Link between Structural and Optical Properties of CoxFe3-xO4 Nanoparticles and Thin Films with Different Co/Fe Ratios

CoxFe3-xO4 nanoparticles (x = 0.4 to x = 2.5) and thin films (x = 0.9 to x = 2.2) are analyzed by Raman, absorption, and photoluminescence spectroscopy to link structural and optical properties to different cobalt to iron (Co/Fe) ratios. Raman spectroscopy shows that with decreasing Co content, the crystal structure changes from a predominantly normal cubic spinel phase to a mixed inverse spinel phase. This finding is supported by absorption spectroscopy that points out that inter valence charge transfer (IVCT) processes between octahedrally coordinated Co2+ and Fe3+ cations become more prominent with increasing Fe content. Independent of the Co/Fe ratio, CoxFe3-xO4 nanoparticles show a broad photoluminescence (PL) band with a maximum at around 510 nm. Time-resolved photoluminescence spectroscopy shows subnanosecond lifetimes and temperature-resolved photoluminescence experiments reveal that the green PL increases with decreasing temperature (300 to 10 K) while showing no temperature-dependent shift in energy. It is proposed that this green PL originates from OH-groups on the particles' surface.

Automated summary

The structural and optical properties of CoxFe3-xO4 nanoparticles change with increasing Fe content, as revealed by Raman and absorption spectroscopy, with inter valence charge transfer processes becoming more prominent. The photoluminescence of the nanoparticles shows a broad band with a peak at around 510 nm, while temperature-resolved experiments indicate that the green photoluminescence is likely due to OH-groups on the surface.