Optical tuning of molecule-induced magnetism at non-magnetic semiconductor nanoparticles

We have demonstrated that it is possible to reversibly photo-control the room temperature magnetism in a diluted magnetic semiconductor based on zinc oxide nanoparticles coated with azobenzene-derivatized thiols (Azo-ZnO NPs). These photochromic ligands provide a photo-function and at the same time act as a trigger for the generation of ferromagnetism at ZnO-S interface. UV irradiation increases the saturated magnetization of the Azo-ZnO NPs due to trans- to cis-isomerization in azobenzene-derivatized ligands, and following irradiation with visible light the magnetization is decreased due to inverse isomerization. X-ray photoelectron spectroscopy reveal that the photo-induced magnetization changes are strongly related to photo-induced changes in the chemical state of zinc atom in the ZnO. These photo-magnetic effects can be attributed to photo-induced changes in surface states of the ZnO due to photo isomerization in azobenzene ligands, which is accompanied by alterations in the lowest unoccupied molecular orbital level of the ligand, since magnetic properties of ZnO depend on carrier concentration, which correlated with surface states of the ZnO.