Thiol-capped CdTe nanocrystals with cubic zinc blende structure are synthesized in aqueous solution. Their steady-state and time-resolved luminescence characteristics are studied at room and liquid nitrogen temperatures. A strong exciton luminescence peak at 2.3 eV dominates the emission spectrum of CdTe nanocrystals at room temperature, whereas the trap band centered at 2.0 eV undergoes substantial temperature quenching. Luminescence excitation spectra reveal different channels leading to radiative recombination via either excitons or traps. The mean luminescence decay time of CdTe nanocrystals at room temperature decreases from 120 ns at 1.94 eV to 20 ns at 2.43 eV. Luminescence decay kinetics of CdTe nanocrystals are strongly nonexponential and are described by extremely broad lifetime distributions lying within the range from a few hundred picoseconds to a few hundred nanoseconds.

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We report on the results of a detailed quantitative experimental evaluation of exciton relaxation pathways as well as direct measurement of singlet oxygen (1O2) generation efficiencies for CdSe/ZnS quantum dot (QD)– porphyrin nanocomposites in toluene at 295 K. QD photoluminescence quenching in nanocomposites is caused by two main factors: electron tunneling in the quantum confined QD (efficiency 0.85–0.90) and Förster resonance energy transfer (FRET) QD→porphyrin (quenching efficiency 0.10–0.15). Efficiencies of 1O2 generation γΔ by nanocomposites are essentially higher with respect to those obtained for QDs alone. For nanocomposites, the nonlinear decrease of 1O2 generation efficiency γΔ on the laser pulse energy is caused by nonradiative intraband Auger processes, realized in the QD counterpart. Finally, FRET efficiencies found from the direct sensitization data for porphyrin fluorescence in nanocomposites (ΦFRET = 0.14 ± 0.02) are in good agreement with the corresponding values obtained via the direct 1O2 generation measurements at low laser excitation (ΦFRET Δ = 0.12 ± 0.03). The obtained quantitative results provide for the first time strong evidence that a FRET process QD→porphyrin is the reason for singlet oxygen generation by nanocomposites.

Self-assembly of only one functionalized porphyrin dye molecule with one CdSe/ZnS quantum dot (QD) not only modifies the photoluminescence (PL) intensity but also creates a few energetically clearly distinguishable electronic states, opening additional effective relaxation pathways. The related energy modifications are in the range of 10-30 meV and show a pronounced sensitivity to the specific nature of the respective dye. We assign the emerging energies to surface states. Time-resolved PL spectroscopy in combination with spectral deconvolution reveals that surface properties of QDs are a complex interplay of the nature of the dye molecule and the topography of the ligand layer across a temperature range from 77 to 290 K. This includes a kind of phase transition of trioctylphosphine oxide ligands, switching the nature of surface states observed below and above the phase transition temperature. Most importantly, our findings can be closely related to recent calculations of ligand-induced modifications of surface states of QDs. The identification of the optical properties emerged from a combination of spectroscopy on single QDs and QDs in an ensemble.