Charge Carrier Dynamics and Thermal Excitations in Nickel and Zinc Ferrite Nanoparticle Films

Abstract

We study charge carrier dynamics in 100 nm thin films of the nanocrystalline spinels γ-Fe2O3, ZnFe2O4, and NiFe2O4 on FTO-coated glass substrates by transient absorption spectroscopy from femtosecond to nanosecond time scales. The induced optical absorption can be attributed to excitations of holes in the valence band, and the relaxation of transient signals is well-described by a sum of exponential decay processes with time constants of 0.54 ps–0.66 ps, 6.2 ps–9.2 ps, and 70 ps–260 ps. We associate these relaxation times with electron–phonon scattering, charge carrier lifetime, and relaxation of trapped carriers/thermal equilibration, respectively. Time constants were longest for NiFe2O4, followed by ZnFe2O4 and γ-Fe2O3. Within less than a nanosecond, the emerging transient spectra resemble differential spectra of samples at elevated temperature from 18 K (ZnFe2O4) to 44 K (NiFe2O4). Our findings point to mid-gap states near the valence band edge that can be thermally populated and result in increased visible excitation but appear to yield charge carriers that are only weakly active electrochemically in comparison with those shown by hematite films.