The synthesis of PbSe/PbS and PbSe/PbSexS1–x core/shell nanocrystals (NCs) with luminescence quantum efficiencies of 45–55 % is reported. PbSe/PbS NCs are prepared via a two‐stage process, while the PbSe/PbSexS1–x NCs are formed in a single‐stage procedure. The core/shell NCs exhibit an energy tuning of the exciton transitions, with respect to that of the core NC, that is dependent on the core diameter, shell thickness, and composition.

The optical properties and functionality of air-stable PbSe/PbS core-shell and PbSe/PbSexS1-x core-alloyed shell nanocrystal quantum dots (NQDs) are presented. These NQDs showed chemical robustness over months and years and band-gap tunability in the near infrared spectral regime, with a reliance on the NQD size and composition. Furthermore, these NQDs exhibit high emission quantum efficiencies of up to 65% and an exciton emission band that is narrower than that of the corresponding PbSe NQDs. In addition, the emission bands showed a peculiar energy shift with respect to the relevant absorption band, changing from a Stokes shift to an anti-Stokes shift, with an increase of the NQD diameter. The described core-shell structures and the corresponding PbSe core NQDs were used as passive Q-switches in eye-safe lasers of Er:glass, where they act as saturable absorbers. The absorber saturation investigations revealed a relatively large ground-state cross-section of absorption (sigma gs = 10(-16) - 10(-15) cm2) and a behavior of a "fast" absorber with an effective lifetime of tau eff approximately 4.0 ps is proposed. This lifetime is associated with the formation of multiple excitons at the measured pumping power. The product of sigma gs and tau eff enables sufficient Q-switching performance and tunability in the near infrared spectral regime. The amplified spontaneous emission properties of PbSe NQDs were examined under continuous illumination by a diode laser at room temperature, suitable for standard device conditions. The results revealed a relatively large gain parameter (g = 2.63 - 6.67 cm-1). The conductivity properties of PbSe NQD self-assembled solids, annealed at 200 degrees C, showed an Ohmic behavior at the measured voltages (up to 30 V), which is governed by a variable-range-hopping charge transport mechanism.

The synthesis and optical properties of core/shell and core/shell/shell nanocrystals based on CdTe nanocrystals are described. The influence of the different charge‐carrier separation in the CdTe/CdS nanocrystals (type‐I heterostructure) and the CdTe/CdSe nanocrystals (type‐II heterostructure) is examined. The behavior of CdTe/CdS/CdSe nanocrystals (see image) in comparison to two core/shell systems is also studied.

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.

Semiconductor nanocrystals (NCs) exhibit size-controlled spectral tunability and chemical flexibility, making them attractive materials for use in new and emerging applications, such as fluorescent tagging, [1] lasing, [2] light-emitting diodes [3] and nanoelectronics.[4] One-dimensional quantum wires (QWs) and quantum rods (QRs) have become a class of attractive materials as their dimensional anisotropic behaviour gives rise to unique physical properties; for example, recent observations of CdSe QRs showed a non-monotonic change of the fluorescence Stokes shift with an increase in the aspect ratio (length/ width), exhibiting a linearly polarized photoluminescence.