Magnetic properties and EPR spectra of [Cu(L-arginine)2](NO3)2·3H2O

Magnetic and EPR data have been collected for complex [Cu(L-Arg)₂](NO₃)₂·3H₂O (Arg=arginine). Magnetic susceptibility χ in the temperature range 2-160 K, and a magnetization isotherm at T=2.29(1) K with magnetic fields between 0 and 9 T were measured. The observed variation of χT with T indicates predominant antiferromagnetic interactions between Cu(II) ions coupled in 1D chains along the b axis. Fitting a molecular field model to the susceptibility data allows to evaluate g=2.10(1) for the average g-factor and J=−0.42(6) cm⁻¹ for the nearest neighbor exchange coupling (defined as H_ex=-∑J_ijS_i·S_j). This coupling is assigned to syn-anti equatorial-apical carboxylate bridges connecting Cu(II) ion neighbors at 5.682 Å, with a total bond length of 6.989 Å and is consistent with the magnetization isotherm results. It is discussed and compared with couplings observed in other compounds with similar exchange bridges. EPR spectra at 9.77 were obtained in powder samples and at 9.77 and at 34.1 GHz in the three orthogonal planes of single crystals. At both microwave frequencies, and for all magnetic field orientations a single signal arising from the collapse due to exchange interaction of resonances corresponding to two rotated Cu(II) sites is observed. From the EPR results the molecular g-tensors corresponding to the two copper sites in the unit cell were evaluated, allowing an estimated lower limit |J |>0.1 cm⁻¹ for the exchange interaction between Cu(II) neighbors, consistent with the magnetic measurements. The observed angular variation of the line width is attributed to dipolar coupling between Cu(II) ions in the lattice.     

Fabrication and characterization of magnetic nanoporous zeolite templated carbon

The effects of heat-treatment and guest adsorption on the structure and electronic/magnetic properties were investigated for zeolite templated carbon (ZTC) by using potassium, bromine, and helium as guest species. ZTC consists of a curved graphene network, which has regularly arranged nanopores with a periodicity of 1.4 nm. X-ray diffraction and Raman spectra reveal that the regular nanopore structure is preserved during vacuum heat-treatment (<380 °C) and potassium adsorption process. On the other hand, bromine adsorption is found to destroy the nanopore structure. Helium atoms adsorbed in the nanospace participate in the energy dissipation of the radical spins in ZTC through collisions with the spins.     

Changes in the optical properties of as-deposited plasma polymerized 2,6-diethylaniline thin films by iodine doping

In this study, plasma polymerized 2, 6-diethylaniline (PPDEA) thin films of different thicknesses were synthesized using a glow discharge plasma polymerization method. Scanning electron microscopy showed that the surface morphology of an as-deposited PPDEA thin film was comparatively smooth after iodine doping. The iodine-doped PPDEA was found to be thermally stable up to ca about 560 K, which was slightly lower than that observed for as-deposited PPDEA. Ultraviolet-visible spectroscopic analyses demonstrated that iodine doping resulted in a significant decrease in the optical energy gap. As the doping period increased, the direct optical transition energy gap was reduced from 3.56 to 2.79 eV and the indirect optical transition energy gap was decreased from 2.23 to 1.97 eV. Thus it is observed that, the optical parameters of as-deposited PPDEA thin films with different thicknesses can be modified with different iodine doping periods.     

Theoretical investigation of zero-field splitting and local structure distortion for a substitution of Fe ion in CdCl2 crystal

A theoretical method for studying the inter-relation between electron and molecule structure is proposed on the basis of the complete energy matrices of the electron-electron repulsion, the ligand-field and the spin-orbit coupling for d⁵ configuration ion in a trigonal ligand-field. As an application, the local distortion structure of (FeCl₆)^3- coordination complex for Fe³⁺ ions doped into CdCl₂ is investigated. Both the second-order zero-field splitting parameter and fourth-order zero-field splitting parameter are considered simultaneously in the structural investigation. By diagonalizing the complete energy matrices, the local structure distortion parameters ΔR=−0.24 Å, Δθ=2.137° at 26 K and ΔR=−0.203 Å, Δθ=2.515° at 225 K for Fe³⁺ ions in CdCl₂ are determined. These results elucidate a microscopic origin of various ligand-field parameters which are usually used empirically for the interpretation of electron paramagnetic resonance results. It is found that the theoretical results are in good agreement with the experimental values.     

Optical absorption, EPR, infrared and Raman spectral studies of clinochlore mineral

Optical absorption, EPR, Infrared and Raman spectral studies have been carried out on natural clinochlore mineral. The optical absorption spectrum exhibits bands characteristic of Fe²⁺ and Fe³⁺ ions. A band observed in the NIR region is attributed to an intervalence charge transfer (Fe²⁺-Fe³⁺) band. The room temperature EPR spectrum of single crystal of clinochlore mineral reveals the dominance of Fe³⁺ ion exhibiting resonance signals at g=2.66; 3.68 and 4.31 besides one isotropic resonance signal at g=2.0. The EPR studies have been carried out for a polycrystalline sample in the temperature range from 103 to 443 K and for a single crystal of clinochlore mineral in the temperature range 123-297 K. The number of spins (N) participating in resonance at g=4.3 signal of the single crystal of clinochlore mineral has been calculated at different temperatures. The paramagnetic susceptibility (χ) is calculated from the EPR data at different temperatures for single crystal of clinochlore mineral. The Curie constant and Curie temperature values are evaluated from 1/χ versus T graph. The infrared spectral studies reveal the formation of Fe³⁺-OH complexes due to the presence of higher amount of iron in this mineral. The Raman spectrum exhibits bands characteristic of Si-O-Si stretching and Si-O bending modes.     

Yttrium nitride thin films grown by reactive laser ablation

Yttrium nitride thin films were grown on silicon substrates by laser ablating an yttrium target in molecular nitrogen environments. The composition and chemical state were determined with Auger electron, X-Ray photoelectron, and energy loss spectroscopies. The reaction between yttrium and nitrogen is very effective using this method. Ellipsometry measurements indicate that the films are metallic. We attribute this behavior to a small oxygen contamination. Each oxygen atom introduces two additional electrons to the unit cell, resulting in a complex semiconductor-ionic-metallic system. These results are corroborated by first principles total energy calculations of clean and oxygen doped YN.     

Core-level and valence-band characteristics of carbon nitride films with high nitrogen content

Carbon nitride films with high nitrogen content were prepared by reactive pulsed-laser deposition at nitrogen partial pressures varying from 0.1 to 20.0 Torr. It was found that the nitrogen content in the films first increases with increase of the nitrogen pressure, reaches a maximum of 46 at. % at 5.0 Torr, and then decreases to 37 at. % at 20.0 Torr. The almost pure carbon nitride films were systematically characterized by using X-ray photoelectron spectroscopy (XPS) concerning the core-level and valence-band structures. Some fingerprint information, which shows the role of nitrogen in controlling the electronic structure of carbon nitride films, was found based on the XPS studies. With enhancing the nitrogen incorporation, both the binding energy and the peak intensity of the core-level and the valence-band spectra vary systematically as a function of nitrogen content in the films. Received: 26 June 2000 / Accepted: 26 June 2000 / Published online: 20 September 2000     

Optical and photoluminescent properties of plasma polymer films used in electroluminescent display structures

The optical and photoluminescent properties of plasma polymer layers synthesized from hexamethyldisiloxane are examined. The value of the polymer layer transparence is in the limits from 55% at 400 nm to 88% at 800 nm. Photoluminescence is stimulated by using the spectral line λ=365 nm emitted by a Hg spectral lamp.The organosilicon plasma polymers are included as protective and capsulating layers in electroluminescent (EL) structures. The structure obtained is characterized by a significant increase in emission brightness, compared to inorganic protective layers. For EL structures with a chalcogenide protective layer the increase is more than 6 times and for structures with heterogeneous matrix on the base of TiO₂ it is more than 20 times. As a stable covering the organosilicon plasma polymer increases the lifetime of the EL structures too.     

Synthesis, crystal structure, Cu doped EPR and voltammetric studies of bis[N-(2-hydroxyethyl)ethylenediamine]zinc(II) squarate monohydrate

Crystal structure of [Zn(hydet-en)₂]·C₄O₄·H₂O (ZHES) (hydet-en is N-(2-hydroxyethyl)ethylenediamine) complex has been synthesized and characterized by analytical, spectroscopic (IR, UV/Vis) and voltammetric techniques. After doping Cu²⁺ ion, its magnetic environment has been identified by electron paramagnetic resonance (EPR) technique. The title complex crystalizes in monoclinic system with space group P2₁/c and with Z=4. Each hydet-en ligand acts as a tridentate ligand through the two N atoms and the hydroxyl O atom, resulting in a six coordinate Zn(II) ion. The EPR spectra were recorded in three perpendicular planes of Cu²⁺ doped ZHES single crystal. The calculated g and A values indicated that the paramagnetic center is rhombic symmetry with the Cu²⁺ ion having distorted octahedral environment. The molecular orbital bond coefficients of the Cu(II) ion in d⁹ state is also calculated by using EPR and optical absorption parameters. The dianion SQ²⁻ is oxidized reversibly in two consecutive steps to the corresponding radical monoanion and neutral form.     

EPR, luminescence and IR studies of Mn activated ZnGa2O4 phosphor

Electron paramagnetic resonance (EPR), luminescence and infrared spectra of Mn²⁺ ions doped in zinc gallate (ZnGa₂O₄) powder phosphor have been studied. The EPR spectra have been recorded for zinc gallate phosphor doped with different concentrations of Mn²⁺ ions. The EPR spectra exhibit characteristic spectrum of Mn²⁺ ions (S=I=5/2) with a sextet hyperfine pattern, centered at g_eff=2.00. At higher concentrations of Mn²⁺ ions, the intensity of the resonance signals decreases. The number of spins participating in the resonance has been measured as a function of temperature and the activation energy (E_a) is calculated. The EPR spectra of ZnGa₂O₄: Mn²⁺ have been recorded at various temperatures. From the EPR data, the paramagnetic susceptibility (χ) at various temperatures, the Curie constant (C) and the Curie temperature (θ) have been evaluated. The emission spectrum of ZnGa₂O₄: Mn²⁺ (0.08 mol%) exhibits two bands centered at 468 and 502 nm. The band observed at 502 nm is attributed to ⁴T₁→⁶A₁ transition of Mn²⁺ ions. The band observed at 468 nm is attributed to the trap-state transitions. The excitation spectrum exhibits two bands centered at 228 and 280 nm. The strong band at 228 nm is attributed to host-lattice absorption and the weak band at 280 nm is attributed to the charge-transfer absorption or d⁵→d⁴s transition band. The observed bands in the FT-IR spectrum are assigned to the stretching vibrations of M-O groups at octahedral and tetrahedral sites.     

Synthesis, X-ray structure, EPR and optical properties of a ferrocene substituted polychlorotriphenylmethyl radical

The optical, magnetic and electrochemical properties of the octamethylferrocene aldehyde substituted polychlorotriphenylmethyl radical 1 are reported. Radical 1 is prepared in a three step synthetic route starting with a Wittig-Horner reaction to yield (E)-1-formyl-1′-{2-{4-[bis(2,3,4,5,6-pentachlorophenyl)methyl]-2,3,5,6-tetrachloro phenyl}ethen-1-yl}-2,2′,3,3′,4,4′,5,5′-octamethyl ferrocene (6), which is subsequently deprotonated to yield the corresponding anion 7 and finally oxidized to (E)-4-[2-(1′-formyl-2,2′,3,3′,4,4′,5,5′-octamethylferrocen)ethen-1-yl]-2,3,5,6-tetrachlorophenyl-bis(2,3,4,5,6-pentachlorophenyl)methyl radical (1). Radical 1 exhibits a charge-transfer band transition in the near infrared region which is associated with an intramolecular electron transfer from the ferrocene unit (donor) to the radical unit (acceptor) of this dyad molecule. The X-ray crystal structure of [K⁺(18-crown-6)] (E)-[4-[2-(1′-formyl-2,2′,3,3′,4,4′,5,5′-octamethylferrocen)ethen-1-yl]-2,3,5,6-tetrachlorophenyl-bis(2,3,4,5,6-pentachlorophenyl) methide] (7) has been determined. This organic salt shows an interesting one-dimensional polymeric structure formed by the coordination of the K⁺ cation with several atoms of the organic carbanion.     

Defects and impurities in nanodiamonds: EPR, NMR and TEM study

EPR, ¹³C NMR and TEM study of ultradisperse diamond (UDD) samples is reported. The compounds show a high concentration of paramagnetic centers (up to 10²⁰ spin/g), which are due to structural defects (dangling C-C bonds) on the diamond cluster surface. The anomalous reduction in the spin-lattice relaxation time of ¹³C (from several hours in natural diamond to ∼150 ms in UDD clusters) is attributed to the interaction between the unpaired electrons of the paramagnetic centers and nuclear spins. ¹³C NMR line-width reflects the fact that the structure of the UDD surface is distorted in comparison to the ‘bulk’ diamond structure.     

Fabrication and field emission properties of multi-walled carbon nanotube/silicon nanowire array

Silicon nanowire (SiNW) arrays were fabricated on silicon wafers by the metal-assisted chemical etching method. Varied average diameters of SiNW arrays were realized through further treatment in a mixed agent of HF and HNO₃ of certain concentrations. After the treatment, there were more than 93% SiNWs with diameters smaller than 100 nm. The tip of each SiNW was subsequently wrapped with multi-walled carbon nanotubes (MWCNTs) with chemical vapor deposition method. The as-fabricated MWCNT/SiNW arrays were fabricated into electric field emitters, with turn-on field of 2.0 V/μm (current density: 10 μA/cm²), much lower than that of SiNW array (5.0 V/μm). The turn-on electric field of MWCNT/SiNW array decreased with the decreasing of the average diameter of SiNWs, indicating the performance of the field emission is relative to the morphology of SiNWs. As the SiNW array is uniform in height and easy to fabricate, the MWCNT/SiNW array shows potential applications in flat electric display.     

Structural characterisation of [Cu2(bptd) (H2O) Cl4] and [Ni2(bptd)2(H2O)4] Cl4·3H2O; evidence of null intramolecular exchanges by EPR and magnetic measurements

Using the 2,5-bis(2-pyridyl)-1,3,4-thiadiazole (bptd), we recently prepared [Cu₂(bptd) (H₂O) Cl₄] and [Ni₂(bptd)₂ (H₂O)₄] Cl₄, 3H₂O in which the magnetic centres are connected through one diazine+one chloro and two diazine ligand bridges, respectively. These two compounds are the first examples that show null intramolecular magnetic interactions despite M-M distances close to 3.7 Å within perfectly planar edifices:Down to , [Cu₂(bptd)Cl₄(H₂O)] is paramagnetic while, below T_t, half of the Cu²⁺ions interact, leading to residual paramagnetism of one Cu²⁺/f.u. Magnetic susceptibility measurements, EPR and pulsed EPR study indicate the original intermolecular nature of AF exchanges.[Ni₂(bptd)₂(H₂O)₄]Cl₄·3H₂O susceptibility obeys a Curie-law involving pure paramagnetism. Moreover, its EPR spectrum can be interpreted on the basis of virtual S=1 monomers. Below 70 K, Zero Field Splitting (D∼275 G) due to dipolar interactions without magnetic exchanges could be responsible for the LT spectra splitting. For both compounds, the thia role is suggested as partially responsible for the null-in-plane magnetic exchanges.     

Magnetic properties of MnO nanocrystals dispersed in a silica matrix

Magnetic nanocrystalline MnO particles have been synthesized in a silica glass matrix by the sol-gel method at calcination temperatures up to 1000 °C. EPR spectra of 0.1 mol% MnO doped silica gel and glasses studied in the temperature range 10-290 K show with the exception of those samples calcined at 900 and 1000 °C 6-line characteristic Mn(II) hyperfine (HF) lines. Additionally five spin-forbidden doublets have been observed at 100 K and below. Small spreads in spin Hamiltonian parameters (D and E) imply that the ligand field environments of Mn(II) ions embedded in the silica glass are nearly uniform. Monotonous decrease in HF linewidth in going from 120 °C gel to 800 °C calcined glass has been interpreted as the continuous decrease in population of isolated Mn²⁺ ions in silica glass matrix resulting in the decrease of magnetic dipolar interactions leading to the observed decrease in HF linewidth. XRD and TEM of sample calcined at 1000 °C shows the presence of nanocrystals of MnO having orthorhombic crystalline phase and sizes about 10 nm. The thermal behavior of magnetization (zero-field-cooled and field-cooled) and magnetic hysteresis of MnO nanocrystals in the 5-300 K temperature interval have demonstrated that the MnO nanocrystals display superparamagnetic-ferromagnetic transition at low temperatures. X-band EPR linewidth data plotted versus inverse of temperature (1/T) for samples calcined at 900 and 1000 °C (EPR recorded in the vicinity of 0.35 T applied field) depict similar transitions.     

EPR parameters and local lattice structure study of V ions in CdCl2 and CsMgCl3 crystals

The local lattice structure and EPR parameters (D, g_‖, g_⊥) have been studied systematically on the basis of the complete energy matrix for a d³ configuration ion in a trigonal ligand field. By simulating the calculated optical and EPR spectra data to the experimental results, the local distortion parameters (ΔR, Δθ) are determined for V²⁺ ions in CdCl₂ and CsMgCl₃ crystals, respectively. The results show that the local lattice structure of CdCl₂:V²⁺ system exhibits a compression distortion (ΔR=−0.0868 Å) while that of CsMgCl₃:V²⁺ system exists an elongation distortion (ΔR=0.0165 Å). The different distortion may be ascribed to the fact that the radius of V²⁺ ion is smaller than that of Cd²⁺ ion or larger than that of Mg²⁺ ion. Moreover, the relationships between EPR parameter D and local structure parameters (R, θ) as well as the orbital reduction factor k and g‐factors (g_‖, g_⊥) are discussed.     

Observation of three sites for vanadyl in a biomineral, zinc sodium phosphate hexahydrate: an EPR investigation

Single crystal EPR study has been carried at room temperature for VO(II) doped zinc sodium phosphate hexahydrate. Single crystal rotations in each of the three mutually orthogonal crystallographic planes namely bc, ac, and ab indicate three chemically inequivalent sites, with intensity ratios of 25:13:1. The spin Hamiltonian parameters obtained for the two intense sites are: Site I: g_xx=1.983, g_yy=1.985, g_zz=1.933; A_xx=7.39 mT, A_yy=7.15 mT, A_zz=19.03 mT; Site. II: g_xx=1.985, g_yy=1.985, g_zz=1.937; A_xx=7.36 mT, A_yy=7.25 mT, A_zz=18.67 mT. The two VO bond directions in the two sites are approximately at right angles to each other. The powder spectrum clearly indicates two chemically inequivalent sites, confirming the single crystal analysis. Admixture coefficients, Fermi contact, and dipolar interaction terms have also been evaluated.     

Theoretical examination of optical and EPR spectra for Cu ion in K2PdCl4 crystal

In this work, two d-d transition spectra and four EPR parameters g_∥, g_⊥, A_∥, A_⊥ of K₂PdCl₄/Cu²⁺ are uniformly interpreted based on Zhao's crystal-field model. The calculation result is in good agreement with the experiment findings. The ligand spin-orbit coupling is neglected on the calculation, which is consistent with the ab initio result by Hillier et al. [J. Am. Chem. Soc. 98 (1976) 95]     

Electron paramagnetic resonance, optical and electrical properties of vanadyl doped alkali germanoborate glasses

Glasses with composition xGeO₂.(0.30−x)M₂O.0.70B₂O₃ (M=Li, K) containing 2.0 mol% of V₂O₅ have been prepared in the range 0.00≤x≤0.15 by normal melt quenching method. Electron paramagnetic resonance (EPR), optical transmission and absorption spectra and dc conductivity of these glasses have been studied. Spin Hamiltonian parameters (SHPs) of VO²⁺ ions, dipolar hyperfine coupling parameter, P, Fermi contact interaction parameter, K and molecular orbital coefficients (α² and γ²) have been calculated. In GeO₂·Li₂O·B₂O₃ glasses there is no change in the tetragonality of the V⁴⁺O₆ complex and the size of 3d_xy orbit also remains unchanged with increase in GeO₂ content. In GeO₂·K₂O·B₂O₃ glasses, there is an increase in the tetragonality of the V⁴⁺O₆ complex and the 3d_xy orbit expands with increase in GeO₂ content. Values of the theoretical optical basicity, Λ_th, have also been reported. Optical band gap decreases with increase in GeO₂ content. The dc conductivity of these glasses decreases and the activation energy increases with increase in GeO₂:M₂O ratio.     

The change from paramagnetic resonance to ferromagnetic resonance for iron nanoparticles made by the sol-gel method

Sol-gel glass embedded with iron nanoparticles provides fascinating features inheriting paramagnetic, ferromagnetic, and superparamagnetic resonance properties under various compositional weight ratios and annealing conditions. Two spectra arising from paramagnetic Fe³⁺ ions and ferrimagnetic Fe₂O₃ particles both centered at g_e=2.0 compete with intensities as the annealing temperature T_A increases. Meanwhile, the resonance line width of Fe₂O₃ particles sharply decreases due to the strong exchange narrowing. The asymmetric line shape and the rather broad line width can be elucidated by the ferromagnetic resonance of the single domain Fe₂O₃ nanoparticles.