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International audience; Cobalt-doped ZnO nanoparticles (NPs) with different Co concentrations are investigated by means of Xand Q-band electron spin resonance (ESR) near liquid-helium temperature in both parallel and perpendicular modes. The high crystal quality of the NPs allows for the hyperfine-structure resolution within the single Co 2+ ions' ESR powder spectra. Depending on cobalt concentration, common additional weak ESR lines are detected which are here demonstrated to arise from some Co 2+ high-spin pairs with a distance of about 4-6 Å. ESR simulations show that these 3/2 spin pairs are weakly coupled by an isotropic Heisenberg Hamiltonian with either ferromagnetic or antiferromagnetic J coupling constants, almost identical to those previously detected in bulk and microwire ZnO:Co. The presence of substantial (axial) single-ion anisotropy in ZnO:Co makes the different pairs' resonance positions strongly depending on the J value. For resonance frequency ν in the microwave range, four cobalt pairs can satisfy the condition |J| ∼ hν/3 to resonate at almost zero magnetic field. Such near-zero-field transitions notably resonate in the parallel ESR mode, which is the signature of the gapped nonlinear Zeeman effect, which is of particular interest for highly stable atomic-clock transitions.

International audience; Using electron paramagnetic resonance (EPR), we investigate the electronic and nuclear magnetic properties of ZnO:Co single crystals, grown by the optical furnace method. The high crystal quality of the studied samples allows for the determination of the full hyperfine and g tensors. We explain how the local magnetic anisotropy of the Co 2+ impurities is used as a very fine probe for the local symmetry and crystal quality of the host. The temperature-and power-study of EPR intensities recorded in three static-and microwave-field configurations give a qualitative insight into the dynamics of spin-lattice and spin-spin relaxations. In addition, in the context of nanostructures, we explain how a detailed analysis of the intensities anisotropy can reveal the proportion of ordered and disordered phases.

International audience; The spectral dependence of the light-induced magnetic states generated in zinc oxide nanoparticles by illumination during an electron paramagnetic resonance (EPR) experiment is studied for three incident light wave lengths (451, 405 and 370 nm). The minimal wavelength excitation is found to be in the visible violet region, very close to 405 nm for all of the light-induced EPR lines. This points to a close relationship between their originating centers, despite the difference in their dynamics previously revealed by saturation recovery measurements. The study of the light-induced signals decay with time (after illumination removal) and with temperature (un-der constant illumination) confirms the existence of two groups of lines characterized by different dynamics. The quite unusual persistence of these magnetic states after excitation removal opens promising possibilities for encoding information within semiconducting nanoparticles.

International audience; Planar metal−organic networks are highly promising materials due to their modular nature and wide-ranging possible applications from spintronics up to biosensing. Spin state transitions connect local magnetic properties with structural modifications. In this paper, we report on ab initio calculations for two metal− organic planar networks, the Fe-phthalocyanine (Pc) polymer and its precursor material Fe-tetracyanobenzene (TCNB). The spin-polarized generalized gradient approximation to density functional theory with an explicit treatment of the Hubbard-U correction (SGGA+U) indicates a spin state transition between the well confirmed S = 1 state for Fe-Pc and a local, high-spin S = 2 state at the Fe site for Fe-TCNB. The high-spin state at the Fe site is confirmed by X-ray absorption spectroscopy (XAS) measurements of the Fe-TCNB network on the Au(111) substrate in connection with a multiplet analysis. We propose a possible spin state transition between Fe-TCNB and Fe-Pc by the on-surface synthesis of the latter compound. The ab initio results prove also a high chemical stability of the Fe-TCNB network, metallic and ferromagnetic behavior, as well as a partial screening of the Fe spin S = 2 by two antiparallel electrons on the ligand sites to a state with total spin of S = 1. All of this makes the Fe-TCNB network an interesting material for spintronics applications.

The effects of white-light irradiation on similar to 15.nm diameter ZnO nanoparticles are investigated by means of electron paramagnetic resonance, near liquid-nitrogen and liquid-helium temperatures. Under dark conditions, usual core- and surface-defects are detected, respectively, at g = 1.960 and g = 2.003. Under white-light illumination, the core-defect signal intensity is strongly increased, which is to be correlated to the light-induced conductivitys augmentation. Beside, a four-lines structure appears, with the same gravity center as that of the surface defects. Simulations and intensity power-dependence measurements show that this four-line-structure is very likely to arise from a localized high spin S = 2, induced by light irradiation, and subjected to a weak axial anisotropy. At 85K, this high-spin state can last several hours after the light-irradiation removal, probably due to highly spin-forbidden recombination process. The possible excited resonant complexes at the origin of this signal are discussed. Other light-induced S = 1/2-like centers are detected as well, which depend on the nanoparticles growth conditions. Funding Agencies| [CNRS-FR3443]

The effects of white-light irradiation on ~ 20 nm diameter ZnO nanoparticles are investigated by means of electron paramagnetic resonance, near liquid-nitrogen and liquid-helium temperatures. Under dark conditions, usual core- and surface-defects are detected, respectively at g=1.960 and g = 2.003. Under white-light illumination, the core-defect signal intensity is strongly increased, which is to be correlated to the light-induced conductivity's augmentation. Beside, a four-lines structure appears, with the same gravity center as that of the surface defects. Simulations and intensity power-dependence measurements show that this four-line-structure is very likely to arise from a localized high spin S=2, induced by light irradiation, and subjected to a weak axial anisotropy. At 85 K, this high-spin state can last several hours after the light-irradiation removal, probably due to highly spin-forbidden recombination process. The possible excited resonant complexes at the origin of this signal are discussed. Other light-induced S=1/2-like centers are detected as well, which depends on the nanoparticles growth conditions.

We demonstrate that the low temperature synthesis chemical route can be utilized to control the functionality of zinc oxide (ZnO) nanoparticles (NPs) and nanorods (NRs) for optical and magneto-optical performance. Different structural, optical, electro- and magneto-optical results will be displayed and analyzed. In the first part, we show how high quality ZnO NPs can be efficient for photodegradation using ultra-violet radiation. In the second part we will present our recent results on the control of the core defects in cobalt doped ZnO NR. Here and by using electron paramagnetic resonance (EPR) measurements, the substitution of Co 2+ ions in the ZnO NRs crystal is shown. The relation between the incorporation and core defects concentration will be discussed. The findings give access to the magnetic anisotropy of ZnO NRs grown by the low temperature chemical route and can lead to demonstrate room temperature ferromagnetism in nanostructures with potential for different device applications.

Zinc oxide (ZnO) nanorods grown by the low-temperature (90 degrees C) aqueous chemical method with different cobalt concentration within the synthesis solution (from 0% to 15%), are studied by electron paramagnetic resonance (EPR), just above the liquid helium temperature. The anisotropic spectra of substitutional Co2+ reveal a high crystalline quality and orientation of the NRs, as well as the probable presence of a secondary disordered phase of ZnO: Co. The analysis of the EPR spectra indicates that the disappearance of the paramagnetic native core-defect (CD) at g similar to 1.96 is correlated with the apparition of the Co2+ ions lines, suggesting a gradual neutralization of the former by the latter. We show that only a little amount of cobalt in the synthesis solution (about 0.2%) is necessary to suppress almost all these paramagnetic CDs. This gives insight in the experimentally observed improvement of the crystal quality of diluted ZnO: Co nanorods, as well as into the control of paramagnetic defects in ZnO nanostructures. Funding Agencies|NATO project Science for Peace (SfP), Novel nanostructures [984735]

We demonstrate an efficient possibility to synthesize vertically aligned pure zinc oxide (ZnO) and Co-doped ZnO nanorods (NRs) using the low-temperature aqueous chemical synthesis (90 degrees C). Two different mixing methods of the synthesis solutions were investigated for the Co-doped samples. The synthesized samples were compared to pure ZnO NRs regarding the Co incorporation and crystal quality. Electron paramagnetic resonance (EPR) measurements confirmed the substitution of Co2+ inside the ZnO NRs, giving a highly anisotropic magnetic Co2+ signal. The substitution of Zn2+ by Co2+ was observed to be combined with a drastic reduction in the core-defect (CD) signal (g similar to 1.956) which is seen in pure ZnO NRs. As revealed by the cathodoluminescence (CL), the incorporation of Co causes a slight red-shift of the UV peak position combined with an enhancement in the intensity of the defect-related yellow-orange emission compared to pure ZnO NRs. Furthermore, the EPR and the CL measurements allow a possible model of the defect configuration in the samples. It is proposed that the as-synthesized pure ZnO NRs likely contain Zn interstitial (Zn-i(+)) as CDs and oxygen vacancy (V-O) or oxygen interstitial (O-i) as surface defects. As a result, Co was found to likely occupy the Zn-i(+), leading to the observed CDs reduction and hence enhancing the crystal quality. These results open the possibility of synthesis of highly crystalline quality ZnO NRs-based diluted magnetic semiconductors using the low-temperature aqueous chemical method. Published by AIP Publishing. Funding Agencies|NATO [984735]

International audience; Pure and cobalt-doped zinc oxide aligned nanorods have been grown by the low-temperature (90 °C) aqueous chemical method on amorphous ZnO seed layer, deposited on a sapphire substrate. High crystallinity of these objects is demonstrated by the electron paramagnetic resonance investigation at liquid helium temperature. The successful incorporation of Co2+ ions in substitution of Zn2+ ones in the ZnO matrix has also been confirmed. A drastic reduction of intrinsic ZnO nanorods core defects is observed in the Co-doped samples, which enhances the structural quality of the NRs. The quantification of substitutional Co2+ ions in the ZnO matrix is achieved by comparison with a reference sample. The findings in this study indicate the potential of using the low-temperature aqueous chemical approach for synthesizing material for spintronics applications.Keywords

Quantum-chemical calculations are performed at the semiempirical Valence-Bond/Hartree–Fock level on model 1D aggregates to assess the electronic structure at pentacene/C60 heterojunctions. We show that the asymmetry of the electrostatic potential at the interface profoundly impacts the energy landscape explored by the charges, as they move away from the interface. Depending on the orientation of the pentacene molecules with respect to the interface, electrostatic effects may favor either the charge recombination or separation process. © 2012 Wiley Periodicals, Inc.

The single ion magnetic anisotropy of S-state transition metal ions (3d 5 ) in wurtzite diluted magnetic semiconductors (DMS) is studied from a microscopic point of view. Starting from a hamiltonian including Coulomb, spin-orbit and effective crystal field interactions, the axial spin hamiltonian parameter is calculated by using an exact diagonalization method. Application to some Mn-doped DMS validates the model.

The electronic and magnetic properties of Mn coordinated to 1,2,4,5-tetracyanobenzene (TCNB) in the Mn-TCNB 2D metal-ligand networks have been investigated by combining scanning tunneling microscopy and X-ray magnetic circular dichroism (XMCD) performed at low temperature (3 K). When formed on Au(111) and Ag(111) substrates the Mn-TCNB networks display similar geometric structures. Magnetization curves reveal ferromagnetic (FM) coupling of the Mn sites with similar single-ion anisotropy energies, but different coupling constants. Low-temperature XMCD spectra show that the local environment of the Mn centers differs appreciably for the two substrates. Multiplet structure calculations were used to derive the corresponding ligand field parameters confirming an in-plane uniaxial anisotropy. The observed interatomic coupling is discussed in terms of superexchange as well as substrate-mediated magnetic interactions., Comment: J. Phys. Chem. C 2014

In conventional analyses of g approximately 5 signals given by [4Fe-4S](+) clusters with S = 3/2, the effective g values that cannot be measured in the electron paramagnetic resonance (EPR) spectrum are deduced from rhombograms calculated by assuming that the g matrix is isotropic with g(x) = g(y) = g(z) = 2.00. We have shown that when the two low-field peaks corresponding to the Kramers doublets are visible in the spectrum, a new, independent piece of information about the system can be obtained by studying the temperature dependence of the ratio of the area under these peaks. By applying this method to the g approximately 5 signals displayed by NarGHI nitrate reductase, we were able to determine all the parameters of the spin Hamiltonian of FS0 centers with S = 3/2 and to measure accurately their number. Our results indicate that simple analyses based on the assumption of an isotropic g matrix can give rise to very large errors.