Ultrafast intraband Auger process in self-doped colloidal quantum dots

Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs. Excess charge accumulation in quantum dots is unavoidable when running electronic devices. Since it is an instant phenomenon happening randomly, there have not been many systematic approaches to investigate it. Also, the excess charge-accumulated state is an adequate model for exploration of higher quantum states. n-Type self-doped quantum dots provide an excellent platform from which to study the electron-accumulated state in the conduction band. In combination with femtosecond mid-infrared spectroscopy, we were able to selectively photoexcite the excess electrons in the lowest energy state of the conduction band and monitor the electron dynamics that have never been directly and experimentally measured. In particular, ultrafast electron dynamics are revealed by this method such as the intraband Auger process, helping us to comprehend the blinking of single quantum dots, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of colloidal quantum dots. Self-doped colloidal quantum dots are fascinating materials in which excess electrons occupy the lowest energy state in the conduction band at steady state. Femtosecond mid-infrared spectroscopy can selectively monitor electron dynamics occurring only in the conduction band of self-doped β-HgS colloidal quantum dots without creating a hole in the valence band. Quantitative analyses make it possible to reveal the hidden electron dynamics: intraband Auger process, inter-sublevel transition, and electronic-to-vibrational energy transfer process.