In the past 30 years, chemists and material scientists have largely capitalized on the grand appeal of utilizing quantum confinement to obtain size-tunable inter-band optical transitions and implementing colloidal quantum dots (CQDs) in optoelectronic applications throughout the electromagnetic spectrum. The infrared (IR) region is particularly exciting with applications in molecular spectroscopy, telecommunications, night-time surveillance, and satellite imaging for agricultural water conservation. Historically, single crystals of InGaAs, InSb, and HgCdTe (MCT), as well as epitaxial superlattice materials have been used as IR detectors. Unfortunately, these detectors use expensive fabrication processes and typically require cryogenic cooling. In this regard, colloidal quantum dot-based IR photodetectors present a promising path toward fabricating sensors and imagers at significantly reduced cost compared to state-of-the-art epitaxial grown quantum-well IR detectors while offering many new modalities for detector fabrication. While most progress with CQDs in the IR has been achieved using inter-band transitions in Pb- and Hg-based heavy metal compounds, intra-band optical transitions originating from external- or self- dopants can potentially expand the library of materials to generate IR-optoelectronic devices with non-toxic materials. In this talk, I will focus on my group’s work on recently discovered silver chalcogenide (Ag2Se) quantum dots that exhibit distinct optical absorption in the mid-IR wavelength spectrum. These CQDs demonstrate a narrow bandgap metastable tetragonal phase, not available in bulk, and contain excess electrons in the lowest level of the conduction band. This allows for intra-band optical transitions between the first and the second conduction energy level which can significantly decrease Auger recombination rates and avoid the need for cryogenic cooling. I will present a detailed study of the size-dependent inter-band to intra-band optical transition and compare the competing effects of quantum confinement, environmental Fermi level and particle stoichiometry to provide guidelines for stable electron occupation of the 1Se state and obtaining tunable mid-IR absorption.

Elucidating the origin of tunable intra-band optical transitions in Ag2Se colloidal quantum dots and their potential utilization for infrared photodetectors

ACS Fall 2021

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