Femtosecond studies of photoinduced electron dynamics in colloidal quantum-confined II-VI semiconductor nanoparticles: CdS, CdSe and CdZnS.

摘要

A variety of synthetic and spectroscopic techniques have been applied to elucidate photoinduced charge carrier processes in II-VI semiconductor quantum dots. These semiconductor nanoparticles exhibit both size-dependent optical tuning due to the quantum-confinement effect and power-dependent absorption, bleach and emission characteristics. Although the tunable-absorption has been well characterized, the subsequent trapping and recombination processes are still under much investigation and are the subject of this dissertation.; Particles with vastly differing surfaces, sizes, energetics and solvents have been characterized using various spectroscopic techniques in unison. The primary technique was transient femtosecond near-IR absorption, which was used to characterize charge carrier processes on the subpicosecond and picosecond time scales. UV-visible spectroscopy was used to characterize the size of the particles. Static fluorescence measurements were used to characterize the surface of the particles and the relative amount of radiative recombination. Nanosecond fluorescence measurements were also used to assist in the assignment of the fast, power-dependent near-IR absorption decay.; The research reported here makes two fundamental contributions to the photophysics of semiconductor nanoparticles. First, the power-dependent, few picosecond decay process has primarily been assigned to electron-hole recombination via exciton-exciton annihilation. As the power increases, higher order, Auger processes may also arise. The exciton-exciton annihilation mechanism was primarily deduced based on power-dependent fluorescence measurements which exhibited the formation of short-lived exciton fluorescence at high powers. Secondly, many nanoparticle properties and environments were varied in order to better understand the observed picosecond processes and the effect of variations on these processes. The systems studied ranged from aqueous acidic and basic quantum dots of differing size and surface, organic dots of differing sizes and surfaces to dots in glass and bulk CdS. Although substantial changes were observed in static absorption and fluorescence measurements, the effects were relatively small on the observed early time dynamics, with the greatest effect being the decrease in the exciton-exciton annihilation time constant from 5 to 1 {dollar}{lcub}1over 2{rcub}{dollar} picoseconds as the number of photoexcitations per particle increases from two to greater than ten. Otherwise, many variations had little effect which should allow considerable freedom in the eventual integration of nanoparticles into devices.