Ultrafast exciton dynamics in CdxHg(1-x)Te alloy quantum dots

Ultrafast transient absorption spectroscopy is used to investigate sub-nanosecond exciton dynamics in CdxHg(1−x)Te alloy colloidal quantum dots. A bleach was observed at the band gap due to state-filling, the mono-exponential decay of which had a characteristic lifetime of 91 ± 1 ps and was attributed to biexciton recombination; no evidence of surface-related trapping was observed. The rise time of the bleach, which is determined by the rate at which hot electrons cool to the band-edge, ranged between 1 and 5 ps depending on the pump photon energy. Measuring the magnitude of the bleach decay for different pump fluences and wavelengths allowed the quantum yield of multiple exciton generation to be determined, and was 115 ± 1% for pump photons with energy equivalent to 2.6 times the band gap

Optical properties of HgTe nanocrystals

HgTe nanocrystals presently receive growing interest because the negative band gap in bulk HgTe enables tunability of the gap from the infrared to the near infrared in quantum dots thanks to the quantum confinement. Therefore we propose a tight-binding model of HgTe which gives an accurate band structure in a wide energy range of energy compared to recent ab initio calculations. The inverted band structure near the Fermi level and its temperature dependence are also very well described. Using this model, we study the effects of the quantum confinement on the electronic structure of HgTe quantum dots. We calculate the optical absorption spectra of quantum dots with various shapes and diameters up to 10 nm, including excitonic effects using a configuration interaction approach [1]. The optical spectra are consistent with recent experimental data. We also simulate the multiple exciton generation and we discuss the results of ultrafast transient absorption spectroscopy experiments performed on HgTe nanocrystals. [1] G. Allan and C. Delerue, Phys. Rev. B 86, 165437 (2012). [2] A. Al-Otaify, S.V. Kershaw, S. Gupta, A.L. Rogach, G. Allan, C. Delerue and D.J. Binks, Phys. Chem. Chem. Phys. 15, 16864 (2013)