Ferromagnetism induced in ZnO nanorods by morphology changes under a nitrogen-carbon atmosphere
Résumé
We use thermal carbonization with acetylene and nitrogen to treat hydrothermally grown ZnO nanorods on silicon substrates. The method is found to be strongly temperature dependent. Treatment temperatures below 800 °C do not induce any morphological changes of the rods, while temperatures above 800 °C cause significant erosion of the rods leading to hollow- and tubular-like structures. The temperature dependent weight decrease confirms the observation of erosion and X-ray photoelectron spectroscopy (XPS) measurements indicate significant decrease in Zn content. Raman spectra evidence the presence of a diamond-like carbon layer around the rods. The coupling of resonant and non-resonant Raman results with photoluminescence measurements allow us to suggest that both nitrogen and carbon are present within the ZnO lattice. The samples treated above 800 °C are also found to be ferromagnetic and the magnetization increases as the treatment temperature increases (up to 1.45 emu cm−3 at 5 K). Thermal annealing in nitrogen atmosphere does not cause either morphological changes or ferromagnetism, suggesting that the erosion results from the additional carbon source (acetylene) of the treatment. Complexes formed by carbon and nitrogen atoms at the surface of the treated and eroded samples are believed to be at the origin of the ferromagnetism.