The investigation of sub-nanosecond exciton dynamics in HgTe colloidal quantum dots using ultrafast transient absorption spectroscopy is reported. The transmittance change spectrum acquired immediately after pumping is dominated by a bleach blue-shifted by ~200-300 nm from the photoluminescent emission band. Comparison with a tight-binding model of the electronic structure allows this feature to be attributed to the filling of band edge states. The form of the pump-induced transmittance transients is dependent on the excitation rate and the rate of sample stirring. For moderate pumping of stirred samples, the transmittance transients are well-described by a mono-exponential decay associated with biexciton recombination, with a lifetime of 49 ± 2 ps. For samples that are strongly-pumped or unstirred, the decay becomes bi-exponential in form, indicating that trap-related recombination has become significant. We also present a new analysis that enables fractional transmittance changes to be related to band edge occupation for samples with arbitrary optical density at the pump wavelength. This allows us to identify the occurrence of multiple exciton generation, which results in a quantum yield of 1.36 ± 0.04 for a photon energy equivalent to 3.1 times the band gap, in good agreement with the results of the model.

No abstract

Sub-nanosecond charge dynamics in monodisperse Au nanoparticles (NPs) supported on TiO2 colloidal spheres are studied as a function of NP diameter using ultrafast transient absorption spectroscopy. The decay of the transmittance changes observed in the region of the plasmon resonance of the Au NPs following photoexcitation of the TiO2 spheres are well-described by a bi-exponential function consisting of a fast component of 2 ps duration associated with electron-phonon scattering, followed by a slow and relatively weak component associated with phonon-phonon scattering. The decay constant characterising the latter component was found to be dependent on the size of the Au NPs, rising from 49 ± 3 to 128 ± 6 ps as the diameter of the Au NPs increased from 12.2 ± 2.2 nm to 24.5 ± 2.8 nm, respectively.

No abstract