Abstract
Semiconductor nanocrystals (NCs), or semiconductor quantum dots, exhibit novel physical properties not observable in bulk materials.1 Three-dimensional carrier confinement results in a discrete atomiclike energy spectrum with a level spacing that can greatly exceed typical phonon energies. This has been expected to significantly inhibit phonon-related mechanisms of energy losses, an effect commonly referred to as a phonon bottleneck.2 Because of a large surface-to-volume ratio, carrier dynamics are strongly affected by surface/interface states. Trapping at surface states can lead to a fast depopulation of quantized states, accompanied by charge separation and generation of local fields that significantly modify the optical properties of NCs.3 In this paper, we report femtosecond (fs) studies of energy relaxation and trapping dynamics in three types of quantum-dot systems: semiconductor NC/glass composites made by high- temperature precipitation, ion-implanted NCs, and colloidal NCs generated via organometallic routes. Comparison of ultrafast data for these three types of samples allows us to separate effects intrinsic to quantum dots from those related to lattice imperfections and interface properties.
© 1998 Optical Society of America
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