Isotropic G-value Research Articles

Monoanionic Molybdenum and Tungsten Tris(dithiolene) Complexes: A Multifrequency EPR Study

Numerous Mo and W tris(dithiolene) complexes in varying redox states have been prepared and representative examples characterized crystallographically: [M(S(2)C(2)R(2))(3)](z) [M = Mo, R = Ph, z = 0 (1) or 1- (2); M = W, R = Ph, z = 0 (4) or 1- (5); R = CN, z = 2-, M = Mo (3) or W (6)]. Changes in dithiolene C-S and C-C bond lengths for 1 versus 2 and 4 versus 5 are indicative of ligand reduction. Trigonal twist angles (Θ) and dithiolene fold angles (α) increase and decrease, respectively, for 2 versus 1, 5 versus 4. Cyclic voltammetry reveals generally two reversible couples corresponding to 0/1- and 1-/2- reductions. The electronic structures of monoanionic molybdenum tris(dithiolene) complexes have been analyzed by multifrequency (S-, X-, Q-band) EPR spectroscopy. Spin-Hamiltonian parameters afforded by spectral simulation for each complex demonstrate the existence of two distinctive electronic structure types. The first is [Mo(IV)((A)L(3)(5-•))](1-) ((A)L = olefinic dithiolene, type A), which has the unpaired electron restricted to the tris(dithiolene) unit and is characterized by isotropic g-values and small molybdenum superhyperfine coupling. The second is formulated as [Mo(V)((B)L(3)(6-))](1-) ((B)L = aromatic dithiolene, type B) with spectra distinguished by a prominent g-anisotropy and hyperfine coupling consistent with the (d(z(2)))(1) paramagnet. The electronic structure disparity is also manifested in their electronic absorption spectra. The compound [W(bdt)(3)](1-) exhibits spin-Hamiltonian parameters similar to those of [Mo(bdt)(3)](1-) and thus is formulated as [W(V)((B)L(3)(6-))](1-). The EPR spectra of [W((A)L(3))](1-) display spin-Hamiltonian parameters that suggest their electronic structure is best represented by two resonance forms {[W(IV)((A)L(3)(5-•))](1-) ↔ [W(V)((A)L(3)(6-))](1-)}. The contrast with the corresponding [Mo(IV)((A)L(3)(5-•))](1-) complexes highlights tungsten's preference for higher oxidation states.

A photosensitive Cr3+ center in photorefractive Bi12SiO20 crystals co-doped with chromium and phosphorus

A trigonal Cr3+ (3d3) defect in sillenite type Bi12SiO20 crystals co-doped with chromium and phosphorus has been identified by means of multi-frequency X-band (9.4 GHz), Q-band (35 GHz) and W-band (94 GHz) electron paramagnetic resonance (EPR). A consistent analysis of the observed spectra and their angular dependence was reached using an S = 3/2 spin Hamiltonian with axial symmetry around a 〈111〉 crystallographic direction, an isotropic g-value g = 1.983, and a zero field splitting parameter B20 = 0.1950 cm−1. The spectra are attributed to a chromium ion in the unusual Cr3+ valence replacing a substitutional Si4+ in tetrahedral oxygen coordination. Evidence is found that the symmetry lowering from tetrahedral to trigonal is not spontaneous but induced by an associated defect, for which a P5+ ion in a nearest-neighbor Si4+ site is the most plausible candidate. Optical excitation results in a reversible charge transfer process directly correlated with a photochromic effect: near-UV light leads to a strong reduction of the Cr3+ EPR signals, and induces a broad band at 675 nm in the absorption spectrum, and both effects are reversed under red light excitation.

Radiation-induced defects in GaN

Radiation-induced defects in n-type GaN irradiated by 2 MeV electrons at room temperature were studied by electron paramagnetic resonance (EPR). Four EPR spectra, labelled D1–D4, were observed in irradiated n-type GaN. The D1 spectrum is a broad line (∼10–12 mT in line width) with an isotropic g-value g∼2.03 and can be detected in all the studied samples in the temperature range of 4–300 K. The D2 centre has an electron spin S=1/2 and shows a clear hyperfine structure due to interaction with three equivalent 14N. The g-values of the axial configuration are determined to be g∥=2.001 and g⊥=1.999. On the basis of the observed hyperfine structure, formation conditions and annealing behaviour, the D2 defect was assigned to the gallium vacancy–oxygen pair in the negative charge state, (VGaON)–.

Characterization, photoluminescence, thermally stimulated luminescence and electron spin resonance studies of Eu 3+ doped LaAlO 3 phosphor

Europium-doped lanthanum aluminate (LaAlO 3) powder was prepared by using a combustion method. The crystallization, surface morphology, specific surface area and luminescence properties of the samples have been investigated. Photoluminescence studies of Eu doped LaAlO 3 showed orange-reddish emission due to Eu 3+ ions. LaAlO 3:Eu 3+ exhibits one thermally stimulated luminescence (TSL) peak around 400 °C. Room temperature electron spin resonance spectrum of irradiated phosphor appears to be a superposition of two centres. One of them (centre I) with principal g-value 2.017 is identified as an O − centre while centre II with an isotropic g-value 2.011 is assigned to an F + centre (singly ionized oxygen vacancy). An additional defect centre observed during thermal annealing around 300 °C grows with the annealing temperature. This centre (assigned to F + centre) originates from an F-centre (oxygen vacancy with two electrons) and the F-centre along with the associated F + centre appear to correlate with the observed TSL peak in LaAlO 3:Eu 3+ phosphor. The activation energy for this peak has been determined to be 1.54 eV from TSL data.

EPR characterization of ascorbyl and sulfur dioxide anion radicals trapped during the reaction of bovine Cytochrome c Oxidase with molecular oxygen

The reaction intermediates of reduced bovine Cytochrome c Oxidase (CcO) were trapped following its reaction with oxygen at 50μs–6ms by innovative freeze-quenching methods and studied by EPR. When the enzyme was reduced with either ascorbate or dithionite, distinct radicals were generated; X-band (9GHz) and D-band (130GHz) CW-EPR measurements support the assignments of these radicals to ascorbyl and sulfur dioxide anion radical (SO2-), respectively. The X-band spectra show a linewidth of 12G for the ascorbyl radical and 11G for the SO2- radical and an isotropic g-value of 2.005 for both species. The D-band spectra reveal clear distinctions in the g-tensors and powder patterns of the two species. The ascorbyl radical spectrum displays approximate axial symmetry with g-values of gx=2.0068, gy=2.0066, and gz=2.0023. The SO2- radical has rhombic symmetry with g-values of gx=2.0089, gy=2.0052, and gz=2.0017. When the contributions from the ascorbyl and SO2- radicals were removed, no protein-based radical on CcO could be identified in the EPR spectra.

Studies of defects in combustion synthesized europium-doped LiAl 5O 8 red phosphor

Europium-doped LiAl 5O 8 red phosphor was prepared using a self-propagating (combustion) synthesis. The formation of crystalline LiAl 5O 8 was confirmed by X-ray diffraction. The morphological aspect of the resulting powders was examined by scanning electron microscopy. Electron spin resonance studies have been carried out in order to study the characteristics of the defect centres and the thermoluminescence (TL) peaks observed in this phosphor. Two types of centres (centre I and centre II) have been identified in LiAl 5O 8:Eu. Centre I is characteristic of a species exhibiting an isotropic g-value 2.0089 with a line width of 70 G and is assigned to a V-centre. Centre II is also characterized by an isotropic g-value 2.0059 with a line width of about 10 G. Centre II is ascribed to a F +-centre. A room temperature photoluminescence study shows a strong emission line at 613 nm corresponding to the 5 D 0→ 7 F 2 transition of Eu 3+ ions.

Electron paramagnetic resonance study on n-type electron-irradiated 3C-SiC

Electron Paramagnetic Resonance (EPR) was used to study defects in n-type 3C-SiC films irradiated by 3-MeV electrons at room temperature with a dose of 2×1018 cm-2. After electron irradiation, two new EPR spectra with an effective spin S = 1, labeled L5 and L6, were observed. The L5 center has C3v symmetry with g = 2.004 and a fine-structure parameter D = 436.5×10-4 cm-1. The L5 spectrum was only detected under light illumination and it could not be detected after annealing at ~550°C. The principal z-axis of the D tensor is parallel to the <111>-directions, indicating the location of spins along the Si-C bonds. Judging from the symmetry and the fact that the signal was detected under illumination in n-type material, the L5 center may be related to the divacancy in the neutral charge state. The L6 center has a C2v-symmetry with an isotropic g-value of g = 2.003 and the fine structure parameters D = 547.7×10-4 cm-1 and E = 56.2×10-4 cm-1. The L6 center disappeared after annealing at a rather low temperature (~200°C), which is substantially lower than the known annealing temperatures for vacancy-related defects in 3C-SiC. This highly mobile defect may be related to carbon interstitials.

Defect centers and thermoluminescence in Al 2O 3:Si,Ti

Al 2O 3:Si,Ti is a sensitive thermoluminescent phosphor. ESR studies have been carried out in order to study the characteristics of the defect centers and the TL peaks observed in this phosphor. Two types of ESR centers (center I and center II) have been identified in Al 2O 3:Si,Ti. Center I is characteristic of a species exhibiting an isotropic g-value 2.0096 with a line width of 50 G. Center II is also characterized by an isotropic g-value 1.9526 with a line width of about 40 G. Thermal decay of center I shows three different annealing stages in the temperature ranges 125 – 225 ∘ C , 430 – 525 ∘ C and 600 – 680 ∘ C , which indicates correlation between center I and TL peaks at 175, 475 and 625 ∘ C , respectively. Decay of center II in the region 600 – 700 ∘ C appears to relate with the TL peak around 625 ∘ C .

A rapid combustion process for the preparation of MgSrAl 10O 17:Eu 2+ phosphor and related luminescence and defect investigations

Blue-emitting europium-ion-doped MgSrAl 10O 17 phosphor, prepared using the combustion method, is described. An efficient phosphor can be prepared by this method in a muffle furnace maintained at 500 °C in a very short time of few minutes. The phosphor is characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy and BET surface area measurements. Photoluminescence (PL) spectra revealed that europium ions were present in divalent oxidation state. The thermoluminescence (TL) glow curve shows two peaks at around 178 and at 354 °C. The defect centres formed in the phosphor are studied using electron spin resonance (ESR). The ESR spectrum indicates the presence of Fe 3+ ions in the non-irradiated system. Irradiated MgSrAl 10O 17:Eu exhibits lines due to radiation-sensitive Fe 3+ ion and a defect centre. The centre is characterized by an isotropic g-value of 2.0012 and is assigned to a F + centre. The radiation-sensitive Fe 3+ ion appears to correlate with the main TL peak at 178 °C. During irradiation an electron is released from Fe 2+ and is trapped at an anion vacancy to form F + centre. During heating, an electron is liberated from the defect centre and recombines with Fe 3+ emitting light.

Unique Coordination of Copper in Hexacyanometallates

Within divalent transition metals hexacyanometallates (III) the copper (2+) salts show unique features. To the copper (2+) salts correspond the shortest unit cell edges, the highest ν(CN) vibration frequencies, the lowest hydration degree and dehydration temperatures, the lowest Mössbauer isomer shift value, and the strongest magnetic exchange interaction (J) between the metal centers. Such unique features were attributed to a particularly strong bond of the copper atom to the N ends of the CN groups. The driving force for such behaviour was ascribed to a combined effect where the copper (2+) shows a high ability to receive electrons in its 3d hole favouring an electronic configuration close to 3d 10 and the CN group complements such ability donating electrons through its 5σ orbital which has certain anti-bonding character. This hypothesis is supported by the obtained structural and spectroscopic data. The occurrence of a cooperative Jahn–Teller effect in this family of materials was discarded. In the pseudo-octahedral coordination for the copper (2+) atom the e g orbital degeneration is initially removed. The collected EPR spectra are characteristic of a cubic environment (isotropic g-values). Analogue evidence was obtained from high resolution X-ray powder patterns recorded in the 12–300K temperature range. All the patterns correspond to a cubic unit cell.

Electron Spin Resonance (ESR) and Microwave Absorption Studies in TbMnO$_3$Multiferroic Compound

We report temperature dependent X-Band (f~9.5 GHz) Electron Spin Resonance (ESR) and microwave absorption measurements in a single crystal of TbMnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> . A single Lorentzian ESR line with an isotropic g-value g~1.96 was observed for Tges120 K up to 600 K. The ESR signal is attributed to the Mn <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ions in a insulator environment. The temperature dependence of the ESR linewidth is investigated for the external field applied along the three crystallographic axes. For all direction the ESR linewidth show a strong broadening as the temperature decreases due to the presence of short range magnetic correlations. The microwave absorption around the ferroelectric transition T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">lock</sub> ~27 K was investigated using a special cavity with a TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">103</sub> mode where the sample sits at the maximum microwave electric-field

Synthesis, photoluminescence, thermoluminescence and electron spin resonance investigations of CaAl 12O 19:Eu phosphor

The present paper describes the synthesis of europium-doped calcium aluminate phosphor using the combustion method. An efficient blue emission phosphor can be prepared at reaction temperatures as low as 500 °C in a few minutes by this method. Characterization of the powder was done by X-ray diffraction, transmission electron microscopy, scanning electron microscope analysis and the optical properties were studied by photoluminescence spectra. Thermoluminescence (TL) studies also have been carried out on CaAl 12O 19:Eu 2+ phosphor. The TL glow curve shows peaks at 174 and 240 °C. Defect centres formed in irradiated phosphor have been studied using the technique of electron spin resonance. Step annealing measurements indicate that one of the annealing stages of a defect centre appear to correlate with the release of carriers resulting in TL peak at 174 °C. The centre is characterized by an isotropic g-value of 2.0046 and is assigned to a F + centre.

High-field (94-GHz) EPR spectroscopy on the S 2 multiline signal of photosystem II

The multiline signal of the S 2 state in Photosystem II was measured both in frozen-solution and single-crystal preparations from the cyanobacterium Thermosynechococcus elongatus. The frozen-solution EPR spectrum shows a gaussian-like line shape without any resolution of Mn hyperfine couplings. This line shape can be understood on the basis of the single-crystal spectra, where a strong orientation dependence of partially resolved hyperfine structures appears. Simulation of the frozen-solution spectrum on the basis of Mn hyperfine couplings taken from published pulse-ENDOR data yields a fully rhombic g-matrix for the multiline signal with principal components 1.997, 1.970, and 1.965. The resulting isotropic g-value g iso = 1.977 is surprisingly small compared to other manganese complexes containing manganese ions in the formal oxidation states three and four.

Luminescence and defect studies of Ce 3+ doped CaS phosphor synthesized via solid state diffusion method

Cerium dope CaS phosphor has been successfully prepared by solid state diffusion method. The phosphor is characterized by X-ray powder diffraction, transmission electron microscopy, selected area electron diffraction, thermogravimetric analysis and photoluminescence. Thermoluminescence and electron spin resonance studies also have been carried out on CaS:Ce phosphor. Irradiated CaS:Ce exhibits electron spin resonance (ESR) lines due to defect centres. The thermal annealing behaviour of one of the defect centres appears to correlate with the thermoluminescence (TL) peak at 135 °C. The centre is characterized by an isotropic g-value of 2.0036 and is assigned to a F + centre.

Preparation, luminescence and defect studies of Eu 2+-activated strontium hexa-aluminate phosphor prepared via combustion method

Preparation of Eu 2+ ions activated strontium hexa-aluminate phosphor using the combustion method is described. An efficient phosphor can be prepared by this method at reaction temperatures as low as 500 °C in a few minutes. Powder X-ray diffraction (XRD), transmission electron microscopy, scanning electron microscope analysis were used to characterize the as prepared product and the optical properties were studied by photoluminescence (PL) spectra. Thermally stimulated luminescence studies also have been carried out on SrAl 12O 19:Eu 2+ phosphor. The TSL glow curve is broad and indicates two dominant peaks at 206 and 345 °C. Defect centres formed in irradiated phosphor have been studied using the technique of electron spin resonance. One of the centres is characterized by an isotropic g-value of 2.0055 and is assigned to a F + centre. The two annealing stages of F + centre in the region 125–230 and 340–390 °C appear to correlate with the release of carriers resulting in TSL peaks at 206 and 345 °C, respectively.

Preferential substitution of Fe on physically equivalent Ga sites in GaN

The EPR spectra of Fe 3 + in high-quality thick freestanding hydride vapor phase grown GaN have been studied in the X- and Q-band. A complex resonance pattern with numerous lines of different intensities provided by three different defects is observed for these nearly stress-free iron-doped samples. The dominating part is due to isolated Fe 3 + on Ga sites and can be well described with a spin Hamiltonian (SH) with S = 5 2 considering the two magnetically inequivalent Ga sites with C 3 v symmetry in the wurtzite structure of GaN. Aside from the displacement of their magnetic axis the two Ga sites are physically equivalent. However, contrary to the expectation, we observed different Fe populations of both sites that varied from a ratio 1:4 to a ratio of nearly 1:1 in different crystals. In addition and in contrast to previously published data we determined nearly isotropic g-values and larger values for the axial fine-structure parameters D , a – F and much larger value for the cubic fine-structure parameter a.

Synthesis and characterization of Ce-doped SrS phosphors

SrS:Ce phosphor was prepared by a solid-state diffusion method. Powder X-ray diffraction, transmission electron microscopy, and thermogravimetric analysis were used to characterize the as-prepared product, and the optical properties were studied by photoluminescence spectra. In addition, thermoluminescence and electron spin resonance studies have been carried out on SrS:Ce phosphor. The TL glow curve is broad and indicates two dominant peaks at 137 and 275°C, with an additional peak, not clearly resolved, appearing as shoulder at ∼362°C. Two defect centers are observed at room temperature. One of them is characterized by an isotropic g-value of 2.0039 and is assigned to an F+ center. Step annealing measurements indicate a possible association between the F+ center and the 137°C TL peak.

Defect centres and thermoluminescence in SrS:Bi phosphor

Thermoluminescence (TL) and electron spin resonance studies have been carried out on SrS:Bi phosphor. The TL glow curve is broad and indicates a dominant peak at 120 °C with two additional peaks, not clearly resolved, appearing as shoulders at around 180 and 250 °C. Two defect centres are observed at room temperature. One of them is characterized by an isotropic g-value 2.0034 and is assigned to an F+ centre. Step annealing measurements indicate a possible association between the F+ centre and the three TL peaks.

Synthesis of silicon oxynitride layers by dual ion-implantation and their annealing behaviour

Single crystal n-type silicon samples were implanted at room temperature sequentially by molecular oxygen ( 16O 2 +) and nitrogen ( 14N 2 +) in different proportions to high fluence levels ranging from 5 × 10 16 to 1 × 10 18 ions cm −2 to synthesize silicon oxynitride layers of various compositions. Rapid thermal annealing (RTA) of some samples was carried out at different temperatures in nitrogen ambient. Fourier transform infrared (FTIR) measurements were performed on as-implanted and on annealed samples. The FTIR studies show that the structure of ion-beam synthesized oxynitride layers are strongly dependent on total ion-fluence as well as on the ratio of implanted oxygen and nitrogen. The Si x O y N z structures formed at lower fluence levels are homogeneous, at intermediate fluence levels are composed of separate phases of oxide, nitride and some complexes of O, N and Si and at higher fluence levels seem to have again homogeneous silicon oxynitride complex structures (Si x O y N z ) due to sputter limited profile of implanted ions. RTA studies show pronounced structural changes of the ion-beam synthesized layers on heat treatment at different temperatures. The spectra of annealed samples showed shift of the peak towards higher wave number. The ESR signal of silicon samples implanted to different fluence levels exhibited an isotropic g-value of 2.0045 corresponding to D-center with line width 8 G. The spin density was found to decrease with increase in ion-fluence.

Negatively Charged Donor Centers in Ultrathin ZnSe:N Layers

Pseudomorphically grown p-ZnSe:N layers being as thin as 100, 20, 10 and 5 nm embedded in ZnMgSSe barriers were investigated by means of photoluminescence and spin-flip-Raman (SFR) scattering spectroscopy. A new donor-acceptor-pair (DAP) band has been observed involving a very shallow donor with a binding energy of (19 ± 2) meV and the well-known N acceptor on Se site. Further evidence for a new donor center is given by transitions in angle-dependent SFR scattering measurements which correspond to an isotropic (donor-like) g-value of (1.49 ± 0.03). The 19 meV donor is interpreted as the negatively charged state of the usually found 52 meV deep donor in ZnSe:N. This is supported by the temperature behavior of the DAP band and, further, by its pronounced appearance for resonant excitation into the barriers quite similar to the behavior of trions.