Excitation-dependent ultrafast carrier dynamics of colloidal TiO₂ nanorods in organic solvent

The relaxation dynamics of charge carriers of organic capped TiO₂ nanorods dispersed in chloroform was investigated by femtosecond transient absorption in a weak-excitation regime. Anisotropic TiO₂ nanocrystals were excited in the UV-vis range, using different pump wavelengths, namely above (300 nm), close to (350 nm), and below (430 nm) the direct band gap of anatase TiO₂. We show that the ultrafast dynamics strongly depends on excitation wavelength and determine the time constants of all the processes entering the relaxation. Moreover, we demonstrate that two transient absorption bands at 500 and 700 nm, typically attributed to trapped h⁺ and e^-, respectively, are accessible only when TiO₂ is photoexcited well above the band gap, while there is no evidence of such bands when TiO₂ is photoexcited close to or below its band gap. In such cases the observed dynamics are attributed to trapped excitons.