Low-temperature Electron Paramagnetic Resonance Research Articles

Coexistence of antiferromagnetism and ferromagnetism in Mn2+/CdS nanocrystals and their photophysical properties

Manganous ion doped CdS nanocrystals with different concentrations of Mn2+ have been synthesized by a chemical synthesis route. The as-synthesized Mn2+/CdS nanocrystals evince interesting optical emission properties; the intensity of cadmium vacancy emissions VCd decreases on the addition of the dopant ion, proving the displacement of cadmium by manganese which is accompanied by the appearance and enhancement of the Mn2+ d–d emission, 4T1 → 6A1. However, the intensity of the latter emission decreases on further increase in [Mn2+] due to strong cluster formation, such as Mn2+–Mn2+. The low temperature electron paramagnetic resonance measurements suggest strong antiferromagnetism due to strong exchange coupling between the spins of nearby Mn2+ centres both in pre- and post-annealed 5% Mn2+/CdS samples. These magnetic properties for pre- and post-annealed 5% Mn2+/CdS samples are explained by a core/shell (antiferromagnetism/ferromagnetism) model. The pre-annealed sample, on one hand, shows antiferromagnetic behavior with weak ferromagnetism below the Neel temperature, the latter due to uncompensated surface spins. On the other hand, annealing leads to the formation of more Mn2+–Mn2+ dimers or clusters due to ionic migration. This will result in increased antiferromagnetic character as compared to the pre-annealed sample. Both are confirmed by magnetization hysteresis curves at various temperatures. The electron microscopy images confirm the morphology prediction by XRD.

Electrochemical formation of MnIII-peroxo complexes supported by pentadentate amino pyridine and imidazole ligands

A novel and efficient method for preparing [Mn(III)(O2)(L)](+) complexes using electrochemically generated superoxide is reported, with the reaction probed by low temperature electronic absorption and electron paramagnetic resonance spectroscopic techniques.

ESR intensity and the Dzyaloshinsky-Moriya interaction of the nanoscale molecular magnet V15

The intensity of electron spin resonance (ESR) of the nanoscale molecular magnet ${\mathrm{V}}_{15}$ is studied. We calculate the temperature dependence of the intensity at temperatures from high to low. In particular, we find that the low-temperature ESR intensity is significantly affected by the Dzyaloshinsky-Moriya interaction.

Electron spin resonance study of point defects in thermal GaAs/GaAs-oxide structures

In an attempt to atomically assess interface and oxide-related point defects, a first basic multifrequency low-temperature electron spin resonance study has been carried out on semi-insulating (Fe compensated) GaAs/oxide structures, implying both powders and (100)GaAs/oxide slices, thermally grown in the range Tox=350–615 °C. Various spectra are observed: As for powders, this includes the 4-line EL2 defect spectrum centered at g~2.043 and characterized by the isotropic hyperfine constant Aiso~ 910 G, ascribed to the 75AsGa+ antisite defect. Observed in freshly crushed powder, it substantially increases in density with oxidation, in line with theoretical expectation; A maximum appears reached for at Tox~440 °C. It is not observed in the parent c-GaAs wafer. A second isotropic signal is observed at g~1.937 in powders for Tox in the range 510–615 °C, but only after additional VUV irradiation; it may correspond to As clusters. In bulk (100)GaAs, we observe the 5-branch spectrum of substitutional Fe impurities (spin S=5/2) in GaAs, with inferred crystal field constant a ≈ 360 G, well in line with previous observations. The results are discussed within the framework of advanced theoretical interface and defect models and previous experimental assessment.

Trigonal Mn3 and Co3 clusters supported by weak-field ligands: a structural, spectroscopic, magnetic, and computational investigation into the correlation of molecular and electronic structure.

Transamination of divalent transition metal starting materials (M(2)(N(SiMe(3))(2))(4), M = Mn, Co) with hexadentate ligand platforms (R)LH(6) ((R)LH(6) = MeC(CH(2)NPh-o-NR)(3) where R = H, Ph, Mes (Mes = Mesityl)) or (H,Cy)LH(6) = 1,3,5-C(6)H(9)(NHPh-o-NH(2))(3) with added pyridine or tertiary phosphine coligands afforded trinuclear complexes of the type ((R)L)Mn(3)(py)(3) and ((R)L)Co(3)(PMe(2)R')(3) (R' = Me, Ph). While the sterically less encumbered ligand varieties, (H)L or (Ph)L, give rise to local square-pyramidal geometries at each of the bound metal atoms, with four anilides forming an equatorial plane and an exogenous pyridine or phosphine in the apical site, the mesityl-substituted ligand ((Mes)L) engenders local tetrahedral coordination. Both the neutral Mn(3) and Co(3) clusters feature S = (1)/(2) ground states, as determined by direct current (dc) magnetometry, (1)H NMR spectroscopy, and low-temperature electron paramagnetic resonance (EPR) spectroscopy. Within the Mn(3) clusters, the long internuclear Mn-Mn separations suggest minimal direct metal-metal orbital overlap. Accordingly, fits to variable-temperature magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange interactions through the bridging anilide ligands with exchange couplings ranging from J = -16.8 to -42 cm(-1). Conversely, the short Co-Co interatomic distances suggest a significant degree of direct metal-metal orbital overlap, akin to the related Fe(3) clusters. With the Co(3) series, the S = (1)/(2) ground state can be attributed to population of a single molecular orbital manifold that arises from mixing of the metal- and o-phenylenediamide (OPDA) ligand-based frontier orbitals. Chemical oxidation of the neutral Co(3) clusters affords diamagnetic cationic clusters of the type [((R)L)Co(3)(PMe(2)R)(3)](+). Density functional theory (DFT) calculations on the neutral (S = (1)/(2)) and cationic (S = 0) Co(3) clusters reveal that oxidation occurs at an orbital with contributions from both the Co3 core and OPDA subunits. The predicted bond elongations within the ligand OPDA units are corroborated by the ligand bond perturbations observed by X-ray crystallography.

Present Status and Future Plan at High Magnetic Field Laboratory in Osaka University

We report on the development of experimental apparatus and the achievement of our research since 2006 when we reported “the present status and future plan of research in high magnetic fields” at the RHMF 2006 conference, after brief introduction of history and facility of the High Magnetic Field Laboratory at KYOKUGEN in Osaka University. As for our high-field and multi-frequency electron spin resonance (ESR) apparatus, we have expanded the magnetic field range up to 65 T and the frequency range up to 6 THz. In addition, ultra low temperature ESR apparatus down to 0.1 K for static fields and down to 0.6 K for pulsed fields have been fabricated. Furthermore, we have constructed a wide bore pulse magnet with the bore diameter of 48 mmϕ that enables us to perform resistivity measurements, which have been improved with the signal to noise ratio more than 100 times, under high pressure utilizing a diamond anvil pressure cell.

Visible Light Active Phosphorus-Doped TiO2 Nanoparticles: An EPR Evidence for the Enhanced Charge Separation

Phosphorus-doped TiO2 nanoparticles with visible light activity were prepared by sol–gel method by using Ti(IV) isopropoxide and phosphoric acid as precursors. As prepared samples were calcined at different temperatures, and the obtained samples were characterized by X-ray diffraction, UV–vis spectroscopy, Brunauer–Emmett–Teller (BET) surface area analysis, X-ray photoelectron spectroscopy, scanning emission microscopy, electron paramagnetic resonance (EPR) spectroscopy, the photodegradation of methyleneblue (MB). The results indicate that phosphorus-doping into TiO2 lattice decreases the particle size, increases the thermal stability of titania, and retards the phase transition from anatase to rutile. UV–vis absorption of the P-doped samples shows the redshift in its absorption edge. Doped phosphorus exists in a pentavalent oxidation state by replacing part of lattice Ti4+ by the formation of Ti–O–P bonds. MB degradation profiles with visible light irradiation show that the photocatalytic activity of P-doped Titania is much enhanced and superior to undoped TiO2 and commercial Degussa P25. Low temperature EPR studies with in situ visible light irradiation on the samples heated at different temperatures clearly demonstrates that enhanced charge separation is the major reason for the enhanced photocatalytic activity.

Local environment of optically active Nd3+ions in the ultratransparent BaMgF4ferroelectric crystal

A comprehensive study of the site location of Nd${}^{3+}$ ions in the BaMgF${}_{4}$ ultratransparent ferroelectric crystal is presented. By combining different low-temperature optical spectroscopies and electron paramagnetic resonance, the crystal field energy levels of Nd${}^{3+}$ ions and the gyromagnetic factors are experimentally determined. These results are employed to perform the crystal field analysis of Nd${}^{3+}$ ions considering a Cs point symmetry. The crystal field calculation yields a small root-mean-square deviation of 18 cm${}^{\ensuremath{-}1}$ and reveals a large crystal field strength (621 cm${}^{\ensuremath{-}1}$), verifying the assignment of the Ba${}^{2+}$ cationic site as the location for Nd${}^{3+}$ ions in this fluoride host. The results suggest a slight displacement of Nd${}^{3+}$ from the barium regular site with a rearrangement of the fluorine ions around it. The work gives a deep insight into the properties of the Nd${}^{3+}$-doped BaMgF${}_{4}$ crystal, a ferroelectric widely ultra-transparent material with potential applications as optical device operating in the Vacuum Ultraviolet-Ultraviolet and midinfrared spectral regions.

Gd3+ electron spin resonance spectroscopy on LaO1 − x F x FeAs superconductors

We report an electron spin resonance (ESR) spectroscopy study on polycrystalline samples of the LaO1 − xFxFeAs (x = 0 and 0.1) compound with small levels of Gd doping (2% and 5%). The Gd ESR signal is found to be sensitive to the magnetic phase transition from the paramagnetic to the spin density wave (SDW) state occurring in the parent LaO1 − xFxFeAs compounds at TSDW ∼ 130 K. Interestingly, the analysis of the low-temperature ESR spectra of the c-axis oriented Gd1 − yLayOFeAs samples gives evidence for the magnetically nonequivalent Gd sites and also for sites having a different local charge environment. The analysis of the temperature dependence of the ESR linewidths gives evidence for a coupling of the localized 4f electrons of Gd to the conduction electrons in the FeAs layers. The ESR data reveal that the fluorine substitution, which provides electron doping, suppresses the SDW order and enhances the density of states in the electronic bands stemming from the xz and yz orbital states of Fe to which the 4f electrons are most strongly coupled.

High spin to low spin change induced by reductive chemistry with iron-substituted Dawson polyoxometalate

Abstract Chemical reduction of an iron(III) substituted Dawson 5-molybdo-12-tungsto-2-phosphate α2-K7[Fe(H2O)P2Mo5W12O61]⋅ 17H2O complex, with dithionite in aqueous solution is reported alongside with similar data for the monolacunary Dawson molybdo-tungsto-diphosphates α2-[P2Mo5W12O61]10 −. UV–vis absorption and low-temperature EPR (electron paramagnetic resonance) spectra indicate several distinct species and an unprecedented change of spin state at the ferric center, induced indirectly by reduction of the polyoxometalate framework.

Interface nature of oxidized single-crystal arrays of etched Si nanowires on (100)Si

Low temperature electron spin resonance studies have been carried out on single crystalline arrays of sub-10 nm Si nanowires (NWs) manufactured on (100)Si by top down etching and oxidation thinning. This reveals the presence of a substantial inherent density of Pb0 (Si3 ≡ Si•) defects (traps) at the NW Si/SiO2 interfaces, due to particular faceting and enhanced interface strain, leaving NW interfaces of reduced electrical quality. Perusal of the specific properties of the occurring Pb-type defect system points to a nanopillar morphology compatible with NWs predominantly bordered by {110} facets, with cross sectional shape of 〈100〉 truncated {110} squares. The inherent interface quality appears limited by the wire-narrowing thermal oxidation procedure.

Structural and optical studies of thick freestanding GaN films deposited by Hydride vapor phase epitaxy

Thick freestanding films or bulk GaN substrates with very low background impurity levels (≤1×1015/cm3) and high crystalline quality are required for a number of electronic device applications. Low pressure chemical vapor and molecular beam epitaxy techniques can systematically deposit films with low residual impurity concentrations. However, their typical slow growth rate prevents their utilization for substrate growth. The hydride vapor phase epitaxy deposition technique can achieve hundreds of microns per hour growth rate, but these films have typically high free carrier concentration (≥3×1017/cm3). It is crucial to verify if this method can reproducibly deliver thick freestanding GaN films of high crystalline quality with exceptionally low free carrier concentration. Low temperature photoluminescence and room temperature Raman scattering experiments carried out on a number of samples indicate that they have high crystalline quality and uncommonly low donor background levels. Reduced concentration of uncompensated shallow donors verified by low temperature electron paramagnetic resonance was confirmed by detailed high sensitive SIMS analyses. In addition, it was verified by X-ray diffraction analysis that relatively low dislocation densities can be achieved.

Low-frequency ultrasound induces oxygen vacancies formation and visible light absorption in TiO 2 P-25 nanoparticles

Low-frequency ultrasound (LFUS) irradiation induces morphological, optical and surface changes in the commercial nano-TiO 2-based photocatalyst, Evonik–Degussa P-25. Low-temperature electron spin resonance (ESR) measurements performed on this material provided the first experimental evidence for the formation of oxygen vacancies ( V o), which were also found responsible for the visible-light absorption. The V o surface defects might result from high-speed inter-particle collisions and shock waves generated by LFUS sonication impacting the TiO 2 particles. This is in contrast to a number of well-established technologies, where the formation of oxygen vacancies on the TiO 2 surface often requires harsh technological conditions and complicated procedures, such as annealing at high temperatures, radio-frequency-induced plasma or ion sputtering. Thus, this study reports for the first time the preparation of visible-light responsive TiO 2-based photocatalysts by using a simple LFUS-based approach to induce oxygen vacancies at the nano-TiO 2 surface. These findings might open new avenues for synthesis of novel nano-TiO 2-based photocatalysts capable of destroying water or airborne pollutants and microorganisms under visible light illumination.

Electron spin resonance investigation of ultra-small double walled carbon nanotubes embedded in zeolite nanochannels

We report on the low temperature electron spin resonance (ESR) properties of ultra-small (0.45 nm) double walled carbon nanotubes (DWCNTs) embedded in zeolite nanochannels. An isotropic ESR signal is observed at gc = 2.002 77 with the spin density (S = 1/2) ∼ 1019 g−1, which is suggested to originate from the carbon related point defects in the DWCNTs. Measurements of the ESR line width and signal intensity as a function of temperature indicate that the spins are of a localized nature as opposed to the conduction type electrons observed in large diameter CNTs. The results are consistent with the suggestion that electrons are trapped at interstitial defects. The observed linear frequency dependence of the ESR line width of embedded DWCNTs points to ‘strain’ as the prime source of broadening. By contrast, the study of free standing DWCNTs shows the presence of a distinctly superlinear frequency dependence of the signal width at low temperatures. The possible origin of the frequency dependence is discussed.

Manifestation of non-centrality effect in the EPR spectrum of Fe3+ ion in the polycrystalline materials

In a number of works [1–6], the study of the electron paramagnetic resonance (EPR) spectrum of polycrystalline non-organic and organic matters revealed presence of EPR spectrum of impurity Fe3+ ion which has essentially different forms at helium and room temperatures. At the helium temperature intensity of the resonance line with g-value close to g1 ≅ 4.3 prevails (Line 1 of the low temperature spectrum). The resonance line with g ≅ 4.3 corresponds to magnetic center with spin S = 5/2 by the spin Hamiltonian parameter D ≫ hv. The resonance line with g-value close to 2.0 (Line 2 of the high temperature spectrum) prevails at the room temperatures. The resonance line with g2 ≅ 2.0 corresponds to magnetic center with spin S = 5/2 by the spin Hamiltonian parameter D ≪ hv. The transition from the low- to high-temperature spectrum is gradual and it is accompanied by a redistribution of the absorption intensity. The observed properties of the temperature dependence of the EPR spectrum are characteristic of systems with a multiminimum potential. The described effect of change of integral intensity of low-temperature and high-temperature EPR spectrum of Fe3+ ion is investigated in substances Na[FeO6(C10H8N)3], C21H14Br4O5S, C25H30ClN3, Al2[Si2O5](OH)4, polyaniline and polyparaphenylene.

Mechanism of protection by natural antioxidant ergothioneine against copper-induced oxidative damage to DNA and protein

Ergothioneine is a naturally occurring amino acid found in up to millimolar concentrations in several biological tissues and fluids. The biological functions of ergothioneine are not completely understood. In this study, the role of ergothioneine in copper-induced oxidative damage to DNA and proteins was investigated using the following two copper-containing systems: Cu(II) with ascorbate and Cu(II) with H2O2. Strand breakage and protein carbonyl formation were used as indicators of oxidative damage to DNA and bovine serum albumin, respectively. Ergothioneine provided strong, dose-dependent protection against oxidation of DNA and proteins in both copper-containing systems. By contrast, only limited protection was observed with the hydroxyl radical scavengers dimethyl sulfoxide and mannitol, even with the scavengers at concentrations as high as 100 mmol/L. UV-visible spectroscopy and low-temperature electron spin resonance spectroscopy showed ergothioneine significantly inhibited copper-catalyzed oxidation of ascorbate and competed effectively with histidine and 1,10-phenanthroline for binding of Cu(I) but not Cu(II). In conclusion, ergothioneine is a potent, natural, sulfur-containing antioxidant that prevents copper-dependent oxidative damage to biological macromolecules by forming a redox-inactive ergothioneine-copper complex.

First-Row Transition-Metal Chloride Complexes of the Wide Bite-Angle Diphosphine iPrDPDBFphos and Reactivity Studies of Monovalent Nickel

The diphosphine 4,6-bis(3-diisopropylphosphinophenyl)dibenzofuran (abbreviated as (iPr)DPDBFphos) has been metalated with transition metal dichlorides of zinc, cobalt, and nickel to yield ((iPr)DPDBFphos)MCl(2) complexes. Within these compounds, the diphosphine (iPr)DPDBFphos adapts a wide range of bite angles (115 to 180°) as determined by X-ray crystallography. A three-coordinate planar Ni(I) species was isolated from the reduction of ((iPr)DPDBFphos)NiCl(2) with KC(8). Low-temperature electron paramagnetic resonance (EPR) measurements of ((iPr)DPDBFphos)NiCl allow the determination of g values (2.09, 2.14, 2.37) and hyperfine coupling constants to two (31)P nuclei, A(iso) = 46 × 10(-4) cm(-1), and one (37)Cl/(35)Cl nucleus, A = (12, 0.7, 35) × 10(-4) cm(-1). Density functional theory (DFT) studies reveal the nature of the magnetic orbital to be d(xy), which has σ-antibonding and π(∥)-antibonding interactions with the phosphorus and chloride atoms, respectively. The monovalent nickel complex reacts with substrates containing C-X bonds; and in the case of vinyl chloride, a Ni(II) vinyl species ((iPr)DPDBFphos)Ni(CH═CH(2))Cl is generated along with the Ni(II) dichloride complex. The monovalent Ni(I) chloride is an active catalyst in the Kumada cross-coupling reaction of vinyl chloride and phenyl Grignard reagent.

Inherent interfacial Si dangling bond point defects in thermal (1 1 0)Si/SiO 2

A low-temperature electron spin resonance (ESR) study has been carried out on thermal (1 1 0)Si/SiO 2 as a function of oxidation temperature T ox with the intend to evaluate on atomic level the interface nature in terms of occurring inherent point defects which are at the origin of technologically detrimental traps. This reveals a remarkably high density of P b-type interface centers (interfacial Si dangling bonds), which variant, based on coinciding ESR parameters, is typified as P b0 (110), close to P b0 (100) at the (1 0 0)Si/SiO 2 interface. Starting at 7.5 × 10 12 cm −2, from T ox ≈ 700 °C onward, the density [P b0 (110)] gradually decreases by about 50% for T ox → 1100 °C – a result of global structural relaxation of the top SiO 2 layer as exposed by several ESR parameters. In terms of P b(0) centers, the (1 1 0) face is found to be the worst of all three low-index Si interfaces, i.e., [P b0 (100)] < [P b (111)] < [P b0 (110)], over the range T ox < ∼900 °C. For T ox ⩾ 900 °C, the (1 1 0)Si face does slightly improve over the (1 1 1)Si one, herewith underlining the importance of T ox not only for such comparative defect studies but also for interface optimization.

Low-temperature electron paramagnetic resonance studies on natural calumetite from Khetri copper mine, Rajasthan, India

We report here for the first time low-temperature (123–295 K) electron paramagnetic resonance (EPR) spectroscopic data on naturally occurring calumetite [Cu(OH,Cl2)·2H2O] from the Khetri copper complex, Jhunjhunu district, Rajasthan, India. The sample was characterized by X-ray powder diffraction (XRD), differential thermal analysis (DTA), and thermogravimetric (TG) analysis techniques. DTA scans show two endothermic reactions at about 378 and 598 K, which are attributed to dehydration of the sample. TG data indicate weight loss of up to 24 wt% in the temperature region between 600 and 700 K. The room-temperature EPR spectrum exhibits parallel and perpendicular components centered at g || = 2.26 and g ⊥ = 2.10, respectively. The evaluated spin-Hamiltonian parameters indicate that these resonance signals are characteristic of Cu2+ ions in distorted octahedral symmetry. The population difference (N) between the Zeeman levels has been evaluated and is found to increase with decreasing temperature.

Thermal and EPR spectroscopic studies of novel coordination compound trans-bis(acesulfamato)tetraaquazinc(II) with Mn(II) impurity

The thermogravimetric analysis (TGA) and electron paramagnetic resonance (EPR) studies of powder and single crystals of bis(acesulfamato)tetraaquazinc(II), Zn(acs) 2(H 2O) 4, a novel coordination compound, are carried out. Previously synthesized bis(acesulfamato) tetraaquamanganese(II), Mn(acs) 2(H 2O) 4, is included into the host in trace amount as a paramagnetic probe for EPR analysis. Single crystal EPR spectra at room temperature are resolved and discussed. Low temperature EPR spectra down to 90 K do not show remarkable change. At higher temperatures, however, the TGA and EPR spectra show changes around 335 K and 395 K; the causes and the mechanisms of changes are discussed.