Investigation of self-trapped excitonic dynamics in hematite nanoforms through non-degenerate pump-probe transmission spectroscopy

Non-degenerate pump-probe transmission spectroscopy is used to examine the ultrafast dynamics of photo-excited carriers in hematite nanoforms at various pump fluences. Using coupled rate equations, the kinetics of self-trapped exciton (STE) formation and its interaction with free excitons resulting in exciton annihilation were studied. It is shown from this model that the majority of the excitons were trapped by polaronic trap states to form self-trapped excitons within & SIM;3.5 ps. The findings indicate that free excitons and STEs interact non-linearly, similar to trap-assisted bi-molecular Auger recombination to annihilate one another. It is observed that there is substantial dependence of kinetics of STE formation and exciton decay on photo-excited exciton density, and the nature of this dependence indicates the reduced screening of electron-phonon interaction. Using the screening model applied to the rate constants of STE formation and decay, we estimate the saturation exciton density to be & SIM;3.3 x 10(17) cm(-3) and the average STE density to be & SIM;3.8 x 10(18) cm(-3) in the hematite nanoforms. We also noticed that doping K and Ni to hematite nanoforms up to 5% did not remarkably change the nature of the exciton dynamics. Published under an exclusive license by AIP Publishing.

Automated summary

Non-degenerate pump-probe transmission spectroscopy was used to study the ultrafast dynamics of photo-excited carriers in hematite nanoforms. A coupled rate equation model was employed to investigate the formation of self-trapped excitons (STE) and their interaction with free excitons leading to exciton annihilation. The results demonstrate that the majority of excitons are trapped by polaronic trap states and form self-trapped excitons within approximately 3.5 ps. There is a nonlinear interaction between free excitons and STEs, similar to trap-assisted bi-molecular Auger recombination.