Light-induced High-spin State in ZnO Nanoparticles

International audienceThe 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

Lesiones adquiridas de los tejidos duros dentarios

Las lesiones de tipo adquirido son de causas predominantemente mecánicas, aunque existen otros agentes capaces de lesionar la pieza dentaria. Así tenemos: fractura, puede ser coronaria (adamantina, amelodentinaria o amelodentinopulpar) o radicular (cemento-dentino-pulpar o cementaria); atricción fisiológica (masticación) o patológica (bruxismo); abrasión, en el sector cervical (por sustancias abrasivas). De causa química: pigmentación endógena (hemorragia interna) o exógena (tabaco); erosión endógena (vómitos) o exógena (ácidos industriales) y de causa multifactorial como la caries dental. Estas lesiones deben diferenciarse de malformaciones, la importancia de su conocimiento facilita el plan de tratamiento adecuado para la salud bucal.Facultad de Odontologí

Core-defect reduction in ZnO nanorods by cobalt incorporation

International audienceZinc oxide (ZnO) nanorods grown by the low-temperature (90 • 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 Co 2+ 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 ∼ 1.96 is correlated with the apparition of the Co 2+ 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

Electronic and nuclear magnetic anisotropy of cobalt-doped ZnO single-crystalline microwires

International audienceUsing 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

Core-defect reduction in ZnO nanorods by cobalt incorporation

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]</p

Coherent spin dynamics of solitons in the organic spin chain compounds (oo-DMTTF)2X_2X (XX = Cl, Br)

14 pages, 8 figuresInternational audienceWe studied the magnetic properties, in particular dynamics, of the correlated spins associated with natural defects in the organic spin chain compounds ($o$-DMTTF)$_2X$ ($X =$ Br, Cl) by means of electron spin resonance (ESR) spectroscopy. Both materials exhibit spin-Peierls transitions at temperatures around 50 K [Foury-Leylekian et al., Phys. Rev. B 84, 195134 (2011)], which allow a separation of the properties of defects inside the chains from the magnetic response of the spin chains. Indeed, continuous wave ESR measurements performed over a wide temperature range evidence the evolution of the spin dynamics from being governed by the spins in the chains at elevated temperatures to a low-temperature regime which is dominated by defects within the spin-dimerized chains. In addition, contributions of triplon excitations to the ESR response below the transition temperature were observed which provides a spectroscopic estimate for the spin-gap of the studied systems. Moreover, details of spin dynamics deep in the spin-Peierls phase were investigated by pulse ESR experiments which revealed Rabi-oscillations as signatures of coherent spin dynamics. We discuss the results obtained from these complementary methods of ESR spectroscopy in terms of solitons localized at the defect sites within the chains. From a comparison of the characteristic damping times of the Rabi oscillations with measurements of the spin relaxation times by means of primary-echo decay and CPMG methods it becomes evident that inhomogeneities in local magnetic fields strongly contribute to the soliton decoherence