X-ray Photoemission Study of the Charge State of Au Nanoparticles on Thin MgO/Fe(001) Films

Abstract

The charge state of Au nanoparticles deposited on MgO/Fe(001) ultrathin films has been investigated experimentally by means of X-ray photoemission spectroscopy and theoretically by performing density functional theory (DFT) calculations. The binding energy of the Au 4f7/2 core-level peak has been studied as a function of the Au coverage and of the MgO film thickness. The Au 4f7/2 peak presents a high binding energy value (about 600 meV higher than that of bulk Au) for the smallest Au nanoparticles deposited on thick MgO films. The binding energy shift is only 200 meV for Au particles on very thin MgO films, indicating a different electronic configuration in this case. For larger Au coverages, the binding energy of the Au 4f7/2 peak converges to the bulk value for both thick and thin MgO films. In view of the results from DFT calculations, we show that this behavior is consistent with the occurrence of a charge flow from the MgO/Fe(001) interface to the Au nanoparticles. We also show that, on ultrathin MgO films, the Au 4f7/2 photoemission line width is much sharper than that for thicker oxide films, a result which is rationalized based on the different growth mode of Au nanoclusters when a charge-transfer process takes place.