An electron paramagnetic resonance study of defects in PECVD silicon oxides

Electron paramagnetic resonance (EPR) has been used to determine how the type and population of paramagnetic defects are altered firstly by changes to the composition of oxides grown on silicon by plasma enhanced chemical vapor deposition (PECVD) and secondly by subsequent anneal treatments of the various oxide films. Silicon oxide films with refractive index, n, of 1.464, 1.487, 1.508 and 1.536 and thickness of 1 m to 3 m were studied; those of higher index have a higher silicon content. Only E centers are detected in films with n=1.464 and 1.487. Raising n to 1.508 leads to the introduction of other types of paramagnetic centers and a large increase in the average total spin concentration. Increasing n further to 1.536 produces a further small increase in average spin concentration. Identities are proposed for the additional types of defect and the dependence of their population on anneal temperature is also reported.     

An in vitro L-band electron paramagnetic resonance study of highly irradiated whole teeth

Regarding in vivo L-band dosimetry with human teeth, a number of preliminary experiments were carried out that were linked to the resonators response and the relative contribution of enamel to the EPR signal intensity of irradiated whole teeth. The sensitivity of the extended loop resonator varies in the antenna plane, but this variation tends to vanish when the sample is moved away from this plane. When the loop antenna is placed just above the highly irradiated molar, around 88% of the dosimetric signal is due to the crown enamel. The sensitivity inside a birdcage cavity is approximately equal over the volume of a molar; only 30% of the molar's total dosimetric signal results from enamel. Some decrease in the intensity of the dosimetric signal from enamel is observed after irradiation. At room temperature, the signal is reduced by about 20% within 90 days and approaches a plateau with a time constant of about 35 days.     

The electron paramagnetic resonance in copper-modified amorphous carbon

Room-temperature EPR spectra of copper-modified α-C:H films exhibit signals due to the isolated paramagnetic copper centers and dangling carbon bonds. An analysis of variation of the EPR signal intensity depending of the annealing temperature (T _a≤300°C) and relaxation of the EPR and IR absorption signals upon annealing indicates that the annealing-induced modification of the α-C:H films involves hydrogen and oxygen impurities in the amorphous carbon matrix.     

Dose reconstruction with electron paramagnetic resonance spectroscopy of deciduous teeth

In the present study the feasibility of using whole, naturally loose deciduous incisors for dose reconstruction with electron paramagnetic resonance (EPR) spectroscopy was investigated. The properties of EPR signals were analysed before and after laboratory irradiation. The parameters of the native EPR signal of deciduous incisors was found to be different from those from enamel of permanent molars. The native EPR signal of deciduous incisors with peak-to-peak line width of 0.65 mT was located at g = 2.0050. The evaluated parameters of the dosimetric EPR signal (CO2-) of deciduous incisors were in agreement with those for enamel of permanent molars. A detection threshold for absorbed dose of about 100 mGy was estimated.     

A mass spectrometry and electron paramagnetic resonance study of photochemical and thermal aging of triterpenoid varnishes

Photochemical and thermal aging of triterpenoid dammar and mastic resins used as varnishes on paintings were studied using graphite-assisted laser desorption/ionization mass spectrometry. This extends an earlier study on similar materials (Zumbühl et al., Anal. Chem. 1998, 70, 707-715) that focused on photoaging. Progressive aging results in development of groups of signals spaced by 14 and 16 Da, indicating incorporation of oxygen as well as simultaneous loss of hydrogen. Oligomers up to tetramers are formed, while cleavage reactions lead to increased signal intensities in the mass ranges between the oligomers and below the monomers. No major differences were found between the mass spectra of samples aged in light or darkness, except that deterioration was faster in light. Electron paramagnetic resonance spectroscopy revealed similar and significant amounts of radicals in films of dammar stored either in light or in darkness. It is concluded that oxidative radical reactions also take place in darkness and that differences in light and dark aging pathways are minor, although rates may differ. These findings lead to a unified explanation for yellowing of natural resin varnishes, one of the major degenerative changes in the appearance of paintings. It is also shown that the commercially available, nominally fresh resins are already in an advanced stage of oxidation and degradation. Energy-rich substances are formed upon irradiation with sunlight and are believed to restart the autoxidative chain reactions, regardless of storage conditions. As a result, varnishes are oxidized quite quickly (months) even when kept in darkness.     

A study of electron paramagnetic resonance and optical absorption in calcium manganese phosphate glasses containing praseodymium

This paper reports on electron paramagnetic resonance (EPR) and optical absorption studies for Mn ions in praseodymium calcium manganese phosphate glasses. The EPR spectra exhibit three resonance signals at g2.0, g3.3 and g4.8. A six line hyperfine structure spectrum centered at g2.0 has been observed in the EPR spectra of all the glasses at lower concentration of Mn ions. The effects of the concentration of Mn ions and praseodymium oxide on resonance signals have been studied. The temperature dependence of EPR signals were also studied. The intensity of the resonance signals decreases with increase in temperature whereas the linewidths are found to be independent of temperature. As temperature is increased small fluctuations in hyperfine splitting are observed. From the optical absorption spectrum, the crystal field parameters and optical band gap energy were evaluated. The optical band gap energy is found to depend quite sensitively on added manganese content. The theoretical values of optical basicity were also evaluated.     

Lateral diffusion of thiol ligands on the surface of au nanoparticles: an electron paramagnetic resonance study

The lateral mobility of the thiolate ligands on the surface of Au nanoparticles was probed by EPR spectroscopy. This was achieved by using bisnitroxide ligands, which contained a disulfide group (to ensure attachment to the Au surface) and a cleavable ester bridge connecting the two spin-labeled branches of the molecule. Upon adsorption of these ligands on the surface of Au nanoparticles, the two spin-labeled branches were held next to each other by the ester bridge as evidenced by the spin-spin interactions. Cleavage of the bridge removed the link that kept the branches together. CW and pulsed EPR (DEER) experiments showed that the average distance between the adjacent thiolate branches on the Au nanoparticle surface only marginally increased after cleaving the bridge and thermal treatment. This implies that the lateral diffusion of thiolate ligands on the nanoparticle surface is very slow at room temperature and takes hours even at elevated temperatures (90 degrees C). The changes in the distance distribution observed at high temperature are likely due to ligands hopping between the nanoparticles rather than diffusing on the particle surface.     

Synthesis and unusual electron paramagnetic resonance spectrum of metastable nanoclusters of ZnO semiconductor crystallites

Metastable nanoclusters of ZnO semiconductor crystallites, 20 to 30 nm diameter, are synthesized by a reconstructive decomposition of a polymer precursor of dispersed Zn2+ cations in poly vinyl alcohol (PVA) polymer molecules. They have EPR (electron paramagnetic resonance) spectrum of distinct excitonic features. Multiple EPR bands appear in prominent intensities in oxygen vacancies VO+ and singly ionized Oi- and Zn(i)+ interstitials. A paramagnetic VO+ vacancy derives from usual diamagnetic O2- vacancy of VO++ (behaves as if doubly charged compared to the lattice) by addition of one electron. The results demonstrate the existence of a surface-interface or surface barrier layer in free-carrier depletion at the crystallite surface in the clusters and its effects on the Oi- and Zn(i)+ ionization states (determine green photoluminescence). Both VO+ and Zn(i)+ are curable by a thermal annealing in O2 gas. A cured sample of equilibrium structure achieved by heating at approximately 550 degrees C has a single EPR in Oi- at g = 1.990. The results are useful in understanding their correlation with EPR and optical properties in ZnO semiconductors and devices.     

Manganese in apatites,chemical, ligand-field and electron paramagnetic resonance spectroscopy studies

Hydroxapatites precipitated with different amounts of carbonate and Mn²⁺ were studied. Infrared spectra, X-ray diffraction lattice parameters and thermal behaviour indicate that the presence of Mn²⁺ does not have any effect on the formation and properties of the carbonate apatites obtained. Interaction of apatite with a Mn²⁺-containing aqueous solution reveals that the molar uptake of Mn²⁺ by the apatite is higher than the Ca²⁺ released by it. Ligand-field and electron paramagnetic resonance spectroscopy results show that in Mn²⁺-doped Cd₅(PO₄)₃Cl, Mn²⁺ replaces Ca²⁺ in the apatite lattice, but in precipitated carbonate apatite it is in an MnO phase. The conclusion that Mn²⁺ in precipitated carbonate apatites is not incorporated in the apatite crystal is discussed in the light of the size differences between Mn²⁺ and Ca²⁺.     

Photoluminescence, thermally stimulated luminescence and electron paramagnetic resonance of europium-ion doped strontium pyrophosphate

Europium-ion doped strontium pyrophosphate was prepared via a chemical precipitation method to investigate the fluorescence of europium ions, the phosphate radical ions formed upon gamma-ray irradiation and their role in the thermally stimulated luminescence (TSL) of this compound. Fluorescence spectra revealed that europium ions were present in divalent as well as trivalent oxidation states. The measurements of fluorescence life time indicated that Eu³⁺ ions existed in two different types of environments in the lattice. Gamma irradiated europium-ion doped Sr₂P₂O₇ showed the presence of two thermo-luminescence glow peaks at 465 and 565 K; however, no glow was observed in the undoped sample. Electron paramagnetic resonance (EPR) studies of europium-ion doped samples showed signals from Mn²⁺ ions (present as impurity) prior to and after gamma irradiation. Upon gamma irradiation, signals originating from PO₂²⁻, PO₃²⁻ and O₂⁻ radical ions were observed in the undoped and doped samples. In the gamma irradiated europium-ion doped samples, additional low-field EPR signals, attributed to Eu²⁺ ions, were observed. By correlating the TSL and EPR results on europium-ion doped Sr₂P₂O₇, the mechanism for the glow peak at 565 K was identified.     

Exploring the rôle of impurities in non-metallic materials by electron paramagnetic resonance

In a penetrating review on defects in diamond [1], Frank draws conclusions about the depth, temperature and time scale of its genesis. He writes, A diamond is a letter to us from the depths but the perfect crystal is a blank page. The individuality of every diamond resides in its imperfections, and these afford our only clue to what has happened to that crystal since its birth.The present paper advances electron paramagnetic resonance (epr) as a technique for detecting these clues in materials, most of which are more mundane than diamond, and discusses applications to a range of materials problems. It is unfortunate that information in the form of epr spectra is still in a sense encoded, so a secondary aim of this paper is to collect together methods for deciphering the message.Glossary S electron spin = 1/2 × number of unpaired electrons - M component of the spin along the axis of quantisation. Takes values from + S to – S - I nuclear spin - m nuclear spin component, takes values from + I to – I - g spectroscopic splitting factor - Bohr magneton - h Planck's constant - microwave frequency - H magnetic field (strictly magnetic induction B should be used) 1 Gauss = 10^–4 Tesla - H ₀ h/g, H _{0 par} = h /g _par etc - W difference in energy between the individual spin states and their mean energy - A parallel nuclear spin coupling energy - B perpendicular nuclear spin coupling energy - D axial crystal field energy. Other equivalent symbols are b ₂ ⁰ = D = 3B₂ ⁰ - E orthorhombic crystal field energy. Other equivalent symbols are 1/3b ₂ ²=E=B ₂ ² - J exchange coupling energy     

A method for applying strong electric fields in electron paramagnetic resonance at low temperatures

A method is described for studying epr spectra in crystals under the action of an external electric field which is applicable to a RE-1301 spectrometer, in the resonator of which a quartz cryostat extension is housed. A viscous electrically insulating grease is used, making it possible to carry out investigations in electric fields up to 700 kV cm^?1 at T = 77 and 4.2 K.