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Origin of Low Energy d–d Excitations Observed on Wet Chemically Prepared Cobalt Bearing Nanoparticles by 2p3d Resonant X‑ray Emission Spectroscopy

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journal contribution
posted on 11.07.2013 by Matti M. van Schooneveld, Amélie Juhin, Carlos Campos-Cuerva, Thorsten Schmitt, Frank M. F. de Groot
The 2p3d resonant X-ray emission spectroscopic (RXES) measurements on 8.4 and 5.0 nm cobalt and 3.6 nm cobalt–nickel nanoparticles coated with oleate molecules are provided. The spectra reveal low energy resonant Raman features at 0.3 and 0.75 eV. In combination with time-dependent density functional theory (TD-DFT) and ligand field multiplet (LFM) calculations, these are ascribed to d–d excitations of cobalt ions in a low symmetry ligand field. Two different chemical environments of the ion may cause the transitions. In the first model cobalt ions in the nanoparticle outer atomic layer, resulting from the adsorbate binding, cause the excitations. These are transitions from a mixture of 4B1g plus 4Eg to 4Eg at 0.3 eV. At 0.75 eV transitions to 4B2g and 4A1g take place. In the alternative model the excitations occur in a cobalt molecular species that might coexist with the nanoparticles. Here the transitions are from 4A2g to 4Eg and to 4Eg plus 4B2g at 0.3 and 0.75 eV, respectively. On the basis of two-dimensional 2p3d RXES planes of the models and the differences between the three different nanoparticle systems, we exclude the first model and conclude that the metallic particles indeed coexist with varying minor degrees of molecular species. We observe however a second type of cobalt species, which is possibly related to surface-ligated cobalt ions.