RESUMO
The thermal activation processes in PbSe colloidal quantum dots and their influence on the ground-state exciton emission are discussed. Activation of a dark exciton occurs at 1.4-7 K, assisted by an acoustic phonon coupling. Activation of a bright exciton occurs at 100-200 K, which appears as a sudden change in the photoluminescence band intensity, energy, and full width at half maximum. This activation overcomes the dark-bright-state splitting, when the activation temperature increases with the decrease of the dots' size. The dark exciton lifetime is found to be approximately 6-12 micros at 1.4 K, while the bright exciton lifetime at 300 K evaluated as 450 ns varies slightly with the change in the size of the dots. In addition, the emission quantum yield of these dots, measured at a variety of temperatures when dissolved in various solvents, reveals information about the influence of the environment on the recombination processes.
Assuntos
Coloides , Chumbo/química , Luz , Pontos Quânticos , Compostos de Selênio/química , Temperatura , Luminescência , Microscopia Eletrônica de Transmissão , Espectrofotometria UltravioletaAssuntos
Cádmio/química , Nanopartículas Metálicas/química , Nanopartículas/química , Nanopartículas/classificação , Telúrio/química , Compostos de Cádmio/química , Cristalização , Eletroquímica , Nanopartículas Metálicas/ultraestrutura , Microscopia Eletrônica de Transmissão , Nanopartículas/ultraestrutura , Nanotecnologia , Pontos Quânticos , Selênio/química , Semicondutores , Espectrofotometria , Sulfetos/químicaRESUMO
The saturable optical absorption properties of PbSe core nanocrystals (NCs), and their corresponding PbSe/PbScore/shell and PbSe/PbSexS(1-x) core/alloyed-shell NCs, were examined at lambda = 1.54 microm. Saturation intensities of approximately 100 MW/cm2 were obtained. The NCs act as passive Q switches in near-infrared pulsed lasers. Q-switched output pulse energies up to 3 mJ, with a pulse duration of 40-55 ns were demonstrated. Analysis of the optical transmission versus pulse light intensity was carried out according to a model that includes ground-state as well as excited-state absorption. For pulses approximately 10 ns long, the NCs act as fast saturable absorbers. The theoretical fits yield a ground-state absorption cross section of 10-16-10-15 cm2, an excited-state absorption cross section of sigma(es) is congruent to 10(-16) cm2, and an effective lifetime of tau(eff) is congruent to 5 x 10(-12) s.
