Magnetic functionalization of ZnO nanoparticles surfaces via optically generated methyl radicals.

The combination of nuclear and electron magnetic resonance techniques, in pulse and continuous wave regimes, is used to unravel the nature and features of the light-induced magnetic state arising at the surface of chemically prepared zinc oxide nanoparticles (NPs) occurring under 120 K when subjected to a sub-bandgap (405 nm) laser excitation. It is shown that the four-line structure observed around g ∼ 2.00 in the as-grown samples (beside the usual core-defect signal at g ∼ 1.96) arises from surface-located methyl radicals (•CH3), originating from the acetate capped ZnO molecules. By functionalizing the as-grown zinc oxide NPs with deuterated sodium acetate, the •CH3 electron paramagnetic resonance (EPR) signal is replaced by trideuteromethyl (•CD3). For •CH3, •CD3, and core-defect signals, an electron spin echo is detected below ∼100 K, allowing for the spin-lattice and spin-spin relaxation-time measurements for each of them. Advanced pulse-EPR techniques reveal the proton or deuteron spin-echo modulation for both radicals and give access to small unresolved superhyperfine couplings between adjacent •CH3. In addition, electron double resonance techniques show that some correlations exist between the different EPR transitions of •CH3. These correlations are discussed as possibly arising from cross-relaxation phenomena between different rotational states of radicals.

Origin of the Magnetic Exciton in the van der Waals Antiferromagnet NiPS_{3}.

An ultrasharp photoluminescence line intimately related to antiferromagnetic order has been found in NiPS_{3}, a correlated van der Waals material, opening prospects for magneto-optical coupling schemes and spintronic applications. Here we unambiguously clarify the singlet origin of this excitation, confirming its roots in the spin structure. Based on a comprehensive investigation of the electronic structure using angle-resolved photoemission and q-dependent electron energy loss spectroscopy as experimental tools we develop, in a first step, an adequate theoretical understanding using density functional theory (DFT). In a second step the DFT is used as input for a dedicated multiplet theory by which we achieve excellent agreement with available multiplet spectroscopy. Our Letter connects the understanding of the electronic structure and of optical processes in NiPS_{3} and related materials as a prerequisite for further progress of the field.

Light-induced high-spin state in ZnO nanoparticles.

The effects of white-light irradiation on ∼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 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 depend on the nanoparticles growth conditions.

Core-defect reduction in ZnO nanorods by cobalt incorporation.

Zinc 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 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 [Formula: see text] 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.

EPR investigation of pure and Co-doped ZnO oriented nanocrystals.

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.

Ligand field parameters and the ground state of Fe(II) phthalocyanine.

A judicious analysis of previously published experimental data leads one to conclude that the ground state of iron(II) phthalocyanine is an orbitally degenerate spin triplet a(1g)(2) e(g)(↑↓↑) b(2g)(↑) ((3)Eg). The ligand field parameters, in relation to Racah's C, are approximately as follows: B20∕C = 0.84, B40∕C = 0.0074. The uniqueness of this result is demonstrated by means of a special diagram in the B20∕C - B40∕C plane (under additional conditions that B44∕B40 = 35∕3 and B∕C = 0.227). The system is in a strong-ligand-field regime, which enables the use of single-determinant techniques corrected for correlations within the 3d shell of Fe.