Air-Stable PbSe/PbS and PbSe/PbSexS1-x Core−Shell Nanocrystal Quantum Dots and Their Applications

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Abstract
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 (σgs = 10-16−10-15 cm2) and a behavior of a “fast” absorber with an effective lifetime of τeff ≈ 4.0 ps is proposed. This lifetime is associated with the formation of multiple excitons at the measured pumping power. The product of σgs and τeff 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°C, showed an Ohmic behavior at the measured voltages (up to 30 V), which is governed by a variable-range-hopping charge transport mechanism.