Co-Doped SnO2 Nanocrystals: XPS, Raman, and Magnetic Studies

Even though doping small sized NCs is a challenging task, 2.7 nm sized tin oxide (SnO2) nanocrystals (NCs) are successfully doped with cobalt (Co). Studies are carried out in dilutely Co-doped SnO2 NCs, so as to avoid Co cluster formation and interference due to extrinsic effect. Co2+ ions are substitutionally doped in SnO2 lattice as seen from the d-d transitions in optical absorption spectra. Further x-ray photoelectron spectroscopic measurements also confirm Co2+ ionic state in SnO2 NCs. In view of the known fact that ferromagnetism in NCs arises due to different defects, Raman measurements are carried out. Raman spectra reveal presence of "in-plane" oxygen vacancies, which is also substantiated by photoluminescence spectra. Even though, Co-doping is not profoundly affecting the structure of SnO2 NCs, subtle ferromagnetic ordering observed in un-doped NCs get destroyed in Co-doped NCs. The present work reiterates the importance of Co-Co anti-ferromagnetic interaction and also indirectly supports the conjecture that oxygen vacancies adjacent with Co ions in SnO2 lattice in a necessary condition for realizing ferromagnetic ordering.