Charge Carrier Dynamics of Nanoparticles
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Nanomaterials, studied extensively during the past two decades, represent a class of materials with at least one relevant dimension on the length scale of a few to a few hundred nanometers. In this size regime, properties of nanomaterials become strongly dependent on factors such as surface characteristics, shape, and size due to the quantum confinement effect. It is, in principle, possible to adjust the properties such as the color and redox potentials of nanomaterials by changing the particle size or shape, without any change of chemical composition. Nanomaterials become increasingly important in applications such as transistors, light emitting diodes, sensors, solar cells, and lasers.
Compared with the static properties such as optical absorption and emission, dynamic properties of charge carrier in nanomaterials are relatively less understood. Considerable progress has been made in the study and understanding of charge carrier dynamics recently. Most studies have focused on issues such as charge carriers trapping, carrier interaction, recombination, and their dependence on particle size, shape, surface characteristics. Studies of dynamic properties provide complementary information to static studies and help to gain a better fundamental understanding of the charge carrier behavior in nanomaterials.
Several powerful time-resolved laser techniques have been employed to investigate charge carrier dynamics, including transient absorption, transient bleach, and time-resolved fluorescence. In a transient absorption measurement, samples are first excited with an ultrafast laser pulse. A second laser pulse, delayed with respect to the first (excitation) pulse, is used to detect the population of charge carriers in excited states. Transient bleach has been used to monitor the population of ground states in a similar pump–probe setup. The excited-state population has also been monitored by time-resolved fluorescence combined with a fluorescence up-conversion method or stimulated emission.
This chapter reviews the recent advances in the studies of charge carrier dynamics and optical properties of semiconductor and metal nanoparticles (NPs). It starts with a general picture of photoexcited electron relaxation in semiconductor NPs. Specific nanoparticle systems are then discussed in some detail, including CdS, CdSe, CdTe, PbS, Cu2S, Ag2S, GaAs, InP, Mn2 +-doped ZnS, Mn2 +-doped ZnSe, TiO2, Fe2O3, AgI, AgBr, MoS2, PbI2, BiI3, and Si. The dynamic properties of metal NPs are then illustrated with examples such as Ag and Au NPs. Possible dependence of charge carrier dynamics on parameters such as size, shape, and surface characteristics is discussed with appropriate examples. A summary is given at the end of the chapter.