Linear and Two-Photon Absorption in Zero- and One-Dimensional CdS Nanocrystals: Influence of Size and Shape

Abstract

Colloidal CdS nanocrystals show strongly enhanced two-photon absorption (TPA) cross sections δ(2) of 104 to 105 GM over a wide NIR spectral range, making them ideal markers for confocal two-photon microscopy. We present a systematic study of the size and shape dependence of the linear and TPA cross sections for colloidal CdS dots and rods of diameters d between 2 and 5 nm. On the basis of z-scan measurements, we observe that the TPA cross section of colloidal CdS dots at 800 nm in and near the absorption continuum is manly shape and volume dependent, whereas it is less influenced by the electronic confinement. In the case of resonance to the lowest excitonic transition, a significant confinement-induced TPA enhancement with respect to the off-resonant case is observed. Colloidal CdS rods can exhibit a factor on the order of 10 larger δ(2) compared to CdS dots of the same diameter. In very small CdS rods a non-negligible three-photon absorption is found and assigned to a change in valence band symmetry.