such as the average site-site distance and site-concentration within resin beads in each solvent system.     

Mixed-valent structure,dielectric properties and defect chemistry of Ca1−3x/2TbxCu3Ti4−xTbxO12 ceramics

Single-phase Ca_1−3x/2Tb_xCu₃Ti_4−xTb_xO₁₂ (0.025≤ x≤0.075) (CTCTT) ceramics with a cubic perovskite-like structure and a fine-grained microstructure (1.6‒2.3 µm) were prepared using a mixed oxides method. The results revealed that mixed valence states of Cu²⁺/Cu⁺, Ti⁴⁺/Ti³⁺, and Tb³⁺/Tb⁴⁺ coexisted in CTCTT. A multiphonon phenomenon in the Raman scattering at 1148, 1323, and 1502 cm⁻¹ was reported for undoped and doped CTTO. Tb was mainly incorporated in the interior of the CTCTT grains rather than on the surface. The dielectric permittivity of CTCTT (ε_r'_RT =3590‒5200) decreased relative to CCTO (ε_r'_RT =10240) at f =1 kHz, but the dielectric loss of CTCTT (the minimum value of tan δ=0.12 at RT) increased as a result of Tb doping. The defect chemistry of CTCTT is discussed. The internal barrier layers capacitance (IBLC) model was adopted for impedance spectroscopy (IS) analysis. The activation energies of the grain boundaries (E_gb) and semi-conductive grains (E_g) for CTCTT were determined to be 0.52 eV and 104 meV, respectively. The IS and defect chemistry analyses confirmed that the decrease in the dielectric permittivity was mainly due to a decrease in conductivity in the semiconducting CTCTT grains caused by the acceptor effect of Tb⁴⁺ at the Ti site, which resulted in a decrease in the IBLC effect.     

In situ EPR spectroelectrochemistry of single-walled carbon nanotubes and C60 fullerene peapods

The first in situ electron paramagnetic resonance (EPR) spectroelectrochemical study of C₆₀ fullerene peapods (C₆₀@SWCNT) as well as that of single-walled carbon nanotubes (SWCNTs) in different electrolyte solutions describes the formation of spin states by charge transfer reactions. Electrochemical reduction of peapods at high negative potentials causes the production of spins at the SWCNT site, while the intratubular fullerene is unchanged.Slightly anisotropic EPR signals were detected during electrochemical reduction of single-walled carbon nanotubes and fullerene peapods in the potential region from −1.75 to −2.15 V vs. decamethylferrocene/decamethylferrocinium couple. They are centered at g = 2.0038 and exhibit a hyperfine structure indicating the presence of functional groups containing N, O, H atoms in neighborhood. They differ from the EPR signals of chemically (potassium) doped SWCNT and C₆₀@SWCNT. As the EPR signal is influenced by the electrolyte counter ions a reaction with electrolysis products of tetraalkylammonium cations is taken into consideration. No EPR lines of fullerene anions were found in electrochemically treated peapods, but these anions are detectable, if a free C₆₀ in solution is cathodically reduced on a SWCNT electrode.     

Characterization of Textured Ceramics by Electron Paramagnetic Resonance Spectroscopy: I, Concepts and Theory

Electron paramagnetic resonance (EPR) spectroscopy is shown to be a powerful and versatile technique for the detection and characterization of preferred orientation effects in insulating ceramics. While the technique is applicable to both cubic and noncubic ceramics, in this work the conceptual and theoretical basis for the analysis of textured cubic materials is established. The practical utility of the EPR technique is illustrated for the specific case of a paramagnetic impurity (Mn²⁺) in the cubic ceramic MgO. For this example, EPR methods show that texturing occurs in a cold pressed, sintered body and that the preferred orientation corresponds to 〈111〉 axes lying nearly parallel to the direction of the cold-pressing force. The advantages of the EPR technique in characterizing preferred orientation effects in ceramic materials are discussed and compared with more traditional diffraction-based and optical characterization methods.     

Nanodispersed iron, tin and antimony in vapour grown carbon fibres for lithium batteries: an EPR and electrochemical study

The use of iron-, tin- and antimony-containing carbon fibres as potential candidates for the negatives electrode of advanced lithium-ion batteries is described. Two different types of fibres are used: non-graphitic and partially graphitized carbon. EPR results show that both carbon and dispersed iron participate in redox processes during the electrochemical reaction with Li. Lithium test cells evidence higher capacities for partially graphitized fibres. The addition of dispersed tin or antimony is used to improve the capacity of the non-graphitic fibres by formation of lithium-tin and lithium-antimony compounds. Reversible capacity values close to 300 mA h g⁻¹ are achieved in both partially graphitized and composite electrodes containing Li-alloying elements. The use of nanodispersed transition metals, i.e. iron, in a carbon matrix is proposed here as an alternative to carbon composites containing Li-alloying elements.     

Combination of non-thermal plasma and heterogeneous catalysis for oxidation of volatile organic compounds: Part 3. Electron paramagnetic resonance (EPR) studies of plasma-treated porous alumina

The effect of plasma processes inside the intra-particle volume of porous materials (especially Al₂O₃) was studied in order to evaluate the potential of the combination of non-thermal plasma (NTP) and in situ heterogeneous catalysis (plasma catalysis), for the improvement of efficiency and selectivity towards total oxidation of organic pollutants in gas cleaning applications. Electron paramagnetic resonance (EPR) spectroscopy was applied as an appropriate method to detect both the formation of radical species by the NTP as well as the initiation of structural changes to the catalyst.

The presence of paramagnetic oxygen or hydroxyl species (O⁻, O₂⁻ or OH) could not be detected by EPR spectroscopy. The observed signal was not significantly influenced either by the type of atmosphere present during NTP treatment or by applying reducing agents to the sample after plasma treatment.

However, by using non-porous and porous alumina (- and γ-Al₂O₃) as model catalysts, the effect of NTP modifying the surface structure in the interior of a porous material could be clearly demonstrated. A paramagnetic species probably related to an AlOO aluminium peroxyl group was formed by NTP processes independently of the oxygen content of the gas atmosphere. It was not formed when the alumina sample was positioned in the off-gas flow of a plasma reactor, i.e. used in the post-treatment mode.

The structure of the paramagnetic site was investigated by employing several spectroscopic tools (X- and Q-band EPR, electron spin echo envelope modulation [ESEEM] and EPR measurements after pre-deuteration).     

Tetrachloridocuprates(II)-Synthesis and Electron Paramagnetic Resonance (EPR) Spectroscopy

Ionic liquids (ILs) on the basis of metal containing anions and/or cations are of interest for a variety of technical applications e.g., synthesis of particles, magnetic or thermochromic materials. We present the synthesis and the results of electron paramagnetic resonance (EPR) spectroscopic analyses of a series of some new potential ionic liquids based on tetrachloridocuprates(II), [CuCl(4)](2-), with different sterically demanding cations: hexadecyltrimethylammonium 1, tetradecyltrimethylammonium 2, tetrabutylammonium 3 and benzyltriethylammonium 4. The cations in the new compounds were used to achieve a reasonable separation of the paramagnetic Cu(II) ions for EPR spectroscopy. The EPR hyperfine structure was not resolved. This is due to the exchange broadening, resulting from still incomplete separation of the paramagnetic Cu(II) centers. Nevertheless, the principal values of the electron Zeemann tensor (g(║) and g(┴)) of the complexes could be determined. Even though the solid substances show slightly different colors, the UV/Vis spectra are nearly identical, indicating structural changes of the tetrachloridocuprate moieties between solid state and solution. The complexes have a promising potential e.g., as high temperature ionic liquids, as precursors for the formation of copper chloride particles or as catalytic paramagnetic ionic liquids.     

A New and Robust Method for In‐situ EPR Electrochemistry

A new and simple design of electrochemical setup for in‐situ generation of free radicals to be measured using X‐band (9.5 GHz) electron paramagnetic resonance (EPR) spectrometer is described. The cell parts are all from commercially available components and requires no specially made glassware. The EPR performance of the setup is demonstrated by in‐situ electrochemical generation of organic radical cations and radical anions such as quinones, perylene‐diimide, pyrene, flavin, tryptophan and tyrosine through oxidation or reduction in solution. The well‐resolved EPR spectra of the radical products were simulated and analyzed and the hyperfine coupling constants have been assigned for the interactions of the unpaired electron with the various ring protons and the nitrogen nuclei.     

Characterization of Textured Ceramics by Electron Paramagnetic Resonance Spectroscopy: II, Formation and Properties of Textured MgO

A method for the formation of macroscopic quantities of faceted, nanophase MgO precursor powders has been developed that permits a degree of control over the particulate size distribution. By employing powders of this type it was possible to form textured MgO ceramics by means of cold-pressing and sintering. The theoretical basis for the application of EPR techniques to texture analysis as developed in Part I of this work was used to establish the nature of the preferred orientation in these MgO ceramics. Both EPR and X-ray analyses showed that preferred orientations of the 〈111〉 axes of the MgO microstructural grains occur along or near the direction of force used to compact the green body. EPR results also show that this texture develops during the sintering process and that no preferred orientation effects are present in the unsintered body formed by cold-pressing the faceted, nanophase MgO powder. Effects of parametric variations such as sintering temperature, compaction force, and faceted particle size on the degree of preferred orientation in these sintered MgO ceramics were investigated using EPR techniques. The results show that the amount of texturing increases with increasing compaction force or temperature up to a limiting value and that it also increases with decreasing faceted particle size.     

Termination kinetics of free-radical polymerization in ionic liquids

Termination rate coefficients, k_t, for free-radical polymerization of 15 vol.-% methyl methacrylate (MMA) dissolved in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)-imide and in 1-butyl-3-methylimidazolium tetrafluoroborate were measured at 10 °C via the single pulse − pulsed laser polymerization − electron paramagnetic resonance (SP-PLP-EPR) technique. Whereas absolute k_t in ionic liquid solution is by about one order of magnitude below the associated bulk MMA value, the chain-length dependence of k_t is very similar in both liquid environments.     

Effect of Zn substitution on the structural and magnetic properties of nanocrystalline NiFe2O4 ferrites

Nanocrystalline Ni_1?xZn_xFe₂O₄ (where, x = 0.0, 0.2, 0.4, 0.6, 0.8 & 1) samples were synthesized through solution combustion technique using oxylyl de-hydrazide (ODH) as a fuel and the effect of dopant and its concentration on the structural and magnetic properties was investigated. As-prepared samples were characterized using different characterization techniques such as, XRD, SEM-EDS, TEM and Raman spectroscopy for their phase-purity, crystallinity, surface morphology and elemental composition; also magnetic properties were investigated through EPR, Mossbauer spectroscopy and vibrating sample magnetometer (VSM). Rietveld fitted XRD and Raman studies confirm the formation of cubic spinel structured ferrites and substitution of Zn ion at Ni site without formation of impurity phases. No other structural changes were observed and the structure remains in cubic phase with increase of Zn concentration. SEM and TEM micrographs reveal that the particles are agglomerated and the particles size were found in the nano range. Also good stoichiometric composition was observed in all the compositions of Zn substituted Ni ferrite samples. Magnetic measurements (VSM) reveal that pure Ni ferrites exhibits soft magnetic behaviour. Further the ferromagnetic behaviour suppressed with the substitution of diamagnetic Zn ion and with increase of its concentration in Ni ferrites, which was further evidenced in the Mossbauer spectroscopic results. At room temperature, electronic paramagnetic resonance spectra exhibits a broad resonance signal with Lande's g factor varies from 2.23 to 1.95 with increase in Zn content, which is attributed to spin exchange interactions between Fe³⁺, Ni²⁺ and Zn²⁺ ions also asymmetric EPR spectra was observed. The investigated results show that, Zn substitution has greater impact on the magnetic properties of Ni ferrites due to the diamagnetic nature of Zn, which inturn alters the cationic distribution and the exchange interactions between Ni-Fe and Fe-Fe.     

Defects characterization of Dy-doped BaTiO3 ceramics via electron paramagnetic resonance

The point defects and the structural and dielectric properties of Dy-doped BaTiO₃ ceramics prepared at 1400 °C were investigated. The solubility of Dy in the self-compensation mode was determined to be x = 0.07 for (Ba_1−xDy_x)(Ti_1−xDy_x)O₃, and no EPR signals associated with the Dy³⁺ Kramers ion or the Ba and Ti vacancies were detected using the electron paramagnetic resonance (EPR) technique. As x increases, the dielectric behavior changed from a first-order phase transition to a diffuse phase transition to a Y7R dielectric-temperature stability. A strong EPR signal at g = 1.974, which is rare among rare-earth-doped BaTiO₃ ceramics appeared unexpectedly in the single-phase (Ba_1−xDy_x)Ti_1−x/4O₃ ceramics with deliberately designed Ti vacancies. This signal was attributed to ionized Ba vacancy defects. A preference for the self-compensation mode of Dy³⁺ ions is responsible for the appearance of Ba vacancies. The real formula of the nominal (Ba_1−xDy_x)Ti_1−x/4O₃ is expressed as (Ba_1−xDy_3x/4)(Ti_1−x/4Dy_x/4)O₃. In addition, the defect chemistry is discussed.     

EPR study on petroleum cokes annealed at different temperatures and used in lithium and sodium batteries

EPR spectroscopy was used for characterisation of petroleum cokes heat-treated in the temperature range of 480-2800 °C and after their electrochemical reactions with lithium and sodium. Unpaired spins with localised character were detected for cokes treated below 1000 °C, while delocalised electrons were found to contribute to the EPR profile of cokes treated between 1400 and 2800 °C. After electrochemical interaction of cokes with lithium, the EPR signal due to delocalised electrons undergoes a strong line narrowing combined with an increase in signal intensity. Localised paramagnetic centres interact with Li and Na, as a result of which there is a line narrowing and decrease in the signal intensity. A new narrow signal is detected for low-temperature cokes after reaction with lithium. The origin of this signal could be associated with ‘near-metallic’ lithium, which is accommodated in non-ordinary carbon sites such as microcavities. The intensity of this signal is higher for samples treated at 600 °C, where a higher hysteresis in the voltage versus composition curve is observed.     

Oxidation of demineralized coal and coal free of pyrite examined by EPR spectroscopy

The influence of oxidation on complex paramagnetic centres system of demineralized coal and coal free of pyrite was compared. Polish orthocoking coal with a carbon content of 87.8 wt% was studied. This coal was oxidized with nitric acid (HNO₃), peroxyacetic acid (PAA) and in O₂/Na₂CO₃ system. Multi-component structure of X-band EPR spectra of the coal samples was numerically analysed. The lineshape and the parameters of the component lines: linewidths and g-factors, were determined. Concentrations of paramagnetic centres were measured. The three groups of paramagnetic centres belonging to different molecular units were found in the studied samples. They were responsible for broad Gauss (ΔB_pp: 0.49-0.84 mT), broad Lorentz 1 (ΔB_pp: 0.18-0.35 mT), and narrow Lorentz 3 (ΔB_pp: 0.04-0.08 mT) EPR lines. Properties of paramagnetic centres of the simple molecular units with broad Gauss and Lorentz 1 lines were changed during demineralization of coal and after pyrite removing from coal. g-Values of Gauss and Lorentz 1 lines increased, Lorentz 1 lines were broadened and concentrations of paramagnetic centres with Lorentz 1 lines increased. Oxidation of both demineralized coal and coal free of pyrite with nitric acid led to the highest decrease of the concentration of paramagnetic centres with Gauss lines, narrowing of these lines and increase of g-factor. The higher effects of oxidation on paramagnetic centres responsible for broad lines were observed for coal free of pyrite. Paramagnetic centres with narrow Lorentz 3 lines belonging to multi-ring aromatic units in demineralized coal and coal free of pyrite were not susceptible for oxidation.