Size and structure effect on optical transitions of iron oxide nanocrystals

The heterogeneity of electronic states of transition metal oxides produces a variety of properties, which may be discerned by the measurements of their nanocrystals. The structure and optical spectra of chemically synthesized pure ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$, $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$, and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanocrystals of different sizes have been investigated. Three types of electronic transitions: ligand to metal charge-transfer transitions, strong magnetic coupled ${\mathrm{Fe}}^{3+}$ ligand field transitions, and pair excitations, occur distinctly in the optical spectra of $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanocrystals except ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$. The quantum size effect (for itinerant carriers) and finite size effect (for local magnetic moments) were observed in our nanocrystals, although their bulks are strong electron correlation systems. The localized to delocalized transformation of electrons occurs with time delay in the femtosecond transient absorption spectra of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ nanocrystal sol, which indicates that heteroresponse times for different transitions agree well with the size-dependent indication of the steady-state absorption bands of these iron oxides nanocrystals. These results are helpful in understanding the relationship among $d\text{\ensuremath{-}}d$ transition, magnetic pairing, and charge-transfer in transition metal oxides.