Giant two-photon absorption of anatase TiO₂ in Au/TiO₂ core-shell nanoparticles

Abstract

We report on deep-to-near-UV transient absorption spectra of core-shell Au / SiO 2 and Au / TiO 2 nanoparticles (NPs) excited at the surface plasmon resonance of the Au core, and of UV-excited bare anatase TiO 2 NPs. The bleaching of the first excitonic transition of anatase TiO 2 at ∼3.8 eV is a signature of the presence of electrons/holes in the conduction band (CB)/valence band (VB) of the material. We find that while in bare anatase TiO 2 NPs, two-photon excitation does not occur up to the highest used fluences (1.34 mJ / cm 2 ), it takes place in the TiO 2 shell at moderate fluences (0.18 mJ / cm 2 ) in Au / TiO 2 core-shell NPs, as a result of an enhancement due to the plasmon resonance. We estimate the enhancement factor to be of the order of ∼10 8 – 10 9 . Remarkably, we observe that the bleach of the 3.8 eV band of TiO 2 lives significantly longer than in bare TiO 2 , suggesting that the excess electrons/holes in the conduction/valence band are stored longer in this material.