Origin of Intraband Optical Transitions in Ag2Se Colloidal Quantum Dots

In the past 30 years, scientists have utilized quantum confinement to obtain size-tunable interband optical transitions in colloidal quantum dots (CQDs) and implemented them in various optoelectronic applications throughout the electromagnetic spectrum. The infrared (IR) region is particularly important with applications in telecommunications, night-time surveillance, and satellite imaging for agricultural water conservation. Nearly all progress with CQDs in the IR region has been achieved using interband transitions in Pb- and Hg-based heavy metal compounds with narrow band gaps. An alternative approach is to exploit intraband optical transitions originating from external- or self-dopants, which could expand the library of materials for IR-optoelectronic devices to include nontoxic materials. Herein, we present a simple two-precursor hot-injection (170 °C) synthesis of 2.6–6.5 nm diameter environmentally benign Ag2Se CQDs that exhibit a crossover from interband near-infrared (NIR) absorption to intraband mid-wave infrared (MWIR) absorption above a critical size of 5.1 nm. CQDs smaller than 5.1 nm are photoactive in the NIR, exhibiting multiple well-defined excitonic peaks and stable room-temperature emission in the NIR and short-wave infrared (SWIR) regions of the electromagnetic spectrum. Films cast from these CQDs and ligand-exchanged with ethanedithiol exhibit NIR photoconductivity. In contrast, CQDs larger than 5.1 nm exhibit MWIR absorbance. Compared to other synthetic methods that generate Ag2Se CQDs over a limited size range, our approach allows access to both ultrasmall and large Ag2Se CQDs, enabling a detailed study of the size-dependent interband to intraband optical transition. We 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 obtain tunable intraband MWIR absorption.