Ultrafast studies of photoexcited electron dynamics in γ- and α-Fe2O3 semiconductor nanoparticles

The ultrafast dynamics of photoexcited electrons in the semiconductor iron oxides, gamma-Fe2O3 and alpha-Fe2O3, have been measured using femtosecond laser spectroscopy. Transmission electron microscopy shows the y-Fe2O3 particles are spherical, with 1-2 nm average diameter, and the alpha-Fe2O3 are spindle-shaped, with average dimensions of 1 x 5 nm. Static electronic absorption measurements of the colloids suggest that they may be in the quantum confined regime. Steady-state emission measurements show that the <400 nm direct transitions are moderately emissive, but the indirect transitions in the visible do not produce measurable emission. The ultrafast transient absorption decay profiles measured for the gamma-Fe2O3 and alpha-Fe2O3 samples synthesized in our lab as well as a commercial sample of gamma-Fe2O3 are the same and are fit best with three exponentials with 0.36, 4.2, and 67 ps time constants. The decay profiles are independent of pump power, probe wavelength, and pH and were not affected by lattice doping with other metals or surface adsorbates for the cases studied. This fast overall decay, in conjunction with very weak fluorescence, suggests extremely efficient nonradiative relaxation, possibly related to the dense band structure, a high density of trap states, or simply strong coupling between trap states. The fast relaxation of photoexcited electrons in the conduction band and trap states is consistent with the low photocurrent efficiency of typical Fe2O3 electrodes.