Infrared Carrier Dynamics in Mercury Chalcogenide Quantum Dots: from Fundamental Spectroscopy to Next-Generation Optoelectronics

The mercury chalcogenide quantum dots are an emerging class of infrared nanomaterial being developed as next-generation infrared photodetectors and light sources. Although considerable progress has been made on the development of near- and mid-infrared detectors and light sources based on these systems, the basic material photophysics remains relatively unexplored. In this thesis, I describe time-resolved spectroscopy investigations of HgTe and HgSe quantum dots in the near- and mid-infrared. Transient absorption measurements on HgTe show that biexciton Auger recombination is much slower than in bulk materials of similar gaps, and comparable to that in quantum dots with gaps an order of magnitude larger. This suggests that the Auger mechanism is modified in quantum dots compared to bulk semiconductors, and possibly scattering-assisted. Using a simple detailed-balance model of thermal carrier generation and recombination in an HgTe quantum dot solid, I then show how the slow Auger recombination leads to thermodynamic detector performance limits which exceed those of commercial devices by an order of magnitude. In HgSe, transient photoluminescence upconversion measurements show near-total Auger suppression in mid-infrared n-type particles and typical quantum dot Auger behavior in near-infrared intrinsic particles. Auger suppression is a fundamental benefit for mid-infrared photodetection and lasing, and this phenomenon is ascribed to the different densities of states in the two situations. In the single-exciton regime, spectrally-resolved photoluminescence dynamics reveal conduction fine structure and direct phonon-mediated intersublevel relaxation at early times. The mid-infrared nonradiative relaxation at longer times is investigated through quantum yield and lifetime measurements on n-type HgSe and HgSe/CdS core/shell structures. Modeling indicates that near-field energy transfer to infrared-absorbing species remains the dominant nonradiative pathway out to the regime of several nanoseconds, and the ~2% quantum yields measured in thick-shell n-type HgSe/CdS are the largest reported to date in mid-infrared colloidal nanomaterials. This thesis highlights several unique aspects of carrier dynamics in small-gap quantum dots, and it fundamentally justifies continued work on devices made from these materials